Constraints on the loss of matrix-sited helium during vacuum crushing of mafic phenocrysts
Constraints on the loss of matrix-sited helium during vacuum crushing of mafic phenocrysts
复制标题
镁铁质斑晶真空破碎过程中基质位氦损失的限制
DOI：
10.1016/j.gca.2008.05.044
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发表时间：
2008
影响因子：
5
通讯作者：
K. Farley
中科院分区：
文献类型：
--
作者：
P. Blard;Nicolas Puchol;K. Farley
真空破碎是一种选择性释放橄榄石和辉石斑晶中幔源氦组分的有效方法。然而，与以前的假设相反，最近的研究表明，这种方法可能会释放出大量的基质宇宙成因3 He（3 Hec）或放射成因4 He（4 He）。由于这种损失可能会使岩浆中3 He/4 He比值的测定和3 He测量的准确性产生偏差，因此有必要了解什么机制是负责任的，以及在什么条件下基质氦损失是明显的。为了解决这个问题，橄榄石和辉石与不同数量的矩阵sited 3 He（从107至1011at.g-1）被粉碎在空气中或在真空中使用几个粉碎设备。在每个压碎实验期间控制样品温度，并且范围为25至325°C。然后将所得粉末过筛以获得300μm之间的几个均匀颗粒部分<10 and >。在每个馏分中测量的3 Hec浓度清楚地表明，显著的3 Hec损失（>20%）仅影响最细的馏分（<10μm），重要的是，仅在热条件下（此处T = 300°C）。即使是最小的部分（<10μm）在低温条件下（T = 25°C）破碎时也能定量地保留基质中的3 Hec，而与破碎的持续时间和能量无关。这些结果使先前由（Yokochi R.，马蒂B皮克河和Burnard P.（2005）High 3 He/4 He ratios in橄榄岩捕虏体from SW Japan revisited：evidence for cosmogenic 3 He released by vacuum crushing.地球化学地球物理学Geosyst.6，doi：10.1029/2004 GC 000836），其中包括spp轨迹，并暗示损失的大小主要是由粒度控制。此外，进行了新的扩散实验，以限制基质位氦在破碎橄榄石中的扩散率。当用于模型diffusive 3 Hecloss作为粉碎过程中的粒度的函数时，这些新的数据相当好地预测了观察到的释放。因此，我们认为温度增强体积扩散是控制3 Hec在破碎过程中释放的主要机制之一。因此，对于未来的应用，应特别注意控制样品的粒度、破碎时间和温度。
Vacuum crushing is an efficient technique to selectively release the mantle-derived helium component trapped within olivine and pyroxene phenocrysts. However, contrary to previous assumptions, recent studies have shown that this method may liberate significant matrix-sited cosmogenic3He (3Hec) or radiogenic4He (4He∗). Because this loss may bias both the determination of magmatic3He/4He ratios and the accuracy of3Hecmeasurements, it is essential to understand what mechanism is responsible and under what conditions matrix helium loss is manifest. To address this question, olivines and pyroxenes with various amounts of matrix-sited3He (from 107to 1011at.g−1) were crushed in air or in vacuum using several crushing devices. Sample temperature was controlled during each crushing experiment, and ranged from 25 to 325°C. The resulting powders were then sieved to obtain several homogeneous grain fractions ranging between <10 and >300μm. The3Hecconcentrations measured in each fraction clearly show that significant3Hecloss (>20%) affects only the finest fraction (<10μm) and, importantly, only under hot conditions (here T ⩾300°C). Even the smallest fractions (<10μm) quantitatively retain matrix-sited3Hecwhen crushed under cold conditions (T ⩽25°C), regardless of the duration and energy of crushing. These results invalidate the model previously proposed by (Yokochi R., Marty B., Pik R. and Burnard P. (2005) High3He/4He ratios in peridotite xenoliths from SW Japan revisited: evidence for cosmogenic3He released by vacuum crushing. Geochem. Geophys. Geosyst.6, doi:10.1029/2004GC000836) that involved spallation tracks and implied that the magnitude of loss was mainly controlled by the grain size. Moreover, new diffusion experiments were carried out to constrain the diffusivity of matrix-sited helium in crushed olivines. When used to model diffusive3Hecloss as a function of grain size during crushing, these new data predict the observed release fairly well. Therefore, we conclude that temperature-enhanced volume diffusion is one of the main mechanisms controlling the release of3Hecduring crushing. For future applications, special attention should thus be paid to control the grain size, the crushing duration, and the temperature of the sample.