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Acknowledgments The authors would like to thank Ángel Perandrés-Villegas for preparing the thin sections used for this study, Fátima Linares Ordóñez for X-ray computed micro-tomography scanning, Mike Tappa for assistance with thermal ionization mass spectrometry analysis, Francisco Alonso-Chaves and Fernando Simancas for sending structural data, Whitney Behr, Johannes Glodny, Sean Mulcahy, and an anonymous reviewer for providing helpful reports, and editors Laurent Jolivet and Federico Rossetti for additional comments that helped improve two earlier manuscript versions. EFB gratefully acknowledges support from the NSF Grants EAR-1250497 and PIRE-1545903 as well as start up funds from the Boston College. DA and ARF gratefully acknowledge financial support through Spanish project “DAMAGE” (CGL2016-80687-R AEI/FEDER), and Junta de Andalucía project RNM148, both directed by Jesús Galindo Zaldivar. ARF acknowledges an PhD Grant (FPU) from the Spanish government. Open access fees have been funded by Universidad de Granada /CBUA., High-resolution microstructural analysis of porphyroblast inclusion trails integrated with Sm-Nd garnet geochronology has provided new insight into the tectonic history of the Betic-Rif orogen. Three principal age groups of porphyroblasts are demonstrated with distinctly oriented inclusion-trails. Inclusion-trail curvature axes or “FIA” (Foliation Inflexion/Intersection Axes) are shown to represent “fossilized” crenulation axes from which a succession of different crustal shortening directions can be deduced. The regional consistency of microstructural orientations and their geometric relationship with multiple sets of macroscopic folds reveal the composite character of the Gibraltar Arc formed by a superposition of different folding directions and associated lineations. Bulk-garnet ages of 35–22 Ma obtained from five micaschist samples of the Alpujarride-Sebtide complex (ASC) and of 35–13 Ma from four micaschists of the Nevado-Filabride complex (NFC) allow to deduce NNE-SSW directed shortening in the Late Eocene changing to NW-SE shortening in the early Oligocene, alternating with suborthogonal NE-SW shortening during the Miocene. These directions can be related to a major swing in the direction of relative Africa-Iberia plate-motion known from kinematic modeling of magnetic seafloor anomalies, and subsequent dynamic interference between plate convergence and suborthogonal “tectonic escape” of the Alboran Domain. Coupled to previously established P-T-t paths, the new garnet ages support a common tectono-metamorphic evolution of the ASC and NFC as laterally equivalent orogenic domains until, in the Miocene, the second became re-buried under the first., NSF Grant EAR-1250497, PIRE-1545903, Spanish project “DAMAGE” (CGL2016-80687-R AEI/FEDER), Junta de Andalucía project RNM148, PhD Grant (FPU) from the Spanish government, Open access fees have been funded by Universidad de Granada /CBUA

The Inyoni shear zone represents an important tectonic boundary between (i) the ca. 3.45 Ga high-pressure amphibolite facies, granite-greenstone domain south of the Barberton greenstone belt, termed the Stolzburg terrane, and (ii) the ca. 3.29 to 3.23 Ga rocks of the trondhjemitic Badplaas pluton to the west. The Stolzburg terrane is separated from the greenschist facies rocks of the rest of the Barberton greenstone belt by the Komati fault, which records >10 km uplift of the Stolzburg terrane relative to the lower-grade rocks of the greenstone belt at ca. 3.23 Ga. A number of studies within the Stolzburg terrane have documented high-pressure amphibolite facies metamorphism that occurred concurrently with exhumation, with the lowest apparent geothermal gradients documented in the Inyoni shear zone, where strong constraints on the age of metamorphism are most limited. In addition, different studies on Inyoni metamorphism have produced significantly different temperature estimates. This study utilizes garnet Sm-Nd geochronology in combination with P-T modelling to directly date the metamorphism and re-evaluate the P-T conditions of the Inyoni shear zone. Two petrologically distinct samples produce similar P-T evolutions. A heterogeneous sample with both garnet-bearing and garnet-absent domains gives up-P evolutions reaching conditions of 550 to 675°C and 7 to 10 kbar, whereas a homogenous sample containing garnet and clinopyroxene produces a similar dominantly up-P evolution reaching peak conditions of 650°C and 8 to 10 kbar. Sm-Nd garnet ages of 3 201.6 ± 4.7 Ma (MSWD = 1.02) and 3 200.3 ± 5.3 Ma (MSWD = 0.44) were obtained from two samples of the homogenous garnet and clinopyroxene-bearing amphibolite. The Sm-Nd garnet geochronology provides accurate ages for the metamorphism of the Inyoni shear zone, with age results suggesting activity on the Inyoni shear zone may have continued after the regional metamorphism at ca. 3.23 Ga previously established by zircon U-Pb geochronology. However, 147Sm decay constant uncertainty leaves open the possibility that Inyoni garnet growth could have coincided with the previously recognized 3.23 Ga regional metamorphism.

Microstructural analysis of porphyroblast inclusion trails in 44 micaschist samples has been combined with Sm-Nd garnet geochronology to investigate the tectonic history of the Betic-Rif orogen. Th...

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Nineteen single-grain detrital garnet ages from a tributary to the French Broad River (North Carolina, USA) establish a novel approach to Sm-Nd detrital garnet geochronology wherein the equilibrium bulk-rock composition lost during weathering and transport is replaced with the composition of inclusions leached from within each garnet grain. Detrital garnet ages were compared to published detrital zircon and monazite ages from the same river tributary system. Results show that 87% of the zircons have inherited Proterozoic ages; only zircon rims give Paleozoic ages. Monazites are exclusively Ordovician (weighted average: 460.9 ± 3.0 Ma). Our new detrital garnet ages (and the detrital zircon rims) record younger ages spanning the Late Ordovician to Early Devonian, likely reflecting prolonged metamorphic heating. The weighted average age of the detrital garnet population is Silurian (430.1 ± 7.2 Ma). Statistical tests confirm that the garnet population is younger than the monazite. The new detrital garnet ages illuminate a previously uninterpreted Silurian tectonometamorphic signal in this region.

