Li as a barometer for bimineralic eclogites: Experiments in natural systems
Li as a barometer for bimineralic eclogites: Experiments in natural systems
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DOI：
10.1016/j.lithos.2009.05.021
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发表时间：
2009-11
期刊：
影响因子：
3.5
通讯作者：
M. Hanrahan;G. Brey;A. Woodland;H. Seitz;T. Ludwig
中科院分区：
文献类型：
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作者：
M. Hanrahan;G. Brey;A. Woodland;H. Seitz;T. Ludwig
在榴辉岩体系中进行的实验表明，在天然榴辉岩块体成分中，Li在石榴石和单斜辉石之间的分配是压力依赖的。这在实验上支持了基于榴辉岩包体和钻石包裹体中Li的分析的假设，即这种特殊元素在单斜辉石和石榴石之间的分配对压力变化非常敏感，可以作为双矿物榴辉岩的晴雨表。为了将这种压力依赖关系校准到气压计中，在4-13 Gpa和1100-1500℃范围内进行了16次多压锤实验，使用的原料来自保存完好的来自Roberts Victor金伯利岩管(南非)的富镁榴辉岩捕虏体。我们的发现证实了我们在CMAS体系中观察到的总体趋势，即DLICPx-GRt for Li随着P的增加而减少，并且在P≥12 Gpa时，石榴石能够比单斜辉石吸收更多的Li。对我们的实验结果进行了多元线性回归，建立了气压计：其中P是以Gpa为单位的压力，T是以℃为单位的温度，DLICpx-grtis定义为Li在cpx中的浓度除以石榴石中的Li的浓度得到的Li的分配系数。该气压计将实验条件重现到±0.2 GPa.适用于富镁榴辉岩包体，也适用于钻石中橄榄岩和榴辉岩包裹体。将气压计应用于含金刚石包体，在钻石稳定区产生了压力。单斜辉石在包体中容易腐蚀，在短暂的交代过程中也优先吸收Li。因此，必须小心分析原生的、未改变的单斜辉石。我们对天然样品的初步应用表明，该气压计可以应用于实验范围以外的压力低至3.5 Gpa的压力。这比在地壳榴辉岩中遇到的压力要高，因此气压计不适用于这些榴辉岩。
Experiments performed in eclogitic systems show that the partitioning of Li between garnet and clinopyroxene is pressure dependent in natural eclogitic bulk compositions. This supports experimentally the hypothesis, based on the analysis of Li in eclogitic xenoliths and inclusions in diamond, that the partitioning of this particular element between clinopyroxene and garnet is very sensitive to changes in pressure and could be calibrated as a barometer for bimineralic eclogites. In order to calibrate this pressure dependence into a barometer, 16 multianvil experiments were performed from 4 to 13 GPa and 1100–1500 °C using starting materials sourced from a well preserved Mg-rich eclogitic xenolith from the Roberts Victor kimberlite pipe (South Africa). Our findings reinforce the general trend we observed in the CMAS system, that DLicpx–grtfor Li decreases with increasing P, and that at P≥12 GPa, garnet is able to incorporate more Li than clinopyroxene. Multiple linear regression was applied to our experimental results to create the barometer:where P is pressure in GPa, T is temperature in °C and DLicpx–grtis defined as the partitioning coefficient of Li obtained by dividing the concentration of Li in cpx by the concentration of Li in garnet. This barometer reproduces the experimental conditions to ±0.2 GPa. It is applicable to Mg-rich eclogitic xenoliths, and to peridotitic and eclogitic inclusions in diamond. Application of the barometer to diamond-bearing xenoliths results in pressures in the diamond stability field. Clinopyroxene is easily corrupted in xenoliths and also preferentially takes in Li during short lived metasomatic processes. Care must be taken therefore to analyse primary, unaltered clinopyroxene. Our preliminary application to natural samples shows that the barometer can be applied beyond the experimental range to pressures down to 3.5 GPa. This is higher than the pressures encountered in crustal eclogites and thus the barometer is not applicable to these eclogites.