The Fidelity of Xenoliths in Recording Mantle Water Concentrations

Abstract:

Mantle xenoliths bear the nominally anhydrous minerals olivine and pyroxene which may contain significant quantities of water and potentially provide information on the water contents (and therefore solidus and strength) of the mantle. Very high diffusivities of water (as H) in olivine and pyroxenes, however, will promote rapid equilibration of water between xenoliths and their host magma. If magma ascends from the mantle (> 40 km) on the order of 10 m/s (an upper bound), minimum timescales available for interaction between melt and xenoliths are on the order of hours. Assuming the fastest diffusivities measured in olivine and clinopyroxene [1,2], water could exchange on the mm to cm lengthscales that are relevant to xenoliths during ascent. In this case, xenoliths would no longer convey reliable information about the water concentration of the upper mantle. However, multiple diffusive mechanisms may operate in olivine [3] and clinopyroxene [4]; some of these mechanisms are orders of magnitude slower than others, and may prevent widespread diffusive exchange [3].

To explore the potential for water exchange between mantle xenoliths and their host magma, we report here data on xenolith-host magma pairs from two cinder cones in the Western US: MO1016 from the Mojave Desert, CA and GCB from the north rim of the Grand Canyon, AZ). The host magma at MO1016 is relatively dry, while the magma at GCB is relatively wet, with 1.3 vs. 3.2 wt% H₂O as measured by SIMS in olivine-hosted melt inclusions. The water concentration of mantle xenolith clinopyroxenes (cpx) in MO1016 also contain lower H₂O (< 225 ppm, by SIMS) than those in GCB (< 560 ppm, from [5]). If we apply an Al-dependent partition coefficient [6] to the cpx water concentrations, the predicted melt concentrations are nearly identical to those recorded in the host magma melt inclusions. This relationship is suggestive of complete diffusive exchange of xenolith cpx with the host magma. Future work will explore grain size, lengthscale and FTIR spectra variations to test for diffusive relationships and mechanisms.

[1] Kohlstedt & Mackwell, 1998; [2] Woods, et al., 2000; [3] Padron-Navarta et al., 2014; [4] Ferriss et al., 2014; [5] Li et al., 2008.; [6] Tenner et al., 2009.