The metamorphic devolatilization of serpentinites during subduction represents the largest potential source of fluids from the subducting slab and influences a range of important processes from subduction zone seismicity to arc magmatism. Obtaining a record of metamorphic dehydration directly from serpentinites, however, is challenging, as serpentinite mineral assemblages are less conducive to extracting a detailed pressure-temperature-time (P-T-t) history. In this study, we use meta-rodingites (metasomatized gabbros) from the central portion of the Voltri Ophiolite in the Ligurian Alps (Italy), to investigate the P-T and geochronological record of subduction metamorphism and dehydration within the adjacent serpentinites. These meta-rodingites underwent eclogite-facies metamorphic recrystallization to a garnet-clinopyroxene-chlorite-ilmenite assemblage, and are cross-cut by several generations of garnet-bearing veins. A comparison of the Voltri meta-rodingites with un-subducted seafloor rodingites from the Apennines reveals textural and geochemical evidence of complete metamorphic recrystallization of the original seafloor metasomatic assemblage during Alpine subduction. Phase equilibria modeling of the Voltri meta-rodingite sample suggests that the peak metamorphic assemblage recorded was stable in the range of ~450–600 °C, which is consistent with previous P-T estimates for gabbroic and ultramafic lithologies from the Voltri Ophiolite. A late prograde to peak metamorphic age of 40.6 ± 2.9 Ma was obtained from the meta-rodingite using Sm Nd garnet geochronology, which closely matches the ages of late prograde to peak metamorphism estimated by companion studies of associated eclogites from the central part of the Voltri Ophiolite. The overlap in P-T conditions and timing of peak metamorphism for samples across the area suggests subduction and exhumation of the central part of the Voltri Ophiolite as a coherent lithospheric slice. Dating of a garnetite vein cross-cutting the main meta-rodingite body suggests an influx of fluid at 37.7 ± 1.4 Ma, at or near peak eclogite-facies conditions. Enrichment in the vein selvage of both compatible transition metals (Cr, Ni, Cu) and fluid-mobile elements (Li, Rb, Cs, Ba) suggests the fluid carried both serpentinite and sedimentary geochemical signatures and interacted with the main meta-rodingite body. We suggest that the most probable explanation for this fluid composition, and thus the source of the fluid, is the dehydration of the Voltri serpentinites. These serpentinites likely experienced an influx of sedimentary-derived, fluid-mobile-element-bearing fluids earlier in their history, either on the seafloor or most likely during the early stages of subduction. The presence of a sedimentary signature in this serpentinite-derived fluid emphasizes the importance of subducted serpentinites and their dehydration to the geochemical cycling of fluid-mobile elements in subduction zones. The widespread development of such vein systems within multiple meta-rodingites across the Voltri Ophiolite points to a large-scale fluid release event, likely resulting from the partial dehydration of brucite and antigorite within the surrounding serpentinite hosts. By utilizing direct dating of vein mineralogy formed during serpentinite dehydration, we provide one of the first geochronological records of serpentinite dehydration and fluid release occurring during subduction.

The problem of how dense high- (HP) and ultrahigh-pressure (UHP) metamorphosed oceanic mafic and ultramafic rocks are exhumed from deep within subduction zones is crucial to understanding processes occurring at the interface between the subducting slab and the overlying plate and mantle wedge. In this study, we use the P-T-t evolution of metamorphosed Fe Ti gabbros hosted in meta-serpentinite from the Voltri Ophiolite (Ligurian Alps, Italy) to evaluate potential exhumation mechanisms and assess whether HP mafic rocks were exhumed as blocks within a complex serpentinite-hosted melange or as coherent sections of oceanic lithosphere. A number of observations provide evidence for subduction and exhumation of the central portion of Voltri Ophiolite as a coherent package of ultramafic and mafic material: (1) a similar evolution in mineral assemblages, textures and compositions suggesting peak metamorphism in the eclogite-facies, followed by a retrograde evolution through blueschist-, amphibolite- and greenschist-facies conditions; (2) uniform P-T conditions of both initial garnet growth (~475–480 °C and 2.0–2.2 GPa) and peak metamorphism (~500–525 °C and 2.3–2.5 GPa); and (3) bulk Sm Nd garnet ages that reveal a very tight grouping of ages for four well-equilibrated foliated Fe Ti meta-gabbros clustering at 39.98 ± 0.84 Ma. A massive undeformed gabbro also from the central Voltri area gave a slightly younger age of 38.15 ± 0.89 Ma, which is interpreted to reflect kinetic overstepping and delayed growth of garnet. By contrast, an Sm Nd garnet age from the NW portion of the Voltri Ophiolite indicates peak eclogite-facies metamorphism occurring ~10–12 Ma earlier at 49.68 ± 0.35 Ma. A compilation of the current geochronological data suggests that the NW portion of the Voltri Ophiolite underwent peak eclogite-facies metamorphism first at ~50 Ma, the sedimentary portion of the Voltri Unit (the Voltri-Rossiglione meta-sediments) experienced peak metamorphism around ~47–44 Ma, whilst subduction and peak metamorphism of the central part of the ophiolite occurred later at 41–38 Ma. Alongside recent studies of other Alpine ophiolites (Zermatt Saas, Monviso, Alpine Corsica), this study suggests that detachment and buoyant exhumation of large kilometric-scale coherent sections of slab material, rather than exhumation within a chaotic serpentinite melange, is the dominant mechanism of preserving and returning eclogite-facies metamorphic rocks to the surface in the Western Alps. In the case of the Voltri Ophiolite, which lacks any associated continental massif, this exhumation was likely aided by the buoyancy of the serpentinite-dominated oceanic lithosphere, rather than by coupling to continental crust.

Subduction zones are key regions of chemical and mass transfer between the Earth’s surface and mantle. During subduction, oxidized material is carried into the mantle and large amounts of water are released due to the breakdown of hydrous minerals such as lawsonite. Dehydration accompanied by the release of oxidizing species may play a key role in controlling redox changes in the subducting slab and overlying mantle wedge. Here we present measurements of oxygen fugacity, using garnet–epidote oxybarometry, together with analyses of the stable iron isotope composition of zoned garnets from Sifnos, Greece. We find that the garnet interiors grew under relatively oxidized conditions whereas garnet rims record more reduced conditions. Garnet δ56Fe increases from core to rim as the system becomes more reduced. Thermodynamic analysis shows that this change from relatively oxidized to more reduced conditions occurred during lawsonite dehydration. We conclude that the garnets maintain a record of progressive dehydration and that the residual mineral assemblages within the slab became more reduced during progressive subduction-zone dehydration. This is consistent with the hypothesis that lawsonite dehydration accompanied by the release of oxidizing species, such as sulfate, plays an important and measurable role in the global redox budget and contributes to sub-arc mantle oxidation in subduction zones. Lawsonite dehydration and release of oxidizing fluids could play an important role in sub-arc mantle oxidation in subduction zones, suggest measurements of changing oxygen fugacity in zoned garnets from Sifnos, Greece.

