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Tectonic models for the Mesozoic evolution of accreted island arcs and subduction complexes in the Blue Mountain Province (BMP) of eastern Oregon and western Idaho vary widely. The BMP is situated between coeval accreted terranes of southern British Columbia and the western U.S., and is critical for interpreting Mesozoic paleogeography of the U.S. Cordillera. In this study, we interpret the Triassic–Jurassic evolution of the BMP using geochemistry as a tool to evaluate the provenance of mudrocks in fine-grained turbidites. Temporal and spatial variations in mudrock geochemistry indicate that during Late Triassic to Early Jurassic time the Wallowa terrane was an intra-oceanic island arc and the Olds Ferry terrane was a pericratonic (continental-fringing) island arc. Subsequently, a regionally extensive Middle to Late Jurassic marine basin received input from continental sources and sediment may also have been derived from coeval terranes to the south. Comparison of provenance data from the BMP with similar data from accreted terranes to the north and south suggests that Triassic–Jurassic basins of the western U.S. share a long-term provenance history within a complex system of continent-fringing and intra-oceanic island arcs. In contrast, it appears that the Quesnel terrane of southern British Columbia had a distinct provenance with a more direct link to Laurentia during Triassic time. Early to Middle Jurassic sedimentary rocks from southern British Columbia and the western U.S. show a high degree of geochemical provenance similarity. This may indicate reorganization of formerly discrete tectonic regions into a nascent, integrated western Laurentian margin prior to complete development of a Cretaceous Andean-type margin. The results of this study demonstrate the utility of mudrock geochemistry in tectonic analyses and show that major- and trace-element geochemistry of mudrocks is useful for developing a comprehensive understanding of regional tectonic evolution.

Ultra-high-pressure eclogites from the Dabie orogen that formed over a range in temperatures (∼600 to > 700 °C) have been investigated with combined Lu–Hf and Sm–Nd geochronology. Three eclogites, sampled from Zhujiachong, Huangzhen and Shima, yield Lu–Hf ages of 240.0 ± 5.0, 224.4 ± 1.9 and 230.8 ± 5.0 Ma and corresponding Sm–Nd ages of 222.5 ± 5.0, 217.6 ± 6.1 and 224.2 ± 2.1 Ma respectively. Well-preserved prograde major- and trace-element zoning in garnet in the Zhujiachong eclogite suggests that the Lu–Hf age mostly reflects an early phase of garnet growth that continued over a time interval of c. 17.5 Myr. For the Huangzhen eclogite, despite preserved elemental growth zoning in garnet, textural study reveals that the Lu–Hf age is biased towards a later garnet growth episode rather than representing early growth. The narrow time interval of

The Lutetium–Hafnium radiogenic isotopic system is widely used as a chronometer and tracer of planetary evolution. In order for this isotopic system to fulfill its potential in planetary studies, the Lu–Hf system parameters need to be more tightly constrained, in particular the Lu–Hf isotopic composition of the chondritic uniform reservoir (CHUR) and, by extension, the bulk silicate Earth (BSE). We present new Lu–Hf and Sm–Nd isotopic compositions of unequilibrated carbonaceous, ordinary, and enstatite chondrites of petrologic types 1, 2, and 3 which define a narrow range of Lu/Hf ratios (3%) identical with that of Sm/Nd. This contrasts with previously published data from mostly equilibrated ordinary chondrites of petrologic types 4, 5, and 6 which have a much larger range in Lu/Hf (28%). This heterogeneity has hampered an unambiguous choice for the Lu–Hf isotopic composition of CHUR. Our new determinations of Lu–Hf CHUR parameters are 176 Lu/ 177 Hf = 0.0336 ± 1 and 176 Hf/ 177 Hf = 0.282785 ± 11 (2 σ m ), which are higher than previous estimates and, together with average Sm–Nd chondrite compositions of unequilibrated chondrites of 147 Sm/ 144 Nd = 0.1960 ± 4 and 143 Nd/ 144 Nd = 0.512630 ± 11 (2 σ m ), now provide firm constraints on the chondritic parameters for both Lu–Hf and Sm–Nd isotopic systems. A comparison of Lu–Hf and Sm–Nd data show that terrestrial planets, as well as early differentiated planetesimals, converge toward a common initial Hf and Nd isotope composition corresponding to the average of chondrites. Finally, a compilation of Lu–Hf isotopic data of unequilibrated and equilibrated chondrites demonstrates that the 176 Lu decay decay-constant value cannot be resolved by age comparison on metamorphosed or shocked planetary materials which have a complex history.

Continent subduction beneath continental upper plates has occurred at a broad range of rates that correlate with the size of the subducted fragment: small slivers subduct and exhume fast and cold while larger fragments undergo much slower and hotter orogenesis (Kylander-Clark et al., 2012). The few extant examples of continental fragments subducted beneath oceanic crust record cold geothermal gradients (

Two perpendicular ultramafic lamprophyre dikes on Little Presque Isle near Marquette, Michigan were studied to compare their intrusive magmatic fabric using anisotropy of magnetic susceptibility (AMS) with the strains preserved by mechanical twins in dike magmatic calcite (δ18O = −21.93, δ13C = −4.53) and dike-margin hydrothermal calcite (δ18O = −9.72, δ13C = −3.76) veins. Phlogopite from the lamprophyres yield an 40Ar–39Ar plateau age of 1877 Ma, and the lamprophyres intrude metadiabase dikes and the Compeau Creek gneiss (2691 ± 25 Ma; LA-ICPMS U–Pb on zircon), all of which is cross-cut by Keweenaw basaltic dikes and unconformably overlain by the undeformed Jacobsville Sandstone (Meso- to Neoproterozoic). The lamprophyre dikes strike east–west and north–south in this small (3 km2), complex outcrop. Analysis of mechanically twinned calcite in the E–W dike (n = 89 grains, 26.97% NEVs) reveals a sub-horizontal dike-acute shortening strain axis (N38°E, 15°). Strain analysis of the N–S dike (n = 107 grains, 22.43% NEVs) reveals a sub-horizontal and dike-parallel shortening strain axis (N17°E, 14°). Calcite veins from the margins of both lamprophyre dikes reveal a NE–SW sub-horizontal shortening, and both the lamprophyres and vein calcite preserve a vertical shortening strain overprint. Differential stress magnitudes for twinning the lamprophyre calcite and adjacent veins was −35 MPa. AMS analyses, used as a proxy for the primary magmatic flow direction, reveal a sub-horizontal and dike-parallel Kmax, with a dike-normal Kmin for the E–W dike. The N–S dike preserves a vertical Kmax and a dike-normal Kmin. Hysteresis studies on both dikes show that this anisotropy is carried by multi-domain ferromagnetic minerals. These lamprophyres preserve a complex, local intrusive fabric (AMS) that was unaltered by Penokean sub-horizontal shortening (∼SW–NE), or the younger, sub-vertical shortening strain overprint ∼200 km inboard of the unrestored Penokean margin. The twinning strain results are consistent with other Penokean finite strains but not with the strain fabrics of younger orogens (e.g., Yavapai, Mazatzal, Keweenaw-Grenville).

