Vapor Saturation as The Cause of Volcanic Eruptions at the Lassen Volcanic Center, California, as Inferred from Crystallization Pressures and Temperatures

Abstract:

The last three silicic eruptions at the Lassen Volcanic Center occurred at Lassen Peak (27 ka and 1915-17) and Chaos Crags (1103 yrs BP). Klemetti and Clynne (2014) showed that felsic eruptions at Lassen reflect remobilization of resident rhyodacitic crystal mush by intrusion of mafic magma. To better understand the rejuvenation and eruption triggering process, we calculate crystallization temperatures and pressures from clinopyroxene-liquid equilibria on mafic enclaves that provide our closest approach to the composition of mafic magmas delivered to the shallow system. Our goal is to examine whether and to what extent cooling and crystallization occur after recharge, which bears on whether recharge, mixing, or partial crystallization (and consequent vapor saturation) provide the trigger for eruption. We use results from the cpx-liq barometer (1.7 kbar) as input to calculate T for other phases (plagioclase, olivine and amphibole) found in mafic enclave samples. Cpx crystallizes at 1100-1150 ^oC and olivine precipitates at similar to slightly higher temperatures. Cpx and ol are followed by plagioclase (1000-1050 ^oC), amphibole (875-1000 ^oC), and Fe-Ti oxides (1030-1050 ^oC). These temperatures indicate that recharge magmas are incompletely crystallized as they enter the shallow reservoir of cooler (~725-750 ^oC, Quinn et al., 2013) felsic crystal mush, and that significant cooling of the mafic magma occurs during mixing and prior to eruption. Such cooling intervals indicate that recharge is not the proximal cause of eruption, but rather that vapor saturation, following a period of mixing and cooling, leads to increased magma overpressure that causes eruption. Interestingly, the Lassen Peak 27 ka volcanics (at 2.09 km³), have a greater volume than either of Chaos Crags (1.2 km³) and the 1915 (0.03 km³) eruption, but our results indicate that their thermal histories are similar. This suggests that while volumes of mafic recharge may control the degree of interaction with felsic crystal mush (e.g., Clynne, 1999), and eventually the volume of erupted magma, recharge volumes do not affect eruption triggering mechanisms.

Clynne MA (1999) J. Petrol, 40: 105–132.

Klemetti and Clynne, 2014, PLoS One (/12:e113157. doi10.1371/journal.pone0113157)

Quinn et al., 2013, AGU Fall Annual Meeting V23C-2848