Li, B and Be Contents of Harzburgites from the Dramala Complex (Pindos Ophiolite, Greece): Evidence for a MOR-type Mantle in a Supra-subduction Zone Environment

Abstract

The Pindos ophiolite represents oceanic lithosphere obducted during theJurassic.The Dramala mantle section mainly consists of highly depleted spinel harzburgite and minor plagioclase-bearing harzburgite. Textural observations and major element compositions of minerals indicate that the harzburgites experienced impregnation by a mafic, depleted melt and subsequent high-temperature (high-T) hydration and cooling (47508C) forming pargasite and edenitic hornblende. During further cooling (from 350^4008C to 51008C), talcþ tremolite serpentine olivine, serpentineþ magnetite, and finally plagioclase alteration phases formed.To test the hypothesis of a supra-subduction zone origin for the Dramala mantle, we measured Li, B and Be contents of minerals by secondary ion mass spectrometry. Whole-rock contents were measured using inductively coupled plasma^mass spectrometry and prompt gamma neutron activation analysis. We observe low Li and B contents of primary minerals (olivine, orthopyroxene, clinopyroxene) consistent with values for unmetasomatized mantle minerals; only Li contents of clinopyroxene (up to 3 7 g/g) are slightly elevated. The bulk Li contents (0 5^1 1 g/g) are in the upper range of values for unmetasomatized mantle, whereas B contents (50 04^1 1 g/g) are variable and slightly elevated compared with the unmetasomatized mantle as a result of serpentinization. Beryllium abundances in all minerals are very low (50 005 g/g), except for pargasite, where a maximum Be content of 0 012 g/g was measured. The selective addition of Li to clinopyroxene can be related to the interaction with a depleted melt, and/or to partitioning of Li into clinopyroxene upon cooling. During high-T hydration and cooling, the fluid calculated to be in equilibrium with the pargasite or edenitic hornblende (based on Li, Be and B) could have been reactionmodified seawater. Low-T hydration may have led to a very minor increase in bulk B content of most samples and to the formation of serpentine with highly variable B contents (0 1^28 g/g). Low-T hydration decreased the Li content of orthopyroxene, and Li was probably leached from some samples.The lack of correlation between degree of serpentinization and bulk B contents as well as the presence of highand low-B serpentine can be explained by low fluid^rock ratios, decreasingTduring serpentinization and lack of equilibrium as a result of fast obduction^exhumation. The low light-element contents of primary minerals and whole-rock samples clearly argue against a supra-subduction zone (SSZ) origin of the Dramala mantle section, and against the previous hypothesis of hydrous melting of the Pindos mantle above a subduction zone.We therefore conclude that the Dramala harzburgites represent a mid-ocean ridge (MOR)-type mantle, and not an SSZ-type mantle, juxtaposed with MOR-type and SSZ-type oceanic crust, either in a back-arc or in an intra-oceanic subduction zone setting.