Rocks may undergo complex pressure–temperature (P–T) histories during orogenesis in response to alternating episodes of synconvergent burial and exhumation. In this study, chemical zoning in garnets combined with textural and chemical evidence from the schist of Willow Creek in the Albion Mountains of south-central Idaho (USA), reveals a complex P–T path during the early stages of Sevier orogenesis. The distribution of quartz inclusions combined with internal resorption features establishes a hiatus in garnet growth. Chemical zoning was simulated using a G-minimization approach to yield a P–T path consisting of three distinct pressure changes during increasing temperature, defining an “N” shape. Lu–Hf isochron ages from multiple garnet fractions and whole-rock analyses in two samples are 132.1 ± 2.4 and 138.7 ± 3.5 Ma. The samples were collected from the hanging wall of the Basin-Elba thrust fault and yielded results similar to those previously obtained from the footwall. This leads to several conclusions: (1) Both the hanging wall and footwall experienced the same metamorphic event, (2) the paths document a previously unrecognized crustal thickening and synorogenic extension cycle that fills an important time gap in the shortening history of the Sevier retroarc, suggesting progressive eastward growth of the orogen rather than a two-stage history, and (3) episodes of extensional exhumation during protracted convergent orogenesis are increasingly well recognized and highlight the dynamic behavior of orogenic belts.

Granitic rocks are commonly used as a means to study chemical evolution of continental crust. In particular, their isotopic compositions reflect the relative contributions of mantle and crustal sources in their genesis. In Laurentia, a distinctive belt of Mesoproterozoic A-type or “anorogenic” granites of ∼1.4 Ga age was emplaced within composite, heterogeneous Proterozoic crust. Zircons are an ideal mineral to constrain the granite petrogenetic history because they are repositories of both age (U–Pb geochronology) and tracer (Lu–Hf isotopic) information. We measured the Hf isotope composition of zircons from 31 previously dated A-type granites intruding Proterozoic basement provinces from the southwest U.S. to the upper mid-continent. Isotopic compositions for all granites are broadly similar, with average 176 Hf/ 177 Hf(i) ratios of 0.281871–0.282153. Averages for granites within different crustal provinces yield present-day eHf values between −31.9 and −21.9. Initial eHf values discriminate the granites by age of the 2.0–1.6 Ga crust which they intrude, but are independent of intrusion age, as follows (basement formation ages in parentheses): southern Granite–Rhyolite (1.5–1.3 Ga), +7.0±0.9; central Yavapai (1.8–1.7 Ga), +5.4±0.9; western Yavapai (1.8–1.7 Ga), +3.3± 1.1; Granite–Rhyolite (1.5–1.3 Ga), +1.4±0.6; Mojave (1.8–1.7 Ga), +0.2±0.8; and Penokean (1.9–1.8 Ga), −0.1±n/d. The narrow ranges of Hf isotopic signatures within these regional groupings of granites reflect the age and isotopic composition of the basement provinces they intrude. Granites in the southern Granite–Rhyolite and central Yavapai provinces have the highest initial eHf, reflecting their more juvenile sources, whereas Mojave and Penokean granites show contributions from more evolved crustal sources. Simple calculations indicate that all the granites represent dominantly crustal melts; although a mantle contribution cannot be ruled out, if present it must be minor. The Hf isotope compositions of the 1.4 Ga granites appear controlled predominantly by widespread melting of heterogeneous 2.0–1.6 Ga lower crust, consistent with other geochemical indicators. In addition to constraining granite petrogenesis, the distinct age and Hf isotope signature of the zircons will prove useful as a provenance tracer in detrital zircon suites from Neoproterozoic and younger siliciclastic deposits worldwide.

We need to understand chemical recycling at convergent margins and how chemical interactions between subducted slab and the overlying mantle wedge affect mantle evolution and magmagenesis. This requires distinguishing contributions from recycled individual subducted components as well as those contributed by the mantle. We do this by examining magmatic products generated at different depths above a subduction zone, in an intra-oceanic arc setting. The Guguan cross-chain in the intra-oceanic Mariana arc overlies subducted Jurassic Pacific plate lithosphere at depths of ~125--230 km and erupts mostly basalt. Basalts from rear-arc volcanoes are more primitive than those from the magmatic front, in spite of being derived by lower degrees of melting of less-depleted mantle. Rear-arc magmas also show higher temperatures and pressures of equilibration. Coexisting mineral compositions become more MORB- or OIB-like with increasing height above the subduction zone. Trace element and isotopic variations indicate that the subduction component in cross-chain lavas diminishes with increasing depth to the subduction zone, except for water contents. There is little support for the idea that melting beneath the Mariana Trough back-arc basin depleted the source region of arc magmas, but melting to form rear-arc volcanoes may have depleted the source of magmatic front volcanoes. Enrichments in rear-arc lavas were not caused by sediment melting; the data instead favor an OIB-type mantle that has been modestly affected by subduction zone fluids. Our most important conclusion is that sediment fluids or melts are not responsible for the K--h relationship and other cross-chain chemical and isotopic variations. We speculate that an increasing role for supercritical fluids released from serpentinites interacting with modestly enriched mantle might be responsible for cross-chain geochemical and isotopic variations.

The Palaeoproterozoic (1.90 - 1.60 Ga) crust of central Fennoscandia was intruded repeatedly by dolerite dikes and sills during the Neo- and Mesoproterozoic eons. We report 17 new baddeleyite U - Pb dates comprising six generations of dolerites ( in Ma): Blekinge-Dalarna dolerites 946 - 978 Protogine Zone dolerites 1,215 - 1,221 Central Scandinavian Dolerite Group 1,264 - 1,271 Tuna dikes and age equivalents in Dalarna 1,461 - 1,462 Varmland dolerites similar to 1,568 Breven-Hallefors dolerites similar to 1,595 The favoured tectonic model implies that the majority of these suites were related to active margin processes somewhere west ( and possibly south) of the Fennoscandian Shield. Dolerite intrusions are interpreted to reflect discrete events of back-arc extension as the arc retreated oceanward. Initial Hf and Nd isotope compositions of the dolerite swarms fall between CHUR and normal-depleted mantle, and suggest a variably depleted and re-enriched mantle as the source for the here investigated 1.6 to 0.95 Ga old mafic rocks. Repeated recycling of older crustal components, mainly sediments ( dominated by material with short residence ages) in earlier subduction systems may have been very efficient at producing geochemically and isotopically variably enriched lithospheric mantle sections beneath the Fennoscandian Shield.

Sm–Nd and Lu–Hf isotopic data are presented for 19 chondritic meteorites: six carbonaceous chondrites, five L-chondrites, seven H-chondrites, and a single enstatite chondrite. The primary goal of the study is to better define the Bulk Silicate Earth (BSE) reference values for Hf isotopes. Except for one sample with lower Sm/Nd, the Sm–Nd data define a cluster around the accepted reference values for chondrites and terrestrial planets, giving a mean 147Sm/144Nd of 0.1960±0.0005, and a mean 143Nd/144Nd of 0.512631±0.000010 (uncertainties are two standard errors). It seems appropriate to retain the presently accepted Sm–Nd reference parameters, 147Sm/144Nd=0.1966 and 143Nd/144Nd=0.512638 (when fractionation-corrected to 146Nd/144Nd=0.7219).Lu–Hf isotopic data are not clustered, but spread along an approximate 4.5-Ga isochron trend, with a range of 176Lu/177Hf from 0.0301 to 0.0354. The data are similar to many of the samples of chondrites presented by Bizzarro et al. [Nature 421 (2003) 931], but lack the range to lower Lu/Hf shown by those authors. Our chondrite data define a regression line of 4.44±0.34 Ga when 1.867×10−11 year−1 is used for the decay constant of 176Lu [Science 293 (2001) 683; Earth Planet. Sci. Lett. 219 (2004) 311–324]. Combining our data with the main population of analyses from Bizzarro et al. [Nature 421 (2003) 931] yields 4.51±0.24 Ga. Unless samples of eucrite meteorites and deviating replicates of chondrites with 176Lu/177Hf less than 0.030 are employed, no combination of the main population of chondrite Lu–Hf data yields a regression with sufficiently low error to constrain the decay constant of 176Lu. Sample heterogeneity seems to hinder the acquisition of reproducible Lu–Hf analyses from small, manually ground pieces of chondrites, and we suggest that analysis of powders prepared from large volumes of meteorite will be needed to adequately characterize the Lu–Hf isotope systematics of chondritic reservoirs and of BSE. Our results for carbonaceous chondrites show higher average 176Lu/177Hf and 176Hf/177Hf than ordinary chondrites, and the mean of carbonaceous chondrites also coincides with replicate analyses of a powder representing a large volume of meteorite, the Allende powder from the Smithsonian Institution. Use of the carbonaceous chondrite mean for BSE Lu–Hf characteristics results in a BSE Hf–Nd point that lies well within the array of terrestrial compositions, and leads to plausible initial eHf values for Precambrian rocks. An improved objective resolution of meteorite data and of meteoritic models for the Earth needs to occur before BSE can be established for Lu–Hf.

