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Twelve apatite samples have been tested as secondary ion mass spectrometry (SIMS) reference materials. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis shows that the SLAP, NUAN and GR40 apatite gems are internally homogeneous, with most trace element mass fractions having 2 standard deviations (2s) ≤ 2.0%. BR2, BR5, OL2, AFG2 and AFB1, which have U > 63 μg g-1, 206Pb/204Pb > 283, and homogeneous SIMS U-Pb data, have respective isotope dilution thermal ionisation mass spectrometry (ID-TIMS) ages of 2053.90 ± 0.18 Ma, 2040.32 ± 0.14 Ma, 868.90 ± 0.22 Ma, 478.74 ± 0.20 Ma and 473.33 ± 0.19 Ma. Minor U-Pb heterogeneity exists and accurate SIMS results require correction with the 3D Concordia-constrained common Pb composition. Among the studied samples, AFG2 and BR5 are the most homogeneous U-Pb reference materials. The SIMS sulfur isotopic compositions of eight of the apatites shows they are homogeneous, with 2s for both 103δ34S and 103δ33S

Precambrian iron oxide copper-gold (IOCG) deposits are generally encountered with multistage hydrothermal overprints and hence have complex isotopic records. Precise dating of ore-forming and overprinting events and assessment of time-resolved metal sources are fundamental for understanding ore genesis. Here, we quantify the evolution history by integrating in situ U-Pb dating of texturally constrained allanite and Sm-Nd isotope data of ores and major rare earth element (REE) minerals in the breccia-hosted Lanniping Fe-Cu deposit in Kangdian region, southwestern China. The economically mineralized breccia in Lanniping Fe-Cu deposit is characterized by pervasive and texturally destructive replacement of polymictic clasts, including host metasedimentary packages, the intruded dolerite, and pre-ore halokinetic breccia. Ore minerals in cements are mainly composed of magnetite, chalcopyrite, bornite, and variable amounts of REE-rich minerals (e.g., apatite and allanite/epidote). Two types of allanite were identified in ores. Type I prismatic allanite texturally intergrown with magnetite has a SHRIMP U-Pb age of 1728 ± 20 Ma (1σ), which matches a zircon U-Pb age of 1713 ± 14 Ma (2σ) for the dolerite clasts and provides the direct age constraint on the Fe-Cu mineralization event. Type II anhedral allanite shows complex zoning and is spatially associated with, but texturally later than, magnetite, apatite, and chalcopyrite. This type of allanite yields significantly younger SHRIMP dates of 1015 ± 33 (1σ) and 800 ± 16 Ma (1σ) for cores and rims, respectively, which correspond to discrete regional magmatic events and hence record hydrothermal overprint/remobilization events of ore minerals in the deposit. Integrated Sm-Nd isotope compositions of type I allanite, apatite, and whole ores generally align along the reference Sm-Nd isochron of 1728 Ma, further confirming the primary ore formation at ~1.7 Ga. Corresponding εNd(1728 Ma) values ranging from –2.8 to 0.3 are significantly higher than those of the host metasedimentary rocks (–9.5 to –6.2) but comparable to those of contemporaneous igneous intrusions (–0.3 to 5.3) in the region, demonstrating that REE components of the primary ores were dominantly sourced from rocks of mantle-derived affinity. Both cores and rims of the younger type II allanite grains have Nd isotope compositions consistent with the unique time-evolved line of the ~1.7 Ga ores, implying that REEs incorporated into type II allanite were ultimately sourced from the primary ores in this deposit. The combined texture, chemical, U-Pb, and Sm-Nd isotope data thus demonstrate that REE remobilization was localized during post-ore hydrothermal overprint with negligible external inputs of REEs to the primary ores in the Lanniping deposit. In this contribution, we not only date primary ore formation but also recognize several younger allanite generations that record internal metal redistributions in response to post-ore tectonothermal events. Our study highlights the potential of ore-associated REE minerals such as allanite for resolving the age of multiple stages of hydrothermal events in complex ore deposits by ion probe, provided that careful examination of textural and paragenetic relationship of ores is conducted. Our finding of these younger allanite generations also exemplifies the significance of evaluation on time-resolved metal input for better characterizing the evolution history of the IOCG deposits.

New SHRIMP U-Pb data from dioritic to granodioritic synmineral intrusions associated with the Jebel Ohier porphyry copper deposit (mineral inventory, including NI43-101-compliant total inferred and indicated resources, of 593 million tonnes [Mt] at 0.33% Cu and 0.05 ppm Au, for 1.953 Mt of contained Cu and 933,600 oz of Au at 0.15% Cu cutoff) in the Red Sea Hills of northeastern Sudan have bracketed the age of mineralization to ca. 816 to 812 Ma. This age range, as well as constraints from new and existing lithogeochemical data, is consistent with the deposit’s formation from a productive parental magma source during the early stages in the evolution of an intra-Mozambique Ocean island arc. The Jebel Ohier porphyry copper deposit bears many similarities to well-documented Phanerozoic analogues elsewhere in terms of (1) the mapped style and zonation of hydrothermal alteration (i.e., proximal K-silicate–dominated, to sericitic, to distal propylitic alteration), (2) the occurrence of intense Cu-bearing A- and B-type vein stockwork, as well as sulfide-only C-type veins, anhydrite veins, and younger, peripheral D-type veins, and (3) the geochemical fingerprint of the associated porphyry, which is akin to those of ore-related Tertiary porphyries in the Escondida area in northern Chile. The multiphase intrusion hosting the Jebel Ohier porphyry copper deposit has been intruded by several generations of mafic to felsic postmineralization dikes and voluminous plutons, with a peak in magmatic activity coinciding with the suturing of the Gebeit terrane at ca. 724 Ma. In spite of, or perhaps because of, the occurrence of extensive postmineralization magmatism, and regardless of subsequent deformation, regional metamorphism, uplift, and erosion, the deposit has remained remarkably intact. The discovery of a relatively ancient, yet well-preserved porphyry copper deposit in the Neoproterozoic Arabian-Nubian Shield has major implications for the exploration potential of this resource-rich geologic terrain.

Hematite (α-Fe2O3) is a common iron-oxide mineral known to incorporate U into its crystal lattice at up to wt% concentrations and has been previously used to date ore formation within iron-oxide copper gold and banded iron formation deposits. However, there has been no detailed evaluation of the potential challenges this novel mineral geochronometer may present for accurate temporal interpretation. We report a multi-technique U Pb geochronological study comprising laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), sensitive high-resolution ion microprobe (SHRIMP) and the first isotope dilution-thermal ionization mass spectrometry (ID-TIMS) procedure and analyses using an in-situ grain micro-sampling technique. We assess intra- and inter-sample data reproducibility, and examine the compatibility of spatial and analytical resolutions to texturally and compositionally diverse samples. Five samples of hydrothermal, primary, oscillatory-zoned hematite were analysed from distinct lithologies throughout the ~1.59 Ga Olympic Dam Cu-U-Au-Ag deposit, with typical U concentrations ranging between ~20 and 1000 ppm. LA-ICP-MS mapping of isotope distributions across mm-sized grains allows pinpointing of U-rich and relatively U/Pb isotopically ‘homogeneous’ domains. Micro-domains were extracted via laser-cut square shaped trenches (100 × 100 μm). U Pb datum range in complexity, with concordant age domains measured by ID-TIMS indicating that hematite can retain a near-closed U Pb system over ~1.6 Ga, while also demonstrating that younger Pb-loss events may be recorded. Significant reverse discordance is common in the data, likely to be a real feature due to both internal decoupling and mobility of Pb within a single grain and U Pb fractionation during microbeam analysis. These effects will obscure the interpretation of real ages for Proterozoic samples where non-zero age Pb-loss events occur, and restricts the evaluation of common Pb mixing components. As a result, microbeam analyses may be biased younger or older in terms of 207Pb/206Pb dates in samples with complex histories. Microbeam datum compare favourably with high precision ID-TIMS ages. Although reproducible ID-TIMS U Pb dates were generally not obtained, in the best-case example, a 207Pb/206Pb weighted mean age of 1589.91 ± 0.91 Ma (MSWD = 1.3, n = 5), within 0.2% of the Olympic Dam granite host rock age (1593.87 ± 0.21 Ma) was generated. All five hematite samples dated by LA-ICP-MS and SHRIMP yield weighted mean 207Pb/206Pb dates within 2% of the host granite age after small degrees of data rejection. The results demonstrate that with careful sample petrography, screening and data interpretation, hematite can be considered a very useful U Pb mineral geochronometer, with potential application to all U-bearing Fe-oxide rich mineral systems.

