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The Moon likely formed as a result of a giant impact between proto-Earth and another large body. The timing of this event and the subsequent lunar differentiation timescales are actively debated. New high-precision Nd isotope data of Apollo mare basalts are used to evaluate the Low-Ti, High-Ti and KREEP mantle source reservoirs within the context of lunar formation and evolution. The resulting models are assessed using both reported 146Sm half-lives (68 and 103 Myr). The linear relationship defined by 142Nd–143Nd systematics does not represent multi-component mixing and is interpreted as an isochron recording a mantle closure age for the Sm–Nd system in the Moon. Using a chondritic source model with present day μ142Nd of −7.3, the mare basalt mantle source reservoirs closed at 4.45−09+10 Ga (t1/2Sm146=68 Myr) or 4.39−14+16 Ga (t1/2Sm146=103 Myr). In a superchondritic, 2-stage evolution model with present day μNd142 of 0, mantle source closure ages are constrained to 4.41−08+10 (t1/2Sm146=68 Myr) or 4.34−14+15 Ga (t1/2Sm146=103 Myr). The lunar mantle source reservoir closure ages

A major change in Earth's geodynamics occurred ~3 billion years (Ga) ago, likely related to the onset of modern and continuous plate tectonics. However, the question of how Earth functioned prior to this time is poorly constrained. Here, we find a resolvable positive 142Nd anomaly in a 2.7 Ga old tholeiitic lava flow from the Abitibi Greenstone Belt indicating that early-formed mantle heterogeneities persisted at least 1.8 Ga after Earth's formation. This result contradicts the expected rapid early (~0.1 Ga), as well as the slower present-day (~1 Ga) mixing rates in the convecting mantle. Using a numerical modeling approach, we show that convective mixing is inefficient in absence of mobile-lid plate tectonics. The preservation of a 142Nd anomaly until 2.7 Ga ago can be explained if throughout the Hadean and Archean, Earth was characterized by a stagnant-lid regime, possibly with sporadic and short subduction episodes. The major change in geodynamics observed around ~3 Ga ago can then reflect the transition from stagnant-lid plate tectonics to modern mobile-lid plate tectonics. Solving the paradox of a convective but poorly-mixed mantle has implications not only for Archean Earth, but also for other planets in the solar system such as Mars.

Tungsten isotope compositions of magmatic iron meteorites yield ages of differentiation that are within ±2 Ma of the formation of CAIs, with the exception of IVB irons that plot to systematically less radiogenic compositions yielding erroneously old ages. Secondary neutron capture due to galactic cosmic ray (GCR) irradiation is known to lower the e182W of iron meteorites, adequate correction of which requires a measure of neutron dosage which has not been available, thus far. The W, Os and Pt isotope systematics of 12 of the 13 known IVB iron meteorites were determined by MC-ICP-MS (W, Os, Pt) and TIMS (Os). On the same dissolutions that yield precise e182W, stable Os and Pt isotopes were determined as in situ neutron dosimeters for empirical correction of the ubiquitous cosmic-ray induced burn-out of 182W in iron meteorites. The W isotope data reveal a main cluster with e182W of ∼−3.6, but a much larger range than observed in previous studies including irons (Weaver Mountains and Warburton Range) that show essentially no cosmogenic effect on their e182W. The IVB data exhibits resolvable negative anomalies in e189Os (−0.6e) and complementary e190Os anomalies (+0.4e) in Tlacotepec due to neutron capture on 189Os which has approximately the same neutron capture cross section as 182W, and captures neutrons to produce 190Os. The least irradiated IVB iron, Warburton Range, has e189Os and e190Os identical to terrestrial values. Similarly, Pt isotopes, which are presented as e192Pt, e194Pt and e196Pt range from +4.4e to +53e, +1.54e to −0.32e and +0.73e to −0.20e, respectively, also identify Tlacotepec and Dumont as the most GCR-damaged samples. In W–Os and W–Pt isotope space, the correlated isotope data back-project toward a 0-epsilon value of e192Pt, e189Os and e190Os from which a pre-GCR irradiation e182W of −3.42±0.09 (2σ) is derived. This pre-GCR irradiation e182W is within uncertainty of the currently accepted CAI initial e182W. The Pt and Os isotope correlations in the IVB irons are in good agreement with a nuclear model for spherical irons undergoing GCR spallation, although this model over-predicts the change of e182W by ∼2×, indicating a need for better W neutron capture cross section determinations. A nucleosynthetic effect in e184W in these irons of −0.14±0.08 is confirmed, consistent with the presence of Mo and Ru isotope anomalies in IVB irons. The lack of a non-GCR Os isotope anomaly in these irons requires more complex explanations for the production of W, Ru and Mo anomalies than nebular heterogeneity in the distribution of s-process to r-process nuclides.

