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2012
Brandon, AD, Puchtel IS, Walker RJ, Day JMD, Irving AJ, Taylor LA.  2012.  Evolution of the martian mantle inferred from the Re-187-Os-187 isotope and highly siderophile element abundance systematics of shergottite meteorites. Geochimica Et Cosmochimica Acta. 76:206-235.   10.1016/j.gca.2011.09.047   AbstractWebsite

Shergottite meteorites are a suite of mafic to ultramafic igneous rocks whose parental magmas probably derived from the martian mantle. In this study, a suite of 23 shergottites, spanning their known range in bulk compositions, Rb-Sr, Sm-Nd, and Lu-Hf isotopes, were measured for Re-187-Os-187 isotopic systematics and highly siderophile element abundances (HSE: including Os, Ir, Ru, Pt, Pd, Re). The chief objective was to gain new insight on the chemical evolution of the martian mantle by unraveling the long-term HSE budget of its derivative melts. Possible effects upon HSEs related to crustal contamination, as well as terrestrial and/or martian surface alteration are also examined. Some of the shergottites are hot arid-desert finds. Their respective acetic acid leachates and residues show that both Re and Os display open-system behavior during sample residence at or near the martian and/or terrestrial surfaces. In some meteorites, the alteration effects can be circumvented by analysis of the leached residues. For those shergottites believed to record robust Re-Os isotopic systematics, calculated initial Os-187/Os-188 are well correlated with the initial Nd-143/Nd-144. Shergottites from mantle sources with long-term melt-depleted characteristics (initial epsilon Nd-143 of + 36 to + 40) have chondritic initial gamma Os-187 ranging from -0.5 to + 2.5. Shergottites with intermediate initial epsilon Nd-143 of + 8 to + 17 have a range in initial gamma Os-187 of -0.6 to + 2.3, which overlaps the range for depleted shergottites. Shergottites from long-term enriched sources, with initial epsilon Nd-143 of similar to-7, are characterized by suprachondritic gamma Os-187 values of + 5 to + 15. The initial gamma Os-187 variations for the shergottites do not show a correlation with indices of magmatic differentiation, such as MgO, or any systematic differences between hot arid-desert finds, Antarctic finds, or observed falls. The strong correlation between the initial epsilon Nd-143 and gamma Os-187 in shergottites from approximately + 40 and 0 to -7 and + 15, respectively, is assessed in models for mixing depleted mantle-derived melts with ancient crust (modeled to be similar to evolved shergottite in composition), and with assimilation-fractional crystallization. These models show that the correlation is unlikely to result from participation of martian crust. More likely, this correlation relates to contributions from depleted and enriched reservoirs formed in a martian magma ocean at ca. 4.5 Ga. These models indicate that the shergottite endmember sources were generated by mixing between residual melts and cumulates that formed at variable stages during solidification of a magma ocean. The expanded database for the HSE abundances in shergottites suggests that their martian mantle sources have similar HSE abundances to the terrestrial mantle, consistent with prior studies. The relatively high HSE abundances in both planetary mantles likely cannot be accounted for by high pressure-temperature metal-silicate partitioning at the bases of magma oceans, as has been suggested for Earth. If the HSE were instead supplied by late accretion, this event must have occurred prior to the crystallization of the last martian magma ocean. (C) 2011 Elsevier Ltd. All rights reserved.

O'Driscoll, B, Day JMD, Walker RJ, Daly JS, McDonough WF, Piccoli PM.  2012.  Chemical heterogeneity in the upper mantle recorded by peridotites and chromitites from the Shetland Ophiolite Complex, Scotland. Earth and Planetary Science Letters. 333:226-237.   10.1016/j.epsl.2012.03.035   Abstract

The timing, causes and extent of mantle heterogeneity preserved in the ∼492 Ma Shetland Ophiolite Complex (Scotland) are evaluated using Re–Os isotope and whole rock highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance measurements of a suite of eight chromitites and 21 serpentinised harzburgites and dunites. Shetland dunites have more variable initial 187Os/188Os, as well as absolute and relative abundances of the HSE, compared to spatially associated harzburgites. As is common for ophiolitic peridotites, the harzburgites (γOs492Ma of −5.3 to +2.6) preserve evidence for a Mesoproterozoic depletion event, but are dominated by contemporary chondritic, ambient upper mantle compositions. The dunites have γOs492Ma values ranging between −3.3 and +12.4, reflecting dunite formation by higher degrees of melt interaction with mantle rock than for the spatially associated harzburgites.

Chromitite seams from three locations separated by <500 m have a large range in HSE concentrations (e.g., 0.09 to ∼2.9 μg g−1 Os) with initial γOs492Ma values ranging only from +0.48 to +3.95. Sulphides, arsenides and platinum-group minerals are the primary hosts for the HSE in the chromitites. Their isotopic variations reflect initial isotopic heterogeneity in their primary magmatic signatures. Coupled with field observations that support chromitite formation in concentrated zones of enhanced melt flow, the isotopic dichotomy between the harzburgites and the chromitites suggests that chromitite 187Os/188Os compositions may better approximate the upper limit, rather than an average value, of the bulk convecting upper mantle.

The Shetland peridotite compositions reflect protracted melt depletion (low-Al2O3) and melt percolation events in a supra-subduction zone (SSZ) setting at ∼492 Ma, following an earlier (Mesoproterozoic) melt-depletion event. These results provide further evidence that ancient chemical complexities can be preserved in the upper mantle during ocean plate formation. Chromitites and peridotites from the Shetland Ophiolite Complex also attest to lithological and geochemical heterogeneities generated at scales of less than tens of metres during the formation of ancient oceanic lithosphere by high-degree SSZ melt extraction, percolation and during chromitite formation in the oceanic lithosphere.