We report the results of Sm–Nd garnet geochronology on a mafic granulite from the metamorphic sole of the Ballantrae Ophiolite, Scotland. The resultant age of 477.6 ± 1.9 Ma (2σ) is interpreted to represent the obduction of the Ballantrae Ophiolite and the onset of the Grampian phase of the Caledonian Orogeny in Scotland. Thermodynamic modelling indicates peak conditions of c . 825°C and at least 10 kbar for the unit. In conjunction with a biostratigraphic constraint on metamorphic garnet deposition in sediments at 465 ± 2.5 Ma, this new age more precisely constrains the duration of Grampian metamorphism in Scotland to 12.6 ± 3.1 myr (2σ). In combination with this new age, previous geochronological work on sillimanite-zone garnet indicates a time-averaged initial heating rate of 106 ± 78°C myr −1 (2σ), a rapid rate that may be inconsistent with orogenic self-heating via crustal overthickening alone. The calculated heating rate for the associated garnet zone is also rapid and more precisely constrained at 47 ± 14°C myr −1 (2σ). These results together with the short overall duration of the event support the idea that the Grampian phase of the Caledonian Orogeny in Scotland included both rapid heating and correspondingly fast exhumation.

Thermal ionization mass spectrometers, or TIMS, were developed by the pioneers of mass spectrometry in the mid-20th century, and have since been workhorses for generating isotopic data for a wide range of elements. Later-developed mass spectrometric techniques have many advantages over TIMS, including higher spatial resolution with in situ techniques, such as secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), and greater versatility in terms the elements that can be easily-and well-measured. The reason TIMS persists as an important method for geochronology is that for some key parent-daughter systems (e.g., U–Pb, Sm–Nd), it can produce isotopic data and resultant dates with 10–100 times higher precision and more quantifiable accuracy than in situ techniques, even when sample sizes are very small (such as those that might result from single crystals, or even small portions of zoned crystals). For many questions in the geosciences, the highest achievable precision and accuracy are required to resolve the timescales of processes and/or correlate events globally. As an example, modern TIMS U–Pb geochronology is capable of producing dates with precision and accuracy better than 0.1% of the age for single crystals with only a few picograms (pg) of Pb. Therefore, it is possible to constrain the durations of single zircon crystal growth in magmatic systems over tens to hundreds of kyr in Mesozoic and younger rocks. If these dates and rates can be connected with other igneous processes such as magma transfer, emplacement and crystallization, then it becomes possible to calibrate thermal and mass budgets in magmatic systems and evaluate competing models for pluton assembly and subvolcanic magma storage. As another example, Sm–Nd geochronology of garnet permits dates with precision better than ±1 million years for garnets of any age, including multiple concentric growth zones in single crystals. Such …

Garnet could be the ultimate petrochronometer. Not only can you date it directly (with an accuracy and precision that may surprise some), but it is also a common rock-forming and porphyroblast-forming mineral, with wide ranging—yet thermodynamically well understood—solid solution that provides direct and quantitative petrologic context. While accessory phase petrochronology is based largely upon establishing links to the growth or breakdown of key rock-forming pressure–temperature–composition ( P–T–X ) indicators (e.g., Rubatto 2002; Williams et al. 2007), garnet is one of those key indicator minerals. Garnet occurs in a great variety of rock types (see Baxter et al. 2013) and is frequently zoned (texturally, chemically) meaning that it contains more than just a snapshot of metamorphic conditions, but rather a semi-continuous history of evolving tectonometamorphic conditions during its often prolonged growth. In this way, garnet and its growth zonation have been likened to dendrochronology: garnet as the tree rings of evolving tectonometamorphic conditions (e.g., Pollington and Baxter 2010). In some ways, the dream of ‘petrochronology’ all started with garnet (Fig. 1). When Atherton and Edmunds (1965) or Hollister (1966) recognized the chemical zonation in garnet, when Rosenfeld (1968) noted the spiral ‘snowball’ of inclusions in rotated garnet, or when Tracy et al. (1976) drew the first 2-D map of garnet chemical zonation, illuminating those ‘tree-rings’ for the first time, they could only imagine what is now a reality decades later—direct zoned garnet geochronology of those concentric rings of growth. Geoscientists soon thereafter attempted the first garnet geochronology (van Breemen and Hawkesworth 1980), though several factors severely limited the development and wider-spread use of garnet geochronology from that point. These factors included 1) contamination of garnet by micro-mineral inclusions, 2) analytical limitations of small sample size, 3) the requirement of anchoring a garnet age analysis with another point on an isochron, and …

Granulite-facies metamorphism of large tracts of continental crust is a characteristic feature of Archean cratons, where metamorphism ultimately led to crustal stabilization and chemical differentiation. Constraining the timing and duration of this metamorphism can provide insight into the processes behind Archean cratonic evolution, and here we present new Sm–Nd garnet ages from granulite-facies metasediments from the eastern Beartooth Mountains of the Wyoming Craton. The eastern Beartooth Mountains are dominantly comprised of a ∼2.8 Ga calc-alkaline batholith known as the Long Lake Magmatic Complex (LLMC). Within this batholith, numerous cm- to km-scale granulite-facies metasedimentary xenoliths and roof pendants have previously been interpreted to represent metamorphism due to contact heating with the LLMC or prior to LLMC emplacement. Garnet ages from five granulite-facies metasediments range from 2669 ± 48 Ma to 2681 ± 58 Ma: significantly younger than the LLMC and consistent with newly described field relations. Preserved major and trace element garnet zoning suggest that these dates are biased towards late stage garnet growth during granulite-facies metamorphism, and that this metamorphism post-dates LLMC emplacement. U–Pb monazite petrochronology of a metasediment and an tonalite reveals distinct periods of monazite (re)crystallization at 2.79–2.78 Ga (consistent with LLMC emplacement) and at 2.69–2.67 Ga (coincident with the garnet ages). The Sm–Nd bulk garnet ages exhibit larger errors and high MSWDs, which may result from sampling a polymetamorphic population, as suggested by REE zoning in garnet and supported by the U–Pb monazite data. Diffusion modeling of preserved major element garnet zoning suggests that the timescale at near-peak temperatures during the 2.69–2.67 Ga event was