The Middleback Ranges is a 60 km-long BIF-endowed iron ore belt located in the southeastern Gawler Craton and is the largest exploited iron resource in South Australia. The belt is hosted within an Archean- to Paleoproterozoic-aged granitic-gneissic basement terrane. Petrographic and geochronological constraints are given for a suite of granites associated with deposits throughout the Middleback Ranges and compared with other regional granites. Granites from the Middleback Ranges show complex mineralogical and geochemical features indicating multiple overprinting events. Sensitive high-resolution ion microprobe U-Pb zircon ages from all granites across the Middleback Ranges yield Archean ages, suggesting the existence and a more extensive distribution of Mesoarchean crust than previously recognized beneath the entire Middleback Ranges BIF sequence. Grey and pink granites from the central Middleback Ranges (Sultan prospect) produced the oldest 207Pb/206Pb weighted mean ages within the sample suite (3241 ± 6 and 3238 ± 9 Ma, respectively). Zircon in the latter granite also features overgrowths dated at ∼2.6 Ga and interpreted to be metamorphic. A second group of younger granites from the Iron Monarch deposit and a sample of Cooyerdoo Granite (northern Middleback Ranges) have a minimum crystallization age of 3165 ± 8 Ma and a 207Pb/206Pb weighted mean magmatic age of 3134 ± 18 Ma, respectively. The youngest granites analyzed here derive from the Iron Baron deposit in the central part of the belt, which display strong deformation and have a minimum crystallization age of 3089 ± 9 Ma, and from the Iron Magnet deposit in the southern part of the belt (207Pb/206Pb weighted mean age of 3023 ± 33 Ma). Petrographic and geochemical features observed within the sample suite indicate that post-Archean overprinting events led to alteration of the granitic basement and may infer that fluid-rock interaction and granite leaching were important for BIF upgrading processes and iron-oxide formation.

Northwest Africa (NWA) 6950 is an olivine-gabbro lunar meteorite that has a distinctly different petrographic texture from other lunar basalts. In this contribution, we report the petrography, mineralogy, and U-Pb geochronology of baddeleyite and phosphate in this lunar meteorite and make a comparison with other NWA 773 paired meteorites. NWA 6950 consists mainly of coarse-grained olivine, pyroxene, and plagioclase, with minor ilmenite, chromite, troilite, baddeleyite, taenite, and apatite. The abundant rounded to euhedral olivine has a limited Fo ranging from 68 to 69. Pyroxenes include both augite and pigeonite, with a narrow range of Mg#=71–80, and composition, respectively, ${\rm~En}_{46\text{-}52}{\rm~Fs}_{12\text{-}16}{\rm~Wo}_{32\text{-}41}$ and ${\rm~En}_{58\text{-}68}{\rm~Fs}_{19\text{-}28}{\rm~Wo}_{7\text{-}20}$. Plagioclase is mainly anhedral, conforming to the crystal margins of olivine and pyroxene. The plagioclase grains are elongated and several hundreds of microns in length. Raman spectra shows that plagioclase has been transformed into maskelynite, which is very calcic (${\rm~An}_{87\text{-}95}$) with low sodium and very low potassium (${\rm~Ab}_{4\text{-}9}{\rm~Or}_{0.6\text{-}3.7}$). The weighted mean $^{207}{\rm~Pb}/^{206}{\rm~Pb}$ ages of baddeleyite and phosphate measured using a sensitive high resolution ion micro probe (SHRIMP II) are $3119\pm16$ Ma ($n=9$, MSWD = 2.5) (meansquared weighted deviates, MSWD) and $3119\pm18$ Ma ($n=15$, MSWD = 2.1), respectively, giving the crystallization age of the meteorite.NWA 6950 has similar mineral compositions, geochemical characteristics, and an almost identical age of crystallization to the magnesian gabbro of the NWA 773 clan. We conclude that NWA 6950 and NWA 773 are paired meteorites, which crystallize at ca. 3119 Ma, and represent magmatic activities in the KREEP (potassium (K), rare earth elements (REE) and phosphorus (P) rich lunar material) region of the Moon.

Zircons from three distinct granites within the Samphire Pluton, South Australia, are characterised at the micron- to nanoscale. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon dating generated 207Pb/206Pb weighted average ages of 1586 ± 9.3 Ma, 1583.2 ± 8.5 Ma and 1578 ± 9.5 Ma, respectively. Although zircons from all three granites display evidence of hydrothermal overprinting, this is most strongly expressed in a distinct, red-colored granite referred to here as Granite C. Alteration is expressed as variation in the concentrations of HREE + Y, Th and Pb, whereby grain margins are relatively enriched in these elements and both Zr and Si are depleted. Altered zircon cores have high U contents relative to grain margins, which are appreciably poorer in this element, whereas Th is strongly enriched within crosscutting microfractures. Coupled with the relative enrichment in non-formula elements and marked disturbance of U-Th-Pb isotope ratios, the nanoscale observations of Pb-bearing nanoparticles (galena?) implying mobility of Pb and other elements, nanofractures and structural defects demonstrate that zircon in Granite C has undergone multi-stage alteration impacting upon accurate dating. Importantly, and with implications for analogous systems elsewhere, our results confirm nanoscale open system U-Pb behaviour in Hiltaba Suite zircon. SHRIMP U-Pb zircon geochronology cannot confidently resolve any statistical differences in the age of the three granites, despite their distinct appearance suggesting they might represent temporally distinct phases of a larger magmatic system. Zircons within the most altered Granite (C), directly underlying the Blackbush uranium prospect contain convincing micron- to nanoscale evidence for an alteration event that triggered a remobilisation of uranium from granite into the cover sequence.

Banded iron formation (BIF)-derived iron ore deposits of the Middleback Ranges, Eyre Peninsula, South Australia, are hosted within Archean to Paleoproterozoic basement in the southeastern Gawler Craton. Mafic sills and dikes are associated with the orebodies throughout the belt and although described previously, have never been studied in detail. Two main types of mafic rocks, amphibolites and dolerites, are distinguishable from field occurrence and mineralogy. Amphibolites interbedded with BIFs from the southern Iron Magnet deposit are characterized by extensive overprinting. These amphibolites contain metamorphic assemblages of coexisting Mg-Fe- and Ca-amphiboles, garnet, corundum, biotite and relict titaniferous magnetite displaying ilmenite exsolution. Amphibolite petrography and geochemistry in the southern part of the belt indicate formation in a complex BIF depositional environment, with possible detrital input of titaniferous magnetite from igneous rocks. In contrast, NW-trending dolerite dikes occur throughout the belt and contain assemblages consisting of igneous magnesiohornblende and plagioclase. SHRIMP U–Pb zircon ages for amphibolites and dolerites yield two age groups. Amphibolite from Iron Magnet yields a 207Pb/206Pb weighted mean age of 2542 ± 15 Ma. The most robust 206Pb/238U weighted mean age of 782 ± 23 Ma for dolerites comes from the Iron Queen deposit in the central part of the Middleback Ranges. LA-ICP-MS U–Pb geochronology of U-bearing hematite from the Iron Knight South deposit yields ages of ~680 Ma. Taken together, these results suggest mafic magmatism contemporaneous with Archean BIF deposition followed by a high-grade metamorphic event (~2.47–2.41 Ga Sleafordian Orogeny). Dikes from Cape Donington, the southernmost point on the Eyre Peninsula are, instead, associated with the 1.73–1.69 Ga Kimban Orogeny, as evidenced by zircon from a pyroxene-rich dolerite, which yields a 206Pb/238U weighted mean age of 1711 ± 14 Ma. The dolerite ages from the Iron Queen deposit provide the first evidence for the presence of the ~800 Ma craton-scale Gairdner Large Igneous Province in the Middleback Ranges. The young U–Pb hematite age from Iron Knight South and post-emplacement alteration of dikes of Gairdner age from the central part of the Middleback Ranges belt provide additional support for multiple events leading to upgrading of iron ores.

Here, we document a detailed characterisation of two zircon gemstones, GZ7 and GZ8. Both stones had the same mass at 19.2 carats (3.84 g) each; both came from placer deposits in the Ratnapura district, Sri Lanka. The U‐Pb data are in both cases concordant within the uncertainties of decay constants and yield weighted mean 206Pb/238U ages (95% confidence uncertainty) of 530.26 Ma ± 0.05 Ma (GZ7) and 543.92 Ma ± 0.06 Ma (GZ8). Neither GZ7 nor GZ8 have been subjected to any gem enhancement by heating. Structure‐related parameters correspond well with the calculated alpha doses of 1.48 × 1018 g−1 (GZ7) and 2.53 × 1018 g−1 (GZ8), respectively, and the (U‐Th)/He ages of 438 Ma ± 3 Ma (2s) for GZ7 and 426 Ma ± 9 Ma (2s) for GZ8 are typical of unheated zircon from Sri Lanka. The mean U mass fractions are 680 μg g−1 (GZ7) and 1305 μg g−1 (GZ8). The two zircon samples are proposed as reference materials for SIMS (secondary ion mass spectrometry) U‐Pb geochronology. In addition, GZ7 (Ti mass fractions 25.08 μg g−1 ± 0.18 μg g−1; 95% confidence uncertainty) may prove useful as reference material for Ti‐in‐zircon temperature estimates., Geostandards and Geoanalytical Research, 42 (4), ISSN:1639-4488, ISSN:1751-908X

Metamorphic mineral compositions of a staurolite-bearing greyschist from the middle reaches of the Moeraki River valley in south Westland reveal peak equilibration at c. 558 ± 50 °C and c. 6.1 ± 1.2 kbar. Two c. 83 Ma U–Pb monazite age populations from the cores of monazite-apatite-allanite-epidote corona structures in mylonitised schists from near Fox Glacier confirm that Alpine Schist metamorphism occurred during the Late Cretaceous. The published spread in Late Cretaceous metamorphic ages indicates that metamorphism was diachronous or was a protracted event. Further dating is required to pin down the cryptic transition into the Jurassic–Early Cretaceous metamorphosed Otago Schist, but the Alpine Schist must extend at least 11 km east of the Alpine Fault in south Westland and overprint the suture between the Pounamu and Rakaia terranes. The P–T–t results imply that the Late Cretaceous crust represented by portions of the Alpine Schist was probably of similar thickness to that beneath the Southern Alps t...

