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

许多构造环境的拉斑玄武岩通常与富硅喷发产物有关。这种组合通常是双峰式的，在玄武岩和A型流纹之间明显缺乏中间成分。A型流纹岩的成因通常有三个过程：（1）母岩玄武岩的长期分离结晶作用;（2）中地壳或上地壳镁铁质源区的部分熔融作用;然而，火山环境中不混溶熔体大规模分离的证据尚未显示。蛇河平原（SRP）岩浆区具有双峰岩浆活动的清晰记录，ICDP Kimama和金伯利岩心提供了一个独特的机会来检验这三个关于A型岩浆起源的假设。在这个项目中，我们将天然岩心样品的联合收割机调查与实验工作相结合，以确定是否可以用分离结晶来最好地解释SRPY中A型韵律的起源，硅酸盐液体的不可渗透性或地壳中的部分熔融。详细的矿物学和岩石学研究的演化玄武岩进行，以确定岩浆储存条件（热压）和岩浆过程的时间尺度。结晶实验模拟流纹质液体的形成从四个可能的玄武岩源进行在300和600 MPa，以确定这些玄武岩的下降和可能的条件下，在可能发生的不可渗透性的液体线。进行了辉长岩和玄武岩的补充部分熔融实验。结果被用来评估这三个过程最好地解释了主要和微量元素组成的SRP皱纹。初步结果表明，玄武岩有一个复杂的历史。橄榄石斑晶经常记录喷发前不久的补给事件（使用成分配置文件的确定时间尺度正在进行中）。结晶实验表明，形成的SRP韵律的分步结晶可能是唯一现实的演变铁玄武组合物。在这样的系统中，熔融不可渗透性过程，无论是在300和600 MPa，预计将发挥作用的岩浆的地球化学特征，实验证明的第一次。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玄武岩相平衡和不可熔性的理解。这也将对熔融反应和干基性岩部分熔融产生的液体施加新的限制。

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.