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2019
Day, JMD, Corder CA, Assayag N, Cartigny P.  2019.  Ferrous oxide-rich asteroid achondrites. Geochimica et Cosmochimica Acta. 266:544-567.   j.gca.2019.04.005   Abstract

Ferrous oxide (FeO)-rich asteroid achondrites can be defined as asteroid-derived samples that experienced incipient partial melting processes in the early Solar System (>4.5 Ga) leading to melt-residues and cumulate and melt rocks that have high FeO in silicate grains (molar Mg/ [Mg + Fe] <80), implying relatively oxidative conditions (fO2 of IW +1 to +3). These achondrites include olivine-dominated brachinite and brachinite-like achondrite meteorites, ungrouped meteorites including Lewis Cliff 88763, Northwest Africa (NWA) 6693 and NWA 6704, Tafassasset, NWA 011/1296, and the oligoclase-rich meteorites Graves Nunataks (GRA) 06128 and GRA 06129. Ferrous oxide-rich asteroidal achondrites differ from other partially-melted achondrites, including ureilites and acapulcoite-lodranites in that the latter have higher molar Mg/ (Mg + Fe) in silicate grains, and lower fO2 (IW 0 to -2). New mineral chemical, whole-rock major- and trace-element and highly siderophile element (HSE: Re, Os, Ir, Ru, Rh, Pt, Pd, Au) abundance data, and O and Os isotope data are presented for FeO-rich achondrite meteorites Allan Hills 84025 (brachinite), Miller Range (MIL) 090206 and MIL 090405 (brachinite-like achondrites), and NWA 6693 (ungrouped). These results, combined with available data for FeO-rich asteroidal achondrites, reveal that these rocks include nearly-pure residues after partial melting, to samples formed by melt-rock reaction and as cumulates, requiring variable to extensive Fe-Ni-S partial melting, and between 1 to 20% silicate partial melting. The FeO-rich asteroidal achondrites originate from at least four distinct parent bodies, based on O-Cr-Ti isotope systematics, and occur in both carbonaceous and non-carbonaceous chondrite precursor sources. The initial water and volatile contents of FeO-rich asteroid achondrites were similar to carbonaceous chondrite groups, implying both carbonaceous and non-carbonaceous precursor materials generated water-rich partially-melted asteroidal bodies. The existence and recognition of FeO-rich asteroid achondrites explains the otherwise enigmatic nature of some iron meteorite groups (e.g., IVA, IVB) that require segregation from an oxidized asteroid parent body. The internal structure of some asteroid parent bodies was likely to be complex, reflecting early differentiation processes of nascent core formation, Fe-Ni-S melt pooling, variable silicate partial melting, igneous differentiation and the important role of melt-rock reaction, melt refertilization and late-stage C- (reduced bodies) or S-rich (oxidized bodies) fluid and vapor reactions.

Pernet-Fisher, JF, Barry PH, Day JMD, Pearson DG, Woodland S, Agashev AM, Pokhilenko LN, Pokhilenko NP.  2019.  Heterogeneous kimberlite metasomatism revealed from a combined He-Os isotope study of Siberian megacrystalline dunite xenoliths. Geochimica et Cosmochimica Acta. 266:220-236.   https://doi.org/10.1016/j.gca.2019.07.054   Abstract

The Siberian sub-continental lithospheric mantle (SCLM) is regionally heterogeneous due to the complex modification of ancient cratonic material by various metasomatic fluids and interaction with the Siberian plume at ∼250 Ma. Here, we assess the extent and origin of this heterogeneity by analysis of helium isotopes, rhenium-osmium isotopes, trace-element, and highly siderophile element [HSE: Os, Ir, Ru, Pt, Pd, Re] abundances in a suite of mantle-derived megacrystalline dunite xenoliths from the 360 Ma Udachnaya East kimberlite pipe, Siberia. This enables assessment of the style and extent of metasomatism acting to modify the volatile budget of the Siberian SCLM. The olivine megacrysts have 3He/4He values that range from 2.3 to 7.4 RA (where RA = the 3He/4He of air), outside the canonical range for the lithospheric mantle (6.1 ± 2.1 RA; Day et al., 2015). High [He] and low U + Th concentrations in these olivine megacrysts indicate that the Udachnaya megacrystalline dunite xenolith suite has undergone minimal post-eruptive modification of He isotopes by 4He recoil. We instead interpret He isotope variations to reflect pre- or syn-eruptive metasomatic signatures during kimberlite emplacement. The dunites can be divided into two groups, ultimately reflecting modification by two distinct metasomatic styles. Group 1 dunites are characterized by highly unradiogenic Os isotope compositions (γOs360Ma -16 to -13) and ancient melt depletion ages (∼3 Ga) typical of ancient cratonic lithospheric mantle; yet display a range of He isotope ratios (2.3 to 5.9 RA). This indicates that group 1 dunites are modified by gas-rich metasomatic fluids acting to modify He abundances and isotope ratios, while preserving unradiogenic Os isotope and HSE abundance systematics. Group 2 dunites extend to significantly more radiogenic Os isotope signatures and higher He isotope values (γOs360Ma > -5 to +53; 3He/4He from 3.7 to 7.4 RA). These two groups define a negative hyperbolic array between Os and He isotopes which we interpret to result from metasomatism by variable mixtures between: (1) a gas-rich and HSE-poor fluid (similar to the fluids acting to modify Group 1 dunites) with helium isotope compositions above the MORB range (> 8 RA) derived from the asthenosphere, and: (2) strongly radiogenic fluids (∼ 2 RA; 187Os/188Os > 0.25) likely sourced from within the SCLM. Thee Udachnaya megacrystalline dunite xenolith suite provides insight into how mantle-plume derived fluids interact with metasomatic fluids within the SCLM.