High seismic velocity layers within the lower crust (i.e. ∼40 km) of the Yangtze Block are interpreted as mafic underplated rocks derived from the Late Permian Emeishan mantle plume. However, the region experienced a previous magmatic event during the Neoproterozoic (∼800 Ma) that produced the Kangdian basalts and associated mafic intrusions and therefore the lower crust seismic velocity layers may represent a mixture of two different magmatic episodes. The identification of inherited Neoproterozoic (i.e. ∼750 to ∼850 Ma) zircons within Emeishan magmatic rocks indicates either assimilation of older material during emplacement or that the rocks could be derived from a mafic Neoproterozoic precursor. Equilibrium partial melt modeling of Neoproterozoic Kangdian basalts can produce compositions similar to Emeishan basalt at a pressure of 1.2 GPa (i.e. ≈40 km depth). It is suggested that the injection of high temperature picritic magmas into the lower crust of the Yangtze Block could be sufficient to induce high degrees of melting of the lower crust and produce magmas that are similar in composition to some Emeishan mafic rocks and/or produce melts that could contaminate ‘pristine’ Emieshan magmas.

Rocks in the Waipuna Valley in Westland show that the area immediately south of the Reefton Goldfield has been dramatically affected by Cretaceous tectonism. The main lithology in the Waipuna Valley is metasedimentary Greenland Group that is here subdivided into four types (I, II, III and IV). Type I contains abundant detrital grains with a weak Palaeozoic greenschist facies overprint and is compositionally indistinguishable from unmineralised Greenland Group within the goldfield. Type II contains detrital grains, but also a weak amphibolite facies foliation or, where proximal to granitoid intrusions, a hornfelsic texture. Type III contains no detrital components and records polyphase recrystallisation. U–Th–Pb monazite dating indicates that metamorphism in the Type III rocks occurred, at least in part, at 108.1 ± 1.2 Ma. Type IV Greenland Group has partially melted. Zircon from granitoids intruding Greenland Group in the Waipuna Valley indicate Cretaceous (c. 111 Ma) emplacement. Greenland Group Types II...

The peak conditions and timing of metamorphism have been determined for the early Paleozoic Greenland Group in south Westland, New Zealand. The Greenland Group, which is the most extensive pre-Cenozoic unit west of the Alpine Fault in Westland, is dominated by greenschist facies greywacke and argillite. However, in the Jackson River valley this unit comprises amphibolite facies pelitic and psammitic gneisses. Psammitic horizons are dominated by biotite, plagioclase and quartz. Pelitic horizons have similar mineralogy but also contain sillimanite and K–feldspar, as well as boudinaged K–feldspar-bearing leucosomes. Conventional geothermometry and thermodynamic modelling indicate that peak metamorphism occurred between 660 and 700 °C and 3.5 and 6.0 kbar. The timing of this moderately high–T/low–P event is constrained to be mid-Paleozoic by U–Th–Pb dating of overgrowths on detrital zircon (334 ± 18 Ma) and confirmed to be Carboniferous by a single population of monazite grains (343 ± 3 Ma). Greenland Group m...

Back-arcs are hot regions of enduring weakness in the hinterland of subduction zones that are commonly sites of strain localization, forming mobile belts that, in some cases, develop into mountain belts. Their evolution results from their strength, a function of thickness, heat flux, and stresses. Contractional back-arcs may be too weak to form thick crusts, whereas in other cases, they can thicken considerably and lead to mountain building. The stress and thermal histories of back-arcs can be retrieved from investigation of the relationships between deformation and metamorphism. The 490−430 Ma Famatinian orogen of the Sierras Pampeanas is associated with a back-arc that has a complex development. We focus on the Sierra de Quilmes within the northern section of the back-arc and present new structural data tied to anatexis and constrained by geochronology. We find that low-pressure−high-temperature conditions prevailed over an ∼35 m.y. thermal peak between 497 and 461 Ma, as indicated by U/Pb ages of monazite. Anatexis coincided with thrusting to the west and northwest, as evidenced by leucosomes in axial planes of folds and in thrust planes. As rocks were thrust upward, they cooled, melt solidified, and deformation localized into two major thrust zones. These record the same thrust-to-the-west or northwest kinematics as the earlier syn-anatectic deformation and ultimately placed hot rocks over cold ones. Continuous thrusting during cooling from peak metamorphism, accompanied by strain localization, is consistent with a number of shear zones in the northern Sierras Pampeanas. The low-pressure metamorphic assemblages indicate that shortening was not accompanied by pronounced crustal thickening, suggesting that the lithosphere was too hot, and the partially melted crust was too weak for substantial stacking. Fil: Finch, Melanie A.. Monash University; Australia Fil: Ferrez Weinberg, Roberto. Monash University; Australia Fil: Hasalová, Pavlína. Czech Geological Survey; República Checa Fil: Becchio, Raul Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Investigaciones en Energía no Convencional. Universidad Nacional de Salta. Facultad de Ciencias Exactas. Departamento de Física. Instituto de Investigaciones en Energía no Convencional; Argentina Fil: Fuentes, María Gabriela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Investigaciones en Energía no Convencional. Universidad Nacional de Salta. Facultad de Ciencias Exactas. Departamento de Física. Instituto de Investigaciones en Energía no Convencional; Argentina Fil: Kennedy, Allen. Curtin University; Australia

Northwest Africa 7533, a polymict Martian breccia, consists of fine‐grained clast‐laden melt particles and microcrystalline matrix. While both melt and matrix contain medium‐grained noritic‐monzonitic material and crystal clasts, the matrix also contains lithic clasts with zoned pigeonite and augite plus two feldspars, microbasaltic clasts, vitrophyric and microcrystalline spherules, and shards. The clast‐laden melt rocks contain clump‐like aggregates of orthopyroxene surrounded by aureoles of plagioclase. Some shards of vesicular melt rocks resemble the pyroxene‐plagioclase clump‐aureole structures. Submicron size matrix grains show some triple junctions, but most are irregular with high intergranular porosity. The noritic‐monzonitic rocks contain exsolved pyroxenes and perthitic intergrowths, and cooled more slowly than rocks with zoned‐pyroxene or fine grain size. Noritic material contains orthopyroxene or inverted pigeonite, augite, calcic to intermediate plagioclase, and chromite to Cr‐bearing magnetite; monzonitic clasts contain augite, sodic plagioclase, K feldspar, Ti‐bearing magnetite, ilmenite, chlorapatite, and zircon. These feldspathic rocks show similarities to some rocks at Gale Crater like Black Trout, Mara, and Jake M. The most magnesian orthopyroxene clasts are close to ALH 84001 orthopyroxene in composition. All these materials are enriched in siderophile elements, indicating impact melting and incorporation of a projectile component, except for Ni‐poor pyroxene clasts which are from pristine rocks. Clast‐laden melt rocks, spherules, shards, and siderophile element contents indicate formation of NWA 7533 as a regolith breccia. The zircons, mainly derived from monzonitic (melt) rocks, crystallized at 4.43 ± 0.03 Ga (Humayun et al. 2013) and a 147Sm‐143Nd isochron for NWA 7034 yielding 4.42 ± 0.07 Ga (Nyquist et al. 2016) defines the crystallization age of all its igneous portions. The zircon from the monzonitic rocks has a higher Δ17O than other Martian meteorites explained in part by assimilation of regolith materials enriched during surface alteration (Nemchin et al. 2014). This record of protolith interaction with atmosphere‐hydrosphere during regolith formation before melting demonstrates a thin atmosphere, a wet early surface environment on Mars, and an evolved crust likely to have contaminated younger extrusive rocks. The latest events recorded when the breccia was on Mars are resetting of apatite, much feldspar and some zircons at 1.35–1.4 Ga (Bellucci et al. 2015), and formation of Ni‐bearing pyrite veins during or shortly after this disturbance (Lorand et al. 2015).

The southern Midyan terrane is a composite Tonian to Ediacaran tectonostratigraphic crustal block in the northern Arabian Shield that prior to Red Sea opening was contiguous with coeval rocks in the Eastern Desert of Egypt and Sinai. Ion microprobe (sensitive high-resolution ion microprobe [SHRIMP]) dating of 12 rock samples described here and the results of other dating programmes establish a clear timeframe for depositional, intrusive, and structural events in the region and provide a chronology of tectonism in this part of the Arabian-Nubian Shield. Deposition of Zaam and Bayda group volcanosedimentary rocks and emplacement of mafic-ultramafic complexes and TTG-type diorite, tonalite, and granodiorite denote formation of the Tonian (780–715 Ma) Zaam arc and fore-arc ophiolite above a possible west-dipping subduction system in the southern part of the Midyan terrane. Convergence with the Hijaz terrane farther south and obduction of ophiolite nappes resulted by ~700 Ma in development of the Yanbu suture. Ongoing or a new subduction system led to a ~705–660 Ma Cryogenian pulse of magmatism represented by I-type calc-alkaline diorite, granodiorite, and granite that have volcanic-arc and syn-collisional granite affinities. This was followed, after a brief end-Cryogenian hiatus, by a 635–~570 Ma period of Ediacaran magmatism marked by monzogranite, syenogranite, and minor gabbro and diorite. These rocks are reported to have within-plate to volcanic-arc and syncollision chemical characteristics but their precise tectonic setting is uncertain. Structurally, the intrusions are diapiric and were evidently emplaced in an extensional regime consistent with an overlap between intrusion and Najd faulting associated, at this time, with transpressional collision and northward extension through much of the ANS. Terminal magmatism in the southern Midyan terrane postdated cessation of Najd faulting at ~575 Ma and resulted in the emplacement of undeformed within-plate A-type alkali-feldspar granites and mafic (lamprophyre) and felsic dikes.

