Magma evolution during the last 23 ky at Popocatepetl Volcano: insights from Sr, Nd, and Pb isotopes in plagioclase, pyroxene and pumice matrix
Abstract
In the last 23 ky Popocatepetl, a dominately andesitic volcano, has erupted at least 5 large volumes of dacitic magma. A common question is how are these dacites related to the andesites and how silicic magmas are affected by magma mixing and crustal assimilation through its eruptive history. The isotopic composition of magma is the cumulative product of its genesis and evolution, including magma mixing and crustal assimilation. To clarify how magma evolution influence Popocatepetl magmatic system we did a textural analysis of plagioclase, measured their compositional variability, analyzed Sr, Nd and Pb isotopes on plagioclase, pumice groundmass, and pyroxene of dacitic Plinian eruptions from the last 23 ky and compare them with older andesites from the same magmatic system. Plagioclase textures and compositional transects were analyzed in order to investigate the evolutionary processes responsible for crystal growth. Sr, Nd, and Pb isotopes were analyzed for each major phase to corroborate observations from plagioclase textural-compositional analysis and to investigate which processes dominate the final isotopic composition of the magmas. Our results show that plagioclase from different eruptions record variable textures and compositions that can be explained by different thermo-compositional fluctuations in the magma reservoir. In order to investigate if magmas have been affected by compositional changes or just thermal fluctuations we analyzed their isotopic compositions. Isotopically, plagioclase, pyroxene and groundmass from dacites from Popocatepetl Plinian eruptions display considerable ranges in 87Sr/86Sr (.70395-.70457) and ɛNd (2.2-5.1), whereas Pb isotopic ratios display more narrow ranges (206Pb/204Pb 18.62-18.68, 207Pb/204Pb 15.56-15.62, 208Pb/204Pb 38.30-38.49). Those dacites younger than 5 ky all pyroxene, plagioclase and groundmass are in isotopic equilibrium for Sr and Pb, suggesting that sieve textures and compositional variability of plagioclase from these magmas were produced by thermal fluctuations with very little mixing or assimilation. Dacitic eruptions within the 14-23 ky range show isotopic disequilibrium for Sr between plagioclase and matrix,and equilibrium for Pb in one case suggesting that magma mixing and or assimilation were possibly involved before eruption. Andesitic magma older than 23 ky at Popocatepetl volcano (Sosa, unpublished M.S. thesis) display a range that overlaps those of the dacitic eruptions suggesting that mixing-assimilation processes have been volumetrically minor during differentiation.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V51A1658S
- Keywords:
-
- 8499 VOLCANOLOGY / General or miscellaneous