Petrogenesis of the Lodranite-Acapulcoite Parent Body

Details Petrogenesis of the Lodranite-Acapulcoite Parent Body Petrogenesis of the Lodranite-Acapulcoite Parent Body

Jul 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.258m&link_type=abstract

Meteoritics, vol. 27, no. 3, volume 27, page 258

Mathematics

Logic

6

Scientific paper

The formation and migration of partial melts on asteroid-sized objects are still poorly understood, in part because of the lack of these melts or their residues in our collections. Recent petrologic (Takeda et al., 1991; McCoy et al., 1992) and isotopic (Clayton et al., 1992) investigations suggest that lodranites and acapulcoites are the residues of varying degrees of partial melting, consistent with an origin on a common parent body. The fine-grained acapulcoites experienced partial melting of metal, troilite and phosphates at or slightly above the Fe,Ni-FeS eutectic, while the coarse-grained lodranites experienced silicate partial melting. We have studied lodranites to define the properties of this group, assess the effects of silicate partial melting and examine whether the lodranites precursor was homogeneous (Nagahara, 1992) or heterogeneous (Clayton et al., 1992). Nine lodranites have now been discovered, including Lodran, MAC 88177, LEW 88280, Y-74357, Y-791491/Y-791493, Y-75274/Y-8002 and a new lodranite--Gibson. In contrast to the small range of average grain sizes (539-702 micrometers), most properties vary dramatically. Silicate compositions range from MAC 88177 (Fa(sub)13.3, Fs(sub)11.5) to Y-8002 (Fa(sub)3.5, Fs(sub)3.7). Lodranites of intermediate compositions exhibit reverse FeO zoning in olivine and/or pyroxene, often with higher Fs than Fa. This could result from partial melting and reduction of a common precursor (e.g., acapulcoites) to form lodranites. Some evidence is inconsistent with this interpretation. Iron-rich lodranites cannot be produced by partial melting of an acapulcoite precursor. Total Fe,Ni + FeS varies from 2.4 to 29.5 vol% and no correlation exists with olivine composition. Such large differences are inconsistent with low degrees of partial melting of a common precursor. A general trend exists among five analyzed lodranites of less negative delta^17O at higher Fa contents. Five acapulcoites fall along this trend. This correlation cannot result from igneous processing or reduction, but must be a primary feature of the precursor material. Other features can be explained by partial melting. Varying degrees of partial melting resulted in a range of plagioclase contents (0-10.3 vol%), although explaining a trend of increasing plagioclase content with decreasing Fa is problematic. Processes during or after slow cooling resulted in Ca zoning and exsolution in pyroxene and a range of shock effects. Divnoe (Zaslavskaya and Petaev, 1990) is a coarser-grained, iron-rich (Fa(sub)26.6) meteorite, possibly a new lodranite or winonaite. It is depleted in plagioclase and contains centimeter- sized interstitial pyroxenes, continuing the lodranite trend of greater partial melting at higher Fa contents. No such trend exists in the winonaites. The winonaite Tierra Blanca also has interstitial pyroxenes, but is finer-grained and contains abundant plagioclase. Divnoe is closer in Fa content to the lodranites (Fa(sub)3-13) than the winonaites (Fa(sub)1-5). New measurements of oxygen isotopes are ambiguous. Divnoe falls within the winonaite-LAB field, but continues the lodranite trend of less negative delta^17O with increasing Fa and falls along a slope-1 mixing line passing through the lodranites. We conclude that Divnoe is an iron-rich lodranite, since no evidence suggests the existence of iron-rich winonaites. Lodranite evolution mirrors that of ureilites, as first noted by Takeda et al. (1991). The lodranite precursor was heterogeneous with increasing Fa content at less negative delta^17O along a slope-1 mixing line parallel to the ureilites. With increasing temperature, iron-rich lodranites melted first because of lower eutectic temperatures, leading to greater depletions in plagioclase and, in the case of Divnoe, a coarser-grained residue. Later reduction resulted in minor zoning and compositional changes in some lodranites. Slow cooling and shock were the final processes recorded by these meteorites. References: Clayton et al. (1992) LPSC XXIII, 231-232; McCoy et al. (1992) LPSC XXIII, 871-872; Nagahara (1992) Proc. NIPR Symp. Ant. Met. 5, pre-print; Takeda et al. (1991) LPSC XXII, 1375-1376; Zaslavskaya and Petaev (1990) LPSC XXI, 1369-1370.

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