The pre-Cenozoic configuration of western New Zealand allows determination of the effects of magmatism and a changing lower crustal rheology on the evolution of a Cretaceous orogen from upper to lower crustal levels (10–50 km). Beginning at ∼126 Ma, a composite batholith dominated by diorite was emplaced into the lower crust. During emplacement, deformation was partitioned into zones weakened by magma and heat, leading to the development of two layer-parallel shear zones at the upper and lower contacts of the batholith. Transient vertical decoupling of the crust above and below the batholith occurred from ∼126 Ma until ∼120 Ma as magma was emplaced into and moved through a weak, thick lower crust. By ∼116 Ma, however, much of the batholith had crystallized and the lowermost crust had cooled from 750 °C

At present, there is large uncertainty in the decay constant of 176Lu needed for Lu–Hf isotopic studies. We have determined λ176Lu by cross-calibration of U–Pb and Lu–Hf isotopic systems on mineral fractions from the Proterozoic Karlshamn and Sorkka dolerites in Sweden and Finland. The dolerites crystallized at shallow depths from homogeneous, high-temperature magmas, carry olivine that is nearly 100% unaltered, and show no signs of post-magmatic isotopic disturbance. The Lu and Hf isotopic compositions of plagioclase, olivine, pyroxene, apatite, ilmenite and baddeleyite were determined by multicollector-inductively coupled plasma mass spectrometry (MC-ICPMS). Calibrating the Lu–Hf results against baddeleyite U–Pb dates of 954.1±1.2 and 1256.2±1.4 Ma for the dolerites yields a mean λ176Lu of 1.867±0.008×10−11 year−1. The pristine character of the rocks and the agreement of the λ176Lu values with those from other terrestrial data sets [E. Scherer et al., Science 293 (2001) 683–687] suggest that the true value of λ176Lu lies between 1.86 and 1.87×10−11 year−1. Calibration experiments on extraterrestrial samples give significantly higher (4–6%) values, a discrepancy that may be due to plotting of non-cogenetic samples on the same Lu–Hf isochron diagram, or may have other, as yet undetermined, causes. The result of this study also indicates that the Lu–Hf method is capable for dating the crystallization of mafic rocks. The high 176Lu/177Hf ratio in apatite suggests that intrusive ages can be determined at a precision of a few million years or better.

New U–Pb dates, combined with Nd and Hf isotopic data, from rocks within the Port Morvan area of the Baie de St Brieuc region of Brittany identify a unique portion of the Neoproterozoic Cadomia terrane. Two gneisses near Port Morvan yielded U–Pb dates of 754.6±0.8 Ma and 746.0±0.9 Ma, ages that are more than 130 Myr older than the oldest units formed during the main phase of early Cadomian magmatism. Two trondhjemite boulders from the monogenetic facies of the Cesson conglomerate yielded identical ages of 665.2±0.5 Ma and 665.5±0.7 Ma, and a cobble from the polygenetic facies yields a 207Pb–206Pb date of 637±2 Ma. Individual detrital zircons from a sandstone associated with the Cesson conglomerates yield concordant U–Pb dates ranging from 650±3 Ma to 624.1±0.6 Ma. Initial ϵNd values for the rocks in this region range from +5.0 to +6.6, indicative of a substantial input from depleted mantle. Initial ϵHf values determined on zircons from these Neoproterozoic rocks, including the detrital zircons, range from +6.7 to +14.5, consistent with the Nd isotopic results. Maximum initial ϵHf values for two 2 Ga Icartian gneisses, considered basement to Cadomia, average +8.4 and +8.7. In contrast to the results of the Port Morvan rocks, 616–608 Ma syn-tectonic intrusions from Normandy and the British Channel Islands all have negative initial ϵNd values (−10.4 to −8.3) consistent with significant contamination by ancient crust such as the 2 Ga gneisses. The oldest arc-related magmas should have interacted most extensively with Cadomian basement, buffering younger mantle-derived magmas that were generated in subsequent magmatic episodes. The rocks within the Port Morvan region are thus inconsistent as examples of the earliest Cadomian intrusions as they show no evidence of interaction with 2 Ga basement. Instead, the older ages and mantle-like isotopic composition of these rocks suggest they are part of an independent terrane that formed prior to, and independently from, the Cadomian arc. Possible terrane-scale structural boundaries have recently been identified, including the newly recognized Port Morvan thrust fault and the NW-dipping Main Cadomian thrust.

We report Hf isotopic data for over 50 well studied lower crustal samples from three Proterozoic and Phanerozoic regions in southwest Europe, eastern Australia and southern Mexico. We use these data to characterize the Lu–Hf isotopic composition of the lower crust and, in combination with existing Sm–Nd data, to constrain coupled Hf–Nd isotopic behavior and evolution within this reservoir. Although most of these samples have present-day parent/daughter (p/d) ratios consistent with Hf–Nd evolution within the terrestrial Hf–Nd array, some samples have divergent p/d ratios that would evolve out of the terrestrial array in 1 Ga or less. The present-day 176Hf/177Hf and 143Nd/144Nd isotopic compositions of all samples, with one lone exception, plot within the terrestrial array. This indicates that (1) some present-day p/d ratios may be a relatively recently acquired characteristic through magmatic or metamorphic processes not related to the time-integrated Sm/Nd and Lu/Hf ratios of their sources, and/or (2) the Lu/Hf and Sm/Nd p/d variations exist on a small hand-size scale but not necessarily on a larger scale. The lower crust, from this initial data set, is broadly similar to the upper crust in terms of both its present-day p/d values and time-integrated Lu–Hf and Sm–Nd evolution. As a result, the lower crust appears to have a Hf and Nd isotopic composition similar to that of all other crust and mantle reservoirs so far characterized.

We report new Hf (and Nd) data for more than 100 sedimentary samples, recent to Archean in age, from a wide range of depositional environments. These data document the behavior of Lu–Hf and Sm–Nd isotopic systems in the global sedimentary system. In conjunction with existing data for mantle-derived rocks, we now have reasonable constraints on coupled Hf–Nd isotopic behavior in the crust and mantle. Lu/Hf and Hf isotopic compositions are strongly fractionated between muds and sands in passive margin sediments due to concentration of low Lu/Hf, low 176Hf/177Hf, Hf-rich zircons in mature sands. In active margin settings, Lu–Hf fractionation due to the `zircon effect' is minor due to the less weathered and more juvenile character of the sediments. Nd isotopic compositions are not highly fractionated by sedimentary sorting because heavy minerals, also rich in REEs, do not fractionate Sm–Nd efficiently. The lack of a large and systematic fractionation at active margins means that no significant Hf–Nd decoupling occurs here. This is important because sediments at active margins are the most likely to be recycled to the mantle. Hf–Nd isotopic data for all terrestrial samples fall along a single coherent trend (eHf=1.36eNd+2.95) which we call the `terrestrial array'. This array is composed of two complementary components: a mantle array (eHf=1.33eNd+3.19, defined by all oceanic basalts; and a crustal array (eHf=1.34eNd+2.82), defined by sediments, continental basalts, granitoids, and juvenile crustal rocks. The similarity of the crustal and mantle arrays indicates that no large-scale Hf–Nd decoupling occurs between the crust and mantle. The coherency of the terrestrial Hf–Nd array implies mixing within the mantle, due to stirring processes, and also within the crust, due to homogenization by collective sedimentary processes. In addition, tight Hf–Nd covariation may also imply that efficient crust to mantle recycling has modulated isotopic correlation in the silicate Earth. All Hf–Nd arrays, including the terrestrial array, lie significantly above (2–3 eHf units) the BSE (bulk silicate Earth) reference. This would appear to require a hidden reservoir in the Earth, heretofore unsampled, to balance the Hf–Nd isotopic composition of the terrestrial array. However, the discrepancy between the terrestrial array and BSE may simply be due to differences in the way the CHUR (chondritic uniform reference) values were determined for the Lu–Hf and Sm–Nd isotope systems.

