Graduate Thesis Or Dissertation
 

Variations in trace element abundances in small fragments from a single terrestrial basaltic flow : applications to 59 Apollo-17 mare basaltic fragments

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  • Eleven hand specimen whole rock (WR) samples (25-200 g each) and 33 fragments (0.5-0.8 g each) from a single basaltic flow at Picture Gorge, Oregon were analyzed via instrumental neutron activation analysis (INAA) for 12 elements (Na, Fe, Cr, Sc, Co, La, Sm, Eu, Yb, Lu, Hf and Ta). Standard deviation, range, and similarity coefficient were used to determine dispersions in element abundances of the samples. The results show only slight variations in composition among the samples suggesting that the flow is homogeneous with respect to the above element abundances. Effect of sampling is evident by the slightly less variability in composition of the powdered whole rock samples as compared to the chipped fragments. Petrographic studies of the whole rock samples also reveal their similarity to one another. All samples consist mainly of plagioclase (60%) and clinopyroxene (40%) with relatively small amounts of opaque oxides and glass. Phenocrysts and microphenocrysts make up about 1-4% of the rock samples. Although the groundmass grain size of the samples coarsen away from the cooling surface, they are nevertheless rather fine-grained (2 microns to 0.6 millimeter). The grain size and compositional data indicate that dispersions in trace element concentrations of fragments from a single flow are less pronounced for fine-grained basalts. On the basis of the above results, calibration of similarity coefficient using 12 elements implies that fine-grained 0.5-0.8 g samples from the same flow should give about 98% of the coefficients in the range 90-99% with about 75% of the coefficients in the 95-99% range. Application of this criterion to 59 fine-grained Apollo-17 mare basalts reinforces the suggestion that these basalts originated from three separate magma types (A, B and C) or flows, although the rare Type C basalts are not well defined in this paper.
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