Rare-earth elements play a crucial role in modern technologies and are necessary for a transition to a green economy. Potentially economic deposits of these elements are typically hosted in minerals such as monazite, bastnasite, and eudialyte (a complex Na-Ca-Fe-Zr silicate mineral with Cl), making these prime targets for geological research. Globally, rare-earth mineral deposits commonly show evidence of polyphase development and mineralisation processes, which need to be better understood to improve exploration strategies. The Norra Karr alkaline complex (Sweden) contains a globally significant deposit of rare-earth elements, hosted in the mineral eudialyte. In this study, we focussed on eudialyte crystals in undeformed, cross-cutting pegmatoid veins from Norra Karr. In order to determine their age, we refined an established micromilling method to enable sampling of minerals rich in rare-earth elements for precise analysis of major and trace elements, Nd isotope ratios, and Sm-Nd geochronology down to a scale of

A quantitative assessment of metamorphic chemical equilibrium derived from correlation of spessartine content and garnet Sm–Nd ages suggests that major element matrix equilibrium was maintained (to a first order) throughout a ca. 40 cm-wide rock sample during garnet growth; however cm-scale Sm–Nd isotopic heterogeneity limits the Sm–Nd age precision required to evaluate more subtle age differences within individual garnet crystals. Central wafers from 1–3 cm diameter garnet grains within a 1.21 × 104 cm3 block of pelitic schist were used to document concentric growth zoning of major elements, with decreasing Mn and Ca and increasing Fe and Mg from cores to rims. Garnets also preserve growth zoning patterns for HREE and MREE and show evidence for resorption and partial recrystallization of the outermost rims. Similar garnet core compositions and identical garnet rim compositions for large like sized porphyroblasts throughout the sample suggest that garnet growth occurred at near equilibrium P–T–X conditions for major elements over the sample volume. Comparison of 28 rock Sm–Nd isotope values from the sample indicates substantial cm-scale heterogeneity, which precludes meaningful use of local garnet rock isotope pairs for isochron age calculation. Therefore, Sm–Nd isotopic compositions of thirty-eight concentric core to rim garnet segments from ten large (1–3 cm) garnets and two small (1–4 mm) bulk garnets, with narrow ranges of Mn content, are paired with sixteen matrix/whole-rock Sm–Nd isotopic compositions collected over the rock volume to define a range of isochron ages from 383.1 ± 6.8 Ma to 324.5 ± 3.3 Ma. Four of the garnets have anomalously young rims that likely result from post-growth alteration. Chlorite, quartz, and xenotime haloes around garnet suggest that anomalously young garnet rim ages reflect post-growth resorption/recrystallization effects. Excluding these young rims yields a range of ages from 383.1 ± 6.8 (oldest core) to 374.9 ± 1.8 Ma (youngest rim). Sm–Nd age precisions > 1.5 m.y. (and high MSWD) result primarily from isotopic heterogeneity in the finely layered metasedimentary rock matrix. However, garnet cores with high Mn (n = 7), mantles with intermediate Mn (n = 14), and rims with low Mn (n = 8; including the 2 smaller bulk garnet analyses), define three distinct multi-grain isochrons of 380.3 ± 2.0 Ma (n = 23, MSWD = 14), 377.3 ± 1.4 Ma (n = 30, MSWD = 18), and 376.5 ± 1.0 Ma (n = 24, MSWD = 18), respectively, yielding an average garnet growth duration of 3.8 ± 2.2 m.y. These three composite Mn-age zones define a Mn vs. age relationship that reflects depletion of Mn in the rock matrix as it is sequestered by growing garnet. Correlation of garnet major element compositions throughout the sample suggests that major element matrix equilibrium was generally maintained (to a first order) throughout the ca. 4 m.y. duration of garnet growth.

We use coupled zoned geochronology and thermodynamic modeling of garnet to elucidate the nature and scale of metamorphic dehydration during Eocene subduction of a quartzofeldspathic lithology from Sifnos, Greece. Two large garnet porphyroblasts were microdrilled to sample concentric growth zones, and these were dated using Sm–Nd geochronology. To put results in a geodynamic context and reveal the causes and consequences of garnet growth, we constructed thermodynamic forward models for a series of prescribed pressure–temperature ( P – T ) paths. Our data reveal three distinct phases of garnet growth: initial growth at 53.4 ± 2.6 Ma (∼0.8 GPa and ∼300 °C), followed by a period of very limited growth until a second phase, at 47.22 ± 0.36 Ma , and then a major pulse of growth, responsible for the majority of the final garnet volume, at 44.96 ± 0.53 Ma (2.06–2.19 GPa and 490–550 °C). This suggests a >2 order of magnitude acceleration in volumetric growth rate from crystal core to rim, with the final growth pulse occurring rapidly ( 75 °C/My. This final pulse was accompanied by net bulk rock dehydration of ∼0.5 wt.%. Rapid heating during early stages of exhumation in the subduction channel, or by sharp thermal gradients related to slab-mantle coupling could be causes for this pulsed metamorphism and dehydration. The garnet data thus record a concentrated pulse of dehydration and heating during the otherwise slow and continuous process of subduction.

The geochemical evolution and chronology of the Breitenau sparry magnesite deposit has been investigated by means of major and trace element abundances, stable C–O isotope, and Sm–Nd isotope analysis. Chemically separated magnesite fractions (leachates) show consistent light-REE depleted REY patterns (marine carbonate normalized) and nearly constant normalized values for the middle and heavy REE without significant Ce, Eu or Y anomalies. Non-carbonate residues (composed of dark matrix material) are high in Al 2 O 3 and show variably light to middle REE enriched REY patterns similar to shales. Leachates of limestone from nearby country rocks have normalized REY patterns similar to marine carbonates. Only Ce and Y are slightly fractionated. Based on their trace element chemistry and normalized REY patterns, leachates of dolomite from the margin of the deposit closely resemble either limestone (Type A) or magnesite (Type B). With one exception, δ 18 O VPDB for both dolomite types is within the range of the respective magnesite values (δ 18 O VPDB c. − 14.2 to − 15.7‰). Leachates of dolomite from druses within the deposit (Type C) show low REY abundances with normalized patterns similar to those of magnesite. Sm–Nd isotope data for magnesite leachates yield a well-defined isochron age of 222.5 ± 9.8 Ma (MSWD 3.1). Isochron calculations for leachate(s) and residue(s) from individual magnesite samples (“internal isochrons”) yield ages between 236 ± 16 Ma and 193.5 ± 8.6 Ma with a low degree of scatter (MSWD 18 O VPDB c. − 15‰), 2) 37.9 ± 5.4 Ma (δ 18 O VPDB c. − 12.5‰), and 3) − 1.7 ± 6.8 Ma (δ 18 O VPDB c. − 11‰). Type C dolomite data indicate that druses were formed by open-system fluid flow throughout the deposit.

