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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.

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.

Day, JMD, Waters CL, Schaefer BF, Walker RJ, Turner S.  2016.  Use of Hydrofluoric Acid Desilicification in the Determination of Highly Siderophile Element Abundances and Re-Pt-Os Isotope Systematics in Mafic-Ultramafic Rocks. Geostandards and Geoanalytical Research. 40(1):49-65.   DOI: 10.1111/j.1751-908X.2015.00367.x   Abstract

Properly combining highly siderophile element (HSE: Re,Pd, Pt, Ru, Ir, Os) abundance data, obtained by isotope dilution, with corresponding 187Os/188Os and 186Os/188Os measurements of rocks requires efficient digestion of finely-ground powders and complete spike-sample equilibration. Yet, because of the nature of commonly used methods for separating Os from a rock matrix, hydrofluoric acid (HF) is typically not used in such digestions. Consequently, some silicates are not completely dissolved, and HSE residing within these silicates may not be fully accessed. Consistent with this, some recent studies of basaltic reference materials (RMs) have concluded that an HF-desilicification procedure is required to fully access the HSE (Ishikawa et al. (2014)Chemical Geology, 384, 27–46; Li et al. (2015) Geostandards and Geoanalytical Research, 39, 17–30). Highly siderophile element abundance and Os isotope studies of intraplate basalts typically target samples with a range of MgO contents (< 8to> 18% m/m, or as mass fractions, < 8 to> 18 g per 100 g), in contrast to the lower MgO mass fractions (< 10 g per 100 g) of basalt and diabase RMs (i.e., BIR-1, BHVO-2, TDB-1). To investigate the effect of HF-desilicification on intraplate basalts, experiments were performed on finely ground Azores basalts (8.1–17 g per 100 g MgO) using a‘standard acid digestion’ (2:1 mixture of concentrated HNO3 and HCl), and a standard acid digestion, followed by HF-desilicification. No systematic trends in HSE abundances were observed between data obtained by standard acid digestion and HF-desilicification. Desilicifi-cation procedures using HF do not improve liberation ofthe HSE from Azores basalts, or some RMs (e.g., WPR-1).We conclude that HF-desilicification procedures are useful for obtaining total HSE contents of some young lavas, but this type of procedure is not recommended for studies where Re-Pt-Os chronological information is desired. The collateral effect of a standard acid digestion to liberate Os, followed by HF-desilicification to obtain Re and Pt abundances in samples, is that the measured Re/Os and Pt/Os may not correspond with measured 187Os/188Os or 186Os/188Os.

Cabral, RA, Jackson MG, Koga KT, Rose-Koga EF, Hauri EH, Whitehouse MJ, Price AA, Day JMD, Shimizu N, Kelley KA.  2014.  Volatile cycling of H2O, CO2, F, and Cl in the HIMU mantle: A new window provided by melt inclusions from oceanic hot spot lavas at Mangaia, Cook Islands. Geochemistry, Geophysics, Geosystems. 15(11):4445-4467.   10.1002/2014GC005473   Abstract

Mangaia hosts the most radiogenic Pb-isotopic compositions observed in ocean island basalts and represents the HIMU (high m5238U/204Pb) mantle end-member, thought to result from recycled oceanic crust. Complete geochemical characterization of the HIMU mantle end-member has been inhibited due to a lack of deep submarine glass samples from HIMU localities. We homogenized olivine-hosted melt inclusions separated from Mangaia lavas and the resulting glassy inclusions made possible the first volatile abundances to be obtained from the HIMU mantle end-member. We also report major and trace element abundances and Pb-isotopic ratios on the inclusions, which have HIMU isotopic fingerprints. We evaluate the samples for processes that could modify the volatile and trace element abundances postmantle melting, including diffusive Fe and H2O loss, degassing, and assimilation. H2O/Ce ratios vary from 119 to 245 in the most pristine Mangaia inclusions; excluding an inclusion that shows evidence for assimilation, the primary magmatic H2O/Ce ratios vary up to 200, and are consistent with significant dehydration of oceanic crust during subduction and long-term storage in the mantle. CO2 concentrations range up to 2346 ppm CO2 in the inclusions. Relatively high CO2 in the inclusions, combined with previous observations of carbonate blebs in other Mangaia melt inclusions, highlight the importance of CO2 for the generation of the HIMU mantle. F/Nd ratios in the inclusions (3069; 2r standard deviation) are higher than the canonical ratio observed in oceanic lavas, and Cl/K ratios (0.07960.028) fall in the range of pristine mantle (0.02–0.08).

