DELICATE OSCILLATORY ZONING IN PYROXENE PRESERVED THROUGH VARIATION IN THE CLOUDING INTENSITY OF EXSOLVED OXIDES: AN INSIGHT INTO MAFIC-ULTRAMAFIC MAGMA CHAMBER DYNAMICS

A tectonic slice of Proterozoic peridotite with cumulate texture is exposed in Ninetyone Mile Creek Canyon in the Upper Granite Gorge of the Grand Canyon, Arizona, USA. Delicate oscillatory zoning defined by variation in the abundance of dusty oxide crystals is preserved in diopside crystals in lherzolite and wehrlite. The oxides are 1- 20µm acicular rods. They resulted either from exsolution or from crystallization directly from the melt. In some grains, up to eleven concentric oxide-rich zones ranging in thickness from 20µm to a few mm can be identified. Although fine scale of zoning patterns are diverse, overall zoning patterns show similarity in many pyroxene grains. Similarity between these sequences from one grain to another suggests the recording of some widely occurring magma chamber event (Wiebe, 1968). This project uses detailed petrography of 72 clinopyroxene grains from 12 samples, along with electron microprobe (EMP) analyses, elemental abundance maps, and a loose application of dendrochronology to address the causes of both the variation in oxide abundance and the similarity in zoning patterns throughout the suite of lherzolite and wehrlite samples. A slight variation in aluminum abundance (with higher Al corresponding with more oxide-rich zones) can be detected in EMP elemental abundance maps. Individual rods imaged using EMP elemental abundance maps with a resolution of 1µm show enrichment in iron and aluminum relative to the surrounding clinopyroxene and similar composition to included chromite grains that have been altered to titanomagnetite. Preservation of oscillatory zoning as documented in plagioclase by Wiebe (1968) is rarer in pyroxene and olivine because higher magma temperatures and more rapid rates of self-diffusion of the compositional components of pyroxene or olivine tend to compositionally homogenize pyroxene and olivine. The preservation of remnant compositional zoning in pyroxene from the Ninetyone Mile Peridotite provides the opportunity to correlate compositional changes, thermal fluctuations, and/or oxygen fugacity excursions with pyroxene growth episodes across a moderately large mafic-ultramafic magma chamber.