Silica-bearing chondrules and clasts in ordinary chondrites

Abstract

Unusual silica-bearing chondrules and clasts have been observed in several unequilibrated ordinary chondrites. There appear to be two distinct types: (I) silica, low-Ca pyroxene assemblages, and (II) silica, fayalite intergrowths coexisting with low-Ca pyroxene. Only the former types are chondrules. The bulk compositions of most of these objects can be expressed in terms of three components—MgO, FeO, and SiO_2—since Ca, Al, and Na are essentially trace elements. Secondary alteration has probably perturbed the Fe/Mg ratios in most objects and produced Ca-rich pyroxenes in some type II clasts, but the silica, low-Ca pyroxene and fayalite are hard to understand as other than primary phases. Such silica-rich bulk compositions can be understood as an intermediate temperature condensate of a solar gas, formed by the loss of refractory solid material by gas-solid separation before the complete condensation of silicon. Alternatively, the silicarich bulk compositions could have been formed by extreme reduction of normal chondritic olivine and orthopyroxene; however, the observed Ca, Al depletion and the lack of Ni-poor metals are not explained by this model. The fayalite-bearing clasts can be explained by a relatively complex two-stage process involving reduction followed by oxidation. Partial reduction of olivine in the presence of S produces Mg-rich pyroxene, silica, and FeFeS; oxidation of metal and sulfide followed by reaction with silica forms fayalite. An alternative is that the fayalite-bearing clasts result from the decomposition of ferrosilite, but this requires the production of metastable ferrosilite during shock or some other high-pressure event. The simplest unflawed alternative is that the fayalite-bearing clasts result from the mixing of a partially molten silica, fayalite liquid with Mg-rich pyroxene.