44th Lunar and Planetary Science Conference (2013) 1892.pdf

PARTIAL MELTING PROCESSES ON THE PRIMITIVE PARENT BODIES FROM A VIEWPOINT OF CHEMICAL COMPOSITION. Y. Hidaka1, A. Yamaguchi2, 3, N. Shirai1 and M. Ebihara1, 1Department of chemistry, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji-shi, Tokyo, Japan 192- 0397, e-mail: 2National Institute of Polar Research, Tachikawa, Tokyo, Japan , 3Department of Polar Science, School of Multidisciplinary Science, Graduate University for Advanced Sciences, Tachikawa, Tokyo, Japan,

Introduction: Primitive are the most analyzed by INAA and ICP-MS for lithophile element suitable for understanding the early differentiation abundances except for Dhofar 290 and Y-74063, processes on the solar system bodies, because they are which were analyzed as bulk powder samples. The the partial melting residues. Two types of partial magnetic fraction was analyzed by ICP-MS and ICP- melting processes (basaltic partial melting and Fe,Ni- AES for siderophile and chalcophile element FeS eutectic melting) have been proposed for abundances. The magnetic fraction of Y-8005 was chondritic materials [1]. devided into two fragments which were analyzed in The basaltic partial melting in primitive this study. Because the magnetic separation was not achondrites can be confirmed as the depletion of practicable for Dhofar 125, Dhofar 290 and NWA plagioclase in the texture, and the depletions of 2235, chemical compositional data of the magnetic plagiophile elements (such as Na, Al and K), light rare fraction could not be obtained for these . earth elements (LREE) and Eu in their composition [1, 2]. The Fe,Ni-FeS eutectic melting can be confirmed Results: The non-magnetic fraction of most as the depletion of in the texture, and the shows nearly chondritic compositions, depletion of chalcophile elements and the fractionation being less depleted in LREEs and plagiophile elements. of siderophile element abundances [1, 3]. Among the acapulcoites analyzed in this study, Y- There are two main groups of primitive achondrites, 74063 has the most chondritic composition. On the - and . Acapulcoites other hand, GRA 95209 and Y-981505 have the largest and lodranites originated from the same , depletion in plagiophile and chalcophile elements. while winonaites originated from a different parent Although these elements are also severely depleted in body [4]. Acapulcoites and lodranites can be lodranites, there is a clear difference in plagiophile distinguished by the difference of their partial melting element abundances between acapulcoites and degrees; acapulcoites suffered small degree of partial lodranites. Winonaites analyzed in this study show melting compared with lodranites [5, 6]. similar chemical compositions to those of Y-74063; In this study, we focus on the chemical they all are similar to in chemical composition of primitive achondrites to understand the composition. partial melting processes on their parent bodies. Each The magnetic fraction of acapulcoite-lodranites is primitive achondrite must have suffered the different widely scattered in platinum group elements (PGEs) degree of partial melting, so they would have their own and W. A Y-791491 has the highest chemical compositions. Therefore, by analyzing a abundances of PGEs and W. On the other hand, an number of primitive achondrites, we tried to evaluate acapulcoite Y-981505 is depleted in PGEs. The how the partial melting processes proceeded on the abundances of siderophile and chalcophile elements primitive achondrite parent bodies. other than PGEs are less variable within acapulcoite- lodranites. Winonaites show similar chemical Samples and Method: We have studied 13 composition to those of an acapulcoite Y-74063 except primitive achondrites; 9 acapulcoites (Dhofar 125, for volatile elements. Two winonaites are similar to Dhofar 290, GRA 95209, MET 01195/01198/01244 each other in PGE and W abundances, but not in the pairs, Y-74063, Y-981505 and Y-981725), 2 lodranites abundances of volatile elements such as Cu, Ga and Sn. (NWA 2235 and Y-791491) and 2 winonaites (NWA 725 and Y-8005). Y-981725 was once classified as a Discussion: We discuss both silicate and metallic lodranite [7], but chemical composition of this partial melting processes of individual primitive was observed to be similar to those of achondrite groups. Chemical compositions of the non- acapulcoites [8]. Therefore, here, we consider Y- magnetic fraction of primitive achondrites show that 981725 is an acapulcoite. Several chips weighing ~300 the partial melting influencing the non-magnetic mg of each meteorite were ground into fine grains and fraction of primitive achondrites is not the basaltic magnetically separated into non-magnetic and partial melting process only. This can be realized in magnetic fractions. The non-magnetic fraction was Fig. 1, where plagiophile elements (Na, Al and K) and 44th Lunar and Planetary Science Conference (2013) 1892.pdf

