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Peters, BJ, Carlson RW, Day JMD, Horan MF.  2018.  Hadean silicate differentiation preserved by anomalous 142Nd/144Nd ratios in the Réunion hotspot source. Nature. 555:89-93.   doi:10.1038/nature25754   Abstract

Active volcanic hotspots can tap into domains in Earth’s deep interior that were formed more than two billion years ago1,2. High-precision data on variability in tungsten isotopes have shown that some of these domains resulted from differentiation events that occurred within the first fifty million years of Earth history3,4. However, it has not proved easy to resolve analogous variability in neodymium isotope compositions that would track regions of Earth’s interior whose composition was established by events occurring within roughly the first five hundred million years of Earth history5,6. Here we report 142Nd/144Nd ratios for Réunion Island igneous rocks, some of which are resolvably either higher or lower than the ratios in modern upper-mantle domains. We also find that Réunion 142Nd/144Nd ratios correlate with helium-isotope ratios (3He/4He), suggesting parallel behaviour of these isotopic systems during very early silicate differentiation, perhaps as early as 4.39 billion years ago. The range of 142Nd/144Nd ratios in Réunion basalts is inconsistent with a single-stage differentiation process, and instead requires mixing of a conjugate melt and residue formed in at least one melting event during the Hadean eon, 4.56 billion to 4 billion years ago. Efficient post-Hadean mixing nearly erased the ancient, anomalous 142Nd/144Nd signatures, and produced the relatively homogeneous 143Nd/144Nd composition that is characteristic of Réunion basalts. Our results show that Réunion magmas tap into a particularly ancient, primitive source compared with other volcanic hotspots7,8,9,10, offering insight into the formation and preservation of ancient heterogeneities in Earth’s interior.

Clay, PL, Burgess R, Busemann H, Ruzie-Hamilton L, Joachim B, Day JMD, Ballentine CJ.  2017.  Halogens in chondritic meteorites and terrestrial accretion. Nature. 551:614-618.   10.1038/nature24625   Abstract

Volatile element delivery and retention played a fundamental part in Earth’s formation and subsequent chemical differentiation. The heavy halogens—chlorine (Cl), bromine (Br) and iodine (I)—are key tracers of accretionary processes owing to their high volatility and incompatibility, but have low abundances in most geological and planetary materials. However, noble gas proxy isotopes produced during neutron irradiation provide a high-sensitivity tool for the determination of heavy halogen abundances. Using such isotopes, here we show that Cl, Br and I abundances in carbonaceous, enstatite, Rumuruti and primitive ordinary chondrites are about 6 times, 9 times and 15–37 times lower, respectively, than previously reported and usually accepted estimates1. This is independent of the oxidation state or petrological type of the chondrites. The ratios Br/Cl and I/Cl in all studied chondrites show a limited range, indistinguishable from bulk silicate Earth estimates. Our results demonstrate that the halogen depletion of bulk silicate Earth relative to primitive meteorites is consistent with the depletion of lithophile elements of similar volatility. These results for carbonaceous chondrites reveal that late accretion, constrained to a maximum of 0.5 ± 0.2 per cent of Earth’s silicate mass2,3,4,5, cannot solely account for present-day terrestrial halogen inventories6,7. It is estimated that 80–90 per cent of heavy halogens are concentrated in Earth’s surface reservoirs7,8 and have not undergone the extreme early loss observed in atmosphere-forming elements9. Therefore, in addition to late-stage terrestrial accretion of halogens and mantle degassing, which has removed less than half of Earth’s dissolved mantle gases10, the efficient extraction of halogen-rich fluids6 from the solid Earth during the earliest stages of terrestrial differentiation is also required to explain the presence of these heavy halogens at the surface. The hydropilic nature of halogens, whereby they track with water, supports this requirement, and is consistent with volatile-rich or water-rich late-stage terrestrial accretion5,11,12,13,14.

