A review of K-Rb fractionation trends by covariance analysis

Details A review of K-Rb fractionation trends by covariance analysis A review of K-Rb fractionation trends by covariance analysis

Jun 1968

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1968gecoa..32..573s&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 32, Issue 6, pp.573-601

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61

Scientific paper

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.

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