We have studied zircon xenocrysts from Mesozoic kimberlites from the Kuoika and Ary–Mastakh fields in Northern Yakutia. Zircon xenocrysts are assumed to originate from crustal rocks. Our SHRIMP (Sensitive High mass Resolution Ion MicroProbe) analyses yielded predominantly concordant U–Th–Pb ages (up to ~ 3570 Ma; Paleoarchean) that clearly predate kimberlite formation. The general U–Th–Pb concordance observed excludes notable disturbance of the zircon xenocrysts U–Th–Pb isotope system during kimberlite ascent and emplacement. In addition, zircon xenocrysts were found to be significantly more radiation-damaged than would correspond to damage accumulation only since the time of kimberlite formation. This observation first indicates that zircon crystals were sampled by the kimberlite magma at comparably shallow depths not exceeding 10–12 km. If, in contrast, zircon crystals originated from deeper levels of the Earth's crust, they would have been exposed to temperatures of 250–300 °C or more. This in turn would have caused long-term thermal annealing of the radiation damage, which was however not observed in our study. Second, our observation contradicts the hypothesis that high temperatures experienced by zircon xenocrysts during kimberlite ascent will cause notable structural reconstitution by short-term thermal annealing. Consequently, zircon crystals cannot have spent more than a few hours at temperatures exceeding ca. 700–800 °C, or more than a few days at temperatures exceeding ca. 500–600 °C. This in turn suggests that (i) temperatures of the ascending kimberlite magmas were rather moderate, and (ii) kimberlite ascent is a comparably short process followed by rapid cooling.

Chemical analysis of the meteorite NWA 7533 indicates that it may be a Martian regolith breccia and, if so, that the crust of Mars may have formed in the first 100 million years of the planet’s history. This paper identifies the NWA 7533 meteorite from Northwest Africa as the first sample of Martian highlands rock in the meteorite collection. Munir Humayun and co-authors show that NWA 7533 has a composition indicative of a highlands breccia. It also contains zircons more than 4.4 billion years old, implying that early crustal differentiation on Mars occurred in the first 100 million years of its history, coeval with earliest crust formation on the Moon and the Earth. The ancient cratered terrain of the southern highlands of Mars is thought to hold clues to the planet’s early differentiation1,2, but until now no meteoritic regolith breccias have been recovered from Mars. Here we show that the meteorite Northwest Africa (NWA) 7533 (paired with meteorite NWA 70343) is a polymict breccia consisting of a fine-grained interclast matrix containing clasts of igneous-textured rocks and fine-grained clast-laden impact melt rocks. High abundances of meteoritic siderophiles (for example nickel and iridium) found throughout the rock reach a level in the fine-grained portions equivalent to 5 per cent CI chondritic input, which is comparable to the highest levels found in lunar breccias. Furthermore, analyses of three leucocratic monzonite clasts show a correlation between nickel, iridium and magnesium consistent with differentiation from impact melts. Compositionally, all the fine-grained material is alkalic basalt, chemically identical (except for sulphur, chlorine and zinc) to soils from Gusev crater. Thus, we propose that NWA 7533 is a Martian regolith breccia. It contains zircons for which we measured an age of 4,428 ± 25 million years, which were later disturbed 1,712 ± 85 million years ago. This evidence for early crustal differentiation implies that the Martian crust, and its volatile inventory4, formed in about the first 100 million years of Martian history, coeval with earliest crust formation on the Moon5 and the Earth6. In addition, incompatible element abundances in clast-laden impact melt rocks and interclast matrix provide a geochemical estimate of the average thickness of the Martian crust (50 kilometres) comparable to that estimated geophysically2,7.

U–Pb dating of the common iron-oxide hematite (α-Fe2O3), using laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICP-MS), provides unparalleled insight into the timing and processes of mineral deposit formation. Until now, the full potential of this method has been negatively impacted by the lack of suitable matrix-matched standards. To achieve matrix-matching, we report an approach in which a U–Pb solution and ablated material from 99.99% synthetic hematite are simultaneously mixed in a nebulizer chamber and introduced to the ICP-MS. The standard solution contains fixed U- and Pb-isotope ratios, calibrated independently, and aspiration of the isotopically homogeneous solution negates the need for a matrix-matched, isotopically homogenous natural iron-oxide standard. An additional advantage of using the solution is that the individual U–Pb concentrations and isotope ratios can be adjusted to approximate that in the unknown, making the method efficient for dating hematite containing low (~10 ppm) to high (>1 wt %) U concentrations. The above-mentioned advantage to this solution method results in reliable datasets, with arguably-better accuracy in measuring U–Pb ratios than using GJ-1 Zircon as the primary standard, which cannot be employed for such low U concentrations. Statistical overlaps between 207Pb/206Pb weighted average ages (using GJ-1 Zircon) and U–Pb upper intercept ages (using the U–Pb mixed solution method) of two samples from iron-oxide copper-gold (IOCG) deposits in South Australia demonstrate that, although fractionation associated with a non-matrix matched standard does occur when using GJ-1 Zircon as the primary standard, it does not impact the 207Pb/206Pb or upper intercept age. Thus, GJ-1 Zircon can be considered reliable for dating hematite using LA-ICP-MS. Downhole fractionation of 206Pb/238U is observed to occur in spot analyses of hematite. The use of rasters in future studies will hopefully minimize this problem, allowing for matrix-matched data. Using the mixed-solution method in this study, we have validated a published hematite Pb–Pb age for Olympic Dam, and provide a new age (1604 ± 11 Ma) for a second deposit in the same province. These ages are further evidence that the IOCG mineralizing event is tied to large igneous province (LIP) magmatism in the region at ~1.6 Ga.

Faculty of Geosciences, Geography and Astronomy, University of Vienna; Beijing SHRIMP Centre, Chinese Academy of Geological Sciences; European Commission [MEXC-CT-2005-024878]; FWF Austrian Science Fund [P20028-N10, P24448-N19]; U.S. National Science Foundation [EAR 1524336]

Martian regolith breccia NWA 7533 (and the seven paired samples) is unique among Martian meteorites in showing accessory pyrite (up to 1% by weight). Pyrite is a late mineral, crystallized after the final assembly of the breccia. It is present in all of the lithologies, i.e., the fine-grained matrix (ICM), clast-laden impact melt rocks (CLIMR), melt spherules, microbasalts, lithic clasts, and mineral clasts, all lacking magmatic sulfides due to degassing. Pyrite crystals show combinations of cubes, truncated cubes, and octahedra. Polycrystalline clusters can reach 200 lm in maximum dimensions. Regardless of their shape, pyrite crystals display evidence of very weak shock metamorphism such as planar features, fracture networks, and disruption into subgrains. The late fracture systems acted as preferential pathways for partial replacement of pyrite by iron oxyhydroxides interpreted as resulting from hot desert terrestrial alteration. The distribution and shape of pyrite crystals argue for growth at moderate to low growth rate from just-saturated near neutral (6 FMQ + 2 log units. It is inferred from the maximum Ni contents (4.5 wt%) that pyrite started crystallizing at 400–500 °C, during or shortly after a short-duration, relatively low temperature, thermal event that lithified and sintered the regolith breccias, 1.4 Ga ago as deduced from disturbance in several isotope systematics.

Gneiss domes involving the South Tibetan Detachment System provide evidence for crustal extension simultaneous with shortening. The Nielaxiongbo gneiss dome is composed of a metamorphic complex of granitic gneiss, amphibolite, and migmatite; a ductilely deformed middle crustal layer of staurolite- or garnet-bearing schist; and a cover sequence of weakly metamorphosed Triassic and Lower Cretaceous strata. The middle crust ductilely deformed layer is separated from both the basement complex and the cover sequence by lower and upper detachments, respectively, with a smaller detachment fault occurring within the ductilely deformed layer. Leucogranites crosscut the basement complex, the lower detachment, and the middle crustal layer, but do not intrude the upper detachment or the cover sequence. Three deformational fabrics are recognized: a N–S compressional fabric (D1) in the cover sequence, a north- and south-directed extensional fabric (D2) in the upper detachment and lower tectonic units, and a deformation...

We have investigated the homogeneity, chemical composition, structure, degree of radiation damage, and post-formation evolution of titanite crystals from skarns of the Grenville Province of the Canadian Shield using SHRIMP, TIMS, Raman and PL spectroscopy, EBSD, and EPMA–WDS. These results are used to assess the potential of the titanite as Reference Material (RM) for micro-analytical U–Th–Pb age dating. The SHRIMP data show that these megacrysts (5–31g) have concordant U–Pb isotope systematics, 60 to 500 ppm U, 120 to 1200 ppm Th, 206 Pb/ 204 Pb between 500 and 2500, ages of ~1 Ga, and excellent homogeneity at the scale of the analytical volume of the ion probe. The ID–TIMS titanite data for OLT1, OLT2 and TCB show that these crystals are essentially concordant. Data for OLT1 and OLT2 show slight scatter ( i.e. , in excess of that expected from the uncertainty in an individual analysis). For OLT1, one of seven analyses shows Pb loss or, possibly, a younger period of growth. Crystals OLT1 and OLT2 have respective TIMS concordia ages of 1014.8 ± 2.0 Ma (2σ, n = 6, MSWD = 1.8) and 998.0 ± 4.5 Ma (2σ, n = 3, MSWD = 3.3) for domains that have not lost Pb. The TIMS analyses of TCB are tightly clustered and give a concordia age of 1018.1 ± 1.7 Ma (2σ, n = 4, MSWD = 0.92). Raman and PL spectra show a low to moderate degree of accumulated radiation-induced damage in the Grenville Skarn Titanite crystals and uniform internal distributions of this damage. The EDSB contrast images indicate little or no crystallographic misorientation. The EMPA–WDS data show that the outer 50–100 μm of the OLT1 and TCB crystals are enriched in Al and F, and depleted in Fe and Nb, when compared with the interior. In spite of the variation in composition and degree of radiation damage amongst samples, there are no identifiable matrix effects in our SHRIMP data. Some Grenville skarn titanite (GST) crystals have potential as RM for micro-analytical U–Th–Pb age dating. Crystal TCB has excellent homogeneity of U–Th–Pb isotopic composition. Crystals OLT1 and OLT2 have minor TIMS age heterogeneity. However, this heterogeneity is smaller than that of the Khan titanite, our current in-house titanite standard. Careful selection of analysis areas during SIMS, and of chips for TIMS analysis, allows high-quality isotopic data to be obtained from these large crystals of titanite.