Filiberto, J, Chin E, Day JMD, Franchi IA, Greenwood RC, Gross J, Penniston-Dorland SC, Schwenzer SP, Treiman AH.  2012.  Geochemistry of intermediate olivine-phyric shergottite northwest Africa 6234, with similarities to basaltic shergottite northwest Africa 480 and olivine-phyric shergottite northwest Africa 2990. Meteoritics and Planetary Science. 47(8):1256-1273.   10.1111/j.1945-5100.2012.01382.x   Abstract

The newly found meteorite Northwest Africa 6234 (NWA 6234) is an olivine (ol)-phyric shergottite that is thought, based on texture and mineralogy, to be paired with Martian shergottite meteorites NWA 2990, 5960, and 6710. We report bulk-rock major- and trace-element abundances (including Li), abundances of highly siderophile elements, Re-Os isotope systematics, oxygen isotope ratios, and the lithium isotope ratio for NWA 6234. NWA 6234 is classified as a Martian shergottite, based on its oxygen isotope ratios, bulk composition, and bulk element abundance ratios, Fe/Mn, Al/Ti, and Na/Al. The Li concentration and δ7Li value of NWA 6234 are similar to that of basaltic shergottites Zagami and Shergotty. The rare earth element (REE) pattern for NWA 6234 shows a depletion in the light REE (La-Nd) compared with the heavy REE (Sm-Lu), but not as extreme as the known “depleted” shergottites. Thus, NWA 6234 is suggested to belong to a new category of shergottite that is geochemically “intermediate” in incompatible elements. The only other basaltic or ol-phyric shergottite with a similar “intermediate” character is the basaltic shergottite NWA 480. Rhenium-osmium isotope systematics are consistent with this intermediate character, assuming a crystallization age of 180 Ma. We conclude that NWA 6234 represents an intermediate compositional group between enriched and depleted shergottites and offers new insights into the nature of mantle differentiation and mixing among mantle reservoirs in Mars.

Moynier, F, Day JMD, Okui W, Yokoyama T, Bouvier A, Walker RJ, Podosek FA.  2012.  Planetary-Scale Strontium Isotopic Heterogeneity and the Age of Volatile Depletion of Early Solar System Materials. The Astrophysical Journal. 758(1):45.   doi:10.1088/0004-637X/758/1/45   Abstract

Isotopic anomalies in planetary materials reflect both early solar nebular heterogeneity inherited from presolar stellar sources and processes that generated non-mass-dependent isotopic fractionations. The characterization of isotopic variations in heavy elements among early solar system materials yields important insight into the stellar environment and formation of the solar system, and about initial isotopic ratios relevant to long-term chronological applications. One such heavy element, strontium, is a central element in the geosciences due to wide application of the long-lived 87Rb-87Sr radioactive as a chronometer. We show that the stable isotopes of Sr were heterogeneously distributed at both the mineral scale and the planetary scale in the early solar system, and also that the Sr isotopic heterogeneities correlate with mass-independent oxygen isotope variations, with only CI chondrites plotting outside of this correlation. The correlation implies that most solar system material formed by mixing of at least two isotopically distinct components: a CV-chondrite-like component and an O-chondrite-like component, and possibly a distinct CI-chondrite-like component. The heterogeneous distribution of Sr isotopes may indicate that variations in initial 87Sr/86Sr of early solar system materials reflect isotopic heterogeneity instead of having chronological significance, as interpreted previously. For example, given the differences in 84Sr/86Sr between calcium aluminum inclusions and eucrites (ε84Sr > 2), the difference in age between these materials would be ~6 Ma shorter than previously interpreted, placing the Sr chronology in agreement with other long- and short-lived isotope systems, such as U-Pb and Mn-Cr.

Riches, AJV, Day JMD, Walker RJ, Simonetti A, Liu Y, Neal CR, Taylor LA.  2012.  Rhenium–osmium isotope and highly-siderophile-element abundance systematics of angrite meteorites. Earth and Planetary Science Letters. 353:208-218.   10.1016/j.epsl.2012.08.006   Abstract

Coupled 187Os/188Os compositions and highly-siderophile-element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundance data are reported for eight angrite achondrite meteorites that include quenched- and slowly-cooled textural types. These data are combined with new major- and trace-element concentrations determined for bulk-rock powder fractions and constituent mineral phases, to assess angrite petrogenesis. Angrite meteorites span a wide-range of HSE abundances from <0.005 ppb Os (e.g., Northwest Africa [NWA] 1296; Angra dos Reis) to >100 ppb Os (NWA 4931). Chondritic to supra-chondritic 187Os/188Os (0.1201–0.2127) measured for Angra dos Reis and quenched-angrites correspond to inter- and intra-sample heterogeneities in Re/Os and HSE abundances. Quenched-angrites have chondritic-relative rare-earth-element (REE) abundances at 10–15×CI-chondrite, and their Os-isotope and HSE abundance variations represent mixtures of pristine uncontaminated crustal materials that experienced addition (<0.8%) of exogenous chondritic materials during or after crystallization. Slowly-cooled angrites (NWA 4590 and NWA 4801) have fractionated REE-patterns, chondritic to sub-chondritic 187Os/188Os (0.1056–0.1195), as well as low-Re/Os (0.03–0.13), Pd/Os (0.071–0.946), and relatively low-Pt/Os (0.792–2.640). Sub-chondritic 187Os/188Os compositions in NWA 4590 and NWA 4801 are unusual amongst planetary basalts, and their HSE and REE characteristics may be linked to melting of mantle sources that witnessed prior basaltic melt depletion. Angrite HSE-Yb systematics suggest that the HSE behaved moderately-incompatibly during angrite magma crystallization, implying the presence of metal in the crystallizing assemblage.

