T237 Tick Tock in the Rock: Elucidating the Time Scales of Geologic Processes and Honoring the Contributions of Bruce Watson, 2018 Roebling Medalist

Wednesday, 7 November 2018
8:00 AM - 12:00 PM
Room 240-241

LI DIFFUSION IN PLAGIOCLASE: A GEOSPEEDOMETER FOR RAPID HEATING EVENTS

Paper number: T237-10
Program: Technical Sessions
Time: 10:15 AM – 10:30 AM

Authors

Abstract

Lithium is an exceptionally fast diffuser in silicate materials, making Li diffusion profiles observed in minerals and melts useful for quantifying short-lived heating phenomena. Plagioclase is a ubiquitous phase in both mafic and felsic magmatic systems and the Li-in-plagioclase diffusion chronometer has great potential for evaluating the timescales of eruptive processes. However, timing heating events using Li concentration gradients in natural plagioclase requires well-constrained, laboratory calibrated Li diffusion coefficients measured under a variety of pressures, temperatures and oxygen fugacities. We present the results of sealed Si tube experiments conducted at 1 atm, 500-950 °C and varying fO2 to measure Li diffusion in polished and oriented crystals of sanidine, albite, oligoclase and anorthite. Lithium concentration gradients in all experiments were characterized using laser ablation ICP-MS. Measured lithium diffusivities increase significantly as plagioclase anorthite content increases. In all plagioclase compositions, Li diffuses by both a “fast” and a “slow” mechanism, with fast-path diffusion coefficients typically an order of magnitude higher than the slow-path. It is likely that Li diffusion in plagioclase operates via jumps to interstitial (fast path) and vacant (slow path) sites, similar to Li diffusion in olivine and clinopyroxene [1-3]. Depending on what diffusion mechanism operates in nature, Li concentration profiles in magmatic plagioclase may record the final hours to days before eruption.

[1] Dohmen et al. (2010) Geochim Cosmochim Acta, 274-292 [2] Richter et al. (2017) Geochim Cosmochim Acta, 124-142 [3] Richter et al. (2014) Geochim Cosmochim Acta, 352-370

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-322019

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