Chemical heterogeneities in the Martian mantle are believed to result from the crystallization of a magma ocean in the first 100 million years of its history. Shergottite meteorites from Mars are thought to retain a compositional record of such early differentiation and the resulting mineralogy at different depths. The coupled 176 Lu– 176 Hf and 147 Sm– 143 Nd isotope systematics in 9 shergottites are used here to investigate these issues. Three compositional groups in the shergottites display distinct isotope systematics. One group, commonly termed as depleted, is characterized by positive ɛ 176 Hf i from + 46.2 to + 50.4 and ɛ 143 Nd i from + 36.2 to + 39.1. Another, termed as enriched, has negative ɛ 176 Hf i = − 16.5 to − 13.2 and ɛ 143 Nd i = − 7.0 to − 6.5. The third group is intermediate between the depleted and enriched groups with positive ɛ 176 Hf i = + 30.0 to + 33.4 and ɛ 143 Nd i = + 16.9. Together, they describe mixing curves between 176 Hf/ 177 Hf, 143 Nd/ 144 Nd, Lu/Hf, and Sm/Nd, implying that they sample two distinct sources in the Martian mantle. All shergottites are characterized by (Sm/Nd) source sample , but (Lu/Hf) source > (Lu/Hf) sample . This decoupling can be explained by two successive partial melting episodes in the depleted shergottite source and localized in the Martian upper mantle. The genesis of shergottites can be modeled using non-modal equilibrium partial melting in a source initially composed of 60% olivine, 21% clinopyroxene, 9% orthopyroxene, and 10% garnet, with degrees of partial melting of 8.8% and 3.9%, respectively, for the two successive events. The enriched end-member of the shergottite mixing curve is best modeled by late-stage quenched residual melt resulting from the crystallization of a magma ocean. The depleted shergottite source may be modeled as a mixture of cumulates and residual melt, as convection in the Martian magma ocean is expected to reduce the incompatible trace element heterogeneity in the final solidified layers. Consequently, equilibrium crystallization is preferred to model the crystallization of the Martian magma ocean. The models that best explain the shergottite data are those where the magma ocean is at a depth of at least 1350 km in Mars.

Mantle plumes are commonly perceived to have both a chemical and dynamic link with the subduction of ocean crust into the mantle. In principle, this should lead to the observation of chemical and isotopic signatures that are characteristic of ocean crust and marine sediments in ocean island basalts. This study investigates the thallium (Tl) isotope systematics of lavas from Iceland and the Azores archipelago, in order to determine if their compositions were affected by admixing of ferromanganese sediments or upper ocean crust altered at low temperature. Such materials are known to display strongly fractionated Tl isotope signatures relative to the ambient upper mantle. Two samples from the island of Terceira in the Azores archipelago have Tl isotope compositions significantly different from normal mantle, and this suggests the presence of Fe–Mn sediments. Combined Pb and Tl isotope modelling indicates that the Tl anomalies are not a feature of the Azores plume but produced by assimilation of modern Fe–Mn sediments during magma ascent through the ocean crust. Excluding these two anomalous lavas from Terceira, the Iceland and Azores samples have identical Tl isotope compositions, with an overall mean of ɛ205Tl = − 1.5 ± 1.4 (2SD, n = 30) that is indistinguishable from the previously estimated upper mantle average (ɛ205Tl = − 2.0 ± 0.5). The near-constant Tl isotope compositions of the Iceland and Azores lavas may indicate that the respective mantle plume sources contain virtually no Fe–Mn sediments or altered upper ocean crust. Alternatively, it is possible that the lack of Tl isotope variation reflects quantitative removal of fractionated Tl from the slab during subduction and dehydration. A less straightforward explanation is that past marine environments produced sediments and altered marine basalts with nearly unfractionated Tl isotope compositions. All three scenarios have important implications and future Tl isotope studies will be able to identify the most feasible interpretation.