The new HSE abundance and 187Os/188Os compositions indicate that the silicate mantle of the angrite parent body(ies) (APB) had HSE abundances in chondritic-relative proportions but at variable abundances at the time of angrite crystallization. The HSE systematics of angrites are consistent with protracted post-core formation accretion of materials with chondritic-relative abundances of HSE to the APB, and these accreted materials were rapidly, yet inefficiently, mixed into angrite magma source regions early in Solar System history.

2011
Sarbadhikari, AB, Goodrich CA, Liu Y, Day JMD, Taylor LA.  2011.  Evidence for heterogeneous enriched shergottite mantle sources in Mars from olivine-hosted melt inclusions in Larkman Nunatak 06319. Geochimica Et Cosmochimica Acta. 75:6803-6820.   10.1016/j.gca.2011.09.001   AbstractWebsite

Larkman Nunatak (LAR) 06319 is an olivine-phyric shergottite whose olivine crystals contain abundant crystallized melt inclusions. In this study, three types of melt inclusion were distinguished, based on their occurrence and the composition of their olivine host: Type-I inclusions occur in phenocryst cores (Fo(77-73)); Type-II inclusions occur in phenocryst mantles (Fo(71-66)); Type-III inclusions occur in phenocryst rims (Fo(61-51)) and within groundmass olivine. The sizes of the melt inclusions decrease significantly from Type-I (similar to 150-250 mu m diameter) to Type-II (similar to 100 mu m diameter) to Type-III (similar to 25-75 mu m diameter). Present bulk compositions (PBC) of the crystallized melt inclusions were calculated for each of the three melt inclusion types based on average modal abundances and analyzed compositions of constituent phases. Primary trapped liquid compositions were then reconstructed by addition of olivine and adjustment of the Fe/Mg ratio to equilibrium with the host olivine (to account for crystallization of wall olivine and the effects of Fe/Mg re-equilibration). The present bulk composition of Type-I inclusions (PBC1) plots on a tie-line that passes through olivine and the LAR 06319 whole-rock composition. The parent magma composition can be reconstructed by addition of 29 mol% olivine to PBC1, and adjustment of Fe/Mg for equilibrium with olivine of Fo(77) composition. The resulting parent magma composition has a predicted crystallization sequence that is consistent with that determined from petrographic observations, and differs significantly from the whole-rock only in an accumulated olivine component (similar to 10 wt%). This is consistent with a calculation indicating that similar to 10 wt% agnesian (Fo(77-73)) olivine must be subtracted from the whole-rock to yield a melt in equilibrium with Fo(77). Thus, two independent estimates indicate that LAR 06319 contains similar to 10 wt% cumulate olivine. The rare earth element (REE) patterns of Type-I melt inclusions are similar to that of the LAR 06319 whole-rock. The REE patterns of Type-II and Type-III melt inclusions are also broadly parallel to that of the whole-rock, but at higher absolute abundances. These results are consistent with an LAR 06319 parent magma that crystallized as a closed-system, with its incompatible-element enrichment being inherited from its mantle source region. However, fractional crystallization of the reconstructed LAR 06319 parent magma cannot reproduce the major and trace element characteristics of all enriched basaltic shergottites, indicating local-to-large scale major-and trace-element variations in the mantle source of enriched shergottites. Therefore, LAR 06319 cannot be parental to the enriched basaltic shergottites. (C) 2011 Elsevier Ltd. All rights reserved.

Day, JMD, Hilton DR.  2011.  Origin of (3)He/(4)He ratios in HIMU-type basalts constrained from Canary Island lavas. Earth and Planetary Science Letters. 305:226-234.   10.1016/j.epsl.2011.03.006   AbstractWebsite

New helium isotope and abundance measurements are reported for olivine and clinopyroxene phenocrysts from HIMU-type (high-mu=elevated (238)U/(204)Pb) lavas and xenoliths spanning the stratigraphies of El Hierro and La Palma, Canary Islands. Some pyroxene phenocrysts have suffered post-eruptive modification, either by less than 1% assimilation of crustal-derived He, or by closed-system ageing of He. Olivine phenocrysts record mantle source (3)He/(4)He compositions, with the average (3)He/(4)He for La Palma olivine (7.6 +/- 0.8R(A), where R(A) is the atmospheric (3)He/(4)He ratio of 1.38 x 10(-6)) being within uncertainty of those for El Hierro (7.7 +/- 0.3R(A)), and the canonical mid-ocean ridge basalt range (MORB: 8 +/- 1R(A)). The new helium isotope data for El Hierro and La Palma show no distinct correlations with whole-rock (87)Sr/(86)Sr, (143)Nd/(144)Nd, (187)Os/(188)Os, or Pb isotopes, but (3)He/(4)He ratios for La Palma lavas correlate with (18)O/(16)O measured for the same phenocryst populations. Despite limited (3)He/(4)He variations for El Hierro and La Palma, their He-O isotope systematics are consistent with derivation from mantle sources containing distinct recycled oceanic basaltic crust (El Hierro) and gabbroic lithosphere (La Palma) components that have mixed with depleted mantle, and a high-(3)He/(4)He component (>9.7R(A)) in the case of La Palma. The new data are consistent with models involving generation of compositionally and lithologically (e.g., pyroxenite, eclogite, peridotite) heterogeneous mantle sources containing recycled oceanic crust and lithosphere entrained within upwelling high-(3)He/(4)He mantle that has been severely diluted by interaction with depleted mantle. We propose that the noble gas systematics of HIMU-type lavas and ocean island basalts (OIB) in general, are most simply interpreted as being controlled by the most gas-rich reservoir involved in mixing to generate their mantle sources. In this scenario, HIMU and enriched mantle (EM) sources are dominated by depleted mantle, or high-(3)He/(4)He mantle, because recycled crust and lithosphere have low He concentrations. Consequently, high-(3)He/(4)He OIB would predominantly reflect derivation from a less depleted mantle source with sub-equal to higher He contents than depleted mantle. The available coupled He-O isotope systematics measured for OIB lavas are consistent with this hypothesis. (C) 2011 Elsevier B.V. All rights reserved.