Lunar breccia 73217 is composed of plagioclase and pyroxene clasts originating from a single gabbronorite intrusion, mixed with a silica-rich glass interpreted to represent an impact melt. A study of accessory minerals in a thin section from this breccia (73217,52) identified three different types of zircon and anhedral grains of apatite which represent distinct generations of accessory phases and provide a unique opportunity to investigate the thermal history of the sample. Equant, anhedral zircon grains that probably formed in the gabbronorite, referred to as type-1, have consistent U–Pb ages of 4332 ± 7 Ma. A similar age of 4335 ± 5 Ma was obtained from acicular zircon (type-2) grains interpreted to have formed from impact melt. A polycrystalline zircon aggregate (type-3) occurs as a rim around a baddeleyite grain and has a much younger age of 3929 ± 10 Ma, similar to the 3936 ± 17 Ma age of apatite grains found in the thin section. A combined apatite-type-3 zircon age of 3934 ± 12 Ma is proposed as the age of the Serenitatis impact event and associated thermal pulse. X-ray mapping and electron probe analyses showed that Ti is inhomogeneous in the zircon grains on the sub-micrometer scale. However, model temperatures estimated from SHRIMP analyses of Ti-concentration in the 10 μm diameter spots on the polished surfaces of type-1 and type-2 zircons range between about 1300 and 900 °C respectively, whereas Ti-concentrations determined for the type-3 zircon are higher at about 1400–1500 °C. A combination of U–Pb ages, Ti-concentration data and detailed imaging and petrographic studies of the zircon grains shows that the gabbronorite parent of the zircon clasts formed shortly before the 4335 ± 5 Ma impact, which mixed the clasts and the felsic melt and projected the sample closer to the surface where fast cooling resulted in the crystallization of acicular zircon (type-2). The 3934 ± 12 Ma Serenitatis event resulted in partial remelting of the glass and formation of polycrystalline zircon (type-3). This event also reset the U–Pb system of apatite, formed merrillite coronas around some apatite grains, and probably re-equilibrated some pyroxenes in the clasts. Although there have been arguments for pre-3.9 Ga impacts based on other types of samples, the age of the acicular zircon at 4335 ± 5 Ma provides the first evidence of impact melt significantly predating the lunar cataclysm. Our data, combined with other chronological results, demonstrate the occurrence of pre-3.9 Ga impacts on the Moon and suggest that the lunar impact history consisted of a series of intense bombardment episodes interspersed with relatively calm periods of low impact flux.

The phenomenon of deficient electron microprobe analyses, with sums of analyzed constituents often below 95 wt%, is assigned to the analysis of altered, porous minerals. With the example of three zircon populations we show that low totals are related to textural features (i.e., numerous pores of tens to hundreds of nanometers size) as well as to the chemical composition (i.e., water content well within the wt% range, which may affect partial sample degradation under the electron beam). The formation of the spongy texture is explained by the alteration of a previously radiation-damaged and, thus, volumeexpanded material in a fluid-driven replacement reaction. The smaller volume of the reaction product (crystalline, non volume- expanded zircon) accounts for the formation of numerous voids and pores, which are perfect candidates for the incorporation of water. The alteration has also resulted in uptake of non-formula elements such as Al, P, Ca, Fe, Y, and REEs whereas Si and Zr are depleted. In one case, strong uptake of non-radiogenic Pb in altered zircon was observed. Because porous, low-total zircon has formed in secondary alteration process, its occurrence can be considered as an indicator of a secondary alteration history of the host rock. Low-total zircon is easily recognized by very low electron back-scatter intensities, which are closely related to the two main causes of the analytical shortfall (i.e., water content and porosity) and often lowered furthermore by the presence of light non-formula elements (especially P and Fe) up to the wt% range.

In order to better understand the formation and evolution processes of ultrahigh pressure (UHP) felsic rocks, we determined the ages of various domains of zircon and monazite crystals from the diamondiferous quartzofeldspathic rocks of the Saxonian Erzgebirge. According to cathodoluminescence imagery and Th/U ratios, three zircon zones were distinguished. Each was dated using several spot analyses from a sensitive high-resolution ion microprobe analysing Pb, U and Th isotopes. The results were: (1) core zone – 21 analyses: Th/U ≤ 40.023 and 337.0±2.7 Ma (2σ, combined 206Pb/238U-207Pb/235U age); (2) diamond-bearing intermediate zone – 23 analyses: Th/U ≥ 50.037 and 336.8±2.8 Ma; and (3) rim zone – 12 analyses: Th/U = 0.015–0.038 (plus one analysis of 0.164) and 330.2±5.8 Ma. The U-Pb obtained ages are virtually concordant. Furthermore, two oscillatory zoned zircon cores (Th/U ≥ 50.8) yielded (~concordant) ages of ~400 Ma. Six SHRIMP analyses of monazites gave an age of 332.4±2.1 Ma. In addition, Pb, Th and U contents in monazite were analysed with an electron microprobe (EMP). A mean age of 324.7±8.0 (2σ) Ma was acquired from 113 analyses.By combining the defined ages with previously published P-T conditions, minimum velocities for burial and exhumation were estimated. In addition, we present a likely geodynamic scenario involving age data from the literature as well as this study: beginning 340 million years ago, gneisses at the base of a thickened continentalcrust (~1.8 GPa, 650ºC) were transported to depths of at least 130 km, possibly as deep as 250 km. Here they were heated (>1050ºC) and partially melted and as a result began to rise rapidly. The burial and subsequent ascent back to a depth of 50 km, where zircon rims and monazite formed, took only a few million years and perhaps significantly less.

The Xuelongbao plutonic complex in the eastern margin of the Tibetan Plateau is dated at 748 ± 7 Ma using the SHRIMP zircon U–Pb method and represents part of the Neoproterozoic igneous assemblage of South China. Rocks in the complex include tonalite and granodiorite and have SiO2 ranging from 62.0 to 74.8 wt.% and Al2O3 from 14.3 to 20.9 wt.%. Their Na2O contents range from 4.2 to 6.7 wt.% and K2O from 0.47 to 1.96 wt.%, indicating that they belong to the Na-series. These rocks show chondrite-normalized REE patterns depleted in HREE and variably enriched in LREE. They have positive Sr and negative Nb and Ti anomalies in the primitive mantle-normalized trace elemental spider diagram. Their Sr contents range from 320 to 780 ppm and Y contents are lower than 10 ppm, resulting in high Sr/Y ratios (52–320), characteristic of typical adakites. Their eNd (t) and initial Sr isotopic compositions range from + 0.36 to + 2.88 and from 0.7033 to 0.7054, respectively. The geochemical features of the Xuelongbao plutonic complex are consistent with an origin from adakitic magmas that were likely derived from partial melting of a subducted oceanic slab. Together with arc signatures of other granites and mafic intrusions in the region, the 750 Ma Xuelongbao adakitic complex provides evidence for a major, subduction-related Neoproterozoic magmatic event of South China.

U-Pb SHRIMP dating of zircons of metamagmatites from the Bayerischer Wald (Germany) reveals a complex evolution of this section of the Moldanubian Zone exposed in the western Bohemian Massif of the central European Variscan belt. In the south-western part of the Bayerischer Wald Upper Vendian magmatism is constrained by pooled 206Pb/238U mean ages of 555±12, 549±7 and 549±6 Ma from metarhyolites and a metabasite. Inherited zircon cores were not observed. Zircon overgrowths, yielding pooled 206Pb/238U ages of 316±10 and 319±5 Ma, provide evidence for Variscan metamorphic zircon growth; cathodoluminescence imaging reveals a two-stage metamorphic overprint. In contrast, Lower Ordovician magmatism and anatexis are documented in the north-eastern parts of the Bayerischer Wald by metagranitoids (480±6, 486±7 Ma), an eclogitic metabasite (481±8 Ma) and a leucosome (491 to 457 Ma). Inherited zircon cores are found in Lower Ordovician metagranitoids and the leucosome, indicating a Palaeoproterozoic-Archaean (ca. 2.7, 2.0 Ga) source region, presumably of Gondwana affinity (West African craton), and documenting Cadomian magmatism (ca. 640 Ma). Post-Cadomian metamorphism is inferred from concordant ages of 433±4 and 431±7 Ma. Upper Vendian magmatism is assumed at an active continental margin with ensialic back-arc development (eNd(t) –3.01 to +1.22); the lack of inherited zircon is due to either derivation from juvenile (?volcanic arc) material or complete isotopic resetting of pre-existing zircon. An active continental margin setting, possibly with some lateral variation (accretion/collision) is envisaged for the Lower Ordovician, producing granitoids, rhyolites and leucosomes (eNd(t) -0.5 to -6.27); MORB-type metabasites may be related to ZEV or Marianske Lazně Complex metabasites. A tentative palaeogeographic reconstruction puts the “Bayerischer Wald” in close relationship with the Habach terrane (proto-Alps), as the “eastern” extension of terranes of the northern Gondwana margin.

