A Study of Plagioclase-bearing Pyroxenites from the Ultraslow-spreading Gakkel Ridge, Arctic Ocean

Abstract:

Mantle pyroxenites play an important role in models on melt petrogenesis at mid-ocean ridges and ocean islands. Thus their study can give extremely valuable insights on mantle heterogeneity and deep melting and melt transport processes but only a limited number of studies exist. A recent study on pyroxenites sampled at the Lena Trough showed that measuring the elemental composition of minerals within pyroxenites can give important information on their formation processes and associated pressures and temperatures.

 Here we build on this recent study by working on fresh plagioclase-bearing pyroxenites from the nearby Gakkel Ridge, Arctic Ocean. Very little has been published on abyssal pyroxenites and plagioclase-bearing pyroxenites in particular, and the ability to contrast our results – including estimates of formation pressures and temperatures – with pyroxenites from a nearby ridge is particularly useful. In this study we determined the chemical and modal composition of three samples of plagioclase-bearing pyroxenites dredged within the Sparsely Magmatic Zone.

These samples are particularly fresh, allowing a detailed study of mineral compositional variation and their textural context. Different generations of pyroxene can be identified and plagioclase occurs as rims around spinel, pl-opx symplectites and lamellae in and around clinopyroxene and crosscutting olivines.

Mineral compositions are variable within a given thin section and distinctly different from pyroxenites from Lena Trough. We established temperature and pressure conditions under which the samples likely formed using mineral equilibria and single mineral thermometers; we then compared and contrasted the studied samples with published data from other plagioclase pyroxenites and peridotites. Pressure estimates show that plagioclase formation occurred shallower relative to Lena Trough but comparable to pyroxenites from the Southwest Indian Ridge.