加拿大Slave克拉通橄榄岩的平衡温压计算

Calculating Equilibrium Temperatures and Pressures of Peridotite Xenoliths from the Slave Craton in Canada

加拿大Slave克拉通Jericho金伯利岩筒携带的橄榄岩包体提供了研究大陆岩石圈地幔物质组成和热结构的窗口。文章总结了地幔岩矿物温压计的研究进展,测量了Jericho金伯利岩携带的9个新鲜橄榄岩包体的矿物主量元素和微量元素,并使用不同的矿物温压计估算了平衡温度和压力。结果表明Nickel和Green(1985)的石榴子石—斜方辉石压力计可以较好地估算含石榴子石橄榄岩形成时的压力,Taylor(1998)二辉石温度计和Nimis和Taylor(2000)单斜辉石温度计的计算结果一致。具有粗粒变晶结构的尖晶石—石榴子石橄榄岩和石榴子石橄榄岩样品的平衡温度为575~843℃,压力为2.4~3.6 GPa,表明Slave克拉通岩石圈地幔温度较低。而残斑结构尖晶石—石榴子石二辉橄榄岩的平衡温度1109℃,压力为5.0 GPa,来源深度为~156 km,可能被早期金伯利岩浆携带到岩石圈地幔中部冷却,然后再被侏罗纪喷发的Jericho金伯利岩筒带到地表。使用石榴子石—单斜辉石稀土元素温压计获得的平衡温度高于主量元素温度计的结果,表明Slave克拉通岩石圈地幔经历了逐渐冷却的过程。此外,Slave克拉通浅部的尖晶石橄榄岩保留了强烈亏损的早期岩石圈地幔特征,而下部的岩石圈地幔经历了金伯利岩熔体和硅酸盐熔体的交代作用。

Peridotite xenoliths from the Jericho kimberlite pipe in the Slave Craton in Canada provides a window to investigate composition and thermal structure of the subcontinental lithospheric mantle.Here we summarized the recent progress in thermometers and barometers of mantle rocks,then measured major and trace element concentrations of 9 fresh peridotite xenoliths from the Jericho kimberlite pipe,and calculated their equilibrium temperatures and pressures using different thermometers and barometers.The results indicate that the garnet-orthopyroxene barometer of Nickel and Green(1985)can yield reliable pressure estimates,and two-pyroxene thermometer of Taylor(1998)and clinopyroxene thermometer of Nimis and Taylor(2000)give consistent temperature estimates.Coarse-grained granoblastic spinel-garnet peridotites and garnet peridotites have equilibrium temperatures of 575-843℃and pressures of 2.4-3.6 GPa.In contrast,prophyroclastic spinel-garnet lherzolite records temperature of 1109℃and pressure of 5.0 GPa,and probably experienced an early exhumation event from~156 km to the middle lithospheric depth by kimberlite magmas and the final exhumation by the eruption of the Jericho kimberlite pipe in the Jurassic.The temperatures estimated from the REE-in-garnet-clinopyroxene are higher than the corresponding values from the major-element based thermometers,indicating a cooling history of the lithospheric mantle beneath the Slave Craton.In addition,spinel peridotites preserved the signature of the high-degree partial melting in the shallow mantle,whereas the lower part of the lithospheric mantle have been modified by metasomatism of kimberlite and silicate melts.