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The Earliest Lunar Magma Ocean Differentiation Recorded in Fe Isotopes ,∗ Kun Wang (王昆) a , Stein B

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The Earliest Lunar Magma Ocean Differentiation Recorded in Fe Isotopes ,∗ Kun Wang (王昆) a , Stein B Earth and Planetary Science Letters 430 (2015) 202–208 Contents lists available at ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl The earliest Lunar Magma Ocean differentiation recorded in Fe isotopes ,∗ Kun Wang (王昆) a , Stein B. Jacobsen a, Fatemeh Sedaghatpour a, Heng Chen b, Randy L. Korotev b a Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA b Department of Earth and Planetary Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA a r t i c l e i n f o a b s t r a c t Article history: Recent high-precision isotopic measurements show that the isotopic similarity of Earth and Moon is Received 25 March 2015 unique among all known planetary bodies in our Solar System. These observations provide fundamental Received in revised form 13 August 2015 constraints on the origin of Earth–Moon system, likely a catastrophic Giant Impact event. However, in Accepted 17 August 2015 contrast to the isotopic composition of many elements (e.g., O, Mg, Si, K, Ti, Cr, and W), the Fe isotopic Available online xxxx compositions of all lunar samples are significantly different from those of the bulk silicate Earth. Such Editor: B. Buffett a global Fe isotopic difference between the Moon and Earth provides an important constraint on the Keywords: lunar formation – such as the amount of Fe evaporation as a result of a Giant Impact origin of the Iron isotopes Moon. Here, we show through high-precision Fe isotopic measurements of one of the oldest lunar rocks Origin of the Moon (4.51 ± 0.10 Gyr dunite 72 415), compared with Fe isotope results of other lunar samples from the Apollo Giant Impact program, and lunar meteorites, that the lunar dunite is enriched in light Fe isotopes, complementing the Lunar Magma Ocean theory heavy Fe isotope enrichment in other lunar samples. Thus, the earliest olivine accumulation in the Lunar Magma Ocean may have been enriched in light Fe isotopes. This new observation allows the Fe isotopic composition of the bulk silicate Moon to be identical to that of the bulk silicate Earth, by balancing light Fe in the deep Moon with heavy Fe in the shallow Moon rather than the Moon having a heavier Fe isotope composition than Earth as a result of Giant Impact vaporization. © 2015 Elsevier B.V. All rights reserved. 1. Introduction the Moon; any successful model must properly explain such sim- ilarity between Earth and Moon. Thus, a model for Earth–Moon Smooth Particle Hydrodynamic (SPH) simulations have been isotopic equilibration subsequent to the Giant Impact was devel- used to support the hypothesis that Earth–Moon system is the oped (Pahlevan and Stevenson, 2007). In contrast, more recently product of an ancient catastrophic Giant Impact event. The orig- SPH simulations have been carried out for different scenarios inal simulations suggested that the Moon was made mostly from and now appear to show that it is possible to make the Moon the impactor material with only minor contributions from Earth primarily out of Earth materials in a Giant Impact scenario if (Canup and Asphaug, 2001; Canup, 2008). In contrast, isotopic the angular momentum constraint could be lifted (Canup, 2012; studies have demonstrated that Earth and Moon are remarkably Cuk´ and Stewart, 2012). Here we explore the case of Fe as many similar in their isotopic compositions for many elements (e.g., O, previous studies have argued that the Fe isotopic compositions of Ti, Cr and W isotopic anomalies and Mg and Si stable isotopes) Earth and Moon are not the same. All published Fe isotope data (Armytage et al., 2012; Clayton and Mayeda, 1996; Qin et al., 2010; of lunar materials, show an at least about 50 ppm/a.m.u. (parts Touboul et al., 2007; Zhang et al., 2012). This similarity between per million/atomic mass units) enrichment of heavy Fe isotopes Earth and Moon is unique in our Solar System when compared relative to the bulk silicate Earth. This difference is well resolved to other planetary bodies (such as Mars, Vesta 4 and other as- within the current analytical uncertainties and has been repeat- teroids represented by the various classes of meteorites). It pro- edly reported in samples ranging from mare basalts, highland vides a fundamental constraint on the theory of the origin of anorthosites, Mg-suite plutonic rocks, to lunar regolith (Craddock et al., 2010; Liu et al., 2010; Moynier et al., 2006; Poitrasson et al., 2004; Wang et al., 2012a, 2012b; Weyer et al., 2005; * Corresponding author. Wiesli et al., 2003). It has been