Garnet is a widespread mineral in crustal metamorphic rocks, a primary constituent of the mantle, a detrital mineral in clastic sediments, and an occasional guest in igneous rocks. Garnet occurs in ultramafic to felsic bulk-rock compositions, and its growth and stability span from

Garnet's potential as a chronometer of tectonometamorphic processes and conditions was first recognized over 30 years ago. The Sm–Nd and Lu–Hf systems have since emerged as the most effective chronometers, permitting age precision of better than ±1 My, even on tiny samples such as concentric growth zones within individual crystals. New, robust analytical methods mitigate the effects of ubiquitous mineral inclusions, improving the precision and accuracy of garnet dates. Important differences between Sm–Nd and Lu–Hf with respect to partitioning, diffusivity, contaminant phases, and isotopic analysis make these two systems powerfully complementary.

The release of volatiles from subducting lithologies is a crucial triggering process for arc magmatism, seismicity, the growth and maturation of continents, and the global geological water-CO 2 cycle. While models exist to predict slab volatile release from hydrous phases, it is challenging to reconstruct and test these fluid fluxes in nature. Here we show that the growth of garnet may be used as a proxy for devolatilization at blueschist to lower eclogite facies conditions in subduction zones. Using thermodynamic analysis including the effects of garnet fractionation and fluid removal, we show the proportional relationship between garnet and water production in two end-member crustal lithologies (pelitic sediment and hydrated mid-oceanic-ridge basalt [MORB]) in three representative subduction geotherms. Dehydrating minerals such as lawsonite, chlorite, amphibole, and epidote contribute to garnet growth, especially between ∼1.4 and 3.0 GPa where geophysical models and observations predict dehydration. The average production ratio for altered MORB compositions is 0.52 (wt% water as fluid per vol% garnet) in cooler geotherms (Honshu [Japan] and Nicaragua) and 0.27 in hotter geotherms (Cascadia [North America]), whereas for pelite the production ratios are about half (0.24 and 0.13, respectively). Garnet growth correlates with production of 3.3–5.9 wt% water in hydrated MORB and 1.8–3.1 wt% water in pelite, representing 42%–100% of the water lost between 0.5 and 6.5 GPa from a fully saturated starting material. Garnet abundance, its pressure-temperature growth span, and its growth chronology may be used to recognize, reconstruct, and test models for progressive subduction zone dehydration.

We report the discovery of granulite facies gneisses that attained ultrahigh temperatures (UHT) above those predicted by typical models of conductive thermal relaxation of over-thickened crust during exhumation. The rocks, which form part of the Acadian (Devonian) metamorphic belt in Connecticut (United States), reached ∼1000 °C and minimum pressures of ∼1 GPa based on Zr-in-rutile thermometry, ternary feldspar compositions, and pseudosection analysis. This is the first regional UHT locality in the United States of which we are aware, and one of relatively few post-Gondwana assembly UHT localities known worldwide. The UHT metamorphism requires heretofore unrecognized contributions to the regional thermal budget. Some possibilities include rapid exhumation from the mantle, underthrusting of extremely radiogenic crust, mechanical strain heating, asthenospheric upwelling, and/or heat input from mantle-derived magmas. The rocks are fairly ordinary looking in outcrop, raising the possibility that other UHT domains remain undiscovered in the orogen.

In this study, we report the first discovery of microdiamond inclusions in kyanite–garnet schists from the Central Rhodope Mountains in Bulgaria. These inclusions occur in garnets from metapelites that are part of a meta-igneous and meta-sedimentary mélange hosted by Variscan (Hercynian) orthogneiss. Ultra-high-pressure (UHP) conditions are further supported by the presence of exsolved needles of quartz and rutile in the garnet and by geothermobarometry estimates that suggest peak metamorphic temperatures of 750–800°C and pressures in excess of 4GPa. The discovery of UHP conditions in the Central Rhodopes of Bulgaria compliments the well-documented evidence for such conditions in the southernmost (Greek) part of the Rhodope Massif. Dating of garnets from these UHP metapelites (Chepelare Shear Zone) using Sm–Nd geochronology indicates a Late Cretaceous age (70.5–92.7Ma) for the UHP metamorphic event. This is significantly younger than previously reported ages and suggests that the UHP conditions are associated with the Late Mesozoic subduction of the Vardar Ocean northward beneath the Moesian platform (Europe). The present-day structure of the RM is the result of a series of subduction–exhumation events that span the Cenozoic, alongside subsequent post-orogenic extension and metamorphic core complex formation.

