Palladium, iridium, and gold contents of mafic and ultramafic rocks drilled from the Mid-Atlantic Ridge, Leg 37, Deep Sea Drilling Project

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Abstract
Basalts and ultramafic rocks from holes 332B and 334 of Leg 37, DSDP, were analyzed for palladium, iridium, and gold by neutron activation. Averages obtained for the basalts are: Pd < 0.69 ppb, Ir < 0.025 ppb, and Au = 2.9 ppb. The samples include one representative of each of the main basaltic lithological units except for Unit III, hole 332B, which is represented by a suite of 14 samples. Averages of 43, 0.60, and 16 ppb were obtained for Pd, Ir, and Au respectively in two ultramafic rocks and a gabbro from hole 334.In comparison with basalts from mid-ocean islands such as Hawaii, Leg 37 sea-floor basalts are significantly depleted in Ir and Pd, but are similar in gold content. These differences may be related to differing degrees of alteration sustained by rocks from these differing environments.The depth profiles for Ir and Pd in hole 332B show no significant trends, but a weak trend of increasing gold content with depth is suggested. Variation of Ir and Pd in adjacent lithological units may be as great as a factor of 50. The Pd distribution in 14 samples from Unit III suggests that these differences probably reflect real differences in average metal content rather than sampling effects. Although alteration of Leg 37 basalts is weak, the low Pd and Ir contents and the variability from flow to flow suggest that reaction with sea water has leached Pd and Ir from these rocks. A possible complementary enhancement of some noble metals including Pd has been noted in the sediments immediately overlying mid-ocean rises. Although gold does not seem to be depleted in Leg 37 basalts there is evidence that the metal is easily mobilized by local heating accompanying igneous intrusive rocks.The average Au/Ir ratio of the Leg 37 basalts is four times higher than that of the ultramafic–mafic rocks of hole 334. Although part of this effect is probably due to secondary alteration, it is considered unlikely that the Leg 37 basalts and ultramafic rocks are petrogenetically related.

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