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Day, JMD, Taylor LA, Floss C, Patchen AD, Schnare DW, Pearson DG.  2006.  Comparative petrology, geochemistry, and petrogenesis of evolved, low-Ti lunar mare basalt meteorites from the LaPaz Icefield, Antarctica. Geochimica Et Cosmochimica Acta. 70:1581-1600.   10.1016/j.gca.2005.11.015   AbstractWebsite

New data is presented for five evolved, low-Ti lunar mare basalt meteorites from the LaPaz Icefield, Antarctica, LAP 02205, LAP 02224, LAP 02226, LAP 02436, and LAP 03632. These basalts have nearly identical mineralogies, textures, and geochemical compositions, and are therefore considered to be paired. The LaPaz basalts contain olivine (Fo(64-2)) and pyroxene (Fs(32)Wo(8)En(60) to Fs(84-86)Wo(15)En(2-0)) crystals that record extreme chemical fractionation to Fe-enrichment at the rims, and evidence for silicate liquid immiscibility and incompatible element enrichment in the mesostasis. The basalts also contain FeNi metals with unusually high Co and Ni contents, similar to some Apollo 12 basalts, and a single-phase network of melt veins and fusion crusts. The fusion crust has similar chemical characteristics to the whole rock for the LaPaz basalts, whereas the melt veins represent localized melting of the basalt and have an endogenous origin. The crystallization conditions and evolved nature of the LaPaz basalts are consistent with fractionation of olivine and chromite from a parental liquid similar in composition to some olivine-phyric Apollo 12 and Apollo 15 basalts or lunar low-Ti pyroclastic glasses. However, the young reported ages for the LaPaz mare basalts (similar to 2.9 Ga) and their relative incompatible element enrichment compared to Apollo mare basalts and pyroclastic glasses indicate they cannot be directly related. Instead, the LaPaz mare basalts may represent fractionated melts from a magmatic system fed by similar degrees of partial melting of a mantle source similar to that of the low-Ti Apollo mare basalts or pyroclastic glasses, but which possessed greater incompatible element enrichment. Despite textural differences, the LaPaz basalts and mare basalt meteorite NWA 032 have similar ages and compositions and may originate from the same magmatic system on the Moon. (c) 2005 Elsevier Inc. All rights reserved.

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.

Day, JMD, Corder CA, Rumble D, Assayag N, Cartigny P, Taylor LA.  2015.  Differentiation processes in FeO-rich asteroids revealed by the achondrite Lewis Cliff 88763. Meteoritics & Planetary Science. 50:1750-1766.   10.1111/maps.12509   AbstractWebsite

Olivine-dominated (70-80 modal %) achondrite meteorite Lewis Cliff (LEW) 88763 originated from metamorphism and limited partial melting of a FeO-rich parent body. The meteorite experienced some alteration on Earth, evident from subchondritic Re/Os, and redistribution of rhenium within the sample. LEW 88763 is texturally similar to winonaites, has a Delta O-17 value of -1.19 +/- 0.10 parts per thousand, and low bulk-rock Mg/(Mg+Fe) (0.39), similar to the FeO-rich cumulate achondrite Northwest Africa (NWA) 6693. The similar bulk-rock major-, minor-, and trace-element abundances of LEW 88763, relative to some carbonaceous chondrites, including ratios of Pd/Os, Pt/Os, Ir/Os, and Os-187/Os-188 (0.1262), implies a FeO-and volatile-rich precursor composition. Lack of fractionation of the rare earth elements, but a factor of approximately two lower highly siderophile element abundances in LEW 88763, compared with chondrites, implies limited loss of Fe-Ni-S melts during metamorphism and anatexis. These results support the generation of high Fe/Mg, sulfide, and/or metal-rich partial melts from FeO-rich parent bodies during partial melting. In detail, however, LEW 88763 cannot be a parent composition to any other meteorite sample, due to highly limited silicate melt loss (0 to << 5%). As such, LEW 88763 represents the least-modified FeO-rich achondrite source composition recognized to date and is distinct from all other meteorites. LEW 88763 should be reclassified as an anomalous achondrite that experienced limited Fe, Ni-FeS melt loss. Lewis Cliff 88763, combined with a growing collection of FeO-rich meteorites, such as brachinites, brachinite-like achondrites, the Graves Nunataks (GRA) 06128/9 meteorites, NWA 6693, and Tafassasset, has important implications for understanding the initiation of planetary differentiation. Specifically, regardless of precursor compositions, partial melting and differentiation processes appear to be similar on asteroidal bodies spanning a range of initial oxidation states and volatile contents.