Komatiite samples from the 2.8-Ga Kostomuksha greenstone belt in the Baltic Shield define Pt–Os and Re–Os isochrons with ages of 2816 ± 190 and 2880 ± 83 Ma, respectively. The mean initial 186Os / 188Os and 187Os / 188Os ratios calculated at T = 2821 Ma are 0.1198341 ± 7 and 0.11042 ± 69 (γ187Os = + 2.5 ± 0.6), respectively. This Os isotopic composition characterizes that of the source of the Kostomuksha komatiite and is 53 ± 6 ppm and 2.5 ± 0.6% more radiogenic in 186Os/188Os and 187Os/188Os, respectively, than the putative contemporary convecting mantle. The coupled 186Os–187Os enrichment cannot be the result of incorporation of the material of aged recycled oceanic crust into the source of the plume, as five to six times more crustal component is required to create the 186Os-enrichment than the 187Os-enrichment observed. Moreover, the radiogenic 186Os / 188Os ratio combined with the subchondritic Pt/Os in this source calculated from the HSE abundance data is inconsistent with any model of Os isotopic evolution that involves crustal recycling, as these models require substantially suprachondritic Pt/Os in the hybrid sources. The coupled 186Os–187Os enrichment in the source of the Kostomuksha komatiite is best explained via derivation of most of the Os from the outer core. This is most consistent with the Kostomuksha komatiites originating from a mantle plume that arose from the core–mantle boundary (CMB). If this interpretation is correct, the data provide minimum constraints on the 186Os / 188Os and 187Os / 188Os ratios of the outer core at 2.8 Ga. The existing models of core crystallization based on experimentally determined solid metal–liquid metal partition coefficients for Pt, Re, and Os can adequately explain the Os isotopic composition of the Kostomuksha mantle source, although require the onset of inner core crystallization several hundred million years prior to formation of the Kostomuksha plume. Based on thermodynamic models, such an early onset of inner core crystallization would require a relatively low heat flux across the CMB of ∼3 TW and the presence of > 100 ppm K in the core. The results of this study combined with the HSE abundance data for the source of komatiites at Kostomuksha indicate that core–mantle interaction, at least in this instance, could occur in the form of isotopic exchange without significant mass transfer from the core to the mantle.

Some hotspot volcanism is likely the surface manifestation of hot upwelling mantle plumes that rise from the core–mantle boundary (CMB) or from compositional boundaries within the mantle. Plumes that rise from the CMB may generate magmas that carry compositional information about the diversity of materials present in the lowermost mantle. Some plume-derived materials have coupled enrichments in 186Os/188Os and 187Os/188Os relative to upper mantle materials. The coupled enrichments are consistent with Os isotopic compositions predicted for the liquid outer core that might result from elevated Pt/Os and Re/Os generated via progressive crystallization of a solid inner core over Earth history. If these enrichments are a reflection of core–mantle interaction, then the mechanisms of material transport between the core and the mantle, rates of whole mantle convection at present and over time, and cooling histories for the core and the mantle can potentially be constrained. However, fractionation may also occur between Pt/Re/Os via mantle and crustal processes and in particular, Pt and Pt/Re may be elevated in some portions of ancient recycled slabs thought to be present in certain plume sources. This possibility brings into question whether the coupled Os isotope enrichments are a product of core–mantle exchange. Hence, a lively debate over the causes of Os isotopic heterogeneities in plume-derived materials has ensued. Resolution of this debate has been hampered by limited constraints on the behavior of these elements in the Earth's core, mantle and crust. A variety of analytical and experimental tasks will have to be pursued to test different aspects of both the core–mantle interaction and crustal recycling models, and to further constrain the behaviors of these elements. These future endeavors may help to unravel the compositional complexities of plume-derived materials and their mantle sources, and ultimately provide important insights to processes operating at the CMB.

