Petrogenesis of Ultramafic Xenoliths from the 1800 Kaupulehu Flow ...

ULTRAMAFIC XENOLITHS FROM THE KAUPULEHU FLOW 191To evaluate the bearing of the REE data on the cumulus vs. postcumulus origin of theclinopyroxene, we examine the consequences of assuming, contrary to the phase equilibriumand textural evidence presented above, that the clinopyroxene in all the xenoliths ispurely cumulus. On this assumption, distribution coefficients can be used to calculate REEpatterns for magmas that crystallized the clinopyroxenes, and these patterns can becompared with those of alkalic and tholeiitic lavas from Hualalai. For these calculations, weuse the preferred distribution coefficients (D) of Frey et al. (1978). As pointed out by Frey etal, these coefficients are near the lower limit of published values, but are preferred becauseany contamination of analyzed mineral grains would lead to high values. Also, the Frey etal. preferred values are consistent with experimentally determined values of 0-19 for D Yb(Watson et al, 1987), 0-16 for D Gd (Green et al, 1971), and 0-13 for D^ (Ray et al, 1983).Figure 9 shows that the calculated REE patterns of presumed magmas that crystallizedclinopyroxenes in the wehrlite and clinopyroxenite samples are approximately parallel tothose of Hualalai alkalic lavas (Clague et al, 19805) but are at higher concentrations.Because REE concentrations of Hawaiian tholeiitic lavas typically are lower than those ofHawaiian alkalic lavas, the match of the calculated liquids with tholeiitic lavas would beeven worse. One might argue that the presence of phenocrysts in the lavas analyzed by20010lavas (7 samples)Kaupulehu flowCe Nd Sm Eu Gd Dy Er YbFIG. 9. Calculated REE patterns for liquids in equilibrium with Kaupulehu wehrlite and clinopyroxenitexenoliths based on the preferred distribution coefficients of Frey et al. (1978). Data for Hualalai mafic alkalic lavasare seven samples from Clague et al. (1980a) and two samples from Lofgren et al. (1981).