Rahib, RH, Udry A, Howarth GH, Gross J, Paquet M, Combs LM, Laczniak DL, Day JMD.  2019.  Mantle source to near-surface emplacement of enriched and intermediate poikilitic shergottites in Mars. Geochimica et Cosmochimica Acta. 266:463-496.   j.gca.2019.07.034   Abstract

Poikilitic shergottites make up >20% of the current martian meteorite collection, with a total of 27 samples. These meteorites are intrusive gabbroic to lherzolitic rocks and represent igneous materials recording important processes in the martian crust. To further constrain petrogenetic relationships amongst enriched and intermediate poikilitic shergottites, we studied a comprehensive suite of poikilitic shergottites — including four newly recovered samples (Northwest Africa [NWA] 11065, NWA 11043, NWA 10961, NWA 10618) — using bulk rock major- and trace-element compositions, mineral major-element compositions, oxygen fugacity (ƒO2) values, crystallization temperatures, phosphorus maps of olivine grains, and quantitative textural analyses. The characteristic bimodal textures (poikilitic and non-poikilitic textures) of poikilitic shergottites record evolving magmatic conditions at different stages of crystallization. Higher temperatures and more reducing conditions during early-stage crystallization are recorded in the poikilitic textures, while lower temperature and more oxidizing conditions are recorded in the non-poikilitic textures during late-stage crystallization. Oxygen fugacity estimates relative to the quartz-fayalite-magnetite (QFM) buffer for early-stage olivine-pyroxene-spinel assemblages of enriched and intermediate poikilitic shergottites suggest decoupling of ƒO2 and the degree of light rare earth element (LREE)-enrichment (i.e., [La/Yb]CI). An increase in ƒO2 exceeding 1 log unit from poikilitic to non-poikilitic textures implies degassing, with possible auto-oxidation, and/or crustal contamination. Quantitative textural analyses support the emplacement of both enriched and intermediate poikilitic shergottites as various shallow intrusive bodies, as well as a potentially widespread emplacement mechanism responsible for a major lithology of the martian crust. In addition, early assemblages (i.e., pyroxene oikocrysts) of all the poikilitic shergottites likely formed close to the crust-mantle boundary, implying a possible widespread presence of magma staging chambers at these depths. Fractional crystallization and magma storage in these chambers could have possibly resulted in all of the different enriched and intermediate shergottites that have been analyzed from Mars.

Combs, LM, Udry A, Howarth GH, Righter M, Lapen TJ, Gross J, Ross DK, Rahib RR, Day JMD.  2019.  Petrology of the enriched poikilitic shergottite Northwest Africa 10169: Insight into the martian interior. Geochimica et Cosmochimica Acta. 266:435-462.   https://doi.org/10.1016/j.gca.2019.07.001   Abstract

The martian meteorite Northwest Africa (NWA) 10169 is classified as a member of the geochemically enriched poikilitic shergottites, based on mineral composition, Lu-Hf and Sm-Nd isotope systematics, and rare earth element (REE) concentrations. Similar to other enriched and intermediate poikilitic shergottites, NWA 10169 is a cumulate rock that exhibits a bimodal texture characterized by large pyroxene oikocrysts (poikilitic texture) surrounded by olivine-rich interstitial material (non-poikilitic texture). Olivine chadacrysts and pyroxene oikocrysts have higher Mg#s (molar Mg/Mg + Fe) than those in the interstitial areas, suggesting that the poikilitic texture represents early-stage crystallization and accumulation, as opposed to late-stage non-poikilitic (i.e., interstitial material) crystallization. Calculated oxygen fugacity values are more reduced (FMQ −2.3 ± 0.2) within the poikilitic regions, and more oxidized (FMQ −1.1 ± 0.1) within the interstitial areas, likely representing auto-oxidation and degassing during magma crystallization. Calculated parental melt compositions using olivine-hosted melt inclusions display a dichotomy between K-poor and K-rich melts, thus possibly indicating mixing of parental melt with K-rich melt. The 176Lu-176Hf crystallization age for NWA 10169 is 167 ± 31 Ma, consistent with the ages reported for other enriched shergottites. Based on the isochron initial 176Hf/177Hf value, the modeled source 176Lu/177Hf composition for NWA 10169 is 0.02748 ± 0.00037, identical within uncertainty to the source compositions of the enriched shergottites Shergotty, Zagami, LAR 06319, NWA 4468, and Roberts Massif (RBT) 04262, suggesting a shared, long-lived geochemical source, and distinct from the source of other enriched shergottites Los Angeles, NWA 856, and NWA 7320. This study reveals that at least two sources are responsible for the enriched shergottites, and that the martian mantle is more heterogeneous than previously thought. Additionally, the enriched shergottites, which share a source with NWA 10169, have consistent crystallization ages and magmatic histories, indicating that a common magmatic system on Mars is likely responsible for the formation of this group.

Tian, Z, Chen H, Fegley B, Lodders K, Barrat J-A, Day JMD, Wang K.  2019.  Potassium isotopic compositions of howardite-eucrite-diogenite meteorites. Geochimica et Cosmochimica Acta. 266:611-632.   https://doi.org/10.1016/j.gca.2019.08.012   Abstract

We report new high-precision stable K isotope data for three martian meteorites, one lunar meteorite, one ordinary chondrite, four terrestrial igneous United States Geological Survey (USGS) reference materials, and twenty howardite–eucrite–diogenite [HED] meteorites. The three martian meteorites define a relatively narrow δ41K range with an average of −0.36 ± 0.12‰ (2 SD) that is slightly heavier than the Bulk Silicate Earth (BSE) K isotopic composition (−0.48 ± 0.03‰). Except for the four Northwest Africa samples which were terrestrially contaminated, all HED meteorites reveal substantial 41K enrichment compared to BSE, lunar samples, martian meteorites, and chondrites. We propose that the average δ41K (+0.36 ± 0.16‰) obtained from HED meteorites is representative of Bulk Silicate 4-Vesta. The coupled volatile depletion and heavy K isotope enrichment in 4-Vesta could be attributed to both nebula-scale processes and parent-body events. The asteroid 4-Vesta is likely to have accreted from planetary feedstocks that have been significantly volatile-depleted prior to the major phases of planetary accretion in the early Solar System, with secondary effects of K loss during accretionary growth and magma ocean degassing.

Day, JMD, Neal CR.  2019.  To the Moon: A scientific tribute to Lawrence A. Taylor. Geochimica et Cosmochimica Acta. 266:1-8.   https://doi.org/10.1016/j.gca.2019.08.033   Abstract

Professor Lawrence (Larry) Taylor (September 14, 1938 to September 18, 2017) was a true ‘lunatic’: a term coined to describe one of the early pioneers who served as part of the science teams for the Apollo missions to the Moon. He later advocated for a return to the Moon to both improve our understanding of the formation of our nearest neighbor and to support the further exploration of space. Larry’s larger-than-life personality is matched by his scientific legacy of launching the careers of over 50 post-doctoral scholars and graduate students, present authors included, and authoring a staggering number of peer-reviewed publications (>540 at last count). Lunar science has lost one of its greatest science advocates for returning to the Moon and building a permanent human settlement on its surface. Amongst many of Larry’s memorable sayings, his epitaph should surely be: “To the Moon”. This special issue of Geochimica et Cosmochimica Acta is a tribute to Larry’s scientific achievements. The manuscripts within the volume provide a flavor of the wide range of research in which he was engaged.