Day, JMD, Pearson DG, Macpherson CG, Lowry D, Carracedo JC.  2009.  Pyroxenite-rich mantle formed by recycled oceanic lithosphere: Oxygen-osmium isotope evidence from Canary Island lavas. Geology. 37:555-558.   10.1130/g25613a.1   AbstractWebsite

Plate tectonic processes result in recycling of crust and lithosphere into Earth's mantle. Evidence for long-term preservation of recycled reservoirs in the mantle comes from the enriched isotopic character of oceanic island basalt (OIB) lavas. Although recycled constituents can explain much of the geochemical variation in the OIB-source mantle, it has been shown that direct melting of these components would lead to magmas with evolved compositions, unlike OIB. Instead, it has been argued that either metasomatic pyroxene-rich peridotite that has inherited the trace element and isotopic character of subducted materials, or high-temperature intramantle metasomatism of lithosphere can explain OIB compositions. To test these models, we present new oxygen and osmium isotope data for lavas from the Canary Islands of El Hierro and La Palma. These islands have distinct (18)O/(16)O and (187)Os/(188)Os compositions that can be explained through melting of pyroxenite-enriched peridotite mantle containing <10% recycled oceanic lithosphere. We also assess O-Os isotope systematics of lavas from Hawai'i and the Azores and show that they also conform to addition of distinct recycled oceanic components, including lithosphere and pelagic sediment. We conclude that enriched isotopic signatures of some OIBs are consistent with pyroxenite-rich mantle sources metasomatized by recycled components.

Day, JMD, Moynier F, Shearer CK.  2017.  Late-stage magmatic outgassing from a volatile-depleted Moon. Proceedings of the National Academy of Sciences. 114(35):9547-9551.   10.1073/pnas.1708236114   AbstractWebsite

The abundance of volatile elements and compounds, such as zinc, potassium, chlorine, and water, provide key evidence for how Earth and the Moon formed and evolved. Currently, evidence exists for a Moon depleted in volatile elements, as well as reservoirs within the Moon with volatile abundances like Earth’s depleted upper mantle. Volatile depletion is consistent with catastrophic formation, such as a giant impact, whereas a Moon with Earth-like volatile abundances suggests preservation of these volatiles, or addition through late accretion. We show, using the “Rusty Rock” impact melt breccia, 66095, that volatile enrichment on the lunar surface occurred through vapor condensation. Isotopically light Zn (δ66Zn = −13.7‰), heavy Cl (δ37Cl = +15‰), and high U/Pb supports the origin of condensates from a volatile-poor internal source formed during thermomagmatic evolution of the Moon, with long-term depletion in incompatible Cl and Pb, and lesser depletion of more-compatible Zn. Leaching experiments on mare basalt 14053 demonstrate that isotopically light Zn condensates also occur on some mare basalts after their crystallization, confirming a volatile-depleted lunar interior source with homogeneous δ66Zn ≈ +1.4‰. Our results show that much of the lunar interior must be significantly depleted in volatile elements and compounds and that volatile-rich rocks on the lunar surface formed through vapor condensation. Volatiles detected by remote sensing on the surface of the Moon likely have a partially condensate origin from its interior.

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.  2016.  Extraordinary World. Nature. 537:310-311.   10.1038/537310a   Abstract

The isotopic compositions of objects that formed early in the evolution of the Solar System have been found to be similar to Earth's composition — overturning notions of our planet's chemical distinctiveness.

Day, JMD, Pearson DG, Hulbert LJ.  2008.  Rhenium-osmium isotope and platinum-group element constraints on the origin and evolution of the 1.27 Ga Muskox layered intrusion. Journal of Petrology. 49:1255-1295.   10.1093/petrology/egn024   AbstractWebsite