We present a method to reconstruct the dehydration flux associated with garnet-forming reactions during subduction. Garnet-bearing blueschists from the island of Sifnos, Greece, in the Attic–Cycladic Blueschist Belt are used as a test case to extract information on the timescales of dehydration during subduction. We use garnet growth as a proxy for the net dehydration reaction. Thermodynamic (pseudosection) analysis of a mafic blueschist in this unit (representative of an altered basalt protolith) indicates that garnet grew via net reaction(s) of the form chlorite + chloritoid + glaucophane + phengite = garnet + pyroxene + lawsonite + paragonite + quartz + H2O. Garnet core and rim chemistry indicates that growth began at 2.0 GPa and 460 °C and ended at 2.2 GPa and 560 °C. The stabilization of matrix lawsonite (promoted by the bulk chemical shift of the matrix due to fractionation of the garnet and its inclusions from the system) throughout garnet growth limits the amount of water liberated from the rock over this P–T span. The average stoichiometry of the garnet-forming reaction(s) indicates an average molar production ratio of garnet to water of ~ 1.0:0.7. Given the 11 vol.% garnet observed in the rock, this analysis indicates a loss of 0.3 to 0.4 wt.% H2O from the bulk rock during garnet growth. Zoned garnet geochronology from these rocks provides a constraint on the dehydration rate from this lithology during the time span of garnet growth. Two garnet grains, roughly 1.5 cm each in diameter, were microdrilled based on major element zoning contours. Three concentric growth zones were extracted from each garnet for Sm–Nd geochronology. Very low Nd concentrations in acid-cleansed garnet (0.03 to 0.09 ppm) yielded very small samples (~ 1 ng Nd) that were analyzed with TIMS using a NdO+ with Ta2O5 activator method. Untreated “garnet” powders rich in mineral inclusions from each garnet zone were also analyzed. The powders fall off of the garnet-matrix isochron, likely indicating age inheritance in the inclusions. Combining acid-cleansed garnet data from each garnet, multi-point garnet-matrix isochron ages of 46.50 ± 0.80 Ma for the core, 46.49 ± 0.53 Ma for the intermediate zone, and 46.46 ± 0.59 Ma for the rim were determined, indicating a brief growth duration of 0.04 Ma with an upper bound (2 SD) on growth duration of 1.0 Myr. This equates to a release of 0.3–0.4 wt.% of water due to this reaction in this lithology, and heating of 100 °C, in less than 1.0 Myr. The short time interval represents a focused, rapid pulse of dehydration and heating within the context of the overall subduction descent timescale of ~ 10–20 Myr.

The Fosdick migmatite-granite complex in West Antarctica records evidence for two high-temperature metamorphic events, the first during the Devonian-Carboniferous and the second during the Cretaceous. The conditions of each high-temperature metamorphic event, both of which involved melting and multiple melt-loss events, are investigated using phase equilibria modelling during successive melt-loss events, microstructural observations and mineral chemistry. In situ SHRIMP monazite and TIMS Sm-Nd garnet ages are integrated with these results to constrain the timing of the two events. In areas that preferentially preserve the Devonian-Carboniferous (M1) event, monazite grains in leucosomes and core domains of monazite inclusions in Cretaceous cordierite yield an age of c. 346 Ma, which is interpreted to record the timing of monazite growth during peak M1 metamorphism (820-870 � C, 7.5-11.5 kbar) and the formation of garnet-sillimanite-biotite-melt-bearing assem- blages. Slightly younger monazite spot ages between c. 331 and 314 Ma are identified from grains located in fractured garnet porphyroblasts, and from inclusions in plagioclase that surround relict garnet and in matrix biotite. These ages record the growth of monazite during garnet breakdown associated with cooling from peak M1 conditions. The Cretaceous (M2) overprint is recorded in compositionally homogeneous monazite grains and rim domains in zoned monazite grains. This monazite yields a protracted range of spot ages with a dominant population between c. 111 and 96 Ma. Rim domains of monazite inclusions in cordierite surrounding garnet and in coarse-grained poikiloblasts of cordierite yield a weighted mean age of c. 102 Ma, interpreted to constrain the age of cordierite growth. TIMS Sm-Nd ages for garnet are similar at 102-99 Ma. Mineral equilibria modelling of the residual protolith composition after Carboniferous melt loss and removal of inert M1 garnet constrains M2 conditions to 830-870 � C and 6-7.5 kbar. The modelling results suggest that there was growth and resorption of garnet during the M2 event, which would facilitate overprinting of M1 compositions during the M2 prograde metamorphism. Measured garnet compositions and Sm-Nd diffusion modelling of garnet in the migmatitic gneisses suggest resetting of major elements and the Sm-Nd system during the Cretaceous M1 overprint. The c. 102-99 Ma garnet Sm-Nd closure ages correspond to cooling below 700 � C during the rapid exhumation of the Fosdick migmatite-granite complex.

Zoned garnet porphyroblasts contain one of the longest continuous records of tectonic processes, with growth histories spanning many millions of years. Well known challenges exist in the physical preparation, purification, and geochronologic analysis of zoned garnets, which have limited the progress and application of this potentially powerful geochronologic tool. Here, we present details of an integrated methodology specifically for the acquisition of high-resolution Sm–Nd geochronologic records from zoned garnets that overcome these challenges. We develop and demonstrate our methods on large test garnets from a shear zone in the Tauern Window in western Austria. Chemically contoured microdrilling permits physical sampling of many discrete age annuli (each as narrow as ~100 μm) within a single garnet. Refined HNO 3 /HF/HClO 4 partial dissolution methods cleanse coarse crushed (75–150 μm size) garnet annuli of detrimental inclusions yielding high 147 Sm/ 144 Nd ratios (0.9 to 4.6) and consequently accurate and precise ages. Experiments on powdered garnet ( 147 Sm/ 144 Nd ( 143 Nd/ 144 Nd with 10–20 ppm (2 RSD) precision, good enough for 147 Sm/ 144 Nd > ~ 1.0. In total, this integrated sampling and analytical protocol permits the acquisition of 3–11 discrete high precision (

The diffusive behavior of argon in quartz was investigated with three analytical depth profiling methods: Rutherford Back-scattering Spectroscopy (RBS), 213 nm laser ablation, and 193 nm (Excimer) laser ablation on the same set of experimental samples. The integration of multiple depth profiling methods, each with different spatial resolution and sensitivity, allows for the cross-checking of methods where data ranges coincide. The use of multiple methods also allows for exploration of diffusive phenomena over multiple length-scales. Samples included both natural clear rock crystal quartz and synthetic citrine quartz. Laser analysis of clear quartz was compromised by poor coupling with the laser, whereas the citrine quartz was more easily analyzed (particularly with 193 nm laser). Diffusivity measured by both RBS and 193 nm laser ablation in the outermost 0.3 μm region of citrine quartz are self-consistent and in agreement with previously published RBS data on other quartz samples (including the clear quartz measured by RBS in this study). Apparent solubilities (extrapolated surface concentrations) for citrine quartz are in good agreement between RBS, 213 nm, and 193 nm laser analyses. Deeper penetration of argon measured up to 100 μm depth with the 213 nm laser reveal contributions of a second, faster diffusive pathway, effective in transporting much lower concentrations of argon into the crystal interiors of both clear and citrine quartz. By assuming such deep diffusion is dominated by fast pathways and approximating them as a network of planar features, the net diffusive uptake can be modeled and quantified with the Whipple-LeClaire equation, yielding delta D-b values of 1.32 x 10-14 to 9.1 x 10-17 cm3/s. While solubility values from the measured profiles confirm suggestions that quartz has a large capacity for argon uptake (making it a potentially important sink for argon in the crust), the slow rate of lattice diffusion may limit its capability to take up argon in shorter lived geologic environments and in experiments. In such shorter-lived systems, bulk argon diffusive uptake will be dominated by the fast pathway and the quartz lattice (including natural isolated defects that may also be storing argon) may never reach its equilibrium capacity.