Day, JMD, Barry PH, Hilton DR, Burgess R, Pearson DG, Taylor LA.  2015.  The helium flux from the continents and ubiquity of low-3He/4He recycled crust and lithosphere. Geochimica et Cosmochimica Acta. 153:116-133.   http://dx.doi.org/10.1016/j.gca.2015.01.008   AbstractWebsite

New helium isotope and trace-element abundance data are reported for pyroxenites and eclogites from South Africa, Siberia, and the Beni Bousera Massif, Morocco that are widely interpreted to form from recycled oceanic crustal protoliths. The first He isotope data are also presented for Archaean peridotites from the Kaapvaal (South Africa), Slave (Canada), and Siberian cratons, along with recently emplaced off-craton peridotite xenoliths from Kilbourne Hole, San Carlos (USA) and Vitim (Siberia), to complement existing 3He/4He values obtained for continental and oceanic peridotites. Helium isotope compositions of peridotite xenoliths vary from 7.3 to 9.6 RA in recently (<10 kyr) emplaced xenoliths, to
0.05 RA in olivine from cratonic peridotite xenoliths of the 1179 Ma Premier kimberlite, South Africa. The helium isotope compositions of the peridotites can be explained through progressive sampling of 4He produced from radiogenic decay of U and Th in the mineral lattice in the older emplaced peridotite xenoliths. Ingrowth of 4He is consistent with generally higher 4He concentrations measured in olivine from older emplaced peridotite xenoliths relative to those from younger peridotite xenoliths. Collectively, the new data are consistent with pervasive open-system behaviour of He in peridotite xenoliths from cratons, mobile belts and tectonically-active regions. However, there is probable bias in the estimate of the helium isotope composition of the continental lithospheric mantle (6.1 ± 2.1 RA), since previously published databases were largely derived from peridotite xenoliths from non-cratonic lithosphere, or phenocrysts/xenocrysts obtained within continental intraplate alkaline volcanics that contain a contribution from asthenospheric sources. Using the new He isotope data for cratonic peridotites and assuming that significant portions (>50%) of the Archaean and Proterozoic continental lithospheric mantle are stable and unaffected by melt or fluid infiltration on geological timescales (>0.1 Ga), and that U and Th contents vary between cratonic lithosphere and non-cratonic lithosphere, calculations yield a 3He flux of 0.25–2.2 atoms/s/cm2 for the continental lithospheric mantle. These estimates differ by a factor of ten from non-cratonic lithospheric mantle and are closer to the
observed 3He flux from the continents (<1 atoms/s/cm2). Pyroxenites and eclogites from the continental regions are all characterized by 3He/4He (0.03–5.6 RA) less than the depleted upper mantle, and relatively high U and Th contents. Together with oceanic and continental lithospheric peridotites, these materials represent reservoirs with low time-integrated 3He/(U + Th) in the mantle. Pyroxenites and eclogites are also characterized by higher Fe/Mg, more radiogenic Os–Pb isotope compositions, and more variable d18O values (3 to 7 per mille), compared with peridotitic mantle. These xenoliths are widely interpreted to be the metamorphic/metasomatic equivalents of
recycled oceanic crustal protoliths. The low-3He/4He values of these reservoirs and their distinctive compositions make themprobable end-members to explain the compositions of some low-3He/4He OIB, and provide an explanation for the low-3He/4He measured in most HIMU lavas. Continental lithospheric mantle and recycled oceanic crust protoliths are not reservoirs for high-3He/4He and so alternative, volumetrically significant, He-rich reservoirs, such as less-degassed (lower?) mantle, are required to explain high-3He/4He signatures measured in some intraplate lavas. Recycling of oceanic crust represents a fundamental process for the generation of radiogenic noble gases in the mantle, and can therefore be used effectively as tracers for volatile recycling.

Stronik, NA, Trumbull RB, Krienitz M-S, Niedermann S, Romer RL, Harris C, Day JMD.  2017.  Helium isotope evidence for a deep-seated mantle plume involved in South Atlantic breakup. Geology. 45(9):827-830.   10.1130/G39151.1   Abstract

Earth history has been punctuated by episodes of short-lived (<10 m.y.), high-volume (>106 km3) magmatism. The origin of these events and their manifestations as large igneous provinces (LIPs) with associated continental flood basalts do not fit in the current plate-tectonic paradigm. Upper-mantle processes have been invoked for some LIPs, whereas the origin of others appears to be related to plumes rising from the deep mantle. The Paraná-Etendeka LIP has remained enigmatic and highly contested in terms of plume versus upper-mantle models. Here, we provide evidence for a plume origin based on new isotopic (He, O, Sr, Nd, Pb) and trace-element data from olivine-rich dikes from Namibia. The composition of the dikes can be explained by mixing at shallow depths between a plume source with high 3He/4He (>26 RA) and ambient asthenospheric mantle, before ascent through the thinning lithosphere.

