High-pressure highly reduced nitrides and oxides from chromitite of a Tibetan ophiolite

Abstract

The deepest rocks known from within Earth are fragments of normal mantle (Ϸ400 km) and metamorphosed sediments (Ϸ350 km), both found exhumed in continental collision terranes. Here, we report fragments of a highly reduced deep mantle environment from at least 300 km, perhaps very much more, extracted from chromite of a Tibetan ophiolite. The sample consists, in part, of diamond, coesite-after-stishovite, the high-pressure form of TiO 2, native iron, high-pressure nitrides with a deep mantle isotopic signature, and associated SiC. This appears to be a natural example of the recently discovered disproportionation of Fe 2؉ at very high pressure and consequent low oxygen fugacity (fO2) in deep Earth. Encapsulation within chromitite enclosed within upwelling solid mantle rock appears to be the only vehicle capable of transporting these phases and preserving their low-fO 2 environment at the very high temperatures of oceanic spreading centers.boron nitride ͉ coesite after stishovite ͉ Luobasa chromitite ͉ TiO2 II ͉ titanium nitride-osbornite U ntil recently, the deepest rock samples recovered from Earth's interior were xenoliths carried to the surface in explosive eruptions of kimberlite and related rocks, with maximum depth of Ϸ300 km (1). However, the advent of microstructural analysis of rocks from continental collision zones has established exhumation from still greater depths [peridotites from Ͼ300-400 km (2-5) and metamorphosed sediments from Ϸ350 km (6)]. More recently, a small fraction of diamonds from kimberlitic rocks has been found to carry inclusions that strongly suggest even greater depths, perhaps 1,700 km (7), but rocks that previously incorporated those diamonds at depth have not been recognized. We report here discovery of unexpected mineral assemblages from an ocean-spreading center environment, representing another window into Earth's deep interior.The discovery consists of very high-pressure and highly reduced phases extracted from chromitite within the mantle portion of an only lightly altered (e.g., small amounts of serpentine) fossil cross-section of oceanic crust and uppermost mantle (an ophiolite) in Tibet. Initial description of this material (8) documented diamonds and coesite-after-stishovite, establishing a minimum depth of origin of Ϸ300 km. This surprising and curious discovery was strengthened by observation of coesite and diopside lamellae within chromite collected from the same outcrop (9) and, very importantly, discovery of diamonds and indicators of very low oxygen fugacity from similar chromitite in a second ophiolite in the Polar Ural Mountains (10), establishing that the Tibetan occurrence is not unique. This discovery strongly enhances the conclusion of ref. 8 that this occurrence of very high-pressure minerals is not the result of meteorite impact. Thus, a subset of ophiolites (currently of unknown abundance) contains high-pressure and reduced phases, despite the overwhelming evidence that the ophiolites themselves formed at oceanic spreading centers under oxidizing conditions...

Iron Carbide Inclusions in Lower-Mantle Diamond From Juina, Brazil

“…However, nitrides (osbornite TiN and BN) are known in chromitites from the Luobasa, Tibet ophiolites, where they associate with diamond (Dobrzhinetskaya et al, 2009). …”

High-pressure polymorph of TiO2-II from the Xiuyan crater of China

“…The formation of natural TiO 2 -II could be related to highpressure endogenic geological processes or bolide impact events. Occurrence of natural TiO 2 -II had been previously reported in ultrahigh-pressure metamorphic rocks [11,12], mantle-derived rocks [13], terrestrial impact structures [14][15][16], and tektite [17]. However, the mineralogical features and the formation conditions of natural TiO 2 -II still needs to be further investigated because of the very limited material that had been found.…”

Ultra-high pressure and ultra-reduced minerals in ophiolites may form by lightning strikes

“…Wavelength dispersive scans on the metal globules with the oxygen sensitive LDE-1 crystal show that the globules are oxide-free. To identify the phases we investigated focussed ion beam (FIB) sections (Wirth, 2009) from metal condensates with TEM (Fig. 3).…”

“…It is now being argued that mantle sections of ophiolites either originate in, or were processed within, the Earth's mantle at depths as great as 600 km Xu et al, 2015;Griffin et al, 2016). Furthermore, these UHP phases are often found associated with ultra-reduced minerals: native metals, silicides, carbides, and nitrides (Dobrzhinetskaya et al, 2009;Xu et al, 2015). Consequently, it is speculated that the mantle source regions of ophiolites must be ultra-reduced.…”