High-precision Nd/Nd measurements in terrestrial rocks: Constraints on the early differentiation of the Earth’s mantle

We present new ultra-high precision ¹⁴²Nd/¹⁴⁴Nd measurements of early Archaean rocks using the new generation thermal ionization mass spectrometer Triton. Repeated measurements of the Ames Nd standard demonstrate that the ¹⁴²Nd/¹⁴⁴Nd ratio can be determined with external precision of 2 ppm (2σ), allowing confident resolution of anomalies as small as 5 ppm. A major analytical improvement lies in the elimination of the double normalization procedure required to correct our former measurements from a secondary mass fractionation effect. Our new results indicate that metasediments, metabasalts, and orthogneisses from the 3.6 to 3.8 Ga West Greenland craton display positive ¹⁴²Nd anomalies ranging from 8 to 15 ppm. Using a simple two-stage model with an initial ε¹⁴³Nd value of 1.9 ± 0.6 ε-units, coupled ¹⁴⁷Sm-¹⁴³Nd and ¹⁴⁶Sm-¹⁴²Nd chronometry constrains mantle differentiation to 50-200 Ma after formation of the solar system. This chronological constraint is consistent with differentiation of the Earth’s mantle during the late stage of crystallization of a magma ocean. We have developed a two-box model describing ¹⁴²Nd and ¹⁴³Nd isotopic evolution of depleted mantle during the subsequent evolution of the crust-mantle system. Our results indicate that early terrestrial protocrust had a lifetime of ca. 0.7-1 Ga in order to produce the observed Nd isotope signature of Archaean rocks. In the context of this two box mantle-crust system, we model the evolution of isotopic and chemical heterogeneity of depleted mantle as a function of the mantle stirring time. Using the dispersion of ¹⁴²Nd/¹⁴⁴Nd and ¹⁴³Nd/¹⁴⁴Nd ratios observed in early Archaean rocks, we constrain the stirring time of early Earth’s mantle to 100-250 Ma, a factor of 5 shorter than the stirring time inferred from modern oceanic basalts.     

Non-chondritic Sm/Nd ratio in the terrestrial planets: Consequences for the geochemical evolution of the mantle-crust system

Super-chondritic ¹⁴²Nd signatures are ubiquitous in terrestrial, Martian and lunar samples, and indicate that the terrestrial planets may have accreted from material with Sm/Nd ratio higher than chondritic. This contradicts the long-held view that chondrites represent a reference composition for the ¹⁴⁷Sm-¹⁴³Nd system. Using coupled ¹⁴⁶Sm-¹⁴²Nd and ¹⁴⁷Sm-¹⁴³Nd systematics in planetary samples, we have proposed a new set of values for the ¹⁴⁷Sm/¹⁴⁴Nd and ¹⁴³Nd/¹⁴⁴Nd ratios of the bulk silicate Earth (Caro et al., 2008). Here, we revise the Bulk Silicate Earth estimates for the ⁸⁷Rb-⁸⁷Sr and ¹⁷⁶Lu-¹⁷⁶Hf systems using coupled Sr-Nd-Hf systematics in terrestrial rocks. These estimates are consistent with Hf-Nd systematics in lunar samples. The implications of a slightly non-chondritic silicate Earth with respect to the geochemical evolution of the mantle-crust system are then examined. We show that the Archean mantle has evolved with a composition indistinguishable from that of the primitive mantle until about 2 Gyr. Positive ε¹⁴³Nd and ε¹⁷⁶Hf values ubiquitous in the Archean mantle are thus accounted for by the non-chondritic Sm/Nd and Lu/Hf composition of the primitive mantle rather than by massive early crustal formation, which solves the paradox that early Archean domains only have a limited extension in the present-day continents. The Sm-Nd and Lu-Hf evolution of the depleted mantle for the past 3.5 Gyr can be entirely explained by continuous extraction of the continents from a well-mixed mantle. Thus, in contrast to the chondritic Earth model, Sm-Nd mass balance relationships can be satisfied without the need to call upon hidden reservoirs or layered mantle convection. This new Sm-Nd mass balance yields a scenario of mantle evolution consistent with trace element and noble gas systematics. The high ³He/⁴He mantle component is associated with ¹⁴³Nd/¹⁴⁴Nd compositions indistinguishable from the bulk silicate Earth, suggesting that the less degassed mantle sources did not experience significant fractionation for moderately incompatible elements.     

Re-evaluating Nd/Nd in lunar mare basalts with implications for the early evolution and bulk Sm/Nd of the Moon

The Moon likely accreted from melt and vapor ejected during a cataclysmic collision between Proto-Earth and a Mars-sized impactor very early in solar system history. The identical W, O, K, and Cr isotope compositions between materials from the Earth and Moon require that the material from the two bodies were well-homogenized during the collision process. As such, the ancient isotopic signatures preserved in lunar samples provide constraints on the bulk composition of the Earth. Two recent studies to obtain high-precision ¹⁴²Nd/¹⁴⁴Nd ratios of lunar mare basalts yielded contrasting results. In one study, after correction of neutron fluence effects imparted to the Nd isotope compositions of the samples, the coupled ¹⁴²Nd-¹⁴³Nd systematics were interpreted to be consistent with a bulk Moon having a chondritic Sm/Nd ratio [Rankenburg K., Brandon A. D. and Neal C. R. (2006) Neodymium isotope evidence for a chondritic composition of the Moon. Science312, 1369-1372]. The other study found that their data on the same and similar lunar mare basalts were consistent with a bulk Moon having a superchondritic Sm/Nd ratio [Boyet M. and Carlson R. W. (2007) A highly depleted Moon or a non-magma origin for the lunar crust? Earth Planet. Sci. Lett.262, 505-516]. Delineating between these two potential scenarios has key ramifications for a comprehensive understanding of the formation and early evolution of the Moon and for constraining the types of materials available for accretion into large terrestrial planets such as Earth.To further examine this issue, the same six lunar mare basalt samples measured in Rankenburg et al. [Rankenburg K., Brandon A. D. and Neal C. R. (2006) Neodymium isotope evidence for a chondritic composition of the Moon. Science312, 1369-1372] were re-measured for high-precision Nd isotopes using a multidynamic routine with reproducible internal and external precisions to better than ±3 ppm (2σ) for ¹⁴²Nd/¹⁴⁴Nd ratios. The measurements were repeated in a distinct second analytical campaign to further test their reproducibility. Evaluation of accuracy and neutron fluence corrections indicates that the multidynamic Nd isotope measurements in this study and the 3 in Boyet and Carlson [Boyet M. and Carlson R. W. (2007) A highly depleted Moon or a non-magma origin for the lunar crust? Earth Planet. Sci. Lett.262, 505-516] are reproducible, while static measurements in the previous two studies show analytical artifacts and cannot be used at the resolution of 10 ppm to determine a bulk Moon with either chondritic or superchondritic Sm/Nd ratios. The multidynamic data are best explained by a bulk Moon with a superchondritic Sm/Nd ratio that is similar to the present-day average for depleted MORB. Hafnium isotope data were collected on the same aliquots measured for their ¹⁴²Nd/¹⁴⁴Nd isotope ratios in order to assess if the correlation line for ¹⁴²Nd-¹⁴³Nd systematics reflect mixing processes or times at which lunar mantle sources formed. Based on the combined ¹⁴²Nd-¹⁴³Nd-¹⁷⁶Hf obtained we conclude that the ¹⁴²Nd-¹⁴³Nd correlation line measured in this study is best interpreted as an isochron with an age of 229⁺²⁴₋₂₀Ma after the onset of nebular condensation. The uncertainties in the data permit the sources of these samples to have formed over a 44 Ma time interval. These new results for lunar mare basalts are thus consistent with a later Sm-Nd isotope closure time of their source regions than some recent studies have postulated, and a superchondritic bulk Sm/Nd ratio of the Moon and Earth. The superchondritic Sm/Nd signature was inherited from the materials that accreted to make up the Earth-Moon system. Although collisional erosion of crust from planetesimals is favored here to remove subchondritic Sm/Nd portions and drive the bulk of these bodies to superchondritic in composition, removal of explosive basalt material via gravitational escape from such bodies, or chondrule sorting in the inner solar system, may also explain the compositional features that deviate from average chondrites that make up the Earth-Moon system. This inferred superchondritic nature for the Earth similar to the modern convecting mantle means that there is no reason to invoke a missing, subchondritic reservoir to mass balance the Earth back to chondritic for Sm/Nd ratios. However, to account for the subchondritic Sm/Nd ratios of continental crust, a second superchondritic Sm/Nd mantle reservoir is required.     