Furi, E, Hilton DR, Murton BJ, Hemond C, Dyment J, Day JMD.  2011.  Helium isotope variations between Reunion Island and the Central Indian Ridge (17 degrees-21 degrees S): New evidence for ridge-hot spot interaction. Journal of Geophysical Research-Solid Earth. 116   10.1029/2010jb007609   AbstractWebsite

We report new helium abundance and isotope results for submarine basaltic glasses from the Central Indian Ridge (CIR) between the Marie Celeste (16.7 degrees S) and Egeria fracture zones (FZ) (20.6 degrees S); the adjacent Gasitao, Three Magi, and Rodrigues ridges; and for olivine separates from lavas and cumulate xenoliths from the Mascarene Islands (Reunion, Mauritius, and Rodrigues). Helium isotope ratios in basaltic glasses range from 7.1 to 12.2 R(A) (where R(A) = air (3)He/(4)He) and lie between values of Mid-Ocean Ridge Basalt (MORB) (8 +/- 1 R(A)) and samples from Reunion Island (11.5 to 14.1 R(A)). The highest (3)He/(4)He values (up to 12.2 R(A)) are found in glasses recovered off axis from the Three Magi and Gasitao ridges. Along the CIR axis, MORB-like (3)He/(4)He ratios are found near the Egeria FZ, and there is a marked increase to values of similar to 11 R(A) between similar to 19 degrees and 20 degrees S. The lowest (3)He/(4)He values (< 8 R(A)) are found immediately south of the Marie Celeste FZ, where incompatible trace element ratios (e. g., La/Sm) are highest. These low (3)He/(4)He ratios can be explained by closed system radiogenic (4)He ingrowth in either (1) a "fossil" Reunion hot spot mantle component, embedded into the subridge mantle when the CIR migrated over the hot spot at similar to 34 Ma or (2) trace element enriched MORB mantle. In contrast, the high (3)He/(4)He ratios observed on the CIR axis adjacent to the Gasitao Ridge, and along the off-axis volcanic ridges, are consistent with flow of hot spot mantle material from Reunion (similar to 1100 km to the west) toward the CIR.

Barry, PH, Hilton DR, Day JMD, Pernet-Fisher JF, Howarth GH, Magna T, Agashev AM, Pokhilenko NP, Pokhilenko LH, Taylor LA.  2015.  Helium isotopic evidence for modification of the cratonic lithosphere during the Permo-Triassic Siberian flood basalt event. Lithos. 216-217:73-80.   10.1016/j.lithos.2014.12.001   Abstract

Major flood basalt emplacement events can dramatically alter the composition of the sub-continental lithospheric mantle (SCLM). The Siberian craton experienced one of the largest flood basalt events preserved in the geologic record — eruption of the Permo-Triassic Siberian flood basalts (SFB) at ~250 Myr in response to upwelling of a deep-rooted mantle plume beneath the Siberian SCLM. Here,we present helium isotope (3He/4He) and concentration data for petrologically-distinct suites of peridotitic xenoliths recovered from two temporally-separated kimberlites:
the 360 Ma Udachnaya and 160 Ma Obnazhennaya pipes, which erupted through the Siberian SCLM and bracket the eruption of the SFB. Measured 3He/4He ratios span a range from 0.1 to 9.8 RA (where RA = air 3He/4He) and fall into two distinct groups: 1) predominantly radiogenic pre-plume Udachnaya samples (mean clinopyroxene 3He/4He = 0.41 ± 0.30 RA (1σ); n = 7 excluding 1 outlier), and 2) ‘mantle-like’ post plume Obnazhennaya samples (mean clinopyroxene 3He/4He=4.20±0.90 RA (1σ); n=5 excluding 1 outlier). Olivine separates from both kimberlite pipes tend to have higher 3He/4He than clinopyroxenes (or garnet). Helium contents in Udachnaya samples ([He] = 0.13–1.35 μcm3STP/g; n = 6) overlap with those of Obnazhennaya
([He]=0.05–1.58 μcm3STP/g; n = 10), but extend to significantly higher values in some instances ([He]=49–349 μcm3STP/g; n = 4). Uranium and thorium contents are also reported for the crushed material from which He was extracted in order to evaluate the potential for He migration from the mineral matrix to fluid inclusions. The wide range in He content, together with consistently radiogenic He-isotope values in Udachnaya peridotites suggests that crustal-derived fluids have incongruently metasomatized segments of the Siberian SCLM, whereas high 3He/4He values in Obnazhennaya peridotites show that this section of the SCLM has been overprinted by Permo-Triassic (plume-derived) basaltic fluids. Indeed, the stark contrast between pre- and post-plume 3He/4He ratios in peridotite xenoliths highlights the potentially powerful utility of He-isotopes for differentiating between various types of metasomatism (i.e., crustal versus basaltic fluids).

