Magma Recharge and Emerging Heterogeneity in Prolonged Strombolian and Vulcanian Eruptions
Abstract
Volcanoes that erupt mafic to intermediate compositions frequently show prolonged eruptive phases of months to years after decades or more of repose. Such eruptions are characterized by quasi-continuous strombolian and vulcanian behavior. Understanding the driving force that sustains this prolonged activity is important for hazard mitigation and may provide important insights into our knowledge of the shallow crustal plumbing system of such volcanoes. While volcanic gases may be indicative of magma recharge, gas measurements are still relatively rare and absent for many past eruptions. Textural and geochemical studies of crystal zoning are an alternative to derive time scales and frequencies of magma recharge. In order to identify distinct magma batches and their temporal relationship to the eruptive sequence, we have obtained by LA-ICPMS multi-element zoning patterns for olivines from the VEI 3, 1963-65 eruption of Irazu volcano (Costa Rica) for elements that span a wide range of diffusivities (Fe-Mg, Mn, Li, Ca, P, Sc, Ti V, Cr, Mn, Fe-Mg, Co, Ni, Zn). The 1963-65 eruption is characterized by vulcanian and strombolian phases involving basaltic andesites of fairly uniform composition (54-57 wt% SiO2). In three samples spanning most of the eruption we identify olivines (Fo70-91) of different origin (mantle-derived, crustal cumulates and 1963-65 growth). The initial phase is dominated by mantle-melt derived olivines with a thin low Fo rim consistent with decompression growth over a few weeks and a slightly normal zoned interior (Fo84-90) resulting from an extended time of diffusive exchange with intermediate compositions on the order of tens of years - a time scale comparable to the repose time of Irazu volcano. Thus, the low amplitude variations observed in the interior of the olivines may record previous recharge activity during the last eruptive phase in 1939-40. The later eruptive sequence of the 1963-65 eruption continues to have olivines from the early stage. However, these mantle-melt derived olivines show more variability in their zoning profile than in the early part of the eruption. While they may continue to have thin growth rims indicating continued magma recharge, they also may develop much thicker rims suggesting prolonged residence and growth of olivines from earlier magma recharge (on the order of months to years at intermediate growth rates of 1e-6 mm/s). In addition to the added complexity of the primitive olivines, other olivines (with < Fo80 cores and anomalous trace element compositions: e.g., high Ni) that have waxing and waning proportions in the later part of the eruption suggest either transient other recharge magmas and/or remobilization of olivine cumulates. Thus, the phenocrysts record evolving magma heterogeneity within the magma plumbing system on the crystal-scale due to continued magma recharge, while the bulk magma is relatively insensitive to these variations. Furthermore, it is unusual for an arc volcano to tap primary mantle melts in their recharge signaling the absence of a deep plumbing system. The 1963-65 eruption of Irazu can therefore potentially be used to constrain mantle melt fluxes through the erupted crystal cargo.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.V31H..03R
- Keywords:
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- 1042 GEOCHEMISTRY / Mineral and crystal chemistry;
- 3618 MINERALOGY AND PETROLOGY / Magma chamber processes;
- 8439 VOLCANOLOGY / Physics and chemistry of magma bodies