Primitive off-rift basalts from Iceland and Jan Mayen: Os-isotopic evidence for a mantle source containing enriched subcontinental lithosphere

New measurements of Os, He, Sr and Nd isotopes, along with major and trace elements, are presented for basalts from the three volcanic flank zones in Iceland and from Jan Mayen Island. The ¹⁸⁷Os/¹⁸⁸Os ratios in lavas with <30 ppt Os (n = 4) are elevated compared to ratios in coexisting olivine and appear to be contaminated at a shallow level. The ¹⁸⁷Os/¹⁸⁸Os ratios in the remaining lavas with >30 ppt Os (n = 17) range between 0.12117 and 0.13324. These values are surprisingly low for oceanic island basalts and include some samples that are less than putative present-day primitive upper mantle (PUM with ¹⁸⁷Os/¹⁸⁸Os of 0.1296). These low ¹⁸⁷Os/¹⁸⁸Os preclude significant shallow-level contamination from oceanic crust. The ¹⁸⁷Os/¹⁸⁸Os ratios for Jan Mayen lavas are less than PUM, severely limiting the presence of any continental crust in their mantle source. A positive correlation between ¹⁴³Nd/¹⁴⁴Nd and ¹⁸⁷Os/¹⁸⁸Os ratios in Iceland and Jan Mayen lavas likely reflects the presence in their source of ancient subcontinental lithosphere that has undergone incompatible trace element enrichment that did not affect the Re-Os system. In addition, the Jan Mayen lava isotopic signature cannot be explained solely by the presence of subcontinental lithospheric mantle, and the influence of another geochemical component, such as a mantle plume appears required. Combined ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, ³He/⁴He and ¹⁸⁷Os/¹⁸⁸Os data indicate a genetic relationship between Jan Mayen Island and the Iceland mantle plume. Material from the Iceland mantle plume likely migrates at depth until it reaches the tensional setting of the Jan Mayen Fracture Zone, where it undergoes low-degree partial melting. At a first-order, isotopic co-variations can be interpreted as broadly binary mixing curves between two primary end-members. One end-member, characterized in particular by its unradiogenic ¹⁸⁷Os/¹⁸⁸Os and ¹⁴³Nd/¹⁴⁴Nd, low ³He/⁴He and high ⁸⁷Sr/⁸⁶Sr, is represented by subcontinental lithospheric mantle stranded and disseminated in the upper mantle during the opening of the Atlantic Ocean. The second end-member corresponds to a hybrid mixture between the depleted-MORB mantle and the enriched Iceland mantle plume, itself resulting from mixing between recycled oceanic crust and depleted lower mantle. This hybrid accounts for the high ³He/⁴He (∼28 Ra), high ¹⁴³Nd/¹⁴⁴Nd (∼0.5132), high ¹⁸⁷Os/¹⁸⁸Os (∼0.14) and low ⁸⁷Sr/⁸⁶Sr (∼0.7026) composition observed in Iceland. Two different models may account for these observed mixing relationships between the end-members. In this first model, the Iceland mantle entrains pristine depleted material when rising in the upper mantle and allows refractory sub-lithospheric fragments to melt because of excess heat derived from the deep plume material. A second model that may better account for the Pb isotopic variations observed, uses the same components but where the depleted-MORB mantle is already polluted by subcontinental lithospheric mantle material before mixing with the Iceland mantle plume. Both cases likely occur. Though only three principal components are required to explain the isotopic variations of the Iceland-Jan Mayen system, the different possible mixing relationships may be accounted for by potentially a greater number of end-members.     

Subduction and melting processes inferred from U-Series, Sr-Nd-Pb isotope, and trace element data, Bicol and Bataan arcs, Philippines

