Geochemical investigations of ordinary chondrites, shergottites, and Hawaiian basalts

by 1971- Reynolds, Valerie Slater

Part I: Quantifying peak temperatures achieved during metamorphism is critical for understanding the thermal histories of ordinary chondrite parent asteroids. I performed two-pyroxene geothermometry, using QUILF95, on the same Type 6 chondrites for which peak temperatures were estimated using the plagioclase geothermometer. Pyroxenes record a narrow, overlapping range of temperatures in H6 (865-926°C), L6 (812-934°C), and LL6 (874-945°C) chondrites. Lower plagioclase temperature estimates may not reflect peak metamorphic temperatures because chondrule glass probably recrystallized to plagioclase prior to reaching the metamorphic peak. The average temperature for H, L, and LL chondrites (~900°C) is at least 50°C lower than peak temperatures used in current asteroid thermal evolution models, which may require minor adjustments. Part II: The light lithophile elements lithium, beryllium, and boron have been used successfully to indicate recycled crust or fluids derived from recycled crust in the source regions of island arc lavas. Radiogenic isotopes and other geochemistry of Mauna Kea lavas and Martian basalts (basaltic shergottites) suggest their source regions may contain a crustal component. The goal of this study is to determine whether Li, Be, and B indicate the presence of a crustal component in the source regions of Mauna Kea and Martian basalts and whether it was altered at low temperatures. Mauna Kea: Although several samples show effects of alteration, our results suggest Li (3.9±0.9 ppm) and Be (0.47±0.09 ppm) preserve mantle compositions. In contrast, highly variable B/K ratios (0.0002-0.008) and B/Be ratios (1-25) suggest postmagmatic alteration has destroyed the mantle B signature. When examined with depth, Li and Be abundances increase in the uppermost portion of the core, in late main shield and post-shield samples, and correspond to decreasing degrees of partial melting as the volcano moved off the plume’s center. Li and Be appear to be well mixed in the Hawaiian source region as evidenced by the lack of correlation between Li/Yb or Be/Nd iv ratios and Pb isotopes or Nb/Zr ratios, which were previously used to identify geochemically distinct Mauna Kea lava groups. Such mixing probably also accounts for the lack of any crustal signature when Li/Yb or Be/Nd are compared with O isotopes. These elements do not appear to vary on the timescale of Hawaiian shield development, possibly reflecting the efficiency with which these elements are homogenized in the mantle. Martian basalts: Although terrestrial alteration minerals (caliche) in Dhofar 019 apparently affected the primary Li and Be concentrations, the remaining basaltic shergottites contain Be (0.09-0.77 ppm) abundances similar to mid-ocean ridge basalts or ocean island basalts, whereas Li abundances (2.7-9.9 ppm) are slightly higher compared to these reservoirs. On diagrams of Li/Yb vs. Dy/Yb and K/Li vs. La/Yb, basaltic shergottites define trends similar to IAB, which are attributed to altered oceanic crust in the IAB source regions. However, the correlation between Li or Be and ?18O for basaltic shergottites is weak, and ?7Li values measured in two geochemically distinct basaltic shergottites, Zagami (+3.97‰) and EETA79001 (+4.37‰), are identical within error. Therefore, although the Martian assimilant appears to be enriched in Li and possibly Be, it either was not altered at low temperatures or the proportion of altered material in basaltic shergottite magmas is too small to be resolved using these crustal indicators. v