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Ophiolitic peridotites represent variously depleted residues of the primitive mantle after multiple episodes of partial melting, melt extraction, and melt-rock interactions. They display a wide range of compositional and geochemical heterogeneities at different scales, and their incompatible bulk-rock compositions and mineral chemistries are commonly inconsistent with their evolution through simple partial melting processes at shallow mantle depths. Approaching these issues from different perspectives, the papers in this volume concentrate on (1) melt evolution and magmatic construction of ophiolites in various tectonic settings, and (2) the occurrence of microdiamonds, ultrahigh-pressure (UHP) minerals, and crustal material as inclusions in ophiolitic chromitites and peridotites. Crustal and mantle rock units exposed in different ophiolites show that the mantle melt sources of ophiolitic magmas undergo progressive melting, depletion, and enrichment events, constantly modifying the melt compositions and the mineralogical and chemical makeup of residual peridotites. Formation and incorporation of microdiamonds and UHP minerals into chromite grains occurs at depths of 350–660 km in highly reducing conditions of the mantle transition zone. Carbon for microdiamonds and crustal minerals are derived from subduction-driven recycling of surface material. Host peridotites with their UHP mineral and diamond inclusions are transported into shallow mantle depths by asthenospheric upwelling, associated with either slab rollback–induced channel flow or superplumes. Decompression melting of transported mantle rocks beneath oceanic spreading centers and their subsequent flux melting in mantle wedges result in late-stage formation of podiform chromitites during the upper mantle petrogenesis of ophiolites. Future studies should demonstrate whether diamonds and UHP minerals also occur in peridotites and chromitites of nonsubduction-related ophiolites. LITHOSPHERE; v. 10; no. 1; p. 3–13 | Published online 10 January 2018 https://doi.org/10.1130/L715.1 OPHIOLITES, DIAMONDS, AND ULTRAHIGH PRESSURE MINERALS Recent geochemical and geochronological data from many ophiolites in different orogenic belts have shown strong evidence for structural and compositional heterogeneity in their crustal and mantle units, and for multiple episodes of melt extraction, melt-rock reactions, and melt migration in their mantle evolution history (Batanova et al., 1998; Morishita et al., 2006; Aldanmaz et al., 2009; Caran et al., 2010; Jean et al., 2010; Dilek and Furnes, 2014; Piccardo et al., 2014; O’Driscoll et al., 2015). These observations are not consistent with a concept of a simple steady-state magmatic accretion at an oceanic spreading center. High silica contents and high ratios of light/heavy rare earth elements (LREE/HREE) in most ophiolitic peridotites are incompatible, for example, with their origin as the residuum of partial melting of primitive mantle. Peridotites that underwent high degrees of melting are generally strongly depleted in incompatible elements in contrast to those that underwent low degrees of melting. Systematic covariations of major and trace element distributions in ophiolitic peridotites are commonly interpreted as artifacts of partial melting processes (Walter, 2003). However, the incompatibilities between the bulk composition and mineral chemistry of peridotites in many ophiolites are inconsistent with simple crystal line of descent in concert with the liquid line of descent for a parental melt that evolved via fractional crystallization. These geochemical characteristics are the manifestations of metasomatic processes between the mantle lithosphere and ascending melts and fluids beneath spreading centers in various tectonic settings (Dilek and Thy, 1998; Parkinson and Pearce, 1998; Braun and Kelemen, 2002; Ohara et al., 2003; Drouin et al., 2009; Dick et al., 2010; Ulrich et al., 2010; Wanless and Shaw, 2012; Dilek and Furnes, 2011; Furnes et al., 2014). They may have resulted, for example, from postcumulus infiltration of melts, which reacted and reequilibrated with the host peridotites, causing refertilization of an already depleted mantle by crystallization of interstitial mineral phases (mainly plagioclase and clinopyroxene) (e.g., Dijkstra et al., 2001). Therefore, the observed mineralogy, textures, and compositions in the upper mantle peridotites of some ophiolites do not reflect their primary features. Likewise, similar melt-peridotite reactions may have affected the lower crustal accretion by producing ultramaficmafic intrusions (i.e., dunite, wehrlite, troctolite) in the layered gabbros (Shallo and Dilek, 2003; Lissenberg and Dick, 2008). Reaction of residual © 2018 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY-NC license. Yildirim Dilek http://orcid.org/0000-0003-2387-9575 THEMED ISSUE: Ophiolites, Diamonds, and UHP Minerals: New Discoveries and Concepts on Upper Mantle Petrogenesis Downloaded from https://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/doi/10.1130/L715.1/4032887/l715.pdf by guest on 30 May 2019