an incompatible lithophile element (Th) are plotted. Meteorite Newsletter 20 (2011) NIPR [8] Hidaka Y. et Plagiophile elements abundance can be an indicator of al. (2012) LPS XXXXIII, #1785. [9] McCoy T. J. et al. the amount of plagioclase and the degree of basaltic (2006) GCA, 70, 516-531. [10] Feldstein S. N. et al. partial melting, whereas Th abundance can be an (2001) & Planet. Sci., 36, 1421-1441. [11] indicator of the amount of phosphate. From this figure, Hunt A. C. et al. (2012) LPS XXXXIII, #1818. [12] we can see that the amounts of plagioclase and Hidaka Y. et al., (2012) 75th Ann. Meteor. Soc. Meeting, phosphate are not well correlated with each other. #5210. [13] Chabot N. L. et al. (2003) Meteorit. Planet. Apparently there must have been two types of melting Sci., 38, 181-196. [14] Chabot N. L. et al. (2009) processes affecting the non-magnetic fraction of Meteoritics & Planet. Sci., 44, 505-519. [15] Schulz T. primitive achondrites; basaltic partial melting and et al. (2009) EPSL, 280, 185-193. [16] Anders E. and phosphate-dominated partial melting. The presence of Grevesse N. (1989) GCA, 53, 197-214. [17] Kong P. et the latter partial melting is supported by observations al. (1997) GCA, 61, 4895-4914. of phosphate veins or metal-sulfide-phosphate veins in 1 some acapulcoites [5, 9] and phosphate-rich partial melt in melting experiment of a [10]. As shown in Fig. 1, the silicate portion of the two plagioclase winonaites analyzed in this study shows chondritic CI Dho 125 composition, implying that these winonaites were Dho 290 MET 01195 scarcely melted. The same comclusion was suggested MET 01198 MET 01244 by the previous study [11]. Y-74063 Chemical compositions of the magnetic fraction of Y-981505 GRA 95209, 251 primitive achondrites show that Fe,Ni-FeS eutectic GRA 95209, 254

melting is the dominant process of metallic partial ((Na+Al+K)/3*Mg) GRA 95209, 256 0.1 Y-981725 melting. In Fig. 2, EL chondrite-normalized values are NWA 2235 Y-791491 compared for Ir/Ni and W/Re ratios. EL chondrite data NWA 725 are used for normalization, considering that the loss Y-8005 0.05 precursor materials of acapulcoites would be similar to 0.1 1 3 EL chondrites in chemical composition [8, 12]. In this (Th/Mg)CI figure, the results of model calculation of Fe,Ni-FeS loss phosphate gain eutectic melting are also shown. Partition coefficient Fig. 1. CI-normalized plagiophile element abundances data of siderophile and chalcophile elements are from vs. Th abundances for the non-magnetic fraction of [13, 14]. Acapulcoite-lodranite data are mostly primitive achondrites. CI values are from [16]. consistent with the results of model calculation. This comparison shows that metals in a lodranite Y-791491 5 residual metal FeNi-FeS eutectic represent the residues of high degree Fe,Ni-FeS 4 compositions eutectic melting while metals of an acapulcoite Y- 3 Y-791491 981505 represent the liquid portion of high degree 0.7 Fe,Ni-FeS eutectic melting. Metal compositions of 2 average of winonaites can’t be explained by Fe,Ni-FeS eutectic GRA 95209 melting. As seen in Fig. 2, there is a large compositional difference between the two winonaites. Y-8005 Considering that a parent body is considered 1 0.9 to have experienced several large impact events that 0.8 0.7

caused secondary heating or the breakup-reassembly EL-normalized Ir/Ni 0.6 Y-981505 [15], such a difference can be explained by the 0.5 contribution of materials from different reservoirs. 0.4 liquid metal compositions 0.995 References: [1] McCoy T. J. et al (1997) GCA, 61, 0.3 639-650. [2] Torigoye N. et al. (1993) Proc. NIPR 0.80.9 1 2 3 4 Symp. Antarct. Meteorites, 6, 100-119. [3] EL-normalized W/Re Mittlefehldt D. W. et al. (1996) GCA, 60, 867-882. [4] Fig. 2. EL chondrite-normalized Ir/Ni vs. W/Re ratios Greenwood R. C. et al. (2012) GCA, 94, 146-163. [5] for the magnetic fraction of primitive achondrites. EL McCoy T. J. et al. (1996) GCA, 60, 2681-2708. [6] chondrite data are from [17]. Legends are the same as McCoy T. J. et al. (1997) GCA, 61, 623-637. [7] shown in Fig. 1.