A review of K-Rb fractionation trends by covariance analysis
A review of K-Rb fractionation trends by covariance analysis
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DOI：
10.1016/0016-7037(68)90050-1
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发表时间：
1968-06
影响因子：
5
通讯作者：
D. Shaw
中科院分区：
文献类型：
--
作者：
D. Shaw
通过对21套火成岩和似火成岩的K-Rb协方差分析，解决了在火成岩分异过程中K：Rb比值是保持不变还是稳定下降的问题。存在三种主要模式。第一种趋势（主趋势）从0.002到8%的K和0.1到500 ppm的Rb：在双对数图上呈线性，可用方程log（ppm Rb）= 1.115 log（%K）+1.597来描述。由于斜率不是1，K：Rb随绝对浓度而变化，在浓度为0.01%、0.1%、1%、10%时，K：Rb分别为433、332、254、195。这一趋势是由12个岩套的次平行行为所定义的，其中包括非常广泛的大陆和海洋岩石类型。第二个趋势是K：Rb从>3500开始下降，与主趋势合并，所有且仅由海洋拉斑玄武岩和无球粒陨石显示。第三个是众所周知的伟晶-热液趋势的极端Rb浓度相对于几乎恒定K和Rb的部分分离最可能的机制是在硅酸盐熔体和（a）黑云母、（B）水相之间分配（减少K：Rb在每一个这些），或（c）闪石大洋拉斑玄武岩似乎是最原始的地球物质，因为大规模的过程只能导致K：Rb的减少。然而，它不包含足够的Rb被认为是大陆地壳-上地幔系统的母物质。大陆地壳演化的一种尝试性模式是通过反复深熔作用的分带细化作用导致低K：Rb。
Analysis of K-Rb covariance in 21 suites of igneous and quasi-igneous rocks has helped resolve the question of whether the ratio K:Rb remains constant or decreases steadily during igneous differentiation. Three principal patterns exist. The first (main trend) extends from 0·002 to 8 per cent K and 0.1 to 500 ppm Rb: it is linear on a log-log plot and is described by the equation log (ppm Rb) = 1·115 log (per cent K) + 1·597. Since the slope is not unity, K:Rb varies with absolute concentration and has the values 433, 332, 254, 195 at concentrations of 0·01, 0·1, 1, 10 per cent K respectively. This trend was defined by the sub-parallel behaviour of 12 of the suites, which include a very wide variety of continental and oceanic rock types.The second trend is of K:Rb decreasing from >3500, merging with the main trend and is shown by all and only by oceanic tholeiites and achondritic meteorites. The third is the wellknown pegmatitic-hydrothermal trend of extreme Rb concentration relative to nearly constant (or even decreasing) K in the range 3 to 10 per cent.The most likely mechanisms for partial separation of K and Rb are partition between silicate melts and one of (a) biotite, (b) aqueous phase (decrease of K:Rb in each of these), or (c) amphibole (increase in K:Rb).Oceanic tholeiitic basalt appears to be the most primitive earth material, since large-scale processes can only lead to decrease in K:Rb. It does not however contain enough Rb to be considered as parent material for the continental crust-upper mantle system. A tentative model of continental crustal evolution leads to lower K:Rb by the zone-refining action of repeated anatexis.