debated whether this Fe isotopic E-mail address: [email protected] (K. Wang). difference between Earth and Moon represents a primary signature http://dx.doi.org/10.1016/j.epsl.2015.08.019 0012-821X/© 2015 Elsevier B.V. All rights reserved. K. Wang et al. / Earth and Planetary Science Letters 430 (2015) 202–208 203 of the Giant Impact due to the evaporative Fe loss (Poitrasson et of Fe using anion-exchange chromatography is following an estab- al., 2004), or of an unidentified mechanism during magma differ- lished protocol (Strelow, 1980). Detailed reviews of this technique entiation on the Moon (Craddock et al., 2010; Liu et al., 2010; are previously presented (Craddock and Dauphas, 2011; Dauphas Weyer et al., 2005). et al., 2009, 2004). 2. Samples and methods 2.3. Iron isotopic analyses with MC-ICP-MS 2.1. Samples Except for five additional analyses of the dunite 72 415, all other samples were analyzed for Fe isotopic ratios with the Neptune Plus To better understand the significance of Fe isotopes in the ori- MC-ICP-MS in the Department of Earth and Planetary Sciences at gin of the Moon, we analyzed a variety of lunar materials for Washington University in St. Louis, using the same procedures pre- Fe isotopes. The lunar dunite 72 415 is one of the oldest rocks viously described (Wang et al., 2012a, 2012b). The Neptune Plus from the Moon (4.51 ± 0.10 Gyr with Rb–Sr dating after cor- MC-ICP-MS was used in medium resolution mode to resolve the 87 −11 −1 rected with the new Rb decay constant of 1.402 × 10 yr , different peaks of Fe and argide species. Samples were introduced or 4.37–4.52 Gyr with U–Pb dating) (Papanastassiou and Wasser- into the plasma ion source with a 100 μL/min PFA MicroFlow neb- burg, 1975; Premo and Tatsumoto, 1992). It is also one of the ulizer and a SSI (Stable Sample Introduction) cyclonic/scott dual most magnesian (Mg#89) rocks from the Moon. It comprises 93% quartz spray chamber. All measurements with the Neptune Plus olivine (coarse-grained olivine embedded in fine-grained olivine were conducted in the medium (M/M ∼ 8500) resolution on the 40 14 matrix), 4% plagioclase, and 3% pyroxene (Dymek et al., 1975). peak shoulder in order to resolve isobaric interference of Ar N 54 40 16 56 40 16 1 57 It has been debated whether it is a deep-origin cumulate of the (on Fe), Ar O (on Fe), and Ar O H (on Fe). The interfer- 54 54 53 primary differentiation of the Moon (Dymek et al., 1975), or a cu- ence of Cr on Fe is corrected by measuring Cr intensities. The mulate in a shallow intrusion (Ryder, 1992). This shallow-origin correction is up to 0.01h for one Cr-rich dunite sample and negli- argument rests on two observations: the slightly zoned olivines gible for other samples. These results are calculated with sample- (Fo86–89) and relatively high CaO contents (0.05–0.12%) in 72 415, standard bracketing and expressed as conventional delta notations, x x 54 x 54 which neither is a reliable indicator of depths, as that there are that δ Fe = [( Fe/ Fe)sample/( Fe/ Fe)IRMM-014 −1] × 1000, where counter examples in terrestrial mantle xenoliths (see Supplemen- x = 56 or 57. Uncertainties are reported as 2SE (standard errors) of tary Materials). A recent experimental petrology study suggests repeated measurements. All the data from the Neptune Plus are 56 57 that dunite 72 415 was most likely formed from a partial melt- plotted in a δ Fe–δ Fe space (see Fig. 1) and all the samples fall ing of the hybridization of a rising solid diapir of low-density on a mass-dependent fractionation line, expressed as the exponen- early cumulates of Lunar Magma Ocean (LMO), KREEP and pla- tial law. gioclase components at the base of the crust (Elardo et al., 2011; The dunite 72 415 first measured with the Neptune Plus shows Shearer et al., 2014). This dunite belongs to the lunar Mg-suite a surprising value compared to all the lunar samples previously re- of plutonic rocks (including dunites, troctolites, norites and gab- ported. To confirm the validity of this negative value, we dissolved bronorites), which are likely relicts of the earliest cumulates of four more chips of this dunite sample (dunite #2 to dunite #5; LMO differentiation or mostly derived from these cumulates. No mass varies from 0.6 to 6.2 mg). The five additional measurements Fe isotopic composition of this dunite and other Mg-suite rocks were conducted with a GV Instruments IsoProbe P MC-ICP-MS at (except of one norite) (Poitrasson et al., 2004) has been pre- the Cosmochemistry Laboratory in the Department of Earth and viously reported. In addition, nearly all the literature data (but Planetary Sciences at Harvard University. The IsoProbe utilizes a one) (Wang et al., 2012a)are from analysis of Apollo samples.
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