EFFECTS OF INCIPIENT WEATHERING IN THE COMPOSITION OF BASALTS AND ANDESITES FROM GUATEMALA

Rocks from Tecuamburro and Moyuta Volcanoes in Guatemala were studied to document chemical changes in the early stages of weathering. The samples were collected from outcrops and large boulders that represent lava flows that range in age from 0.8 to 2.6 Ma (for Tecuamburro). Samples contain outer weathered rinds and fresh inner cores, which were analyzed individually.

Bulk density, chemical and petrographic analyses were used to verify that the samples were in the early stages of alteration. The bulk density of the rinds is consistently lower than that of the cores, but the overall ranges are very similar. To infer the extent of weathering of the Guatemalan samples, we used the Weathering Index of Parker (1970). Both rinds and cores have Parker weathering indices that range from 60 to 80. A similar range is observed in fresh volcanic rocks from the Central American arc. Although there are not large differences in the weathering indices between the rinds and the cores, there are differences in the chemical composition of the samples.

To study the chemical variations between the rinds and the cores, we used the isovolumetric approach proposed by Gardner et al. (1978), where the concentrations are converted to volumetric concentrations. In most instances, the volumetric concentrations of the rinds are lower than the cores. No one major element can be assigned as immobile for all the units. Aluminum (traditionally assumed to be immobile) is being mobilized at similar rates in the samples from the different units; it shows a tight linear arrangement in weathering progress diagrams. Easily mobile elements like calcium are lost at different rates; the trends in the weathering progress diagrams have different slopes. The behavior of trace elements is variable. Some elements (Sr) behave like calcium. Other trace elements, like La, can be considered immobile in some units, but highly mobile in others.

During incipient weathering the differences in vesicularity are not the controlling factor in element mobility. This was evident when comparing the composition of samples from two different units that were collected from lava flows that had different vesicularities at the outcrop scale. The changes in the composition during the early stages of weathering seem to depend on petrographic characteristics of the samples.