Helium distribution in a mantle shear zone from the Josephine Peridotite
Helium distribution in a mantle shear zone from the Josephine Peridotite
复制标题
约瑟芬橄榄岩地幔剪切带中的氦分布
DOI：
10.1016/j.epsl.2012.09.046
复制
发表时间：
2012
影响因子：
5.3
通讯作者：
J. Curtice
中科院分区：
文献类型：
--
作者：
A. Recanati;M. Kurz;J. Warren;J. Curtice
以前的研究表明，大洋糜棱岩的橄榄岩氦浓度与地幔高温韧性变形的程度。为了验证这一结果，本研究结合氦测量的变形状态的表征哈氏玄武岩样品在一个小的（6米宽）韧性地幔剪切带从约瑟芬橄榄岩，俄勒冈州，美国。所有的测量都是通过真空破碎和熔融耦合进行的，表明大部分氦（>80%）存在于固相中，而不是流体或熔体包裹体中。本研究证实了变形对氦含量的影响，但只有在最高剪切应变（γ>20）下氦含量才显著升高。最高的氦浓度，大约是2倍，在最中心的样品中发现，其晶粒尺寸也减小了1.24倍。位错和亚晶界存在于所有样品中，并不与氦浓度。矿物学似乎也有一个可以忽略不计的影响，在这个剪切带，因为模态矿物学是相对均匀的，与所有的样品方辉橄榄岩。这些观测结果表明，氦浓度的增加与晶粒尺寸的减小有关，晶界被认为是地幔中氦的额外储存场所。约瑟芬橄榄岩的同位素组成为：3 He/4 He =6.7±0.2Ra（n=33，在6.3 ~ 7.1Ra之间）。熔融释放的氦同位素组成表明基质中存在宇宙成因的3 He：3 He/4 He =8.5±0.3Ra（n=10;从7.9到10.9）。这相当于10 Kyr的暴露年龄，这与末次盛冰期的结束大致一致。样品中放射成因氦含量极少，表明铀和钍含量极低（[U]<0.3ppb）。剪切带外的四个样品中的氦同位素测量表明，矿物学和熔体渗透也是理解地幔中氦储存的重要因素。
A previous study of oceanic mylonites suggested that peridotite helium concentrations are correlated with the degree of high-temperature ductile deformation in the mantle. In order to test this result, this study combines helium measurements with characterization of the deformation state of harzburgite samples in a small (6m width) ductile mantle shear zone from the Josephine Peridotite, Oregon, USA. All measurements were made by coupled in vacuo crushing and melting, demonstrating that most of the helium (>80%) resides within the solid phases rather than fluid or melt inclusions. The present study confirms the influence of deformation on helium contents, but only at the highest shear strain (γ>20) are helium contents significantly higher. The highest helium concentration, by roughly a factor of two, is found in the center-most sample, which also has grain size reduction by a factor of ∼4. Dislocations and sub-grain boundaries are present in all samples and do not correlate with helium concentrations. Mineralogy also appears to have a negligible influence in this shear zone, as modal mineralogy is relatively homogeneous, with all samples being harzburgites. These observations suggest that the increase in helium concentration is related to grain size reduction, with grain boundaries proposed as an additional storage site for helium in the mantle. The present data also characterize the isotopic composition of the Josephine Peridotite:3He/4He=6.7±0.2Ra (n=33, between 6.3 and 7.1Ra). The presence of cosmogenic3He in the matrix is indicated by the helium isotopic composition released by melting:3He/4He=8.5±0.3Ra (n=10; from 7.9 to 10.9). This corresponds to an exposure age of 10Kyr, which is approximately concordant with the end of the last glacial maximum. Very little radiogenic helium is present in the samples, suggesting extremely low uranium and thorium contents ([U]<0.3ppb). Helium isotope measurements in four samples outside the shear zone suggest that mineralogy and melt infiltration are also important factors for understanding helium storage in the mantle.