Chemistry of the lavas and tephra from the recent (A.D. 1631-1944) Vesuvius (Italy) volcanic activity (original) (raw)

INTRODUCTION AND GEOLOGIC SETTING The A.D. 1631 eruption of volcano Vesuvius, in Naples, Italy, began more than 300 years of nearly constant eruptive activity. The succeeding activity was predominately effusive with only minor pyroclastic events usually at the end of an eruptive cycle. This continuous, relatively low energy activity made Vesuvius an ideal laboratory for volcanologists and petrologists until its last eruption in AD. 1944. The literature pertaining to Vesuvius begins with Pliny the Younger's letter to Tacitus describing the A.D. 79 "Pompei" eruption and continues today as new studies continue to add to the body of work used to interpret and model its past and perhaps future activity. Systematic study and chemical analysis of the Vesuvius products began with Johnston-Laves (1884) and Washington (1906), respectively. Recent studies are discussed by Barberi et al., (1981) and Santacroce (1983). Modern analytical work is reviewed by Joron et al. (1987). This report presents the comprehensive chemical database which has been determined as part of an ongoing study (Belkin et al., 1991; Belkin et al., 1993) of the petrogenesis of the recent AD. 1631-1944 Vesuvius activity. The data generated by such an analytical program will provide a base for use in petrologic analysis as well as in derivative and complementary studies such as detailed modal and chemical petrography and isotopic analysis. The importance and utility of such a unified data set arises from the fact that although Vesuvius has been extensively studied, few workers have presented complete analyses. Mt. Somma-Vesuvius is part of the Roman potassic province of Washington (1906) located east of Naples, at the southern boundary of the Campanian plain. Mt. Somma-Vesuvius is a composite volcano that has erupted silica-undersaturated and potassium-rich lavas and pyroclastics for at least 25,000 years. The eruptive history of Somma-Vesuvius can be divided into three periods; (1) the early historic period before the AD. 79 "Pompei" plinian eruption, (2) the middle period covering A.D. 79 to 1631 and (3) the recent period of activity from A.D. 1631 to 1944. The most recent eruptive period represents an almost continuous series of mild, mostly effusive lavas ranging in composition from phonolitic-leucitite to tephritic-leucitite. The average SiO2 content is 48.0 wt.% and the rocks are classified as tephriphonolites according to their alkali (I^O + Na2O) content. All of the lavas are silica-undersaturated and are nepheline, leucite, and olivine normative. The proximity of Vesuvius to the resources and study of renaissance and postrenaissance Europe plus recent detailed mapping (Rosi et al., 1987) has provided historical documentation for the reconstruction of this recent period. We have used as a base for sampling (figs. 1, 2, 3, and 4) the recent map of Rosi et al., (1987). A recent compilation and discussion of the eruptive history, chemistry, petrography, and geophysics of Mt. Somma-Vesuvius can be found in Santacroce (1987) and references therein. All the flows consist of moderately viscous lavas with either aa or pahoehoe surfaces. They are homogeneous in appearance, either vesicular or massive, and phaneritic with well developed leucite or pyroxene phenocrysts. Arno et al., (1987) has divided the recent period activity into 18 eruptive cycles. These cycles start with the A.D. 1638 effusive activity. The question of the existence of lavas erupted with the A.D.