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2016
Harvey, J, Day JMD.  2016.  Highly siderophile and strongly chalcophile elements in high temperature geochemistry and cosmochemistry. (81):774pp.: Mineralogical Society of America   10.2138/rmg.2015.81.00  
Gannoun, A, Burton KW, Day JMD, Harvey J, Schiano P, Parkinson I.  2016.  Highly Siderophile Element and Os Isotope Systematics of Volcanic Rocks at Divergent and Convergent Plate Boundaries and in Intraplate Settings. Reviews in Mineralogy and Geochemistry. 81:651-724.   10.2138/rmg.2016.81.11   Abstract

Terrestrial magmatism is dominated by basaltic compositions. This definition encompasses mid-ocean ridge basalts (MORB), which account for more than eighty percent of Earth’s volcanic products and which are formed at divergent oceanic plate margins, as well as intraplate volcanic rocks such as ocean island basalts (OIB), continental flood basalts (CFB) and continental rift-related basalts, and highly magnesian ultramafic volcanic rocks that dominantly occur in Archean terranes, termed komatiites. All of these broadly basaltic rocks are considered to form by partial melting of the upper mantle, followed by extraction from their source regions and emplacement at the Earth’s surface. For these reasons, basalts can be used to examine the nature and extent of partial melting in the mantle, the compositions of mantle sources, and the interactions between the crust and mantle. Because much of Earth’s mantle is inaccessible, basalts offer some of the best ‘proxies’ for examining mantle composition, mantle convection and crust–mantle interactions. By contrast, at arcs, volcanism is dominated by andesitic rock compositions. While some arcs do have basaltic and picritic magmatism, these magma types are rare in convergent plate margin settings and reflect the complex fractional crystallization and often associated concomitant assimilation processes occurring in arcs. Despite the limited occurrence of high MgO magmas in arc volcanic rocks, magmas from this tectonic setting are also important for elucidating the behavior of the HSE from creation of basaltic compositions at mid-ocean ridges to the subduction of this crust beneath arcs at convergent plate margins.

Day, JMD, Brandon AD, Walker RJ.  2016.  Highly Siderophile Elements in Earth, Mars, the Moon, and Asteroids. Reviews in Mineralogy and Geochemistry. 81:161-238.   10.2138/rmg.2016.81.04   Abstract

The highly siderophile elements (HSE: Os, Ir, Ru, Rh, Pt, Pd, Re, Au) are key tracers of planetary accretion and differentiation processes due to their affinity for metal relative to silicate. Under low-pressure conditions the HSE are defined by having metal–silicate partition coefficients in excess of 104 (e.g., Kimura et al. 1974; Jones and Drake 1986; O’Neill et al. 1995; Borisov and Palme 1997; Mann et al. 2012). The HSE are geochemically distinct in that, with the exception of Au, they have elevated melting points relative to iron (1665 K), low vapour pressures, and are resistant to corrosion or oxidation. Under solar nebular conditions, Re, Os, Ir, Ru, Rh, and Pt, along with the moderately siderophile elements (MSE) Mo and W, condense as refractory-metal alloys. Palladium and Au are not as refractory and condense in solid solution with FeNi metal (Palme 2008). Assuming abundances of the HSE in materials that made up the bulk Earth were broadly similar to modern chondrite meteorites, mass balance calculations suggest that >98% of these elements reside in the metallic core (O’Neill and Palme 1998). In practical terms, the resultant low HSE abundance inventories in differentiated silicate crusts and mantles enables the use of these elements in order to effectively track metallic core formation and the subsequent additions of HSE-rich impactors to planets and asteroids (Fig. 1). In detail, the absolute and relative abundances of the HSE in planetary materials are also affected by mantle and crustal processes including melting, metasomatism, fractional crystallization, and crust-mantle remixing, as well as later impact processing, volatility of Re under oxidizing conditions, and low-temperature secondary alteration (cf., Day 2013; Gannoun et al. 2016, this volume). In the absence of metal, the HSE are chalcophile, so these elements are also affected by processes

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

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

Kato, C, Moynier F, Valdes M, Dhaliwal JK, Day JMD.  2015.  Extensive volatile loss during formation and differentiation of the Moon. Nature Communications. 6(7617)   10.1038/ncomms8617   Abstract

Low estimated lunar volatile contents, compared with Earth, are a fundamental observation for Earth–Moon system formation and lunar evolution. Here we present zinc isotope and abundance data for lunar crustal rocks to constrain the abundance of volatiles during the final stages of lunar differentiation. We find that ferroan anorthosites are isotopically heterogeneous, with some samples exhibiting high d66Zn, along with alkali and magnesian suite samples. Since the plutonic samples were formed in the lunar crust, they were not subjected to degassing into vacuum. Instead, their compositions are consistent with enrichment of the silicate portions of the Moon in the heavier Zn isotopes. Because of the difference in d66Zn between bulk silicate Earth and lunar basalts and crustal rocks, the volatile loss likely occurred in two stages: during the proto-lunar disk stage, where a fraction of lunar volatiles accreted onto Earth, and from degassing of a differentiating lunar magma ocean, implying the possibility of isolated, volatile-rich regions in the Moon’s interior.

