Fluorine and chlorine behaviour during progressive dehydration melting: Consequences for granite geochemistry and metallogeny

Abstract

Dehydration melting of biotite is the main control on crustal differentiation in the mid to lower continental crust because this reaction produces the most voluminous and most mobile granitic melts. Biotite breaks down over a broad temperature interval, so the partitioning behaviour of elements between biotite and melt is likely to vary. It has been hypothesized that fluorine may stabilize biotite to higher melting temperatures because biotite is typically F-rich in ultra-high temperature (UHT) metamorphic rocks. If true, F would be an important influence on crustal differentiation because not only would it broaden the temperature range of melting but also elevated F concentration decreases melt viscosity. Furthermore, ligand partitioning between biotite and melt may be an important influence on the metallogeny of magmas. This study used electron microprobe analysis of biotite in rocks from the Ballachulish and Rogaland metamorphic aureoles to investigate the concentration of F and Cl in biotite heated to 600-1,000°C. Results show a broad increase in biotite F content (up to 5.04% F) with temperature until 850-920°C, beyond which F content decreases (<2.5% F). Chlorine concentrations in biotite are consistently lower (<1% Cl), and show a progressive decrease after the onset of partial melting. It was found that Mg content, and the processes that control Mg distribution, are most strongly correlated with F and Cl concentration in biotite. Calculations based on these results indicate that F-enriched biotite could be a significant source of F for continental crust-derived melts. Generation of a hot, F-rich melt at UHT conditions could be important for transporting lower crustal metals to the upper crust.

Ultrahigh‐temperature osumilite gneisses in southern Madagascar record combined heat advection and high rates of radiogenic heat production in a long‐lived high‐T orogen

“…Fluorine can increase the stability of biotite to higher temperature (Dooley & Patiño Douce, ; Finch & Tomkins, ; Motoyoshi & Hensen, ), but is not included in the solution model of White, Powell, Holland, et al. ().…”

“…Fluorine can increase the stability of biotite to higher temperature (Dooley & Patiño Douce, 1996;Finch & Tomkins, 2017;Motoyoshi & Hensen, 2001), but is not included in the solution model of White, Powell, Holland, et al (2014). In all modelled samples, biotite occurs as coronae around other mafic phases or as xenoblastic grains in the matrix and is interpreted to be exclusively retrograde in these samples; the P-T interpretations of this study do not rely on reactions involving biotite.…”

P–T–t conditions of Early Palaeozoic low‐P high‐T granulite facies metamorphism in the southern Truong Son Belt, Central Vietnam

“…However, calculated y (opx) (0.13–0.15) isopleths match the peak assemblage without biotite. It is possible that biotite could be stable at higher temperatures with the addition of fluorine (Bose et al, 2005; Brigatti and Guggenheim, 2002; Finch & Tomkins, 2017). Although the sample has been extensively overprinted by fine‐grained mineral assemblages, the modal proportions of coarse‐grained garnet and orthopyroxene seem not to be affected by mylonitization.…”

The Role of Fluids in Melting the Continental Crust and Generating Granitoids: An Overview

“…However, Cl-bearing fluids might have a negative influence on the release of trace elements into melt. On account of Cl-OH exchange, Cl-bearing fluids usually improve the stability of hydrous minerals (e.g., biotite and muscovite) by way of stronger Fe-Cl bonds [109]. To a certain extent, the stabilities of these minerals will hinder the incorporation of some rare metal elements into melt to a certain extent, as micas are important enrichment carriers of many rare metal elements in the crust.…”