Tectonic discrimination of peridotites using fO 2–Cr# and Ga–Ti–Fe III systematics in chrome–spinel

Abstract

The accessory mineral chrome–spinel, [(Mg,Fe) (Cr,Al,Fe) 2O 4], is well-established as a useful, alteration-resistant petrogenetic and tectonic setting indicator for mafic–ultramafic rocks. This study of unaltered spinels from 58 peridotites from oceanic and ophiolitic settings further develops the tectonic discrimination of peridotites from mid-ocean ridges (MOR) and supra-subduction zone (SSZ) settings. The geochemical data include the standard suite of elements, together with accurate ferric iron obtained by correcting electron microprobe data with Mössbauer spinel standards, and gallium (Ga) obtained by Laser Ablation-ICP-MS. The results show that discrimination using the established oxygen fugacity ( fO 2)–Cr# [Cr/(Cr + Al)] diagram remains effective, but does depend on melt–rock reaction and magmatic differentiation of the reacting melt, in addition to tectonic setting. In particular, the MOR–SSZ boundary for dunites is displaced to higher oxygen fugacities than that for residual harzburgites, with some partially reacted harzburgites proving hard to fingerprint. Ga–Ti–Fe 3# [Fe 3/(Fe 3 + Cr + Al)] systematics provide a potential way to improve the discrimination. Significantly, Ga and Fe 3+ have similar ionic radii but only Fe 3+ is redox-dependent, so Ga/Fe 3# is largely independent of magmatic differentiation while retaining the capability to separate MOR peridotites from the higher Fe 3# SSZ peridotites. Whereas Ga/Fe 3# ratios decrease during melt–rock reaction, Ti/Fe 3# ratios stay constant or increase according to the relative compositions of the mantle lithosphere and the interacting melt. In consequence, the Ti/Fe 3# vs. Ga/Fe 3# diagram requires only a single boundary to separate MOR harzburgites and dunites (with high ratios) from SSZ harzburgites and dunites (with low ratios). Podiform chromitites can only be fingerprinted if they share a similar history of olivine–spinel subsolidus re-equilibration as the peridotites. Unlike the oxygen fugacity plot, the Ti/Fe 3# vs. Ga/Fe 3# diagram can also fingerprint peridotites that have no co-existing olivine or host rock information, such as serpentinites and detrital spinels. Testing these methods using spinels in mantle peridotites from the northern Oman–U.A.E. ophiolite confirms independent indicators that the ophiolite records the switch in tectonic setting from MOR to SSZ and hence magma genesis during the initiation of subduction.