The covariant behavior of Lu-Hf and Sm-Nd isotopes during most magmatic processes has long been recognized, but the details of this behavior in the depleted mantle reservoir have not been adequately examined. We report new whole-rock Hf and Nd isotope data for 1) juvenile, mantle-derived rocks, mid-Archean to Mesozoic in age, and 2) early Archean gneisses from West Greenland. Hf and Nd isotopic compositions of the juvenile rocks are well correlated, with the best fit corresponding to the equation e Hf = 1.40 e Nd + 2.1, and is similar to the collective Hf-Nd correlation for terrestrial samples of e Hf = 1.36 e Nd + 3.0. The early Archean Greenland gneisses, in contrast, have an extreme range in e Nd values ( − 4.4 to + 4.2; Bennett et al., 1993 ) that is not mirrored by the Hf isotopic system. The e Hf values for these rocks are consistently positive and have much less variation (0 to + 3.4) than their e Nd counterparts. The information from the Hf isotopic compositions of the West Greenland gneisses portrays an early Archean mantle that is relatively isotopically homogeneous at 3.8 to 3.6 Ga and moderately depleted in incompatible elements. There is no evidence that any of these gneisses have been derived from an enriched reservoir. The Hf isotopic data are in stark contrast to the Nd isotopic record and strongly imply that the picture of extreme initial isotopic heterogeneity indicated by Nd isotopes is not a real feature of the West Greenland gneisses but is rather an artifact produced by disturbances in the Sm-Nd isotope system of these rocks. Although Hf and Nd isotopic data do not uniquely constrain either the nature of the earliest crust or the timing of crustal growth, the most probable candidate for the enriched reservoir complementary to the depleted mantle in the pre-4.0 Ga Earth is a mafic, oceanic-type crust. In order to explain the predominantly positive e Hf and e Nd values for the early Archean rocks, this crust must have had a short residence time at the surface of the Earth before returning to the mantle where it was isolated from mixing with the depleted mantle for several hundred million years. The following period from 3.5 to 2.7 Ga may mark a transition during which this early formed mafic crust was mixed progressively back into the depleted mantle reservoir. While a present-day volume of continental crust at 4.0 Ga cannot be excluded on isotopic grounds, we find such a scenario unlikely based on the lack of direct isotopic and physical evidence for its existence. An important aspect of crustal growth and evolution, therefore, may be the transformation of the enriched reservoir from being predominantly mafic in the early Earth to becoming progressively more sialic through time.

The 176Hf/177Hf ratio was measured on 34 ferromanganese nodules, mostly from the Atlantic ocean. The different ocean basins are isotopically distinct with the extreme compositions being less radiogenic in the Atlantic (єHf ∼ +1) than in the Pacific (єHf ∼ +9). A good correlation of єHf and єNd is observed amongst most samples which supports that Hf isotopic compositions in nodules reflect those of ambient seawater. For a given єNd, єHf is more radiogenic in ferromanganese nodules than in rocks from either the mantle or the crust. This correlation makes the coupled Hf-Nd systems a potential paleoceanographic tool. It is argued that a zircon-free clayish component of probable eolian origin may account for the radiogenic Hf in nodules.

The southernmost exposed rocks of the North American Midcontinent rift system (1100 Ma) consist of 3000 m of mafic volcanic flows and minor interflow sediment exposed along the St. Croix River in Minnesota and Wisconsin. The flows are mostly high-Fe tholeiitic basalt with plagioclase phenocrysts and ophitic to subophitic clinopyroxene. Abundant secondary chlorite, epidote, and actinolite indicate the group was metamorphosed to greenschist facies (~350 °C). Low sodium (M4 site) and tetrahedral aluminum (AlIV) contents of actinolite indicate low-pressure metamorphism (0.25 GPa) and imply a geothermal gradient of 45 – 50 °C/km. Low magnesium (Mg# = 0.37–0.58) and Ni contents (36–185 ppm) indicate the basalts have undergone significant fractionation and are not primary mantle melts. Incompatible element abundances are inversely correlated with Mg#, and most samples plot within either high or low trace element groups (e.g., Ti, P, Zr). The basalts are enriched in the light rare earth elements and Th, and are variably depleted in Ta and Nb relative to La and Th. Initial 143Nd/144Nd compositions of the group range from 0.51099 to 0.51122 (initial εNd = −4.5 to +0.1). Most flows have isotopic compositions within a relatively limited range (initial εNd = −2.5 to −1.6), but exhibit variable trace element abundances. Flows with the highest and lowest initial 143Nd/144Nd ratios have isotopic compositions that are inversely correlated with trace element abundances and ratios (e.g., La/Yb, Th/La, Th/Ta). The combined geochemical data suggest that the Chengwatana basalts originated from plume-derived melts and underwent variable fractional crystallization and crustal contamination. These melts may have interacted with lithospheric mantle enriched during Penokean subduction.

The North Shore Volcanic Group (NSVG) of northeast Minnesota is a thick (9 km) sequence of plateau volcanic rocks that constitutes an important part of the Midcontinent rift system. This volcanic sequence is unique among the Midcontinent rift lavas, because it is composed of up to 25% rhyolite flows. We have analyzed Sm- and Nd-isotope compositions of 20 of the largest rhyolite and icelandite flows from the NSVG and seven comparably sized granophyres in the subjacent Duluth and Beaver Bay complexes. The lavas vary in composition from primitive basalt and basaltic andesite to icelandite and rhyolite, with a bimodal distribution. The rhyolites have much lower initial εNd values (−2 to −15, most samples < −10) than either the icelandites (0 to −6) or granophyres (0 to −8). Most rhyolites cannot be related to either the icelandites or more mafic magmas by simple fractionation, but rather have been produced by melting and assimilation of older, evolved crust. We suggest that the bimodal magmatism in the NSVG, and probably throughout the Midcontinent rift, has been produced by two fundamentally different processes. The bulk of the magmatism is basaltic; magmas originate in the mantle and migrate through the lithosphère with minor compositional change. Assimilation and fractional crystallization occur to varying degrees in the crust and, in some cases, produce icelandites, some small-volume rhyolites, and the granophyres, with Nd compositions dominated by the mantle component. The melting that produced the large-volume rhyolites is the result of a multistage process induced by these mantle-derived magmas that pond within the crust. This process appears to occur during a period of slowed extension and causes widespread heating and eventually localized extensive melting of the crust.