O'Driscoll, B, Walker RJ, Day JMD, Ash RD, Daly JS.  2015.  Generations of Melt Extraction, Melt–Rock Interaction and High-Temperature Metasomatism Preserved in Peridotites of the∼ 497 Ma Leka Ophiolite Complex, Norway. Journal of Petrology. 56(9):1797-1828.   10.1093/petrology/egv055   Abstract

Ophiolites allow spatial and temporal assessment of the causes and length-scales of upper mantle compositional heterogeneity because they permit field-based observations to be coupled with geochemical investigations of upper mantle lithologies. The ∼497 Ma Leka Ophiolite Complex (Norway) comprises a section of early Palaeozoic (Iapetus) oceanic lithosphere with well-exposed mantle and lower crustal sections and generally low degrees of serpentinization. The Leka upper mantle section is heterogeneous at the centimetre to metre scale, manifested by abundant dunite lenses and sheets in harzburgitic host-rock, especially within ∼500 m of the palaeo Moho. Abundant chromitite (≥60 vol. % Cr-spinel) and pyroxenite lenses and layers also occur in the uppermost 200–300 m of the mantle section. These diverse mantle lithologies probably developed in a suprasubduction-zone (SSZ) setting, as a result of fluid-assisted melt extraction, offering an opportunity to interrogate the nature of chemical heterogeneities developed in such rocks. At ∼497 Ma, the Os isotopic compositions of Leka harzburgites averaged ∼2% more radiogenic than the projected average for abyssal peridotites at that time, yet they exhibit nearly chondritic relative abundances of the highly siderophile elements (HSE). Several of the harzburgites are characterized by low initial 187Os/188Os (<0·121), reflecting Proterozoic melt depletion. Preservation of Os isotopic compositions consistent with ancient (<0·5 to 2 Ga) melt depletion episodes is a common characteristic of melt-depleted oceanic peridotites. There is no clear evidence that SSZ melt extraction had a discernible impact on the bulk Os isotopic composition of the Iapetus oceanic mantle, as represented by the Leka harzburgites. By contrast, non-harzburgitic lithologies are generally characterized by more radiogenic initial 187Os/188Os and more variable HSE abundances. The dunites, chromitites and pyroxenites of the LOC can be separated into two groups on the basis of their trace element geochemistry and the Re-Os isotope errorchrons that they define, yielding ages of 485 ± 32 Ma and 589 ± 15 Ma, respectively. The former age corresponds, within error, to the accepted age of the ophiolite (497 ± 2 Ma). The meaning of the latter age is uncertain, but possibly corresponds to the early stages of Iapetus opening. The Leka ophiolite reveals the importance of oceanic lithosphere formation processes for mantle heterogeneity at metre to kilometre scales, but also emphasizes the robustness of Os isotopes in recording older melt-depletion events.

Day, JMD, Barry PH, Hilton DR, Burgess R, Pearson DG, Taylor LA.  2015.  The helium flux from the continents and ubiquity of low-3He/4He recycled crust and lithosphere. Geochimica et Cosmochimica Acta. 153:116-133.   http://dx.doi.org/10.1016/j.gca.2015.01.008   AbstractWebsite

New helium isotope and trace-element abundance data are reported for pyroxenites and eclogites from South Africa, Siberia, and the Beni Bousera Massif, Morocco that are widely interpreted to form from recycled oceanic crustal protoliths. The first He isotope data are also presented for Archaean peridotites from the Kaapvaal (South Africa), Slave (Canada), and Siberian cratons, along with recently emplaced off-craton peridotite xenoliths from Kilbourne Hole, San Carlos (USA) and Vitim (Siberia), to complement existing 3He/4He values obtained for continental and oceanic peridotites. Helium isotope compositions of peridotite xenoliths vary from 7.3 to 9.6 RA in recently (<10 kyr) emplaced xenoliths, to
0.05 RA in olivine from cratonic peridotite xenoliths of the 1179 Ma Premier kimberlite, South Africa. The helium isotope compositions of the peridotites can be explained through progressive sampling of 4He produced from radiogenic decay of U and Th in the mineral lattice in the older emplaced peridotite xenoliths. Ingrowth of 4He is consistent with generally higher 4He concentrations measured in olivine from older emplaced peridotite xenoliths relative to those from younger peridotite xenoliths. Collectively, the new data are consistent with pervasive open-system behaviour of He in peridotite xenoliths from cratons, mobile belts and tectonically-active regions. However, there is probable bias in the estimate of the helium isotope composition of the continental lithospheric mantle (6.1 ± 2.1 RA), since previously published databases were largely derived from peridotite xenoliths from non-cratonic lithosphere, or phenocrysts/xenocrysts obtained within continental intraplate alkaline volcanics that contain a contribution from asthenospheric sources. Using the new He isotope data for cratonic peridotites and assuming that significant portions (>50%) of the Archaean and Proterozoic continental lithospheric mantle are stable and unaffected by melt or fluid infiltration on geological timescales (>0.1 Ga), and that U and Th contents vary between cratonic lithosphere and non-cratonic lithosphere, calculations yield a 3He flux of 0.25–2.2 atoms/s/cm2 for the continental lithospheric mantle. These estimates differ by a factor of ten from non-cratonic lithospheric mantle and are closer to the
observed 3He flux from the continents (<1 atoms/s/cm2). Pyroxenites and eclogites from the continental regions are all characterized by 3He/4He (0.03–5.6 RA) less than the depleted upper mantle, and relatively high U and Th contents. Together with oceanic and continental lithospheric peridotites, these materials represent reservoirs with low time-integrated 3He/(U + Th) in the mantle. Pyroxenites and eclogites are also characterized by higher Fe/Mg, more radiogenic Os–Pb isotope compositions, and more variable d18O values (3 to 7 per mille), compared with peridotitic mantle. These xenoliths are widely interpreted to be the metamorphic/metasomatic equivalents of
recycled oceanic crustal protoliths. The low-3He/4He values of these reservoirs and their distinctive compositions make themprobable end-members to explain the compositions of some low-3He/4He OIB, and provide an explanation for the low-3He/4He measured in most HIMU lavas. Continental lithospheric mantle and recycled oceanic crust protoliths are not reservoirs for high-3He/4He and so alternative, volumetrically significant, He-rich reservoirs, such as less-degassed (lower?) mantle, are required to explain high-3He/4He signatures measured in some intraplate lavas. Recycling of oceanic crust represents a fundamental process for the generation of radiogenic noble gases in the mantle, and can therefore be used effectively as tracers for volatile recycling.