The Westonia Formation comprises clayey quartz sand commonly exposed in interfluve breakaways in the southwestern Yilgarn Craton, Western Australia. As is typical of Cenozoic sediments in the region, it is texturally immature, poorly stratified and unfossiliferous, making it difficult to distinguish from the underlying deeply weathered Archaean granite. In order to verify the difference in origin and elucidate provenance, the varietal properties and U–Pb ages of zircons from the Westonia Formation and underlying granitic saprolite were compared. Zircons from the Westonia Formation comprise 11 distinct morphological varieties, seven of which are not present within the granitic saprolite. SHRIMP U–Pb ages of zircons from the granitic saprolite range from 2657 ± 13 to 2554 ± 10 Ma, corresponding to published age determinations of zircons from the Darling Range Batholith. Angular zircons from the Westonia Formation range in age from 2898 ± 12 to 2631 ± 11 Ma, reflecting a mixed provenance including local gran...

In-situ U-Th-Pb analyses by ion-microprobe on zircon in intact textural relationships are combined with backscatter and cathodoluminescence imaging and trace element analyses to provide evidence for growth episodes of zircon. This approach helps: (a) to unravel the polymetamorphic history of aluminous migmatitic and granitoid gneisses of the regional contact aureole around the Rogaland anorthosite-norite intrusive complex; and (b) to constrain the age of M 2 ultrahigh-temperature (UHT) metamorphism and the subsequent retrograde M 3 event. All samples yield magmatic inherited zircon of c. 1035 Ma, some an additional group at c. 1050 Ma. This suggests that loss of Pb by volume diffusion in non-metamict zircon is not an important factor even under extreme crustal conditions. Furthermore, the identical inheritance patterns in aluminous (garnet, cordierite ± osumilite-bearing) migmatites and orthogneisses indicate a metasomatic igneous instead of a sedimentary protolith for the migmatite. Results for the M 1 metamorphic event at c. 1000 Ma BP are consistent in all samples, including those from outside the orthopyroxene-in isograd. The latter do not show evidence for zircon growth during the M 2 metamorphic episode. Zircon intergrown with or included within M 2 metamorphic minerals (magnetite, spinel, orthopyroxene) give an age of 927 ± 7 Ma (2 σ, n = 20). The youngest observed results are found in zircon outside M 2 minerals, some overgrown by M 3 mineral assemblages (late garnet coronas, garnet + quartz and orthopyroxene + garnet symplectites) and yield a slightly younger pooled age of 908 ± 9 Ma (2 σ, n = 6). These textures are relative time markers for the crystallization of zircon overgrowths during discrete stages of the UHT event. These youngest age groups are consistent with the emplacement age of the Rogaland intrusive complex and the last magmatic activity (Tellnes dyke intrusion), respectively. This is direct and conclusive evidence for UHT metamorphism in the regional aureole being caused by the intrusions, and corrects earlier notions that the events are not linked. Trace element behaviour of zircon (Tb/U and Y content) has been tracked through time in the samples and shows variations both within and between samples. This heterogeneous behaviour at all scales appears to be common in metamorphic rocks and precludes the use of ‘rules of thumb’ in the interpretation of zircon chemistry, but chemical tracers are useful for recognition of zircon growth or recrystallization during metamorphism.

SHRIMP U–Pb ages have been obtained for zircon in granitic gneisses from the aureole of the Rogaland anorthosite–norite intrusive complex, both from the ultrahigh temperature (UHT; >900 °C pigeonite-in) zone and from outside the hypersthene-in isograd. Magmatic and metamorphic segments of composite zircon were characterised on the basis of electron backscattered electron and cathodoluminescence images plus trace element analysis. A sample from outside the UHT zone has magmatic cores with an age of 1034 ± 7 Ma (2σ, n = 8) and 1052 ± 5 Ma (1σ, n = 1) overgrown by M1 metamorphic rims giving ages between 1020 ± 7 and 1007 ± 5 Ma. In contrast, samples from the UHT zone exhibit four major age groups: (1) magmatic cores yielding ages over 1500 Ma (2) magmatic cores giving ages of 1034 ± 13 Ma (2σ, n = 4) and 1056 ± 10 Ma (1σ, n = 1) (3) metamorphic overgrowths ranging in age between 1017 ± 6 Ma and 992 ± 7 Ma (1σ) corresponding to the regional M1 Sveconorwegian granulite facies metamorphism, and (4) overgrowths corresponding to M2 UHT contact metamorphism giving values of 922 ± 14 Ma (2σ, n = 6). Recrystallized areas in zircon from both areas define a further age group at 974 ± 13 Ma (2σ, n = 4). This study presents the first evidence from Rogaland for new growth of zircon resulting from UHT contact metamorphism. More importantly, it shows the survival of magmatic and regional metamorphic zircon relics in rocks that experienced a thermal overprint of c. 950 °C for at least 1 Myr. Magmatic and different metamorphic zones in the same zircon are sharply bounded and preserve original crystallization age information, a result inconsistent with some experimental data on Pb diffusion in zircon which predict measurable Pb diffusion under such conditions. The implication is that resetting of zircon ages by diffusion during M2 was negligible in these dry granulite facies rocks. Imaging and Th/U–Y systematics indicate that the main processes affecting zircon were dissolution-reprecipitation in a closed system and solid-state recrystallization during and soon after M1.

South China has been considered as part of the Rodinian supercontinent during Neoproterozoic time, although its paleogeographic position within this supercontinent is still a matter of debate. The Wangjiangshan and Bijigou complexes along the northern margin of South China are among the largest mafic intrusions in China. New SHRIMP zircon U‐Pb results indicate that these two intrusions have crystallization ages of 820 Ma and 780 Ma, respectively. Enrichment of large ion lithophile and light rare earth elements and depletion of high field‐strength elements in these intrusions suggest derivation from an active arc along a continental margin. This interpretation of these intrusions as part of a continental arc assemblage is in contrast with the previous view that they were products of a Neoproterozoic mantle plume that initiated the breakup of Rodinia. The presence of a Neoproterozoic magmatic arc suggests that Rodinian oceanic lithosphere was subducted beneath the (present) northern margin of South...

Previous studies have suggested that there were two mass extinction events in the Late Permian: one that occurred at the Permo-Triassic (P/T) boundary (251 Ma) and a second, smaller mass extinction that occurred 5–8 Myr earlier at the end of the Guadalupian. Many workers have argued that there is a causal relationship between large-scale volcanic activity and mass extinctions. The major mass extinction event at the P/T boundary coincides with the outpouring of huge quantities of lava that formed the Siberian flood basalt province in Russia. Courtillot et al. [Earth Planet. Sci. Lett. 166 (1999) 177–195] and Wignall [Earth Sci. Rev. 53 (2001) 1–33] suggested that the earlier Late Permian mass extinction coincided with the eruption of the lavas that formed the Emeishan flood basalt (EFB) province in SW China. However, the age of eruption of the EFB lavas is poorly constrained. Using the Sensitive High-Resolution Ion Microprobe to analyze zircons, we have established the age of the Xinjie intrusion, believed to be a feeder to the main phase of EFB volcanism, to be 259±3 Ma. Hence, the formation of the EFB is coincident with a proposed extinction event at 256–259 Ma. This result supports a temporal link between the Emeishan large igneous province and the end-Guadalupian mass extinction.

The magmatic and tectonic history of the Yangtze Block and its possible affinity with other Neoproterozoic arc terranes are important in the reconstruction of Neoproterozoic plate tectonics. In the Panxi Belt, adjacent to the eastern margin of the Tibetan Plateau, there are many metamorphic complexes associated with Neoproterozoic granites. These are granitic gneisses of upper greenschist to amphibolite metamorphic facies, which have traditionally been considered the Archean basement of the Yangtze Block, although their origin and age of formation were poorly understood. This study provides the first reliable, SHRIMP U–Pb zircon dating results for the gneissic complexes and the Neoproterozoic granites. Three samples of the Kangding gneissic complex yielded identical ages of 797±10, 795±13 and 796±14 Ma. The Gongcai gneissic complex has zircons dated to be 824±14 Ma with metamorphic rims of 177±3 Ma, whereas the Gezong granite has an older age of 864±8 Ma. Other gneissic complexes include the Miyi complex that has a younger age of 764±9 Ma. Geochemical data show that the Kangding gneissic complex has arc signatures, representing metamorphic products of Neoproterozoic, arc-related acidic plutons. This scenario suggests subduction of oceanic lithosphere eastward (present-day orientation) underneath the Yangtze Block. There is a well-defined arc assemblage with an identical Neoproterozoic age along the eastern margin of the Yangtze Block. Thus, during Neoproterozoic time, both the western and eastern margins of the block were active arcs separated by the Trans-Yangtze basin. The Yangtze Block must, therefore, have been an isolated continent, although it was presumably located near the Rodinian supercontinent.