Due to the large partition coefficients of garnet for Lu compared to Sm, Nd, and Hf, garnet residual from crustal melting events has a large potential for retaining Lu and, over time, producing high 176Hf177Hf reservoirs in the lower crust. Therefore, melts derived from such garnet-bearing residual assemblages may be high in 176Hf177Hf relative to 143Nd144Nd. In order to determine the extent of potential hafnium isotopic heterogeneities in the lower crust from residual garnet, and to detail coupled HfNd isotopic behavior in older crustal rocks in general, we have determined the hafnium and neodymium isotopic compositions of thirty-two Precambrian granites and rhyolites from diverse suites of crustally derived magmatic suites. The majority of these rocks are known, from major-element and neodymium isotopic evidence, to have been derived from older (>300 m.y.) crust. Modeling of LuHf and SmNd partitioning during melting events in the lower crust indicates that anomalously high 176Hf177Hf compositions should develop within 300–400 m.y. in residual assemblages provided melting was at least 25–30% and 10% or more garnet was left in the residue. In spite of the diverse older crustal sources for the granites and rhyolites in this study, and the potential for garnet to be present in their source regions, none of the granitoids have anomalously high initial 176Hf177Hf compositions: all samples (with one exception) have initial Hf and Nd compositions that plot within a ±8 ϵHf unit wide band of the reference line (ϵHf = 2ϵNd + 2) for juvenile crust. The absence of high initial 176Hf177Hf compositions in these granites and rhyolites most likely implies that either (1) garnet is not a common residual phase in the lower crust or (2) garnet-bearing restite is not easily incorporated into later melts or (3) not enough time was available to develop anomalous Hf compositions.

INFERENCES about the early evolution of the Earth's crust and mantle have come largely from the study of isotope systematics— in particular, those of neodymium1–5. Neodymium isotope data from the oldest preserved rocks have been interpreted4,6–8 as reflecting early large-scale chemical depletion of the mantle (presumably resulting from the extraction of continental crust), but these data have remained controversial, in view of the potential for disturbances to the samarium–neodymium system during these rocks' long history9–11. Here we provide an independent evaluation of the Nd isotope compositions of ten early Archaean (3.6–3.8 Gyr old) gneisses, by investigating the hafnium isotope systematics of zircons from these rocks. The Hf data are consistent with the Nd record in indicating early depletion of the mantle, but fail to verify the scale and variability of this depletion. We conclude that Nd isotopes of early Archaean gneisses do not faithfully record isotopic variations in the early Earth, and therefore that these data need to be examined more critically before they can be used to constrain the early history of crust–mantle differentiation.

Many orogenic belts experience alternations in shortening and extension (tectonic mode switches) during continuous plate convergence. The geodynamics of such alternations are not well understood. We present a record of Late Cretaceous to Eocene alternations of shortening and extension from the interior of the retroarc Sevier-Laramide orogen of the western United States. We integrate new Lu-Hf garnet geochronometry with revised PT paths utilizing differential thermobarometry combined with isochemical G-minimization plots, and monazite Th-Pb inclusion geochronometry to produce a well-constrained “M” shaped PTt path. Two burial events (86 and 65 Ma) are separated by ∼3 kbar of decompression. The first burial episode is Late Cretaceous, records a 2 kbar pressure increase at ∼515–550 °C and is dated by a Lu-Hf garnet isochron age of 85.5 ± 1.9 Ma (2σ); the second burial episode records ∼1 kbar of pressure increase at ∼585–615 °C, and is dated by radially decreasing Th-Pb ages of monazite inclusions in garnet between ∼65 and 45 Ma. We propose a synconvergent lithospheric delamination cycle, superimposed on a dynamic orogenic wedge, as a viable mechanism. Wedge tapers may evolve from critical to subcritical (amplification), to supercritical (separation), and back to subcritical (re-equilibration) owing to elevation changes resulting from isostatic adjustments during the amplification and separation of Rayleigh-Taylor instabilities, and post-separation thermal and rheological re-equilibration. For the Sevier-Laramide hinterland, the sequence of Late Cretaceous delamination, low-angle subduction, and slab rollback/foundering during continued plate convergence explains the burial-exhumation-burial-exhumation record and the “M-shaped” PTt path.

Nd and Pb isotopic compositions were determined for 39 samples from mafic and felsic volcanic and subvolcanic units that underlie volcanogenic massive sulfide deposits in the Noranda and Matagami districts of the Abitibi greenstone belt. At Noranda, Pb and Nd isotopic data plot along a single linear array which implies derivation from an isotopically homogeneous source (average ϵNd = +2.5). At Matagami, in contrast, the Pb and Nd data define two linear arrays. The more compositionally evolved rhyolites have less radiogenic Pb isotopic compositions and higher average initial ϵNd values (ϵNd = +3.2) than the more primitive compositions (ϵNd = +2.4). We interpret these isotopic differences at Matagami, as well as those between Noranda and Matagami, as not due to assimilation of crustal components, but rather primary mantle heterogeneity in the Abitibi mantle at 2.7 Ga. This implies that, although the Abitibi mantle was broadly uniform in terms of initial ϵNd values (+2.5 ± 1), it was characterized by small heterogeneities on both local and regional scales in the Archean. On the basis of Pb and Nd isotopic data, we conclude that the Pb isotopic compositions of galena in Noranda and Matagami ores represent initial Pb isotopic compositions of the underlying volcanic rocks. Pb isotopic compositions of such massive sulfide ores throughout the Abitibi greenstone belt plot along a coherent linear PbPb array and Noranda and Matagami galenas are the most and least radiogenic examples, respectively. This array is nearly coincident with the geochron at 2.7 Ga, using an age for the Earth of 4.53 Ga, and indicates not only that the Abitibi mantle was isotopically heterogeneous at 2.7 Ga but also that these heterogeneities were developed within the first few hundred million years of Earth's history. Available data on initial ϵNd and initial 207Pb204Pb for the Superior Province are negatively correlated, implying that this early UPb fractionation was accompanied by SmNd fractionation and involved incompatible element depletion of the mantle.

As one of two well-exposed intraoceanic arcs, the Talkeetna arc of Alaska affords an opportunity to understand processes deep within arcs. This study reports new Lu-Hf and Sm-Nd garnet ages, Ar-40/Ar-39 hornblende, mica and whole-rock ages, and U-Th/He zircon and apatite ages from the Chugach Mountains, Talkeetna Mountains, and Alaska Peninsula, which, in conjunction with existing geochronology, constrain the thermal history of the arc. Zircon U-Pb ages establish the main period of arc magmatism as 202-181 Ma in the Chugach Mountains and 183-153 Ma in the eastern Talkeetna Mountains and Alaska Peninsula. Approximately 184 Ma Lu-Hf and similar to 182 Ma Sm-Nd garnet ages indicate that 25-35 km deep sections of the arc remained above similar to 700 degrees C for as much as 15 Myr. The Ar-40/Ar-39 hornblende ages are chiefly 194-170 Ma in the Chugach Mountains and 175-150 Ma in the Talkeetna Mountains and Alaska Peninsula but differ from zircon U-Pb ages in the same samples by as little as 0 Myr and as much as 33 Myr, documenting a spatially variable thermal history. Mica ages have a broader distribution, from similar to 180 Ma to 130 Ma, suggesting local cooling and/or reheating. The oldest U-Th/He zircon ages are similar to 137 to 129 Ma, indicating no Cenozoic regional heating above similar to 180 degrees C. Although the signal is likely complicated by Cretaceous and Oligocene postarc magmatism, the aggregate thermochronology record indicates that the thermal history of the Talkeetna arc was spatially variable. One-dimensional finite difference thermal models show that this kind of spatial variability is inherent to intraoceanic arcs with simple construction histories. Citation: Hacker, B. R., P. B. Kelemen, M. Rioux, M. O. McWilliams, P. B. Gans, P. W. Reiners, P. W. Layer, U. Soderlund, and J. D. Vervoort (2011), Thermochronology of the Talkeetna intraoceanic arc of Alaska: Ar/Ar, U-Th/He, Sm-Nd, and Lu-Hf dating, Tectonics, 30, TC1011, doi:10.1029/2010TC002798. (Less)

