Zircon in tephra as a novel tool to decrypt geologic and archaeological archives: a case study from Central America

点击翻译按钮获取中文摘要

Atitlan, Amatitlan, Coatepeque and Ilopango (Guatemala, El Salvador) are four neighboring calderas in the northern part of the Central American Volcanic Arc (CAVA). These calderas have produced numerous explosively erupted tephras which are regionally deposited on- and off-shore where they form geologically and archaeologically important marker horizons throughout the Pleistocene-Holocene. Their most recent eruption, the Tierra Blanca Joven event of Ilopango volcano at around 536 CE, was disastrous for the local Maya civilization and had tangible effects on global climate. Often indistinguishable chemical and mineralogical compositions and overlapping depositional distributions, however, have largely hampered unambiguous correlations between these tephras and their respective source calderas. In consequence, the temporal evolution of magma compositions and eruptive recurrence for these calderas, an important aspect for hazard analysis, remain incompletely understood. We propose to apply high spatial resolution analysis of zircon extracted from medial and distal tephra in comparison with zircon from proximal samples. The chemical stability of zircon is a major advantage compared to the more conventionally used analysis of volcanic glasses and less retentive minerals. This propensity of zircon is the key to meet the three major objectives of this proposal: (1) to reconstruct the temporal evolution of the Ilopango magma system and identify the roles of key magmatic processes (fractional crystallization, assimilation, and magma recharge) at high temporal resolution; (2) to complement and improve existing methods for tephra correlation by using the ages and compositions of dust-sized zircon as a fingerprint for distal tephras; and (3) to compare zircon ages from natural tephra deposits with those associated with volcanic ash used as a temper in late-classic Maya pottery with the goal to clarify the enigmatic provenance of these geomaterials in non-volcanic Maya habitats. The anticipated results are critical for better understanding magma origins and storage in magmatic arcs, as well as the crustal structure of the CAVA. Improved tephrochonology will permit geologic and stratigraphic correlations in terrestrial and marine depositional environments in the region, which can set a benchmark for other volcanic arcs. Lastly, the provenance of geomaterials in Maya pottery is a clue to re-evaluate existing models for Maya societal organization.