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

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

Riches, AJV, Liu Y, Day JMD, Puchtel IS, III RD, McSween HY, Walker RJ, Taylor LA.  2011.  Petrology and geochemistry of Yamato 984028: A cumulate lherzolitic shergottite with affinities to Y 000027, Y 000047, and Y 000097. Polar Science. 4(4):497-514.   10.1016/j.polar.2010.04.009   Abstract

We report the petrography, mineral and whole-rock chemistry (major-, trace-, and highly-siderophile element abundances, and osmium and oxygen isotope compositions) of a newly recognized lherzolitic shergottite, Yamato (Y) 984028. Oxygen isotopes (Δ17O = 0.218‰) confirm a martian origin for this meteorite. Three texturally distinctive internal zones and a partially devitrified fusion crust occur in the polished section of Y 984028 studied here. The zones include: 1) a poikilitic region with pyroxene enclosing olivine and chromite (Zone A); 2) a non-poikilitic zone with cumulate olivine, interstitial pyroxene, maskelynite and Ti-rich chromite (Zone B) and; 3) a monomict breccia (Zone C). The pyroxene oikocryst in Zone A is chemically zoned from Wo3–7En76–71 in the core region to Wo33–36En52–49 at the rim, and encloses more Mg-rich olivine (Fo74–70) in the core, as compared with olivines (Fo69–68) located at the oikocryst rim. Constraints from Fe–Mg partitioning between crystals and melt indicate that constituent minerals are not in equilibrium with the corresponding bulk-rock composition, implying that Y 984028 represents a cumulate. The whole-rock major- and trace-element compositions, and initial 187Os/188Os value (0.1281 ± 0.0002) of Y 984028 are similar to other lherzolitic shergottites and this sample is probably launch-paired with Y 793602, Y 000027, Y 000047, and Y 000097. The Os isotopic composition and highly-siderophile element (HSE) abundances of Y 984028 and other lherzolitic shergottites are consistent with derivation from a martian mantle source that evolved with chondritic Re/Os.

2010
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.

Liu, Y, Spicuzza MJ, Craddock PR, Day JMD, Valley JW, Dauphas N, Taylor LA.  2010.  Oxygen and iron isotope constraints on near-surface fractionation effects and the composition of lunar mare basalt source regions. Geochimica Et Cosmochimica Acta. 74:6249-6262.   10.1016/j.gca.2010.08.008   AbstractWebsite

Oxygen and iron isotope analyses of low-Ti and high-Ti mare basalts are presented to constrain their petrogenesis and to assess stable isotope variations within lunar mantle sources. An internally-consistent dataset of oxygen isotope compositions of mare basalts encompasses five types of low-Ti basalts from the Apollo 12 and 15 missions and eight types of high-Ti basalts from the Apollo 11 and 17 missions. High-precision whole-rock delta(18)O values (referenced to VSMOW) of low-Ti and high-Ti basalts correlate with major-element compositions (Mg#, TiO(2), Al(2)O(3)). The observed oxygen isotope variations within low-Ti and high-Ti basalts are consistent with crystal fractionation and match the results of mass-balance models assuming equilibrium crystallization. Whole-rock delta(56)Fe values (referenced to IRMM-014) of high-Ti and low-Ti basalts range from 0.134 parts per thousand to 0.217 parts per thousand. and 0.038 parts per thousand, to 0.104 parts per thousand, respectively. Iron isotope compositions of both low-Ti and high-Ti basalts do not correlate with indices of crystal fractionation, possibly owing to small mineral-melt iron fractionation factors anticipated under lunar reducing conditions. The delta(18)O and delta(56)Fe values of low-Ti and the least differentiated high-Ti mare basalts are negatively correlated, which reflects their different mantle source characteristics (e.g., the presence or absence of ilmenite). The average delta(56)Fe values of low-Ti basalts (0.073 +/- 0.018 parts per thousand), n = 8) and high-Ti basalts (0.191 +/- 0.020 parts per thousand, n = 7) may directly record that of their parent mantle sources. Oxygen isotope compositions of mantle sources of low-Ti and high-Ti basalts are calculated using existing models of lunar magma ocean crystallization and mixing, the estimated equilibrium mantle olivine delta(18)O value, and equilibrium oxygen-fractionation between olivine and other mineral phases. The differences between the calculated whole-rock delta(18)O values for source regions, 5.57 parts per thousand for low-Ti and 5.30 parts per thousand for high-Ti mare basalt mantle source regions, are solely a function of the assumed source mineralogy. The oxygen and iron isotope compositions of lunar upper mantle can be approximated using these mantle source values. The delta(18)O and delta(56)Fe values of the lunar upper mantle are estimated to be 5.5 +/- 0.27. (2 sigma) and 0.085 +/- 0.040 parts per thousand (2 sigma), respectively. The oxygen isotope composition of lunar upper mantle is identical to the current estimate of Earth's upper mantle (5.5 0.2 parts per thousand), and the iron isotope composition of the lunar upper mantle overlaps within uncertainty of estimates for the terrestrial upper mantle (0.044 +/- 0.030 parts per thousand.). (C) 2010 Elsevier Ltd. All rights reserved.

