Vanadium magnetite–melt oxybarometry of natural, silicic magmas: a comparison of various oxybarometers and thermometers
Vanadium magnetite–melt oxybarometry of natural, silicic magmas: a comparison of various oxybarometers and thermometers
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DOI：
10.1007/s00410-017-1369-6
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发表时间：
2017-06
影响因子：
3.5
通讯作者：
Róbert Arató;A. Audétat
中科院分区：
文献类型：
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作者：
Róbert Arató;A. Audétat
为了检验最新研制的基于钒在磁铁矿和硅酸盐熔体之间分配的硅质岩浆氧压计，对22个天然流纹岩到英安岩进行了全面的氧压计和测温研究。所调查的样品要么是玻璃斑岩，要么是全晶岩，其中的部分矿物和熔体组合仅作为斑晶内的包裹体保存下来。所采用的分析方法有钒磁铁矿熔体氧压法、铁钛氧化物测温法、锆石饱和测温法和双长石测温法，所有分析方法均采用激光烧蚀电感耦合等离子体质谱进行分析。根据分析次数、结果的重现性和被分析矿物和硅酸盐熔体的同时性确定性，将样品分为三类可靠性等级。在最可靠(n=5)和中等可靠(n=10)样品中，钒磁铁熔体氧压法测定的fO_2值与Fe-Ti氧化压力法测定的fO_2值除两个中等可靠组样品外，其余均在0.5个对数单位以内。在最不可靠的样品中(n=8.7)，大多数样品显示有岩浆混合的证据，计算的fO2值在0.75个对数单位以内。三种不同温度计的比较表明，在大多数情况下，锆石饱和测温法测得的温度与双长石测温法测得的温度在不确定度范围内一致，而铁钛氧化物测温法获得的温度通常偏离≥50°C，这是由于大多数硅质岩浆在T-fO2条件下与Fe-钛氧化物模型有关的较大不确定度。这项研究的另一个结果是，岩浆混合是硅质火山岩中一种常见但容易被忽视的现象，这意味着在应用和解释温度计和氧气压计时必须非常小心。
To test a recently developed oxybarometer for silicic magmas based on partitioning of vanadium between magnetite and silicate melt, a comprehensive oxybarometry and thermometry study on 22 natural rhyolites to dacites was conducted. Investigated samples were either vitrophyres or holocrystalline rocks in which part of the mineral and melt assemblage was preserved only as inclusions within phenocrysts. Utilized methods include vanadium magnetite–melt oxybarometry, Fe–Ti oxide thermometry and -oxybarometry, zircon saturation thermometry, and two-feldspar thermometry, with all analyses conducted by laser-ablation ICP–MS. Based on the number of analyses, the reproducibility of the results and the certainty of contemporaneity of the analyzed minerals and silicate melts the samples were grouped into three classes of reliability. In the most reliable (n= 5) and medium reliable (n= 10) samples, allfO2values determined via vanadium magnetite–melt oxybarometry agree within 0.5 log units with thefO2values determined via Fe–Ti oxide oxybarometry, except for two samples of the medium reliable group. In the least reliable samples (n= 7), most of which show evidence for magma mixing, calculatedfO2values agree within 0.75 log units. Comparison of three different thermometers reveals that temperatures obtained via zircon saturation thermometry agree within the limits of uncertainty with those obtained via two-feldspar thermometry in most cases, whereas temperatures obtained via Fe–Ti oxide thermometry commonly deviate by ≥50 °C due to large uncertainties associated with the Fe–Ti oxide model at T-fO2conditions typical of most silicic magmas. Another outcome of this study is that magma mixing is a common but easily overlooked phenomenon in silicic volcanic rocks, which means that great care has to be taken in the application and interpretation of thermometers and oxybarometers.