The Sierra de Pie de Palo of northwest Argentina preserves middle to lower crustal metamorphic rocks that were penetratively deformed during Ordovician accretion of the Precordillera terrane to the Gondwana margin. New structural, petrologic, and geochronologic data from a 40 km structural transect reveals that the Sierra de Pie de Palo preserves a middle to lower crustal ductile thrust complex consisting of individual structural units and not an intact ophiolite and cover sequence. Top-to-the-west thrusting occurred intermittently on discrete ductile shear zones from ∼515 to ∼417 Ma and generally propagated toward the foreland with progressive deformation. Ordovician crustal shortening and peak metamorphic temperatures in the central portion of the Sierra de Pie de Palo were synchronous with retro-arc shortening and magmatic flare-up within the Famatina arc. Accretion of the Precordillera terrane resulted in the end of arc flare-up and the onset of synconvergent extension by ∼439 Ma. Continued synextensional to postextensional convergence was accommodated along progressively lower grade shear zones following terrane accretion and the establishment of a new plate margin west of the Precordillera terrane. The results support models of Cordilleran orogens that link voluminous arc magmatism to periods of regional shortening. The deformation, metamorphic, and magmatic history within the Sierra de Pie de Palo is consistent with models placing the region adjacent to the Famatina margin in the middle Cambrian and not as basement to the Precordillera terrane. Fil: Mulcahy, S. R.. University of Nevada; Estados Unidos. University of California at Davis; Estados Unidos Fil: Roeske, S. M.. University of California at Davis; Estados Unidos Fil: McClelland, W. C.. University of Iowa; Estados Unidos Fil: Jourdan, F.. Curtin University; Australia. Berkeley Geochronology Center; Estados Unidos Fil: Iriondo, A.. Universidad Nacional Autónoma de México; México Fil: Renne, P. R.. Berkeley Geochronology Center; Estados Unidos. University of California at Berkeley; Estados Unidos Fil: Vervoort, J. D.. Washington State University; Estados Unidos Fil: Vujovich, Graciela Irene. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Geología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

The Pb isotope compositions of the footwall sequences of the Noranda and Matagami volcanogenic massive sulfide deposits in the Abitibi greenstone belt have been examined. At Noranda, whole-rock Pb isotope data for the Flavrian, Northwest, Rusty Ridge, and Amulet Formations and the subvolcanic Flavrian intrusion scatter about a line that corresponds to a Pb-Pb age of 2638 + or - 14 Ma. Pb-Pb ages calculated for the individual units are all within error of this age but are significantly younger than the 2700 + or - 2 Ma U-Pb zircon age for the Flavrian intrusion. The Pb-Pb array passes above a tightly constrained field for the galena Pb from ore deposits within the Mine sequence. The younger Pb-Pb ages and the lack of collinearity with the sulfide ores reflect rehomogenization of the Pb isotope compositions in these rocks approximately 60 m.y. after their formation.At Matagami, whole-rock Pb isotope data fall along two arrays. The first, which includes the porphyritic metarhyolite of the Watson Lake Group from the footwall and the key tuffite, corresponds to a Pb-Pb age of 2584 (super +19) (sub -20) Ma. The second array includes the Bell River igneous complex and the overlying Wabassee Group basalts and corresponds to an age of 2647 + or - 14 Ma. These Pb-Pb ages are approximately 75 and 140 m.y. younger than corresponding zircon U-Pb ages of 2725 + or - 2 and 2725 (super +3) (sub -2) Ma. Both arrays pass above a tightly constrained field for galena Pb from several Matagami ore deposits and are probably the result of differences in the initial Pb isotope compositions of the respective rocks. The apparent initial Pb values most likely represent heterogeneities in the mantle source regions for these rocks. The younger Pb-Pb ages for the two arrays indicate Pb isotope homogenization in these rocks approximately 100 m.y. after their formation. The cause of the different ages for the two arrays remains controversial but may be the result of either incomplete isotopic homogenization of Pb or an earlier closure of the U-Pb systems in the rocks constituting the older array.The Pb homogenization implied by the young Pb-Pb ages may be the result of the pervasive circulation of hydrothermal solutions that may have been derived from the dewatering of the lower parts of a newly formed, thickened Archean crust. Such large-scale processes apparently occurred 50 to 150 m.y. after this crust was formed and assembled.

Hot Tropical Explosion The Paleocene-Eocene Thermal Maximum (PETM), 55 million years ago, was a unique episode of rapid global warming (∼5°C), often used as an ancient analog for future global climate change. Climate alteration during the PETM has been extensively studied in the marine realm, and from a few temperate to polar terrestrial localities, but little is known about how the tropics responded to the high temperatures and high levels of CO 2 . Using evidence from pollen analysis, Jaramillo et al. (p. 957 ) show that rapid tropical forest diversification occurred during the PETM, without plant extinction or regional aridity. Unexpectedly, diversity seemed to increase at higher temperatures, contradicting previous assumptions that tropical flora will succumb if temperatures become excessive.

The 176Hf/177Hf ratio of seawater is measured directly with 143Nd/144Nd in a composite vertical profile in the NE Atlantic. The value of ɛHf of intermediate and deep water exhibits little variation and averages +1.0 ± 0.8; surface water shows a wider range, −5.7 to +3.3. While intermediate and deep water samples plot on, or close to, the seawater Hf-Nd isotope array, defined by ferromanganese deposits, near-surface seawater samples plot between the seawater and terrestrial rock ɛHf − ɛNd arrays. It is likely that the shallow water variability in ɛHf arises from a short residence time of Hf in surface water and isotopically heterogeneous inputs. The difference between surface and intermediate-deep water ɛHf may be maintained by the release of surface scavenged Hf at depth from a subset of sinking particles; the composition of eNd at middepths reflects overlying surface water and water advected along isopycnals from areas where they outcrop in winter. Uniform ɛHf values below 1000 m suggest the residence time of Hf may be longer than that of Nd. The radiogenic isotopic composition of seawater Hf relative to Nd and terrestrial rocks suggests that minerals with high Lu/Hf ratios may be preferentially weathered, providing a source of radiogenic Hf to the oceans.

The crystallization age of shergottites is currently not agreed upon. Although mineral 87Rb–87Sr, 147Sm–143Nd, 176Lu–176Hf, and U–Pb isochrons all give very young ages, typically in the range of 160–180 Ma, 207Pb–206Pb data support a much older crystallization age at 4.1 Ga, which is consistent with published whole-rock 87Rb–87Sr data on basaltic shergottites. Different isotopic systems present different complexities, but crater-counting chronology, which shows that a substantial fraction of the Martian surface was resurfaced during the late heavy bombardment, is in favor of an old Martian lithosphere with ages in accordance with Pb–Pb and Rb–Sr isotopic data. A ∼ 4.1 Ga Pb–Pb age of shergottites also agrees with the 142Nd and 182W anomalies found in these rocks and concur with the presence of an actively convecting mantle during the first 500 Myr of the planet's history. We here present new Sm–Nd, Lu–Hf, and Pb–Pb mineral isochrons for the basaltic shergottites Shergotty and Los Angeles complementing our previous results on Zagami [Bouvier A., Blichert-Toft J., Vervoort J.D. and Albarede F. (2005). The age of SNC meteorites and the antiquity of the Martian surface, Earth Planet. Sci. Lett. 240, 221–233]. The internal 147Sm–143Nd and 176Lu–176Hf isochrons give young ages of, respectively, 172 ± 40 (MSWD = 2.0) and 188 ± 91 (MSWD = 3.1) for Shergotty, and 181 ± 13 (MSWD = 0.14) and 159 ± 42 (MSWD = 0.01) for Los Angeles. In contrast, the Pb isotope compositions of the leached whole-rock fragments and maskelynite separates of Shergotty and Los Angeles fall on the whole-rock isochron previously established for Zagami and other shergottite samples and collectively yield a Pb–Pb age of 4050 ± 70 Ma for the crystallization of the basaltic shergottite suite. The contrast between the ∼ 170 Ma ages of internal isochrons and the 4.1 Ga age supported by Pb–Pb and 87Rb–87Sr on whole-rocks simply reflects that the younger age dates the perturbation of a suite of rocks of Noachian age. The internal Rb–Sr, Sm–Nd, Lu–Hf, and U–Pb errorchrons are heavily biased by the presence of disseminated phosphate minerals and inclusions, for which D/H ratios (δD~ + 4600‰) indicate strong interaction with Martian subsurface waters. In contrast, baddeleyite, occasionally present in SNCs and also having extremely young U–Pb ages, reflect resetting under the shock conditions that prevailed either during shergottite extraction from the planet or from impacts associated with a major break-up event of planetesimals in the main asteroid belt. We finally also re-examine 39Ar–40Ar data on SNC meteorites and suggest that they as well support old crystallization ages.

