THE VALENCE OF Ti IN ENSTATITE CHONDRITES: NOT WHAT YOU MIGHT THINK

Introduction: Because they are dominated by enstatite, a nearly FeO-free pyroxene, and Si-bearing FeNi metal, along with minor amounts of unusual monosulfides such as niningerite (MgS), oldhamite (CaS) and alabandite (MnS), the enstatite chondrites have long been thought to have formed under extremely reducing conditions [1,2]. Like the ordinary chondrites, enstatite chondrites exhibit a range of petrologic types from type 3, with sharply defined chondrules, through type 6, with no discernable chondrules. Detailed study of unequilibrated enstatite chondrites (UECs) shows that they contain minor amounts of olivine and pyroxene with significant (>3 wt%) FeO contents which has been described as a ubiquitous component of the UECs [3,4]. On the basis of cathodoluminescence (CL) characteristics three generations of enstatite have been identified within UECs [4]. Some of the FeO-rich grains have different oxygen isotopic compositions from typical UEC enstatite grains while others do not [5]. This body of work shows that enstatite chondrites are not homogeneous, highly reduced rocks, but instead are complex objects, containing materials from multiple sources that record different redox histories. We have undertaken a study of the valence of Ti in E chondrites of grades 3-6 to better understand redox conditions of the source regions of their components and to see how Ti valence and coordination vary with metamorphic grade in these samples. Methods: The samples being investigated in the initial phase of the study are the falls Qingzhen (EH3), Indarch (EH4), St. Mark’s (EH5) and the find Y980211 (EH6). Chondrules and areas to be analyzed were selected based on examination with the scanning electron microscope (backscattered electron imaging and energy-dispersive analysis) and were analyzed by electron probe. The valence of Ti in olivine (Ol) and pyroxene was determined directly by X-ray absorption near edge structure (XANES) spectroscopy using methods previously described [7]. Spots analyzed by XANES were also analyzed by electron microprobe and found to contain up to 0.2 wt% TiO2. Results: Six chondrules in Qingzhen (QZ) and two in Indarch (IN) have been analyzed by XANES thus far. In QZ, CH6 is a type IAB with subhedral, 10-50 μm Ol grains enclosed in enstatite (En) and in devitrified mesostasis. CH8 is a radial pyroxene chondrule. CH10 also is a type IAB, with coarser Ol and En and less mesostasis than CH6. CH11 is a compound chondrule. The main body is dominated by coarse, subhedral En, with minor Ol and mesostasis. The small, attached object has anhedral En grains with sulfide/metal rims. CH12 is very unusual, with coarse, dusty laths of FeO-rich pyroxene with interstitial En, albite and FeS. CH13 is a compound fragment with coarse En laths and a small radial pyroxene chondrule attached. The IN chondrules have coarse En and little mesostasis. Phase compositions. Enstatite corresponding to the red and blue CL groups of [4] and [6] was identified in Qingzhen and Indarch, plus “black” (no CL) enstatite [4] in Qingzhen on the basis of electron albedo and minor element contents. Electron probe analysis of six Indarch chondrules shows that most of the “light” (high-albedo) En has higher Al2O3, CaO, Cr2O3, MnO and FeO contents than most “dark” En. In Fig. 1, CaO and MnO contents of light and dark En (this study) are compared to those of red and blue En from [8]. The plot shows good overlap between dark and blue En on one hand and between light and red En on the other. A difference between the present results and those of [6,8] is that here, lower FeO contents (0.4 wt% avg) are observed in dark/blue En than in red/light En (0.7 wt% avg), and both are lower than the average of 0.9 wt% reported for both types by [6]. QZ CH12 has FeO-rich pyroxene with 9-20 wt% FeO. This material would fall into the “black” CL category of [4]. Excluding CH12, FeO contents of analyzed pyroxene range from below detection to 1.5 wt%.