Barry, PH, Hilton DR, Day JMD, Pernet-Fisher JF, Howarth GH, Magna T, Agashev AM, Pokhilenko NP, Pokhilenko LH, Taylor LA.  2015.  Helium isotopic evidence for modification of the cratonic lithosphere during the Permo-Triassic Siberian flood basalt event. Lithos. 216-217:73-80.   10.1016/j.lithos.2014.12.001   Abstract

Major flood basalt emplacement events can dramatically alter the composition of the sub-continental lithospheric mantle (SCLM). The Siberian craton experienced one of the largest flood basalt events preserved in the geologic record — eruption of the Permo-Triassic Siberian flood basalts (SFB) at ~250 Myr in response to upwelling of a deep-rooted mantle plume beneath the Siberian SCLM. Here,we present helium isotope (3He/4He) and concentration data for petrologically-distinct suites of peridotitic xenoliths recovered from two temporally-separated kimberlites:
the 360 Ma Udachnaya and 160 Ma Obnazhennaya pipes, which erupted through the Siberian SCLM and bracket the eruption of the SFB. Measured 3He/4He ratios span a range from 0.1 to 9.8 RA (where RA = air 3He/4He) and fall into two distinct groups: 1) predominantly radiogenic pre-plume Udachnaya samples (mean clinopyroxene 3He/4He = 0.41 ± 0.30 RA (1σ); n = 7 excluding 1 outlier), and 2) ‘mantle-like’ post plume Obnazhennaya samples (mean clinopyroxene 3He/4He=4.20±0.90 RA (1σ); n=5 excluding 1 outlier). Olivine separates from both kimberlite pipes tend to have higher 3He/4He than clinopyroxenes (or garnet). Helium contents in Udachnaya samples ([He] = 0.13–1.35 μcm3STP/g; n = 6) overlap with those of Obnazhennaya
([He]=0.05–1.58 μcm3STP/g; n = 10), but extend to significantly higher values in some instances ([He]=49–349 μcm3STP/g; n = 4). Uranium and thorium contents are also reported for the crushed material from which He was extracted in order to evaluate the potential for He migration from the mineral matrix to fluid inclusions. The wide range in He content, together with consistently radiogenic He-isotope values in Udachnaya peridotites suggests that crustal-derived fluids have incongruently metasomatized segments of the Siberian SCLM, whereas high 3He/4He values in Obnazhennaya peridotites show that this section of the SCLM has been overprinted by Permo-Triassic (plume-derived) basaltic fluids. Indeed, the stark contrast between pre- and post-plume 3He/4He ratios in peridotite xenoliths highlights the potentially powerful utility of He-isotopes for differentiating between various types of metasomatism (i.e., crustal versus basaltic fluids).

Day, JMD, Walker RJ.  2015.  Highly siderophile element depletion in the Moon. Earth and Planetary Science Letters. 423:114-124.   10.1016/j.epsl.2015.05.001   Abstract

Coupled 187Os/188Os and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance data are reported for Apollo 12 (12005, 12009, 12019, 12022, 12038, 12039, 12040), Apollo 15 (15555) and Apollo 17 (70135) mare basalts, along with mare basalt meteorites La Paz icefield (LAP) 04841 and Miller Range (MIL) 05035. These mare basalts have consistently low HSE abundances, at ∼2×10−5 to 2×10−7 the chondritic abundance. The most magnesian samples have broadly chondrite-relative HSE abundances and chondritic measured and calculated initial 187Os/188Os. The lower abundances and fractionated HSE compositions of more evolved mare basalts can be reproduced by modeling crystal–liquid fractionation using rock/melt bulk-partition coefficients of ∼2 for Os, Ir, Ru, Pt and Pd and ∼1.5 for Re. Lunar mare basalt bulk-partition coefficients are probably higher than for terrestrial melts as a result of more reducing conditions, leading to increased HSE compatibility. The chondritic-relative abundances and chondritic 187Os/188Os of the most primitive high-MgO mare basalts cannot readily be explained through regolith contamination during emplacement at the lunar surface. Mare basalt compositions are best modeled as representing ∼5–11% partial melting of metal-free sources with low Os, Ir, Ru, Pd (∼0.1 ng g−1), Pt (∼0.2 ng g−1), Re (∼0.01 ng g−1) and S (∼75 μg g−1), with sulphide-melt partitioning between 1000 and 10,000.

