Diamonds and other unusual minerals from peridotites of the Myitkyina ophiolite, Myanmar
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Peridotite
Ophiolites are conventionally regarded as fragments of former oceanic lithosphere. Mineralogical and field evidence indicates that peridotite of the Neyriz ophiolite was intruded at high temperature into folded crystalline limestones, forming skarns. This excludes the formation of the ophiolite at a mid-ocean ridge but is consistent with its origin by intrusion during continental rifting.
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Obduction
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Abstract The proton microprobe has been used to determine contents of Ca, Ti, Ni, Mn and Zn in the olivine of 54 spinel lherzolite xenoliths from Australian and Chinese basalts. These data are compared with proton-probe data for Ni, Mn and Zn in the olivine of 180 garnet peridotite xenoliths from African and Siberian kimberlites. Fe, Mn, Ni and Zn contents are well-correlated; because the spinel lherzolite olivines have higher mean Fe contents than garnet peridotite olivines (average Fo 89.6 vs. Fo 90–92 ) they also have lower Ni and higher Mn contents. Zn and Fe are well-correlated in garnet peridotite olivine, but in spinel peridotites this relationship is perturbed by partitioning of Zn into spinel. None of these elements shows significant correlation with temperature. Consistent differences in trace-element contents of olivines in the two suites is interpreted as reflecting the greater degree of depletion of Archean garnet peridotites as compared to Phanerozoic spinel lherzolites. Ca and Ti contents of spinel-peridotite olivine are well correlated with one another, and with temperature as determined by several types of geothermometer. However, Ca contents are poorly correlated with pressure as determined by the Ca-in-olivine barometer of Köhler and Brey (1990). This reflects the strong T -dependence of this barometer: the uncertainty in pressure (calculated by this method) which is produced by the ±50°C uncertainty expected of any geothermometer is ca ± 8 kbar, corresponding to the entire width of the spinel-lherzolite field at 900–1200°C.
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Xenolith
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Peridotite
Platinum group
Chromitite
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The northeast Indian ophiolite belt is exposed in parts of Nagaland and Manipur states. We present the results of an investigation into the petrology and constituent mineral chemistry of peridotite from the ophiolite belt of Phek District, Nagaland. Based on relict primary mineral compositions, the studied rocks are identified as abyssal peridotite. Spinel composition shows that the rocks have undergone low degree of partial melting (5 11%). Equilibration of the ophiolite mantle sequence peridotite was calculated at a temperature range of 850–1165oC, pressure ranging between 19 to 25 kbar, and an oxygen fugacity between 0.437 to 0.657 log units above the FMQ buffer, whereas for the xenolith peridotite the temperature was estimated to be between 850 to 1100 oC, pressure ranging from 12 to 17 kbar and oxygen fugacity between 0.577 to 0.706 log units above the FMQ buffer. Equilibrium condition suggests that the rocks were equilibrated in an upper mantle environment.
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Mineral redox buffer
Fugacity
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Peridotite
Pyroxene
Brucite
Forsterite
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