35. XRF ANALYSES OF VOLCANIC ROCKS FROM LEG 152 BY LABORATORIES IN EDINBURGH AND COPENHAGEN: IMPLICATIONS FOR THE MOBILITY OF YTTRIUM AND OTHER ELEMENTS DURING ALTERATION1Lotte Melchior Larsen,2 J. Godfrey Fitton,3 John C. Bailey,4 and Jørgen Kystol2 |
ABSTRACTNeighboring (parallel) samples from 72 lava flows, splits of identical samples from five lava flows, and splits of 10 shipboard powders from Ocean Drilling Program Leg 152 were analyzed by X-ray fluorescence (XRF) in both Edinburgh and Copenhagen. The agreement between the two data sets is very good, although systematic differences are apparent in some cases. Correlation between the results for the identical powders is very high. Correlation between the results for the parallel samples is somewhat lower, mainly because of element redistribution during secondary alteration, but also due to primary lava inhomogeneity. The elements show differences in the degree of scatter due to alteration, with K, Rb, and Ba being the most mobile, and Si, Sc, Al, Fe, Zn, V, and Nb the least mobile. Y is immobile in the Site 917 lavas but distinctly mobile in the Site 918 lavas where it is both leached and precipitated locally. The mobility of Y is related to secondary breakdown of clinopyroxene at Site 918, in contrast to Site 917 where the clinopyroxene is generally fresh. The hydrothermal alteration conditions at the two sites appear to have been different. |
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