Chemically zoned garnets provide a record of the changing conditions within the tectonically evolving crust where metamorphic mineral growth occurs. Recent advances in Sm–Nd geochronology of zoned garnets now permit the measurement of a higher resolution temporal record than previously attainable. Here, we present precise ( P – T changes. These data show that tectonometamorphic processes leading to garnet growth (such as exhumation, fluid flow, and/or heating) were pulse-like in nature, rather than slow and steady, during the waning stages of the Alpine orogeny 28–20 million years ago.

Whether quartzofeldspathic rocks transform to (U)HP minerals during subduction - and back to low-pressure minerals upon exhumation - remains one of the more profound questions pertaining to collisional orogenesis. Garnet-bearing quartzofelds- pathic gneisses from the Western Gneiss Region, Norway provide an opportunity to answer this question. High-precision Sm-Nd garnet geochronology of these gneisses documents garnet growth from 418 to 398 Ma. Garnet zoning in two samples implies growth during subduction-related increase in pressure from 0.5 GPa and 550 � C to 1.7 GPa and 700 � C. Zoning in all other garnets suggests growth over rather narrow P-T ranges from 1.0-1.6 GPa and 725-800 � C during decompression, possibly accompanying melting. If these samples constitute a representative suite, (i) the dearth of (U)HP garnets suggests that most of the quartzofeldspathic gneisses in the Western Gneiss Region did not transform to eclogite-facies parageneses during subduction; (ii) the abundance of garnets grown during decompression indicates that the major period of densification was during exhumation. The widespread metastability of quartzofeldspathic rocks during subduction is substantially different from the findings of previous work and suggests commensu- rately less subduction of continental crust before the slab is positively buoyant.

The study of the diffusion of the six noble gas elements, helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn) in minerals has spanned many decades with the primary geoscience focus on argon and helium (see below). The primary aim of this chapter is to acquaint the reader with the breadth of themes and perspectives that have accompanied the study of noble gas diffusion in minerals, and in addition, to attempt to bring the reader up to date with a focus on the most recent data and ideas from the literature of the past decade wherever possible. In this chapter, the available data for the diffusivity of each noble gas will be reviewed in turn, beginning with helium, then argon, followed by the rest of the gases. For specific minerals that have received considerably more attention than others, individual sections of this paper will be devoted to them. The data tables which accompany this chapter list the diffusion data cited herein; these tables are also available as an online supplement and from the author in excel format upon request. In general, every effort was made to compile all modern data where diffusivities have been quantified (i.e., where robust measurements of both activation energy and pre-exponential factor have been derived), though some early studies which have been superseded by more recent work will not be discussed. To begin, the first section describes the interpretive challenges associated with the most common method of observation for evaluation of noble gas diffusion in minerals: bulk degassing experiments. Then, following the review of the noble gas diffusion data, the remaining sections of the chapter focus on several broad themes that arise from the data that may span several noble gases or many minerals. The applicability of noble gases and noble gas …

Technological and scientific developments have demonstrated both the attainability and the utility of very high precision (i.e.

Recent work in Barrovian metamorphic terranes has found that rocks experience peak metamorphic temperatures across several grades at similar times. This result is inconsistent with most geodynamic models of crustal over-thickening and conductive heating, wherein rocks which reach different metamorphic grades generally reach peak temperatures at different times. Instead, the presence of additional sources of heat and ⁄ or focusing mechanisms for heat transport, such as magmatic intrusions and ⁄ or advection by metamorphic fluids, may have contributed to the contemporaneous development of several different metamorphic zones. Here, we test the hypothesis of temporally focussed heating for the Wepawaug Schist, a Barrovian terrane in Connecticut, USA, using Sm-Nd ages of prograde garnet growth and U-Pb zircon crystallization ages of associated igneous rocks. Peak temperature in the biotite-garnet zone was dated (via Sm-Nd on garnet) at 378.9 ± 1.6 Ma (2r), whereas peak temperature in the highest grade staurolite-kyanite zone was dated (via Sm-Nd on garnet rims) at 379.9 ± 6.8 Ma (2r). These garnet ages suggest that peak metamorphism was pene-contemporaneous (within error) across these metamorphic grades. Ion microprobe U-Pb ages for zircon from igneous rocks hosted by the metapelites also indicate a period of syn-metamorphic peak igneous activity at 380.6 ± 4.7 Ma (2r), indistinguishable from the peak ages recorded by garnet. A 388.6 ± 2.1 Ma (2r) garnet core age from the staurolite-kyanite zone indicates an earlier episode of growth (coincident with ages from texturally early zircon and a previously published monazite age) along the prograde regional metamorphic T-t path. The timing of peak metamorphism and igneous activity, as well as the occurrence of extensive syn-metamorphic quartz vein systems and pegmatites, best supports the hypothesis that advective heating driven by magmas and fluids focussed major mineral growth into two distinct episodes: the first at c. 389 Ma, and the second, corresponding to the regionally synchronous peak metamorphism, at c. 380 Ma.

We use the records of Sr diffusion preserved in apatite and corroborating multicomponent Fe-Mg–Ca–Mn diffusion preserved in garnet from the classic Barrovian metamorphic zones (Scotland) to quantify the timescale for the thermal peak of crustal heating c. 465 Ma. We show that Sr diffusion in apatite is a powerful means to determine thermal timescales, and provide the first set of diffusion-basedtimescaleestimatesacrossawiderangeofmetamorphicgradesforBarrovianmetamorphism.Theresultsdemonstrate that the thermal peak was extremely brief and lasted a few hundred thousand years. This timescale is one to two orders of magnitude shorter than peak timescales predicted by conventional models based on conductive relaxation of overthickened crust. The short peak thermal pulse or pulses probably involved advective heat transfer driven by magmas and associated fluid flow, followed by rapid exhumation.Peakthermalpulsesmayrepresentaveryshortpart(b1Ma)ofoverallmountainbuildingcycleslasting ∼10 7 yrormore, but play a dominant role in determining mineralogy, geochemical fluxes, and fluid production in mountain belts. © 2007 Elsevier B.V. All rights reserved.

