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Day, JMD, Taylor LA, Floss C, McSween HY.  2006.  Petrology and chemistry of MIL 03346 and its significance in understanding the petrogenesis of nakhlites on Mars. Meteoritics & Planetary Science. 41:581-606. AbstractWebsite

Antarctic meteorite Miller Range (MIL) 03346 is a nakhlite composed of 79% clinopyroxene, similar to 1% olivine, and 20% vitrophyric intercumulus material. We have performed a petrological and geochemical study of MIL 03346, demonstrating a petrogenetic history similar to previously discovered naklilites. Quantitative textural study of MIL 03346 indicates long (> 1 x 10(1) yr) residence times for the Cumulus augite, whereas the skeletal Fe-Ti oxide, fayalite, and sulfide in the vitrophyric intercumulus matrix suggest rapid cooling, probably as a lava flow. From the relatively high forsterite contents of olivine (up to Fo(43)) compared with other nakhlites and compositions of augite cores (Wo(38-42)En(35-40)Fs(22-28)) and their hedenbergite rims, we suggest that MIL 03346 is part of the same or a similar Martian Cumulate-rich lava flow as other nakhlites. However, MIL 03346 has experienced less equilibration and faster cooling than other nakhlites discovered to date. Calculated trace element concentrations based upon modal abundances of MIL 03346 and its constituent minerals are identical to whole rock trace element abundances. Parental melts for augite have REE patterns that are approximately parallel with whole rock and intercumulus melt using experimentally defined partition coefficients. This parallelism reflects closed-system crystallization for MIL 03346, where the only significant petrogenetic process between formation of augite and eruption and emplacement of the nakhlite flow has been fractional crystallization. A model for the petrogenesis of MIL 03346 and the naklilites (Nakhla, Governador Valadares, Lafayette, Yamato-000593, Northwest Africa (NWA) 817, NWA 998) Would include: 1) partial melting and ascent of melt generated from a long-term LREE depleted mantle Source, 2) crystallization of cumulus augite (+/- olivine, +/- magnetite) in a shallow-level Martian magma chamber, 3) eruption of the crystal-laden naklilite magma onto the surface of Mars, 4) cooling, crystal settling, overgrowth, and partial equilibration to different extents within the flow, 5) secondary alteration through hydrothermal processes, possibly immediately succeeding or during emplacement of the flow. This model might apply to single-or multiple-flow models for the nakhlites. Ultimately, MIL 03346 and the other nakhlites preserve a record of magmatic processes in volcanic rocks oil Mars with analogous petrogenetic histories to pyroxene-rich terrestrial lava flows and to komatiites.

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.

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

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

Herzberg, C, Cabral RA, Jackson MG, Vidito C, Day JMD, Hauri EH.  2014.  Phantom Archean crust in Mangaia hotspot lavas and the meaning of heterogeneous mantle. Earth and Planetary Science Letters. 396:97-106.   10.1016/j.epsl.2014.03.065   Abstract

Lavas from Mangaia in the Cook–Austral island chain, Polynesia, define an HIMU (or high μ , where View the MathML source) global isotopic end-member among ocean island basalts (OIB) with the highest 206,207,208Pb/204Pb. This geochemical signature is interpreted to reflect a recycled oceanic crust component in the mantle source. Mass independently fractionated (MIF) sulfur isotopes indicate that Mangaia lavas sampled recycled Archean material that was once at the Earth's surface, likely hydrothermally-modified oceanic crust. Recent models have proposed that crust that is subducted and then returned to the surface in a mantle plume is expected to transform to pyroxenite/eclogite during transit through the mantle. Here we examine this hypothesis for Mangaia using high-precision electron microprobe analysis on olivine phenocrysts. Contrary to expectations of a crustal component and, hence pyroxenite, results show a mixed peridotite and pyroxenite source, with peridotite dominating. If the isotopic compositions were inherited from subduction of recycled oceanic crust, our work shows that this source has phantom-like properties in that it can have its lithological identity destroyed while its isotope ratios are preserved. This may occur by partial melting of the pyroxenite and injection of its silicic melts into the surrounding mantle peridotite, yielding a refertilized peridotite. Evidence from one sample reveals that not all pyroxenite in the melting region was destroyed. Identification of source lithology using olivine phenocryst chemistry can be further compromised by magma chamber fractional crystallization, recharge, and mixing. We conclude that the commonly used terms mantle “heterogeneities” and “streaks” are ambiguous, and distinction should be made of its lithological and isotopic properties.

