Abstract

The combined Fe–Ti oxide geothermometry and oxygen barometry is an important tool in petrology and volcanology. However, its appropriate application to natural magmatic systems is still challenged by poorly constrained kinetics of the re-equilibration processes between magnetite, ilmenite, melt and other magmatic phases. In this study, we experimentally investigated how fast Fe–Ti oxides can re-equilibrate and how fast their compositions can adapt to changing temperature and/or redox conditions. A series of equilibrium crystallization experiments were conducted in internally heated gas pressure vessels using an evolved hydrous basaltic composition. These starting reference experiments were conducted at 200 MPa, at 900 and 1000°C and at two redox conditions, i.e. FMQ + 1 and FMQ + 3·3. The products of the starting experiments, all containing magnetite and/or ilmenite, were then placed at a different temperature (T) and/or oxygen fugacity (fO₂) for time dependent experimental series (1, 10 and 100 hours) in an attempt to check for the time needed for re-equilibration of the oxide composition. The experimental results demonstrate that both magnetite and ilmenite start to respond chemically to the changing conditions quite rapidly in less than 1 h. The largest compositional deviations from the equilibrium compositions were observed in the runs with 1 and 10 h duration. After 100 h, the Fe–Ti oxide compositions were approaching the expected equilibrium values in almost all kinetic series, but still with significant deviation. Moreover, our results clearly show that the Mg/Mn ratio in magnetite and ilmenite can follow the nominal ‘equilibrium’ trend, although the Fe–Ti distribution between oxides may not have reached equilibrium. This observation implies that the use of the Mg/Mn criterion to check for the equilibrium distribution of Fe–Ti between magnetite and ilmenite should be reconsidered or at least applied with caution. The quick, within-100-hours re-equilibration of the Fe–Ti oxides at magmatic conditions imposes limitations on the reliable application of oxide thermobarometry to natural systems. We suggest that in basaltic to andesitic volcanic rocks ascending and cooling relatively slowly (more than 5 days), the compositional T–fO₂ record in oxides is representative of a late evolution stage rather than the pre-eruptive conditions in a magmatic reservoir at depth. This ‘frozen’ late-stage T–fO₂ record is controlled by changing element diffusion rates with cooling and degassing. Only magmatic/volcanic systems, which underwent very rapid cooling, resulting from magma ascent within minutes or few hours (e.g. Plinian eruptions), can deliver Fe–Ti oxides reflecting the pre-eruptive magmatic T–fO₂ signature.

中文翻译：

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岩浆系统中Fe-Ti氧化物重新平衡的动力学：对热氧法的影响

摘要

Fe-Ti氧化物地热法和氧气气压法的组合是岩石学和火山学的重要工具。然而，其在磁铁矿，钛铁矿，熔体和其他岩浆相之间重新平衡过程动力学的约束很有限，仍然挑战着其在天然岩浆系统中的适当应用。在这项研究中，我们通过实验研究了Fe-Ti氧化物可以快速重新平衡的过程，以及它们的成分可以适应变化的温度和/或氧化还原条件的速度。使用析出的含水玄武岩成分在内部加热的气压容器中进行了一系列平衡结晶实验。这些初始参考实验是在200 MPa，900和1000°C以及两个氧化还原条件下进行的，即FMQ + 1和FMQ + 3·3。起始实验的产品，˚F Ò ₂）（取决于时间）的实验系列（1、10和100小时），以尝试检查氧化物组合物重新平衡所需的时间。实验结果表明，磁铁矿和钛铁矿在不到1小时的时间内就开始对变化的条件做出化学反应。在持续1小时和10小时的运行中，观察到与平衡成分的最大成分偏差。100小时后，几乎所有动力学序列中的Fe-Ti氧化物组成都接近预期的平衡值，但仍存在显着偏差。此外，我们的结果清楚地表明，尽管氧化物之间的Fe-Ti分布可能尚未达到平衡，但磁铁矿和钛铁矿中的Mg / Mn比可以遵循标称的“平衡”趋势。该观察结果暗示，应重新考虑或至少应谨慎使用Mg / Mn准则检查Fe-Ti在磁铁矿和钛铁矿之间的平衡分布。在岩浆条件下，Fe-Ti氧化物在100小时内快速重新平衡，这限制了氧化物热压法在自然系统中的可靠应用。我们建议在玄武岩到安山岩火山岩上升和冷却相对较慢（超过5天）时，其成分T –氧化物中的f O ₂记录代表了晚期演化阶段，而不是深部岩浆储层的喷发前条件。通过冻结和除气来改变元素扩散速率，从而控制了这种“冻结”的后期T– f O ₂记录。只有在几分钟或几小时内由于岩浆上升而迅速冷却的岩浆/火山系统（例如普利尼山喷发），才能释放出反映喷发前岩浆T- f O ₂签名的Fe-Ti氧化物。