Peters, BJ, Day JMD, Greenwood RC, Hilton DR, Gibson J, Franchi IA.  2017.  Helium-oxygen-osmium isotopic and elemental constraints on the mantle sources of the Deccan Traps. Earth and Planetary Science Letters. 478:245-257.   10.1016/j.epsl.2017.08.042   Abstract

The Deccan Traps, a 65 million-year-old continental flood basalt province located in western India, is the result of one of the largest short-lived magmatic events to have occurred on Earth. The nature and composition of its mantle source(s), however, have been difficult to resolve due to extensive assimilation of continental crust into the ascending Traps magmas. To circumvent this issue, using high-precision electron microprobe analysis, we have analyzed olivine grains from MgO-rich (up to 15.7wt.%) lavas that likely erupted before substantial crustal assimilation occurred. We compare olivine, pyroxene and plagioclase mineral chemistry and He–O–Os isotope compositions with bulk rock major-and trace-element abundances and 187Os/188Os for both bulk-rocks and mineral separates. Helium isotope compositions for the olivine grains generally show strong influence from crustal assimilation (<3 RA), but one ankaramite from the Pavagadh volcanic complex has a 3He/4He ratio of 10.7 RA, which is slightly lower than the range of 3He/4He measured for present-day Réunion Island volcanism (∼12–14 RA). Olivine-dominated mineral separates span a more restricted range in 187Os/188Os (0.1267 to 0.1443) compared with their host lavas (0.1186 to 0.5010), with the separates reflecting a parental magma composition less affected by lithospheric or crustal interaction than for the bulk-rocks. Despite significant He–Os isotopic variations, D17O is relatively invariant (−0.008 ±0.014 per mil)and indistinguishable from the bulk mantle, consistent with high-3He/4He hotspots measured to-date.

Compositions of olivine grains indicate the presence of up to 25% of a pyroxenite source for Deccan parental magmas, in good agreement with ∼20% predicted from isotopic data for the same samples. Modeled pyroxenite signatures appear like geochemical signatures expected to arise due to other types of mantle differentiation or due to assimilation of continental crust; however, we show that crustal assimilation cannot account for all of the compositional features of the olivine. Weak correlations exist between a global compilation of Xpx(Deccan: 0.2–0.7) and 3He/4He, δ18O (Deccan olivine: 4.9–5.2 per mil) and 187Os/188Os. Robust relationships between these parameters may be precluded due to a lack of two-reservoir source mixing, instead involving multiple mantle domains with distinct compositions, or because Xpxmay reflect both source features and crustal assimilation. Notwithstanding, geochemical similarities exist between Deccan Traps olivine (3He/4He =10.7 RA; 187Os/188Osi=0.1313 ±45, 2σ) and Réunion igneous rocks (3He/4He =12–14 RA; 187Os/188Osi=0.1324 ±14). These relationships imply that a characteristic geochemical ‘fingerprint’ may have persisted in the mantle plume that fed the Deccan Traps, since its inception at 65 Ma, to ongoing eruptions occurring on Réunion up to the present-day.

Day, JMD, Pearson DG, Nowell GM.  2003.  High precision rhenium and platinum isotope dilution analyses by plasma ionisation multi-collector mass spectrometry. Plasma Source Mass Spectrometry: applications and emerging technologies. ( Holland G, Tanner SD, Eds.)., London: RSC Publishing   10.1039/9781847551689  
Macpherson, CG, Hilton DR, Day JMD, Lowry D, Gronvold K.  2005.  High-He-3/He-4, depleted mantle and low-delta O-18, recycled oceanic lithosphere in the source of central Iceland magmatism. Earth and Planetary Science Letters. 233:411-427.   10.1016/j.epsl.2005.02.037   AbstractWebsite

