Globular Silicate Inclusions in IIE Irons and Sombrerete: Highly Fractionated Minimum Melts

HIGHLY FRACTIONATED MINIMUM MELTS M. Prinz? C.E. ~ e h r u ? ) *J.S. ~ e l a n e y j M. weisberg; 1 *E. 01sen; (1) Dept. Min. Sci., Amer. Mus. Nat. Hist., NY, NY 10024, ( 2 ) Dept. Geology, Brooklyn College, CUNY, B'kiyn, NY 11210, (3) Field Mus. Nat. Hist., Chicago, IL 60605. Four of t h e IIE irons, a small group of 12, have globular o r rounded s i l ica te inclusions. These a r e Kodaikanal (KO), Elga (El), Colomera (Co) and Weekeroo St. (WS). Orme IIE iron, ~ e t s c h a e v o (Ne), has angular inclusions with poorly preserved chondrules and chondri t ic mineralogy (1,2). Sombrere te (So), a new ungrsuped iron, has highly albi t ic and phosphatic globular s i l icate inclusions (3); meta l and si l icates have general s imilar i t ies t o IIE irons with globular silicates, but oxygen isotopes differ (4). IIE irons with globular s i l icates have been studied t o varying degrees, but the i r origin and significance is s t i l l poorly understooci. The me ta l is considered magmat ic , but Ga, G e and Ni show small weak correlat ions (5). The globular s t ruc tu re of t h e s i l icates indicates metal-silicate liquid immiscibility and t h e mineralogy is considered f rac t ionated from chondritic (5,6). Isotopic ages a r e 4.5 bey. for WS and Co, 3.8 b.y. for ~d and Ne (7). The Ne a g e i s surprising eince t h e s i l icates a r e chondri t ic and unmelted, and i t i s not c lear t h a t t h e I-Xe and Ar-Ar clocks c a n be r e se t by shock. Most workers suggest a shock origin for th,: mixture of t h e m e t a l and si l icates (e.g. 5-9). Oxygen isotopic da t a (10, this volume) indicate t h a t a l l 3IE s i l ica tes formed f rom t h e same reservoir as H chondrites. The purpose of this s tudy i s t o ga ther new mineralogic and petrologic da t a , especially modes of inclusions, in order t o be t t e r understand IIE si l icates prior t o carrying ou t consortium studies. In all , 50 inclusions have been analyzed; 4 in Ne, 6 in KO, 4 in El, 1 3 in Co, 8 in WS (lo), and 1 3 in So (3). Average modal compositions for each me teo r i t e were determined, and major e lement compositions calculated. Modes and Bulk Compositions: Modal d a t a for 50 inclusions and their average modes a r e shown in Fig. 1. The major charac ter i s t ic of t h e globular IIE si l icates is their e x t r e m e enr ichment in plag, feldspathic glass and tridymite. These components a r e of ten int imately intergrown and t h e overall average for t h e four IIE irons are: KO, 73%; El, 68%; Co, 67%; WS, 58.5%; So has 76%. Phosphate is also enriched, f rom 0.4-2%, with inclusions up t o 5.8%; So has 8%, inclusions up t o 13%. Yagiite i s found only in Co, 2.7%; one inclusion has 47%. Cpx is t h e dominant pyroxene in th ree IIE irons; KO, El and Co have 21-28% cpx, 2-4% opx. WS is dominated by opx (24%) over cpx (16%). So has only opx (15%). Thus the globular IIE si l icates a r e Na-K feldspar-tridymite-phosphate-cpx assemblages, minimum mel t compositions in chondri t ic and basal t ic systems. Bulk compositions of globular IIE si l icates have high S i02 (6 1-65%), A1203 (9-1 2%), C a O (6-8%), N a 2 0 (3.5-6%), K 2 0 (0.4-3.8%), P 2 0 5 (0.2-1%) and low FeO (1.8-7.5%) and MgO (6-11%) in comparison with chondri t ic compositions. So has similar concentrat ions, but P2O5 (4%) is ex t remely high. Ne d i f fers radically. It has typical chondritic mineralogy, with 33% 01, 52% opx, 12% plag. Unusual a spec t s a r e t he absence of cpx, and high phosphate (2.8%). Mineralogy: Although modes a r e highly variable, mineral compositions for KO, El and C o a r e uniform for each meteor i te , with opx from En75-81 (Fig. 2). WS mineralogy varies, f rom En61-81. So i s uniform at En68. Ne has Engq, slightly more Mg-rich than H chondrites. Feldspar in globular IIE si l icates is strikingly different than in IAB silicates. KO, El and C o have Na and K feldspar, with minor An; WS has a larger An component. Ne has sodic plag a s in IAB si l icates and chondrites. So has ca lc ic plag, An75, and sodic quench glass. Ca-Cr-Zr a rmalcol i te was found in Colomera as a very minor component. It contains 1.8% CaO, 11.8% Cr2O3; 6.1% 2 0 2 , 1.8% MgO, as well as FeO and T i 0 3 Discussion:(l) Globular s i l icates in IIE irons have remarkable textures, modes, mineralogy and bulk compositions. They a r e igneous in their outer s t ruc tu re