MINERALOGICAL AND GEOCHEMICAL INDICATORS OF SUBAERIAL WEATHERING IN THE POZZOLANE ROSSE IGNIMBRITE (ALBAN HILLS VOLCANIC DISTRICT, ITALY)

Trends in geochemistry, mineralogy and clay development can help to identify processes that contribute to particular soil profiles as well as elucidate the processes that contributed to ancient soil development. This study uses geochemistry and mineralogy to explore paleosol development and chemical/ mineralogical changes within the Pozzolane Rosse ignimbrite (456±3ka) in the Alban Hills Volcanic District, Rome, Italy. These data contribute to ongoing geoarchaeological research on this important ignimbrite and reconstruction of the region’s mid-Pleistocene paleoclimate.

Pozzolane Rosse [PR] is found in the potassium-rich/silica-undersaturated Roman Comagmatic Province. Primary unaltered mineralogy is leucite, clinopyroxene, and biotite. Previous petrographic study of PR reveals vertically stratified alteration zones presumably produced by meteoric waters. Much of this ignimbrite was exposed ~40 ka prior to burial by the Pozzolane Nere ignimbrite at 407±2 ka; some localities show evidence for colluvial activity during this time. The purpose of this study is to determine if the alteration of PR carries distinct geochemical trends that could indicate aspects of incipient paleosol development or other mechanisms of alteration.

XRF, XRD and clay separation results from PR whole rock samples derived from boreholes at the Istituto Nazionale di Geofisica e Vulcanologia [INGV], at Santa Maria delle Mole, Italy [CA1], and from Castel di Leva quarry [Castel] show evidence of paleopedogenesis. All samples are compared to published analyses of so called PR juvenile scoria clasts. In the upper 2 to 5 m, CA1 and Castel display significant loss of base cations, up to 75 wt%, and INGV base cation loss up to 25 wt%. Loss of K is consistent with XRD data showing dissolution of leucite or its alteration to analcime. Accumulation of Al, 40-50 wt% in CA1 and Castel, and high L.O.I., support XRD evidence of 1:1 clay species at upper depth. This style of geochemical alteration contrasts with other observed PR intervals in which sulfides or their derivatives, low concentrations of major elements (Si, Al, Ti, Mg, Na, K) compared to high amounts of Fe, and very high L.O.I. (30+ wt%) are interpreted to represent hydrothermal alteration. Trends in all sampled areas suggest geochemistry data can be used to determine alteration extent.