Platinum-group element (PGE: Os, Ir, Ru, Pt, Pd) and Re-Os isotope systematics determined for the entire preserved stratigraphy of the 1.27 Ga Muskox intrusion provide an exceptional view of magma chamber processes and mineralization in the main plutonic system of the Mackenzie large igneous province (LIP). We present new Re-Os isotope data for the intrusion, together with PGE and trace element abundances, and oxygen and Sm-Nd isotope data on samples that include local crustal materials, layered series peridotites, stratiform chromitites, marginal and roof zone rocks, and the Muskox Keel feeder dyke. Intrusive rocks span wide ranges in initial isotopic compositions (gamma(Os)i=+1.0 to +87.6; epsilon(Nd)i = -0.4 to -6.6; delta(18)O(Ol) =+ 5.5 to + 6.9 parts per thousand) and highly siderophile element abundances (HSE: PGE and Re; Re =0.02-105 ppb; Pt =0.23-115 ppb; O(s)= 0.02 to > 200 ppb). HSE and fluid-immobile trace element abundance variations are consistent with relative compatibilities expected for cumulate rocks. The most radiogenic Os and unradiogenic Nd isotope compositions occur in the Muskox marginal and roof zones. Negative gamma(Os)i values in these rocks and their non-isochronous relations result from mobilization of Re in the intrusion through post-magmatic hydrothermal processes. The most significant process causing Os and Nd isotope variations in the layered series of the intrusion is crustal contamination of mantle-derived magma batches feeding individual cyclic units. This process may be directly responsible for formation of chromitite horizons within the intrusion. Accounting for crustal assimilation, the Muskox intrusion parental magma has gamma(Os)i = +1.2 +/- 0.3, epsilon(Nd)i > -1.0 +/- 0.4, delta(18)O similar to +5.5 parts per thousand and HSE abundances similar to those expected from >= 15% partial melting of the Mackenzie LIP mantle source. This composition is similar to that calculated for 1.27 Ga primitive upper mantle. Parental magmas were probably derived from a mantle source unaffected by long- term, large-scale melt depletion, with no appreciable input from recycled crust and lithosphere, or putative core contributions.

Day, JMD, Moynier F, Meshik AP, Pradivtseva OV, Pettit DR.  2017.  Evaporative fractionation of zinc during the first nuclear detonation. Science Advances. 3(2):e1602668.   10.1126/sciadv.1602668   Abstract

Volatile element and compound abundances vary widely in planets and were set during the earliest stages of solar system evolution. Experiments or natural analogs approximating these early conditions are limited. Using silicate glass formed from arkosic sands during the first nuclear detonation at the Trinity test site, New Mexico, we show that the isotopes of zinc were fractionated during evaporation. The green silicate glasses, termed “trinitite,” show +0.5 ± 0.1‰/atomic mass unit isotopic fractionation from ~200 m to within 10 m of ground zero of the detonation, corresponding to an α fractionation factor between 0.999 and 0.9995. These results confirm that Zn isotopic fractionation occurs through evaporation processes at high temperatures. Evidence for similar fractionations in lunar samples consequently implies a volatile-depleted bulk Moon, with evaporation occurring during a giant impact or in a magma ocean.

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.

Day, JMD.  2016.  Evidence against an ancient non-chondritic mantle source for North Atlantic Igneous Province lavas. Chemical Geology. 440:91-100.   10.1016/j.chemgeo.2016.07.002   Abstract

North Atlantic Igneous Province (NAIP) lavas host olivine with the highest 3He/4He ever measured for terrestrial
igneous rocks (up to 50 RA, or 4He/3He = ~15,300). The relationship of high-3He/4He with Pb isotope compositions
close to the terrestrial geochron and 143Nd/144Nd plausibly consistentwith supra-chondritic mantle Sm/Nd
in Baffin Island and West Greenland lavas has been interpreted to reflect an ancient ‘non-chondritic’ mantle
source signature. Alternatively, assimilation of continental crustal rocks with unradiogenic Pb isotope compositions
and low 143Nd/144Nd, into magmaswith high-3He/4He, and derived from variably depleted mantle sources,
could impart similar geochemical signatures. Radiogenic and stable isotope data for NAIP lavas are consistent
with origins as melts from upper mantle sources that contain low-18O/16O recycled lithosphere and/or hydrothermally
altered crust, or that have experienced pervasive contamination by crustal gneisses. Olivines from
NAIP lavas with 3He/4He spanning from 8 to 48 RA have δ18O ranging from 3.5 to 5.5‰. These compositions
are consistent with sources of ambient mantle and low-δ18O recycled lithosphere, or with concomitant crustal
assimilation and He-loss during fractional crystallization. Limited assimilation (≤1%) of incompatible element
rich crustal gneisses with low 206Pb/204Pb and 143Nd/144Nd by melts from variably depleted mantle sources
can explain Nd-Pb isotope compositions of Baffin Island and West Greenland picrites. Icelandic lavas provide
supporting evidence that the ancestral mantle plume responsible for generating NAIP magmatism sampled variably
enriched and depletedmantle,with no evidence for ancient non-chondriticmantle sources. Pervasive crustal
contamination and partial melting of heterogeneous mantle sources, generated by plate tectonic processes, can
account for the compositions of continental flood basalts (CFB),without the requirement of a non-chondritic terrestrial
reservoir. Combined with evidence that the 142Nd/144Nd composition of the bulk silicate Earth is due to
nucleosynthetic S-process deficits in chondrite meteorites, these observations cast doubt thatNAIP lavas sampled
a non-chondritic mantle source with Sm/Nd higher than in chondrites. If short-lived radiogenic (e.g.,
146Sm-142Nd, 182Hf-182W)isotope anomalies are found in CFB, theymust either reflect assimilation of isotopically
anomalous crustal materials, or partial melting of early-formed mantle heterogeneities produced by differentiation
and late accretion.

