The accessory minerals rutile and apatite are rare or absent in the convecting upper mantle but occur in shallow, cooler, metasomatized continental lithospheric mantle (CLM) where they serve as carrier phases for the trace elements Ta (in rutile) and Th (in apatite). Because both minerals crystallize near‐solidus and are eliminated early during partial mantle melting, the relative abundances of rutile and apatite should control the Ta and Th abundances of mantle melts and provide a means of identifying the involvement of rutile‐ and/or apatite‐bearing metasomatized CLM in mafic continental magmatism. As a test, we investigated published Ta and Th abundances data from ~2,000 whole‐rock samples of mafic to intermediate composition, Cenozoic volcanic rocks in southwestern North America. Roughly half of the samples have Ta/Th values similar to those of island arc volcanic rocks (<0.2) or ocean island and mid‐ocean ridge basalts (>0.6). The remaining samples have intermediate and variable Ta/Th values between 0.2 and 0.6, independent of specific indices of crustal interaction (e.g., wt% P₂O₅/wt% K₂O). We interpret the intermediate Ta/Th rocks as the products of direct melting of, or of extensive melt‐rock interaction with, rutile‐ and/or apatite‐bearing CLM. Intermediate Ta/Th rocks also have uniformly high ⁸⁷Sr/⁸⁶Sr (0.706 to 0.708) compared to oceanic basalts that, unlike their Nd isotopic compositions, do not covary with lithospheric age. These observations are consistent with widespread metasomatism of the CLM by Sr‐rich, Nd‐poor, aqueous fluids generated by dehydration of oceanic lithosphere, and its overlying tectonic mélange during early Cenozoic subduction beneath southwestern North America.

中文翻译：

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从Ta / Th值确定北美洲西南西南部交代的大陆岩石圈地幔参与新生代岩浆作用

在对流的上地幔中稀有或不存在辅助金红石和磷灰石，但发生在浅，凉爽，交代的大陆岩石圈地幔（CLM）中，它们充当痕量元素Ta（金红石）和Th（磷灰石）的载体相。因为两种矿物都结晶接近固相线，并且在部分地幔融化的早期就消除了，所以金红石和磷灰石的相对丰度应控制地幔熔体的Ta和Th丰度，并提供一种识别金红石和/或磷灰石含矿量的手段。镁铁质大陆性岩浆作用的交代化CLM。作为一项测试，我们调查了北美西南部约2,000个镁铁质至中间成分的新生代火山岩全岩石样品中已发表的Ta和Th丰度数据。大约一半的样品的Ta / Th值与岛弧火山岩（<0.2）或大岛和中海脊玄武岩（> 0.6）相似。其余样品的Ta / Th值介于中间值和可变值之间，介于0.2和0.6之间，与地壳相互作用的特定指标（例如，wt％P₂ O ₅ / wt％K ₂ O）。我们将中间的Ta / Th岩石解释为含金红石和/或磷灰石的CLM的直接熔化或与熔化的岩石相互作用的产物。与海洋玄武岩相比，中间的Ta / Th岩石也具有均匀高的⁸⁷ Sr / ⁸⁶ Sr（0.706至0.708），与它们的Nd同位素组成不同，它不随岩石圈年龄而变。这些观察结果与北美西南地区早期新生代俯冲作用中富集的，贫钕的，贫化的，由海洋岩石圈的脱水产生的含水流体及其上覆的构造混杂岩广泛存在的CLM交代作用相一致。