We present U-series, Sr-Nd-Pb isotope, and trace element data from the two principal volcanic chains on Luzon Island, developed over oppositely dipping subduction zones, to explore melting and mass transfer processes beneath arcs. The Bataan (western) and Bicol (eastern) arcs are currently subducting terrigenous and pelagic sediments, respectively, which have different trace element and isotopic compositions. The range of (²³⁰Th/²³⁸U) disequilibria for both arcs is 0.85-1.15; only lavas from Mt. Mayon (Bicol arc) have ²³⁰Th activity excesses. Bataan lavas have higher ⁸⁷Sr/⁸⁶Sr and lower ¹⁴³Nd/¹⁴⁴Nd than Bicol lavas (⁸⁷Sr/⁸⁶Sr = 0.7042-0.7046, ¹⁴³Nd/¹⁴⁴Nd = 0.51281-0.51290 vs. ⁸⁷Sr/⁸⁶Sr = 0.70371-0.70391, ¹⁴³Nd/¹⁴⁴Nd = 0.51295-0.51301) and both arcs show steep linear arrays towards sediment values on ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb diagrams. Analysis of incompatible element and isotopic data allows identification of a sediment component that, at least in part, was transferred as a partial melt to the mantle wedge peridotite. Between 1% and 5% sediment melt addition can explain the isotopic and trace element variability in the rocks from both arcs despite the differences in sediment supply. We therefore propose that sediment transfer to the mantle wedge is likely mechanically or thermally limited. It follows that most sediments are either accreted, reside in the sub-arc lithosphere, or are recycled into the convecting mantle. However, whole-sale sediment recycling into the upper mantle is unlikely in light of the global mid-ocean ridge basalt data. Fluid involvement is more difficult to characterize, but overall the Bicol arc appears to have more fluid influence than the Bataan arc. Rock suites from each arc can be related by a dynamic melting process that allows for ²³⁰Th ingrowth, either by dynamic or continuous flux melting, provided the initial (²³⁰Th/²³²Th) of the source is ∼0.6-0.7. The implication of either model is that inclined arrays on the U-Th equiline diagram may not have chronologic significance. Modeling also suggests that U-series disequilibria are influenced by the tectonic convergence rate, which dictates mantle matrix flow. Thus with slower matrix flow there is a greater degree of ²³⁰Th ingrowth. While other factors such as prior mantle depletion and addition of a subducted component may explain some aspects of U-series data, an overall global correlation between tectonic convergence rate and the extent of U-Th disequilibria may originate from melting processes.     

Pt-Re-Os and Sm-Nd isotope and HSE and REE systematics of the 2.7 Ga Belingwe and Abitibi komatiites

High-precision Pt-Re-Os and Sm-Nd isotope and highly siderophile element (HSE) and rare earth element (REE) abundance data are reported for two 2.7 b.y. old komatiite lava flows, Tony’s flow (TN) from the Belingwe greenstone belt, Zimbabwe, and the PH-II flow (PH) from Munro Township in the Abitibi greenstone belt, Canada. The emplaced lavas are calculated to have contained ∼25% (TN) and ∼28% (PH) MgO. These lavas were derived from mantle sources characterized by strong depletions in highly incompatible lithophile trace elements, such as light REE (Ce/Sm_N = 0.64 ± 0.02 (TN) and 0.52 ± 0.01 (PH), ε¹⁴³Nd(T) = +2.9 ± 0.2 in both sources). ¹⁹⁰Pt-¹⁸⁶Os and ¹⁸⁷Re-¹⁸⁷Os isochrons generated for each flow yield ages consistent with respective emplacement ages obtained using other chronometers. The calculated precise initial ¹⁸⁶Os/¹⁸⁸Os = 0.1198318 ± 3 (TN) and 0.1198316 ± 5 (PH) and ¹⁸⁷Os/¹⁸⁸Os = 0.10875 ± 17 (TN) and 0.10873 ± 15 (PH) require time-integrated ¹⁹⁰Pt/¹⁸⁸Os and ¹⁸⁷Re/¹⁸⁸Os of 0.00178 ± 11 and 0.407 ± 8 (TN) and 0.00174 ± 18 and 0.415 ± 5 (PH). These parameters, which by far represent the most precise and accurate estimates of time-integrated Pt/Os and Re/Os of the Archean mantle, are best matched by those of enstatite chondrites. The data also provide evidence for a remarkable similarity in the composition of the sources of these komatiites with respect to both REE and HSE. The calculated absolute HSE abundances in the TN and PH komatiite sources are within or slightly below the range of estimates for the terrestrial Primitive Upper Mantle (PUM). Assuming a chondritic composition of the bulk silicate Earth, the strong depletions in LREE, yet chondritic Re/Os in the komatiite sources are apparently problematic because early Earth processes capable of fractionating the LREE might also be expected to fractionate Re/Os. This apparent discrepancy could be reconciled via a two-stage model, whereby the moderate LREE depletion in the sources of the komatiites initially occurred within the first 100 Ma of Earth’s history as a result of either global magma ocean differentiation or extraction and subsequent long-term isolation of early crust, whereas HSE were largely added subsequently via late accretion. The komatiite formation, preceded by derivation of basaltic magmas, was a result of second-stage, large-degree dynamic melting in mantle plumes.     

Correlated nucleosynthetic isotopic variability in Cr, Sr, Ba, Sm, Nd and Hf in Murchison and QUE 97008

Acid leaching of the primitive C-chondrite Murchison and O-chondrite QUE 97008 reveal nucleosynthetic anomalies in Cr, Sr, Ba, Nd, Sm and Hf. The anomalies in all but Cr and Sm are best explained by variable additions of pure s-process nuclides to a background nebular composition slightly enriched in r-process isotopes compared to average Solar System material. Leaching leaves a residue in Murchison that is strongly enriched in s-process nuclides with depletions of over 0.1% in ¹³⁵Ba and seven parts in 10,000 in ⁸⁴Sr. If there are p-process anomalies in these two elements, they are lost in the variability caused by different r-, s-process contributions to the normalizing isotopes. The concentration and isotope systematics are consistent with the Ba and Sr isotopic composition in the Murchison residue being strongly influenced by s-process-rich presolar SiC. In general, the nucleosynthetic isotope anomalies are 2- to 5-fold smaller in QUE 97008 than in Murchison. The different magnitudes of isotope anomalies are similar to the difference in matrix abundance between CM and O chondrites consistent with the suggestion that the carriers of nucleosynthetically anomalous material preferentially reside in the matrix and that some of this material has been distributed throughout the O-chondrite minerals as a result of thermal metamorphism.Neodymium, Sm and Hf display variable s-, r-process nuclide abundances as in Ba and Sr, but the anomalies are much smaller (e.g. ε¹⁴⁸Nd, ε¹⁴⁸Sm = −5.7, 2.1, respectively, in Murchison and −0.43, 0.16, respectively in QUE 97008 residues). After correcting Nd and Sm for s-, r-process variability, Sm in whole rock chondrites shows variable relative abundances of the p-process isotope ¹⁴⁴Sm that correlate weakly with ¹⁴²Nd suggesting that the direct p-process contribution to ¹⁴²Nd is small (∼7-9%). Nucleosynthetic variability in Nd explains the range in ¹⁴²Nd/¹⁴⁴Nd seen between C and O, E-chondrites, but not the difference between chondrites and all modern Earth rocks, leaving decay of ¹⁴⁶Sm and a superchondritic Sm/Nd ratio as the likely explanation for Earth’s high ¹⁴²Nd/¹⁴⁴Nd.     

