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S and the Formation of Pallasites with Rounded Olivine Grains

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S and the Formation of Pallasites with Rounded Olivine Grains Available online at www.sciencedirect.com ScienceDirect Geochimica et Cosmochimica Acta 162 (2015) 259–275 www.elsevier.com/locate/gca Fast grain growth of olivine in liquid Fe–S and the formation of pallasites with rounded olivine grains Giulio F.D. Solferino a,b,⇑, Gregor J. Golabek a,c, Francis Nimmo d, Max W. Schmidt a a Department of Earth Sciences, ETH Zurich, 8092 Zurich, Switzerland b School of Biological, Earth and Environmental Sciences, University College Cork, Cork, Ireland c Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany d Department of Earth and Planetary Sciences, University of California Santa Cruz, 95064 Santa Cruz, CA, USA Received 4 September 2014; accepted in revised form 10 April 2015; Available online 18 April 2015 Abstract Despite their relatively simple mineralogical composition (olivine + Fe–Ni metal + FeS ± pyroxene), the origin of pallasite meteorites remains debated. It has been suggested that catastrophic mixing of olivine fragments with Fe–(Ni)–S followed by various degrees of annealing could explain pallasites bearing solely or prevalently fragmented or rounded olivines. In order to verify this hypothesis, and to quantify the grain growth rate of olivine in a liquid metal matrix, we performed a series of annealing experiments on natural olivine plus synthetic Fe–S mixtures. The best explanation for the observed olivine grain size distributions (GSD) of the experiments are dominant Ostwald ripening for small grains followed by random grain bound- ary migration for larger grains. Our results indicate that olivine grain growth in molten Fe–S is significantly faster than in solid, sulphur-free metal. We used the experimentally determined grain growth law to model the coarsening of olivine sur- rounded by Fe–S melt in a 100–600 km radius planetesimal. In this model, an impact is responsible for the mixing of olivine and Fe–(Ni)–S. Numerical models suggest that annealing at depths of up to 50 km allow for (i) average grain sizes consistent with the observed rounded olivine in pallasites, (ii) a remnant magnetisation of Fe–Ni olivine inclusions as measured in nat- ural pallasites and (iii) for the metallographic cooling rates derived from Fe–Ni in pallasites. This conclusion is valid even if the impact occurs several millions of years after the differentiation of the target body was completed. Ó 2015 Elsevier Ltd. All rights reserved. 1. INTRODUCTION average. An occasionally present additional phase is phos- phoran olivine. Texturally, pallasites consist of olivine sur- Pallasites are a group of stony-iron meteorites with a rounded by a metal matrix with a few additional minor simple, yet vexing mineralogical composition. Buseck phases. Recent findings provided pallasites bearing up to (1977) found pallasites to contain 64.9 vol% olivine, 40 vol% pyroxene, e.g., North West Africa NWA 1911 31.0% metal, 2.3% troilite (FeS), 1.2% schreibersite (Bunch et al., 2005), changing the classic definition. ((Fe,Ni)3P), 0.4% chromite, and 0.2% phosphates on Originally separated into a Main Group (MG) and an Eagle Station group (ES) (Scott, 1977a), pallasites (the Meteoritical Bulletin website lists 103 pallasite records) ⇑ Corresponding author at: School of Biological, Earth and Environmental Sciences, University College Cork, Cork, Ireland. are nowadays subdivided into 5–7 groups based on the oxy- Tel.: +353 21 490 4582. gen isotopes signatures, trace element concentrations in the E-mail addresses: [email protected] (G.F.D. Solferino), Fe–Ni metal, and mineralogical composition (Scott, 2007; [email protected] (G.J. Golabek), [email protected]. Yang et al., 2010; Boesenberg et al., 2012). edu (F. Nimmo), [email protected] (M.W. Schmidt). http://dx.doi.org/10.1016/j.gca.2015.04.020 0016-7037/Ó 2015 Elsevier Ltd. All rights reserved. 260 G.F.D. Solferino et al. / Geochimica et Cosmochimica Acta 162 (2015) 259–275 A further characteristic feature is the morphology of the 18 °C/Ma and 13–16 °C/Ma, determined for Main Group olivines, which varies between angular and rounded, and and Eagle Station pallasites, respectively, are not compati- constitutes a key to deciphering the mechanism of pallasite ble with annealing in proximity of the CMB (Yang et al., formation. Macroscopically, there are pallasite specimens 2010). bearing solely or prevalently well-rounded, well-sorted oli- Tarduno et al. (2012) detected a remnant magnetisation vine grains, and samples with almost only angular frag- in microscopic Fe–Ni inclusions in olivine crystals of two ments of olivine (fragmental pallasites; Buseck, 1977; Main Group pallasites (Bryson et al., 2015). The measured Scott, 1977a). The former are commonly attributed to pro- magnetic intensities were ascribed to the presence of longed annealing, the latter could be the result of a violent dynamo action in their parent body, proposed to be active mixing or fracturing event. Among pallasites bearing frag- throughout their formation and cooling below the Curie mental