Diffusive Loss of Water in Mantle Olivine and Pyroxene during Xenolith Emplacement

Nominally anhydrous minerals (NAMs), such as olivine and pyroxene, whose chemical formulas do not contain hydrogen, may contain trace amounts of hydrogen in their crystal lattice defects bonded to structural oxygen, i.e., water. Studies of mantle xenoliths show that water in olivine is often partially lost through diffusive degassing during peridotite emplacement. However, water in coexisting pyroxene is generally homogeneously distributed, indicating no loss of water during emplacement. These observations suggest that water diffuses much faster in mantle xenolithic olivine than in coexisting pyroxene. By contrast, laboratory experiments show no systematic or significant difference between water diffusivity in olivine and pyroxene. This apparent paradox has remained unsolved for a long time.

We determined water contents of olivine and pyroxene in mantle xenoliths collected from Lashaine, Eledoi, and Kisite volcanoes erupted adjacent to the Tanzanian craton using Fourier transform infrared spectroscopy (FTIR). Water diffusion profiles are observed in both olivine and coexisting pyroxene in the xenoliths. Water at the grain centers in coexisting olivine and pyroxene, seen as "plateaus" in diffusion profiles, is in equilibrium, evidenced by the ratios of water between olivine and pyroxene are similar to experimentally determined partition coefficients. This suggests that the diffusive loss of water in coexisting olivine and pyroxene could have occurred during magma emplacement. Water diffusivities in our olivine and pyroxene are similar, and match experimental constraints, in contrast to previous studies of xenoliths showing that water diffuses much faster in olivine than in coexisting pyroxene. Combining with literature experimental diffusivity data, we propose that temperature is the main factor that causes this discrepancy. Water diffuses at similar rates in olivine and pyroxene at relative low temperatures (lower than 920 °C), while water diffusion in mantle xenoliths from previous studies may have occurred at high temperatures (≥ 1200 °C).