Day, JMD, Sossi PA, Shearer CK, Moynier F.  2019.  Volatile distributions in and on the Moon revealed by Cu and Fe isotopes in the ‘Rusty Rock’ 66095. Geochimica et Cosmochimica Acta. 266:131-143.   10.1016/j.gca.2019.02.036   Abstract

The Apollo 16 ‘Rusty Rock’ impact melt breccia 66095 is a volatile-rich sample, with the volatiles inherited through vapor condensation from an internal lunar source formed during thermo-magmatic evolution of the Moon. We report Cu and Fe isotope data for 66095 and find that bulk-rocks, residues and acid leaches span a relatively limited range of compositions (3.0 ±1.3 wt.% FeO [range = 2.0-4.8 wt.%], 5.4 ±3.1 ppm Cu [range = 3-12 ppm], average δ56Fe of 0.15 ± 0.05‰ [weighted mean = 0.16‰] and δ65Cu of 0.72 ± 0.14‰ [weighted mean = 0.78‰]). In contrast to the extreme enrichment of light isotopes of Zn and heavy isotopes of Cl in 66095, δ65Cu and δ56Fe in the sample lie within the previously reported range for lunar mare basalts (0.92 ± 0.16‰ and 0.12 ± 0.02‰, respectively). The lack of extreme isotopic fractionation for Cu and Fe isotopes reflects compositions inherent to 66095, with condensation of a cooling gas from impact-generated fumarolic activity at temperatures too low to lead to the condensation of Cu and Fe, but higher than required to condense Zn. Together with thermodynamic models, these constraints suggest that the gas condensed within 66095 between 700 and 900 °C (assuming a pressure of 10-6 and an fO2 of IW-2). That the Cu and Fe isotopic compositions of sample 66095 are within the range of mare basalts removes the need for an exotic, volatile-enriched source. The enrichment in Tl, Br, Cd, Sn, Zn, Pb, Rb, Cs, Ga, B, Cl, Li relative to Bi, Se, Te, Ge, Cu, Ag, Sb, Mn, P, Cr and Fe in the ‘Rusty Rock’ is consistent with volcanic outgassing models and indicates that 66095 likely formed distal from the original source of the gas. The volatile-rich character of 66095 is consistent with impact-generated fumarolic activity in the region of the Cayley Plains, demonstrating that volatile-rich rocks can occur on the lunar surface from outgassing of a volatile-poor lunar interior. The ‘Rusty Rock’ indicates that the lunar interior is significantly depleted in volatile elements and compounds and that volatile-rich lunar surface rocks likely formed through vapor condensation. Remote sensing studies have detected volatiles on the lunar surface, attributing them dominantly to solar wind. Based on the ‘Rusty Rock’, some of these surface volatiles may also originate from the Moon’s interior.

Paquet, M, Day JMD, Castillo PR.  2019.  Osmium isotope evidence for a heterogeneous 3He/4He mantle plume beneath the Juan Fernandez Islands. Geochimica et Cosmochimica Acta. 261:1-19.   https://doi.org/10.1016/j.gca.2019.06.039   Abstract

Mantle plume models have been widely applied to explain the formation of ocean island basalts (OIB), with high-3He/4He in their lavas being explained by sampling of a primitive deep mantle source. The Juan Fernandez Islands have 3He/4He (7.8–18 RA) similar to or higher than in mid-ocean ridge basalts (MORB; 8 ± 1 RA) and have been used to both support and refute the mantle plume hypothesis. Ambiguity regarding the origin of the Juan Fernandez Islands primarily originates from interpretation of mantle source signatures between the lava series from the two main islands, Robinson Crusoe and Alexander Selkirk. To examine this issue, we report new whole-rock and olivine separate 187Os/188Os ratios and major-, trace-, and highly siderophile-element (HSE: Re, Pd, Pt, Ru, Ir, Os) abundances. The HSE and trace element abundances in Juan Fernandez main shield lavas can be explained by up to 30% olivine removal, together with spinel crystallization at 1–5 kbar, whereas Robinson Crusoe rejuvenated lavas can be reproduced by higher-pressure fractional crystallization (up to 10 kbar). An assemblage of 30 modal % olivine and 1–5 modal % spinel, combined with the additional contribution of primary melt trapped in olivine inclusions reproduces the range of HSE compositions observed in Juan Fernandez Archipelago olivine grains. Ratios of 187Os/188Os for Juan Fernandez lavas are generally less radiogenic than global OIB and show no correlation with indices of fractionation, indicating that they reflect mantle source compositions. Younger basanite lavas from Robinson Crusoe represent rejuvenated volcanism dominantly from a depleted lithospheric mantle source (187Os/188Os < 0.13) mixed with a high-3He/4He component from the main shield building stage. These lavas are similar in origin and composition to other Pacific rejuvenated lavas (e.g., Samoa, Hawaii). Robinson Crusoe main shield lavas are from a high-3He/4He (>18 RA) and enriched mantle source (187Os/188Os = 0.1312) similar to the ‘C’ or ‘FOZO’ component, whereas Alexander Selkirk lavas are consistent with a dominant contribution from a depleted, low-3He/4He (<10 RA) mantle component. The mantle sources of the shield lavas yield subtle variations in Sr-Nd-Os-Pb isotope space and have no clear variations with relative or absolute abundances of the HSE or trace elements. These results are consistent with a heterogeneous mantle plume model, with initial eruption of lavas from a primitive high-3He/4He mantle source ∼4 million years ago (Ma) to form Robinson Crusoe Island, which also led to 3He enrichment of the oceanic lithosphere, followed by eruption of Alexander Selkirk lavas from a more depleted mantle source at ∼2 Ma. Juan Fernandez lavas show that mantle heterogeneity preserved in OIB can occur over short-timescales (<2 Ma) and can impact lithospheric compositions, leading to eruption of rejuvenated lavas with unusual isotopic characteristics.

Day, JMD, O'Driscoll B.  2019.  Ancient high Pt/Os crustal contaminants can explain radiogenic 186Os in intraplate magmas. Earth and Planetary Science Letters. 519:101-108.   https://doi.org/10.1016/j.epsl.2019.04.039   Abstract