An experimental procedure has been developed that permits measurement of the partitioning of Ar and He between crystal interiors and the intergranular medium (ITM) that surrounds them in synthetic melt-free polycrystalline diopside aggregates. ^(37)Ar and ^(4)He are introduced into the samples via neutron irradiation. As samples are crystallized under sub-solidus conditions from a pure diopside glass in a piston cylinder apparatus, noble gases diffusively equilibrate between the evolving crystal and intergranular reservoirs. After equilibration, ITM Ar and He is distinguished from that incorporated within the crystals by means of step heating analysis. An apparent equilibrium state (i.e., constant partitioning) is reached after about 20 h in the 1450 °C experiments. Data for longer durations show a systematic trend of decreasing ITM Ar (and He) with decreasing grain boundary (GB) interfacial area as would be predicted for partitioning controlled by the network of planar grain boundaries (as opposed to ITM gases distributed in discrete micro-bubbles or melt). These data yield values of GB-area-normalized partitioning, K¯^(Ar)_(ITM), with units of (Ar/m^3 of solid)/(Ar/m^2 of GB) of 6.8 x 10^3 – 2.4 x 104 m^(-1). Combined petrographic microscope, SEM, and limited TEM observation showed no evidence that a residual glass phase or grain boundary micro-bubbles dominated the ITM, though they may represent minor components. If a nominal GB thickness (δ) is assumed, and if the density of crystals and the grain boundaries are assumed equal, then a true grain boundary partition coefficient (K^(Ar)_(GB) = X^(Ar)_(crystals)/X^(Ar)_(GB) may be determined. For reasonable values of δ, K^(Ar)_(GB) is at least an order of magnitude lower than the Ar partition coefficient between diopside and melt. Helium partitioning data provide a less robust constraint with K¯^(He)_(ITM) between 4 x 10^3 and 4 x 10^4 cm^(-1), similar to the Ar partitioning data. These data suggest that an ITM consisting of nominally melt free, bubble free, tight grain boundaries can constitute a significant but not infinite reservoir, and therefore bulk transport pathway, for noble gases in fine grained portions of the crust and mantle where aqueous or melt fluids are non-wetting and of very low abundance (i.e.

Recent years have seen a rapid expansion in the acquisition and use of information on diffusive transport in phases relevant to the solid Earth (crystals, melts and fluids). Although far from complete, the data base on diffusion coefficients is now sufficiently large that broad constraints can be placed upon the length- and time scales of many natural transport phenomena in which diffusion plays a role. Conversely, observations of diffusion progress in specific natural samples can be used to extract time–temperature information for a variety of geologic and geochemical processes, ranging from sediment burial and crustal erosion to fluidmediated reactions and biosignature retention. Despite this undeniable progress, several major challenges remain that largely define the frontiers of research in solid-Earth diffusion. Perhaps foremost among these is the need to address and understand the multi-scale, multi-path aspects of diffusion in many systems—a complication that is not limited to polyphase materials (individual mineral grains can exhibit clear indications of multi-path behavior even when visible evidence of such paths is lacking). Many other diffusion frontiers are linked in one way or another to this multi-scale issue; they include: diffusion of molecular H2O and the effect of H species on diffusion in minerals and rocks; diffusive fractionation of multiple isotopes of a single element; diffusion at the extreme conditions of the deep Earth; reconciliation of observations from natural samples and laboratory studies; and development of theoretical approaches to ‘predict’

Available constraints on metamorphic reaction rates derived from the study of natural systems are similar to, or slightly lower than, the bulk strain rates measured in the same rocks. Here, we explore whether this apparent relationship is merely coincidence or due to a more fundamental mechanistic link between reaction and strain. Grain boundary migration accommodated dislocation creep (GBMDC) or grain boundary diffusion creep (GBDC) (i.e. pressure solution), both of which involve dissolution-precipitation as we define it, will occur simultaneously with mineral reactions involving dissolution-precipitation in the presence of a non-zero deviatoric stress. The exact relationships between reaction and strain are different depending on whether GBMDC or GBDC is controlling strain, but the mechanistic link exists in both cases. We present theoretical arguments which show that bulk strain by GBMDC or GBDC, which may additionally be accommodated by processes not involving dissolution-precipitation, such as dislocation glide and climb or grain boundary sliding, should in most cases be somewhat faster than the bulk reaction rates as observed. With few exceptions, for natural metamorphic systems undergoing plastic deformation, strain rates provide an upper limit for bulk reaction rates occurring simultaneously in the same rocks. The data suggest that mineral reaction rates may typically be within one order of magnitude of the strain rate.

A quantitative physical model is presented which includes the factors that control the presence, or absence, of internally derived excess 40Ar or excess 4He in geological systems. In particular, the model incorporates the transport and partitioning properties of the rock surrounding the mineral of thermochronologic interest and illuminates the related effects on the amount of excess 40Ar or 4He preserved in the system. Modeling of a simplified 1-D rock column bounded by an external sink for 40Ar or 4He shows that a steady-state excess 40Ar or 4He profile develops, the magnitude of which is determined by a system parameter called the ‘transmissive timescale’, τT. The characteristic time required to reach this steady state depends upon τT and the ‘total local sink capacity’, TLSC, wherein the important role of local matrix mineral and fluid phases is incorporated. Together, these two system parameters (τT and TLSC) determine the evolution of excess 40Ar or 4He buildup within a system above the closure temperatures of all minerals involved. An analytical expression for the 1-D system describing the evolution of excess 40Ar (or by analogy 4He) in a particular potassium-bearing (or U–Th-bearing) mineral located at a distance, L, from an external sink has been derived empirically from model results: 40 Ar age - equivalent (L,t)≅ τ T Ar 2 1− exp 5 2 · t τ T Ar (1+ TLSC Ar ) Local matrix minerals, perhaps most notably quartz, may act as important sinks for 40Ar (except in the most fluid-rich systems where fluids dominate) and thus are fundamental in controlling, and limiting, thermochronologically problematic excess 40Ar in neighboring potassium-bearing minerals. In general, the model provides a rigorous means of predicting excess noble gas content, residence, release, and transport within the compositionally variable and thermally evolving crust.