Primary Igneous Anhydrite: Progress Since Its Recognition In The 1982 El Chichon Trachyandesite

摘要

Primary igneous anhydrite was first identified in 1982 El Chichon pumices. Analysis of the sulfur budget for the eruption provided compelling evidence that the pre-eruptive magma contained a significant gas phase at ～ 7 km depth in order to account for the "excess gas release" of ～ 5-9 million tons of SO_2 to the stratosphere by the eruption. Primary igneous anhydrite and a larger "excess gas release" of ～20 million tons of SO_2 were noted for the significantly larger eruption of Mount Pinatubo in 1991, for which a separate gas phase at ～7-9 km depth was also required by the sulfur budget. Pumices from both eruptions have mineral assemblages dominated by plagioclase and hornblende, with minor biotite, and show evidence for co-nucleation and mutual inclusions of anhydrite and apatite. Both magmas were also very water-rich and highly oxidized, with oxygen fugacities $1 log unit above the synthetic Ni-NiO buffer. Furthering the similarities between these two eruptions, ion-microprobe analyses of sulfur isotopic compositions of anhydrites in pumices from El Chichon and Mount Pinatubo both showed that individual crystals are isotopically homogeneous, but inter-crystalline variations in δ~(34)S are well beyond analytical error. This study reviews the history of thought regarding primary igneous anhydrite. Ten volcanic and plutonic analogues to the El Chichon and Mount Pinatubo magmas are discussed. All were similarly water-rich and highly oxidized magmas. Hornblende is the dominant mafic phenocryst in all but one plutonic example; in that case so much early anhydrite precipitated that no calcium remained to form hornblende. Biotite is very abundant in that exceptional plutonic case, and is present in six of the other examples with primary igneous anhydrite. Sphene is known as an indicator mineral of high magmatic oxygen fugacity; it was present in the 1982 El Chichon pumices, and in 6 of the other anhydrite-bearing samples. Hornblende and biotite phenocrysts are also characteristic of the intermediate-to-felsic plutonic rocks that typically host porphyry-Cu deposits. Such magmas are also known to be water-rich and highly oxidized. A close spatial and petrogenetic connection exists between magmas with primary igneous anhydrite and magmatic-hydrothermal ore deposits.