Day, JMD, Walker RJ, Ash RD, Liu Y, Rumble D, Irving AJ, Goodrich CA, Tait K, McDonough WF, Taylor LA.  2012.  Origin of felsic achondrites Graves Nunataks 06128 and 06129, and ultramafic brachinites and brachinite-like achondrites by partial melting of volatile-rich primitive parent bodies. Geochimica Et Cosmochimica Acta. 81:94-128.   10.1016/j.gca.2011.12.017   AbstractWebsite

New major- and trace-element abundances, highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances, and oxygen and rhenium-osmium isotope data are reported for oligoclase-rich meteorites Graves Nunataks 06128 and 06129 (GRA 06128/9), six brachinites (Brachina; Elephant Morraine 99402/7; Northwest Africa (NWA) 1500; NWA 3151; NWA 4872; NWA 4882) and three olivine-rich achondrites, which are referred to here as brachinite-like achondrites (NWA 5400; NWA 6077; Zag (b)). GRA 06128/9 represent examples of felsic and highly-sodic melt products from an asteroid that may provide a differentiation complement to brachinites and/or brachinite-like achondrites. The new data, together with our petrological observations, are consistent with derivation of GRA 06128/9, brachinites and the three brachinite-like achondrites from nominally volatile-rich and oxidised 'chondritic' precursor sources within their respective parent bodies. Furthermore, the range of Delta O-17 values (similar to 0 parts per thousand to -0.3 parts per thousand) among the meteorites indicates generation from isotopically heterogeneous sources that never completely melted, or isotopically homogenised. It is possible to generate major-and trace-element compositions similar to brachinites and the three studied brachinite-like achondrites as residues of moderate degrees (13-30%) of partial melting of primitive chondritic sources. This process was coupled with inefficient removal of silica-saturated, high Fe/Mg felsic melts with compositions similar to GRA 06128/9. Melting of the parent bodies of GRA 06128/9, brachinites and brachinite-like achondrites halted well before extensive differentiation, possibly due to the exhaustion of the short-lived radionuclide Al-26 by felsic melt segregation. This mechanism provides a potential explanation for the cessation of run-away melting in asteroids to preserve achondrites such as GRA 06128/9, brachinites, brachinite-like achondrites, acapulcoite-lodranites, ureilites and aubrites. Moderate degrees of partial melting of chondritic material and generation of Fe-Ni-S-bearing melts are generally consistent with HSE abundances that are within factors of similar to 2-10 x CI-chondrite abundances for GRA 06128/9, brachinites and the three brachinite-like achondrites. However, in detail, brachinite-like achondrites NWA 5400, NWA 6077 and Zag (b) are interpreted to have witnessed single-stage S-rich metal segregation, whereas HSE in GRA 06128/9 and brachinites have more complex heritages. The HSE compositions of GRA 06128/9 and brachinites require either: (1) multiple phases in the residue (e. g., metal and sulphide); (2) fractionation after generation of an initial melt, again involving multiple phases; (3) fractional fusion, or; (4) a parent body with non-chondritic relative HSE abundances. Petrological and geochemical observations permit genetic links (i.e., same parent body) between GRA 06128/9 and brachinites and similar formation mechanisms for brachinites and brachinite-like achondrites. (C) 2011 Elsevier Ltd. All rights reserved.

