新西兰陶波火山带奥卡泰纳火山中心爆炸性地震平岩浆体的浅层贮存:相平衡实验的证据

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS Contributions to Mineralogy and Petrology Pub Date : 2024-07-25 DOI:10.1007/s00410-024-02151-y
Elizabeth R. G. Grant, Dawnika L. Blatter, Thomas W. Sisson, Kari M. Cooper
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引用次数: 0

摘要

流纹凝灰岩的晶体含量范围很广,从近似斑状到富含晶体,其晶体含量为上地壳岩浆储层的概念提供了依据。Earthquake Flat 热成岩(新西兰陶波火山带 Okataina 火山中心)是 10 km3 的流纹凝灰岩,含有丰富的(约 40 vol.%)斜长石和石英,少量的斜长石、角闪石和正长石,以及附属的铁钛氧化物、磷灰石和锆石,镶嵌在高硅流纹岩玻璃中。主要矿物形成大型八面体表晶和大量团晶,除一些微弱的吸收石英外,几乎没有不平衡纹理。斜长石表晶具有厚厚的边缘,其成分在 An30 附近几乎保持不变,角闪石呈弱分带或无分带。丰富而质地复杂的矿物组合与附近(约 25 千米)的罗托提火山口流纹凝灰岩形成鲜明对比,两者几乎同步,但体积更大,晶体更温和。关于喷发的地震平熔体(玻璃)的新的 H2O 饱和相平衡结果确定了它与斜长石、石英、斜长石和铁钛氧化物的部分表晶组合在 140 兆帕、755 摄氏度的条件下的共饱和度:140 兆帕、755 ºC。这与喷发前岩浆体的条件非常接近,假定其饱和度为近乎纯净的 H2O,fO2 为 ~ Ni-NiO 。合成的集合体中没有角闪石和正长石,可能是因为这些矿物与熔体的包晶反应关系是生成生物橄榄石,因此它们不会从作为起始材料的液体中生长出来。对 Rotoiti 流纹岩的实验结果(Nicholls 等人,1992 年)表明,这两个岩体所处的压力、温度和 fO2s 相似。罗托伊蒂凝灰岩的晶体丰度较低可能是由于成分上的细微差别造成的。我们的解释是,地震平原的火成岩来源于淤泥质储层系统中晶体丰富的外围,而罗托伊蒂凝灰岩则位于熔融物更丰富的中心位置。目前还不确定这是一个结晶度连续分区的单一侵入体,还是相邻的离散侵入体,但我们的研究结果说明并量化了岩浆在相对较短距离内储存的复杂性。
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Shallow storage of the explosive Earthquake Flat Pyroclastics magma body, Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand: evidence from phase-equilibria experiments

Rhyolitic tuffs range widely in their crystal contents from nearly aphyric to crystal-rich, and their crystal cargoes inform concepts of upper crustal magma reservoirs. The Earthquake Flat pyroclastics (Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand) are 10 km3 of rhyolitic tuffs with abundant (~ 40 vol.%) plagioclase and quartz, minor biotite, hornblende, and orthopyroxene, and accessory Fe-Ti oxides, apatite, and zircon, set in high-silica rhyolitic glass. Major minerals form large, euhedral phenocrysts and abundant glomerocrysts with few disequilibrium textures excepting some faintly resorbed quartz. Plagioclase phenocrysts have thick rims of nearly constant composition near An30, and hornblende is weakly zoned or unzoned. The abundant and texturally complex mineral assemblage contrasts with the nearby (~ 25 km), nearly synchronous, but more voluminous and crystal-moderate rhyolite tuffs from Rotoiti caldera. New H2O-saturated phase-equilibria results on the erupted Earthquake Flat melt (glass) determine its co-saturation with the partial phenocryst assemblage of plagioclase, quartz, biotite, and Fe-Ti oxides at: 140 MPa, 755 ºC. These closely approximate the conditions of the pre-eruptive magma body assuming it was saturated with nearly pure H2O and at an fO2 of ~ Ni–NiO. Absence of hornblende and orthopyroxene from the synthesized assemblages may result from those minerals being in a peritectic reaction relation with melt to produce biotite, so they would not grow from the liquid used as starting material. Experimental results on Rotoiti rhyolite (Nicholls et al. 1992) show that the two bodies resided at similar pressures, temperatures, and fO2s. Lower crystal abundance of the Rotoiti tuffs may result from slight compositional differences. We interpret that the Earthquake Flat pyroclastics were sourced from the crystal-rich periphery of a mushy reservoir system with the Rotoiti occupying a more melt-rich central location. Uncertain is whether this was a single intrusion zoned continuously in crystallinity, or discrete adjacent intrusions, but our results illustrate and quantify complexities of magma storage across relatively short distances.

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来源期刊
Contributions to Mineralogy and Petrology
Contributions to Mineralogy and Petrology 地学-地球化学与地球物理
CiteScore
6.50
自引率
5.70%
发文量
94
审稿时长
1.7 months
期刊介绍: Contributions to Mineralogy and Petrology is an international journal that accepts high quality research papers in the fields of igneous and metamorphic petrology, geochemistry and mineralogy. Topics of interest include: major element, trace element and isotope geochemistry, geochronology, experimental petrology, igneous and metamorphic petrology, mineralogy, major and trace element mineral chemistry and thermodynamic modeling of petrologic and geochemical processes.
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