New helium and oxygen isotope data and trace element concentrations are reported for volcanic rocks from central Iceland. Basalts that are depleted in the most incompatible trace elements possess a wide range in He-3/He-4 but most ratios are similar to or higher than those of mid-ocean ridge basalt (MORB:similar to 8R(A)[1] [D.W. Graham, Noble gas geochemistry of mid-ocean ridge and ocean island basalts: characterisation of mantle source reservoirs, in: D.P. Porcelli, C.J. Ballentine, R. Wieler (Eds.), Noble gases in Geochemistry and Cosmochemistry, Rev. Mineral. Geochem., vol. 47, 2002, pp. 247-317]). The low concentrations of helium in these rocks suggest that significant degassing has made them susceptible to contamination by low-He-3/He-4 crust, therefore all measured He-3/He-4 are considered minimum estimates for their sources. Elevated helium isotope ratios in the source of these rocks result from interaction with high-He-3/He-4 mantle. The highest oxygen isotope ratios in the depleted rocks are similar to those in melts from typical depleted upper mantle and the range of delta(18)O values is consistent with variable, limited amounts of contamination by Icelandic crust. Most of the incompatible trace element-enriched rocks possess He-3/He-4 ratios that are similar to or lower than those in MORB. These rocks were erupted close to the postulated centre of the Iceland plume. This observation contradicts models in which high-He-3/He-4 characterises the focus of mantle upwelling. A source with MORB-like He-3/He-4 ratios may also be common to other parts of the North Atlantic Igneous Province. The highest delta(18)O values in the enriched rocks are lower than those in MORB and do not appear to have been affected by interaction with low-delta(18)O Icelandic crust. Recycling of hydrothermally altered oceanic crust that has been subducted into the mantle provides a plausible mechanism for generating an O-18-poor source with the trace element and isotopic characteristics of the enriched lavas. (C) 2005 Elsevier B.V All rights reserved.

Harvey, J, Day JMD.  2016.  Highly siderophile and strongly chalcophile elements in high temperature geochemistry and cosmochemistry. (81):774pp.: Mineralogical Society of America   10.2138/rmg.2015.81.00  
Gannoun, A, Burton KW, Day JMD, Harvey J, Schiano P, Parkinson I.  2016.  Highly Siderophile Element and Os Isotope Systematics of Volcanic Rocks at Divergent and Convergent Plate Boundaries and in Intraplate Settings. Reviews in Mineralogy and Geochemistry. 81:651-724.   10.2138/rmg.2016.81.11   Abstract

Terrestrial magmatism is dominated by basaltic compositions. This definition encompasses mid-ocean ridge basalts (MORB), which account for more than eighty percent of Earth’s volcanic products and which are formed at divergent oceanic plate margins, as well as intraplate volcanic rocks such as ocean island basalts (OIB), continental flood basalts (CFB) and continental rift-related basalts, and highly magnesian ultramafic volcanic rocks that dominantly occur in Archean terranes, termed komatiites. All of these broadly basaltic rocks are considered to form by partial melting of the upper mantle, followed by extraction from their source regions and emplacement at the Earth’s surface. For these reasons, basalts can be used to examine the nature and extent of partial melting in the mantle, the compositions of mantle sources, and the interactions between the crust and mantle. Because much of Earth’s mantle is inaccessible, basalts offer some of the best ‘proxies’ for examining mantle composition, mantle convection and crust–mantle interactions. By contrast, at arcs, volcanism is dominated by andesitic rock compositions. While some arcs do have basaltic and picritic magmatism, these magma types are rare in convergent plate margin settings and reflect the complex fractional crystallization and often associated concomitant assimilation processes occurring in arcs. Despite the limited occurrence of high MgO magmas in arc volcanic rocks, magmas from this tectonic setting are also important for elucidating the behavior of the HSE from creation of basaltic compositions at mid-ocean ridges to the subduction of this crust beneath arcs at convergent plate margins.

