Aims and background
Source: IGCP 649   Publish Time: 2015-04-28 19:25   3712 Views   Size:  16px  14px  12px

 Ophiolites are 5- to 10-km-thick fragments of ancient oceanic lithosphere emplaced on continental margins, and provide 3-dimensional exposures allowingexamination of the internal structure, chemical makeup and the processes of formation of oceanic rocks(Dilek and Furnes 2014). They also hostore depositsof valuable commodities, such as chromites, gold and platinum group minerals, and copper. Conventional models interpret ophiolites as manifestations ofpartial melting of the upper mantle at shallow depths (60-80 km). Podiformchromitites in ophioltesare regarded asmagmatic deposits formed due tomelt-rock reactions beneath seafloor spreading centers (Zhou et al. 1994; Arai 1997). However, our recent discoveries of in-situ diamonds and other ultrahigh pressure (UHP) minerals, highly reduced phases, native elements, andcrustal minerals in ophioliticperidotites and chromitites of southern Tibet, northern Russia and Burma-Myanmar, suggest that these bodies form at mantle depths of 150-300 km or perhaps deeper, near the mantle transition zone (Bai et al. 2003; Dobrzhinetskaya et al. 2009; Trumbull et al. 2009; Robinson et al. 2014; Yang et al. 2014a, b). These ophiolite-hosted diamondsare distinctly different in their morphology, carbon isotopes and mineral inclusions from diamonds occurring in kimberlites and UHP metamorphic belts, indicating a new environment for diamond formation and carbon reservoir in the deep mantle.

The widely accepted models for the formation of podiformchromititespropose their precipitation from melt-rock reactions between tholeiitic to boninitic magmas andtheir host peridotites, which commonly consist of harzburgitesdepleted in mantle wedges at 60-80 km at depth. These models infer that during melt-rock reactions both clinopyroxene and orthopyroxeneare removed, leaving behind a residuum of dunite, enveloping the chromitite bodies (Zhou et al. 1994; Arai 1997; Dilek and Morishita, 2009). Although these models explainsome of the field observations and geochemical features in peridotites and associated chromitites, they do not provide a plausible explanation for the existence in these ultramafic rock suites of diamonds and other UHP minerals that clearly originated near the Transition Zone in the mantle. Thus, an outstanding problem in ophiolite and oceanic lithosphere petrogenesis is how to reconcile the apparently conflicting partial-melting and crystallization histories of peridotites and chromitites recorded in them.In this project, we aim to resolve this conundrum by integrating field-based petrological, geochemical and geochronological analyses of a wide-range of peridotite rocks and chromitites from world ophiolites in different orogenic belts.

We have separateddiamond and moissanite crystals, as well as many highly reduced phases from peridotites and chromotites of manyophiolites in the Yarlung-Zangbo suture zone of Tibet, the Ray-Izophiolite of Polar Russia, and the Naga ophiolite of Myanmar(Yang et al. 2007,2010; Xu et al.2014).  Some of this material includes carbides such as SiC, WC, and alloys such as Cr-Fe, Si-Al-Fe, Ni-Cu, Ag-Au, Ag-Sn, Fe-Si, Fe-P, Ag-Zn-Sn, PGE and wüstite  (Robinson et al. 2004; Yang et al. 2007; Yang et al. 2008). Mineral separates also contain minerals such as kyanite, zircon, corundum, almandine garnet, K-feldspar and rutile, which are commonly found in the continental crust (Robinson et al. 2014). Many of the highly reduced mineral phases suggest their derivation from deep mantle sources, whereas the crustal mineralsindicate their recycling into the deep mantle from the surface via subduction.

We have analysed the carbon isotopes of in-situ and separated diamonds from chromitites and peridotites of the Tibetan and Russian ophiolitesby SIMS (secondary ion mass spectrometry) in the GeoForschungsZentrum (Germany) and the University of Western Australia. The δ13CPDBvaluesrange from -18 to -28 (Trumbull et al. 2009), which are much lighter than those of diamonds from eitherkimberlites(-4 to -8) or UHP metamorphic belts (-5 to -18).  The source of this light carbon is unknown, but the most likely candidates are either organic carbon subducted from the surface or fractionation of the main mantle reservoir, which has an average δ13CPDB value of about -5‰ (Deines 2002). During dissociation of CH4 in the deep mantle to C, CO2 and H2O, the escape of isotopically heavy CO2 could produce fluids with lower δ13C (Maruoka et al. 2004; Cartigny 2005).Whatever the source, the widespread occurrence of minerals with these values suggests the existence of some type of separate carbon reservoir in the deep mantle, and its recycling through a combination of complex Earth processes. A crucial goal of this project is to determine how widespread this type of carbon is in the deep mantle and how it becomes incorporated into ophioliticperidotites and chromitites. This aspect of the project requires systematic fieldwork and sampling in a large number of ophiolites with ages ranging from Precambrian to Cenozoic and tectonic settings of igneous construction ranging from mid-ocean ridges (Zabargad Island, Egypt, ~21 Ma) to forearc and incipient island arc (WadiGadhir, Egypt, ~700 Ma; Mirditaophiolite, Albania, 165 Ma; Tethyanophiolites in Turkey, 170-95 Ma; Oman ophiolite, 95 Ma) and backarc (Central Tibetan ophiolites, 130 Ma; Polar Ural ophiolites 340 Ma) environments. If we find that diamonds and other UHP minerals are common in ophiolites of different ages and tectonic settings, we can show that these unusual mineral assemblages occur extensively throughout the mantle and contribute to mantle heterogeneity at all scales. This would implythe existence of an enormous carbon reservoir in the deep mantle, and suggest an efficient system of recycling and exchange between the crustal and mantle layers of the Earth via subduction, asthenospheric upwelling and updraft mechanisms.

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