Cryptic Metamorphic Effects in Chondrules from Highly Unequilibrated Ordinary Chondrites: an Insidious Parent-body Process

Introduction: Type 3 ordinary chondrites display a wide variety of metamorphic grades, ranging from little metamorphosed type-3.0 chondrites up to nearly equilibrated type 3.9's. Researchers commonly assume that pristine-looking chondrules found in the lowest petrological subtypes have been unaffected by metamorphic processes, and can be used to constrain pre-accretionary processes. Last year, we reviewed various effects that result from parent-body processing in chondrules from highly unequilibrated ordinary chondrites [1]. Some are easily observed with a microscope or backscattered-electron imagery, such as "bleaching" and formation of hydrated minerals in mesostasis, but some are cryptic, such as the chemical zoning of chondrule glass that can only be observed using techniques like x-ray mapping. We presented preliminary results on another cryptic effect that might be related to secondary processing of chondrules, the formation of "inhomogeneous" mesostasis. We found that mesostasis in certain chondrules is composed of a mixture of two alkali-bearing phases: glass with high K/Na, and crystalline albitic feldspar with very low K/Na. Herein, we report the results of further studies of the distribution and composition of chondrule mesostasis in LL chondrites with petrologic type ≤3.4. We show that albite-bearing mesostasis forms during the early stages of metamorphism, altering the primary record of pre-accretionary processes in all ordinary chondrites, including type-3.0 Semarkona. The silica-rich glass found in type II chondrules is extraordinarily sensitive to metamorphic alteration, and, for related reasons, is far more difficult to analyze using electron-beam methods than many researchers have realized. Experimental: We continued our study [1] of al-bite-bearing chondrules using the TEM at the University of New Mexico. Two chondrules each from Se-markona and Chainpur were examined. A survey of alkali abundances in chondrule mesostasis in the least equilibrated ordinary chondrites was also carried out to measure changes in the abundance and composition of minerals as a function of metamorphic grade. Mesostasis and silicates from 50-75 chondrules selected at random from each of six meteorites, Semark-(LL3.4), were analyzed by electron microprobe. Special care was used in the measurement of sodium to