We have analyzed the Pb isotopic compositions of whole-rocks and various components (CAIs, chondrules, and/or mineral separates) of two carbonaceous chondrites, Allende (CV3) and Murchison (CM2), and nine ordinary chondrites, Sainte Marguerite (H4), Nadiabondi and Forest City (H5), Kernouve (H6), Bjurbole (L/LL4), Elenovka and Ausson (L5), Tuxtuac (LL5), and Saint-Severin (LL6) by MC-ICP-MS. Three CAI fractions from Allende define an isochron with an age of 4568.1 ± 9.4 Ma (MSWD = 0.08) and plot on the same isochron as fragments of the Efremovka inclusion E60 analyzed by Amelin et al. [Amelin, Y., Krot, A. N., Hutcheon, I. D., and Ulyanov, A. A. (2002a). Lead isotopic ages of chondrules and calcium–aluminum-rich inclusions. Science 297 , 1679–1683]. When these two groups of samples are combined, the isochron yields an age of 4568.5 ± 0.5 (MSWD = 0.90), which is our best estimate of the age of the Solar System. Chondrules and pyroxene–olivine fractions from the ordinary chondrites yield ages that reflect the blocking of Pb isotope equilibration with the nebular gas. The combination of these ages with the corresponding metamorphic phosphate ages provides constraints on the thermal history of the different chondrite parent bodies. Among the H chondrites, Sainte Marguerite cooled to below ∼1100 K within a few My at 4565 Ma and to ∼800 K at 4563 Ma. Nadiabondi appears to have experienced a slightly more protracted cooling history with the corresponding interval lasting from 4559 to 4556 Ma. The data from Forest City and Kernouve show evidence of late-stage perturbation with resulting U/Pb fractionation. Likewise, Pb isotopes in Tuxtuac (LL5) record a cooling history lasting from ∼4555 to 4544 Ma, which may indicate that the cooling history for the LL parent body was more prolonged than for the H parent body. We suggest a thermal evolution model for the growth of the planetary bodies based on the release of radiogenic heat from 26 Al and 60 Fe. This model incorporates the accretion rate, which determines the time at which the radiogenic heat becomes efficiently trapped, and the terminal size of the parent body, which controls its overall thermal inertia. The parent bodies of carbonaceous chondrites, which show little indication of metamorphic transformation, collect cooler nebular material at a relatively late stage. Small asteroids of ∼10–50 km radius accreting within 1–3 My could be the parent bodies of H and LL chondrites. The parent body of the L chondrites is likely to be a larger asteroid ( r > 100 km) or possibly the product of collisions of smaller planetary bodies.

We analyzed detrital zircons in seven samples of metasedimentary rock from north-central Idaho, U.S.A., to test the previous assignment of these rocks to the Mesoproterozoic Belt–Purcell Supergroup. Correlating these rocks with known sedimentary units through field observations is difficult if not impossible due to the high metamorphic grade (amphibolite facies) and intensity of deformation. Zircon analysis by laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS) reveals that five of the seven samples contain multiple zircon populations between 1700 and 1400 Ma and a scatter of Paleoproterozoic and Archean ages, similar to results reported from the Belt Supergroup to the north and east. These results indicate that the likely protoliths of most high-grade metamorphic rocks northwest of the Idaho batholith were upper strata of the Belt Supergroup. In contrast, a quartzite at Bertha Hill north of Pierce lacks grains younger than 1600 Ma and thus is distinctly unlike the Ravalli Group of the Belt Supergroup, with which it was previously correlated. Possible correlatives that contain similarly old populations of zircons and feldspar-poor quartzite include the Neihart Formation (lowermost Belt Supergroup in Montana), Neoproterozoic quartzite (Syringa metamorphic sequence), and Cambrian quartzite. A sample from the North Fork of the Clearwater River yielded a large number of zircons with concordant Neoproterozoic ages, all of which had low Th/U ratios that suggest either a Neoproterozoic metamorphic event or the transport and deposition of zircons that were metamorphosed in the Neoproterozoic. SHRIMP (sensitive high-resolution ion microprobe) dating of a granite (now augen gneiss) that intruded sedimentary rocks west of Pierce, Idaho, yields an age of 1379 ± 12 Ma based on seven of fourteen analyses; this provides a lower age limit for sediment deposition of some rocks mapped as metamorphosed Belt Supergroup, and which had detrital zircon populations in the 1700 to 1400 Ma range. Additional analyses of three zircon rims yield an age range of 87–82 Ma, which is similar to the youngest ages from the North Fork sample. We interpret these ages to reflect the time of zircon overgrowth synchronous with the emplacement of the Cretaceous Idaho Batholith. None of the metasedimentary rocks dated can be older than Mesoproterozoic, and, with the exception of the Bertha Hill quartzite, none can be older than the Belt–Purcell Supergroup.

Zircon U-Pb analyses of 18 orthogneisses from the Clearwater and Priest River complexes of northern Idaho have identified important exposures of Neoarchean and Paleoproterozoic basement rocks in northwest Laurentia. All samples have ages that fall into two tightly defined age ranges: in the Neoarchean from 2.67 to 2.65 Ga and in the Paleoproterozoic from 1.88 to 1.84 Ga. The Neoarchean orthogneisses show no evidence of older components, whereas some of the Paleoproterozoic orthogneisses have xenocrystic zircon cores with ages overlapping with those of the Neoarchean gneisses. Collectively, these two packages of rocks represent the two main periods of crust formation in northwest Laurentia, one wholly juvenile in the Neoarchean, and the other a mixture of juvenile Paleoproterozoic and inherited Neoarchean components. Based on existing data, the ages of orthogneisses in the Clearwater and Priest River complexes are identical and probably represent one continuous crustal block; we refer to this basement domain as the Clearwater block. The Paleoproterozoic 1.86 Ga magmatism described here is distributed throughout the northwest Laurentian margin and does not coincide with the proposed trend of the Great Falls tectonic zone. Therefore, a model of a single linear arc preserved within the Great Falls tectonic zone is inadequate in describing the majority of the known 1.86 Ga crust in the region.

