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

蛇河平原大量、18O 严重耗尽的高温流纹岩的起源

• 批准号: 0610081
• 负责人: John Wolff
• 金额: $ 20万
• 依托单位: Washington State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2006
• 资助国家: 美国
• 起止时间: 2006-07-15 至 2010-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0610081&HistoricalAwards=false
• 关键词: Origin; Very; Voluminous; Strongly; 18

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.

花岗岩是地球大陆的重要组成部分，代表了地质时期岩浆过程的终端阶段，这些岩浆过程将大陆从地球的地幔中蒸馏出来。 在花岗岩形成的许多地区，岩浆在地表喷发，同时在地下冷却。 花岗岩和相当的火山流纹岩的起源是由于岩浆过程（部分熔融状态下的质量重新分配）在预先存在的地壳内，直到现在，大多数工作者都同意岩浆是由早期岩石在中下部大陆地壳的熔融产生的，那里温度很高。 在蛇河平原，ID，我们已经发现，大量的流纹岩，相当数量的花岗岩基，并在约1100万年前爆发的黄石热点的影响下，具有强烈的影响雨水的氧同位素组成。 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非常浅，也许需要修改现有的花岗岩和流纹岩起源模型。 因此，本研究的目的是了解这些岩浆形成过程中获得的不寻常条件。 蛇河平原中部约7,000 km 3流纹岩喷发的平均d18 O值为2 ‰，约有30个流纹岩单位。这种18 O贫化程度和大量流纹岩的组合是前所未有的。 我们将建立在我们的试点研究，以确定在Bruneau-Jarbidge和邻近的火山中心强烈的18 O耗尽的总的空间和时间范围，并使用微量元素和同位素分析的整个岩石和矿物相，以及潜在的地壳原岩，以限制岩浆源。 这将使我们能够确定是否在流纹岩强烈的18 O耗尽是热液和岩浆过程与破火山口循环（目前接受的解释），热液蚀变爱达荷州岩基的再循环（如我们的初步数据所建议的），或其他一些过程的结果。 了解这些岩石的成因将是一个重要的贡献，以了解整个现象的大规模大陆岩浆活动，并将提供有价值的解释，其他科学家研究的起源和演化的蛇河平原-黄石热点轨道的各种各样的问题。