Opx–Cpx exsolution textures in lherzolites of the Cretaceous Purang Ophiolite (S. Tibet, China), and the deep mantle origin of Neotethyan abyssal peridotites
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Abstract:
We investigated lherzolitic peridotites in the Cretaceous Purang ophiolite along the Yarlung Zhangbo suture zone (YZSZ) in SW Tibet to constrain their mantle–melt evolution history. Coarse-grained Purang lherzolites contain orthopyroxene (Opx) and olivine (Ol) porphyroclasts with embayments filled by small olivine (Ol) neoblasts. Both clinopyroxene (Cpx) and Opx display exsolution textures represented by lamellae structures. Opx exsolution (Opx1) in clinopyroxene (Cpx1) is made of enstatite, whose compositions (Al2O3 = 3.85–4.90 wt%, CaO = <3.77 wt%, Cr2O3 = 0.85–3.82 wt%) are characteristic of abyssal peridotites. Host clinopyroxenes (Cpx1) have higher Mg#s and Na2O, with lower TiO2 and Al2O3 contents than Cpx2 exsolution lamellae in Opx, and show variable LREE patterns. Pyroxene compositions of the lherzolites indicate 10–15% partial melting of a fertile mantle protolith. P–T estimates (1.3–2.3 GPa, 745–1067°C) and the trace element chemistry of pyroxenes with exsolution textures suggest crystallization depths of ~75 km in the upper mantle, where the original pyroxenes became decomposed, forming exsolved structures. Further upwelling of lherzolites into shallow depths in the mantle resulted in crystal–plastic deformation of the exsolved pyroxenes. Combined with the occurrence of microdiamond and ultrahigh-pressure (UHP) mineral inclusions in chromites of the Purang peridotites, the pyroxene exsolution textures reported here confirm a multi-stage partial melting history of the Purang lherzolites and at least three discrete stages of P-T conditions in the course of their upwelling through the mantle during their intra-oceanic evolution.Keywords:
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We report the first natural occurrence of keatite, also known as silica K, discovered as a precipitate in the core of ultrahigh-pressure (UHP) clinopyroxene (Cpx) within garnet pyroxenite from the Kokchetav Massif, Kazakhstan. High-resolution transmission electron microscopy and electron diffraction demonstrate that sub-micrometer and nano-scale keatite precipitates have a definite crystallographic relationship with the host pyroxene (diopside = ~Di90) Cpx (100) || keatite (100) and Cpx (010) || keatite (001). Clinopyroxene provides a template for keatite nucleation due to the close structural relationship and excellent lattice match between the diopside and keatite. We propose that keatite micro-precipitates are formed in localized low-pressure micro-environments produced as a result of exsolution of extra silica and vacancies held within UHP host diopside and stabilized by the pyroxene lattice. Low-density metastable keatite and its relationship to the host pyroxene likely reflects the important influence of pyroxene/precipitate interfacial energy on the micro- and nano-scales in controlling the nature of exsolved phases in exhumed UHP minerals.
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Enstatite
Pyroxene
Solidus
Wollastonite
Magnesite
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Abstract Although pyroxene has been detected remotely across the Solar System, limited information is available from infrared remote sensing about the Mg‐Fe composition of pyroxene, and distinguishing between augite (20 < CaSiO 3 < 45) and diopside‐hedenbergite (CaSiO 3 > 45) remains challenging. The characteristics of pyroxene in the intermediate infrared range (4–8 μm), meanwhile, have not been documented. Using reflectance spectra of 72 samples ranging across the pyroxene quadrilateral, we investigate the effect of variations in Mg# (Mg/[Mg + Fe] × 100) and Ca‐content on the positions of strong and well‐defined spectral bands at ∼5.1 and ∼5.3 μm in high‐Ca pyroxene and ∼5.2 in low‐Ca pyroxene. We find that the 5.1, 5.2, and 5.3 μm bands move to shorter wavelengths as Mg# increases, whereas Ca‐content does not significantly affect the positions of these bands, enabling the determination of pyroxene Mg# directly from band positions alone. We also find that the ∼5.1 μm band is significantly more distinctive in diopside‐hedenbergite and the ∼5.3 μm band significantly more so in augite. Therefore, the 5.1, 5.2, and 5.3 μm spectral bands enable discrimination among diopside‐hedenbergite, low‐Ca pyroxene, and augite. Additionally, the 5.1, 5.2, and 5.3 μm bands enable direct determination of Mg# of diopside‐hedenbergite, low‐Ca pyroxene, and augite within ±23, ±10, and ±29 mol% Mg‐Fe, respectively.
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