Apollo 12 olivine-, pigeonite- and ilmenite normative mare basalts define an imprecise 187Re–187Os age of 3.0±0.9 Ga with an initial 187Os/188Os of 0.107±0.010. This age is within uncertainty of 147Sm–143Nd ages for the samples. The initial Os isotopic composition of Apollo 12 samples indicates that the source of these rocks evolved with Re/Os within ∼10% of chondrite meteorites, from the time that the mantle source became a system closed to siderophile additions, to the time that the basalts erupted. Similarity in absolute HSE abundances between mare basalts from the Apollo 12, 15 and 17 sites, and from unknown regions of the Moon (La Paz mare basalts, MIL 05035), indicates relatively homogeneous and low HSE abundances within the lunar interior. Low absolute HSE abundances and chondritic Re/Os of mare basalts are consistent with a late accretion addition of ∼0.02 wt.% of the Moon's mass to the mantle, prior to the formation of the lunar crust. Late accretion must also have occurred significantly prior to cessation of lunar mantle differentiation (>4.4 Ga), to enable efficient mixing and homogenization within the mantle. Low lunar HSE abundances are consistent with proportionally 40 times more late accretion to Earth than the Moon. Disproportional late accretion to the two bodies is consistent with the small 182W excess (∼21–28 ppm) measured in lunar rocks, compared to the silicate Earth.

Magna, T, Day JMD, Mezger K, Fehr MA, Dohmen R, Aoudjehane HC, Agee CB.  2015.  Lithium isotope constraints on crust–mantle interactions and surface processes on Mars. Geochimica et Cosmochimica Acta. 162:46-65.   10.1016/j.gca.2015.04.029   Abstract

Lithium abundances and isotope compositions are reported for a suite of martian meteorites that span the range of petrological and geochemical types recognized to date for Mars. Samples include twenty-one bulk-rock enriched, intermediate and depleted shergottites, six nakhlites, two chassignites, the orthopyroxenite Allan Hills (ALH) 84001 and the polymict breccia Northwest Africa (NWA) 7034. Shergottites unaffected by terrestrial weathering exhibit a range in δ7Li from 2.1 to 6.2‰, similar to that reported for pristine terrestrial peridotites and unaltered mid-ocean ridge and ocean island basalts. Two chassignites have δ7Li values (4.0‰) intermediate to the shergottite range, and combined, these meteorites provide the most robust current constraints on δ7Li of the martian mantle. The polymict breccia NWA 7034 has the lowest δ7Li (−0.2‰) of all terrestrially unaltered martian meteorites measured to date and may represent an isotopically light surface end-member.

The new data for NWA 7034 imply that martian crustal surface materials had both a lighter Li isotope composition and elevated Li abundance compared with their associated mantle. These findings are supported by Li data for olivine-phyric shergotitte NWA 1068, a black glass phase isolated from the Tissint meteorite fall, and some nakhlites, which all show evidence for assimilation of a low-δ7Li crustal component. The range in δ7Li for nakhlites (1.8 to 5.2‰), and co-variations with chlorine abundance, suggests crustal contamination by Cl-rich brines. The differences in Li isotope composition and abundance between the martian mantle and estimated crust are not as large as the fractionations observed for terrestrial continental crust and mantle, suggesting a difference in the styles of alteration and weathering between water-dominated processes on Earth versus possibly Cl–S-rich brines on Mars. Using high-MgO shergottites (>15 wt.% MgO) it is possible to estimate the δ7Li of Bulk Silicate Mars (BSM) to be 4.2 ± 0.9‰ (2σ). This value is at the higher end of estimates for the Bulk Silicate Earth (BSE; 3.5 ± 1.0‰, 2σ), but overlaps within uncertainty.

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.

