Comparing Residual Mantle Peridotites from Ophiolites and Mid-Ocean Ridges

Abstract:

In comparing mid-ocean ridge peridotites (MORP) and ophiolite peridotites (OPHP), sampling bias must be considered. Most (all?) MORP are exposed along fracture zones and detachment faults. Sampling via dredging and even drilling is relatively random. Conversely, most OPHP samples are taken by geologists (who oversample “peculiar” lithologies) with hammers, from outcrops exposed by erosion, often deep within intact massifs far from the paleo-seafloor.

Tectonic exhumation of MORP from (?) 10-20 km to the seafloor probably results in rapid cooling from 1000 to 0°C. OPHP probably cool more slowly in this interval, as reflected in systematic offset of spinels to lower Mg/(Mg+Fe) or Mg#, at a given Cr/(Cr+Al) or Cr# in OPHP compared to MORP (Hanghoj et al JPet 2010). Both suites record rapid cooling from 1350 to 1000°C (Dygert & Liang EPSL 2015), with cooling rate in Oman decreasing with increasing depth below the crust-mantle transition zone (Hanghøj et al. JPet 2010; Dygert et al. this meeting 59980 in Session V010). Rapid high-T cooling in the different MORP and OPHP settings suggests heat extraction via hydrothermal convection to depths of 7 km or more, rather than tectonic exhumation.

Most OPHP massifs contain some high Cr# spinels, compared to max Cr# ca 65% in MORP, often attributed to high degrees of melting of OPHP, perhaps due to aqueous fluid-fluxing in a subducting setting. However, leaving aside ultra-depleted New Caledonia and Papuan ophiolites, this fails to account for overlapping spinel Cr#, olivine Mg#, and pyroxene content in MORP and OPHP. The wide range of Cr# and pyroxene content over short distances in both MORP and OPHP suites reflects open system processes and/or subsolidus re-equilibration with pyroxene and alteration phases. The high Cr# suite locally present in most OPHP probably results from late-stage reaction with magmas saturated in Cr-rich spinel, absent or rare in ridge settings but observed in western Pacific arcs (Arai & co-workers; Suhr G-cubed 2003). This, together with higher Ca/(Ca+Na) in plagioclase and early crystallization of Fe-Ti oxides leading to SiO2-enrichment in lava suites, suggests a subduction setting for most ophiolites. However, few ophiolites are overlain by ash layers from subaerial volcanoes, and most were emplaced soon after magmatism. Subduction = obduction?