Korhonen, FJ, Saito S, Brown M, Siddoway CS, Day JMD.  2010.  Multiple Generations of Granite in the Fosdick Mountains, Marie Byrd Land, West Antarctica: Implications for Polyphase Intracrustal Differentiation in a Continental Margin Setting. Journal of Petrology. 51:627-670.   10.1093/petrology/egp093   AbstractWebsite

Production of granite in the middle to lower crust and emplacement into the middle to upper crust at convergent plate margins is the dominant mechanism of crustal differentiation. The Fosdick Mountains of West Antarctica host migmatitic paragneisses and orthogneisses corresponding to the middle to lower crust and granites emplaced as dikes, sills and small plutons, which record processes of intracrustal differentiation along the East Gondwana margin. U-Pb chronology on magmatic zircon from granites reveals emplacement at c. 358-336 Ma and c. 115-98 Ma, consistent with a polyphase tectonic evolution of the region during Devonian-Carboniferous continental arc activity and Cretaceous continental rifting. The gneisses and granites exposed in the Fosdick migmatite-granite complex were derived from Early Paleozoic quartzose turbidites of the Swanson Formation and Ford Granodiorite suite calc-alkaline plutonic rocks, both of which are widely distributed outside the Fosdick Mountains and have affinity with rock elsewhere in East Gondwana. Granites of both Carboniferous and Cretaceous age have distinct chemical signatures that reflect different melting reactions and accessory phase behavior in contrasting sources. Based on whole-rock major and trace element geochemistry and Sr-Nd isotope compositions, Carboniferous granites with low Rb/Sr are interpreted to be products of melting of the Ford Granodiorite suite. Extant mineral equilibria modeling indicates that the Ford Granodiorite suite compositions produce melt volumes > 10 vol. % at temperatures above biotite stability, involving the breakdown of hornblende + plagioclase, consistent with the high CaO and Na(2)O contents in the low Rb/Sr granites. The Carboniferous low Rb/Sr granites show a sequence from near-melt compositions to compositions with increasing amounts of early crystallized biotite and plagioclase and evidence for apatite dissolution in the source. Carboniferous granites derived from the Swanson Formation are scarce, suggesting that the significant quantities of melt produced from the now residual paragneisses were emplaced at shallower crustal levels than are now exposed. The Cretaceous granites are divided into two distinct groups. An older group of granites (c. 115-110 Ma) has compositions consistent with a dominant Ford Granodiorite source, and characteristics that indicate that they may be less evolved equivalents of the regionally exposed Byrd Coast Granite suite at higher crustal levels. The younger group of granites (c. 109-102 Ma) has distinct light rare earth element depleted signatures. The chemical and isotopic data suggest that these granites were derived from partial melting of both fertile and residual Swanson Formation and had low water contents, indicating that the source rocks may have been dehydrated prior to anatexis as the Byrd Coast Granite suite magmas were transferring through and accumulating at higher crustal levels. The Cretaceous granites derived from the Swanson Formation make up a prominent horizontally sheeted leucogranite complex. The accumulation of these melts probably facilitated melt-induced weakening of the crust during a well-documented transition from regional shortening to regional extension, the formation of a detachment structure, and rapid exhumation of the Fosdick migmatite-granite complex. These multiple episodes of melting along the East Gondwana margin resulted in initial stabilization of the continental crust in the Carboniferous and further intracrustal differentiation in the Cretaceous.

Day, JMD, Walker RJ, James OB, Puchtel IS.  2010.  Osmium isotope and highly siderophile element systematics of the lunar crust. Earth and Planetary Science Letters. 289:595-605.   10.1016/j.epsl.2009.12.001   AbstractWebsite

Coupled (187)Os/(188)Os and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundance data are reported for pristine lunar crustal rocks 60025, 62255, 65315 (ferroan anorthosites, FAN) and 76535, 78235, 77215 and a norite clast in 15455 (magnesian-suite rocks, MGS). Osmium isotopes permit more refined discrimination than previously possible of samples that have been contaminated by meteoritic additions and the new results show that some rocks, previously identified as pristine, contain meteorite-derived HSE. Low HSE abundances in FAN and MGS rocks are consistent with derivation from a strongly HSE-depleted lunar mantle. At the time of formation, the lunar floatation crust, represented by FAN, had 1.4 +/- 0.3 pg g(-1) Os, 1.5 +/- 0.6 pg g(-1) Ir, 6.8 +/- 2.7 pg g(-1) Ru, 16 +/- 15 pg g(-1) Pt,33 +/- 30 pg g(-1) Pd and 0.29 +/- 0.10 pg g(-1) Re (similar to 0.00002 x Cl) and Re/Os ratios that were modestly elevated ((187)Re/(188)Os = 0.6 to 1.7) relative to Cl chondrites. MGS samples are, on average, characterised by more elevated HSE abundances (similar to 0.00007 x Cl) compared with FAN. This either reflects contrasting mantle-source HSE characteristics of FAN and MGS rocks, or different mantle-crust HSE fractionation behaviour during production of these lithologies. Previous studies of lunar impact-melt rocks have identified possible elevated Ru and Pd in lunar crustal target rocks. The new results provide no supporting evidence for such enrichments. If maximum estimates for HSE in the lunar mantle are compared with FAN and MGS averages, crust-mantle concentration ratios (D-values) must be <= 0.3. Such D-values are broadly similar to those estimated for partitioning between the terrestrial crust and upper mantle, with the notable exception of Re.Given the presumably completely different mode of origin for the primary lunar floatation crust and tertiary terrestrial continental crust, the potential similarities in crust-mantle HSE partitioning for the Earth and Moon are somewhat surprising. Low HSE abundances in the lunar crust, coupled with estimates of HSE concentrations in the lunar mantle implies there may be a 'missing component' of late-accreted materials (as much as 95%) to the Moon if the Earth/Moon mass-flux estimates are correct and terrestrial mantle HSE abundances were established by late accretion. (C) 2009 Elsevier B.V. All rights reserved.

