Extreme geochemical variability through the dunitic transition zone of the Oman ophiolite: Implications for melt/fluid-rock reactions at Moho level beneath oceanic spreading centers

Abstract

The Maqsad area in the Oman ophiolite exposes a >300 m thick dunitic mantle-crust transition zone (DTZ) that developed above a mantle diapir. The Maqsad DTZ is primarily f "p " w h scattered chromite and chromite seams) and " p g " which exhibit a significant lithological variability, including various kinds of clinopyroxene-, plagioclase-, orthopyroxene-, amphibole (hornblende/pargasite)bearing dunites. These minerals are interstitial between olivine grains and their variable abundance and distribution suggest that they crystallized from a percolating melt. Generally studied through in-situ mineral characterization, the whole rock composition of dunites is poorly documented. This study reports on whole rock and minerals major and trace element *Manuscript contents on 79 pure to variably impregnated dunites collected systematically along cross sections from the base to the top of the DTZ. In spite of its high degree of depletion, the olivine matrix is selectively enriched in the most incompatible trace elements such as LREE, HFSE, Th, U, Rb and Ba. These data support the view that this enrichment has been acquired early in the magmatic evolution of the DTZ, during the dunitization process itself. The dissolution of orthopyroxene from mantle harzburgites enhanced by the involvement of hydrothermal fluids produced low amounts of melts enriched in silica and in some trace elements that re-equilibrated with the olivine matrix. This pristine signature of the DTZ dunite was eventually variably altered by percolation of melts with a Mid-Ocean Ridge Basalt (MORB) affinity but displaying a wide spectrum of composition attributable to evolution by fractional crystallization and hybridization with the silica enriched, hydrated melts. The olivine matrix has been partially or fully re-equilibrated with these melts, smoothing the early strong concave-upward REE pattern in dunite. The chemical variability in the interstitial minerals bears witness of the percolation of MORB, issued from the mantle decompression melting, variably hybridized with melt batches produced within the DTZ by melt-rock reaction and poorly homogenized before reaching the lower crust. Our results lead to the conclusion that pure and impregnated dunites are end-members that recorded different stages of the same initial igneous processes: pure dunites are residues left after extraction of a percolating melt while impregnated dunites correspond to a stage frozen before complete melt extraction. Therefore dunites trace elements contents allow deciphering the multi-stage processes that led to their formation at the mantle-crust transition zone. Keywords Oman ophiolite; dunitic mantle-crust transition zone; trace elements; melt-rock reactions; melt percolation; refertilization 1995a; Kelemen et al., 1997b). In this way, the DTZ appears to be a reactive interface where melts are focused, transformed and potentially accumulated, then distributed beneath the midocean ridge. Crystallization associated to melt migration through the mantle-crust transition zone accou...

Multi-scale development of a stratiform chromite ore body at the base of the dunitic mantle-crust transition zone (Maqsad diapir, Oman ophiolite): The role of repeated melt and fluid influxes

“…7a-c). This is reminiscent of the trends observed globally all across the DTZ (Abily and Ceuleneer, 2013;Rospabé et al, 2018Rospabé et al, , 2019. Parallel trends are observed in the composition of silicate inclusions in chromite, as in the TiO 2 content of amphibole and alkalis of mica (Fig.…”

“…This supports the view that the stratiform chromitite formed during the same tectonic/igneous episode as the one that built the overlying crust, and that it preserved its primary structure. The lateral extent of the layers cannot be determined precisely due to erosion, but it does not exceed several hundred metres as revealed by our general survey of this area (Rospabé, 2018;Rospabé et al, 2018). The studied samples preserve delicate igneous textures and display no evidence for postcrystallization deformation (Fig.…”

“…incongruent dissolution of the mantle harzburgites' orthopyroxene), melt percolation associated with chemical re-equilibration with the surrounding olivine matrix, refertilization by variably evolved melt batches, compaction, syn-magmatic faulting, later low temperature alteration, etc. (Rospabé et al, 2018(Rospabé et al, , 2019.…”

“…recorded by the troctolite and ol-gabbro cumulates in the mantle section - Benoit et al, 1996;Ceuleneer et al, 1996;Python et al, 2003), and a hydrated component coming from above, probably seawaterderived. The volatile-, Si-, Na-and trace elements-rich hybrid lithospheric melt formed during or/and contributed to the harzburgite dunitization process (Rospabé et al, 2017(Rospabé et al, , 2018(Rospabé et al, , 2019.…”

“…Plagioclase is typically absent in the stratiform chromitite, although common as an interstitial phase between olivine grains in the surrounding DTZ dunites (e.g. Abily and Ceuleneer, 2013;Boudier and Nicolas, 1995;Koga et al, 2001;Rospabé et al, 2017Rospabé et al, , 2018. Among other minor phases, diopside (hybrid between magmatic and hydrothermal clinopyroxene, see Rospabé et al, 2017) is observed in the half lower part of the stratiform chromitite while amphibole and garnet, appearing together above the two highest magmatic breccias at the altitude 1050 m, are restricted to the upper part of the ore body (Fig.…”