Origin of Compositional Gradients with Temperature in the High-SiO2 Rhyolite Portion of the Bishop Tuff: Constraints on Mineral–Melt–Fluid Reactions in the Parental Mush
Origin of Compositional Gradients with Temperature in the High-SiO2 Rhyolite Portion of the Bishop Tuff: Constraints on Mineral–Melt–Fluid Reactions in the Parental Mush
复制标题
主教凝灰岩高 SiO2 流纹岩部分的成分梯度随温度的起源：对母体糊状物中矿物-熔融-流体反应的限制
DOI：
10.1093/petrology/egab087
复制
发表时间：
2021
影响因子：
3.9
通讯作者：
Lange, Rebecca A
中科院分区：
文献类型：
--
作者：
Jolles, Jameson S;Lange, Rebecca A
毕晓普凝灰岩(BT)由加州长谷火山口喷发，具有两种与温度有关的地球化学梯度：一种与岩浆混合有关，另一种位于毕晓普凝灰岩的高二氧化硅流纹岩部分，具有与温度密切相关的两倍或更低的微量元素浓度变化特征。在斑晶生长之前的后一种分带作用被认为是矿物熔体在母质中的间隙熔体和周围晶体之间分配的结果，从中分离出不同的熔体分数。为了验证这一假设，从毕晓普凝灰岩高二氧化硅流纹岩部分的单碎屑浮石样品公布的数据中获得的元素浓度随温度增加或减少的趋势(作为熔体分数的替代)，被用来推断它们在母质泥浆中晶体与熔体之间的不相容与相容的相对程度。所有元素的相对相容值(RCVi)，定义为浓度与温度的斜率除以平均浓度，表明与它们各自的整体分配系数(BulkDi)线性相关。用文献中的矿物熔体分配系数来约束母质泥浆中结晶/熔融反应的平均化学计量比：32%石英+34%斜长石+31%钾长石+1.60%黑云母+0.42%钛磁铁矿+0.34%钛铁矿+0.093%尿安石+0.024%锆石+0.025%磷灰石 = 100%液体。反应中构造硅酸盐的比例(即共晶位置)与熔体分离深度(~400~550℃)和H2O活度(~0.4~0.6℃)一致。~lt；770~780℃的温度受控于反应中的尿晶石。在形成毕晓普凝灰岩高二氧化硅流纹岩部分的分离间隙熔体中，H2O和CO2的含量随温度的降低而增加，这是母岩中存在流体相的证据。过量流体相的存在是解释二氧化碳相容行为所必需的，而流体丰度必须很低才能解释H2O的不相容行为。计算了间隙熔体的脱气路径，这些熔体从母质泥浆中分离出来，上升到较浅的深度以生长斑晶，与已发表的石英熔体包裹体中的挥发性分析相匹配，并将泥浆中的流体丰度限制在≤1wt%。母质泥浆中挥发分的来源，无论它是由结晶或部分熔融形成的，都必须主要来自伴生的玄武岩，因为花岗岩类地壳太不容易挥发。需要大约两倍于流纹岩的玄武岩来提供必需的挥发物。块状Difor几种元素的测定给出了母体淡色花岗岩杂岩的块体成分，并与预期的中生代Sierran弧状花岗岩不同。总而言之，这项研究的结果为整个第四纪复杂的、多阶段的过程模型提供了新的约束，包括来自地幔的玄武岩和先前存在的地壳，导致母体起源于主教凝灰岩。
The Bishop Tuff (BT), erupted from the Long Valley caldera in California, displays two types of geochemical gradients with temperature: one is related to magma mixing, whereas the other is found in the high-SiO2rhyolite portion of the Bishop Tuff and is characterized by twofold or lower concentration variations in minor and trace elements that are strongly correlated with temperature. It is proposed that the latter zonation, which preceded phenocryst growth, developed as a result of mineral–melt partitioning between interstitial melt and surrounding crystals in a parental mush, from which variable melt fractions were segregated. To test this hypothesis, trends of increasing vs decreasing element concentrations with temperature (as a proxy for melt fraction), obtained from published data on single-clast pumice samples from the high-SiO2rhyolite portion of the Bishop Tuff, were used to infer their relative degrees of incompatibility vs compatibility between crystals and melt in the parental mush. Relative compatibility values (RCVi) for all elementsi, defined as the concentration slope with temperature divided by average concentration, are shown to be linearly correlated with their respective bulk partition coefficients (bulkDi). Mineral–melt partition coefficients from the literature were used to constrain the average stoichiometry of the crystallization/melting reaction in the parental mush: 32 % quartz + 34 % plagioclase + 31 % K-feldspar + 1·60 % biotite + 0·42 % titanomagnetite + 0·34 % ilmenite + 0·093 % allanite + 0·024 % zircon + 0·025 % apatite = 100 % liquid. The proportions of tectosilicates in the reaction (i.e. location of eutectic) are consistent with depths of melt segregation of ~400–550 MPa and an activity of H2O of ~0·4–0·6. Temperatures of <770–780 °C are constrained by allanite in the reaction. Evidence that a fluid phase was present in the parental mush is seen in the decreasing versus increasing H2O and CO2contents with temperature in the segregated interstitial melt that formed the high-SiO2rhyolite portion of the Bishop Tuff. The presence of an excess fluid phase, which strongly partitions CO2relative to the melt, is required to explain the compatible behavior of CO2, whereas the fluid abundance must have been low to explain the incompatible behavior of H2O. Calculated degassing paths for interstitial melts, which segregated from the parental mush and ascended to shallower depths to grow phenocrysts, match published volatile analyses in quartz-hosted melt inclusions and constrain fluid abundances in the mush to be ≤1 wt%. The source of volatiles in the parental mush, irrespective of whether it formed by crystallization or partial melting, must have been primarily from associated basalts, as granitoid crust is too volatile poor. Approximately twice as much basalt as rhyolite is needed to provide the requisite volatiles. The determination of bulkDifor several elements gives the bulk composition of the parental leucogranitic mush and shows that it is distinct from Mesozoic Sierran arc granitoids, as expected. Collectively, the results from this study provide new constraints for models of the complex, multi-stage processes throughout the Plio-Quaternary, involving both mantle-derived basalt and pre-existing crust, that led to the origin of the parental body to the Bishop Tuff.