Origin of Very Voluminous, Strongly 18-O Depleted, High-Temperature Rhyolites of the Snake River Plain

花岗岩是地球大陆的重要组成部分，代表了岩浆作用的最后阶段，岩浆作用在地质时期将大陆从地球的地幔中提炼出来。在花岗岩形成的许多地区，岩浆在地表喷发，在地下冷却。花岗岩和类似的火山流纹岩的起源是由于岩浆作用（部分熔融状态下的物质重新分配）在原有的地壳中产生的，直到现在，大多数工人都同意岩浆是由温度较高的中下大陆地壳早期岩石的融化产生的。在德克萨斯州的蛇河平原，我们发现了大量流纹岩，其体积与花岗岩基岩相当，在大约1100万年前在黄石热点的影响下喷发，其氧同位素组成受到雨水的强烈影响。雨水通常不会渗透到地壳中超过几公里，这表明这些岩浆的起源可能确实很浅，这可能需要对现有的花岗岩和流纹岩起源模型进行修订。因此，这项研究的目的是了解在这些岩浆形成过程中获得的不寻常条件。从蛇河平原中部喷出的流纹岩约30个单位，约7000 km3的平均d18O值为2 / mil，其18O耗损程度和体积之大是前所未有的。我们将在前期研究的基础上，确定布鲁诺-贾比奇及其邻近火山中心强烈18O耗竭的总时空范围，并对整个岩石和矿物相以及潜在的地壳原岩进行微量元素和同位素分析，以约束岩浆来源。这将使我们能够确定流纹岩中强烈的18O损耗是与火山口循环相关的热液和岩浆过程的结果（目前接受的解释），是热液改变的爱达荷州基岩的再循环（正如我们的初步数据所表明的），还是其他一些过程。了解这些岩石的岩石成因将对理解大规模大陆硅质岩浆活动的整个现象作出重要贡献，并将为其他科学家研究有关蛇河平原-黄石热点轨道的起源和演化的各种问题提供有价值的解释。

Granite is an important component of Earth's continents, and represents a terminal stage of the magmatic processes that have distilled the continents from the planet's mantle over geologic time. In many areas of granite formation, the magmas erupt at the surface as well as cooling undeground. The origin of granites and equivalent volcanic rhyolites is due to magmatic processes (redistribution of mass in the partially molten state) within pre-existing crust, and until now most workers have agreed that the magmas are produced by melting of earlier rocks in the middle to lower continental crust, where temperatures are high. In the Snake River Plain, ID, we have discovered that a large quantity of rhyolite, of comparable volume to granitic batholiths and erupted at around 11 million years ago under the influence of the Yellowstone hotspot, has oxygen isotopic compositions that are strongly influenced by rainwater. Rainwater normally does not penetrate more than several km into the crust, suggesting that the origins of these magmas may be very shallow indeed, and perhaps requiring revision of existing models of granite and rhyolite origins. The aim of this research is therefore to understand the unusual conditions that obtained during the formation of these magmas. The average d18O value of ~7,000 km3 of rhyolite erupted from the central Snake River Plain, represented by some 30 units, is 2 per mil. This combination of degree of 18O depletion and large volume are unprecedented. We shall build on our pilot study to determine the total spatial and temporal extent of strong 18O depletion at Bruneau-Jarbidge and neighboring volcanic centers, and use trace element and isotope analysis of whole rocks and mineral phases, as well as potential crustal protoliths, in order to constrain magma sources. This will enable us to determine whether strong 18O depletion in rhyolites is a result of hydrothermal and magmatic processes associated with caldera cycles (the currently accepted explanation), recycling of hydrothermally altered Idaho batholith rocks (as is suggested by our preliminary data), or some other process. An understanding of the petrogenesis of these rocks will be an important contribution to understanding the whole phenomenon of large-scale continental silicic magmatism, and will provide interpretations valuable to other scientists examining a wide variety of questions regarding the origin and evolution of the Snake River Plain - Yellowstone hotspot track.