Beschreibung
The Moho transition zone, also referred to as dunitic transition zone (DTZ) in the oceanic crust, is the horizon where ascending melt from mantle begins to crystallize to form the crust. Normally, DTZ is developed at the top of mantle diapirs and ranges in thickness from a few meters to more than 1 kilometer. In this proposal, the DTZ is envisaged for investigating the metal flux and melt fractionation in the mantle-to-crust-passage. We will assess abundances of gold and other highly siderophile elements (HSE: PGE+Re), chalcogen (S, Se and Te), and osmium isotope composition of sulfides in profiles across well-studied DTZ sections in Tethyan ophiolites. Knowledge of geochemical behavior of gold in these environments helps to understand crust-mantle differentiation. A comprehensive petrographic study will be combined with ultra-trace element composition measurements. Samples from accessible exposures in Iran and Oman and from soon available Oman drill hole MD1 will be used. Iran ophiolites and northern massif Oman ophiolites have geochemical characteristics of supra-subduction zone ophiolites, whereas southern massif Oman ophiolites exemplify MORB-like ophiolites (plume-ridge). Our objective is to integrate the trace element composition and osmium isotope data of the DTZ-hosted sulfides with comprehensive petrographic and geochemical data to provide new insights on gold transfer and sulfide segregation by melt-peridotite reaction in plume-ridge and supra-subduction zone settings. Re-Os dating of sulfides will be applied to resolve timing of metal flux in relationship to the evolution of these fossil oceanic spreading ridges. Moreover, detailed examination of exposed Moho sections is anticipated to complement with the drill cores to reveal the 3-D variation of sulfide-bearing zones/rocks.