Day, JMD, Pearson GD, Hulbert LJ.  2013.  Highly siderophile element behaviour during flood basalt genesis and evidence for melts from intrusive chromitite formation in the Mackenzie large igneous province. Lithos. 182-183:242-258.   http://dx.doi.org/10.1016/j.lithos.2013.10.011   Abstract

The 1.27 Ga Coppermine continental flood basalt (CFB) province in northern Canada represents the extrusive manifestation of the 2.7 Mkm2 Mackenzie large igneous province (LIP) that includes the Mackenzie dyke swarm and the Muskox layered intrusion. New Re–Os isotope and highly siderophile element (HSE: Re, Pd, Pt, Ru, Ir, Os) abundance data are reported together with whole-rock major- and trace-element abundances and Nd isotopes to examine the behaviour of the HSE during magmatic differentiation and to place constraints on the extent of crustal interaction with mantle-derived melts. Mineral chemistry and petrography are also reported for an unusual andesite glass flow (CM19; 4.9 wt.% MgO) found in close proximity to newly recognised picrites (> 20 wt.% MgO) in the lowermost stratigraphy of the Coppermine CFB. Compositions of mineral phases in CM19 are similar to the same phases found in Muskox Intrusion chromitites and the melt composition is equivalent to inclusions trapped within Muskox chromites. The apparently conflicting elevated HSE contents (e.g., 3.8 ppb Os) and mantle-like initial 187Os/188Os (γOs = + 2.2), versus stable isotope (δ18O = + 12‰) and lithophile element evidence (εNdi = − 12.8) for extensive crustal contamination, implicate an origin for CM19 as a magma mingling product formed within the Muskox Intrusion during chromitite genesis. Combined with Nd isotope data that places the feeder for lower Coppermine CFB picrites and basalts within the Muskox Intrusion, this result provides compelling evidence for direct processing of some CFB within upper-crustal magma chambers. The Coppermine CFB defines a 187Re–187Os isochron with an age of 1263 + 16/− 20 Ma and initial γOs = + 2.2 ± 0.8. The initial Os isotope composition for the Coppermine CFB is slightly higher than the near-primitive-mantle initial 187Os/188Os for the Muskox Intrusion (γOs = + 1.2 ± 0.3). This result is interpreted to reflect greater crustal contamination in extrusive CFB and the sensitivity of Os isotopes, compared with absolute HSE concentrations, for tracking crustal contributions.

Modelling of absolute and relative HSE abundances in global CFB reveals that HSE concentrations decrease with increasing fractionation for melts with < 8 ± 1 wt.% MgO, with picrites (> 13.5 wt.% MgO) from CFB (n = 98; 1.97 ± 1.77 ppb) having higher Os abundances than ocean island basalt (OIB) equivalents (n = 75; 0.95 ± 0.86 ppb). The differences between CFB and OIB picrite absolute Os abundances may result from higher degrees of partial melting to form CFB but may also reflect incorporation of trace sulphide in CFB picrites from magmas that reached S-saturation in upper-crustal magma chambers. Significant inter-element fractionation of (Re + Pt + Pd)/(Os + Ir + Ru) are generated during magmatic differentiation in response to strongly contrasting partitioning of these two groups of elements into sulphides and/or HSE-rich alloys. Furthermore, fractional crystallization has a greater role on absolute and relative HSE abundances than crustal contamination under conditions of CFB petrogenesis due to the dilution effect of continental crust, which has low total abundances of the HSE. Combined data for the basaltic and intrusive portions of the Mackenzie LIP indicate a mantle source broadly within the range of the primitive upper mantle. The majority of Archaean komatiites and Phanerozoic CFB also require mantle sources with primitive upper mantle to chondritic Re/Os evolution, with exceptions typically being from analyses of highly-fractionated MgO-poor basalts.

Day, JMD, Pearson DG, Taylor LA.  2007.  Highly siderophile element constraints on accretion and differentiation of the Earth-Moon system. Science. 315:217-219.   10.1126/science.1133355   AbstractWebsite

A new combined rhenium-osmium- and platinum-group element data set for basalts from the Moon establishes that the basalts have uniformly low abundances of highly siderophile elements. The data set indicates a lunar mantle with long-term, chondritic, highly siderophile element ratios, but with absolute abundances that are over 20 times lower than those in Earth's mantle. The results are consistent with silicate-metal equilibrium during a giant impact and core formation in both bodies, followed by post-core-formation late accretion that replenished their mantles with highly siderophile elements. The lunar mantle experienced late accretion that was similar in composition to that of Earth but volumetrically less than (similar to 0.02% lunar mass) and terminated earlier than for Earth.