2014
Franz, HB, Kim ST, Farquhar J, Day JMD, Economos RC, McKeegan KD, Schmitt AK, Irving AJ, Hoek J, Dottin J.  2014.  Isotopic links between atmospheric chemistry and the deep sulphur cycle on Mars. Nature. 508:364-+.   10.1038/nature13175   AbstractWebsite

The geochemistry of Martian meteorites provides a wealth of information about the solid planet and the surface and atmospheric processes that occurred on Mars. The degree to which Martian magmas may have assimilated crustal material, thus altering the geochemical signatures acquired from their mantle sources, is unclear(1). This issue features prominently in efforts to understand whether the source of light rare-earth elements in enriched shergottites lies in crustal material incorporated into melts(1,2) or in mixing between enriched and depleted mantle reservoirs(3). Sulphur isotope systematics offer insight into some aspects of crustal assimilation. The presence of igneous sulphides in Martian meteorites with sulphur isotope signatures indicative of mass-independent fractionation suggests the assimilation of sulphur both during passage of magmas through the crust of Mars and at sites of emplacement. Here we report isotopic analyses of 40 Martian meteorites that represent more than half of the distinct known Martian meteorites, including 30 shergottites (28 plus 2 pairs, where pairs are separate fragments of a single meteorite), 8 nakhlites (5 plus 3 pairs), Allan Hills 84001 and Chassigny. Our data provide strong evidence that assimilation of sulphur into Martian magmas was a common occurrence throughout much of the planet's history. The signature of mass-independent fractionation observed also indicates that the atmospheric imprint of photochemical processing preserved in Martian meteoritic sulphide and sulphate is distinct from that observed in terrestrial analogues, suggesting fundamental differences between the dominant sulphur chemistry in the atmosphere of Mars and that in the atmosphere of Earth(4).

Peters, BJ, Day JMD.  2014.  Assessment of relative Ti, Ta, and Nb (TITAN) enrichments in ocean island basalts. Geochemistry, Geophysics, Geosystems. 15(11):4424-4444.   10.1002/2014GC005506   Abstract

The sensitivity of trace element concentrations to processes governing solid-melt interactions has made them valuable tools for tracing the effects of partial melting, fractional crystallization, metasomatism, and similar processes on the composition of a parental melt. Recent studies of ocean island basalts (OIB) have sought to correlate Ti, Ta, and Nb (TITAN) anomalies to isotopic tracers, such as 3He/4He and 187Os/188Os ratios, which may trace primordial deep mantle sources. A new compilation of global OIB trace element abundance data indicates that positive TITAN anomalies, though statistically pervasive features of OIB, may not be compositional features of their mantle sources. OIB show a range of Ti (Ti/Ti* = 0.28–2.35), Ta (Ta/Ta* = 0.11–93.4), and Nb (Nb/Nb* = 0.13–17.8) anomalies that show negligible correlations with 3He/4He ratios, indicating that TITAN anomalies are not derived from the less-degassed mantle source traced by high-3He/4He. Positive TITAN anomalies can be modeled using variable degrees (0.1–10%) of nonmodal batch partial melting of garnet-spinel lherzolite at temperatures and pressures considered typical for OIB petrogenesis, and subjecting this partial melt to fractional crystallization and assimilation of mid-ocean ridge basalt-like crust (AFC). Correlations of TITAN anomalies with modal abundances of olivine and clinopyroxene in porphyritic Canary Islands lavas provide empirical support for this process and indicate that high abundances of these phases in OIB may create misleading trace element anomalies on primitive mantle-normalized spider diagrams. Because partial melting and AFC are common to all mantle-derived magmas, caution should be used when attributing TITAN anomalies to direct sampling of recycled or deep mantle sources by hotspots.

Hyde, BC, Day JMD, Tait KT, Ash RD, Holdsworth DW, Moser DE.  2014.  Characterization of weathering and heterogeneous mineral phase distribution in brachinite Northwest Africa 4872. Meteoritics and Planetary Science. 49(7):1141-1156.   10.1111/maps.12320   Abstract

Terrestrial weathering of hot desert achondrite meteorite finds and heterogeneous phase distributions in meteorites can complicate interpretation of petrological and geochemical information regarding parent-body processes. For example, understanding the effects of weathering is important for establishing chalcophile and siderophile element distributions within sulfide and metal phases in meteorites. Heterogeneous mineral phase distribution in relatively coarsely grained meteorites can also lead to uncertainties relating to compositional representativeness. Here, we investigate the weathering and high-density (e.g., sulfide, spinel, Fe-oxide) phase distribution in sections of ultramafic achondrite meteorite Northwest Africa (NWA) 4872. NWA 4872 is an olivine-rich brachinite (Fo63.6 ± 0.5) with subsidiary pyroxene (Fs9.7 ± 0.1Wo46.3 ± 0.2), Cr-spinel (Cr# = 70.3 ± 1.1), and weathered sulfide and metal. Raman mapping confirms that weathering has redistributed sulfur from primary troilite, resulting in the formation of Fe-oxide (-hydroxide) and marcasite (FeS2). From Raman mapping, NWA 4872 is composed of olivine (89%), Ca-rich pyroxene (0.4%), and Cr-spinel (1.1%), with approximately 7% oxidized metal and sulfide and 2.3% marcasite-dominated sulfide. Microcomputed tomography (micro-CT) observations reveal high-density regions, demonstrating heterogeneities in mineral distribution. Precision cutting of the largest high-density region revealed a single 2 mm Cr-spinel grain. Despite the weathering in NWA 4872, rare earth element (REE) abundances of pyroxene determined by laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) indicate negligible modification of these elements in this mineral phase. The REE abundances of mineral grains in NWA 4872 are consistent with formation of the meteorite as the residuum of the partial melting process that occurred on its parent body. LA-ICP-MS analyses of sulfide and alteration products demonstrate the mobility of Re and/or Os; however, highly siderophile element (HSE) abundance patterns remain faithful recorders of processes acting on the brachinite parent body(ies). Detailed study of weathering and phase distribution offers a powerful tool for assessing the effects of low-temperature alteration and for identifying robust evidence for parent-body processes.