More than 100 Os–Ir–Ru alloy grains from southwestern Oregon, presumably originating in the Mesozoic Josephine ophiolite, were examined for major and minor element compositions and 187Os/188Os isotopic compositions. Sixteen grains spanning the range of 187Os/188Os ratios present in the larger suite were analyzed for high-precision 186Os/188Os ratios. The ranges of major and minor element abundances of individual grains were, in general, similar to compositions previously reported for such grains worldwide. Within-suite compositional variations were quite large. Most grains were likely disaggregated from larger intergrown clusters, and were subdivided based on texture into matrix and lamellae. Composition variations within individual matrix grains were limited, whereas variations within individual lamellae grains were considerably larger. The 187Os/188Os ratios of individual grains were isotopically homogeneous at the ±0.1% level of resolution. The 187Os/188Os ratios of the entire suite of grains ranged from 0.1173 to 0.1468. The average ratio for all grains combined was 0.1241, similar to the composition of three chromitites sampled from the Josephine Creek, and consistent with formation in the Mesozoic upper mantle. The average 186Os/188Os ratio of the subset of 16 grains was 0.119838. All grains but one had isotopic ratios within ±20 ppm of that ratio. Results indicate that 187Os/188Os variations on a scale of mm3 may be quite large within the upper mantle, mimicking heterogeneities observed in both hand-and outcrop-scale samples of upper mantle materials. Highly depleted compositions relative to ambient upper mantle with chondritic 187Os/188Os require that some of the heterogeneities must be >1 Ga. The homogeneity of the 186Os/188Os ratios lend no support to the contention that isotopic heterogeneities within the upper mantle may explain coupled 186Os–187Os enrichments that have been observed in putative plume derived lavas.

Precise Pt–Re–Os isotopic and highly siderophile element (HSE) data are reported for 10 drill core samples collected across a 2.7 m thick, differentiated komatiite lava flow at Pyke Hill in Ontario, Canada. The Pt–Re–Os isotopic data for five cumulate samples yield Re–Os and Pt–Os isochrons with ages of 2725F33 and 2956F760 Ma and initial 187 Os/ 188 Os=0.10902F16 and 186 Os/ 188 Os=0.1198318F18, respectively. These ages are consistent with the time of lava emplacement determined by other geochronometers. The initial 187 Os/ 188 Os isotopic composition of the Pyke Hill source is similar to the 187 Os/ 188 Os isotopic compositions of enstatite and ordinary chondrites and to that of PUM at 2725 Ma. Using the Pt–Os isotopic data for the Pyke Hill source, the present-day 186 Os/ 188 Os isotopic composition of PUM was calculated to be 0.1198387F18. The regression of global 187 Os/ 188 Os isotopic data for terrestrial materials shown to represent Os isotopic composition of the contemporary mantle indicates that the LILE-depleted mantle evolved from the solar system starting composition and, over Earth’s history, was on average f8% depleted in Re vs. Os relative to PUM. From the HSE composition of the emplaced Pyke Hill komatiite lava, the mantle source was inferred to contain Re (0.30 ppb), Os (3.9 ppb), Ir (3.6 ppb), Ru (5.4 ppb), Pt (6.4 ppb), and Pd (6.3 ppb) in relative proportions similar to those in average enstatite chondrites, except for Pd, which was enriched over Pt by f30% in the Pyke Hill source compared to enstatite chondrites. From these data, the 190 Pt/ 188 Os of PUM was calculated to be 0.00157. If the HSE abundances in the terrestrial mantle were inherited from chondritic material of a late veneer, the latter should have had the HSE composition of ordinary or enstatite chondrites. Alternatively, if the reenrichment of the mantle with HSE was the result of an open-system transport of material across the core–mantle boundary, it was likely a global event that should have occurred shortly after core formation. D 2004 Elsevier B.V. All rights reserved.

