Publications

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2012
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.

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

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

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

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

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

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

2007
Spicuzza, MJ, Day JMD, Taylor LA, Valley JW.  2007.  Oxygen isotope constraints on the origin and differentiation of the Moon. Earth and Planetary Science Letters. 253:254-265.   10.1016/j.epsl.2006.10.030   AbstractWebsite

We report new high-precision laser fluorination three-isotope oxygen data for lunar materials. Terrestrial silicates with a range of delta O-18 values (-0.5 to 22.9 parts per thousand) were analyzed to independently determine the slope of the terrestrial fractionation line (TFL; lambda = 0.5259 +/- 0.0008; 95% confidence level). This new TFL determination allows direct comparison of lunar oxygen isotope systematics with those of Earth. Values of Delta O-17 for Apollo 12, 15, and 17 basalts and Luna 24 soil samples average 0.01 parts per thousand and are indistinguishable from the TFL. The delta O-18 values of high- and low-Ti lunar basalts are distinct. Average whole-rock delta O-18 values for low-Ti lunar basalts from the Apollo 12 (5.72 +/- 0.06 parts per thousand) and Apollo 15 landing sites (5.65 +/- 0.12 parts per thousand) are identical within error and are markedly higher than Apollo 17 high-Ti basalts (5.46 +/- 0.11 parts per thousand). Evolved low-Ti LaPaz mare-basalt meteorite delta O-18 values (5.67 +/- 0.05 parts per thousand) are in close agreement with more primitive low-Ti Apollo 12 and 15 mare basalts. Modeling of lunar mare-basalt source composition indicates that the high- and low-Ti mare-basalt mantle reservoirs were in oxygen isotope equilibrium and that variations in delta O-18 do not result from fractional crystallization. Instead, these differences are consistent with mineralogically heterogeneous mantle sources for mare basalts, and with lunar magma ocean differentiation models that result in a thick feldspathic crust, an olivine-pyroxene-rich mantle, and late-stage ilmenite-rich zones that were convectively mixed into deeper portions of the lunar mantle. Higher average delta O-18 (WR) values of low-Ti basalts compared to terrestrial mid ocean ridge basalts (Delta=0.18 parts per thousand) suggest a possible oxygen isotopic difference between the terrestrial and lunar mantles. However, calculations of the delta O-18 of lunar mantle olivine in this study are only 0.05 parts per thousand higher than terrestrial mantle olivine. These observations may have important implications for understanding the formation of the Earth-Moon system. (c) 2006 Elsevier B.V. All rights reserved.

2006
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.