Daly Gaps at South Sister, Oregon, USA, generated via partial melting
Daly Gaps at South Sister, Oregon, USA, generated via partial melting
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美国俄勒冈州南姐妹的达利间隙，由部分熔化产生
DOI：
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发表时间：
2021
影响因子：
3.5
通讯作者：
H. Frey
中科院分区：
文献类型：
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作者：
L. Waters;B. Andrews;H. Frey
我们对南姐妹火山(OR)的6个贫晶英安岩(<； 12vol%)进行了详细的岩石学研究，以确定在一个具有显著DALY缺口的系统中有利于中间岩浆形成和提取(即喷发)的条件。我们给出了六个英安岩(63.2-65.1wt%SiO_2)和六个镁铁质岩流(51.3-56.1wt%SiO_2)的全岩成分，这些英安岩构成了South Sister的底部，而六个基性岩流(51.3-56.1wt%SiO_2)在外围火山田喷发。英安石在plagioclase + orthopyroxene + clinopyroxene + ilmenite + titanomagnetite；中饱和，未观察到角闪石。全岩微量元素分析表明，端元混合(玄武岩与流纹岩)不是英安岩的主要成岩机制，英安岩的矿物成分与实验平衡的矿物成分对比表明，英安岩中的大部分晶体为可能的斑晶。对英安岩中辉石和Fe-Ti氧化物组成的多路温度计分析表明，英安岩的结晶开始于较高的温度(980-1045℃)，斑晶组合中适度的分带作用可归因于少量闭合体系结晶 ± 脱气 ± 冷却的影响。当将英安岩的喷发温度、斜长石和全岩成分纳入斜长岩湿度计时，我们获得了喷发前最大H2O含量，范围为2.1~3.3wt%，这些H2O含量与由相平衡实验测定的斜长石-In曲线相一致。通过回顾文献中实验运行产物中平衡的玻璃，我们发现，与South Sister英安岩相似温度(980-1045℃)的英安质熔体成分仅限于富镁(>； 5wt%MgO)玄武岩的部分熔融实验，而英安质熔体成分在低水逸度下进行的部分熔融实验中最常见。玄武岩的部分熔融解决了南姐妹火山记录中玄武岩和英安岩之间的成分空白，因为在部分熔融过程中，英安岩(1)可以在不需要形成组成中间体的情况下产生，(2)在可喷发的熔体组分中产生(例如，20-25%)，(3)如果它们发生在流体不饱和的条件下(由于在没有流体相的情况下，Clapeyron斜率为正)，则有利于喷发。尽管由于产生熔体所需的热量，部分熔融的效率存在争议，但由于地温梯度升高，周围地区喷发的原始玄武岩的喷发前温度较高，以及该地区最近的火山历史，部分熔融是一个可行的过程。我们的结果表明，英安岩的喷发饱和在两种辉石中，没有角闪石，可能是部分熔融的信号。
We conducted a detailed petrologic study on six crystal-poor (< 12 vol%) dacites from South Sister volcano (OR) to determine the conditions that favor the formation and extraction (i.e., eruption) of intermediate magmas in a system that features prominent Daly Gaps. We present whole rock compositions for six dacites (63.2–65.1 wt% SiO2) that form the base of South Sister and six mafic flows (51.3–56.1 wt% SiO2) that erupted in the peripheral volcanic field. The dacites are saturated in plagioclase + orthopyroxene + clinopyroxene + ilmenite + titanomagnetite; hornblende is not observed. Evaluation of trace element concentrations in whole rock analyses reveals that end member mixing (basalt with rhyolite) is not the primary petrogenetic mechanism for generating the dacites, and comparison of compositions of minerals in the dacites with those equilibrated in experiments suggests that the majority of crystals in the dacites are plausible phenocrysts. Application of multiple thermometers to pyroxene and Fe–Ti oxide compositions in the dacites shows that crystallization in the dacites initiated at elevated temperatures (980–1045 °C) and that modest zonation in the phenocryst assemblage can be attributed to the effects of minor closed system crystallization ± degassing ± cooling. When pre-eruptive temperatures, plagioclase and whole rock compositions from the dacites are incorporated into the plagioclase hygrometer, we obtain maximum pre-eruptive H2O contents that range from 2.1 to 3.3 wt%; these H2O contents are consistent with those associated with plagioclase-in curves determined from phase equilibrium experiments. Through review of glasses equilibrated in experimental run products in the literature, we find that dacitic melt compositions with temperatures similar to the South Sister dacites (i.e., 980–1045 °C) are restricted to partial melting experiments on MgO-rich (> 5 wt% MgO) basalts and that dacitic melt compositions most common in partial melting experiments conducted under low H2O fugacities. Partial melting of basalt resolves the compositional gaps between the basalts and dacites in the South Sister volcanic record because, during partial melting, dacites (1) can be generated without requiring the formation of a compositional intermediate, (2) are produced in eruptible melt fractions (e.g., 20–25%), (3) are favorably erupted if they occur in fluid undersaturated conditions (owing to the positive Clapeyron slope in the absence of a fluid phase). Though the efficiency of partial melting is debated because of the amount of heat required to generate melt, it is a viable process beneath South Sister because of the elevated geothermal gradient, high pre-eruptive temperatures of primitive basalts erupting in the surrounding areas and the recent history of volcanism in the region. Our results suggest that eruption of dacites saturated in two pyroxenes, absent hornblende, may signal partial melting.
从玄武岩到围岩的热量和挥发分传递的高分辨率数值模拟：在加利福尼亚州长谷火山口下的地壳柱中的应用
DOI：
10.1029/2018jb016773
发表时间：
2020
期刊：
Journal of Geophysical Research: Solid Earth
影响因子：
--
作者：
Calogero, M. A.;Hetland, E. A.;Lange, R. A.
通讯作者：
Lange, R. A.