International audience; We report new Sm–Nd, Lu–Hf, and Pb–Pb mineral and whole-rock isotope data for the basaltic shergottite Zagami, as well as Pb–Pb whole-rock isotope data for the basaltic shergottite Los Angeles, the lherzolitic shergottite Dar-al-Gani 476 (DaG 476), and the clinopyroxenite Nakhla. In agreement with previous findings, our new Sm–Nd and Lu–Hf mineral ages on the Martian meteorite Zagami are young (155 and 185 Ma, respectively). The 207Pb/206Pb–204Pb/206Pb compositions of the insoluble fractions of shergottites (Zagami, Los Angeles, and literature data for Shergotty and EETA79001) form an excellent alignment indicative of a 4.0 Ga crystallization age. The range of Pb isotope compositions observed in the leachates of these samples attests to negligible contamination of the shergottites by terrestrial Pb and argues against mixing relationships. The age of 4.048 ± 0.017 Ga (MSWD = 1.5) provided by the Pb isotope compositions of the Zagami whole-rock and residues is therefore taken to date the crystallization of this rock, which, so far, was believed to be only not, vert, similar 180 Ma old. Based on this result, we argue that the lithosphere of Mars is extremely old and that most mineral ages were reset recently by acidic aqueous solutions percolating through the Martian surface. This interpretation is consistent with photographic interpretations of erosional features on Mars. It also relieves the constraint imposed by the presence of anomalies of 142Nd and 182W (both products of extinct radioactive nuclides) that the Martian mantle should have preserved primordial isotopic heterogeneities, thus allowing for the planet interior to be actively convecting.

[1] We report Yb isotopic data for the purpose of accurately correcting measurements of Lu and Lu/Hf ratios during Multicollector Inductively Coupled Plasma Mass Spectrometer (MC-ICPMS) analysis. The Yb isotopic ratios agree with recent results of Chu et al. [2002] and Amelin and Davis [2004], when corrected for mass fractionation to the same reference values. Protocols for Lu measurement involving simultaneous subtraction of Yb interference and application of a fractionation correction based on Yb are described. It is shown that Lu concentrations and Lu/Hf ratios determined by MC-ICPMS using this methodology are more precise and potentially more accurate than results based on analysis of Lu by Thermal Ionization Mass Spectrometry (TIMS). A number of MC-ICPMS Lu-Hf isotopic analyses of standard rocks and other geological samples, previously analyzed by TIMS, are presented. These document reproducibility of Lu/Hf within 0.2%, where most discrepancies can be assigned to the unknown Lu mass fractionation during TIMS analysis.

[1] We present new field, structural, petrographic, and geochronologic data on a rare midcrustal (� 25 km) exposure of a Cordilleran arc, the Coast Ridge Belt, located in the Santa Lucia Mountains of central California. The study area is composed primarily of a deformed suite of upper amphibolite to granulite facies rocks (the ‘‘Sur Series’’), which is dominated by metaigneous tonalites, diorites, and gabbros with subordinate metasedimentary quartzite and marble. Inherited zircons in magmatic rocks suggest that the provenance of framework rocks is drawn heavily from miogeoclinal formations and that sedimentation occurred in the late Paleozoic or later. Minor magmatism in the Coast Ridge Belt began in the Early or Middle Cretaceous, but magmatic activity was most intense during a short period time from 93 to 81 Ma, based on U-Pb zircon ages of a felsic gneiss and two less-deformed diorites. The time period 93– 81 Ma also brackets a period of extensive thickening and high-temperature ductile deformation. While a thrusting cause for ductile deformation cannot be ruled out, we favor the hypothesis that the exposed rocks correspond to a zone of return flow of supracrustal rocks locally displaced by granitoid plutons in the shallower crust. Magmatism ended throughout Salinia between81and76Ma,coincidentwiththeattainmentof peak pressure and temperature conditions of 0.75 GPa and 800� C. Exhumation followed immediately, bringing the Coast Ridge Belt to the surface within 8 My at a rate of at least 2–3 mm/yr. Exhumation was coincident with an episode of extensional collapse that has been documented elsewhere in the southern California arc during the early Laramide orogeny and that may be related to underthrusting of the forearc at that time. INDEX TERMS: 8102 Tectonophysics: Continental contractional orogenic belts; 8035 Structural Geology: Pluton emplacement; 3640 Mineralogy and Petrology: Igneous petrology; 8030 Structural Geology: Microstructures; 8124 Tectonophysics: Earth’s interior—composition and state (1212)

Modern basalts have seemingly lost all ‘memory’ of the primitive Earth's mantle except for an ambiguous isotopic signal observed in some rare gases1,2. Although the Earth is expected to have reached a thermal steady state within several hundred million years (refs 3, 4) of accretion, it is not known how and when the initial chemical fractionations left over from planetary accretion (and perhaps a stage involving a magma ocean) were overshadowed by fractionations imposed by modern-style geodynamics. Because of the lack of samples older than 4 Gyr, this early dynamic regime of the Earth is poorly understood. Here we compare published Hf–Nd isotope data on supracrustals from Isua, Greenland, with similar data on lunar rocks and the SNC (martian) meteorites, and show that, about 3.8 Gyr ago, the geochemical signature of the Archaean mantle was partly inherited from the initial differentiation of the Earth. The observed features seem to indicate that the planet at that time was still losing a substantial amount of primordial heat. The survival of remnants from an early layering in the modern deep mantle may account for some unexplained seismological, thermal and geochemical characteristics of the Earth as observed today.

Book Review: Geochemistry: Pathways and Processes, 2nd Edition, Harry Y. McSween. Columbia University Press (2003).

Widespread base- and precious-metal anomalies, oxidized sulfi de veins, silicifi ed calcareous shales and carbonates, and altered porphyry intrusions occur in the northeastern Sulphur Spring Range, Nevada, 80 km south of important gold deposits in the Carlin trend. The small historic mines and prospects in the area are spatially and perhaps genetically related to a suite of variably altered dikes, small lava fl ows, silicic domes, and related pyroclastic rocks. New major- and trace-element data and U-Pb zircon ages show that the East Sulphur Spring volcanic suite is Eo-Oligocene in age (36‐31 Ma) and ranges in composition from high MgO- basaltic andesite to peraluminous rhyolite. The major- and trace-element compositions of the volcanic rocks are characteristic of continental margin subduction zone magmas and form a high-K, calc-alkaline suite with low Fe/Mg ratios. In addition, the rocks have negative Nb and Ti anomalies and elevated Ba, K, and Pb on normalized trace-element diagrams. Crustal melting is indicated by the eruption of a peraluminous garnet-bearing ignimbrite and as a component in hybridized andesite. The nature of this suite and its potential for mineralization is elucidated via comparisons to other Eocene age volcanic rocks associated with much larger gold and copper deposits in the Great Basin. The East Sulphur Spring suite is more similar to Eocene igneous rocks found along and near the Carlin trend than it is to those erupted while the Bingham porphyry copper deposit developed 300 km farther to the east. For example, the East Sulphur Spring suite and the Eocene magmatic rocks along the Carlin trend are less alkaline than the Bingham suite and lack its unusual enrichment of Cr, Ni, and Ba in intermediate composition rocks (58‐68 wt% silica). Nonetheless, the Bingham and East Sulphur Spring volcanic suites both preserve evidence of mixing that created intermediate compositions. For example, an andesite has obvious mineral disequilibrium with plagioclase, biotite, clinopyroxene, orthopyroxene, olivine, and amphibole coexisting with extensively resorbed megacrysts of quartz, K-feldspar, and garnet—indicative of mixing basaltic andesite or andesite and largely crystallized garnet-bearing rhyolite. On the other hand, we found no evidence for mixing with a mafi c alkaline magma like that in the Bingham Canyon magma-ore system. We conclude that: (1) an unusual tectonic setting prevailed during the Eocene and Oligocene of the western United States that promoted the production of oxidized mafi c magma in an arclike setting, but far inland as a result of the rollback of the Farallon slab; (2) the mafi c magmas intruded or erupted separately, or mixed with more silicic magma generated by fractional crystallization and assimilation of crustal materials; and (3) these mafi c magmas may have delivered signifi cant amounts of sulfur and chalcophile metals to upper crustal magma chambers and eventually to Paleogene ore deposits in the eastern Great Basin.