The Mobility of Major and Trace Elements in EOC Minerals on Parent Chondrite Bodies

Abstract

The combined EPMA and SIMS study of the geochemical features of olivine, low-Ca pyroxene and plagioclase in equilibrated ordinary chondrites (EOCs) has revealed the effect of thermal metamorphism on trace element concentrations in EOC silicate minerals. In ordinary chondrites of petrological type 6, trace element composition is homogenized in olivine (Zr, Al, Ti, Ca, Cr, Sr and Ba) and low-Ca pyroxene (Zr, Hf, Y, Ti, Ca, Cr, Sr, Ba and Rb). Trace element concentrations in silicate minerals of petrological types 4 and 5 remain unequilibrated. Although variations in trace element concentrations have decreased, their concentrations in EOC minerals remain unchanged with an increase in the grade of thermal metamorphism. Plagioclase in equilibrated ordinary chondrites displays a consecutive decrease in trace element concentrations with a rise in the peak temperature of thermal metamorphism measured with an olivine-Cr spinel geothermometer. Chondrites of petrological type 4 show a metamorphic temperature of 670–682 °C, and meteorites of petrological type 5 reached equilibrium at a temperature of 687–700 °C. Chondrites of petrological type 6 show the highest metamorphic temperature of 734 °C. The temperatures are generally consistent with the concentric model of parent chondrite bodies.

Keywords

• trace elements
• ordinary chondrites
• olivine
• low-Ca pyroxene
• plagioclase