New Mexico Geological Society Annual Spring Meeting — Abstracts

The Tshirege member (Qbt) of the Bandelier Tuff is the youngest (ca. 1.0 Ma) large volume volcanic rock in the Jemez Mts.of northern New Mexico. Complete exposures reveal that Qbt was deposited from multiple pyroclastic flows fed by tephra fountains associated with the collapse of the Valles caldera. Analytical data show Qbt as a porphyritic, dominantly high silica rhyolite with phenocrystic sanidine (Or45) and low quartz.

In all exposures visited, Qbt consists of a lower vitric zone (glassy pumice and shards) overlain by a secondarily crystallized (sillar) zone of silicate minerals in place of glass. Powder XRD and electron microprobe analyses indicate the sillar assemblage comprises mainly micron size crystals of low cristobalite and a scapolite (tentatively identified as marialite). The abundance of marialite has two implications. The first is that marialite crystallized instead of albite in the sillar assemblage due to the high chlorine content (ca. 2000 ppm) of the melt. Another is that marialite may be common as a sillar mineral in chlorine rich rhyolitic ignimbrites. Indeed, the mineral is reported to occur in the Bishop Tuff (California) and is perhaps present in the Rhyolite Canyon Tuff (Arizona).

Field and experimental observations have furtherimplications for secondary mineral paragenesis. First, the preponderance of delicate, but intact, crystallized pumice suggests negligible post- to syncrystallization strain. That is, secondary crystallization is post-depositional and the deposit suffered no appreciable compaction or welding after crystallization. Second, a thick (~ 2.0 m) unconsolidated vitric zone, devoid of secondary minerals, overlies the sillar zone in an exposure in the town of Los Alamos. This field relation suggests that precipitation of "vapor-phase" minerals by upward percolating gases is insignificant at that exposure and perhaps throughout Qbt. Finally, experiments show the time scale for crystallization of a cooling silicate melt is roughly the same as for heat conduction in a porous silicate medium. Therefore, under plausible cooling conditions, the amorphous component (pumice and shards) in hotter parts ofI the deposit possibly crystallized without reaching the metastable glassy state. If so, use of the term 'devitrification' is inappropriate in this case.