Wider use of radiogenic isotopes in geosciences has been enabled by developments in massspectrometry at the beginning of the 21st century. Nowadays, radiogenic isotope geochemistry forms an integrated part of geosciences in a range of applications, starting from formation of planetary systems, genesis, and the evolution of Earth's lithosphere and associated mineral and oil deposits, as well as environmental tracers. Two primary types of information are available from radiogenic isotopes studies: age determination and isotopic source tracing. In this chapter, the range of isotope systematics currently used in geosciences and their applications are reviewed, together with progress in analytical technologies. The chapter brings together internationally recognised researchers whohave been at the forefront of analytical technologies in the field of geochemistry of radiogenic isotopes.

Mars exhibits ample evidence for an ancient surfacehydrosphere. The oxygen isotope compositions of carbonateminerals and alteration products in martian meteoritessuggest that this ancient hydrosphere was not in isotopicequilibrium with the martian lithosphere1–4. Martian meteoriteNWA 7533 is composed of regolith breccia from the heavilycratered terrains of ancient Mars and contains zircon grainsfor which U–Pb ages have been reported5. Here we reportvariations between the oxygen isotopic compositions offour zircon grains from NWA 7533. We propose that thesevariations can be explained if the mantle melts from whichthe zircon crystallized approximately 4.43Gyr ago hadassimiliated 17O-enriched regolith materials, and that someof the zircon grains, while in a metamict state, were lateraltered by low-temperature fluids near the surface less than1.7Gyr ago. Enrichment of the martian regolith in 17O beforethe zircon crystallized, presumably through exchange withthe 17O-enriched atmosphere or hydrosphere during surfacealteration, suggests that the thick primary atmosphere ofMars was lost within the first 120Myr after accretion. Weconclude that the observed variation of 17O anomalies in zirconfrom NWA 7533 points to prolonged interaction between themartian regolith, atmosphere and hydrosphere. Early Planetary Evolution

The Wildschonau ophiolite complex represents the lowermost and probably oldest rock unit of the western part of the Greywacke Supergroup. Within this ophiolite complex, zircon grains from a metamorphosec gabbro yield a U-Pb SHRIMP age of 477±9 Ma (2σ), interpreted as age of crystallization. This age reflects the magmatic activity in Cambro-Ordovician times and is probably related to a continental rift environment during breakup of Gondwana. The 477±9 Ma age is a few m.y. younger than oldest ages estimated from biostratigraphy; reasons for this difference are evaluated.

U/Pb SHRIMP ages of nine Variscan leucocratic orthogneisses from the central Tauern Window (Austria) reveal three distinct pulses of magmatism in Early Carboniferous (Visean), Late Carboniferous (Stephanian) and Early Permian, each involving granitoid intrusions and a contemporaneous opening of volcano-sedimentary basins. A similar relationship has been reported for the Carboniferous parts of the basement of the Alps further to the west, e.g. the “External massifs” in Switzerland. After the intrusion of subduction-related, volcanic-arc granitoids (374 ± 10 Ma; Zwolferkogel gneiss), collisional intrusive-granitic, anatectic and extrusive-rhyolitic/dacitic rocks were produced over a short interval at ca. 340 Ma (Augengneiss of Felbertauern: 340 ± 4 Ma, Hochweisenfeld gneiss: 342 ± 5 Ma, Falkenbachlappen gneiss: 343 ± 6 Ma). This Early Carboniferous magmatism, which produced relatively small volumes of melt, can be attributed to the amalgamation of the Gondwana-derived “Tauern Window” terrane with Laurussia–Avalonia. Probably due to the oblique nature of the collision, transtensional phenomena (i.e. volcano-sedimentary troughs and high-level intrusives) and transpressional regimes (i.e. regional metamorphism and stacked nappes with anatexis next to thrust planes) evolved contemporaneously. The magmas are mainly of the high-K I-type and may have been generated during a short phase of decompressional melting of lithospheric mantle and lower crustal sources. In the Late Carboniferous, a second pulse of magmatism occurred, producing batholiths of calc-alkaline I-type granitoids (e.g. Venediger tonalite: 296 ± 4 Ma) and minor coeval bodies of felsic and intermediate volcanics (Heuschartenkopf gneiss: 299 ± 4 Ma, Peitingalm gneiss: 300 ± 5 Ma). Prior to this magmatism, several kilometres of upper crust must have been eroded, because volcano-sedimentary sequences hosting the Heu- schartenkopf and Peitingalm gneisses rest unconformably on 340-Ma-old granitoids. The youngest (Permian) period of magma generation contains the intrusion of the S-type Granatspitz Central Gneiss at 271 ± 4 Ma and the extrusion of the rhyolitic Schonbachwald gneiss protolith at 279 ± 9 Ma. These magmatic rocks may have been associated with local extension along continental wrench zones through the Variscan orogenic crust or with a Permian rifting event. The Permian and the above-mentioned Late Carboniferous volcano-sedimentary sequences were probably deposited in intra-continental graben structures, which survived post-Variscan uplift and Alpine compressional tectonics.

SHRIMP ages of 12 volcanic samples indicate intense magmatic activity to have occurred at the Carboniferous/Permian boundary throughout much of the NE German Basin. Rhyolitic crystal-rich samples have been taken from quarries in the Halle Volcanic Complex (HVC) and in the Flechtingen Ignimbrite, and from drill cores of the Kotzen, Mirow, Friedland and Penkun areas. Ten samples yielded 206Pb/238U ages between 302 and 297 Ma (±3 Ma, respectively) which indicate that the magmatic activity took place concentrated in a relatively short time span throughout much of the NE German Basin. Two HVC samples have ages of 307 and 294 Ma. This remarkably synchronous magmatic activity occurred during the initial phase of the basin development. Considering published volume estimates (48,000 km3), extrusion rates during this time period probably were in the order of 0.01 km3 a−1, implying magma production rates of 0.1 km3 a−1. The calc-alkaline SiO2-rich volcanic rocks originated from magmas that presumably formed during anatexis and subordinate magma mixing in an intra-continental transtensional setting. The basaltic magma which must have provided the thermal input into the lower crust probably formed during decompressional melting of lithospheric mantle fertilised by previous magmatic processes. The dated volcanic rocks occur in three geotectonic provinces, namely the (a) Mid German Crystalline Rise, which forms the northern part of the Internal Variscides, the (b) External Variscides and the Variscan foreland which is considered as (c) Eastern Avalonia. Many of the old zircons found in the HVC samples reflect the magmatic activity of the Mid German Crystalline Rise (325–400 Ma). However, Cadomian (500–650 Ma) and older Gondwanian elements (1674–2373 Ma) are also present. Although the Flechtingen and Kotzen areas are located within the External Variscides, it seems more likely that the middle to lower crust, that experienced anatexis, formed part of the Eastern Avalonia Plate overridden by the Variscan Front. Thus the old zircons found in Flechtingen (350 and 538 Ma) and Kotzen (345 Ma), but certainly those present in the Penkun drill cores (1483 Ma) represent Eastern Avalonia tectonic history. The anatectic magmas in the Friedland area probably formed partly from Baltic crust thrusted below Eastern Avalonia during the Caledonian Orogeny. Among the old zircons from the Friedland samples, in addition to Proterozoic ages (1456 Ma), we found testimony of Caledonian (443 Ma) and of clearly post-Caledonian (387 Ma) magmatism.

The performance characteristics of a double focusing mass spectrometer designed specifically for geochronology are described. The sensitive high resolution ion micro probe (SHRIMP II) mass spectrometer is ideally suited for in-situ analysis of uranium and/or thorium-bearing minerals to provide geochronological information on micron-sized domains of the minerals. SHRIMP II simultaneously meets the requirements of high mass resolution and high ion transmission efficiency together with good abundance sensitivity. SHRIMP II’s excellent spatial and mass resolution, coupled with its high sensitivity, has enabled zircon geochronology to be revolutionized.

The 40Ar/39Ar and U‐Pb methods of isotopic dating have been used to determine the age of alteration minerals and host rocks from a number of Permo‐Carboniferous Au deposits in the Tasman Fold Belt system of northeast Queensland. There was a continuum in porphyry‐style Au mineralisation from ca 330 to 290 Ma, which post‐dates epithermal Au in the Eastern Basin sequence of the Drummond Basin. Porphyry‐style Au at Kidston, and the Buck Reef, Ravenswood, formed at ca 330 Ma. The Red Dome and Mungana skarns were deposited at ca 320–310 Ma and ca 310 Ma, respectively. The Au mineralisation at Mt Wright, and in the quartz‐sulfide veins, Ravenswood, was emplaced at ca 310–305 Ma. The youngest porphyry‐style Au deposit is Mt Leyshon, which probably formed at ca 290 Ma. Where high‐level intrusions are associated with Au, magmatism and mineralisation are, in general, essentially coeval. The age of the host rocks to the Permo‐Carboniferous Au deposits is variable, and may be more than 150 million years older than the...