Nd and Sr isotope compositions in modern and fossil bones - Proxies for vertebrate provenance and taphonomy

Rare earth elements (REE), while not essential for the physiologic functions of animals, are ingested and incorporated in ppb concentrations in bones and teeth. Nd isotope compositions of modern bones of animals from isotopically distinct habitats demonstrate that the ¹⁴³Nd/¹⁴⁴Nd of the apatite can be used as a fingerprint for bedrock geology or ambient water mass. This potentially allows the provenance and migration of extant vertebrates to be traced, similar to the use of Sr isotopes. Although REE may be enriched by up to 5 orders of magnitude during diagenesis and recrystallization of bone apatite, in vivo¹⁴³Nd/¹⁴⁴Nd may be preserved in the inner cortex of fossil bones or enamel. However, tracking the provenance of ancient or extinct vertebrates is possible only for well-preserved archeological and paleontological skeletal remains with in vivo-like Nd contents at the ppb-level. Intra-bone and -tooth REE analysis can be used to screen for appropriate areas. Large intra-bone Nd concentration gradients of 10¹-10³ are often measured. Nd concentrations in the inner bone cortex increase over timescales of millions of years, while bone rims may be enriched over millenial timescales. Nevertheless, ε_Nd values are often similar within one ε_Nd unit within a single bone. Larger intra-bone differences in specimens may either reflect a partial preservation of in vivo values or changing ε_Nd values of the diagenetic fluid during fossilization. However, most fossil specimens and the outer rims of bones will record taphonomic ¹⁴³Nd/¹⁴⁴Nd incorporated post mortem during diagenesis. Unlike REE patterns, ¹⁴³Nd/¹⁴⁴Nd are not biased by fractionation processes during REE-uptake into the apatite crystal lattice, hence the ε_Nd value is an important tracer for taphonomy and reworking. Bones and teeth from autochthonous fossil assemblages have small variations of ±1 ε_Nd unit only. In contrast, fossil bones and teeth from over 20 different marine and terrestrial fossil sites have a total range of ε_Nd values from -13.0 to 4.9 (n = 80), often matching the composition of the embedding sediment. This implies that the surrounding sediment is the source of Nd in the fossil bones and that the specimens of this study seem not to have been reworked. Differences in ε_Nd values between skeletal remains and embedding sediment may either indicate reworking of fossils and/or a REE-uptake from a diagenetic fluid with non-sediment derived ε_Nd values. The latter often applies to fossil shark teeth, which may preserve paleo-seawater values. Complementary to ε_Nd values, ⁸⁷Sr/⁸⁶Sr can help to further constrain the fossil provenance and reworking.     

Chemical and chronologic complexity in the convecting upper mantle: Evidence from the Taitao ophiolite, southern Chile

Exposure of the ca. 6 Ma Taitao ophiolite, Chile, located 50 km south of the Chile Triple Junction, allows detailed chemical and isotopic study of rocks that were recently extracted from the depleted mantle source of mid-ocean ridge basalts (DMM). Ultramafic and mafic rocks are examined for isotopic (Os, Sr, Nd, and O), and major and trace element compositions, including the highly siderophile elements (HSE). Taitao peridotites have compositions indicative of variable extents of partial melting and melt extraction. Low δ¹⁸O values for most whole rock samples suggest some open-system, high-temperature water–rock interaction, most likely during serpentinization, but relict olivine grains have δ¹⁸O values consistent with primary mantle values. Most of the peridotites analyzed for Nd–Sr isotopes have compositions consistent with estimates for the modern DMM, although several samples are characterized by ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd indicative of crustal contamination, most likely via interactions with seawater. The peridotites have initial ¹⁸⁷Os/¹⁸⁸Os ratios that range widely from 0.1168 to 0.1288 (γ_Os = −8.0 to +1.1), averaging 0.1239 (γ_Os = −2.4), which is comparable to the average for modern abyssal peridotites. A negative correlation between the Mg^# of relict olivine grains and Os isotopic compositions of whole rock peridotites suggests that the Os isotopic compositions reflect primary mantle Re/Os fractionation produced by variable extents of partial melting at approximately 1.6 Ga. Recent re-melting at or near the spatially associated Chile Ridge further modified these rocks, and Re, and minor Pt and Pd were subsequently added back into some rocks by late-stage melt–rock or fluid–rock interactions.In contrast to the peridotites, approximately half of the mafic rocks examined have whole rock δ¹⁸O values within the range of mantle compositions, and their Nd and Sr isotopic compositions are all generally within the range of modern DMM. These rocks have initial ¹⁸⁷Os/¹⁸⁸Os ratios, calculated for 6 Ma, that range from 0.126 (γ_Os = −1) to as high as 0.561 (γ_Os = +342). The Os isotopic systematics of each of these rocks may reflect derivation from mixed lithologies that include the peridotites, but may also include pyroxenites with considerably more radiogenic Os than the peridotites. This observation supports the view that suprachondritic Os present in MORB derives from mixed mantle source lithologies, accounting for some of the worldwide dichotomy in ¹⁸⁷Os/¹⁸⁸Os between MORB and abyssal peridotites.The collective results of this study suggest that this >500 km³ block of the mantle underwent at least two stages of melting. The first stage occurred at 1.6 Ga, after which the block remained isolated and unmixed within the DMM. A final stage of melting recently occurred at or near the Chile Ridge, resulting in the production of at least some of the mafic rocks. Convective stirring of this mantle domain during a >1 Ga period was remarkably inefficient, at least with regard to Os isotopes.     

Os and Os enrichments and high-He/He sources in the Earth’s mantle: Evidence from Icelandic picrites