Day, JMD.  2015.  Planet formation processes revealed by meteorites. Geology Today. 31(1):12-20.   10.1111/gto.12082   Abstract

The history of the solar system is locked within the planets, asteroids and other objects that orbit the Sun. While remote observations of these celestial bodies are essential for understanding planetary processes, much of the geological and geochemical information regarding solar system heritage comes directly from the study of rocks and other materials originating from them. The diversity of materials available for study from planetary bodies largely comes from meteorites; fragments of rock that fall through Earth’s atmosphere after impact-extraction from their parent planet or asteroid. These extra-terrestrial objects are fundamental scientific materials, providing information on past conditions within planets, and on their surfaces, and revealing the timing of key events that affected a planet’s evolution. Meteorites can be sub-divided into four main groups: (1) chondrites, which are unmelted and variably metamorphosed ‘cosmic sediments’ composed of particles that made up the early solar nebula; (2) achondrites, which represent predominantly silicate materials from asteroids and planets that have partially to fully melted, from a broadly chondritic initial composition; (3) iron meteorites, which represent Fe-Ni samples from the cores of asteroids and planetesimals; and (4) stony-iron meteorites such as pallasites and mesosiderites, which are mixtures of metal and dominantly basaltic materials. Meteorite studies are rapidly expanding our understanding of how the solar system formed and when and how key events such as planetary accretion and differentiation occurred. Together with a burgeoning collection of classified meteorites, these scientific advances herald an unprecedented period of further scientific challenges and discoveries, an exciting prospect for understanding our origins.

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.

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

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

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

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

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

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

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

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

Howarth, GH, Day JMD, Pernet-Fisher JF, Goodrich CA, Pearson DG, Luo Y, Ryabov VV, Taylor LA.  2017.  Precious metal enrichment at low-redox in terrestrial native Fe-bearing basalts investigated using laser-ablation ICP-MS. Geochimica et Cosmochimica Acta.   10.1016/j.gca.2017.01.003   Abstract

Primary native Fe is a rare crystallizing phase from terrestrial basaltic magmas, requiring highly reducing conditions (fO2

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.

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

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

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.

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.

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.

Pringle, EA, Moynier F, Savage PS, Jackson MG, Moriera M, Day JMD.  2016.  Silicon isotopes reveal recycled altered oceanic crust in the mantle sources of Ocean Island Basalts. Geochimica et Cosmochimica Acta. 189:282-295.   10.1016/j.gca.2016.06.008   Abstract

The study of silicon (Si) isotopes in Ocean Island Basalts (OIB) has the potential to discern between different models for the origins of geochemical heterogeneities in the mantle. Relatively large (several per mil per atomic mass unit) Si isotope fractionation occurs in low-temperature environments during biochemical and geochemical precipitation of dissolved Si, where the precipitate is preferentially enriched in the lighter isotopes relative to the dissolved Si. In contrast, only a limited range (tenths of a per mil) of Si isotope fractionation has been observed from high-temperature igneous processes. Therefore, Si isotopes may be useful as tracers for the presence of crustal material within OIB mantle source regions that experienced relatively low-temperature surface processes in a manner similar to other stable isotope systems, such as oxygen.

Characterizing the isotopic composition of the mantle is also of central importance to the use of the Si isotope system as a basis for comparisons with other planetary bodies (e.g., Moon, Mars, asteroids). Here we present the first comprehensive suite of high-precision Si isotope data obtained by MC-ICP-MS for a diverse suite of OIB. Samples originate from ocean islands in the Pacific, Atlantic, and Indian Ocean basins and include representative endmembers for the EM-1, EM-2, and HIMU mantle components. On average, d30Si values for OIB (0.32 ± 0.09‰, 2 sd) are in general agreement with previous estimates for the d30Si value of Bulk Silicate Earth (0.29 ± 0.07‰, 2 sd; Savage et al., 2014). Nonetheless, some small systematic variations are present; specifically, most HIMU-type (Mangaia; Cape Verde; La Palma, Canary Islands) and Iceland OIB are enriched in the lighter isotopes of Si (d30Si values lower than MORB), consistent with recycled altered oceanic crust and lithospheric mantle in their mantle sources.