We used the Panurge ion microprobe to measure concentrations of the rare earth elements (REEs), Ba, Hf, and Sr in melilite, clinopyroxene, plagioclase, and perovskite and Mg isotopes in plagioclase, spinel, melilite, fassaite, hibonite, grossular, and monticellite from the Allende Type B1 calcium-, aluminum-rich inclusion (CAI), USNM 3655A. The distribution and concentration of Ba and the REE in melilite from the melilite-rich mantle of 3655A are unlike those predicted from melilite-melt REE partitioning experiments for closed system crystal fractionation. REE concentrations are lower than expected in the first crystallized gehlenitic melilite, increase rapidly to higher than expected concentrations in melilite with intermediate akermanite contents (Ak30–Ak40), and decrease as expected only during the late stage of mantle crystallization. Barium concentrations in melilite are 10–50 times those expected, and the LREE/HREE ratio increases continuously rather than remaining constant. The unexpected distribution of trace elements in melilite reflects a progressive enrichment of trace elements in the melt during the early stages of crystallization. A partial explanation for this observation is the dissolution of precursor perovskite that contained half or more of the total REE budget of the inclusion. In addition, there are large trace element enrichments adjacent to included spinel in melilite and similar but smaller enrichments adjacent to spinet in clinopyroxene. These enrichments are consistent with the existence of trace element enriched boundary layers at the mineral/melt interfaces. The fact that kinetic processes partially control trace element abundances and distributions suggests rapid cooling during crystallization of the melilite-rich mantle. Similar trace element signatures are ubiquitous in Type B1 CAI, suggesting that each experienced a similar thermal history. The Mg isotope record of 3655A is distinguished by four salient features: (1) large ^(26)Mg excesses correlated with the respective AI/Mg ratios in plagioclase, melilite, and hibonite, (2) F_(Mg), the mass-dependent fractionation of Mg, is positive, with enrichment of the heavier Mg isotopes in all primary phases, (3) a heterogeneous distribution of F_(Mg) values, with F_(Mg) in melilite systematically greater than in either spinel or fassaite, and (4) isotopically normal Mg in the secondary alteration phases, grossular and monticellite. The occurrence of ^(26)Mg^∗, the decay product of ^(26)Al, in anorthite implies early formation of 3655A, while ^(26)Al was extant at nearly the canonical solar system value of ∼5 × 10^(−5). The trace element and Mg isotope heterogeneities suggest a formation scenario for 3655A which includes (1) flash heating to partially melt a solid precursor, (2) rapid cooling to allow survival of relict phases, (3) diffusive exchange of Mg between melilite and a nebular reservoir, and (4) alteration at low temperature. Although this model explains most of the trace element and isotopic characteristics of 3655A and other Type BI CAIs, it does not provide an explanation for the rapid change of LREE/HREE ratios of melilite during crystallization.

New chemical data for both whole-rock and individual mineral samples from the Acapulco meteorite are reported. Results of instrumental neutron activation analysis (INAA) of bulk samples show large variation in La, Cr, Se, and Fe contents reflecting inhomogeneous distribution of the corresponding host phases: phosphate, chromite, sulfide, and FeNi metal. In contrast, Mn, Sc, and Na contents are uniformly distributed reflecting constant fractions of orthopyroxene, olivine, and plagioclase in bulk samples. INAA data were obtained on single olivine and orthopyroxene grains with numerous tiny metal inclusions. The siderophile elements in these inclusions, in particular the low Ir contents, suggest that the inclusions formed by partial melting of matrix metal. The composition of the metal inclusions and the absence of such inclusions in clinopyroxene suggests an upper limit of about 20% partial melting of Acapulco at around 1200°C. During melting and closed-system crystallization the inhomogeneous distribution of metal, chromite, sulfide, and phosphates was established. Acapulco phosphates are of igneous origin. Most of them were not formed by oxidation of metal. The uniform K contents of bulk Acapulco samples and the comparatively high contents of volatiles (including rare gases) in bulk samples demonstrate a closed-system behavior. Neither loss nor gain of volatile elements has occurred. Ion microprobe data of rare earth elements (REEs) in Acapulco minerals show an equilibrium distribution for heavy REEs and nonequilibrium for light REEs. Bulk Acapulco samples have large excesses of light REEs and U with phosphates being the major carrier. Absolute REE contents of phosphates are variable. The observed enrichments of REEs and U in bulk Acapulco samples cannot be explained by the addition of phosphates, since the approximately chondritic ratios of Ca/Mg and P/Mg exclude major gains of Ca and P through the addition of phosphates. It is suggested that a fluid phase rich in incompatible elements infiltrated Acapulco and that REEs and U were extracted by phosphates. The remaining fluid phase subsequently must have drained away. From temperatures determined by two-pyroxene equilibria, spinel-olivine and Ca zonation in olivine, a thermal history of Acapulco is constructed. Cooling rates at 900°C of about 100 K/Ma are estimated. The concentration profiles of Ca in olivine, showing constant Ca throughout grain interiors and sharp decreases at the rims, are difficult to explain. If the Ca zonation in olivine exclusively reflects cooling, slow cooling must have been followed by fast cooling at temperatures below 650°C. Alternatively, the low Ca contents of olivine rims may reflect other processes, such as, for example, formation of phosphates. A model for the evolution of Acapulco is presented: The parent body of Acapulco accreted earlier than that of the ordinary chondrites (OC), thus more 26Al was available for heating, leading to higher peak temperatures than in OC. Extensive solid state equilibration at 900°C completely erased the 26Mg signature in plagioclase. Acapulco is, despite the absence of chondrules, essentially a chondritic meteorite, attesting to the variety of planetesimals of chondritic composition in the asteroid belt.

The concentrations of 28 elements were measured using an ion microprobe in perovskite, hibonite and coexisting melts, in isothermal crystallization experiments on chemical compositions similar to those of Compact Type A (CTA) Ca-, Al-rich inclusions (CAI) and to a hibonite-glass microspherule. The mineral/melt partition coefficients (D) calculated from the measured concentrations for both minerals define reliable D-values. Perovskite and hibonite D's have ranges of 10^−2 for Si to 20 for Th and 3·10^−3 for Si to ∼8 for La, respectively. There are regular relationships between the ionic radius, the valence of the trace element and the partition coefficients in perovskite and hibonite. While there are differences in the D-values between perovskite and hibonite, they follow very similar trends with perovskite typically having D-values that are 5–10 times higher for the same element. Perovskite and hibonite D's are almost identical for the divalent cations Ba (0.02 and 0.03, respectively) and Sr (1.1 and 0.8, respectively) in our experiments. D_Mg for perovskite is low, 0.03, when compared with the value for hibonite, 0.5. Mineral/melt D's for the REE decrease continuously from D_La = 6 to D_Lu = 0.03 in hibonite. For perovskite, REE D's increase slightly from D_La = 10 to D_Nd = 15 and then decrease continuously to D_Lu = 1.0 and D's for trivalent cations with smaller ionic radii than the REE are lower, with D_Al = 0.08 and D_Sc = 0.15 lower than D_Cr = 0.8 and D_V = 1.0. With the exception of D_Th and D_Si in perovskite and DSi in hibonite, the D-values for tetravalent cations and Nb, the only pentavalent element, fall within the range of D's for the REE. D_th/D_U equals 3 in perovskite and ∼ 15 in hibonite. Our data can be applied to the genesis and evolution of hibonite in refractory meteorite inclusions. For example, low Ba relative to other refractory elements, such as Hf, Zr, La, etc., in hibonite has been observed in some hibonite-bearing inclusions. Since D_Ba ⪡ D_Hf, ⪡ D_Zr and ⪡ D_La in our experiments low Ba may result from the incompatibility of Ba in hibonite rather than the increased volatility of Ba under oxidizing conditions during condensation. In addition, since D_La/D_Lu > 50 for hibonite. LREE/HREE ratios of 1 in hibonite in some CTA CAI from Leoville and Allende are inconsistent with hibonite equilibrating with the melts that formed these inclusions and the hibonite is relict. Similar applications are possible with our perovskite partitioning data. For example, it is likely that high-REE (500−1000 × chondritic) perovskite with Th/U of 3–4 that are found in the outer region of Type B 1 CAI have not been in equilibrium with the CAI melt that contains ∼ 20 × ch REE and a Th/U ratio of 3 and they are probably relics that survived the most recent partial melting event.

Recent experimental work has shown that the homogenization of elemental concentrations can be much slower than that of isotopic ratios when there are strong concentration gradients in SiO_2 and Al_2O_3. The ramifications of this result for magma homogenization and other petrological problems related to diffusion are significant. We report here a comparison of experimental profiles of elemental concentrations and isotopic fractions of K and Ca in rhyolite-andesite (large concentration gradients) and rhyolite-rhyolite (small concentration gradients) melt couples. When the concentration profile and the isotopic profile of the same element in a single couple are compared, the former is much shorter than the latter in the rhyolite-andesite couple, consistent with other recent studies. However, the lengths of both concentration and isotopic profiles are similar in the rhyolite-rhyolite couple. Therefore, diffusion of an element or oxide may be decoupled from or coupled with isotopic ‘diffusion’, depending on whether large concentration gradients of major components are present. When the two couples are compared, the intrinsic effective binary diffusivities obtained from isotopic profiles are similar for each element in the two couples, whereas the effective binary diffusivity of K obtained from the concentration profile in the rhyolite-rhyolite couple is 37 times that in the rhyolite-andesite couple. Therefore, isotopic homogenization is roughly independent of elemental homogenization and the presence of SiO_2, Al_2O_3, and other concentration gradients, whereas elemental homogenization is strongly affected by concentration gradients. Our experimental data (isotopic and concentration profiles including uphill diffusion profiles) can be modeled quantitatively to a good approximation using a modified effective binary diffusion model in which the flux of a component is assumed to be proportional to its activity gradient instead of its concentration gradient. Therefore, the multicomponent diffusion effect in the silicate systems of our experiments seems to be largely due to contributions of non-ideal mixing to the cross-terms of the diffusivity matrix.