Clinopyroxene–liquid geothermobarometry is a widely used tool for estimating the conditions under which mafic magmas are stored before they erupt. However, redox variability, sector zoning and disequilibrium crystallization present major challenges to the robust estimation of magma storage conditions. Moreover, most recent studies seeking to address these challenges have focused on clinopyroxenes from alkalic systems and are thus of limited use for understanding clinopyroxenes from the tholeiitic systems that dominate global magma budgets. Here we combine observations on natural clinopyroxenes from the 2014–2015 Holuhraun lava in Iceland with observations on experimental clinopyroxenes synthesized during high-pressure, high-temperature experiments on the same lava in order to investigate clinopyroxene–liquid equilibria in tholeiitic systems and optimize of geothermobarometric strategies. Natural clinopyroxenes from the 2014–2015 Holuhraun lava are sector zoned, with {1-11} hourglass sectors being enriched in the enstatite–ferrosillite component at the expense of all other components with respect to {hk0} prism sectors. In contrast with observations on clinopyroxenes from alkalic systems, sector zoning in clinopyroxenes from the 2014–2015 Holuhraun lava is characterized by differences in Ca and Na contents as well as in Ti and Al contents. The products of crystallization experiments performed at 100–600 MPa and 1140–1220 °C on a powdered starting glass at two sets of melt H₂O content–oxygen fugacity conditions (∼0·1 wt % H₂O and close to the graphite-oxygen redox buffer, and 0·5–1·0 wt % H₂O and approximately one and half log units above the quartz–fayalite–magnetite redox buffer) demonstrate that clinopyroxene crystals from nominally equilibrium experiments can preserve strongly disequilibrium compositions. The compositional systematics of experimental clinopyroxenes are consistent with the presence of sector zoning. Furthermore, the magnitude of compositional variability increases with decreasing melt H₂O content and increasing deviations of experimental temperatures below clinopyroxene liquidus temperatures (i.e. degrees of undercooling sensu lato), indicating that kinetic processes play a key role in controlling clinopyroxene compositions, even under notionally equilibrium conditions. Few published analyses of experimental clinopyroxene crystals may thus represent truly equilibrium compositions. Stoichiometric calculations on natural and experimental clinopyroxenes show that Fe³⁺ is a major constituent of clinopyroxenes from tholeiitic magmas under naturally relevant oxygen fugacity conditions. They also show that Fe³⁺ is most likely incorporated as Ca- and Al- bearing Ca–Fe-Tschermak’s component rather than Na-bearing aegirine component at oxygen fugacities up to one and a half log units above the quartz–fayalite–magnetite buffer. Elevated oxygen fugacities are thus less likely to compromise clinopyroxene–liquid geothermobarometry than previously thought. Guided by our experimental results, we combined published descriptions of clinopyroxene–liquid equilibria with geothermobarometric equations to develop an internally consistent and widely applicable method for performing geothermobarometry on tholeiitic magmas that does not require equilibrium zones to be selected a priori. Applying this method to natural clinopyroxene crystals from the 2014–2015 Holuhraun lava that formed under low but variable degrees of undercooling (perhaps 25 °C or less) returns values in excellent agreement with those from independent methods (232 ± 86 MPa, 1161 ± 11 °C). Robust estimates of magma storage conditions can thus be obtained by performing clinopyroxene–liquid geothermobarometry on tholeiitic magmas when disequilibrium is suitably accounted for.

中文翻译：

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天然和实验性变黄岩中的斜辉石-液体平衡和地热气压法：2014-2015年冰岛霍卢汉火山喷发