Bottke, WF, Walker RJ, Day JMD, Nesvorny D, Elkins-Tanton L.  2010.  Stochastic Late Accretion to Earth, the Moon, and Mars. Science. 330:1527-1530.   10.1126/science.1196874   AbstractWebsite

Core formation should have stripped the terrestrial, lunar, and martian mantles of highly siderophile elements (HSEs). Instead, each world has disparate, yet elevated HSE abundances. Late accretion may offer a solution, provided that >= 0.5% Earth masses of broadly chondritic planetesimals reach Earth's mantle and that similar to 10 and similar to 1200 times less mass goes to Mars and the Moon, respectively. We show that leftover planetesimal populations dominated by massive projectiles can explain these additions, with our inferred size distribution matching those derived from the inner asteroid belt, ancient martian impact basins, and planetary accretion models. The largest late terrestrial impactors, at 2500 to 3000 kilometers in diameter, potentially modified Earth's obliquity by similar to 10 degrees, whereas those for the Moon, at similar to 250 to 300 kilometers, may have delivered water to its mantle.

Riches, AJV, Liu Y, Day JMD, Spetsius ZV, Taylor LA.  2010.  Subducted oceanic crust as diamond hosts revealed by garnets of mantle xenoliths from Nyurbinskaya, Siberia. Lithos. 120:368-378.   10.1016/j.lithos.2010.09.006   AbstractWebsite

The similar to 380 Ma Nyurbinskaya kimberlite pipe Yakutia Siberia sampled a highly-diamondiferous and unusual mantle xenolith population dominated by eclogites New in-situ major- and trace-element data for garnets previously analyzed for oxygen isotope compositions show that Group A eclogitic garnets have Mg# >68 and are LREE-depleted Group B and Group C eclogitic garnets cover a range of Mg# and are each divided into two types based on their trace-element characteristics Type B1 and Cl eclogitic garnets are dominant and are LREE-depleted Less common Type B2 and C2 garnets generally have Mg# >60 and convex-upward REE profiles Harzburgitic garnets are a minor component of the Nyurbinskaya xenolith suite and have high Mg# (similar to 84) high Cr contents (similar to 11 wt% Cr(2)O(3)) and sinusoidal REE-patterns Group A Type B1 and Cl eclogitic garnets define a broad negative correlation between Mg# and Yb abundances consistent with a shallow origin as basaltic and gabbroic portions of oceanic crust Harzburgitic Type B2 and C2 eclogitic garnets have trace-element characteristics indicative of interaction with a C-O-H-N-S-rich fluid in lithospheric environments These results provide clear evidence for the presence of subducted crustal materials in the Siberian mantle lithosphere and support models of craton formation by subduction zone stacking (C) 2010 Elsevier BV All rights reserved

2009
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, Ash RD, Liu Y, Bellucci JJ, Rumble D, McDonough WF, Walker RJ, Taylor LA.  2009.  Early formation of evolved asteroidal crust. Nature. 457:179-182.   10.1038/nature07651   AbstractWebsite

Mechanisms for the formation of crust on planetary bodies remain poorly understood(1). It is generally accepted that Earth's andesitic continental crust is the product of plate tectonics(1,2), whereas the Moon acquired its feldspar- rich crust by way of plagioclase flotation in a magma ocean(3,4). Basaltic meteorites provide evidence that, like the terrestrial planets, some asteroids generated crust and underwent large- scale differentiation processes(5). Until now, however, no evolved felsic asteroidal crust has been sampled or observed. Here we report age and compositional data for the newly discovered, paired and differentiated meteorites Graves Nunatak ( GRA) 06128 and GRA 06129. These meteorites are feldspar- rich, with andesite bulk compositions. Their age of 4.5+/-0.06 Gyr demonstrates formation early in Solar System history. The isotopic and elemental compositions, degree of metamorphic re-equilibration and sulphide- rich nature of the meteorites are most consistent with an origin as partial melts from a volatile- rich, oxidized asteroid. GRA 06128 and 06129 are the result of a newly recognized style of evolved crust formation, bearing witness to incomplete differentiation of their parent asteroid and to previously unrecognized diversity of early- formed materials in the Solar System.

Liu, Y, Floss C, Day JMD, Hill E, Taylor LA.  2009.  Petrogenesis of lunar mare basalt meteorite Miller Range 05035. Meteoritics & Planetary Science. 44:261-284. AbstractWebsite

Miller Range (MIL) 05035 is a low-Ti mare basalt that consists predominantly of pyroxene (62.3 vol%) and plagioclase (26.4 vol%). Pyroxenes are strongly shocked and complexly zoned from augite (Wo(33)) and pigeonite (Wo(17)) cores with Mg# = 50-54 to hedenbergite rims. Coexisting pyroxene core compositions reflect crystallization temperatures of 1000 to 1100 degrees C. Plagioclase has been completely converted to maskelynite with signs of recrystallization. Maskelynite is relatively uniform in composition (An(94)Ab(6)-An(91)Ab(9)), except at contacts with late-stage mesostasis areas (elevated K contents, An(82)Ab(15)Or(3)). Symplectites (intergrowth of Fe-augite, fayalite, and silica) of different textures and bulk compositions in MIL 05035 suggest formation by decomposition of Ferro-pyroxene during shock-induced heating, which is Supported by the total maskelynitization of plagioclase, melt pockets, and the presence of a relict pyroxferroite grain. Petrography and mineral chemistry imply that crystallization of MIL 05035 Occurred in the sequence of Fe-poor pyroxenes (Mg# = 50-54), followed by plagioclase and Fe-rich pyroxenes (Mg# = 20-50), and finally hedenbergite, Fe-Ti oxides, and minor late-stage phases. Petrography, bulk chemistry, mineral compositions, and the age of MIL 05035 Suggest it is possibly Source crater-paired with Asuka (A-) 881757 and Yamato (Y-) 793169, and may also be launch-paired with Meteorite Hills (MET) 01210. MIL 05035 represents an old (similar to 3.8-3.9 Ga), incompatible element-depleted low-Ti basalt that was not sampled during the Apollo or Luna missions. The light-REE depleted nature and lack of Eu anomalies For this meteorite are consistent with an origin distant from the Procellarum KREEP Terrane, and genesis from an early Cumulate mantle-source region generated by extensive differentiation of the Moon.

