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

Abstract

A stratiform chromite ore body crops out in the lower part of the dunitic mantle-crust transition zone (DTZ) that developed at the top of a mantle diapir in the Maqsad area in the Oman ophiolite. It is made of layers ranging in thickness from a few mm to a maximum of 3 m, and in modal composition from massive to antinodular and disseminated ore. The ore body is about 50 m thick and its lateral extent does not exceed several hundred meters. The layering dips gently to the southeast, parallel to that of the overlying gabbroic cumulates. The chromite composition is typical of a MORB kindred-moderate XCr (100 × Cr/(Cr + Al) atomic ratio), ranging from 48 to 60, and relatively high TiO 2 content, ranging from about 0.3 to 0.5 wt%-, a characteristic shared by most lithologies issued from the igneous activity of the Maqsad diapir. The silicate matrix is essentially made of slightly serpentinized olivine with minor clinopyroxene and rare pargasitic amphibole, orthopyroxene and garnet. This strongly contrasts with the nature of the mineral inclusions mostly made of the assemblage amphibole-orthopyroxene-mica, enclosed in the chromite grains and represented in abundance all along the ore body whatever the ore grade. The inclusions demonstrate the involvement of a silica-and water-rich melt and/or fluid, in addition to MORB, in the early stages of chromite crystallization. The chemical composition of chromite, silicate matrix, together with the one of silicate inclusions display well-defined evolutions vertically along the stratiform chromitite. At the scale of the ore body, the compositional trends are independent of the ore concentration but the major kinks in these trends are well-correlated with levels of magmatic breccias. This shows that abrupt chemical changes can be attributed to sudden melt +/-fluids injection events followed mainly by melt-fluid-rock interaction and in a lesser extent by quieter evolution by fractional crystallization. At the thin section scale, second order chemical variations, essentially in the Mg# (100 × Mg/(Mg + Fe 2+) atomic ratio) of chromite and Fo of olivine, are clearly attributable to re-equilibration between these two solid phases, possibly in the presence of an interstitial melt/fluid.

Hydrated Peridotite – Basaltic Melt Interaction Part I: Planetary Felsic Crust Formation at Shallow Depth

“…This latter scenario is of primary interest in early Earth history (and possibly also for Mars), because the ultramafic-mafic protocrust formed from the crystallization of a magma ocean was most likely hydrated, either by volatiles released by the magma ocean itself or through interactions with liquid water available on the planetary surface (Albarède and Blichert-Toft, 2007;Elkins-Tanton, 2012). A present-day analogue can be observed at the level of the mantle/petrologic Moho boundary in the oceanic lithosphere (at depths of 6 km), where interactions of basaltic magmas with peridotites produce chromitite-dunite associations (Kelemen et al, 1995;Arai, 1997;Borisova et al, 2012;Zagrtdenov et al, 2018;Rospabé et al, 2019) and serpentinized peridotite (hydrated residual peridotite) undergoes partial melting (Benoit et al, 1999). Furthermore, such shallow conditions of serpentinite melting in the presence of basaltic melt might also occur in mantle plumes (Bindeman, 2008;Reimink et al, 2014Reimink et al, , 2016Borisova et al, 2020a), in Hadean heat-pipe volcanoes (Moore and Webb, 2013) or Hadean proto-rift volcanoes (Capitanio et al, 2020) and/or during meteorite impacts (Marchi et al, 2014).…”

Hydrothermal Chromitites from the Oman Ophiolite: The Role of Water in Chromitite Genesis

“…Deep-seated rocks from the lower curst to the mantle are especially well exposed along Wadi Fizh [29], where we found the hydrothermal chromitites examined in this article (Figure 1). has been proposed [9][10][11][12][13][14][15]. Selective severe alteration (serpentinization or metamorphism) in and around chromitite [16,17] reminded early geologists of an essential role of aqueous fluids in formation of chromitite and associated dunite [18][19][20].…”

“…As for the podiform chromitite formation, an igneous origin during a harzburgite-magma reaction in the mantle has been proposed and appears widely accepted [4,8]. The essential role of aqueous solutions in the formation of podiform chromitites in the mantle and other chromitites has been the subject of controversy [4], and the essential involvement of water in chromitite formation has been proposed [9][10][11][12][13][14][15]. Selective severe alteration (serpentinization or metamorphism) in and around chromitite [16,17] reminded early geologists of an essential role of aqueous fluids in formation of chromitite and associated dunite [18][19][20].…”

High resolution X–ray computed tomography and scanning electron microscopy studies of multiphase solid inclusions in Oman podiform chromitite: implications for post–entrapment modification

“…4). MSIs in the banded chromitite samples from Oman ophiolite have also been described in detail by Rospabé et al (2019).…”

“…Multiphase solid inclusions in chromite (chromiumrich spinel) are common and have been reported from podiform chromitite (Schiano et al, 1997;Borisova et al, 2012;Akmaz et al, 2014;Rollinson and Adetunji, 2015;Khedr and Arai, 2016;Rollinson et al, 2018;Rospabé, et al, 2019), stratiform chromitite (Li et al, 2005;Spandler et al, 2005;Vukmanovic et al, 2013), abyssal peridotite (Matsukage and Arai, 1998;Tamura et al, 2014), and other environments. In those studies, pargasite, enstatite, and aspidolite (the Na analogue of phlogopite) are found to be the most common daughter minerals.…”