Diffusion of Water through Olivine and Clinopyroxene: Implications for Melt Inclusion Fidelity
Abstract
The maximum H2O concentrations measured in olivine-hosted melt inclusions (MIs) from arc tephra fall within a narrow range of 3-5 wt%. A major question is whether this reflects parental water concentrations or diffusive exchange through the host crystal during storage and ascent. Laboratory experiments have shown that water can diffuse through 500 micron olivine in minutes to days at 1100°C. We have tested these predictions with a natural experiment using volatile (H2O, CO2, S) diffusion along melt embayments to constrain ascent rates during the 1974 eruption of Volcan Fuego to 5-8 minutes from 7 km depth [1]. Thus, olivine-hosted MIs may move from their storage region to the surface during some eruptions rapidly enough to retain almost all of their original water. Only the smallest MIs (< 30 microns) will lose any water during such fast ascent, even for the fastest diffusion mechanism through olivine. We have also assessed the potential for clinopyroxene (cpx) to retain water (as H+) during magma ascent. In the same 1974 Fuego deposits, cpx crystals show H-loss on their rims and even from their interiors. Such diffusive loss in 5-8 minutes requires rapid diffusion of H in cpx, comparable to olivine and melt, and consistent with our recent laboratory experiments dehydrating Fe-bearing cpx [2]. Although H-diffusion is dependent on the site occupancy, all sites may lose H rapidly in cpx with Mg# < 92.5. While cpx and olivine may lose H during ascent and degassing, olivine-hosted MIs stand a better chance of retaining water due to the very low partitioning of water in olivine (D 0.001). The most favorable conditions for faithful retention of parental water concentrations involve a) rapid ascent (< hr.) from H2O-undersaturated reservoirs (prior to major water degassing), b) minerals with low partition coefficients for water, c) large crystals (>500 microns) and large melt inclusions (>50 microns), and 4) rapid post-eruptive cooling (< 1min, clast sizes < 1 cm). The rapid diffusion of H through olivine and cpx presents a challenge to MI fidelity, but not necessarily if the above conditions are met. [1] Lloyd et al., 2014, JVGR. [2] Ferriss et al., 2016, AmMin.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.V34C..08P
- Keywords:
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- 3612 Reactions and phase equilibria;
- MINERALOGY AND PETROLOGYDE: 3618 Magma chamber processes;
- MINERALOGY AND PETROLOGYDE: 8408 Volcano/climate interactions;
- VOLCANOLOGYDE: 8430 Volcanic gases;
- VOLCANOLOGY