Partitioning of excess argon between alkali feldspars and glass in a young volcanic system
Partitioning of excess argon between alkali feldspars and glass in a young volcanic system
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DOI：
10.1016/j.chemgeo.2011.07.005
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发表时间：
2011-10
期刊：
影响因子：
3.9
通讯作者：
P. Clay;S. Kelley;S. Sherlock;T. Barry
中科院分区：
文献类型：
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作者：
P. Clay;S. Kelley;S. Sherlock;T. Barry
通过对碱性长石斑晶和玻璃分离物的分析，研究了年轻火山岩系中Ar体系的行为，以限制过剩Ar对40 Ar-39 Ar年龄和晶体/熔体分配系数的潜在影响。同步和后侵位过程的重要性进行了调查，取样玻璃和浮石碎屑来自不同的喷发后的条件，包括焊接部分，气相蚀变部分和严重风化的部分熔结凝灰岩。在所有玻璃样品中都检测到过量的~（40）Ar，其含量在1- 10 ppb之间，尽管如此，对共存的碱性长石进行逐步加热，得到的总气体年龄为0.644±0.038Ma，而普遍接受的Arico组侵位年龄为0.668±0.004Ma。这表明，尽管高浓度的Ar溶解在玻璃中淬火，碱长石结晶从过量的富Ar熔体含有非常少的过量Ar。在玻璃中的过剩Ar与从共存的碱性长石中获得的总气体年龄之间没有观察到相关性，长石和玻璃之间的分配系数（KD）被限制为小于10−3。过剩Ar和大气中衍生的36 Ar的玻璃（表现为一个有限的范围内的40 Ar/36 Ar高达~320）的浓度之间的相关性表明，在喷发前的所有样品中测量的这些水库的Ar的合并的统一机制。在对喷发后因素的评估中，在焊接发生的相关时间尺度上，在与焊接过程相关的高温（约高达670°C）期间，Ar的扩散损失并不显著。由于岩浆房中晶体和熔体之间的扩散交换发生在很长一段时间内，因此在火山碱晶石上测定的年龄反映了由于KD非常低而导致的喷发年龄，但在含有过量Ar的系统中，这种年龄的最终准确性将受到KD实际值的限制。我们观察到，黑云母，一种具有较高KD的矿物，在这样的系统中产生非常古老的年龄，这是以前对其他火山系统的观察。扩散损失的Ar从火山玻璃发生的时间尺度接近火山爆发，这表明多余的Ar测量的玻璃分离是代表的脱气状态，并表明更高的喷发前Ar浓度存在于岩浆房喷发前。因此，我们测量的KD可能是一个最大值。物理污染的长石玻璃的意义也突出和量化表明，按重量计小于0.01%的粘附响岩玻璃需要提高测得的长石年龄。这对于年轻的样品特别重要，其中任何这种污染效应将与少量的内生放射性Ar成比例地放大。
Argon system behavior in a young volcanic system has been investigated through analysis of alkali feldspar phenocrysts and glass separates in order to constrain the potential effects of excess Ar upon40Ar–39Ar ages on feldspars and the crystal/melt partition coefficient. The importance of syn-and post-emplacement processes was investigated by sampling glass and pumice clasts derived from varying post-eruptive conditions, including welded sections, vapor-phase altered sections and heavily weathered portions of the ignimbrite. Excess40Ar was detected in all glass samples at 1–10ppb level, yet despite this, step-heating of co-existing alkali feldspar revealed a total gas age of 0.644±0.038Ma compared with the generally accepted age of 0.668±0.004Ma for the timing of emplacement of the Arico Formation. This suggests that despite high concentrations of Ar dissolved in the glass at quenching, the alkali feldspar crystallizing from excess Ar-rich melt contain very little excess Ar. No correlation was observed between excess Ar in the glass and total gas age obtained from the co-existing alkali feldspar and a partition coefficient (KD) between feldspar and glass was constrained to be less than 10−3. A correlation between the concentrations of excess Ar and atmospheric derived36Ar in the glass (manifested as a restricted range of40Ar/36Ar up to ~320) suggests a uniform mechanism of incorporation of Ar from these reservoirs measured in all samples prior to eruption. In an assessment of post-eruptive factors, diffusional loss of Ar was not significant during the elevated temperatures (~ up to 670°C) associated with welding processes at the relevant timescales over which welding occurs. Since diffusive exchange occurs between crystals and melt over long periods in the magma chamber, ages determined on volcanic alkali feldspars reflect the eruption age as a result of the very low KD, but in systems containing excess Ar the ultimate accuracy of such ages will be limited by the actual value of KD. We observe that biotite, a mineral with a higher KD, yields anomalously old ages in such systems, an observation made previously for other volcanic systems. Diffusive loss of Ar from volcanic glass occurs on timescales approximating that of volcanic eruptions, suggesting that excess Ar measured in the glass separates is representative of a degassed state, and indicates even higher pre-eruptive Ar concentrations present in the magma chamber prior to eruption. Thus our measured KDis likely to be a maximum value. The significance of physical contamination of feldspar by glass is also highlighted and quantified demonstrating that less than 0.01% by weight of adhered phonolite glass was required to elevate a measured feldspar age. This is particularly significant for young samples where any such contamination effect would be magnified proportionally to the small amount of in-grown radiogenic Ar.