Day, JMD, Moynier F.  2014.  Evaporative fractionation of volatile stable isotopes and their bearing on the origin of the Moon. Phil. Trans. R. Soc. A. 20130259   10.1098/rsta.2013.0259   Abstract

The Moon is depleted in volatile elements relative to the Earth and Mars. Low abundances of volatile elements, fractionated stable isotope ratios of S, Cl, K and Zn, high mu (238U/204Pb) and long-term Rb/Sr depletion are distinguishing features of the Moon, relative to the Earth. These geochemical characteristics indicate both inheritance of volatile-depleted materials that formed the Moon and planets and subsequent evaporative loss of volatile elements that occurred during lunar formation and differentiation. Models of volatile loss through localized eruptive degassing are not consistent with the available S, Cl, Zn and K isotopes and abundance data for the Moon. The most probable cause of volatile depletion is global-scale evaporation resulting from a giant impact or a magma ocean phase where inefficient volatile loss during magmatic convection led to the present distribution of volatile elements within mantle and crustal reservoirs. Problems exist for models of planetary volatile depletion following giant impact. Most critically, in this model, the volatile loss requires preferential delivery and retention of late-accreted volatiles to the Earth compared with the Moon. Different proportions of late-accreted mass are computed to explain present-day distributions of volatile and moderately volatile elements (e.g. Pb, Zn; 5 to >10%) relative to highly siderophile elements (approx. 0.5%) for the Earth. Models of early magma ocean phases may be more effective in explaining the volatile loss. Basaltic materials (e.g. eucrites and angrites) from highly differentiated airless asteroids are volatile-depleted, like the Moon, whereas the Earth and Mars have proportionally greater volatile contents. Parent-body size and the existence of early atmospheres are therefore likely to represent fundamental controls on planetary volatile retention or loss.

Wang, K, Day JMD, Korotev RL, Zeigler RA, Moynier F.  2014.  Iron isotope fractionation during sulfide-rich felsic partial melting in early planetesimals. Earth and Planetary Science Letters. 392:124-132.   10.1016/j.epsl.2014.02.022   Abstract

New Fe isotope data of feldspar-rich meteorites Graves Nunataks 06128 and 06129 (GRA 06128/9) reveal that they are the only known examples of crustal materials with isotopically light Fe isotope compositions (View the MathML source; δ 56Fe is defined as the per mille deviation of a sample's 56Fe/54Fe ratio from the IRMM-014 standard) in the Solar System. In contrast, associated brachinites, as well as brachinite-like achondrites, have Fe isotope compositions (View the MathML source) that are isotopically similar to carbonaceous chondrites and the bulk terrestrial mantle. In order to understand the cause of Fe isotope variations in the GRA 06128/9 and brachinite parent body, we also report the Fe isotope compositions of metal, silicate and sulfide fractions from three ordinary chondrites (Semarkona, Kernouve, Saint-Séverin). Metals from ordinary chondrites are enriched in the heavier isotopes of Fe (average View the MathML source), sulfide fractions are enriched in the lighter isotopes of Fe (average View the MathML source), and the δ 56Fe values of the silicates are coincident with that of the bulk rock (average View the MathML source).

The enrichment of light isotopes of Fe isotopes in GRA 06128/9 is consistent with preferential melting of sulfides in precursor chondritic source materials leading to the formation of Fe–S-rich felsic melts. Conceptual models show that melt generation to form a GRA 06128/9 parental melt occurred prior to the onset of higher-temperature basaltic melting (<1200 °C) in a volatile-rich precursor and led to the generation of buoyant felsic melt with a strong Fe–S signature. These models not only reveal the origin of enrichment in light isotopes of Fe for GRA 06128/9, but are also consistent with petrological and geochemical observations, experimental studies for the origin of Fe–S-rich felsic melts, and for the cessation of early melting on some asteroidal parent bodies because of the effective removal of the major radioactive heat-source, 26Al. The mode of origin for GRA 06128/9 contrasts strongly with crust formation on Earth, the Moon, Mars and other asteroids, where mantle differentiation and/or oxygen activity are the major controls on crustal Fe isotope compositions.

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

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

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.

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).

