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Geochemistry of Some Volatile Siderophile and Lithophile Elements in the Lunar Highlands

It has been shown elsewhere [l] that the major element composition of the magma from which the lunar highland crust originated (PLC magma) was similar to that of terrestrial oceanic tholeiites, except for depletion of the two most volatile elements, Na and Si. Moreover, the abundances of the rare earth elements were similar in both cases [1,2]. In a companion abstract [3], we have studied the abundances of seven siderophile elements (W,Ni,Co,P,Au,S, Se; Set 1 in Table 1) in the PLC magma, and concluded (a) that large proportions are indigenous to the moon and (b) that the abundances in the PLC magma were similar (i.e. within a factor of 2) to those in terrestrial tholeiitic magmas. The Set 1 siderophile elements (Table 1) are characterised by relatively low volatilities, either in absolute terms (W,Ni,Co,P,Au) or in nebula environments where low hydrogen fugacities (S,Se) may have prevailed [I]. As an extension of the discussion, we shall consider the calculated abundances of some relatively volatile, siderophile elements (As,Cu,Ga,Ag,Sb,Ge; Set 2 in Table 1) in the PLC magma as compared with terrestrial oceanic tholeiites. Since it is well-known that lunar rocks are depleted in volatile elements relative to corresponding terrestrial samples [e.g. 41, we have also calculated the abundances of five moderately volatile, lithophile elements (Li,Mn,Na,K, Zn; Set 3 in Table 1) in the PLC magma in order to readily distinguish this effect of depletion controlled by volatility. The abundances of these Set 2 siderophile elements (Table 1) in the PLC magma have been calculated using the simple two-component mixing model discussed by Delano and Ringwood [3]. This model assumes the Apollo 16 highlands to be largely a mixture of H-group chondrite contamination plus plagioclase cumulates containing intercumulus PLC magma. Since this PLC magma was assumed to contain no Ir, the Ir abundance in a rock was a direct indicator of H-group chondrite contamination. The abundances of Mn,As,Cu,Ga,Ag,Sb,Ge and Zn in the PLC magma (Table 1) have been calculated in this way. Abundances of P,Li,K and Na were obtained from well-established, inter-element correlations in highland rocks, such as La/p [5] , Li/zr [ G I r K/La [7] and Na/~Estimated abundances of these elements in terrestrial oceanic tholeiites [I] are also provided in Table 1. We proceeded to compare the calculated abundances of the Set 2 and Set 3 elements in the PLC magma with the observed abundances in terrestrial oceanic tholeiites (Table 1). In making this comparison for each of the elements, we divided the calculated PLC abundances by the terrestrial abundances (Table 1) and plotted these ratios against "condensation temperatures" as a measure of relative volatilities (Fig. 1). The condensation temperatures were those appropriate to the solar nebula, based on a summary [l] of the results from several workers. However, S and Se were not plotted in Fig. 1 since the condensation temperatures of these two elements are extremely sensitive to hydrogen fugacity. We do not consider it firmly established that a solar hydrogen abundance prevailed in the nebulae from which the moon and Earth formed. The remarkable characteristic in Fig. 1 is that our Set 2 siderophile