Water contents in mantle xenoliths from the Colorado Plateau and vicinity: Implications for the mantle rheology and hydration‐induced thinning of continental lithosphere
Water contents in mantle xenoliths from the Colorado Plateau and vicinity: Implications for the mantle rheology and hydration‐induced thinning of continental lithosphere
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DOI：
10.1029/2007jb005540
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发表时间：
2007-12
影响因子：
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通讯作者：
Z. Li;Cin-Ty A. Lee;A. Peslier;A. Lenardic;S. Mackwell
中科院分区：
文献类型：
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作者：
Z. Li;Cin-Ty A. Lee;A. Peslier;A. Lenardic;S. Mackwell
b[1]采用傅里叶变换红外光谱法对北美西部科罗拉多高原和南部盆地及山脉的橄榄岩捕虏体中名义上无水矿物（橄榄石、斜辉石、正辉石）的含水量进行了系统分析。测量的含水量范围为橄榄石的2至45 ppm，正辉石的53至402 ppm，斜辉石的171至957 ppm。科罗拉多高原的含水量最高（橄榄石中高达45 ppm H2O，正辉石中为402 ppm H2O，斜辉石中为957 ppm H2O），而南部盆地和山脉的圣卡洛斯含水量最低（橄榄石中高达4 ppm H2O，正辉石中为82 ppm H2O，斜辉石中为178 ppm H2O）。除了圣卡洛斯，其他地区的橄榄石和辉石（迪什山、大峡谷和纳瓦霍）的含水量接近或高于软流圈地幔的含水量。我们认为，这里测量到的高含水量是在新生代早期北美法拉龙板块的俯冲作用下，通过再水合作用引入岩石圈地幔底部的。将更新的湿橄榄石位错蠕变流动规律应用于科罗拉多高原下岩石圈地幔条件，预测在给定背景剪切应力下，仅水化作用就能导致岩石圈底部有效粘度下降约1个数量级。如果仅用粘度来区分岩石圈和下面的软流圈，这表明水合作用可能导致岩石圈变薄超过10公里。当考虑到较厚的岩石圈（如太古宙克拉通）和较大的含水量（达到水饱和条件）时，预测黏度降低和岩石圈变薄的程度甚至更大（可达~ 100公里）。如果我们的解释是正确的，那么我们的研究的意义就超出了北美西部，并暗示了一种将大陆地幔（包括克拉通地幔）再循环回对流地幔的可能方法。
[1] Nominally anhydrous minerals (e.g., olivine, clinopyroxene, and orthopyroxene) in peridotite xenoliths collected from the Colorado Plateau and southern Basin and Range in western North America were systematically analyzed by Fourier transform infrared spectroscopy for water contents. Measured water contents range from 2 to 45 ppm for olivine, from 53 to 402 ppm for orthopyroxene, and from 171 to 957 ppm for clinopyroxene. The Colorado Plateau has the highest water contents (up to 45 ppm H2O in olivine, 402 ppm H2O in orthopyroxene, and 957 ppm H2O in clinopyroxene), while San Carlos in the southern Basin and Range has the lowest water contents (up to 4 ppm H2O in olivine, 82 ppm H2O in orthopyroxene, and 178 ppm H2O in clinopyroxene). With the exception of San Carlos, the olivine and pyroxenes from all other localities (Dish Hill, Grand Canyon, and Navajo) have water contents close to or higher than that inferred for the fertile asthenospheric mantle. We interpret the high water contents measured here to have been introduced into the base of the lithospheric mantle by rehydration associated with the subduction of the Farallon plate beneath North America during the early Cenozoic. Application of an updated flow law for dislocation creep of wet olivine to lithospheric mantle conditions beneath the Colorado Plateau predicts that for a given background shear stress, hydration alone can result in approximately 1 order of magnitude drop in the effective viscosity at the base of the lithosphere. If viscosity alone is used to distinguish the lithosphere from underlying asthenosphere, this suggests that hydration could have resulted in more than 10 km of lithospheric thinning. Viscosity reduction and lithospheric thinning of even larger extents (up to ∼100 km) are predicted when thicker lithosphere (such as Archean cratons) and larger water contents (up to water-saturated conditions) are considered. If our interpretations are correct, the implications of our study go beyond western North America and hint at a possible way of recycling continental mantle, including cratonic mantle, back into the convecting mantle.