olivines, it is possible to distinguish those with a temperature of these Fe–Ni particles (360 °C). Since the prevalence of large euhedral or subhedral crystals, often temperature in the proximity of the CMB, while the broken into several pieces and/or crossed by multiple frac- dynamo is active, is many hundreds of degrees above the tures (e.g., the Esquel pallasite), and those with a prepon- Curie temperature of Fe–Ni alloy (Tarduno et al., 2012), derance of shattered fragments in the sub-millimetre the region where pallasite-type material was formed has range (Huckitta). Even in highly fragmental pallasites, the to be placed at shallower depths. smallest fragments are invariably moderately- to In this model a collision event resulted in the fragmenta- well-rounded. Specimens bearing exclusively one type of tion of olivines in the target body mantle mixing with the olivine are uncommon, while a combination of two mor- metal from the impactor. Fragmentation is followed by oli- phologies is frequent. The Seymchan pallasite is one of vine annealing at relatively shallow depths. In this scenario, the rare samples containing all morphological olivine types. the depth range at which the mixed material cools down A classification of pallasites based on olivine textures was determines the olivine textures, with fragmented or shat- presented by Scott (1977b). Nevertheless, there is hardly tered olivines being little annealed at the shallowest depths any correlation between olivine type and chemical and min- while rounded olivines form by longer and hotter annealing eralogical features, only a tendency that pallasites with deeper in the planetesimal’s mantle. This hypothesis could more differentiated metal compositions and higher sulphur be tested, if annealing of small fragments in the shallow contents (Yang et al., 2010) contain rounded olivine slightly mantle is shown to result in rounded grains with a grain size more frequently than fragmented olivine (Buseck, 1977; comparable to that observed in pallasite meteorites before Scott, 1977a). temperatures decrease such that the annealing kinetics The peculiar characteristics of this meteorite family led becomes insignificant. to a number of formation theories. On one hand, the gene- Saiki et al. (2003) performed annealing experiments on a sis of pallasites was ascribed to magmatic differentiation mixture of olivine and solid Fe–Ni, demonstrating that within a single chondritic body in a region located near rounding of angular olivine fragments in hot solid metal the core mantle boundary (CMB) (Ringwood, 1961; is efficient for the smallest fragments, but inefficient over Lovering, 1962; Mason, 1963; Anders, 1964; Scott, meaningful time spans for large fragments >1 mm. Saiki 1977a,b; Wasson and Choi, 2003; Boesenberg et al., et al. (2003) also pointed out that the absence of sulphur 2012). Other authors suggested formation scenarios involv- in their experiments might have slowed down the rate of ing an external source of heat and/or matter related to textural annealing. impacts of various asteroids into the pallasite parent body A different approach is to test whether grain growth of (Mittlefehldt, 1980; Malvin et al., 1985; Scott and Taylor, olivine fragments in a metal matrix might yield the texture 1990; Ulff-Møller et al., 1998; Scott, 2007; Yang et al., and maximum grain size of olivines observed in pallasites 2010). with rounded olivine. Guignard et al. (2012) exposed for- The composition of the metal fraction in pallasites, in sterite plus solid nickel aggregates to high temperatures, particular the low Ir content, is consistent with fractional finding that forsterite growth in a solid Ni matrix could crystallization of the liquid in the IIIAB iron meteorite par- yield textures similar to those of pallasites, but growth rates ent body after crystallization of 80% of the metal (Scott, were too small. A direct comparison of Guignard et al.’s 1977c; Wasson and Choi, 2003). Nevertheless, there is no (2012) experiments with actual pallasites is difficult, since unequivocal evidence of a common origin for IIIAB iron their starting material was Fe- and S-free. meteorites and pallasites, as indicated by oxygen isotopes Our investigation aims to assess the grain growth kinet- (Scott et al., 2009), the concentration of siderophile ele- ics of olivine fragments in Fe–S melt. Moreover, we evalu- ments (Yang et al., 2010), and cosmic ray exposures ages ate the grain size distribution (GSD) of the experimentally (Eugster et al., 2006). Moreover, minor element abundances grown olivines. For comparison, we determine the GSD of in olivine and REE concentrations in phosphates are best three pallasites bearing rounded olivine and two with frag- modelled by low pressure crystallization (Davis and mented olivines in order to discuss their origin and forma- Olsen, 1991; Hsu, 2003), difficult to reconcile with incom- tion conditions. Finally, we elaborate a model of olivine plete silicate-metal separation at the boundary of a growing growth in a Fe–S matrix in a planetesimal (following the core or with mixing of an olivine layer and metal at the impact of a differentiated body).
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