Day, JMD, Pearson DG, Macpherson CG, Lowry D, Carracedo JC.  2010.  Evidence for distinct proportions of subducted oceanic crust and lithosphere in HIMU-type mantle beneath El Hierro and La Palma, Canary Islands. Geochimica Et Cosmochimica Acta. 74:6565-6589.   10.1016/j.gca.2010.08.021   AbstractWebsite

Shield-stage high-MgO alkalic lavas from La Palma and El Hierro (Canary Islands) have been characterized for their O-Sr-Nd-Os-Pb isotope compositions and major-, trace-, and highly siderophile-element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances. New data are also reported for associated evolved rocks, and entrained xenoliths. Clear differences in Pd/Ir and isotopic ratios for high Os (>50 ppt) lavas from El Hierro (delta(18)O(olivine) = 5.17 +/- 0.08 parts per thousand; (87)Sr/(86)Sr = 0.7029 to 0.7031; epsilon(Nd) = +5.7 to +7.1; (187)Os/(188)Os = 0.1481 to 0.1750; (206)Pb/(204)Pb = 19.1 to 19.7; Pd/Ir = 6 +/- 3) versus those from La Palma (delta(18)Oo(livine) = 4.87 +/- 0.18 parts per thousand; (87)Sr/(86)Sr = 0.7031 to 0.7032; epsilon(Nd) = +5.0 to +6.4; (187)Os/(188)Os = 0.1421 to 0.1460; (206)Pb/(204)Pb = 19.5 to 20.2; Pd/Ir = 11 +/- 4) are revealed from the dataset. Crustal or lithospheric assimilation during magma transport cannot explain variations in isotopic ratios or element abundances of the lavas. Shallow-level crystal-liquid fractionation of olivine, clinopyroxene and associated early-crystallizing minerals (e.g., spinel and USE-rich phases) controlled compatible element and HSE abundances; there is also evidence for sub-aerial degassing of rhenium. High-MgO lavas are enriched in light rare earth elements, Nb, Ta, U, Th, and depleted in K and Pb, relative to primitive mantle abundance estimates, typical of HIM U-type oceanic island basalts. Trace element abundances and ratios are consistent with low degrees (2-6%) of partial melting of an enriched mantle source, commencing in the garnet stability field (>= 110 km). Western Canary Island lavas were sulphur undersaturated with estimated parental melt HSE abundances (in ppb) of 0.07 +/- 0.05 Os, 0.17 +/- 0.16 Ir, 0.34 +/- 0.32 Ru, 2.6 +/- 2.5 Pt, 1.4 +/- 1.2 Pd, 0.39 +/- 0.30 Re. These estimates indicate that Canary Island alkali basalts have lower Os, Ir and Ru, but similar Pt, Pd and Re contents to Hawai'ian tholeiites. The HIMU affinities of the lavas, in conjunction with the low delta(18)O(olivine) and high (206)Pb/(204)Pb for La Palma, and elevated (187)Os/(188)Os for El Hierro implies melting of different proportions of recycled oceanic crust and lithosphere. Our preferred model to explain isotopic differences between the islands is generation from peridotitic mantle metasomatised by <10% pyroxenite/eclogite made from variable portions of similar aged recycled oceanic crust and lithosphere. The correspondence of radiogenic (206)Pb/(204), (187)Os/(188)Os, elevated Re/Os and Pt/Os, and low-delta(18)O in western Canary Island lavas provides powerful support for recycled oceanic crust and lithosphere to generate the spectrum of HIMU-type ocean island basalt signatures. Persistence of geochemical heterogeneities throughout the stratigraphies of El Hierro and La Palma demonstrate long-term preservation of these recycled components in their mantle sources over relatively short-length scales (similar to 50 km). (C) 2010 Elsevier Ltd. All rights reserved.

