Continuous mixing of crystal mush and replenished magma in the ongoing Unzen eruption

Abstract

Research Article| September 01, 1995 Continuous mixing of crystal mush and replenished magma in the ongoing Unzen eruption Michihiko Nakamura Michihiko Nakamura 1Department of Earth and Planetary Sciences, Faculty of Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152, Japan Search for other works by this author on: GSW Google Scholar Author and Article Information Michihiko Nakamura 1Department of Earth and Planetary Sciences, Faculty of Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152, Japan Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1995) 23 (9): 807–810. https://doi.org/10.1130/0091-7613(1995)023<0807:CMOCMA>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Michihiko Nakamura; Continuous mixing of crystal mush and replenished magma in the ongoing Unzen eruption. Geology 1995;; 23 (9): 807–810. doi: https://doi.org/10.1130/0091-7613(1995)023<0807:CMOCMA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Zoning profiles of magnetite phenocrysts in the successively effused dacite of the ongoing Unzen eruption from 1991 wereanalyzed to estimate the time scale of magma mixing. The dacite was formed by mixing of relatively high- and low-temperature (T) end-member magmas. The magnetite phenocrysts derived from the low-T magma are reversely zoned by the mixing with high-T magma. A diffusion calculation for reequilibration of the reverse zonings gives the time interval from magma mixing to quenching. For the mixed dacite erupted from May 1991 to May 1993, the typical diffusion time was estimated to be a few months regardless of the effused sequence for 2 yr. This indicates that the mixing was continuous during the effusion. The invariability of the other mixing signatures, such as the thickness of reaction rims around biotite phenocrysts, also supports the continuous mixing model. Low-T end-member magma is estimated by mass-balance calculation to be a crystal-rich mush of dacitic composition. These observations lead to a model wherein the highly crystallized remnant magma of the preceding activity has been mixed with the newly injected hot magma of similar bulk composition just prior to the effusion. The proposed mechanism implies that this type of magma mixing is an inevitable process in periodically erupting polygenetic volcanoes. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.