Melt percolation monitored by Os isotopes and HSE abundances: a case study from the mantle section of the Troodos Ophiolite

摘要

Combined siderophile and lithophile element systematics in mantle rocks can be used to monitor melt percolation processes in the Earth's mantle. Here we present a coherent dataset from a single melt channel from the mantle section of the Troodos Ophiolite Complex on Cyprus. The melt channel is composed of a dunite vein that is surrounded by harzburgite. Dunite and harzburgite both have refractory Cr-spinel (Cr/Cr + AlC of 0.58-0.60). Likewise, clinopyroxeness in both the dunites and harzburgites have strongly depleted REE patterns with (Gd/Yb)_N values varying from 0.03 to 0.07. Such consistent lithophile element patterns suggest that the harzburgite and dunite interacted with the same melt during the melt percolation process. The distribution of the high siderophile elements (HSEs) (Os, Ir, RU, Pt, Pd and Re) in the melt channel cannot be explained by conventional partial melting models, but can be explained by melt-peridotite reaction. THe harzburgites have slightly suprachondritic Os isotope ratios (~(187)Os/~(188)Os_(t=0 Ma) = 0.1288-0.1311) compared to the ~(187)Os/~(188)Os_(t=90 Ma) of the carbonaceous chondrite reference (0.1264), the HSE concentrations overlap with the range observed for lherzolites and harzburgites world-wide. In contrast, the dunites are significantly enriched in ~(187)Os(~(187)Os/~(188)Os_(t=90 Ma) = 0.1335-0.1374), like volcanic rocks from island arcs world-wide. HSE patterns in the dunites are also typical for mantle melts, in that they are enriched in Pd, Pt and Re relative to Ir, Os and Ru, which are lower than in the primitive mantle. Hence, the harzburgites and dunites have complementary HSE concentrations and ratios. In addition, HSE ratios such as Ir/Os, Re/Os, systematically increase from the hrzburgite towards the dunite (Ir/Os)_N:0.36-1.8; (Re/Os)_N: 0.14-9.5). This implies that Ir, Os and Ru behave incompatibly and become fractionated from each other during the melt percolation process. These features are interpreted to reflect the progressive reaction of a mantle melt with spinel-lherzolite to form harzburgite and eventually dunite. We suggest that an upper mantle peridotite was infiltrated by a radiogenic mantle melt typical for subduction-related volcanism. At low melt/rock a harzoburgite residue is left behind and its HSE distribution and the REE pattern of cpx can be explained by open-system melting if one amuses the HSEs to behave incompatibly. Continued melt percolation eventually produces dunites, and all mantle sulfides are removed from the peridotite. Thus, the sulfides and the HSE distribution in the dunites are not of residual origin but are dominated by sulfides that segregated from a sulfide-saturated melt with a radiogenic Os signature. The HSE variation in harzburgites and dunites from the melt channel can be interpreted as a mixing lien that has HSE-bearing sulfides from the melt and from the residual mantle as end members. We conclude that HSEs become significantly mobilized and fractioanted during melt percoaltion processes, thus providing useful proxies for melting and enrichment processes in the Earth's mantle.