Sarbadhikari, AB, Day JMD, Liu Y, Rumble D, Taylor LA.  2009.  Petrogenesis of olivine-phyric shergottite Larkman Nunatak 06319: Implications for enriched components in martian basalts. Geochimica Et Cosmochimica Acta. 73:2190-2214.   10.1016/j.gca.2009.01.012   AbstractWebsite

We report on the petrography and geochemistry of the newly discovered olivine-phyric shergottite Larkman Nunatak (LAR) 06319. The meteorite is porphyritic, consisting of megacrysts of olivine (<= 2.5 mm in length, F(O77-52)) and prismatic zoned pyroxene crystals with Wo(3)En(71) in the cores to Wo(8-30)En(23-45) at the rims. The groundmass is composed of finer grained olivine (<0.25 mm, Fo(62-46)), Fe-rich augite and pigeonite, maskelynite and minor quantities of chromite, ulvospinel, magnetite, ilmenite, phosphates, sulfides and glass. Oxygen fugacity estimates, derived from the olivine-pyroxene-spinel geo-barometer, indicate that LAR 06319 formed under more oxidizing conditions (QFM -1.7) than for depleted shergottites. The whole-rock composition of LAR 06319 is also enriched in incompatible trace elements relative to depleted shergottites, with a trace-element pattern that is nearly identical to that of olivine-phyric shergottite NWA 1068. The oxygen isotope composition of LAR 06319 (Delta(17)O = 0.29 +/- 0.03) confirms its martian origin. Olivine megacrysts in LAR 06319 are phenocrystic, with the most Mg-rich megacryst olivine being close to equilibrium with the bulk rock. A notable feature of LAR 06319 is that its olivine megacryst grains contain abundant melt inclusions hosted within the forsterite cores. These early-trapped melt inclusions have similar trace element abundances and patterns to that of the whole-rock, providing powerful evidence for closed-system magmatic behavior for LAR 06319. Calculation of the parental melt trace element composition indicates a whole-rock composition for LAR 06319 that was controlled by pigeonite and augite during the earliest stages of crystallization and by apatite in the latest stages. Crystal size distribution and spatial distribution pattern analyses of olivine indicate at least two different crystal populations. This is most simply interpreted as crystallization of megacryst olivine in magma conduits, followed by eruption and subsequent crystallization of groundmass olivine. LAR 06319 shows close affinity in mineral and whole-rock chemistry to olivine-phyric shergottite, NWA 1068 and the basaltic shergottite NWA 4468. The remarkable features of these meteorites are that they have relatively similar quantities of mafic minerals compared with olivine-phyric shergottites (e.g., Y-980459, Dho 019), but flat and elevated rare earth element patterns more consistent with the LREE-enriched basaltic shergottites (e.g., Shergotty, Los Angeles). This relationship can be interpreted as arising from partial melting of an enriched mantle source and subsequent crystal-liquid fractionation to form the enriched olivine-phyric and basaltic shergottites, or by assimilation of incompatible-element enriched martian crust. The similarity in the composition of early-trapped melt inclusions and the whole-rock for LAR 06319 indicates that any crustal assimilation must have occurred prior to crystallization of megacryst olivine, restricting such processes to the deeper portions of the crust. Thus, we favor LAR06319 forming from partial melting of an "enriched" and oxidized mantle reservoir, with fractional crystallization of the parent melt upon leaving the mantle. (c) 2009 Elsevier Ltd. All rights reserved.

Day, JMD, Ash RD, Liu Y, Bellucci JJ, Rumble D, McDonough WF, Walker RJ, Taylor LA.  2009.  Asteroids and andesites. Nature. 459:E2.   doi:10.1038/nature08078   Abstract

Arculus et al.1 raise an important question regarding the use of terrestrial rock nomenclature to characterize extraterrestrial materials. Here the issue relates to the definition of ‘andesite’, and whether the felsic achondrite meteorites GRA 06128 and GRA 06129 (GRA 06128/9) can and should be classified using this term2. More broadly, the question is whether extraterrestrial rocks should be classified using standard petrologic and geochemical criteria, such as mineralogy and major-element bulk composition, developed for the description of terrestrial rocks3,4. The approach of Arculus et al.1 is that petrogenetic process is of equal or greater importance when classifying a rock. This question must ultimately be decided by the scientific community.

