Pre-eruptive Conditions of the Huerto Andesite (Fish Canyon System, San Juan Volcanic Field, Colorado): Influence of Volatiles (C–O–H–S) on Phase Equilibria and Mineral Composition
Pre-eruptive Conditions of the Huerto Andesite (Fish Canyon System, San Juan Volcanic Field, Colorado): Influence of Volatiles (C–O–H–S) on Phase Equilibria and Mineral Composition
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韦尔托安山岩（科罗拉多州圣胡安火山田鱼峡谷系统）喷发前条件：挥发物 (C–O–H–S) 对相平衡和矿物成分的影响
DOI：
10.1093/petrology/egn011
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发表时间：
2008
影响因子：
3.9
通讯作者：
S. Feig
中科院分区：
文献类型：
--
作者：
F. Parat;F. Holtz;S. Feig
400 MPa氧化条件下的结晶实验（Δ logfO 2 = NNO + 1，其中NNO是镍-氧化镍缓冲层）和温度范围内（850-950°C）和流体组成（XH_2O_2 = 0·3-1）来约束胡尔托安山岩富硫岩浆的储存条件（硬石膏、磁黄铁矿和富含硫的含磷灰石、后菲什峡谷凝灰岩基性熔岩）。结果被用来评估的作用，从岩浆的结晶，如韦尔托安山岩上的大部分结晶英安岩鱼峡谷岩浆体的再活化释放的流体。使用添加和不添加硫的天然安山岩本体组合物进行实验。硫的存在通过改变斜长石、辉石和钛铁矿的相比例、稳定性场而轻微影响相平衡，并且还影响斜长石的组成。相平衡和矿物成分数据表明，岩浆熔体中可能含有4.5wt%的水，喷发前的温度为875 ± 25°C。假设岩浆与流体相平衡，熔体的CO2浓度估计为2000-4000 ppm（400 MPa）。在喷发前，安山岩的氧化态非常接近或略在NNO + 1·1的安山岩-磁黄铁矿共稳场之内。在这些条件下，熔体中的硫含量为500 ppm。假设开放系统脱气是由于在深部持续结晶而产生的，溶解在安山质熔体中的大部分CO2应该在岩浆结晶<10体积%后释放，相当于从875 ℃冷却到825-850 ℃。因此，由于结晶过程而释放的流体应主要由温度低于825°C的水组成。
Crystallization experiments at 400 MPa, oxidized condition (ΔlogfO2 = NNO + 1, where NNO is nickel–nickel oxide buffer) and over a range of temperatures (850–950°C) and fluid composition (XH2Oin = 0·3–1) have been carried out to constrain the storage conditions of the sulphur-rich magma of the Huerto Andesite (an anhydrite, pyrrhotite, and S-rich apatite-bearing, post-Fish Canyon Tuff mafic lava). The results are used to evaluate the role of fluids released from the crystallization of magmas such as the Huerto Andesite on the remobilization of the largely crystallized dacitic Fish Canyon magma body. Experiments were performed using the natural andesitic bulk composition with and without added sulphur. The presence of sulphur slightly affects the phase equilibria by changing the phase proportions, stability fields of plagioclase, pyroxenes and ilmenite, and also affects the plagioclase composition. Phase equilibria and mineral composition data indicate that the magma may have contained 4·5 wt % water in the melt and that the pre-eruptive temperature was 875 ± 25°C. Assuming that the magma was in equilibrium with a fluid phase, the CO2 concentration of the melt is estimated to be in the range 2000–4000 ppm (at 400 MPa). Before eruption, the andesite had an oxidation state very close to, or slightly within, the co-stability field of anhydrite–pyrrhotite at NNO + 1·1. At these conditions, the sulphur content in the melt is ∼500 ppm. Assuming open-system degassing resulting from continuing crystallization at depth, most of the CO2 dissolved in the andesitic melt should be released after the crystallization of <10 vol. % of the magma, corresponding to a cooling from 875 to 825–850°C. Thus, the fluids released owing to crystallization processes should be mainly composed of water at temperatures below 825°C.