Experimental determination of lithium partitioning between plagioclase and hydrous rhyolitic melt

Abstract

Lithium concentrations and partition coefficients were determined in plagioclase and Li-doped (300, 500, and 1500 ppm) H₂O-saturated rhyolitic melts experimentally produced at temperature of 800–875 °C and pressure of 50–150 MPa.

High bulk Li concentration of 1500 ppm have remarkable effects on the crystallization of rhyolitic melts, comparatively to lower concentrations: i) Ca-rich clinopyroxene crystallizes at the expense of plagioclase and Ca-poor clinopyroxene, ii) plagioclase anorthite content decreases, and iii) melt H₂O solubility increases significantly, resulting in lower crystallinities.

The Li content of plagioclase (70–900 ppm) and rhyolitic melt (300–1700 ppm) both increase with bulk Li content. The Li content in plagioclase increases with decreasing plagioclase anorthite content. The aqueous phase contains a negligible fraction of Li.

The plagioclase-rhyolitic melt partition coefficients (K_Li^plag/rhy) range from 0.2 to 0.6 and are mostly constant with bulk Li concentration. At 875 °C, the K_Li^plag/rhy increase from ∼0.3 to 0.6 with pressure decreasing from 150 to 50 MPa, while constant (0.2–0.4) with pressure at 800 °C. At 50–100 MPa, the K_Li^plag/rhy increase from 0.3 to 0.4 at 800 °C to 0.5–0.6 at 875 °C, while constant (0.3–0.4) with temperature at 150 MPa.

These results give information to interpret Li gradients in natural plagioclases with regard to processes such as magma degassing, decompression, and cooling. In particular, opposite Li gradients are expected in plagioclase during decompression-induced degassing (Li outward-gradients) and isobaric cooling (Li inward-gradients).