Picrites from the neovolcanic zones in Iceland display a range in ¹⁸⁷Os/¹⁸⁸Os from 0.1297 to 0.1381 (γ_Os = + 2.1 to +8.7) and uniform ¹⁸⁶Os/¹⁸⁸Os of 0.1198375 ± 32 (2σ). The value for ¹⁸⁶Os/¹⁸⁸Os is within uncertainty of the present-day value for the primitive upper mantle of 0.1198398 ± 16. These Os isotope systematics are best explained by ancient recycled crust or melt enrichment in the mantle source region. If so, then the coupled enrichments displayed in ¹⁸⁶Os/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os from lavas of other plume systems must result from an independent process, the most viable candidate at present remains core-mantle interaction. While some plumes with high ³He/⁴He, such as Hawaii, appear to have been subjected to detectable addition of Os (and possibly He) from the outer core, others such as Iceland do not.A positive correlation between ¹⁸⁷Os/¹⁸⁸Os and ³He/⁴He from 9.6 to 19 Ra in Iceland picrites is best modeled as mixtures of 1 Ga or older ancient recycled crust mixed with primitive mantle or incompletely degassed depleted mantle isolated since 1-1.5 Ga, which preserves the high ³He/⁴He of the depleted mantle at the time. These mixtures create a hybrid source region that subsequently mixes with the present-day convecting MORB mantle during ascent and melting. This multistage mixing scenario requires convective isolation in the deep mantle for hundreds of million years or more to maintain these compositionally distinct hybrid sources. The ³He/⁴He of lavas derived from the Iceland plume changed over time, from a maximum of 50 Ra at 60 Ma, to approximately 25-27 Ra at present. The changes are coupled with distinct compositional gaps between the different aged lavas when ³He/⁴He is plotted versus various geochemical parameters such as ¹⁴³Nd/¹⁴⁴Nd and La/Sm. These relationships can be interpreted as an increase in the proportion of ancient recycled crust in the upwelling plume over this time period.The positive correlation between ¹⁸⁷Os/¹⁸⁸Os and ³He/⁴He demonstrates that the Iceland lava He isotopic compositions do not result from simple melt depletion histories and consequent removal of U and Th in their mantle sources. Instead their He isotopic compositions reflect mixtures of heterogeneous materials formed at different times with different U and Th concentrations. This hybridization is likely prevalent in all ocean island lavas derived from deep mantle sources.     

Crustal contamination and mantle source characteristics in continental intra-plate volcanic rocks: Pb, Hf and Os isotopes from central European volcanic province basalts

We report new Os-Pb-Hf isotope data for a suite of alkaline to basaltic (nephelinites, basanites, olivine tholeiites to quartz-tholeiites) lavas from the Miocene Vogelsberg (Germany), the largest of the rift-related continental volcanic complexes of the Central European Volcanic Province (CEVP). ¹⁸⁷Os/¹⁸⁸Os in primitive (high-MgO) alkaline lavas show a much wider range than has been observed in alkaline basalts and peridotite xenoliths from elsewhere in the CEVP, from ratios similar to those in modern MORB and OIB (0.1260-0.1451; 58.9-168 ppt Os) to more radiogenic ratios (0.1908 and 0.2197; 27.6-15.1 ppt Os). Radiogenic Os is associated with high ε_Hf and ε_Nd, low ⁸⁷Sr/⁸⁶Sr and does not correlate with Mg^∗ or incompatible trace elements (e.g. Ce/Pb), suggesting the presence of a radiogenic endmember in the mantle rather than crustal contamination as the source of radiogenic Os. This contrasts with another high-Mg alkaline lava characterized by highly radiogenic ¹⁸⁷Os/¹⁸⁸Os (0.4344, 10.3 ppt Os), lower ε_Hf and ε_Nd, higher ⁸⁷Sr/⁸⁶Sr, and Pb isotope signatures than the other alkaline lavas with similar trace element composition suggestive of contamination with crustal material. Hafnium (ε_Hf: +8.9 to +5.0) and Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb: 19.10-19.61; ²⁰⁷Pb/²⁰⁴Pb: 15.56-15.60) of the alkaline rocks fall within the range of enriched MORB and some OIB. The Vogelsberg tholeiites show even more diverse ¹⁸⁷Os/¹⁸⁸Os, ranging from 0.1487 in Os-rich olivine tholeiite (31.7 ppt) to ratios as high as 0.7526 in other olivine-tholeiites and in quartz-tholeiites with lower Os concentrations (10.3-2.0 ppt). Low-¹⁸⁷Os/¹⁸⁸Os tholeiites show Pb-Hf isotope ratios (²⁰⁶Pb/²⁰⁴Pb:18.81; ²⁰⁷Pb/²⁰⁴Pb: 15.61; ε_Hf: +2.7) that are distinct from those in alkaline lavas with similar ¹⁸⁷Os/¹⁸⁸Os and originate from a different mantle source. By contrast, the combination of radiogenic Os and low ²⁰⁶Pb/²⁰⁴Pb and ε_Hf in the other tholeiites probably reflects crustal contamination.The association at Vogelsberg of primitive alkaline and tholeiitic lavas with a range of MORB- to OIB-like Os-Pb-Hf-Nd-Sr isotopic characteristics requires at least two asthenospheric magma sources. This is consistent with trace element modelling which suggests that the alkaline and tholeiitic parent magmas represent mixtures of melts from garnet and spinel peridotite sources (both with amphibole), implying an origin of the magmas in the garnet peridotite-spinel peridotite transition zone, probably at the asthenosphere-lithosphere interface. We propose that uncontaminated Vogelsberg lavas originated in ‘metasomatized’ mantle, involving a 3-stage model: (1) early carbonatite metasomatism several 10-100 Ma before the melting event (2) deposition of low-degree asthenospheric melts from carbonated peridotite at the lithosphere-asthenosphere thermal boundary produces hydrous amphibole-bearing veins or patches, and (3) remobilization of this modified lithospheric mantle into other asthenospheric melts passing through the same area later. In keeping with ‘metasomatized’ mantle models for other continental basalt provinces, we envisage that stage (2) is short-lived (few Ma), thus producing a prominent lithospheric trace element signature without changing the asthenospheric isotopic signatures. Models of this type can explain the peculiar mix of lithospheric (prominent depletions of Rb and K) and asthenospheric (OIB-like high ¹⁸⁷Os/¹⁸⁸Os, ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf) signatures observed in the Vogelsberg and many other continental basalt suites.     

Osmium mass balance in peridotite and the effects of mantle-derived sulphides on basalt petrogenesis

Analyses of enriched mantle (EM)-basalts, using lithophile element-based isotope systems, have long provided evidence for discrete mantle reservoirs with variable composition. Upon partial melting, the mantle reservoir imparts its isotopic fingerprint upon the partial melt produced. However, it has increasingly been recognised that it may not be simple to delimit these previously well-defined mantle reservoirs; the “mantle zoo” may contain more reservoirs than previously envisaged.Here we demonstrate that a simple model with varying contributions from two populations of compositionally distinct mantle sulphides can readily account for the observed heterogeneities in Os isotope systematics of such basalts without additional mantle reservoirs. Osmium elemental and isotopic analyses of individual sulphide grains separated from spinel lherzolites from Kilbourne Hole, New Mexico, USA demonstrate that two discrete populations of mantle sulphide exist in terms of both Re-Os systematics and textural relationship with co-existing silicates. One population, with a rounded morphology, is preserved in silicate grains and typically possesses high [Os] and low [Re] with unradiogenic, typically sub-chondritic ¹⁸⁷Os/¹⁸⁸Os attributable to long term isolation in a low-Re environment. By contrast, irregular-shaped sulphides, preserved along silicate grain boundaries, possess low [Os], higher [Re] and a wider range of, but generally supra-chondritic ¹⁸⁷Os/¹⁸⁸Os ([Os] typically ? 1-2 ppm, ¹⁸⁷Os/¹⁸⁸Os ? 0.3729; this study). This population is thought to represent metasomatic sulphide.Uncontaminated silicate phases contain negligible Os (<100 ppt) therefore the Os elemental and isotope composition of basalts is dominated by volumetrically insignificant sulphide ([Os] ? 37 ppm; this study). During the early stages of partial melting, supra-chondritic interstitial sulphides are mobilised and incorporated into the melt, adding their radiogenic ¹⁸⁷Os/¹⁸⁸Os signature. Only when sulphides armoured within silicates are exposed to the melt through continued partial melting will enclosed sulphides add their high [Os] and unradiogenic ¹⁸⁷Os/¹⁸⁸Os to the aggregate melt. Platinum-group element data for whole rocks are also consistent with this scenario. The sequence of (i) addition of all of the metasomatic sulphide, followed by (ii) the incorporation of small amounts of armoured sulphide can thus account for the range of both [Os] and ¹⁸⁷Os/¹⁸⁸Os of EM-basalts worldwide without the need for contributions from additional silicate mantle reservoirs.     

