The Evolution of the Halogen Budget in Ophiolites from the Western Alps

Abstract:

During subduction, halogens are liberated from the down-going slab via dehydration reactions. The addition of halogens to aqueous fluids alters fluid properties and element mobility. The degree to which halogens are devolatilized and the conditions at which such reactions take place are a topic of active debate. This study investigates halogen abundances and ratios within a suite of petrogenetically-related, exhumed subduction zone rocks from the Western Alps. This sample suite includes metabasalts and metagabbros from the Chenaillet, Queyras, and Monviso meta-ophiolites, which represent different peak metamorphic conditions from sub-greenschist, blueschist, and eclogite facies. Bulk F and Cl concentrations are measured by ion chromatography. Bulk Br and I are measured using ICP-MS.

Preliminary data reveal relatively consistent F concentrations between the lowest- (80-160 μg/g) and highest-grade (30-170 μg/g) samples, suggesting moderate F retention within mafic rocks during prograde metamorphism. This inference is in contrast to the observed behavior of Cl, Br, and I, which display progressively lower concentrations with increasing metamorphic grade. There is a large reduction in bulk heavy halogen (Cl, Br, I) concentrations between sub-greenschist to blueschist facies samples, and with the formation of eclogite. Comparing blueschist facies to eclogite facies samples, average abundances of Cl, Br, and I decrease by ~60%, 20%, and 50%, respectively. The F/Cl and Br/Cl ratios of Monviso eclogites overlap with the values reported for OIBs [1, 2]. The I/Cl ratio does not resemble values reported for OIBs [2], arc volcanics [3], or MORBs [4].

Results of this study point to continuous, rather than episodic, devolatilization of halogens from subducting ocean crust. This observation supports the view that the halogen budget is not controlled by any single phase, instead, halogens are likely distributed among various phases or within fluid inclusions, grain boundaries, or structural defects.

[1] Hauri, Chem Geol, 2002. [2] Kendrick et al., EPSL, 2015. [3] John et al., EPSL, 2011. [4] Kendrick et al., GCA, 2012.