Day, JMD, Peters BJ, Janney PE.  2014.  Oxygen isotope systematics of South African olivine melilitites and implications for HIMU mantle reservoirs. Lithos. 202-203:76-84.   10.1016/j.lithos.2014.05.009   Abstract

Oxygen isotopes are useful tracers of silicate melt generation processes because of the relatively constant abundance of oxygen in silicate reservoirs and the large isotopic fractionation that can occur between 18O and 16O during low (< 350 °C) and high (> 350 °C) temperature alteration processes at Earth's surface. Studies of oceanic island basalts (OIB) have demonstrated the important role of assimilation of hydrothermal altered crust on 18O/16O ratios, as well as evidence that some OIB mantle sources contain recycled oceanic or continental crust and lithosphere based on correlations between oxygen and radiogenic isotopes. To further investigate how oxygen isotope signatures may be used as tracers in intraplate volcanic rocks, we report olivine compositions from South African olivine melilitites. Olivine melilitites are considered to be related to Group 1 kimberlites and form from asthenospheric melting beneath mature oceanic islands or under off-craton continental lithosphere. South African olivine melilitites also exhibit radiogenic isotopic signatures similar to high-μ (HIMU; high-238U/204Pb) OIB, suggesting sources containing subducted oceanic lithosphere. Olivine from South African melilitites has trace element compositions that are consistent with a magmatic origin from a HIMU-type mantle melt and have a remarkably restricted range in primary 18O/16O ratios (δ18O = 4.99–5.26‰; Average = 5.14 ± 0.17‰, 2σ) that are within the mantle olivine range (δ18O = 5.2 ± 0.3‰). These compositions indicate that South African olivine melilitites require a HIMU mantle source with the oxygen isotope characteristics of ambient peridotite mantle and can be explain through either: (1) intra-mantle differentiation processes that fractionate U(and Th) from Pb, but not 18O/16O ratios, or (2) a dominantly peridotitic source with HIMU-like trace-element and radiogenic isotope characteristics inherited from equilibration and remixing of ancient recycled oceanic lithosphere. In contrast, some HIMU ocean island basalts require mantle sources with low-δ18O, indicating that they originate from distinct recycled mantle lithologies (e.g., pyroxenite/eclogite).

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, Qiu L, Ash RD, McDonough WF, Teng F-Z, Rudnick RL, Taylor LA.  2016.  Evidence for high-temperature fractionation of lithium isotopes during differentiation of the Moon. Meteoritics and Planetary Science. 51(6):1046-1062.   10.1111/maps.12643   Abstract

Lithium isotope and abundance data are reported for Apollo 15 and 17 mare basalts and the LaPaz low-Ti mare basalt meteorites, along with lithium isotope data for carbonaceous, ordinary, and enstatite chondrites, and chondrules from the Allende CV3 meteorite. Apollo 15 low-Ti mare basalts have lower Li contents and lower δ7Li (3.8 ± 1.2‰; all uncertainties are 2 standard deviations) than Apollo 17 high-Ti mare basalts (δ7Li = 5.2 ± 1.2‰), with evolved LaPaz mare basalts having high Li contents, but similar low δ7Li (3.7 ± 0.5‰) to Apollo 15 mare basalts. In low-Ti mare basalt 15555, the highest concentrations of Li occur in late-stage tridymite (>20 ppm) and plagioclase (11 ± 3 ppm), with olivine (6.1 ± 3.8 ppm), pyroxene (4.2 ± 1.6 ppm), and ilmenite (0.8 ± 0.7 ppm) having lower Li concentrations. Values of δ7Li in low- and high-Ti mare basalt sources broadly correlate negatively with 18O/16O and positively with 56Fe/54Fe (low-Ti: δ7Li ≤4‰; δ56Fe ≤0.04‰; δ18O ≥5.7‰; high-Ti: δ7Li >6‰; δ56Fe >0.18‰; δ18O <5.4‰). Lithium does not appear to have acted as a volatile element during planetary formation, with subequal Li contents in mare basalts compared with terrestrial, martian, or vestan basaltic rocks. Observed Li isotopic fractionations in mare basalts can potentially be explained through large-degree, high-temperature igneous differentiation of their source regions. Progressive magma ocean crystallization led to enrichment in Li and δ7Li in late-stage liquids, probably as a consequence of preferential retention of 7Li and Li in the melt relative to crystallizing solids. Lithium isotopic fractionation has not been observed during extensive differentiation in terrestrial magmatic systems and may only be recognizable during extensive planetary magmatic differentiation under volatile-poor conditions, as expected for the lunar magma ocean. Our new analyses of chondrites show that they have δ7Li ranging between −2.5‰ and 4‰. The higher δ7Li in planetary basalts than in the compilation of chondrites (2.1 ± 1.3‰) demonstrates that differentiated planetary basalts are, on average, isotopically heavier than most chondrites.

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.