Nd, Sr and Os isotope systematics in young, fertile spinel peridotite xenoliths from northern Queensland, Australia: A unique view of depleted MORB mantle?

Northeastern Queensland, a part of the Phanerozoic composite Tasman Fold Belt of eastern Australia, has a Paleozoic to Mesozoic history dominated by subduction zone processes. A suite of 13 peridotite xenoliths from the <3 Ma Atherton Tablelands Volcanic Province, predominantly from Mount Quincan, comprise fertile (1.8-3.4 wt.% Al₂O₃ and 38.7-41.9 wt.% MgO) spinel lherzolites free from secondary volatile-bearing phases and with only weak metasomatic enrichment of incompatible trace elements (Sm_N/Yb_N = 0.23-1.1; La_N/Yb_N = 0.11-4.9). The suite is isotopically heterogeneous, with measured Sr (⁸⁷Sr/⁸⁶Sr = 0.7027-07047), Nd (¹⁴³Nd/¹⁴⁴Nd = 0.51249-0.51362), and to a lesser extent, Os (¹⁸⁷Os/¹⁸⁸Os = 0.1228-0.1292) compositions broadly overlapping MORB source mantle (DMM) and extending to more depleted compositions, reflecting evolution in a time-integrated depleted reservoir. Major and rare earth element systematics are consistent with mantle that is residual after low to moderate degrees of melt extraction predominantly in the spinel facies, but with a few samples requiring partial melting at greater pressures in the garnet field or near the garnet-spinel transition. In contrast to most previously studied suites of continental lithospheric mantle samples, the incompatible trace element contents and Sr and Nd isotopic systematics of these samples suggest only minimal modification of the sampled lithosphere by metasomatic processes.Five of six Mount Quincan xenoliths preserving depleted middle to heavy REE patterns form a whole rock Sm-Nd isochron with an age of ∼275 Ma (ε_Ndi = +9), coincident with widespread granitoid emplacement in the overlying region. This isochron is interpreted to indicate the timing of partial melting of a DMM-like source. Xenoliths from other Atherton localities scatter about the isochron, suggesting that the sampled mantle represents addition of DMM mantle to the lithosphere in the Permian, when the region may have broadly been within a subduction zone setting. A sixth middle to heavy REE-depleted Mount Quincan xenolith has a distinct Nd and Os isotopic composition consistent either with an earlier, possibly Precambrian melt extraction event, or with Permian derivation from a mantle source with a less depleted (time-averaged lower Sm/Nd) Nd isotopic composition, but a more depleted (low Re/Os) Os isotopic composition.The range in measured whole rock Os isotopic compositions cannot solely be the result of time-integrated effects of variable melt extraction, especially considering the coherent Sm-Nd systematics of the suite. The Os heterogeneity more likely reflects either a heterogeneous ∼275 Ma DMM source that would have a present-day Os composition (¹⁸⁷Os/¹⁸⁸Os ∼ 0.1265-0.1287) overlapping both abyssal peridotites and chondrites, or significant and variable enrichment within the lithospheric mantle by secondary sulfides carrying radiogenic Os in a cryptic chalcophile enrichment event. Regardless of the origin of the Os isotopic variability, these data highlight the mantle Re-Os isotopic heterogeneity that may be present over small length scales where the lithophile Sm-Nd system may be relatively homogeneous.     

Application of precise 142Nd/144Nd analysis of small samples to inclusions in diamonds (Finsch,South Africa) and Hadean Zircons (Jack Hills,Western Australia)

¹⁴⁶Sm–¹⁴²Nd and ¹⁴⁷Sm–¹⁴³Nd systematics were investigated in garnet inclusions in diamonds from Finsch (S. Africa) and Hadean zircons from Jack Hills (W. Australia) to assess the potential of these systems as recorders of early Earth evolution. The study of Finsch inclusions was conducted on a composite sample of 50 peridotitic pyropes with a Nd model age of 3.3 Ga. Analysis of the Jack Hills zircons was performed on 790 grains with ion microprobe ²⁰⁷Pb/²⁰⁶Pb spot ages from 3.95 to 4.19 Ga. Finsch pyropes yield 100 × ?¹⁴²Nd = ? 6 ± 12 ppm, ?¹⁴³Nd = ? 32.5, and ¹⁴⁷Sm/¹⁴⁴Nd = 0.1150. These results do not confirm previous claims for a 30 ppm ¹⁴²Nd excess in South African cratonic mantle. The lack of a ¹⁴²Nd anomaly in these inclusions suggests that isotopic heterogeneities created by early mantle differentiation were remixed at a very fine scale prior to isolation of the South African lithosphere. Alternatively, this result may indicate that only a fraction of the mantle experienced depletion during the first 400 Myr of its history. Analysis of the Jack Hills zircon composite yielded 100 × ?¹⁴²Nd = 8 ± 10 ppm, ?¹⁴³Nd = 45 ± 1, and ¹⁴⁷Sm/¹⁴⁴Nd = 0.5891. Back-calculation of this present-day ?¹⁴³Nd yields an unrealistic estimate for the initial ?¹⁴³Nd of ? 160 ?-units, clearly indicating post-crystallization disturbance of the ¹⁴⁷Sm–¹⁴³Nd system. Examination of ^146,147Sm–^142,143Nd data reveals that the Nd budget of the Jack Hills sample is dominated by non-radiogenic Nd, possibly contained in recrystallized zircon rims or secondary subsurface minerals. This secondary material is characterized by highly discordant U–Pb ages. Although the mass fraction of altered zircon is unlikely to exceed 5–10% of total sample, its high LREE content precludes a reliable evaluation of ¹⁴⁶Sm–¹⁴²Nd systematics in Jack Hills zircons.     

