PETROGENETIC PROCESSES IN THE MAGMA CHAMBER BENEATH FURNAS VOLCANO, SAO MIGUEL, AZORES

Furnas volcano is an active trachytic stratovolcano on the island of Sao Miguel, Azores. Over the last 5,000 years, there have been 10 highly explosive eruptions from Furnas, including the most recent in 1630 AD. In the last 1200 years, Furnas has erupted five times suggesting a recurrence interval of 240 years. Since 375 years have passed since the last eruption, Furnas may be overdue to erupt. There are 3,000 people who live in the village of Furnas and several thousand in the immediate vicinity who are at risk if an eruption were to occur. In order to better understand the nature of the magmatic processess leading to the highly evolved and explosive eruptions of Furnas volcano, we have analyzed samples from the Furnas C eruption (80 AD) and 1630 AD eruptions. These data represent the first comprehensive geochemical and isotopic study of Furnas volcano.

A complete suite of samples has been collected from throughout the Furnas C and Furnas 1630 AD deposits. In both deposits, all samples are trachytic. Major element variability throughout the eruptive sequences in both Furnas C and 1630 AD is limited, with SiO2 ranging from 57.24 to 63.59 wt%. In 1630 AD, the trace elements exhibit significant chemical variability: Zr= 850 -1670 ppm, Nb= 164-268 ppm, Rb= 157-251 ppm, La= 135-193, Nd=104-131 ppm. Furnas C exhibits similar concentrations of these elements with the exception of Zr, which ranges from 1390-1765 ppm. In 1630 AD, the ~2-fold variation in Zr suggests that the samples may be related by ~50% fractional crystallization. In both Furnas C and 1630 AD, the Sr and Ba concentrations are extremely low with Sr ranging from undectable to 12 ppm and Ba from 2 to 43 ppm. The samples are light rare earth enriched with negative Eu anomalies that increase with increasing Zr, consistent with extensive feldspar (sanidine) fractionation. Unlike many chemically zoned volcanic deposits, the chemical variability in Furnas C and 1630 AD is not stratigraphically controlled. Non-systematic chemical variations within and between tephra layers may be due to the complex manner in which the magma chamber is being tapped and erupted.

New microprobe and Sr, Nd, and Pb isotope data will be presented, with quantitative modeling of potential open and closed system magmatic processes based on these analyses.