The origin of variations in 186Os/188Os ratios amongst mantle-derived basaltic and komatiitic lavas remains controversial, with opposing models arguing for deep core-mantle versus shallow mantle sources. Crustal contamination has generally not been favoured due to the low Os contents of such sources, meaning that variations in 186Os/188Os would require involvement of extremely high proportions of crustal material. Here we re-examine crustal contamination as an effective means for generating significant 186Os/188Os variations in Earth materials. Using chromitites and peridotites from the Stillwater, Muskox and Rum layered intrusions, we show that radiogenic 186Os/188Os ratios are correlated with 187Os/188Os ratios and can only be explained by shallow-level mixing processes and crustal contamination. The samples have 186Os ([{(186Os/188Ossample[t]/186Os/188OsPM(t)) -1} × 1000], where the modern primitive mantle [PM] 186Os/188Os is 0.1198388) values ranging between 0.04 to 0.15 for the ~2.7 Ga Stillwater Igneous Complex, -0.05 to 0.17 for the ~1.27 Ga Muskox Intrusion, and 0.02 to 0.13 for the ~0.06 Ga Rum Layered Suite. The highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) contents of the chromitites and peridotites can be modelled through high sulfide-melt partitioning (typically >8000) and emphasise the role of S-saturation and HSE scavenging. Considering the high sulfide-melt partitioning and accounting for high silicate melt to sulfide melt ratios (R-factor), it is possible to explain the variations in 186Os-187Os in layered intrusions using calculated Os isotope crustal evolution growth models. These calculations indicate that <4% of ancient high Pt/Os crustal contributions can explain the composition of the chromitites and peridotites that were examined. Our observations are consistent with published models for chromitite genesis that invoke either crustal melt-primitive melt mixing, or cumulate assimilation. A crustal origin for radiogenic 186Os is a possible cause for 186Os/188Os ratio variations observed in some komatiites. It is more difficult to explain radiogenic 186Os/188Os measured in Hawaiian lavas by crustal contamination processes. Instead, ancient high Pt/Os oceanic crust, shallow mantle sources such as metasomatic sulfide, or metal-rich large low-shear wave velocity provinces at the core-mantle boundary, all remain valid explanations.

Day, JMD.  2019.  Low retention of impact material by the Moon. Nature. 571:177-178.   doi: 10.1038/d41586-019-02066-w   Abstract

Simulations demonstrate that the Moon’s ability to retain material from striking impactors is lower than was previously assumed. This finding helps to explain the scarcity of precious metals in the Moon relative to Earth

Day, JMD, Harvey RP, Hilton DR.  2019.  Melt-modified lithosphere beneath Ross Island and its role in the tectono-magmatic evolution of the West Antarctic Rift System. Chemical Geology. 518:45-54.   https://doi.org/10.1016/j.chemgeo.2019.04.012   Abstract

Mantle lithosphere influences rift system tectonic evolution, yet the age and composition of rifted lithosphere is typically difficult to constrain due to limited sampling. In the West Antarctic Rift System (WARS), Cenozoic to recent alkaline volcanic rocks yield a variety of peridotite and pyroxenite xenoliths that allow sampling of lithosphere. We report osmium and helium isotope data, elemental abundances, and petrology, for a suite of xenoliths and lavas from the Hut Point Peninsula of Ross Island. Recently (<1.3 Ma) erupted basanites yield fresh dunite and harzburgite (olivine forsterite [Fo] 90.1-88.2), lherzolite (Fo90.6-87.4), and pyroxenite xenoliths (Fo89.3-87.3). The basanite lavas contain abundant large olivine xenocrysts (Fo89.7-88.0), with more ferroan matrix olivine grains (Fo83.7-81.2) and have HIMU-like incompatible trace-element signatures. The 3He/4He ratios (6.8 ±0.3RA; 2SD) defined by co-existing He-rich xenoliths indicate a mantle source distinct from high-3He/4He plume mantle. Pyroxenite and lherzolite xenoliths have similar relative abundances of incompatible trace elements to host lavas, whereas dunite xenoliths have refractory compositions. Melt-rock reaction occurring in the xenoliths is demonstrated by replacement by amphibole or clinopyroxene to form pyroxenite and lherzolite lithologies, or as amphibole-impregnated dunites. The 187Re-187Os systematics of the lavas, pyroxenites and lherzolites define an apparent isochron, with initial 187Os/188Os ratio of 0.1286 ±0.0001. The initial 187Os/188Os is within uncertainty of dunite and harzburgite xenolith Os isotope compositions (0.1279-0.1303). Pervasive evidence for melt-rock interaction indicates that the straight-line relationship in 187Re/188Os-187Os/188Os space is a mixing line between high Re/Os lavas with radiogenic 187Os/188Os, and dunite and harzburgite. Petrological and geochemical evidence indicates that dunite and harzburgite xenoliths represent young lithosphere, with rhenium depletion ages up to ~250 Ma. The timing of formation and composition of the Hut Point Peninsula xenoliths are consistent with both destruction and creation of mantle lithosphere during or after subduction along the Gondwana margin, prior to WARS formation. Modification of mantle lithosphere by subduction is also consistent with generation of HIMU-like metasomatized mantle reservoirs that fed Cenozoic to recent alkali volcanism of Mount Erebus and the WARS. The presence of young lithosphere within the WARS has collateral implications for rift dynamics and melting processes, especially beneath Mount Erebus, contrasting with older lithospheric mantle beneath the Trans-Antarctic Mountains and Marie Byrd Land.

Day, JMD, Koppers AAP, Mendenhall BC, Oller B.  2019.  The ‘Scripps Dike’ and its implications for mid-Miocene volcanism and tectonics of the California Continental Borderland. SEPM Special Publication. 110:43-55.: Society for Sedimentary Geology   10.2110/sepmsp.110.02   Abstract

New field observations, petrology, geochemistry, and 40Ar/39Ar geochronology are reported for the Scripps Dike, which crops out at the coast north of La Jolla, California. The northeast–southwest-trending and laterally discontinuous dike has a basaltic–trachyandesite bulk composition,
with an emplacement age of 13.89 6 0.13 Ma. Modeling of the dike composition indicates that it formed from 0.5 to 1.5% partial melting of a primitive mantle-type source, metasomatized by slab fluids, predominantly in the garnet stability field. The composition of the dike, including relatively high MgO (6.6 wt.%) and Sr/Y (~105), makes it akin to magnesian andesites in Baja California, Mexico, termed ‘‘bajaites.’’ Field evidence indicates
that the current exposure of the dike is close to the original stalling depth, it was probably associated with explosive volcanism, and the dike flowed laterally. After accounting for alteration, the dike has an initial 87Sr/86Sr composition of 0.70390, with limited evidence for crustal contamination, consistent with derivation from a slab-fluid-metasomatized mantle source. The composition of the dike places it broadly in the range of Miocene California Continental Borderland (hereafter referred to as Borderland) volcanic rocks studied previously. A comparison of ages of volcanic rocks occurring along the Borderland margin reveals an approximately age-progressive trend to the southeast. This represents an opposite sense to the apparent ageprogressive trend for Miocene to Recent volcanic rocks north of the Western Transverse Ranges. Possible models to explain the compositions and age relationships of Miocene to Recent volcanic rocks of the Borderland region include southeasterly migration of volcanism in response to Rivera Triple Junction movement and slab window formation, or the presence of a weak ‘‘hotspot’’ that has been active since at least the Miocene. Identification of the process(es) responsible for Borderland volcanism is currently limited by dissection and northwestward movement of Borderland rocks in response to northwest–southeast shearing of the Pacific–North American plate boundary, and by the quality and quantity of reported age-dates and paleomagnetic information. The formation processes of volcanism in the Borderland have ramifications for palinspastic reconstruction of the margin, as well as for the thermal and magmatic evolution of western California in response to a change in plate motion in a subduction to transform setting. The Scripps Dike provides evidence that regions of the mantle beneath the California Continental Borderland were metasomatized by slab fluids in a manner similar to portions of mantle beneath central Baja California, Mexico.