O'Driscoll, B, Day JMD, Daly JS, Walker RJ, McDonough WF.  2009.  Rhenium-osmium isotope and platinum-group elements in the Rum Layered Suite, Scotland: Implications for Cr-spinel seam formation and the composition of the Iceland mantle anomaly. Earth and Planetary Science Letters. 286(1-2):41-51.   10.1016/j.epsl.2009.06.013   Abstract

The Rum Layered Suite is a layered mafic–ultramafic body that was emplaced during Palaeogene North Atlantic margin rifting. It is a classic open-system magma chamber, constructed of 16 repeated coupled peridotite–troctolite units, some of which have laterally extensive ~ 2 mm-thick platinum-group element (PGE) enriched (~ 2 µg g− 1) Cr-spinel seams at their bases. In order to investigate Cr-spinel seam petrogenesis and enrichment of the PGE, abundances of these elements and Re–Os isotopes have been determined at three stratigraphic levels of the Rum Layered Suite that represent major magma replenishment events. Individual units preserve a range of initial 187Os/188Os ratios, demonstrating heterogeneity in the composition of replenishing magmas. Data for both the Cr-spinel seams and overlying silicates reveal that the processes that formed the Cr-spinel also concentrated the PGE, following magma replenishment. There is no evidence for structurally-bound PGE in Cr-spinel. Instead, the PGE budget of the Rum Layered Suite is linked to base metal sulphides, especially pentlandite, and to PGE alloys contained within the Cr-spinel seams, but which exist as separate phases at Cr-spinel grain boundaries. The range in initial Os isotope compositions (γOs = 3.4 to 36) in the Rum Layered Suite can be successfully modelled by 5–8% assimilation of Lewisian gneiss coupled with changing PGE contents in the replenishing magmas associated with sulphide removal. Initial 187Os/188Os ratios for Rum rocks range from 0.1305 to 0.1349 and are atypical of the convecting upper mantle, but are within the range for recently erupted picrites and basalts from Iceland and Palaeogene picrites and basalts from Baffin Island, Greenland and Scotland. Thus, the Os isotope data suggest that the North Atlantic Igneous Province magmas were collectively produced from a mantle source with components that remained relatively unchanged in Os isotopic composition over the past 60 Ma, and that likely contain a recycled lithospheric component.

2008
Schnare, DW, Day JMD, Norman MD, Liu Y, Taylor LA.  2008.  A laser-ablation ICP-MS study of Apollo 15 low-titanium olivine-normative and quartz-normative mare basalts. Geochimica Et Cosmochimica Acta. 72:2556-2572.   10.1016/j.gca.2008.02.021   AbstractWebsite

Apollo 15 low-Ti mare basalts have traditionally been subdivided into olivine- and quartz-normative basalt types, based on their different SiO(2), FeO, and TiO(2) whole-rock compositions. Previous studies have reconciled this compositional diversity by considering the olivine- and quartz-normative basalts as originating from different lunar mantle source regions. To provide new information on the compositions of Apollo 15 low-Ti mare basalt parental magmas, we report a study of major and trace-element compositions of whole rocks, pyroxenes, and other phases in the olivine-normative basalts 15016 and 15555 and quartz-normative basalts 15475 and 15499. Results show similar rare-earth-element patterns in pyroxenes from all four basalts. The estimated equilibrium parental-melt compositions from the trace-element compositions of pyroxenes are similar for 15016, 15555 and 15499. Additionally, an independent set of trace-element distribution coefficients has been determined from measured pyroxene and mesostasis compositions in sample 15499. These data suggest that fractional crystallization may be a viable alternative to compositional differences in the mantle source to explain the similar to 25% difference in whole-rock TiO(2), and corresponding differences in SiO(2) and FeO between the Apollo 15 olivine- and quartz-normative basalts. In this model, the older (similar to 3.35 Ga) quartz-normative basalts, with lower TiO(2) experienced olivine, chromite, and Cr-ulvospinel fractionation at 'crustal levels' in magma chambers or dikes, followed by limited near-surface mineral fractionation, within the lava flows. In contrast, the younger (similar to 3.25 Ga) olivine-normative basalts experienced only limited magmatic differentiation at 'crustal-levels', but extensive near-surface mineral fractionation to produce their evolved mineral compositions. A two-stage mineral-fractionation model is consistent with textural and mineralogical observations, as well as the mineral trace-element constraints developed by this study. (C) 2008 Elsevier Ltd. All rights reserved.

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.

2007
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, Taylor LA.  2007.  On the structure of mare basalt lava flows from textural analysis of the LaPaz Icefield and Northwest Africa 032 lunar meteorites. Meteoritics & Planetary Science. 42:3-17. AbstractWebsite

Quantitative textural data for Northwest Africa (NWA) 032 and the LaPaz (LAP) mare basalt meteorites (LAP 02205, LAP 02224, LAP 02226, and LAP 02436) provide constraints on their crystallization and mineral growth histories. In conjunction with whole-rock and mineral chemistry, textural analysis provides powerful evidence for meteorite pairing. Petrographic observations and crystal size distribution (CSD) measurements of NWA 032 indicate a mixed population of slowly cooled phenocrysts and faster cooled matrix. LaPaz basalt crystal populations are consistent with a single phase of nucleation and growth. Spatial distribution patterns (SDP) of minerals in the meteorites highlight the importance of clumping and formation of clustered crystal frameworks in their melts, succeeded by continued nucleation and growth of crystals. This process resulted in increasingly poor sorting, during competition for growth, as the melt crystallized. Based on CSD and SDP data, we suggest a potential lava flow geometry model to explain the different crystal populations for NWA 032 and the LaPaz basalts. This model involves crystallization of early formed phenocrysts at hypabyssal depths in the lunar crust, followed by eruption and flow differentiation on the lunar surface. Lava flow differentiation would allow for formation of a cumulate base and facilitate variable cooling within the stratigraphy, explaining the varied textures and modal mineralogies of mare basalt meteorites. The model may also provide insight into the relative relationships of some Apollo mare basalt suites, shallow-level crystal fractionation processes, and the nature of mare basalt volcanism over lunar history.