2013
Day, JMD.  2013.  Hotspot volcanism and highly siderophile elements. Chemical Geology. 341:50-74.   10.1016/j.chemgeo.2012.12.010   AbstractWebsite

Hotspot volcanic rocks are formed under conditions that differ from conventional plate tectonic boundary magmatic processes and are compositionally distinct from mid-oceanic ridge basalts. Hotspot volcanic rocks include - but are not limited to - ocean island basalts (OIB), continental flood basalts (CFB), komatiites, oceanic plateau and some intraplate alkaline volcanic rocks. Studies of the highly siderophile element (HSE) geochemistry of hotspot volcanic rocks have provided new perspectives into mantle convection, mantle heterogeneity, core-mantle interactions, crustal and mantle lithospheric recycling, melting processes and crust-mantle interactions. The HSE, comprising Os, Ir, Ru, Rh, Pt, Pd, Re and Au, have strong affinities for metal and sulphide relative to silicate. These elements also have variable partitioning behaviour between highly compatible Os, Ir, Ru and Rh relative to compatible Pt and Pd and to moderately incompatible Re and Au during melting and crysta! The HSE can be utilised to understand sub-aerial volcanic degassing and crustal assimilation processes in hotspot volcanic rocks such as CFB and OIB, as well as for quantitative assessment of fractional crystallisation. Mantle melting studies have highlighted the strong control of sulphide in the mantle prior to exhaustion of S and generation of Os Ir Ru metal alloys at similar to>25% partial melting; a behaviour of the HSE that is fundamental to understanding terrestrial hotspot volcanism. Perhaps the most exciting utility of the HSE, however, lies in their ability to reveal both short- and long-term fractionation processes acting on hotspot volcanic sources from inter-element HSE fractionations and Os-187/Os-188-Os-186/Os-188 systematics. The growing database for HSE abundances and Os-187/Os-188 in hotspot volcanic rocks is consistent with their generation from a heterogeneous upper mantle generated by melt differentiation and recycling of crust and mantle lithosphere d! The HSE provide geochemical evidence for how lithological and chemical heterogeneities are sampled within the mantle. Modeling of HSE abundances and Os isotopes show that large apparent recycled contributions (50% to 90%) in some OIB can be explained by the preferential melting of volumetrically minor (<10%) pyroxenite in their sources. Preferential melting of more fusible materials in the mantle also explains why low-degree partial melts, such as alkali basalts and basanites, may exhibit more extreme isotopic variations than tholeiites or komatiites, which likely contain a higher contribution from peridotite in a hybridised pyroxenite-peridotite mantle source. High-precision Os-188/Os-188 data for hotspot volcanism are limited, but the combined variations in long-term Re/Os and Pt/Os retained in some mantle sources may reflect either the long-term fractionation of Re and Pt from Os between the inner and outer core, or ancient sulphide segregation and lithological variati! Study of the HSE in hotspot volcanic rocks from Solar System bodies also informs on planetary-scale processes, indicating that Earth, the Moon, Mars and fully differentiated asteroids all have HSE abundances in their mantles that are higher than expected from low-pressure metal-silicate partitioning. Furthermore, the HSE are in broadly chondritic-relative abundances for these planetary bodies, at similar to 0.0002 (Moon), to similar to 0.007 (Mars), to similar to 0.009 (Earth) x carbonaceous chondrite Ivuna (CI) composition. The timing of addition of the HSE to planetary bodies preserved in their magmas and volcanic products is consistent with Solar-System-wide late accretion no later than 3.8 Ga for Earth, and even earlier based on evidence from the Moon (similar to 4.4 Ga), Mars (similar to 4.5 Ga) and asteroids (>4.56 Ga). (C) 2013 Elsevier B.V. All rights reserved.DELMON.A, 1972, AMERICAN JOURNAL OF SCIENCE, V272, P805

Cabral, RA, Jackson MG, Rose-Koga EF, Koga KT, Whitehouse MJ, Antonelli MA, Farquhar J, Day JMD, Hauri EH.  2013.  Anomalous sulphur isotopes in plume lavas reveal deep mantle storage of Archaean crust. Nature. 496:490-+.   10.1038/nature12020   AbstractWebsite

Basaltic lavas erupted at some oceanic intraplate hotspot volcanoes are thought to sample ancient subducted crustal materials(1,2). However, the residence time of these subducted materials in the mantle is uncertain and model-dependent(3), and compelling evidence for their return to the surface in regions of mantle upwelling beneath hotspots is lacking. Here we report anomalous sulphur isotope signatures indicating mass-independent fractionation (MIF) in olivine-hosted sulphides from 20-million-year-old ocean island basalts from Mangaia, Cook Islands (Polynesia), which have been suggested to sample recycled oceanic crust(3,4). Terrestrial MIF sulphur isotope signatures (in which the amount of fractionation does not scale in proportion with the difference in the masses of the isotopes) were generated exclusively through atmospheric photochemical reactions until about 2.45 billion years ago(5-7). Therefore, the discovery of MIF sulphur in these young plume lavas suggests that sulphur-probably derived from hydrothermally altered oceanic crust-was subducted into the mantle before 2.45 billion years ago and recycled into the mantle source of Mangaia lavas. These new data provide evidence for ancient materials, with negative Delta S-33 values, in the mantle source for Mangaia lavas. Our data also complement evidence for recycling of the sulphur content of ancient sedimentary materials to the subcontinental lithospheric mantle that has been identified in diamond-hosted sulphide inclusions(8,9). This Archaean age for recycled oceanic crust also provides key constraints on the length of time that subducted crustal material can survive in the mantle, and on the timescales of mantle convection from subduction to upwelling beneath hotspots.