Chen, H, Meshik AP, Pravdivtseva OV, Day JMD, Wang K.  2019.  Potassium isotope fractionation during the high-temperature evaporation determined from the Trinity nuclear test. Chemical Geology. 522:84-92.   https://doi.org/10.1016/j.chemgeo.2019.04.02   Abstract

Trinitite materials are the post-detonation glassy residues formed from melting and evaporation of arkosic sands during the first nuclear detonation at the Trinity test site, New Mexico on 16th July, 1945. These trinitites provide useful materials for studying elemental and isotopic behaviors associated with high temperature melting and evaporation that is otherwise difficult to be achieved under laboratory conditions. Using a high-precision method, we measured the potassium (K) isotopic compositions of six bulk trinitite samples taken at different distances from the epicenter of detonation of the Gadget (ground zero). 15 leachates and etchates of trinitite samples were also analyzed to examine the distribution of K within the samples. All trinitites but IF_m (taken within 10 m from the epicenter) show no resolvable K loss and span a narrow range in K isotopic compositions (δ41K: -0.42 ± 0.05‰ to -0.48 ± 0.05‰), revealing no discernible K isotopic fractionation from the Bulk Silicate Earth (BSE) value (-0.48 ± 0.03‰). Residues and etchates of the trinitite material are identical in composition to the bulk samples implying that K isotopes were homogeneous with the arkosic sand at the Trinity test site prior to the nuclear detonation. The most strongly melted green trinitite IF_m, is the only trinitite that shows loss of K (~7%) coupled with resolvable heavier K isotope composition (0.2‰ higher in δ41K than the BSE value). This coupled K loss and isotopic fractionation corresponds to a 55 fractionation factor(avapor-melt) between 0.995 and 0.998 during the Trinity nuclear detonation. These results confirm that K isotopic fractionation occurs through evaporation processes at high temperatures. We also show that, compared with Zn isotopes measured in the same samples, the isotopes of K were significantly less fractionated during evaporation, indicating that K is less volatile during processes such as magma ocean degassing, volcanic outgassing, and impact volatile loss with the relative order of sensitivity being Cu > Zn > K. Our findings support the concept that the heavy K isotopic composition observed in lunar mare basalts reflects the primary signature imprinted by the Moon-forming giant impact event.

Inglis, EC, Moynier F, Creech J, Deng Z, Day JMD, Teng F-Z, Bizzarro M, Jackson M, Savage P.  2019.  Isotopic fractionation of zirconium during magmatic differentiation and the stable isotope composition of the silicate Earth. Geochimica et Cosmochimica Acta. 250:311-323.   10.1016/j.gca.2019.02.010   Abstract

High-precision double-spike Zr stable isotope measurements (expressed as δ94/90ZrIPGP-Zr, the permil deviation of the 94Zr/90Zr ratio from the IPGP-Zr standard) are presented for a range of ocean island basalts (OIB) and mid-ocean ridge basalts (MORB) to examine mass-dependent isotopic variations of zirconium in Earth. Ocean island basalt samples, spanning a range of radiogenic isotopic flavours (HIMU, EM) show a limited range in δ94/90ZrIPGP-Zr (0.046 ± 0.037‰; 2sd, n = 13). Similarly, MORB samples with chondrite-normalized La/Sm of >0.7 show a limited range in δ94/90ZrIPGP-Zr (0.053 ± 0.040‰; 2sd, n = 8). In contrast, basaltic lavas from mantle sources that have undergone significant melt depletion, such as depleted normal MORB (N-MORB) show resolvable variations in δ94/90ZrIPGP-Zr, from −0.045 ± 0.018 to 0.074 ± 0.023‰. Highly evolved igneous differentiates (>65 wt% SiO2) from Hekla volcano in Iceland are isotopically heavier than less evolved igneous rocks, up to 0.53‰. These results suggest that both mantle melt depletion and extreme magmatic differentiation leads to resolvable mass-dependent Zr isotope fractionation. We find that this isotopic fractionation is most likely driven by incorporation of light isotopes of Zr within the 8-fold coordinated sites of zircons, driving residual melts, with a lower coordination chemistry, towards heavier values. Using a Rayleigh fractionation model, we suggest a αzircon-melt of 0.9995 based on the whole rock δ94/90ZrIPGP-Zr values of the samples from Hekla volcano (Iceland). Zirconium isotopic fractionation during melt-depletion of the mantle is less well-constrained, but may result from incongruent melting and incorporation of isotopically light Zr in the 8-fold coordinated M2 site of orthopyroxene. Based on these observations lavas originating from the effect of melt extraction from a depleted mantle source (N-MORB) or that underwent zircon saturation (SiO2 > 65 wt%) are removed from the dataset to give an estimate of the primitive mantle Zr isotope composition of 0.048 ± 0.032‰; 2sd, n = 48. These data show that major controls on Zr fractionation in the Earth result from partial melt extraction in the mantle and by zircon fractionation in differentiated melts. Conversely, fertile mantle is homogenous with respect to Zr isotopes. Zirconium mass-dependent fractionation effects can therefore be used to trace large-scale mantle melt depletion events and the effects of felsic crust formation.

Peters, BJ, Shahar A, Carlson RW, Day JMD, Mock TD.  2019.  A sulfide perspective on iron isotope fractionation during ocean island basalt petrogenesis. Geochimica et Cosmochimica Acta. 245:59-78.   https://doi.org/10.1016/j.gca.2018.10.015   Abstract