Day, JMD, Walker RJ.  2015.  Highly siderophile element depletion in the Moon. Earth and Planetary Science Letters. 423:114-124.   10.1016/j.epsl.2015.05.001   Abstract

Coupled 187Os/188Os and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance data are reported for Apollo 12 (12005, 12009, 12019, 12022, 12038, 12039, 12040), Apollo 15 (15555) and Apollo 17 (70135) mare basalts, along with mare basalt meteorites La Paz icefield (LAP) 04841 and Miller Range (MIL) 05035. These mare basalts have consistently low HSE abundances, at ∼2×10−5 to 2×10−7 the chondritic abundance. The most magnesian samples have broadly chondrite-relative HSE abundances and chondritic measured and calculated initial 187Os/188Os. The lower abundances and fractionated HSE compositions of more evolved mare basalts can be reproduced by modeling crystal–liquid fractionation using rock/melt bulk-partition coefficients of ∼2 for Os, Ir, Ru, Pt and Pd and ∼1.5 for Re. Lunar mare basalt bulk-partition coefficients are probably higher than for terrestrial melts as a result of more reducing conditions, leading to increased HSE compatibility. The chondritic-relative abundances and chondritic 187Os/188Os of the most primitive high-MgO mare basalts cannot readily be explained through regolith contamination during emplacement at the lunar surface. Mare basalt compositions are best modeled as representing ∼5–11% partial melting of metal-free sources with low Os, Ir, Ru, Pd (∼0.1 ng g−1), Pt (∼0.2 ng g−1), Re (∼0.01 ng g−1) and S (∼75 μg g−1), with sulphide-melt partitioning between 1000 and 10,000.

Apollo 12 olivine-, pigeonite- and ilmenite normative mare basalts define an imprecise 187Re–187Os age of 3.0±0.9 Ga with an initial 187Os/188Os of 0.107±0.010. This age is within uncertainty of 147Sm–143Nd ages for the samples. The initial Os isotopic composition of Apollo 12 samples indicates that the source of these rocks evolved with Re/Os within ∼10% of chondrite meteorites, from the time that the mantle source became a system closed to siderophile additions, to the time that the basalts erupted. Similarity in absolute HSE abundances between mare basalts from the Apollo 12, 15 and 17 sites, and from unknown regions of the Moon (La Paz mare basalts, MIL 05035), indicates relatively homogeneous and low HSE abundances within the lunar interior. Low absolute HSE abundances and chondritic Re/Os of mare basalts are consistent with a late accretion addition of ∼0.02 wt.% of the Moon's mass to the mantle, prior to the formation of the lunar crust. Late accretion must also have occurred significantly prior to cessation of lunar mantle differentiation (>4.4 Ga), to enable efficient mixing and homogenization within the mantle. Low lunar HSE abundances are consistent with proportionally 40 times more late accretion to Earth than the Moon. Disproportional late accretion to the two bodies is consistent with the small 182W excess (∼21–28 ppm) measured in lunar rocks, compared to the silicate Earth.

Day, JMD, Brandon AD, Walker RJ.  2016.  Highly Siderophile Elements in Earth, Mars, the Moon, and Asteroids. Reviews in Mineralogy and Geochemistry. 81:161-238.   10.2138/rmg.2016.81.04   Abstract

The highly siderophile elements (HSE: Os, Ir, Ru, Rh, Pt, Pd, Re, Au) are key tracers of planetary accretion and differentiation processes due to their affinity for metal relative to silicate. Under low-pressure conditions the HSE are defined by having metal–silicate partition coefficients in excess of 104 (e.g., Kimura et al. 1974; Jones and Drake 1986; O’Neill et al. 1995; Borisov and Palme 1997; Mann et al. 2012). The HSE are geochemically distinct in that, with the exception of Au, they have elevated melting points relative to iron (1665 K), low vapour pressures, and are resistant to corrosion or oxidation. Under solar nebular conditions, Re, Os, Ir, Ru, Rh, and Pt, along with the moderately siderophile elements (MSE) Mo and W, condense as refractory-metal alloys. Palladium and Au are not as refractory and condense in solid solution with FeNi metal (Palme 2008). Assuming abundances of the HSE in materials that made up the bulk Earth were broadly similar to modern chondrite meteorites, mass balance calculations suggest that >98% of these elements reside in the metallic core (O’Neill and Palme 1998). In practical terms, the resultant low HSE abundance inventories in differentiated silicate crusts and mantles enables the use of these elements in order to effectively track metallic core formation and the subsequent additions of HSE-rich impactors to planets and asteroids (Fig. 1). In detail, the absolute and relative abundances of the HSE in planetary materials are also affected by mantle and crustal processes including melting, metasomatism, fractional crystallization, and crust-mantle remixing, as well as later impact processing, volatility of Re under oxidizing conditions, and low-temperature secondary alteration (cf., Day 2013; Gannoun et al. 2016, this volume). In the absence of metal, the HSE are chalcophile, so these elements are also affected by processes

