Transfer of melts in the sub-arc mantle: Insights from high-pressure experiments and from the New Caledonia ophiolite

Abstract

Listen

Translate article

High-pressure experiments were aimed to study the interaction between slab melts and the sub-arc mantle at depth of ~100km in a range of P-T condition which are representative of the mantle wedge above subducted slab. Experiments have shown that porous reactive flow induces the formation of micas and amphibole in the pressure range of 25 to 45 kbar for 800 to 1000 degrees. This leads to a strong depletion in LILE and LREE in the residual fluid. The wet solidus obtained in our experiments is 975 degrees at 35 kbar. The composition and the melting conditions of metasomatised peridotite highlight the importance of hydrous phases in the sub-arc mantle. Experimental reproduction of focused flow in the mantle wedge show that orthopyroxenite walls form an interface between slab melt and olivine. The trace element composition of the slab melt is only slightly modified in this transfer process. Therefore, the transfer of slab melt by focused flow to the locus of partial melting in the mantle wedge provides a much more efficient way to conserve the slab signature than porous flow. The New Caledonia ultramafic ophiolite provides further insights on the interactions which can exist between melts and the upper mantle. Here we provide the first recent petrological overview of the Massif du Sud ophiolite on the basis of petrological and geochemical data. This thesis show that this highly depleted ophiolite of 3000m thick mantle section is related to an arc environment associated with a spreading ridge origin. A suite of discordant rocks (dunite, pyroxenite, gabbro) presents in New Caledonia show the evolution of H2O-SiO2 rich melts transferred by focused flow. These melts are feeding the arc crust through a network of channels and dykes. Melt evolution at sub-Moho levels provide critical information in solving the arc paradox as large amount of pyroxenite rocks can be accumulated in the mantle prior to their incorporation in the crust. A late generation of pyroxenite and amphibole dykes has been connected to the emplacement of large felsic intrusions at the crust-mantle boundary. U-Pb dating on separated zircons from these intrusions and associated dykes provides Eocene ages confirming that these rocks are a pre-obductive feature of the Massif du Sud. These rocks represent a late fore-arc igneous activity and provide new information on melt-mantle interaction and the composition of melts in a colder part of the mantle wedge. The relative timing of events recorded in the New Caledonia ophiolite also imposes new constraints on the geodynamics of the South West Pacific from the middle Cretaceous to recent times.