Iron isotopic compositions are demonstrably powerful tracers of foundational planetary processes, including crust and core formation. In many volcanic environments, however, geochemical vestiges of these processes are obscured by the effects of magmatic differentiation on Fe isotopic compositions. Recent decades have witnessed continued refinement of observational and experimental approaches to Fe isotope fractionation during silicate differentiation. In contrast, the influence of sulfide fractionation on Fe isotopic compositions in terrestrial environments is known only from theoretical approaches and limited experimental data for relatively siliceous magmatic systems. One reason for this may be that sulfide fractionation is difficult to definitively trace using traditional major and minor element variation patterns. We utilize well-characterized lavas and cumulate xenoliths from Piton de la Fournaise and Piton des Neiges, Réunion Island, that have previously been examined for their highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) contents to investigate the effect of sulfide fractionation on Fe isotopes. The Fe isotopic compositions of the basalts range from δ56Fe values of 0.04 to 0.15‰ (average: 0.10‰) and the compositions of the cumulate xenoliths range from δ56Fe values of -0.07 to 0.08‰ (average: 0‰). In the absence of metal, HSE preferentially partition into sulfide phases, making them important tracers of sulfide segregation during magmatic differentiation. We find that commonly-observed co-variations between Fe isotopic compositions and major element oxide abundances are relatively underdeveloped for Réunion lavas. The correlation between Fe isotopic composition and MgO, for example, has a similar statistical significance to the correlation between Fe isotopic composition and Pd/Ir ratios, suggesting an important role of sulfides during Fe isotopic fractionation. After accounting for sulfide segregation, we determine that the parental magma Fe isotopic composition calculated for Piton de la Fournaise would be overestimated by 0.04‰ (within propagated error, 0.01-0.06‰) when considering silicate differentiation alone. An analogous calculation for Kilauea Iki basalts, for which there is available Fe isotopic and HSE data, yields a somewhat smaller difference of 0.02‰ (0-0.03‰). These differences may partially explain Fe isotopic compositions in other settings that could not previously be reconciled with a dominantly peridotitic and/or chondritic mantle source. This discovery may warrant discussion of the apparent decoupling between Fe and radiogenic isotopes in ocean island basalts, where the latter shows significant global variations and the former may show little or none. Our work highlights the need for additional constraints on the behavior of Fe isotopes during crustal recycling processes and reinforces the notion that consideration must be given to the effect of magmatic differentiation on Fe isotopic compositions.

2018
Day, JMD.  2018.  Geochemical constraints on residual metal and sulfide in the sources of lunar mare basalts. American Mineralogist. 103:1734-1740.   10.2138/am-2018-6368   Abstract

Low oxygen fugacity (fO2) in the lunar interior (one log unit below the iron-wüstite buffer [IW-1]) offers the possibility that stable Fe-metal and sulfide phases exist as restites within lunar mare basalt source regions. Metal and sulfide phases have high metal-melt and sulfide-melt partition coefficients for chalcophile, siderophile (>100), and highly siderophile elements (>>100,000 - HSE: Os, Ir, Ru, Rh, Pt, Pd, Re, Au). If these phases are residual after mare basalt extraction, they would be expected to retain significant quantities of these elements, likely generating non-chondritic HSE inter-element ratios, including Re/Os in the silicate magma. If such phases were present, the estimated HSE abundances of the bulk silicate Moon (BSM) would be proportionally higher than current estimates (0.00023 ±2 × CI chondrite), and perhaps closer to the bulk silicate Earth (BSE) estimate (0.009 ±2 × CI chondrite). Here I show that relationships between elements of similar incompatibility but with siderophile (W), chalcophile (Cu) and lithophile tendencies (Th, U, Yb) do not deviate from expected trends generated by magmatic differentiation during cooling and crystallization of mare basalts. These results, combined with chondrite-relative HSE abundances and near-chondritic measured 187Os/188Os compositions of primitive high-MgO mare basalts, imply that lunar mantle melts were generated from residual metal- and sulfide-free sources, or experienced complete exhaustion of metal and sulfides during partial melt extraction. Evidence for the loss of moderately volatile elements during lunar formation and early differentiation indicates that the BSM is >4 to 10 times more depleted in S than BSE. Because of an S-depleted BSM, mare basalt melts are unlikely to have reached S saturation, even if sulfide concentration at sulfide saturation (SCSS) was lowered relative to terrestrial values due to low lunar fO2. In the absence of residual sulfide or metal, resultant partial melt models indicate that a lunar mantle source with 25 to 75 ug g-1 S and high sulfide-melt partition coefficients can account for the chondritic relative abundances of the HSE in mare basalts from a BSM that experienced <0.02% by mass of late accretion.

Day, JMD, Tait KT, Udry A, Moynier F, Liu Y, Neal CR.  2018.  Martian magmatism from plume metasomatized mantle. Nature Communications. 9:4799.   10.1038/s41467-018-07191-0   Abstract

Direct analysis of the composition of Mars is possible through delivery of meteorites to Earth. Martian meteorites include ∼165 to 2400 Ma shergottites, originating from depleted to enriched mantle sources, and ∼1340 Ma nakhlites and chassignites, formed by low degree partial melting of a depleted mantle source. To date, no unified model has been proposed to explain the petrogenesis of these distinct rock types, despite their importance for understanding the formation and evolution of Mars. Here we report a coherent geochemical dataset for shergottites, nakhlites and chassignites revealing fundamental differences in sources. Shergottites have lower Nb/Y at a given Zr/Y than nakhlites or chassignites, a relationship nearly identical to terrestrial Hawaiian main shield and rejuvenated volcanism. Nakhlite and chassignite compositions are consistent with melting of hydrated and metasomatized depleted mantle lithosphere, whereas shergottite melts originate from deep mantle sources. Generation of martian magmas can be explained by temporally distinct melting episodes within and below dynamically supported and variably metasomatized lithosphere, by long-lived, static mantle plumes.

Udry, A, Day JMD.  2018.  1.34 billion-year-old magmatism on Mars evaluated from the co-genetic nakhlite and chassignite meteorites. Geochimica et Cosmochimica Acta. 238:292-315.   https://doi.org/10.1016/j.gca.2018.07.006   Abstract

Nakhlite and chassignite martian meteorites have similar crystallization (1340 ± 40 Ma) and ejection (∼11 Ma) ages, and 87Rb-87Sr and 143Sm-144Nd compositions. Using a near-comprehensive suite of these rocks, we place further constraints on nakhlite and chassignite petrogenesis, utilizing bulk rock and mineral major- and trace-element compositions, and quantitative textural data for 17 samples, including three recent finds (Northwest Africa [NWA] 10153, NWA 10645, and NWA 11013). Bulk rock and mineral compositions indicate that nakhlites and chassignites originated from <5% partial melting of a highly depleted source, in the presence of residual garnet. Significant fractionation of olivine and pyroxene from parental magmas led to formation of cumulate dunites (chassignites), and augite-rich cumulates with relatively low abundances of interstitial material (nakhlites). We show that two nakhlite groups exist with high and low absolute trace-element abundances, which are consistent with groupings from previous studies based on mesostasis content and volatile element contents. The discrepancy between the parental melt and cumulate bulk rock compositions indicates that a missing fractionated melt composition complementary to nakhlites and chassignites should exist on Mars. Quantitative textural analyses of both nakhlites and chassignites are consistent with emplacement as distinct lava flows and/or magmatic bodies close to the martian surface, rather than from a single sill or lava flow sequence. Although originating from the same parental melt to nakhlites, chassignites likely represent cumulates that were either erupted as xenoliths, or occurred as crystal settling pods within dikes or sills and thus represent a different batch of flow/magma from the nakhlites. Determination of an ancient 207Pb-206Pb age (3.95 ± 0.16 Ga) for an apatite grain in NWA 998 is consistent with hydrothermal alteration of nakhlites by ancient crustal-derived fluids immediately following their emplacement. We interpret the apatite age, which is highly distinct from the crystallization age of nakhlites, to indicate addition of Cl-rich fluids driven by hydrothermal circulation of martian crustal brines during emplacement of the nakhlites and chassignites. Although the spatial location of nakhlites and chassignites at the martian surface remains unconstrained, our results indicate similar emplacement features to those observed in terrestrial volcano-magmatic systems.