Os, Pb, and Nd isotope geochemistry of the Permian Emeishan continental flood basalts: Insights into the source of a large igneous province

The nature of the source of continental flood basalts (CFB) is a highly debated topic. Proposed mantle sources for CFBs, including both high- and low-Ti basalts, include subcontinental lithospheric mantle (SCLM), asthenospheric mantle, and deep, plume-related mantle. Re-Os isotope systematics can offer important constraints on the sources of both ocean island basalts (OIB) and CFB, and may be applied to distinguish different possible melt sources. This paper reports the first Re-Os isotope data for the Late Permian Emeishan large igneous province (LIP) in Southwest China. Twenty one CFB samples including both low- and high-Ti basalts from five representative sites within the Emeishan LIP have been analyzed for Os, Nd, and Pb isotopic compositions. The obtained Os data demonstrate that crustal assimilation affected Os isotopic compositions of some Emeishan basalt samples with low Os concentrations but not all of the samples, and the Emeishan basalts with high Os contents likely experienced the least crustal contamination. The low and high-Ti basalts yield distinct Os signatures in terms of ¹⁸⁷Os/¹⁸⁸Os and Os content. The low-Ti basalt with the highest Os concentration (400 ppt) has a radiogenic Os isotopic composition (γOs(t), +6.5), similar to that of plume-derived OIB. Because the Os isotopic composition of basalts with relatively high Os concentrations (typically >50 ppt) likely represents that of their mantle source, this result implies a plume-derived origin for the low-Ti basalts. On the other hand, the high-Ti basalts with high Os concentration (over 50 ppt) have unradiogenic Os isotopic signatures (γOs(t) values range from −0.8 to −1.4), suggesting that a subcontinental lithosphere mantle (SCLM) component most likely contributed to the generation of these magmas. Combining Pb and Nd isotopic tracers with the Os data, we demonstrate that the low-Ti basaltic magmas in the Emeishan CFB were mainly sourced from a mantle plume reservoir, whereas the high-Ti basaltic magmas were most likely derived from a SCLM reservoir or were contaminated by a significant amount of lithospheric mantle material during plume-related magma ascent through the SCLM.     

A high field strength element perspective on early lunar differentiation

Lunar rocks are inferred to tap the different fossil cumulate layers formed during crystallisation of a lunar magma ocean (LMO). A coherent dataset, including Zr isotope data and high precision HFSE (W, Nb, Ta, Zr, Hf) and REE (Nd, Sm, Lu) data, all obtained by isotope dilution, can now provide new insights into the processes active during LMO crystallisation and during the petrogenesis of lunar magmas. Measured ⁹²Zr and ⁹¹Zr abundances agree with the terrestrial value within 0.2 ε-units. Incompatible-trace-element enriched rocks from the Procellarum KREEP Terrane (PKT) display Nb/Ta and Zr/Hf above the bulk lunar value (ca. 17), and mare basalts display lower ratios, generally confirming the presence of complementary enriched and depleted mantle reservoirs on the Moon. The full compositional spectrum of lunar basalts, however, also requires interaction with ilmenite-rich layers in the lunar mantle. Notably, the high-Ti mare basalts analysed display the lowest Nb/Ta and Zr/Hf of all lunar rocks, and also higher Sm/Nd at similar Lu/Hf than low-Ti basalts. The high-Ti basalts also exhibit higher and strongly correlated Ta/W (up to 25) and Hf/W (up to 140), at similar W contents, which is difficult to reconcile with ortho- and clinopyroxene-controlled melting. Altogether, these patterns can be explained via assimilation of up to ca. 20% of ilmenite- and clinopyroxene-rich LMO cumulates by more depleted melts from the lower lunar mantle. Direct melting of ilmenite-rich cumulates or the possible presence of residual metals in the lunar mantle both cannot easily account for the observed Ta/W and Hf/W patterns. Cumulate assimilation is also a viable mechanism that can partially buffer the Lu/Hf of mare basalts at relatively low values while generating variable Sm/Nd. Thus, the dichotomy between low Lu/Hf of lunar basalts and high time integrated source Lu/Hf as inferred from Hf isotope compositions can potentially be explained. The proposed assimilation model also has important implications for the short-lived nuclide chronology of the Earth-Moon system. The new Hf/W and Ta/W data, together with a compilation of existing W-Th-U data for lunar rocks, indicate that the terrestrial and lunar mantles are indistinguishable in their Hf/W. Virtually identical εW and Hf/W in the terrestrial and lunar mantle suggest a strong link between final core-mantle equilibration on Earth and the Moon forming giant impact. Previously, linear arrays of lunar samples in ¹⁸²W vs. Hf/W and ¹⁴²Nd vs. Sm/Nd spaces have been interpreted as isochrons, arguing for LMO crystallisation as late as 250 Myrs after solar system formation. Based on the proposed assimilation model, the ¹⁸²W and ¹⁴²Nd in many lunar magmas can be shown to be decoupled from their ambient Hf/W and Sm/Nd source compositions. As a consequence, the ¹⁸²W vs. Hf/W and ¹⁴²Nd vs. Sm/Nd arrays would constitute mixing lines rather than isochrons. Hence, the lunar ¹⁸²Hf-¹⁸²W and ¹⁴⁶Sm-¹⁴²Nd data would be fully consistent with an “early” crystallisation age of the LMO, even as early as 50 Myrs after solar system formation when the Moon was probably formed.     

Production and Certification of the Reference Material GSB 04‐3258‐2015 as a 143Nd/144Nd Isotope Ratio Reference

A new certified reference material, labelled GSB 04‐3258‐2015, for use as a ¹⁴³Nd/¹⁴⁴Nd isotope ratio reference has been prepared by the Institute of Geology, Chinese Academy of Geological Sciences, Beijing. Standardization Administration of the People's Republic of China provided the certification for this reference material. This report presents the reference ¹⁴³Nd/¹⁴⁴Nd isotope ratio and supporting production and certification procedures. The reference value was determined by an interlaboratory comparison of results from eleven participating laboratories using MC‐TIMS or MC‐ICP‐MS. The calibration of mass fractionation was conducted by using the exponential law, and the ¹⁴³Nd/¹⁴⁴Nd isotope ratios were normalised to the ¹⁴⁶Nd/¹⁴⁴Nd isotope ratio value of 0.7219. Isobaric interference of ¹⁴⁴Sm on ¹⁴⁴Nd was corrected using an interference‐free ¹⁴⁷Sm/¹⁴⁹Sm isotope ratio value for mass fractionation. GSB 04‐3258‐2015 shows sufficient homogeneity and stability for use as an international isotopic reference material. The certified value was calculated from the unweighted means of the results submitted by the participating laboratories. The ¹⁴³Nd/¹⁴⁴Nd isotope ratio value for GSB 04‐3258‐2015 is 0.512438, with a combined expanded uncertainty (k = 2) of 5 × 10^?6. Reference material GSB 04‐3258‐2015 is available upon request from the Institute of Geology, Chinese Academy of Geological Sciences, and may be used for accurate interlaboratory calibration of Nd isotope analysis.     