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

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

Dhaliwal, JK, Day JMD, Moynier F.  2018.  Volatile element loss during planetary magma ocean phases. Icarus. 300:249-260.   10.1016/j.icarus.2017.09.002   Abstract

Moderately volatile elements (MVE) are key tracers of volatile depletion in planetary bodies. Zinc is an especially useful MVE because of its generally elevated abundances in planetary basalts, relative to other MVE, and limited evidence for mass-dependent isotopic fractionation under high-temperature igneous processes. Compared with terrestrial basalts, which have δ66Zn values (per mille deviation of the 66Zn/64Zn ratio from the JMC-Lyon standard) similar to some chondrite meteorites (∼+0.3‰), lunar mare basalts yield a mean δ66Zn value of +1.4 ± 0.5‰ (2 st. dev.). Furthermore, mare basalts have average Zn concentrations ∼50 times lower than in typical terrestrial basaltic rocks. Late-stage lunar magmatic products, including ferroan anorthosite, Mg- and Alkali-suite rocks have even higher δ66Zn values (+3 to +6‰). Differences in Zn abundance and isotopic compositions between lunar and terrestrial rocks have previously been interpreted to reflect evaporative loss of Zn, either during the Earth-Moon formatting Giant Impact, or in a lunar magma ocean (LMO) phase. To explore the mechanisms and processes under which volatile element loss may have occurred during a LMO phase, we developed models of Zn isotopic fractionation that are generally applicable to planetary magma oceans. Our objective was to identify conditions that would yield a δ66Zn signature of ∼ +1.4‰ within the lunar mantle. For the sake of simplicity, we neglect possible Zn isotopic fractionation during the Giant Impact, and assumed a starting composition equal to the composition of the present-day terrestrial mantle, assuming both the Earth and Moon had zinc ‘consanguinity’ following their formation. We developed two models: the first simulates evaporative fractionation of Zn only prior to LMO mixing and crystallization; the second simulates continued evaporative fractionation of Zn that persists until ∼75% LMO crystallization. The first model yields a relatively homogenous bulk solid LMO δ66Zn value, while the second results in a stratification of δ66Zn values within the LMO sequence. Loss and/or isolation mechanisms for volatiles are critical to these models; hydrodynamic escape was not a dominant process, but loss of a nascent lunar atmosphere or separation of condensates into a proto-lunar crust are possible mechanisms by which volatiles could be separated from the lunar interior. The results do not preclude models that suggest a lunar volatile depletion episode related to Giant Impact. Conversely, LMO models for volatile loss do not require loss of volatiles prior to lunar formation. Outgassing during planetary magma ocean phases likely played a profound part in setting the volatile inventories of planets, particularly for low mass bodies that experienced the greatest volatile loss. In turn, our result suggest that the initial compositions of planets that accreted from smaller, highly differentiated planetesimals were likely to be severely volatile depleted.

Paniello, RC, Day JMD, Moynier F.  2012.  Zinc isotopic evidence for the origin of the Moon. Nature. 490:376-U104.   10.1038/nature11507   AbstractWebsite

Volatile elements have a fundamental role in the evolution of planets. But how budgets of volatiles were set in planets, and the nature and extent of volatile-depletion of planetary bodies during the earliest stages of Solar System formation remain poorly understood(1,2). The Moon is considered to be volatile-depleted and so it has been predicted that volatile loss should have fractionated stable isotopes of moderately volatile elements(3). One such element, zinc, exhibits strong isotopic fractionation during volatilization in planetary rocks(4,5), but is hardly fractionated during terrestrial igneous processes(6), making it a powerful tracer of the volatile histories of planets. Here we present high-precision zinc isotopic and abundance data which show that lunar magmatic rocks are enriched in the heavy isotopes of zinc and have lower zinc concentrations than terrestrial or Martian igneous rocks. Conversely, Earth and Mars have broadly chondritic zinc isotopic compositions. We show that these variations represent large-scale evaporation of zinc, most probably in the aftermath of the Moon-forming event, rather than small-scale evaporation processes during volcanism. Our results therefore represent evidence for volatile depletion of the Moon through evaporation, and are consistent with a giant impact origin for the Earth and Moon.

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

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

Day, JMD, Neal CR.  2019.  To the Moon: A scientific tribute to Lawrence A. Taylor.   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.