TRACE-ELEMENT VARIATIONS INDUCED BY INCIPIENT WEATHERING OF VOLCANIC ROCKS: IMPLICATIONS FOR INTERPRETATIONS OF CENTRAL AMERICAN VOLCANISM

Trace-element signatures (correlations, variations, &c.) are used to infer along-arc, across-arc, and intravolcano magmatic phenomena, and spatial and temporal variation in those phenomena, in the Central American Volcanic Arc. Incipient weathering of volcanic rocks is known to produce variations in trace-element abundances (Patino & al., 2003). This study examines trace-element and REE variations thought to be of significance for interpreting petrogenetic processes in the Central American Volcanic Arc and compares them with those produced by incipient weathering of island-arc volcanic rocks at Volcan Tecuamburro and Volcan Moyuta in southeast Guatemala, and similar weathering of intraplate volcanic rocks from Oahu and Hawai'i, to establish the extent to which incipient weathering might have superimposed a non-petrogenetic overprint on elemental abundances of petrogenetic importance.

Along-arc and across-arc variations in Ba/La, and La/Yb, along-arc variations in U/Th, and across-arc variations in Ba/Th, have been observed and attributed to variations in the contribution of subducted sediment to the magma. Ba/La is invariant with modest degrees of weathering. However, U/Th and Ba/Th tend to decrease and increase (respectively) slightly but systematically with incipient weathering, and La/Yb increases in some incipiently weathered sample suites. Along-arc petrogenetic trends in Ce/Pb vs. Ba/La, and across-arc trends in Ba/La vs. La/Yb and U/La vs. Ba/Th (attributed to the influence of fluid flux on melting), appear unaffected by incipient weathering. Differences in Ce/Pb vs. Ba/La between Miocene-Pliocene and Pleistocene-Holocene volcanic rocks, and variations in V. Arenal and V. Telica in La/Th vs. U/La, and Ba/Th vs. U/La, are all larger than the variations in the trace-element ratios associated with incipient weathering.

Early stages of volcanic-rock weathering produce variations in REE and other trace elements. However, incipient weathering of volcanic rocks does not result in large enough variations in REE and other trace elements to require modification of published interpretations of along-arc, across-arc, temporal, and intravolcano trace-element variations in Central American volcanic rocks.