The Curacautín ignimbrite: a rare look at the large-volume, explosive endmember of mafic volcanism
Abstract
Mafic volcanism accounts for 80% of magmas erupted on Earth. The majority of these eruptions are effusive to Strombolian; however, highly explosive mafic eruptions do occur. In fact, recent work by others suggests that all mafic magmas can erupt explosively. Therefore, highly explosive mafic eruptions should not be treated as anomalous events but instead as important systems that provide insight into mafic eruption dynamics. Llaima volcano in Chile is a Quaternary stratovolcano that produced a voluminous mafic explosive eruption in the late Pleistocene. Despite the far-reaching extent of these deposits, little has been done to investigate the conditions that drove such an unusual eruption. Here, we report on the Curacautín ignimbrite (Ci) at Llaima and compare it to similar well-documented mafic explosive eruptions to better understand these rare eruptions. The Ci consists of four flow units of m- to c-ash pyroclastic deposits. Previous studies mapped the Ci as two eruptions, but our 13C dates and new field observations suggest the Ci is the result of a single eruptive episode at ~12.6 ka. We estimate the bulk volume between 7.6 and 8.6 km3 (3.4-3.9 km3 DRE), which is ~2.9-3.3x smaller than previous estimates. Even with our lower estimate, the Ci is still larger than the Masaya Triple Layer, Pucón, Tarawera 1886, and Etna 122 BC mafic eruptions. Two types of juvenile clasts are observed: frothy microvesicular clasts and dense, radially jointed clasts. Clast agglutination is common and evident of high eruption temperatures. These characteristics, along with granulometry and ash morphologies, suggest fragmentation was magmatic rather than phreatomagmatic in nature. Vesicularities range from 32-48% suggesting high volatile contents were not the driver of the Ci eruption. The groundmass consists of high microlite number densities dominated by acicular plagioclase and tabular clinopyroxene. High microlite content is indicative of large degrees of undercooling from rapid magma ascent. We propose the Ci melt rose too fast to degas, and the resulting decompression-induced microlite crystallization increased melt viscosity enough for magmatic fragmentation. Because the primary driver of mafic explosive eruptions appears to be rapid magma ascent, eruptions similar to the Curacautín are possible at any mafic volcano.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMV010...05M
- Keywords:
-
- 8404 Volcanoclastic deposits;
- VOLCANOLOGY;
- 8428 Explosive volcanism;
- VOLCANOLOGY;
- 8445 Experimental volcanism;
- VOLCANOLOGY;
- 8486 Field relationships;
- VOLCANOLOGY