Sato, KN, Andersson AJ, Day JMD, Taylor JRA, Frank MB, Jung JY, McKittrick J, Levin LA.  2018.  Response of sea urchin fitness traits to environmental gradients across the Southern California oxygen minimum zone. Frontiers in Marine Science. 5   10.3389/fmars.2018.00258   AbstractWebsite

Marine calcifiers are considered to be among the most vulnerable taxa to climate-forced environmental changes occurring on continental margins with effects hypothesized to occur on microstructural, biomechanical, and geochemical properties of carbonate structures. Natural gradients in temperature, salinity, oxygen, and pH on an upwelling margin combined with the broad depth distribution (100-1,100 m) of the pink fragile sea urchin, Strongylocentrotus (formerly Allocentrotus) fragilis, along the southern California shelf and slope provide an ideal system to evaluate potential effects of multiple climate variables on carbonate structures in situ. We measured, for the first time, trait variability across four distinct depth zones using natural gradients as analogues for species-specific implications of oxygen minimum zone (OMZ) expansion, deoxygenation and ocean acidification. Although S. fragilis may likely be tolerant of future oxygen and pH decreases predicted during the twenty-first century, we determine from adults collected across multiple depth zones that urchin size and potential reproductive fitness (gonad index) are drastically reduced in the OMZ core (450-900 m) compared to adjacent zones. Increases in porosity and mean pore size coupled with decreases in mechanical nanohardness and stiffness of the calcitic endoskeleton in individuals collected from lower pH(Total) (7.57-7.59) and lower dissolved oxygen (13-42 mu mol kg(-1)) environments suggest that S. fragilis may be potentially vulnerable to crushing predators if these conditions become more widespread in the future. In addition, elemental composition indicates that S. fragilis has a skeleton composed of the low Mg-calcite mineral phase of calcium carbonate (mean Mg/Ca = 0.02 mol mol(-1)), with Mg/Ca values measured in the lower end of values reported for sea urchins known to date. Together these findings suggest that ongoing declines in oxygen and pH will likely affect the ecology and fitness of a dominant echinoid on the California margin.

Tait, KT, Day JMD.  2018.  Chondritic late accretion to Mars and the nature of shergottite reservoirs. Earth and Planetary Science Letters. 494:99-108.   https://doi.org/10.1016/j.epsl.2018.04.040   AbstractFree for 50 days

Mars is considered to have formed as a planetary embryo that experienced extensive differentiation early in its history. Shergottite meteorites preserve evidence for this history, and for late accretion events that affected their mantle sources within Mars. Here we report the first coupled 187Re–187Os, 87Sr/86Sr, highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) and major element abundance dataset for martian shergottites that span a range of MgO contents, from 6.4 to 30.3 wt.%. The shergottites range from picro-basalt to basaltic-andesite compositions, have enriched to depleted incompatible trace-element compositions, and define fractional crystallization trends, enabling the determination of HSE compatibility for martian magmatism in the order: Os > Ir ≥ Ru ≫ Pt ≥ Pd ≥ Re. This order of compatibility is like that defined previously for Earth and the Moon, but the fractionation of strongly compatible Os, Ir and Ru appears to take place at higher MgO contents in martian magmas, due to early onset of sulfide fractionation. In general, enriched shergottites have lower MgO contents than intermediate or depleted shergottites and have fractionated HSE patterns (Re + Pd + Pt > Ru + Ir + Os) and more radiogenic measured 87Sr/86Sr (0.7127–0.7235) and 187Os/188Os (0.140–0.247) than intermediate or depleted shergottite meteorites (87Sr/86Sr = 0.7010–0.7132; 187Os/188Os = 0.127–0.141). Osmium isotope compositions, corrected for crystallization age, define compositions that are implausibly unradiogenic in some enriched shergottites, implying recent mobilization of Re in some samples. Filtering for the effects of alteration and high Re/Os through crystal-liquid fractionation leads to a positive correlation between age-corrected Sr and Os isotope compositions. Mixing between hypothetical martian crustal and mantle reservoirs are unable to generate the observed Sr–Os isotope compositions of shergottites, which require either distinct and discrete long-term incompatible-element depleted and enriched mantle sources, or originate from hybridized melting of deep melts with metasomatized martian lithosphere. Using MgO-regression methods, we obtain a modified estimate of the bulk silicate Mars HSE composition of (in ng g−1) 0.4 [Re], 7.4 [Pd], 9.6 [Pt], 6.2 [Ru], 3.7 [Ir], 4 [Os], and a long-term chondritic 187Os/188Os ratio (∼0.1312). This result does not permit existing models invoking high-pressure and temperature partitioning of the HSE. Instead, our estimate implies 0.6–0.7% by mass of late accretion of broadly chondritic material to Mars. Our results indicate that Mars could have accreted earlier than Earth, but that disproportional accretion of large bodies and a relative constant flux of accretion of available materials in the first 50–100 Ma of Solar System led to the broad similarity in HSE abundances between Earth and Mars.

O'Driscoll, B, Garwood R, Day JMD, Wogelius RA.  2018.  Platinum-group element remobilisation and concentration in the Cliff chromitites of the Shetland Ophiolite Complex, Scotland. Mineralogical Magazine. 82:471-490.   https://doi.org/10.1180/minmag.2017.081.108   Abstract

The ~492 Ma Shetland Ophiolite Complex (SOC) contains an extensive mantle section, within which numerous podiform chromitite bodies formed during melt percolation in a supra-subduction zone setting. One of the SOC chromitite localities has an unusual style of platinum-group element (PGE) mineralisation. Specifically, the Cliff chromitite suite has relatively high (>250 ppm) Pt plus Pd, compared to other SOC chromitites. In this study, we use petrographic observation, mineral chemistry and X-ray microtomography to elucidate the petrogenesis of PGE-bearing phases at Cliff. The combined data reveal that the PGE at Cliff have likely been fractionated by an As-rich fluid, concentrating Pt and Ir into visible (0.1-1 μm) platinum-group minerals (PGM) such as sperrylite and irarsite, respectively. The high (>1 ppm) bulk rock concentrations of the other PGE (e.g., Os) in the Cliff chromitites suggests the presence of abundant fine-grained unidentified PGM in the serpentinised groundmass. The spatial association of arsenide phases and PGM with alteration rims on Cr-spinel grains suggests that the high Pt and Pd abundances at Cliff result from a late-stage low-temperature (e.g., 200-300°C) hydrothermal event. This conclusion highlights the potential effects that secondary alteration processes can have on modifying and upgrading the tenor of PGE deposits.

