The principal point of issue between Dr. Urey and myself seems to be whether the carbonaceous chondrites represent a source material from which the ‘common’ olivine-pyroxene chondrites have been formed by dehydration and partial reduction at temperatures above 600°C, as I suggested. Dr. Urey believes this is impossible and suggests that “some water containing compounds, hydrogen sulphide, etc., infiltrated some high iron group chondritic material, oxidized the metallic iron to magnetite and the sulphide, deposited carbon compounds, sulfate, sulphur, etc., and removed some sodium and potassium, and partly destroyed the chondrites,” thereby producing the carbonaceous chondrites. Dr. Urey evidently held opinions similar to my own some years ago, but rejected them because the carbonaceous chondrites and the olivine-pyroxene chondrites do not have constant composition with respect to the nonvolatile elements. Nevertheless, the correspondence is close, as Dr. Urey himself admits when he writes, “It is difficult in some cases to decide as to whether the variations (between the two groups) are real or due to analytical error.” He points out differences between the percentages of Na and K and the Mg/Si and Fe/Si ratios for carbonaceous chondrites and other types of chondrites. It is doubtful how much weight be given to these differences. Apart from possible analytical error (probably significant in view of the extremely difficult problems involved in the analysis of carbonaceous chondrites), it should be borne in mind that the carbonaceous chondrites contain water-soluble compounds of sodium, potassium, and magnesium, and hence these elements may readily be lost, as pointed out by Ringwood [1961, p. 170].
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Dec 15, 2009
Open Access
Ursula B Marvin 