THE LODRAN PRIMITIVE ACHONDRITE - PETROGENETIC INSIGHTS FROM ELECTRONAND ION MICROPROBE ANALYSIS OF OLIVINE AND ORTHO-PYROXENE

The Lodran primitive achondrite is thought to represent some of the ea rliest events in the differentiation of chondritic asteroids. Most res earchers who studied Lodran believe that it is a restite from which a melt, enriched in the incompatible trace and minor elements, was extra cted. This process is reflected by a lack of feldspar in the Lodran mi neral assemblage. Presumably, after or during melting, a reduction eve nt took place resulting in an increase in Mg/Fe near the rims of both olivine and orthopyroxene. The reduction process is far more advanced in olivine than in pyroxene because of the more rapid Fe-Mg diffusion rates in olivine. Narrow reaction rims (<10 mu m) around orthopyroxene grains show a depletion in Ca, Al, Cr, Ti, REEs, and Y and an increas e in Mg (reverse zoning). These systematics are largely the result of melting and to a lesser extent of reduction. Significant reaction of o livine has taken place by reduction and/or sulfidation and to a lesser extent by melting. This is reflected by a decrease in Fe, Co, and Ni and an increase in Mg towards the rims of olivine. The reactions by wh ich reduction and/or sulfidation of olivine took place are not confirm ed. However, the following represent two possible reactions: (1) Fe2Si O4 --> 2Fe(0) + O-2 + SiO2; (2) Fe2SiO4 + S-2 --> 2FeS + O-2 + SiO2.