Differentiation: Building the Internal Architecture of Planets 2018 (LPI Contrib. No. 2084) 4020.pdf

AN EXPERIMENTAL ANALOG FOR METAL-SULFIDE PARTITIONING IN ACAPULCOITE- LODRANITE METEORITES. J. K. Dhaliwal1, N. L. Chabot2, R. D. Ash3 and T.J. McCoy4. 1Dept. of Geosci- ences, Pennsylvania State University, University Park, PA ([email protected]) 2Johns Hopkins University Ap- plied Physics Laboratory, Laurel, MD, 3Dept. Of Geology, University of Maryland, College Park, MD, 4Dept. of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC.

Introduction: The primitive achondrites are a 3.3 to 8.3 wt. %. The interstitial material has variable unique and valuable class of meteorites that are partial- Ni content, ranging from negligible to as much as 5.7 ly differentiated and reflect early metal-sulfide segre- wt. %. Because Ni should be excluded from sulfur-rich gation in planetesimals in our Solar System. These phases, this indicates the presence of fine-grained meteorites include the ureilites, brachinites and aca- FeNi, which is not distinguishable at 1500x magnifica- pulcoite-lodranites; recent work on highly siderophile tion (Fig. 1a) element (HSE: Re, Os, Ir, Ru, Pt, Pd) relative abun- dances in these samples show unusual partitioning of Pt and Pd [1-3]. These studies suggest that the inter- element fractionation among the HSE can be described by solid metal-liquid metal partitioning in the presence of sulfur [4]. The present study builds on this prior work, and seeks to replicate this HSE partitioning through high temperature melting experiments of natu- rally occurring metal and troilite separates. Methods: The experiments were conducted at the John Hopkins University Applied Physics Laboratory Figure 1: (a) Back-scattered electron image of FeNi- (APL) using previously established techniques for FeS melting experiment at 1100°C for 7 min (b) Plain investigating trace-element partitioning in solid metal- polarized light image of EET 84302 showing pooling liquid metal systems [e.g., 5]. The major difference metal [3]. between this study and prior work was the use of natu- Discussion: The results of this study can help un- ral materials; FeNi and FeS aliquots were isolated from derstand metal-sulfide partitioning that may have oc- the IAB iron meteorite, Mundrabilla (courtesy of the curred on the acapulcoite-lodranite parent body. This Smithsonian), and placed adjacent to each other in is evident in the petrographic similarity between metal silica glass tubes for each experiment. The experi- melting and pooling between experimental melts and mental set-up consisted of eight runs at 1 bar pressure: the transitional acapulcoite-lodranite, EET 84302 (Fig. (1) 1100°C for 1 hr; (2) 1100°C for 16 hrs; (3) 1000°C 1). The variable Ni content of the melted metal and for 1 hr; (4) 1000°C for 15 hrs; (5) 1100°C for 30 min; interstitial phases suggests that siderophile contents are (6) 1100°C for 15 min; (7) 1100°C for 7 min; (8) affected by sulfur interactions with the melting metal; 950°C for 1 hr. this may be analogous to the decrease in Pt/Os and Results: Petrography: At all temperatures, there is Pd/Os ratios observed for acapulcoite-lodranite mete- incipient metal-melting, evident in the rounded edges orites with decreasing sulfur contents [3]. of FeNi along the edges at the interface with FeS. The We have conducted in situ laser ablation- quenched liquid metal phase has a dendritic texture inductively coupled plasma mass spectrometer (LA- for higher temperatures, 1000°C and 1100°C, and ICPMS) analyses of HSE of these experiments at Uni- longer durations (1 hour and overnight). This texture is versity of Maryland and will present these results. similar to prior solid metal-liquid metal experiments These HSE partitioning results will provide insight [e.g., 6], and becomes progressively less dendritic with into how these experiments approximate the conditions shorter durations. In the experimental runs at 1100°C of very early metal-sulfide partitioning on the parent for 7 min, the quenched material is suggestive of an bodies of primitive achondrites. interstitial sulfur-rich material (Fig. 1a). References: [1] Rankenburg, K. et al. (2008) GCA, Geochemical Analyses: The Mundrabilla meteorite 72, 4642-4659. [2] Day, J.M.D. et al. (2012) GCA, 81, contains both kamacite and taenite phases, with an 94-128. [3] Dhaliwal, J.K. et al. (2017) GCA, 216, average Ni content of 8.3 ± 2.2 wt.% in bulk solid 115-140 [4] Chabot, N.L. and Jones, J.H. (2003), Me- metal grains (measured using an Hitachi 300 TM teorit. Planet. Sci., 38, 1425-1436 [5] Chabot, N.L. et benchtop SEM at APL). In the experimental melts, the al. (2007) Meteorit. Planet. Sci., 42, 1735-1750 [6] deformed solid FeNi at the interface with the melted Chabot, N.L., et al. (2017) Meteorit. Planet. Sci., 52, FeS has a noticeably lower Ni content, with a range of 1133-1145.