Abstract

The Colorado Plateau has undergone as much as 1·8 km of uplift over the past 80 Myr, but never underwent the pervasive deformation common in the neighboring tectonic provinces of the western USA. To understand the source, timing and distribution of mantle hydration, and its role in plateau uplift, garnets from four eclogite xenoliths of the Moses Rock diatreme (Navajo Volcanic Field, Utah, USA) were analyzed in situ for δ¹⁸O by secondary ion mass spectrometry. These garnets have the largest reported intra-crystalline oxygen isotope zoning to date in mantle-derived xenoliths with core-to-rim variations of as much as 3 ‰. All samples have core δ¹⁸O values greater than that of the pristine mantle (∼5·3 ‰, mantle garnet as derived from mantle zircon in earlier work) consistent with an altered upper oceanic crust protolith. Oxygen isotope ratios decrease from core to rim, recording interaction with a low-δ¹⁸O fluid at high temperature, probably derived from serpentinite in the foundering Farallon slab. All zoned samples converge at a δ¹⁸O value of ∼6 ‰, regardless of core composition, suggesting that fluid infiltration was widely distributed. Constraints on the timing of this fluid influx, relative to diatreme emplacement, can be gained from diffusion modeling of major element zoning in garnet. Modeling using best estimates of peak metamorphic conditions (620 °C, 3·7 GPa) yields durations of <200 kyr, suggesting that fluid influx and diatreme emplacement were temporally linked. These eclogite xenoliths from the Colorado Plateau record extensive fluid influx, pointing to complex hydration–dehydration processes related to flat-slab subduction and foundering of the Farallon plate. Extensive hydration of the lithospheric mantle during this fluid influx may have contributed to buoyancy-driven uplift of the Colorado Plateau and melt-free emplacement of Navajo Volcanic Field diatremes.

中文翻译：

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科罗拉多高原以下的大量流体涌入（美国）与平板清除和极端沉积有关：榴辉岩异岩中氧同位素分区的证据

摘要

在过去的80 Myr，科罗拉多高原经历了长达1·8 km的隆升，但从未经历过美国西部邻近构造省份普遍发生的普遍形变。为了理解来自摩西岩火山道（纳瓦霍火山字段，犹他州，USA）四个榴包体源，定时和地幔水合的分布，并且其在高原隆起作用，石榴石分析原位为δ ¹⁸ ö通过二次离子质谱光谱法。迄今为止，这些石榴石在地幔衍生的异岩中具有最大的晶体内氧同位素分区，其芯间差异高达3‰。所有样品都具有核心δ ¹⁸O值大于原始地幔的值（〜5·3‰，地幔石榴石，是早期工作中由地幔锆石衍生而来），与上层洋壳原岩的变化一致。氧同位素比率从核心降低至轮辋，记录与相互作用低δ ¹⁸ O不连到高温，可能是由蛇纹岩在沉没法拉隆板坯衍生流体。所有样品划会聚于δ ¹⁸无论岩心成分如何，O值均约为6‰，表明流体渗透广泛分布。可以从石榴石中主要元素分区的扩散模型获得相对于非正常进水的这种流体涌入时间的约束。使用最佳变质条件峰值（620°C，3·7 GPa）进行建模可得出<200 kyr的持续时间，这表明流体涌入和异常安置在时间上是相关的。这些来自科罗拉多高原的榴辉岩异岩记录到大量的流体涌入，表明与平板俯冲和Farallon板的沉陷有关的复杂的水化-脱水过程。