The presence of coupled enrichments in 186 Os/ 188 Os and 187 Os/ 188 Os in some mantle-derived materials reflects longterm elevation of Pt/Os and Re/Os relative to the primitive upper mantle. New Os data for the 89 Ma Gorgona Island, Colombia komatiites indicate that these lavas are also variably enriched in 186 Os and 187 Os, with 186 Os/ 188 Os ranging between 0.1198397 7 22 and 0.1198470 7 38, and with QOs correspondingly ranging from +0.15 to +4.4. These data define a linear trend that converges with the previously reported linear trend generated from data for modern Hawaiian picritic lavas and a sample from the ca. 251 Ma Siberian plume, to a common component with a 186 Os/ 188 Os of approximately 0.119870 and QOs of +17.5. The convergence of these data to this Os isotopic composition may imply a single ubiquitous source in the Earth’s interior that mixes with a variety of different mantle compositions distinguished by variations in QOs. The 187 Os- and 186 Os-enriched component may have been generated via early crystallization of the solid inner core and consequent increases in Pt/Os and Re/Os in the liquid outer core, with time leading to suprachondritic 186 Os/ 188 Os and QOs in the outer core. The presence of Os from the outer core in certain portions of the mantle would require a mechanism that could transfer Os from the outer core to the lower mantle, and thence to the surface. If this is the process that generated the isotopic enrichments in the mantle sources of these plume-derived systems, then the current understanding of solid metal^liquid metal partitioning of Pt, Re and Os requires that crystallization of the inner core began prior to 3.5 Ga. Thus, the Os isotopic data reported here provide a new source of data to better constrain the timing of inner core formation, complementing magnetic field paleointensity measurements as data sources that constrain models based on secular cooling of the Earth. Published by Elsevier Science B.V.

Rhenium–osmium isotopic systematics of primitive calc-alkaline lavas from the Lassen region appear to be controlled by mantle wedge processes. Lavas with a large proportion of slab component have relatively low Re and Os abundances, and have radiogenic Os and mid ocean ridge basalt-like Sr and Pb isotopic compositions. Lavas with a small proportion of slab component have higher Re and Os elemental abundances and display mantle-like Os, Sr, Nd, and Pb isotopic compositions. Assimilation with fractional crystallization can only generate the Re–Os systematics of the Lassen lavas from a common parent if the distribution coefficient for Re in sulfide is ∼40–1100 times higher than most published estimates and if most incompatible element abundances decrease during differentiation. High Re/Os ratios in mid ocean ridge basalts makes subducted oceanic crust a potential source of radiogenic Os in volcanic arcs. The slab beneath the southernmost Cascades is estimated to have 187Os/188Os ratios as high as 1.4. Mixing between a slab component and mantle wedge peridotite can generate the Os isotopic systematics of the Lassen lavas provided the slab component has a Sr/Os ratio of ∼7.5×105 and Os abundances that are 100–600 times higher than mid ocean ridge basalts. For this model to be correct, Os must be readily mobilized and concentrated in the slab component, perhaps as a result of high water and HCl fugacities in this subduction environment. Another possible mechanism to account for the correlation between the magnitude of the subduction geochemical signature and Os isotopic composition involves increasing the stability of an Os-bearing phase in mantle wedge peridotites as a result of fluxing with the slab component. Melting of such a source could yield low Os magmas that are more susceptible to crustal contamination, and hence have more radiogenic Os isotopic compositions, than magmas derived from sources with a smaller contribution from the slab. Thus, the addition of the slab component to the mantle wedge appears to result in either the direct or indirect addition of radiogenic Os to arc magmas.

Abyssal peridotites are normally thought to be residues of melting of the mid-ocean ridge basalt (MORB) source and are presumably a record of processes affecting the upper mantle. Samples from a single section of abyssal peridotite from the Kane Transform area in the Atlantic Ocean were examined for 190Pt–186Os and 187Re–187Os systematics. They have uniform 186Os/188Os ratios with a mean of 0.1198353±7, identical to the mean of 0.1198340±12 for Os–Ir alloys and chromitites believed to be representative of the upper mantle. While the Pt/Os ratios of the upper mantle may be affected locally by magmatic processes, these data show that the Pt/Os ratio for the bulk upper mantle has not deviated by more than about ±30% from a chondritic Pt/Os ratio over 4.5 billion years. These observations are consistent with the addition of a chondritic late veneer after core separation as the primary control on the highly siderophile element budget of the terrestrial upper mantle. The 187Os/188Os of the samples range from 0.12267 to 0.12760 and correlate well with Pt and Pt/Os, but not Re/Os. These relationships may be explained by variable amounts of partial melting with changing DRe, reflecting in part garnet in the residue, with a model-dependent melting age between about 600 and 1700 Ma. A model where the correlation between Pt/Os and 187Os/188Os results from multiple ancient melting events, in mantle peridotites that were later juxtaposed by convection, is also consistent with these data. This melting event or events are evidently unrelated to recent melting under mid-ocean ridges, because recent melting would have disturbed the relationship between Pt/Os and 187Os/188Os. Instead, this section of abyssal peridotite may be a block of refractory mantle that remained isolated from the convecting portions of the upper mantle for 600 Ma to >1 Ga. Alternatively, Pt and Os may have been sequestered during more recent melting and possibly melt/rock reaction processes, thereby preserving an ancient melting history. If representative of other abyssal peridotites, then the rocks from this suite with subchondritic 187Os/188Os are not simple residues of recent MORB source melting at ridges, but instead have a more complex history. This suite of variably depleted samples projects to an undepleted present-day Pt/Os of about 2.2 and 187Os/188Os of about 0.128–0.129, consistent with estimates for the primitive upper mantle.

