Petrogenesis, Sampling Depth, Thermal and Redox Evolution of Spinel Peridotite Xenoliths From Jiaohe, NE China: Insights From Trace Elements in All Rock-Forming Silicate Minerals

Olivine and orthopyroxene (Opx) are the most abundant minerals in lherzolitic to harzburgitic peridotites, and their compositions represent an important-and sometimes only-source of information for the origin and physicochemical evolution of the subcontinental lithospheric mantle (SCLM). We report a high-quality in situ trace element data set for Opx and olivine of well-characterized spinel-peridotite xenoliths, including eight lherzolites and four harzburgites, which were entrained in the Cenozoic basalts from Jiaohe, NE China. While the lherzolites and harzburgites show contrasting evolution involving various metasomatic agents, reflected in different trace element relationships, both experienced early cooling followed by recent (pre-entrainment) heating, as unraveled by a combination of mineral thermometers with inherently different diffusion rates. The sampling depth of spinel-facies peridotites is obtained by olivine-spinel thermobarometry, taking into account post-melting subsolidus processes and pre-emplacement thermal disturbance, using an empirical correction for the Ca content in olivine, which yields a pressure range of 1.1-2.0 GPa. The V contents (92-116 & mu;g/g) and V/Sc ratios (5.7-6.4) in lherzolitic Opx are higher than those in harzburgitic Opx (V: 56-76 & mu;g/g; V/Sc: 3.4-5.9), qualitatively reflecting the reducing and oxidizing effect, respectively of refertilization versus metasomatic interaction with a small-volume melt. Thus, comparative thermobarometry, together with trace element systematics of Opx and olivine can be effective tools to reveal the architecture, chemical, thermal, and redox evolution of the shallow SCLM, as sampled by spinel peridotite xenoliths. Peridotite xenoliths entrained in basalts are direct samples of the subcontinental lithospheric mantle, archiving a wealth of information on the genesis, structure, and evolution of this long-lived terrestrial reservoir. Orthopyroxene and olivine constitute the only rock-forming minerals in some mantle rocks, such as harzburgites and dunites from which petrogenetic information may be gained. However, until relatively recently, trace element abundances in these minerals have been under-utilized due to their low abundances and analytical challenges. Moreover, pressure (i.e., depth) estimates for spinel-facies peridotites have been hampered by the lack of accurate and precise geobarometers. In this study, we determined trace element abundances in orthopyroxene and olivine of well-characterized spinel lherzolite and harzburgite xenoliths from Jiaohe, NE China by in situ high-precision analyses. We demonstrate their utility to obtain the xenoliths' depth of origin, and to illuminate the chemical, thermal, and redox evolution of the shallow continental lithosphere, providing a new approach, perspective and impetus to comprehensively study this reservoir. Trace element compositions of all minerals are used to unravel the petrogenesis of peridotitesEmpirical correction for Ca in olivine applied for quantitative olivine-spinel thermobarometryUse of multiple thermometers reveals short-term perturbations superposed on long-term continental thermal trends globally