Atmospheric argon contamination of ocean island basalt olivine phenocrysts
Atmospheric argon contamination of ocean island basalt olivine phenocrysts
复制标题
海岛玄武岩橄榄石斑晶的大气氩污染
DOI：
10.1016/0016-7037(94)90027-2
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发表时间：
1994
影响因子：
5
通讯作者：
H. Craig
中科院分区：
文献类型：
--
作者：
K. Parley;H. Craig
在胡安费尔南德斯热点的一个拉斑玄武岩（PIN-12）中，对橄榄石斑晶进行了40 Ar/36 Ar和氦、氩浓度的重复测量。40橄榄石分裂进行了分析，通过破碎的散装样品或激光熔化的单晶。测得的40 Ar/36 Ar比跨越一个非常大的范围（400-7700），并与两个氩组分的二元混合一致。与氩气不同，氦在重复测量这一单一玄武岩流具有可重复的同位素比，17倍的空气3 He/4 He值。40 Ar/36 Ar的大幅度变化不可能代表微尺度地幔的不均一性。相反的结果表明，高度可变的混合比例的地幔衍生的放射性氩成分（40 Ar/36 Ar 7700）和同位素空气样的端元，几乎可以肯定是一种大气污染物。这种类似空气的成分不能通过物理和化学处理去除橄榄石。通过激光熔融对单个晶体的分析表明，放射成因和污染成分都存在于流体包裹体中。要确定污染物在包裹体中的位置，就需要在侵位之前或侵位过程中向热点岩浆中加入源自空气的惰性气体，这一过程可能是在地壳储存过程中同化蚀变地壳，也可能是向岩浆中直接加入空气或海水。在任何一种情况下，橄榄石必须继续捕获氩，大概是通过断裂退火和/或气泡封闭，在污染事件之后。如果大气污染是一种普遍现象，那么橄榄石（可能还有玄武岩玻璃）的40 Ar/36 Ar组成一定只是地幔源区比值的下限。除氦外，其他惰性气体也可能受到类似的损害。我们的研究结果支持这样的论点，即具有近大气惰性气体成分的熔岩反映了严重的大气污染，而不是未脱气的原始地幔储层的类似空气的特征。
40Ar/36Ar and helium and argon concentrations have been repeatedly measured on olivine phenocrysts in a single tholeiitic basalt (PIN-12) from the Juan Fernandez hotspot. Forty olivine splits were analyzed by crushing of bulk samples or laser fusion of single crystals. The measured40Ar/36Ar ratios span a very large range (400–7700) and are consistent with binary mixing of two argon components. Unlike argon, helium in repeated measurements of this single basalt flow has a reproducible isotopic ratio, 17 times the air3He/4He value. It is unlikely that such large variations in40Ar/36Ar represent microscale mantle heterogeneity. Rather the results indicate highly variable mixing proportions of a mantle-derived radiogenic argon component (40Ar/36Ar ⪢ 7700) and an isotopically air-like endmember that is almost certainly an atmospheric contaminant. This air-like constituent cannot be removed by physical and chemical treatments of the olivines.Analysis of individual crystals by laser fusion shows that both the radiogenic and the contaminant components are in fluid inclusions. Siting of the contaminant in inclusions requires the addition of airderived noble gases to hotspot magmas prior to or during emplacement, a process that may occur by assimilation of altered crust during crustal storage or, alternatively, by direct addition of air or seawater to the magma. In either case the olivines must continue to trap argon, presumably by fracture annealing and/or bubble enclosure, after the contaminating event. If atmospheric contamination is a general phenomenon, the40Ar/36Ar composition of olivines (and possibly of basalt glasses as well) must be only a lower limit for the mantle source ratio. With the exception of helium, the other noble gases may be similarly compromised. Our results support contentions that lavas with near-atmospheric noble gas compositions reflect severe atmospheric contamination, rather than the air-like signature of an undegassed primitive mantle reservoir.