Primitive Hawaiian picrites have 187Os/188Os as high as ∼0.145 and are more radiogenic than the depleted upper mantle, reflecting a time-integrated suprachondritic Re/Os ratio. The high Re/Os may be explained either by an ancient recycled crustal component or an evolved outer core component in the Hawaiian plume. New high precision 186Os/188Os measurements for these picrites, combined with previous analyses, show that the Hawaiian plume source has 186Os/188Os that range from chondritic mantle values of ∼0.119834 to more radiogenic values as high as ∼0.119848. The higher 186Os/188Os reflects long-term suprachondritic Pt/Os and is coupled with higher 187Os/188Os in all but the Koolau picrites. The latter have near-chondritic 186Os/188Os but with radiogenic 187Os/188Os. The Pt/Re of crustal materials that may make up ancient recycled slabs ranges from ∼0.1 to 33. Recycled slab material with such Pt/Re ratios evolved for 1–3 Ga and added to the plume source may explain the Koolau Os isotopic compositions. Pt/Re ratios of 88–100, however, are required for the ancient recycled crust to generate the coupled enrichments of 186Os/188Os and 187Os/188Os in picrites from Loihi and Hualalai. These high Pt/Re ratios do not occur in any known crustal materials, but they are consistent with the observed partition coefficients for Os>Re>Pt, during metal crystallization from an initially chondritic molten core (Pt/Re ∼21–24). Such partitioning may have produced an evolved outer core with suprachondritic Pt/Os, Re/Os and Pt/Re, resulting in the production of suprachondritic 186Os/188Os and 187Os/188Os over time. Small amounts of outer core metal (≤1.2%) mixed into the Hawaiian plume source can explain the coupled 186Os/188Os and 187Os/188Os enrichment in some of the Hawaiian picrites. In addition, the most radiogenic 186Os/188Os in the Hawaiian picrites is correlated with higher 3He/4He, consistent with an undegassed, and likely, lower mantle source. These data provide compelling geochemical evidence that the Hawaiian plume was generated at the core-mantle boundary.

Most of the recent discussions on the number and kinds of components which can be distinguished in the Columbia River Basalt Group (CRBG) magmas have used arguments developed from isotopic evidence. In this paper, we consider relative contents of excluded trace elements, interpreted by means of spidergrams and abundance ratios, and compare data from the CRBG and related lavas with those of selected oceanic basalts. Although many recent models for CRBG petrogeneses do not include a plume component, spidergrams for the American Bar flows of the Imnaha Basalt of the CRBG have broad humps at Ba, Th, and Nb (Ta). This feature suggests that an enriched mantle plume component is present in the American Bar magmas, an inference which is weakly confirmed by 87Sr/86Sr data. This plume may have been identical to that presently beneath Yellowstone. The inferred hot spot track lies almost directly east-west, and requires a vector of motion of the Pacific Northwest part of the North American plate significantly different from that inferred by Minster and Jordan (1978) [31]. Our proposed vector for the North American plate agrees, however, with that suggested by the Anahim Belt, a volcanic lineament in southern British Columbia that has been proposed as a hot spot track. This proposed vector requires alterations in current models for Pacific and North American plate movement during the Cenozoic.