Day, JMD, Maria-Benavides J, McCubbin FM, Zeigler RA.  2018.  The potential for metal contamination during Apollo lunar sample curation. Meteoritics and Planetary Science. 53:1283-1291.   https://doi.org/10.1111/maps.13074   Abstract

Curation and preparation of samples for chemical analysis can occasionally lead to significant contamination. This issue is of concern in the study of lunar samples, especially those from the Apollo sample collection, where available masses are finite. Here we present compositional data for stainless steels that have commonly been used in the processing of Apollo lunar samples at NASA Johnson Space Center, including a chisel and a vessel typically used to transfer Apollo samples to principal investigators. The Type 304 stainless steels are Cr-rich, with high concentrations of Mn (4000–18,000 ug g1), Cu (1000–22,900 ug g1), Mo (1030–1120 ug g1), and W (72-193 ug g1). They have elevated highly siderophile element (HSE) concentrations (up to 92 ng g1 Os), 187Os/188Os ranging from 0.1310 to 0.1336, and negligible lithophile element abundances. We find that, while metal contamination is possible, significant (≫0.01% by mass) addition of stainless steel is required to strongly affect the composition of the HSE, W, Mo, Cr, or Cu for most Apollo lunar samples. Nonetheless, careful appraisal on a case-by-case basis should take place to ensure contamination introduced through sample processing during curation is at acceptably low levels. A survey of lunar mare basalts and crustal rocks indicates that metal contamination plays a negligible role in the compositional variability of the HSE and W compositions preserved in these samples. Further work to constrain contamination for other properties of Apollo samples is required (e.g., organics, microbes, water, noble gases, and magnetics), but the effect of metal contamination can be well-constrained for the Apollo lunar collection.

O'Driscoll, B, Walker RJ, Clay PL, Day JMD, Ash RD, Daly JS.  2018.  Length-scales of chemical and isotopic heterogeneity in the mantle section of the Shetland Ophiolite Complex, Scotland. Earth and Planetary Science Letters. 488:144-154.   https://doi.org/10.1016/j.epsl.2018.02.020   Abstract

Kilometre to sub-metre scale heterogeneities have been inferred in the oceanic mantle based on sampling of both ophiolites and abyssal peridotites. The ∼492 Ma Shetland Ophiolite Complex (SOC) contains a well-preserved mantle section that is dominated by harzburgite (∼70 vol.%) previously reported to have variable major and trace element compositions, yet dominantly chondritic initial 187Os/188Os compositions. To assess the preservation of compositional heterogeneities at sub-metre length-scales in the oceanic mantle, a ∼45 m2 area of the SOC mantle section was mapped and sampled in detail. Harzburgites, dunites and a pyroxenite from this area were analysed for lithophile and highly-siderophile element (HSE) abundances, as well as for 187Os/188Os ratios. Lithophile element data for most rocks are characteristic of supra-subduction zone (SSZ) metasomatic processes. Two dunites have moderately fractionated HSE patterns and suprachondritic γOs(492 Ma) values (+5.1 and +7.5) that are also typical of ophiolitic dunites generated by SSZ melt–rock interactions. By contrast, six harzburgites and four dunites have approximately chondritic-relative abundances of Os, Ir and Ru, and γOs(492 Ma) values ranging only from −0.6 to +2.7; characteristics that imply no significant influence during SSZ processes. Two harzburgites are also characterised by significantly less radiogenic γOs(492 Ma) values (−3.5 and −4), and yield Mesoproterozoic time of Re depletion (TRD) model ages. The range of Os isotope compositions in the studied area is comparable to the range reported for a suite of samples representative of the entire SOC mantle section, and approaches the total isotopic variation of the oceanic mantle, as observed in abyssal peridotites. Mechanisms by which this heterogeneity can be formed and preserved involve inefficient and temporally distinct melt extraction events and strong localised channelling of these melts.

Amsellam, E, Moynier F, Day JMD, Moriera M, Puchtel IS, Teng F-Z.  2018.  The stable strontium isotopic composition of ocean island basalts, mid-ocean ridge basalts, and komatiites. Chemical Geology. 483:595-602.   https://doi.org/10.1016/j.chemgeo.2018.03.030   Abstract

The radiogenic 87Rb-87Sr system has been widely applied to the study of geological and planetary processes. In contrast, the stable Sr isotopic composition of the bulk silicate Earth (BSE) and the effects of igneous differentiation on stable Sr isotopes are not well-established. Here we report the stable Sr isotope (88Sr/86Sr, reported as δ88/86Sr, in parts per mil relative to NIST SRM 987) compositions for ocean islands basalts (OIB), mid-ocean ridge basalts (MORB) and komatiites from a variety of locations. Stable Sr isotopes display limited fractionation in a OIB sample suite from the Kilauea Iki lava lake suggesting that igneous processes have limited effect on stable Sr isotope fractionation (±0.12‰ over 20% MgO variation; 2sd). In addition, OIB (δ88/86Sr = 0.16–0.46‰; average 0.28 ± 0.17‰), MORB (δ88/86Sr = 0.27–0.34‰; average 0.31 ± 0.05‰) and komatiites (δ88/86Sr = 0.20–0.97‰; average 0.41 ± 0.16‰) from global localities exhibit broadly similar Sr stable isotopic compositions. Heavy stable Sr isotope compositions (δ88/86Sr > 0.5‰) in some Barberton Greenstone belt komatiites may reflect Archean seawater alteration or metamorphic processes and preferential removal of the lighter isotopes of Sr. To first order, the similarity among OIBs from three different ocean basins suggests homogeneity of stable Sr isotopes in the mantle. Earth's mantle stable Sr isotopic composition is established from the data on OIB, MORB and komatiites to be δ88/86Sr = 0.30 ± 0.02‰ (2sd). The BSE δ88/86Sr value is identical, within uncertainties, to the composition of carbonaceous chondrites (δ88/86Sr = 0.29 ± 0.06‰; 2sd) measured in this study.

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.