On edge melting under the Colorado Plateau margin
On edge melting under the Colorado Plateau margin
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DOI：
10.1002/2016gc006349
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发表时间：
2016-07
期刊：
影响因子：
3.7
通讯作者：
Sean Rudzitis;M. Reid;J. Blichert‐Toft
中科院分区：
文献类型：
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作者：
Sean Rudzitis;M. Reid;J. Blichert‐Toft
在盆岭地区相对较薄的岩石圈之下的软流圈似乎以阶梯状的方式与科罗拉多高原厚的岩石圈龙骨并列。在美国西部亚利桑那州中北部的圣弗朗西斯科-摩曼山火山区，在这一边缘之上发现了原生至近原生玄武岩。我们发现，至少有两个不同的橄榄岩为主的地幔端元的玄武岩的起源作出了贡献。一个具有成对的Nd和Hf同位素特征，集群附近的地幔阵列和微量元素的模式，如预期的熔体中产生的软流圈，可能在石榴石的存在下。第二个具有在εHf-εNd地幔阵列上方移位的同位素组成，连同其特定的微量元素特征，表明来自氢源沉积物和/或熔体（碳酸盐岩或硅酸盐）相关交代作用的贡献。从Si-和Mg-热压法获得的熔体平衡温度大多为1340-1425°C，并考虑了水的影响（假设为2 wt.%）CO2（变量）熔融平衡深度集中在推断的岩石圈-软流圈边界位置，在科罗拉多高原西南缘下方约70-75 km处，但分散到稍大的值（约100 km）。熔体生成可能是在石榴石-尖晶石相过渡带内或下方通过边缘驱动对流开始的，并随着地幔和/或熔体上涌而继续，同化，有时与较浅的污染地幔平衡，直到熔体最终被提取。
Asthenosphere beneath the relatively thin lithosphere of the Basin and Range province appears to be juxtaposed in step‐like fashion against the Colorado Plateau's thick lithospheric keel. Primary to near‐primary basalts are found above this edge, in the San Francisco‐Morman Mountain volcanic fields, north central Arizona, western USA. We show that at least two distinct peridotite‐dominated mantle end‐members contributed to the origin of the basalts. One has paired Nd and Hf isotopic characteristics that cluster near the mantle array and trace element patterns as expected for melts generated in the asthenosphere, possibly in the presence of garnet. The second has isotopic compositions displaced above the εHf ‐ εNd mantle array which, together with its particular trace element characteristics, indicate contributions from hydrogenous sediments and/or melt (carbonatite or silicate)‐related metasomatism. Melt equilibration temperatures obtained from Si‐ and Mg‐thermobarometry are mostly 1340–1425°C and account for the effects of water (assumed to be 2 wt.%) and estimated CO2 (variable). Melt equilibration depths cluster at the inferred location of the lithosphere‐asthenosphere boundary at ∼70–75 km beneath the southwestern margin of the Colorado Plateau but scatter to somewhat greater values (∼100 km). Melt generation may have initiated in or below the garnet‐spinel facies transition zone by edge‐driven convection and continued as mantle and/or melts upwelled, assimilating and sometimes equilibrating with shallower contaminated mantle, until melts were finally extracted.