CLAY MINERALS IN VOLCANIC ASHES OF CENTRAL ITALY (COLLI ALBANO AND MONTI SABATINI): INSIGHTS INTO QUATERNARY WEATHERING AND ALTERATION

The volcanic stratigraphy of the region around Rome hosts a complex history with multiple volcanic centers and eruptive episodes. Central Italian magmas are generally ultrapotassic, with violent explosive eruptions. Here we report preliminary findings on the clay mineralogy of several volcanic ashes in the Roman region, principally from the Alban Hills and Monti Sabatini volcanic centers. Paleosols are variably preserved, in some cases providing primary evidence of earth-surface weathering during the time of exposure.

We extracted clay minerals from a number of altered volcaniclastic units ranging in age from ~800 ka to ~450 ka. Bulk samples were soaked in deionized water, sonicated, and centrifuged to extract the size fraction finer than 0.5µm. Extracts were oriented on glass slides and analyzed by X-ray diffraction air-dried, ethylene glycol solvated, with n-formamide treatment, and with heating to 500˚C. Bulk analyses were also done with randomly oriented powders.

The younger deposits (<~500 ka) are dominated by halloysite, with minor kaolinite content. 1:1 clays are absent from the older deposits, which instead are dominated by 2:1 clays such as smectite and minor illite. This suggests that the younger deposits are significantly more weathered. This is consistent with supporting bulk mineralogical and geochemical analyses showing better preservation of primary volcaniclastic feldspars and feldspathoids, and the occurrence of secondary calcite in the older deposits. Also, we have described paleo-ultisols in the younger deposits, although comparitive paleopedological analyses of the older deposits has not yet been carried out. This is also broadly consistent with Quaternary terrestrial records of generally increasing precipitation on the Italian peninsula, with glacial-interglacial cycles superimposed on the trend.

An alternative explanation is that exposure times, and hence degree of weathering, were greater for the younger eruptive deposits. Ongoing detailed stratigraphic analysis, as well as continued mineralogical and geochemical analyses of the clays and other fractions, will continue to address these questions.