Day, JMD.  2013.  Hotspot volcanism and highly siderophile elements. Chemical Geology. 341:50-74.   10.1016/j.chemgeo.2012.12.010   AbstractWebsite

Hotspot volcanic rocks are formed under conditions that differ from conventional plate tectonic boundary magmatic processes and are compositionally distinct from mid-oceanic ridge basalts. Hotspot volcanic rocks include - but are not limited to - ocean island basalts (OIB), continental flood basalts (CFB), komatiites, oceanic plateau and some intraplate alkaline volcanic rocks. Studies of the highly siderophile element (HSE) geochemistry of hotspot volcanic rocks have provided new perspectives into mantle convection, mantle heterogeneity, core-mantle interactions, crustal and mantle lithospheric recycling, melting processes and crust-mantle interactions. The HSE, comprising Os, Ir, Ru, Rh, Pt, Pd, Re and Au, have strong affinities for metal and sulphide relative to silicate. These elements also have variable partitioning behaviour between highly compatible Os, Ir, Ru and Rh relative to compatible Pt and Pd and to moderately incompatible Re and Au during melting and crysta! The HSE can be utilised to understand sub-aerial volcanic degassing and crustal assimilation processes in hotspot volcanic rocks such as CFB and OIB, as well as for quantitative assessment of fractional crystallisation. Mantle melting studies have highlighted the strong control of sulphide in the mantle prior to exhaustion of S and generation of Os Ir Ru metal alloys at similar to>25% partial melting; a behaviour of the HSE that is fundamental to understanding terrestrial hotspot volcanism. Perhaps the most exciting utility of the HSE, however, lies in their ability to reveal both short- and long-term fractionation processes acting on hotspot volcanic sources from inter-element HSE fractionations and Os-187/Os-188-Os-186/Os-188 systematics. The growing database for HSE abundances and Os-187/Os-188 in hotspot volcanic rocks is consistent with their generation from a heterogeneous upper mantle generated by melt differentiation and recycling of crust and mantle lithosphere d! The HSE provide geochemical evidence for how lithological and chemical heterogeneities are sampled within the mantle. Modeling of HSE abundances and Os isotopes show that large apparent recycled contributions (50% to 90%) in some OIB can be explained by the preferential melting of volumetrically minor (<10%) pyroxenite in their sources. Preferential melting of more fusible materials in the mantle also explains why low-degree partial melts, such as alkali basalts and basanites, may exhibit more extreme isotopic variations than tholeiites or komatiites, which likely contain a higher contribution from peridotite in a hybridised pyroxenite-peridotite mantle source. High-precision Os-188/Os-188 data for hotspot volcanism are limited, but the combined variations in long-term Re/Os and Pt/Os retained in some mantle sources may reflect either the long-term fractionation of Re and Pt from Os between the inner and outer core, or ancient sulphide segregation and lithological variati! Study of the HSE in hotspot volcanic rocks from Solar System bodies also informs on planetary-scale processes, indicating that Earth, the Moon, Mars and fully differentiated asteroids all have HSE abundances in their mantles that are higher than expected from low-pressure metal-silicate partitioning. Furthermore, the HSE are in broadly chondritic-relative abundances for these planetary bodies, at similar to 0.0002 (Moon), to similar to 0.007 (Mars), to similar to 0.009 (Earth) x carbonaceous chondrite Ivuna (CI) composition. The timing of addition of the HSE to planetary bodies preserved in their magmas and volcanic products is consistent with Solar-System-wide late accretion no later than 3.8 Ga for Earth, and even earlier based on evidence from the Moon (similar to 4.4 Ga), Mars (similar to 4.5 Ga) and asteroids (>4.56 Ga). (C) 2013 Elsevier B.V. All rights reserved.DELMON.A, 1972, AMERICAN JOURNAL OF SCIENCE, V272, P805