LI DIFFUSION IN PLAGIOCLASE: A GEOSPEEDOMETER FOR RAPID HEATING EVENTS

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