Clinopyroxene –液体地热气压测量法是一种广泛使用的工具，用于估计铁镁质岩浆爆发前的储藏条件。但是，氧化还原变异性，扇区分区和不平衡结晶化对岩浆储藏条件的可靠估计提出了重大挑战。而且，最新的寻求解决这些挑战的研究都集中在碱性系统中的斜辉石上，因此在理解全球岩浆预算中占主导地位的生烃系统的斜辉石上用途有限。在这里，我们结合了对2014-2015年冰岛Holuhraun熔岩中天然clinopyroxenes的观察结果以及高压下合成的实验clinopyroxenes的观察结果，在同一个熔岩上进行高温实验，以研究高渗体系中的斜辉石-液体平衡并优化地热气压测量策略。将2014-2015年Holuhraun熔岩的天然斜辉石按扇形分区，其中{1-11}个沙漏扇形体富含顽辉石-硅藻土成分，而与{hk0}棱镜扇形体相比，所有其他成分的消耗都很大。与碱性系统中的斜辉石的观测结果相反，2014-2015年的霍卢伦火山熔岩的斜辉石的分区带的特征是钙和钠含量以及钛和铝含量不同。结晶实验的产物是在粉末状起始玻璃上在两组熔融H下在100–600 MPa和1140–1220°C的温度下进行的 将2014-2015年Holuhraun熔岩的天然斜辉石按扇形分区，其中{1-11}个沙漏扇形体富含顽辉石-硅藻土成分，而与{hk0}棱镜扇形体相比，所有其他成分的消耗都很大。与碱性系统中的斜辉石的观测结果相反，2014-2015年的霍卢伦火山熔岩的斜辉石的分区带的特征是钙和钠含量以及钛和铝含量不同。结晶实验的产物是在两组熔融H下在粉末状起始玻璃上于100–600 MPa和1140–1220°C的温度下进行的 将2014-2015年Holuhraun熔岩的天然斜辉石按扇形分区，其中{1-11}个沙漏扇形体富含顽辉石-硅藻土成分，而与{hk0}棱镜扇形体相比，所有其他成分的消耗都很大。与碱性系统中的斜辉石的观测结果相反，2014-2015年的Holuhraun熔岩中的斜辉石的分区带的特征是钙和钠含量以及钛和铝含量不同。结晶实验的产物是在粉末状起始玻璃上在两组熔融H下在100–600 MPa和1140–1220°C的温度下进行的 2014–2015年Holuhraun熔岩中斜向辉石的区域划分以Ca和Na含量以及Ti和Al含量不同为特征。结晶实验的产物是在粉末状起始玻璃上在两组熔融H下在100–600 MPa和1140–1220°C的温度下进行的 2014–2015年Holuhraun熔岩中斜向辉石的区域划分以Ca和Na含量以及Ti和Al含量不同为特征。结晶实验的产物是在粉末状起始玻璃上在两组熔融H下在100–600 MPa和1140–1220°C的温度下进行的₂ O含量-氧逸度条件（〜0·1 wt％H ₂ O并接近石墨-氧氧化还原缓冲液，和0· 5-1〜0 wt％H ₂ O和大约在石英上方一个半对数单位-铁橄榄石-磁铁矿氧化还原缓冲液）表明，名义上平衡实验获得的斜辉石晶体可以保留强烈的不平衡成分。实验性斜py的组成系统与扇形区划的存在是一致的。此外，随着熔体H _2的减少，组成变异性的大小增加。O含量和低于斜向辉石液相线温度（即过冷的感觉程度）的实验温度的偏差不断增加，表明动力学过程在控制斜向辉石的组成中也起着关键作用，即使在理论上平衡的条件下也是如此。因此，很少有公开发表的实验性斜py晶体的分析可以代表真正的平衡组成。对天然和实验性斜吡咯的化学计量计算表明，Fe ³⁺是在自然相关的氧逸度条件下来自胆囊性岩浆的斜吡咯的主要成分。他们还表明Fe ³⁺在氧逸度高出石英-铁橄榄石-磁铁矿缓冲液一个半个对数单位的情况下，最有可能是作为含钙和含铝的钙铁切克马克的成分，而不是含钠的己胺成分。因此，比以前认为的，升高的氧气逸散度不太可能损害环吡咯烯-液体地热气压计。在我们的实验结果的指导下，我们将已公开的关于斜柏油-液体平衡的描述与地热气压方程式结合起来，以开发一种内部一致且广泛适用的方法，用于对胶质岩浆进行地热气压计，而无需事先选择平衡区。将这种方法应用于2014-2015年Holuhraun熔岩中的天然斜辉石晶体，该晶体在较低但可变程度的过冷度（可能为25°C或更低）下形成，其返回值与独立方法（232±86 MPa，1161±11）非常一致°C）。因此，在适当考虑了不平衡的情况下，可以通过对稠密岩浆进行斜辉石-液体地热气压法来获得岩浆储存条件的可靠估计。