Highly depleted Hadean mantle reservoirs in the sources of early Archean arc-like rocks, Isua supracrustal belt, southern West Greenland

Growing evidence from the accessible geological record reveals that crust-mantle differentiation on Earth started as early as 4.4 Ga. In order to assess the extent of early Archean mantle depletion, we obtained ¹⁷⁶Lu-¹⁷⁶Hf, ¹⁴⁷Sm-¹⁴³Nd, and high field strength element (HFSE) concentration data for the least altered, well characterized boninite-like metabasalts and associated metasedimentary rocks from the Isua supracrustal belt (southern West Greenland). The metasediments exhibit initial εHf₍₃₇₂₀₎ values from −0.7 to +1.5 and initial εNd₍₃₇₂₀₎ values from +1.6 to +2.1. Initial εHf₍₃₇₂₀₎ values of the least altered boninite-like metabasalts span a range from +3.5 to +12.9 and initial εNd₍₃₇₂₀₎ values from −0.3 to +3.2. These initial Hf-isotope ratios display coherent trends with SiO₂, Al₂O₃/TiO₂ and other relatively immobile elements, indicating contamination via assimilation of enriched components, most likely sediments derived from the earliest crust in the region. This model is also consistent with previously reported initial γOs₍₃₇₂₀₎ values for some of the samples. In addition to the positive εHf₍₃₇₂₀₎ values, the least disturbed samples exhibit positive εNd₍₃₇₂₀₎ values and a co-variation of εHf₍₃₇₂₀₎ and εΝd₍₃₇₂₀₎ values. Based on these observations, it is argued, that the most depleted samples with initial εHf₍₃₇₂₀₎ values of up to +12.9 and high ¹⁷⁶Lu/¹⁷⁷Hf of ∼0.05 to ∼0.09 tap a highly depleted mantle source with a long term depletion history in the garnet stability field. High precision high field strength element (HFSE) data obtained for the Isua samples confirm the contamination trend. Even the most primitive samples display negative Nb-Ta anomalies and elevated Nb/Ta, indicating a subduction zone setting and overprint of the depleted mantle sources by felsic melts generated by partial melting of eclogite. Collectively, the data for boninite-like metabasalts support the presence of strongly depleted mantle reservoirs as previously inferred from Hf isotope data for Hadean zircons and combined ¹⁴²Nd-¹⁴³Nd isotope data for early Archean rocks.     

Experimental study of polybaric REE partitioning between olivine, pyroxene and melt of the Yamato 980459 composition: Insights into the petrogenesis of depleted shergottites

A synthetic composition representing the Yamato 980459 martian basalt (shergottite) has been used to carry out phase relation, and rare earth element (REE) olivine and pyroxene partitioning experiments. Yamato 980459 is a sample of primitive basalt derived from a reduced end-member among martian mantle sources. Experiments carried out between 1-2 GPa and 1350-1650 °C simulate the estimated pressure-temperature conditions of basaltic melt generation in the martian mantle. Olivine-melt and orthopyroxene-melt partition coefficients for La, Nd, Sm, Eu, Gd and Yb (D_REE values) were determined by LA-ICPMS, and are similar to the published values for terrestrial basaltic systems. We have not detected significant variation in D-values with pressure over the range investigated, and by comparison with previous studies carried out at lower pressure.We apply the experimentally obtained olivine-melt and orthopyroxene-melt D_REE values to fractional crystallization and partial melting models to develop a three-stage geochemical model for the evolution of martian meteorites. In our model we propose two ancient (∼4.535 Ga) sources: the Nakhlite Source, located in the shallow mantle, and the Deep Mantle Source, located close to the martian core-mantle boundary. These two sources evolved distinctly on the ε¹⁴³Nd evolution curve due to their different Sm/Nd ratios. By partially melting the Nakhlite Source at ∼1.3 Ga, we are able to produce a slightly depleted residue (Nakhlite Residue). The Nakhlite Residue is left undisturbed until ∼500 Ma, at which point the depleted Deep Mantle Source is brought up by a plume mechanism carrying with it high heat flow, melts and isotopic signatures of the deep mantle (e.g., ε¹⁸²W, ε¹⁴²Nd, etc.). The plume-derived Deep Mantle Source combines with the Nakhlite Residue producing a mixture that becomes a mantle source (herein referred to as “the Y98 source”) for Yamato 980459 and the other depleted shergottites with the characteristic range of Sm/Nd ratios of these meteorites. The same hot plume provides a heat source for the formation of enriched and intermediate shergottites. Our model reproduces the REE patterns of nakhlites and depleted shergottites and can explain high ε¹⁴³Nd in depleted shergottites. Furthermore, the model results can be used to interpret whole rock Rb-Sr and Sm-Nd ages of shergottites.     

Isotope systematics of Li, Sr, Nd, and volatiles in Indian Ocean MORBs of the Rodrigues Triple Junction: Constraints on the origin of the DUPAL anomaly

The DUPAL anomaly, a radiogenic isotope anomaly discovered in the Indian Ocean mantle, has been interpreted as due to a large-scale mantle heterogeneity. To provide new constraints on the DUPAL origin, we analyzed isotope ratios of Li, Sr, and Nd in fresh N-MORB glasses recovered from the Rodrigues Triple Junction in the Indian Ocean, and from the North Atlantic. The Li isotopic compositions of the Indian Ocean DUPAL N-MORBs were comparable to those of the North Atlantic non-DUPAL N-MORBs. The source of the DUPAL signature in Indian Ocean MORBs and the E-MORB-type enriched mantle source have quite different Li isotopic compositions. The ¹⁴³Nd/¹⁴⁴Nd values of both sources are significantly lower than those of the North Atlantic N-MORBs. The δ⁷Li values of most oceanic island basalts with similar low ¹⁴³Nd/¹⁴⁴Nd signatures are also higher than those of the North Atlantic N-MORBs, except for several Koolau lavas. The Li isotope results support the recent proposal that significant amounts of recycled lower continental crust might produce the radiogenic isotope signatures of the Indian Ocean DUPAL source.