Comment on “Abundance and distribution of gallium in some spinel and garnet lherzolites” by D. B. McKay and R. H. Mitchell

Abstract

MCKAY and MITCHELL (1988) recently reported Ga concentrations in whole rocks and mineral separates from gametand spinel-bearing peridotite xenoliths. They used these data to estimate the average Ga content of spine1 and garnet lherzolites and suggested that an upper mantle, consisting of a 30:70% mixture of these respective components, would have an average Ga content of I .90 ppm with a minimum of 1.20 and a maximum of 2.58 ppm. They used this upper mantle estimate as a guide to the Ga abundance in the primitive mantle and from this perspective compared their estimate with the primitive mantle estimates of JAGOUTZ et al. (1979) and SUN (1982). They also discussed the relative distribution of Ga between the core and mantle. In view of the facts that (1) Ga is moderately incompatible during melting and (2) the peridotites studied by McKay and Mitchell are refractory, it is necessary to allow for the effects of melt extraction before the data can be used to discuss the issues relevant to the primitive mantle. By neglecting this procedure, the authors have reached some erroneous conclusions. McKay and Mitchell’s estimated mantle Ga content of 1.9 ppm is much too low; when the peridotite data are corrected for the effects of melt extraction, a figure of 3.9 ppm Ga is obtained. This value agrees with that of JAGOUTZ et al. (1979) and SUN ( 1982). A fundamental question is, “How representative of the upper mantle are the McKay and Mitchell samples?” Their low A1203 and SC contents indicate that these peridotites, especially the garnet-bearing ones, are depleted relative to a pyrolite and primitive mantle. It is therefore necessary to compare their compositions with those of other peridotites, to determine how well they represent the present upper mantle and how depleted they are relative to the primitive mantle. If peridotites are to be used to determine a primitive mantle abundance of Ga, what happened to them since accretion and core formation must be established. Most xenolith samples, including those studied by McKay and Mitchell, have lost a melt component. This is reflected in their low AO SUN,