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

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

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

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

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

Amsellam, E, Moynier F, Pringle EA, Bouvier A, Chen H, Day JMD.  2017.  Testing the chondrule-rich accretion model for planetary embryos using calcium isotopes. Earth and Planetary Science Letters. 469:75-83.   10.1016/j.epsl.2017.04.022   Abstract

Understanding the composition of raw materials that formed the Earth is a crucial step towards
understanding the formation of terrestrial planets and their bulk composition. Calcium is the fifth most
abundant element in terrestrial planets and, therefore, is a key element with which to trace planetary
composition. However, in order to use Ca isotopes as a tracer of Earth’s accretion history, it is first
necessary to understand the isotopic behavior of Ca during the earliest stages of planetary formation.
Chondrites are some of the oldest materials of the Solar System, and the study of their isotopic
composition enables understanding of how and in what conditions the Solar System formed. Here we
present Ca isotope data for a suite of bulk chondrites as well as Allende (CV) chondrules. We show that
most groups of carbonaceous chondrites (CV, CI, CR and CM) are significantly enriched in the lighter Ca
isotopes (δ44/40Ca= +0.1 to +0.93) compared with bulk silicate Earth (δ44/40Ca= +1.05 ± 0.04,
Huang et al., 2010) or Mars, while enstatite chondrites are indistinguishable from Earth in Ca isotope
composition (δ44/40Ca = +0.91 to +1.06). Chondrules from Allende are enriched in the heavier
isotopes of Ca compared to the bulk and the matrix of the meteorite (δ44/40Ca = +1.00 to +1.21).
This implies that Earth and Mars have Ca isotope compositions that are distinct from most carbonaceous
chondrites but that may be like chondrules. This Ca isotopic similarity between Earth, Mars, and
chondrules is permissive of recent dynamical models of planetary formation that propose a chondrulerich
accretion model for planetary embryos.

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

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

Truong, TB, Castillo PR, Hilton DR, Day JMD.  2018.  The trace element and Sr-Nd-Pb isotope geochemistry of Juan Fernandez lavas reveal variable contributions from a high-3He/4He mantle plume. Chemical Geology. 476:280-291.   10.1016/j.chemgeo.2017.11.024   Abstract

The Juan Fernandez Islands in the southeastern Pacific are an atypical linear volcanic chain that exhibits a considerable range in 3He/4He ratios (8 to 18 RA, where RA is the 3He/4He ratio of air), but limited ranges of 87Sr/86Sr and 143Nd/144Nd. Here we report new trace element abundance data and Sr-Nd-Pb isotope data for mafic lavas previously analyzed for their 3He/4He and He contents from the two main islands of Robinson Crusoe and Alexander Selkirk. Lavas from these islands have been previously grouped based on geochemical and petrological classification into Group I and III basalts, and Group II basanites. In general, samples have overlapping Sr-Nd-Pb isotope compositions that suggest a common, albeit slightly heterogeneous mantle source. In detail, the Group I and III tholeiitic and alkalic basalts have nearly identical incompatible trace element patterns, whereas the Group II basanites show elevated incompatible trace element abundances. Major and incompatible trace element modeling indicates that Group III basalts (3He/4He = 7.8–9.5 RA) from younger Alexander Selkirk Island were produced by the highest degree of partial melting (> 10%) of a common mantle source, followed by Group I basalts (13.6–18.0 RA) and Group II basanites (11.2–12.5 RA) from older Robinson Crusoe Island. The 206Pb/204Pb of Group I basalts and Group II basanites are slightly more radiogenic and limited in range (19.163 to 19.292) compared with those of Group III (18.939 to 19.221). The Group I and II lavas from Robinson Crusoe are consistent with an origin from the so-called focus zone (FOZO) mantle component, whereas the Alexander Selkirk basalts additionally contain contributions from a less-enriched or relatively depleted mantle component. Juan Fernandez lavas reveal limited ranges of Sr-Nd-Pb isotopes but variable 3He/4He as their parental magmas originated mainly from the FOZO component with high 3He/4He (> 9 RA) and variably polluted with a depleted component with lower 3He/4He (ca. 8 RA). Contributions from high-3He/4He mantle sources to ocean island basalts can therefore vary both spatially and temporally, over meter to kilometer lengths and hundred to million-year time scales, and may not be strongly correlated to radiogenic lithophile isotope systematics.

Mundl, A, Touboul M, Jackson MG, Day JMD, Kurz MD, Lekic V, Helz RT, Walker RJ.  2017.  Tungsten-182 heterogeneity in modern ocean island basalts. Science. 356(6333):66-69.   10.1126/science.aal4179   Abstract

New tungsten isotope data for modern ocean island basalts (OIB) from Hawaii, Samoa, and Iceland reveal variable 182W/184W, ranging from that of the ambient upper mantle to ratios as much as 18 parts per million lower. The tungsten isotopic data negatively correlate with 3He/4He. These data indicate that each OIB system accesses domains within Earth that formed within the first 60 million years of solar system history. Combined isotopic and chemical characteristics projected for these ancient domains indicate that they contain metal and are repositories of noble gases. We suggest that the most likely source candidates are mega–ultralow-velocity zones, which lie beneath Hawaii, Samoa, and Iceland but not beneath hot spots whose OIB yield normal 182W and homogeneously low 3He/4He.