CORRELATED MINERALOGY, CHEMICAL COMPOSITIONS, OXYGEN ISOTOPIC COMPOSITIONS AND SIZE OF CHONDRULES

Abstract

We report results of a study into the size, chemical composition, mineralogy and oxygen isotopic compositions of 40 chondrules, in the size range 1-9 mm diameter, separated from Chainpur (LL3.4) and Parnallee (LL3.6). These 4 parameters were found to be interdependent. The main control on the bulk compositions of chondrules is the relative volatilities of the elements. The major elements Mg-Al-Ti-Ca show strong correlations among one other, and most of the remaining, more volatile elements form another grouping. Increasing proportions of refractory to volatile elements correlate with chondrule size. Smaller chondrules tend to have more glassy, SiO2-rich mesostases than larger chondrules, which have a higher proportion of mesostasis with feldspathic compositions. Chondrules with high-SiO2, glassy mesostases have exchanged with 16O-poor gases to a greater degree than low-SiO2 mesostases containing crystalline feldspar. The state of mesostases of the chondrules is the most important control on their oxygen isotopic compositions, although there also seems to be a size effect in Chainpur. Smaller chondrules in Chainpur have more 16O-poor compositions, suggesting that the relatively large surface areas of smaller chondrules facilitated gas-solid exchange. This correlation is obscured in Parnallee because it has undergone more extensive devitrification than Chainpur, leading to an increase in the proportion of crystalline mesostasis. The model we propose to explain the interdependence of bulk compositions, size and oxygen isotopes is in two stages. Chondrules which were still partially molten experienced an influx of relatively volatile elements from ambient gases. Smaller chondrules experienced a relatively greater influx than larger chondrules because of their greater surface area/volume ratio. The second stage, following crystallisation of the chondrules and limited devitrification of the mesostases, is the influx of an 16O-poor gas which led to the isotopic exchange. We believe the most likely setting for the formation of these chondrules and the associated volatile-rich and 16O-poor gas influxes is a planetary regolith setting.