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, O'Driscoll B, Strachan RA, Daly JS, Walker RJ.  2017.  Identification of mantle peridotite as a possible Iapetan ophiolite sliver in south Shetland, Scottish Caledonides. Journal of the Geological Society. 174(1):88-92.   http://dx.doi.org/10.1144/jgs2016-074   Abstract

The Neoproterozoic Dunrossness Spilite Subgroup of south Shetland, Scotland, has been interpreted as a series of komatiitic and mafic lava flows formed in a marginal basin in response to Laurentian continental margin rifting. We show that ultramafic rocks previously identified as komatiites are depleted mantle peridotites that experienced seafloor hydrothermal alteration. The presence of positive Bouguer gravity and aeromagnetic anomalies extending from the Dunrossness Spilite Subgroup northward to the Shetland Ophiolite Complex suggests instead that these rocks may form part of an extensive ophiolite sliver, obducted during Iapetus Ocean closure in a forearc setting.

Day, JMD, Floss C, Taylor LA, Anand M, Patchen AD.  2006.  Evolved mare basalt magmatism, high Mg/Fe feldspathic crust, chondritic impactors, and the petrogenesis of Antarctic lunar breccia meteorites Meteorite Hills 01210 and Pecora Escarpment 02007. Geochimica Et Cosmochimica Acta. 70:5957-5989.   10.1016/j.gca.2006.05.001   AbstractWebsite

Antarctic lunar meteorites Meteorite Hills 01210 and Pecora Escarpment 02007 are breccias that come from different regolith lithologies on the Moon. MET 01210 is composed predominantly of fractionated low-Ti basaltic material and is classified as an immature, predominantly basaltic glassy matrix regolith breccia. PCA 02007 is a predominantly feldspathic regolith breccia consisting of metamorphosed feldspathic, noritic, troctolitic and noritic-anorthosite clasts, agglutinate and impact-glasses, as well as a number of basaltic clasts with mare and possible non-mare affinities. The basalt clasts in MET 0 12 10 have undergone 'Fenner' trend enrichments in iron and may also have witnessed late-stage crystallization of zircon or a zirconium-rich mineral. Some of the features of MET 0 1210 are similar to other basaltic lunar breccia meteorites (e.g., Northwest Africa 773; Elephant Moraine 87521/96008; Yamato 793274/981031), but it is not paired with them. The presence of metamorphic anorthositic clasts as well as agglutinates indicates a small regolith component. Similarities with previously discovered evolved (e.g., LaPaz Icefield 02205; Northwest Africa 032) and ferroan (e.g., Asuka 881757; Yamato 793169) basaltic lunar meteorites suggest a similar mare source region for MET 01210. Despite lack of evidence for pairing, PCA 02007 shares many features with other feldspathic regolith breccias (e.g., Yamato 791197, Queen Alexandra Range 94281), including a high Mg/Fe whole-rock composition, glass spherules, agglutinate fragments and a diverse clast inventory spanning the range of ferroan anorthosite and high magnesium suite rocks. Some of the basalt fragments in this sample are fractionated and have an igneous origin. However, the majority of the basalt fragments are impact melt clasts. PCA 02007 supports previous studies of feldspathic lunar meteorites that have suggested an aluminous crust for the Moon, with compositions more similar to magnesium granulite breccias than ferroan anorthosites. A 'chondrule-like' fragment found in PCA 02007 and unlike any previously described lunar material is described and tentatively identified as the remnants of a chondritic lunar impactor. This clast is porphyritic with equant olivines that have forsterite-rich cores (Fo(> 98)), extreme normal zonation to more fayalitic rims (Fo(> 44)), and a mineral assemblage with rare earth element abundances distinct from described lunar material and more similar to chondrules found in ordinary or carbonaceous chondrites. Its discovery and description is significant for understanding the composition of lunar impactors. Previously, the main evidence for chondritic lunar impactors was from chondritic relative abundances and near chondritic ratios of highly siderophile elements in lunar impact melt breccias. However, the presence of this clast, along with two other chondritic clasts from Apollo soils 12037 and 15602, provides clues to the identity of ancient meteorite impactors on the Moon. (c) 2006 Elsevier Inc. All rights reserved.

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, 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

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.  2016.  Siderophile Elements. Encyclopedia of Geochemistry. ( White WM, Ed.).: Springer   10.1007/978-3-319-39193-9_234-1