PRESSURE AND TEMPERATURE EVOLUTION OF THE CHAOS CRAGS MAGMATIC SYSTEM, MT LASSEN; INSIGHTS FROM CL TEXTURES AND TRACE ELEMENTS IN QUARTZ PHENOCRYSTS

The 1,100-year old Chaos Crags eruptive sequence is preserved as a group of rhyodacite to dacite lava domes and associated pyroclastic deposits in the northwestern corner of Lassen Volcanic National Park, California. The volcanic sequence was produced after the addition of a mafic magma to the felsic magmas below Mt. Lassen. The relationship between the timing of the mafic magma injection and the Chaos Crags eruptive sequence is not clear.

To better constrain the thermal history of magma intrusion and eruption, we are collecting color scanning electron microscope-cathodoluminescent (CL) images of quartz phenocrysts from several of the Chaos Crags units. The CL images, combined with in-situ quartz trace element analyses will yield insight into the temperature and pressure variations throughout the magmatic history during quartz growth.

CL reveals otherwise unobservable growth zonations in quartz pheoncrysts that are parallel to external crystal faces. The oscillatory zonations vary in their intensity of blue CL and apparently do not show any consistent pattern within a single phenocryst or from phenocryst to phenocryst. CL intensity in the quartz phenocrysts is directly correlated with Ti concentrations, which are consistently between 75 and 100 ppm in the phenocrysts examined to date. Like CL intensity, there is no consistent pattern of increase or decrease in Ti concetrations from the core to the rim. No CL-bright, Ti-rich quartz rims have yet been observed that might indicate heating of the felsic magma immediately prior to eruption.

Using the Ti in quartz thermobarometer, these Ti concentrations suggest crystal growth conditions between 1.5 kbars at ~700 degrees and ~5 Kbars at 800 degrees, with oscillating, rather than progressive changes in temperature and pressure conditions. Our preliminary results, presented here, suggest higher pressures or lower temperatures than have previously been suggested for the eruptive conditions of the Chaos Crags sequence. Further investigation of quartz phenocrysts from Mt, Lassen will reveal whether there is a progressive change in magma temperature through time recorded by the quartz phenocrysts and whether all quartz phenocrysts reflect similar conditions of growth.