Experimental study on the production of rhyolites from basaltic sources in the bimodal Snake River Plain-Yellowstone province

来自许多构造环境的拉斑玄武岩通常与富含硅的喷发产物有关。这种联系通常是双峰的，明显缺乏介于玄武岩和a型流纹岩之间的中间成分。a型流纹岩的成因通常有三种解释：(1)母岩玄武岩的长时间分离结晶；(2)中上部地壳基性源的部分熔融；(3)硅酸盐液体的不混溶作用。然而，在火山环境中大规模分离非混溶熔体的证据尚未得到证实。蛇河平原（SRP）岩浆省具有清晰的双峰岩浆活动记录，ICDP Kimama和Kimberley钻探岩心为验证这三种关于a型岩浆起源的假设提供了独特的机会。在本项目中，我们将自然岩心样品的调查与实验工作相结合，以确定SRPY中a型流纹岩的起源是否最好地解释为分离结晶，硅酸盐液体不混溶或地壳部分熔融。对演化玄武岩进行了详细的矿物学和岩石学研究，以确定岩浆储存条件（热气压）和岩浆作用的时间尺度。在300和600 MPa下进行了模拟4种可能的玄武岩源流纹岩液体形成的结晶实验，以确定这些玄武岩的液体下降线和可能发生不混溶的条件。进行了辉长岩和玄武岩的互补部分熔融实验。结果用于评价这三种过程中哪一种最能解释SRP流纹岩的主微量元素组成。初步结果表明，玄武岩具有复杂的历史。橄榄石斑晶通常记录喷发前不久的补充事件（利用成分剖面确定时间尺度正在进行中）。结晶实验表明，分离结晶形成的SRP流纹岩可能只有从演化的铁玄武岩成分来看才是真实的。在这样的体系中，300和600 MPa的熔融不混相过程有望在岩浆的地球化学特征中发挥作用，这是首次通过实验证明的。部分融化实验将在未来18个月内进行。该项目还将增加目前对拉斑铁玄武岩相平衡和不混溶性的理解。它还将对熔融反应和干燥基性岩石部分熔融产生的液体提出新的限制。

Tholeiitic basalts from many tectonic environments are commonly associated with silica-rich eruptive products. This association is commonly bimodal, with a noticeable dearth of intermediate compositions between basalts and A-type rhyolites. Three processes are usually invoked to explain the origin of A-type rhyolites: (1) protracted fractional crystallization of parental basalts, (2) partial melting of a mafic source in the middle or upper crust, (3) silicate liquid immiscibility. However, evidence for large-scale separation of immiscible melts in volcanic setting has yet to be shown. The Snake River Plain (SRP) magmatic province, with a clear record of bimodal magmatism, and the ICDP Kimama and Kimberley drillcores offers a unique opportunity to test these three hypotheses proposed for the origin of A-type magmas.In this project, we combine investigations of natural core samples with experimental work to determine whether the origin of A-type rhyolites in SRPY is best explained by fractional crystallization, silicate liquid immiscibility or partial melting in the crust. A detailed mineralogical and petrographical investigation of evolved basalts is conducted to determine magma storage conditions (thermobarometry) and time scales of magmatic processes. Crystallization experiments simulating the formation of rhyolitic liquids from four possible basaltic sources are conducted at 300 and 600 MPa to identify the liquid line of descent of these basalts and possible conditions at which immiscibility may occur. Complementary partial melting experiments of gabbro and basalt are conducted. The results are used to evaluate which of these three processes best explains the major and trace element composition of SRP rhyolites. Preliminary results show that basalts have a complex history. Olivine phenocrysts often records replenishment events shortly prior to eruption (the determination time scales using compositional profiles is in progress). Crystallization experiments indicate that formation of SRP rhyolites by fractional crystallization is probably only realistic from evolved ferrobasaltic compositions. In such systems, melt immiscibility processes, both at 300 and 600 MPa, are expected to play a role in the geochemical signature of the magmas, as demonstrated experimentally for the first time. Partial melting experiments will be conducted in the next 18 months. This project will also add to the current understanding of phase equilibria and immiscibility in tholeiitic ferrobasalts. It will also put new constraints on melting reactions and the liquids produced by partial melting of dry mafic rocks.