Research Training Program

James Corboy
New Mexico Institute of Mining & Technology
Socorro, New Mexico

James F. Luhr, Ph.D.
Project Advisor
Department of Mineral Sciences

"Although my research topic was a totally new field of study and I had to do a lot of reading, it was a wonderful challenge. The RTP is a great opportunity and I would love to promote this program."

How Frequent Are High-Sulfur Eruptions?

After a high-sulfur eruption, like El Chichón and Mt. Pinatubo, very little sulfur remains behind in the volcanic deposits to identify the eruption as having a high-sulfur magma. While satellite measurements by TOMS of the El Chichón eruption determined total SO2 gas at around 5 Mt, estimates of the crystalline sulfur content of the magma was less than 1.3 wt. % (as SO3) was found as microphenocrysts of anhydrite in fresh pumice (Luhr, 1984). The problem is a majority of these anhydrite crystals were washed away after only the first raining season. The fact that anhydrite (CaSO4) is water soluble in surface waters makes it highly unlikely to be preserved in the geological record and therefore it has limited use as a petrological indicator of ancient high-sulfur eruptions.

Is there then another mineral that can be used to indicate if a volcanic magma was high-sulfur or low-sulfur in composition? The most likely candidate to display such a relationship is the mineral apatite [Ca5 (PO4)3 (F,C,OH)]. It is well known that sulfur can replace phosphorus (McConnell, 1938) in the tetrahedron site, this P5+ to S6 replacement is typically compensated by the coupled substitution of Ca2 by Na or it may be balanced by Si4 for P5. Furthermore, the apatite crystal would, like anhydrite (Luhr, 1984), most likely crystallize from the melt prior to eruption and thus contain a signature of the original magmatic composition.

This research was supported by a grant from the National Science Foundation Research Experiences for Undergraduates (Award: BIR-9300225).