Day, JMD, Pearson GD, Hulbert LJ.  2013.  Highly siderophile element behaviour during flood basalt genesis and evidence for melts from intrusive chromitite formation in the Mackenzie large igneous province. Lithos. 182-183:242-258.   http://dx.doi.org/10.1016/j.lithos.2013.10.011   Abstract

The 1.27 Ga Coppermine continental flood basalt (CFB) province in northern Canada represents the extrusive manifestation of the 2.7 Mkm2 Mackenzie large igneous province (LIP) that includes the Mackenzie dyke swarm and the Muskox layered intrusion. New Re–Os isotope and highly siderophile element (HSE: Re, Pd, Pt, Ru, Ir, Os) abundance data are reported together with whole-rock major- and trace-element abundances and Nd isotopes to examine the behaviour of the HSE during magmatic differentiation and to place constraints on the extent of crustal interaction with mantle-derived melts. Mineral chemistry and petrography are also reported for an unusual andesite glass flow (CM19; 4.9 wt.% MgO) found in close proximity to newly recognised picrites (> 20 wt.% MgO) in the lowermost stratigraphy of the Coppermine CFB. Compositions of mineral phases in CM19 are similar to the same phases found in Muskox Intrusion chromitites and the melt composition is equivalent to inclusions trapped within Muskox chromites. The apparently conflicting elevated HSE contents (e.g., 3.8 ppb Os) and mantle-like initial 187Os/188Os (γOs = + 2.2), versus stable isotope (δ18O = + 12‰) and lithophile element evidence (εNdi = − 12.8) for extensive crustal contamination, implicate an origin for CM19 as a magma mingling product formed within the Muskox Intrusion during chromitite genesis. Combined with Nd isotope data that places the feeder for lower Coppermine CFB picrites and basalts within the Muskox Intrusion, this result provides compelling evidence for direct processing of some CFB within upper-crustal magma chambers. The Coppermine CFB defines a 187Re–187Os isochron with an age of 1263 + 16/− 20 Ma and initial γOs = + 2.2 ± 0.8. The initial Os isotope composition for the Coppermine CFB is slightly higher than the near-primitive-mantle initial 187Os/188Os for the Muskox Intrusion (γOs = + 1.2 ± 0.3). This result is interpreted to reflect greater crustal contamination in extrusive CFB and the sensitivity of Os isotopes, compared with absolute HSE concentrations, for tracking crustal contributions.

Modelling of absolute and relative HSE abundances in global CFB reveals that HSE concentrations decrease with increasing fractionation for melts with < 8 ± 1 wt.% MgO, with picrites (> 13.5 wt.% MgO) from CFB (n = 98; 1.97 ± 1.77 ppb) having higher Os abundances than ocean island basalt (OIB) equivalents (n = 75; 0.95 ± 0.86 ppb). The differences between CFB and OIB picrite absolute Os abundances may result from higher degrees of partial melting to form CFB but may also reflect incorporation of trace sulphide in CFB picrites from magmas that reached S-saturation in upper-crustal magma chambers. Significant inter-element fractionation of (Re + Pt + Pd)/(Os + Ir + Ru) are generated during magmatic differentiation in response to strongly contrasting partitioning of these two groups of elements into sulphides and/or HSE-rich alloys. Furthermore, fractional crystallization has a greater role on absolute and relative HSE abundances than crustal contamination under conditions of CFB petrogenesis due to the dilution effect of continental crust, which has low total abundances of the HSE. Combined data for the basaltic and intrusive portions of the Mackenzie LIP indicate a mantle source broadly within the range of the primitive upper mantle. The majority of Archaean komatiites and Phanerozoic CFB also require mantle sources with primitive upper mantle to chondritic Re/Os evolution, with exceptions typically being from analyses of highly-fractionated MgO-poor basalts.

2012
Day, JMD, Walker RJ, Qin LP, Rumble D.  2012.  Late accretion as a natural consequence of planetary growth. Nature Geoscience. 5:614-617.   10.1038/ngeo1527   AbstractWebsite

Core formation should strip highly siderophile elements (HSEs) from planetary mantles according to the expected metal-silicate partitioncoefficients(1,2). However, studies of Earth(3), the Moon(4) and Mars(5) indicate mantles with HSE abundances in chondrite-relative proportions that exceed the values expected from metal-silicate partitioning. Competing hypotheses have been proposed to account for these observations, including metal-silicate partitioning at higher pressures and temperatures(6) and late accretion(7). Here we present petrological and geochemical analyses of diogenite meteorites that represent mantle and crustal materials from two or more differentiated asteroids. We find that diogenites show HSE abundances that are consistent with metal-silicate equilibration, followed by minor continued accretion. Isotope chronometry supports diogenite crystallization ages within 2-3 million years of Solar System formation, indicating that late accretion occurred earlier than postulated for Earth, the Moon and Mars. The early timing and occurrence on differentiated asteroids, as well as on the larger terrestrial planets, therefore ties late accretion to planetary growth. On asteroidal bodies, such as the diogenite parent bodies, variations in HSE compositions may reflect regional rather than global effects. In contrast, for Earth, the Moon and Mars, compositional variations in mantle materials seem to be consistent with more homogeneous distributions through prolonged melting and/or solid-state convection.

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.