Collaborative Research: Contribution of mafic magmatism to upper crustal batholiths: A case study of the Sierra Nevada batholith

合作研究：镁铁质岩浆作用对上地壳基岩的贡献：内华达山脉基岩的案例研究

• 批准号: 2105371
• 负责人: Claire Bucholz
• 金额: $ 40.51万
• 依托单位: California Institute of Technology
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2021
• 资助国家: 美国
• 起止时间: 2021-08-01 至 2024-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2105371&HistoricalAwards=false
• 关键词: Collaborative; Research; Contribution; mafic; magmatism

The dense crust beneath Earth’s oceans is regularly driven beneath the continents in a tectonic process called subduction, which results in the formation of magmas. Such magmas ascend and create long chains of volcanoes like the Cascades of the northwest United States or the Andes in South America. Over time, magmatism at subduction zones has helped build Earth’s continents. These magmatic processes concentrate silica to create thick and buoyant continents that stand higher than surrounding oceans and oceanic crust, which is a unique feature of our planet. This continental crust is an important source for resources essential to human existence, but the processes that concentrate silica in magmas are not fully understood. This research will study magmatic processes in the Sierra Nevada mountain range of California, which is the ancient “plumbing system” from the insides of subduction zone volcanoes from hundreds of millions of years ago, now exposed at earth’s surface. This work will study the chemistry of mafic (more magnesium and iron-rich, lower silica) rocks that represent an important compositional ingredient to create the high-silica rocks that form the bulk of the continents. Extensive existing work on the high-silica rocks at this location will provide context for new measurements of the mafic end-member composition to understand the magmatic processes that build continents. The research will support collaboration between Caltech and Pomona College, including the mentoring of a female graduate student (Caltech) and multiple undergraduate/post-baccalaureate students (Pomona), as well as early career support for a female faculty member (Caltech). In addition, Earth Science classroom lessons and field trips for middle and high school students from the Big Pine Unified School District (BPUSD) in Owens Valley, located within study area will be developed and conducted. BPUSD serves a student population that is ~50% Native American and 40% Latinx, two under-represented groups in geosciences. The ultimate goal is to increase participation and interest of under-represented students in geosciences through place-based and culturally appropriate lessons that successfully aligned Indigenous ways of knowing and scientific practices with Western science modelsThe formation of high-silica arc batholiths is an enduring petrologic problem. During flux-melting of the mantle wedge at subduction zones primitive basalts are produced. Upon ascent into the crust, further differentiation of these basalts is required to form more silicic derivative melts. Although field and experimental studies highlight the importance of lower crustal (0.7 GPa) fractional crystallization of primitive basalts in generating high-silica melts, this process in detail cannot produce the composition of arc batholiths. In particular, deep crustal fractional crystallization generates peraluminous intermediate and silicic melts, compositions that are not widely observed in arc batholiths. To reconcile these observations, this research will test the following hypothesis: Deep crustal differentiation produces high-Al, low-Mg basalts, as well as, evolved mildly peraluminous granitic melts. These melts represent endmembers that can mix to form the compositional diversity of granitoids observed in arc batholith. Testing this mixing-model hypothesis has been limited due to the relative lack of studies focusing on the mafic endmember. Although volumetrically minor and relatively less-studied compared to high-silica granodiorites to granites that dominate batholiths, mafic plutons (non-primitive gabbros and diorites) are widely present in the upper crust of accreted arc sections. Through a collaboration between Caltech and Pomona College this research will investigate the bulk-rock and mineral major/trace element chemistry, geochronology, and oxygen & strontium isotopic compositions mafic plutonic bodies across a transect from a classic continental arc locality, the Sierra Nevada batholith. This data will be placed in the context of both existing and new granitoid data, as well as, quantitative geochemical and rheologic models to understand whether these mafic plutonic bodies represent suitable mixing endmembers in the production of batholithic granitoids.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

地球海洋下方致密的地壳经常在称为俯冲的构造过程中被推入大陆下方，从而导致岩浆的形成。这些岩浆上升并形成长长的火山链，例如美国西北部的喀斯喀特山脉或南美洲的安第斯山脉。随着时间的推移，俯冲带的岩浆作用帮助形成了地球的大陆。这些岩浆过程浓缩了二氧化硅，形成厚而有浮力的大陆，其高度高于周围的海洋和洋壳，这是我们星球的独特特征。大陆地壳是人类生存所必需的资源的重要来源，但岩浆中二氧化硅富集的过程尚不完全清楚。这项研究将研究加利福尼亚州内华达山脉的岩浆过程，这是数亿年前俯冲带火山内部的古老“管道系统”，现在暴露在地球表面。这项工作将研究镁铁质（富含镁和铁，二氧化硅含量较低）岩石的化学性质，这些岩石是形成大部分大陆的高硅岩石的重要组成成分。对该地点高硅岩石的广泛现有工作将为镁铁质端元成分的新测量提供背景，以了解形成大陆的岩浆过程。该研究将支持加州理工学院和波莫纳学院之间的合作，包括对一名女研究生（加州理工学院）和多名本科/学士后学生（波莫纳）的指导，以及对一名女教员（加州理工学院）的早期职业支持。此外，还将为研究区内欧文斯谷大松联合学区 (BPUSD) 的中学生和高中生开发和开展地球科学课堂课程和实地考察。 BPUSD 的学生群体中约 50% 是美洲原住民，40% 是拉丁裔，这是地球科学领域代表性不足的两个群体。最终目标是通过基于地点和文化适当的课程，提高代表性不足的学生对地球科学的参与和兴趣，成功地将土著的认识方式和科学实践与西方科学模型结合起来。高硅弧岩基的形成是一个持久的岩石学问题。在俯冲带地幔楔的熔剂熔化过程中，产生了原始玄武岩。上升到地壳后，需要进一步分化这些玄武岩以形成更多的硅衍生物熔体。尽管现场和实验研究强调了原始玄武岩下地壳（0.7 GPa）分步结晶在生成高硅熔体中的重要性，但这一过程详细地无法产生弧基岩的成分。特别是，深部地壳分步结晶会产生过铝质中间体和硅质熔体，这些成分在弧岩基中并未广泛观察到。为了协调这些观察结果，本研究将检验以下假设：地壳深部分异产生高铝、低镁玄武岩，以及轻度演化的过铝质花岗岩熔体。这些熔体代表可以混合形成在弧基岩中观察到的花岗岩成分多样性的端元。由于相对缺乏针对镁铁质端元的研究，测试这种混合模型假设受到了限制。尽管与主要基岩的高硅花岗闪长岩和花岗岩相比，体积较小且研究相对较少，但镁铁质岩体（非原始辉长岩和闪长岩）广泛存在于增生弧段的上地壳中。通过加州理工学院和波莫纳学院之间的合作，这项研究将调查经典大陆弧地区内华达山脉岩基横断面的大块岩石和矿物主量/微量元素化学、地质年代学以及镁铁质深成体的氧和锶同位素组成。这些数据将置于现有和新的花岗岩数据以及定量地球化学和流变模型的背景下，以了解这些镁铁质深成体是否代表岩基花岗岩生产中合适的混合端元。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。