Magma Mixing, Recharge and Eruption Histories Recorded in Plagioclase Phenocrysts from El Chichón Volcano, Mexico

JOURNAL OF PETROLOGY VOLUME 41 NUMBER 9 PAGES 1397–1411 2000 Magma Mixing, Recharge and Eruption Histories Recorded in Plagioclase Phenocrysts from El Chichón Volcano, Mexico F. J. TEPLEY III1∗, J. P. DAVIDSON1, R. I. TILLING2 AND J. G. ARTH2 1 DEPARTMENT OF EARTH AND SPACE SCIENCES, UNIVERSITY OF CALIFORNIA, LOS ANGELES, LOS ANGELES, CA 90095, USA 2 US GEOLOGICAL SURVEY, MENLO PARK, CA 94025, USA RECEIVED JULY 7, 1999; REVISED TYPESCRIPT ACCEPTED JANUARY 25, 2000 Consistent core-to-rim decreases of 87Sr/86Sr ratios and coincident increases in Sr concentrations in plagioclase phenocrysts of varying size (>1 cm to 2 mm) are reported from samples of the 1982 and pre-1982 (>200 ka) eruptions of El Chichón Volcano. Maximum 87 Sr/86Sr ratios of >0·7054, significantly higher than the wholerock isotopic ratios (>0·7040–0·7045), are found in the cores of plagioclase phenocrysts, and minimum 87Sr/86Sr ratios of >0·7039 are found near some of the rims. Plagioclase phenocrysts commonly display abrupt fluctuations in An content (up to 25 mol %) that correspond to well-developed dissolution surfaces. The isotopic, textural and compositional characteristics suggest that these plagioclase phenocrysts grew in a system that was periodically recharged by higher-temperature magma with a lower 87Sr/86Sr ratio and a higher Sr concentration. Rim 87Sr/86Sr ratios in plagioclase phenocrysts of rocks from the 200 ka eruption indicate that, at that time, the magma had already attained the lowest recorded 87Sr/86Sr value of the system (>0·7039). In contrast, cores from plagioclase phenocrysts of the 1982 eruption, inferred to have grown in the past few thousand years, have the highest recorded 87 Sr/86Sr ratios of the system. Collectively, the Sr isotopic data (for plagioclase and whole rock), disequilibrium textural features of the phenocrysts, known eruption frequencies, and inferred crystalresidence times of the plagioclases are best interpreted in terms of an intermittent magma chamber model. Similar processes, including crustal contamination, magma mixing, periodic recharge by addition of more mafic magma to induce plagioclase disequilibrium (possibly triggering eruption) and subsequent re-equilibration, apparently were The bulk compositions of volcanic rocks typically represent the integrated effects of fractional crystallization, magma mixing and contamination acting on magmas since they originally separated from their source. However, to characterize magma sources and magma-generation mechanisms, a more complete understanding and quantification of these processes and the timescales over which they operate is required. Bulk isotopic analyses of magmatic differentiates provide only an ‘averaged’ result that is unlikely to identify end-member compositions, and are difficult to interpret. Isotopic signatures of endmembers may be preserved in early-formed crystals from magmas that have subsequently hybridized, melt inclusions from precursor, less-differentiated magmas, or xenocrysts from wallrock assimilants (Davidson et al., ∗Corresponding author. Telephone: +1-310-825-3880. Fax: +1-310825-2779. e-mail:  Oxford University Press 2000 operative throughout the 200 ky history of the El Chichón magma system. KEY WORDS: El Chichón Volcano; magma mixing; microdrilling; pla- gioclase zonation; recharge magmas INTRODUCTION JOURNAL OF PETROLOGY VOLUME 41 1998; Knesel et al., 1999; Tepley et al., 1999). Crystalisotope stratigraphy (textural, chemical and isotopic analysis of single crystals and growth zones within crystals) is especially successful in identifying the endmembers involved and recording the pathways of interactions. Here, we report results of crystal-isotope stratigraphy studies on several plagioclase phenocrysts in rocks from an eruption >200 ky ago and one in 1982 from El Chichón Volcano. Previous work has shown that El Chichón magmas are remarkably similar to one another throughout the entire known span of eruptive activity (276 ka to 1982; Table 1); whole-rock Sr and Nd isotopic compositions plot in a very small compositional field (Rose et al., 1984; McGee et al., 1987; Tilling et al., 1987; Tilling & Arth, 1994; Fig. 1). However, phenocryst phases are in isotopic disequilibrium with their host rock, with their 87Sr/86Sr ratios increasing in the order from groundmass to clinopyroxene, apatite, anhydrite, bulk rock and plagioclase (Tilling & Arth, 1994). Crystal-isotope stratigraphy of plagioclase phenocrysts discussed below has revealed systematic core-to-rim decreases in 87Sr/ 86 Sr ratios, increases in Sr concentration, large variations in An content and obvious textural discontinuities. Using these isotopic, compositional and textural features, it is our goal to unravel the complex story of disequilibrium evolution of the El Chichón magmatic system. El Chichón is a small trachyandesite volcano located in the state of Chiapas in southeastern Mexico (Fig. 2). This volcanic edifice forms part of the Chiapanecan Volcanic Arc, a NW-trending volcanic zone of Pliocene to Recent volcanic centers. It is situated near the junction of the North American plate and the Caribbean and Cocos plates (McGee et al., 1987), which constitutes a region of complex tectonic structure, possibly producing magmatism associated with a subduction and/or a transform fault setting (Duffield et al., 1984). The volcano overlies a Jurassic to Miocene sequence of carbonate, sandstone and evaporite deposits. El Chichón is a complex of trachyandesitic, plagioclaseporphyritic domes and associated pyroclastic flow and airfall deposits (McGee et al., 1987). In addition to plagioclase, phenocryst phases include hornblende, and lesser amounts of clinopyroxene, biotite, quartz, apatite and anhydrite (observed in fresh samples from the 1982 eruption). The juvenile products of the volcano have essentially maintained the same chemical bulk composition during the past 0·3 My (Rose et al., 1984; McGee et al., 1987; Tilling et al., 1987; Table 1). El Chichón has erupted at least 11 times within the past 8000 years with repose times of 100–600 years (Tilling et al., 1984; Espı́ndola et al., 2000). The oldest SEPTEMBER 2000 Table 1: Chemical and isotopic analyses of pre-1982 and 1982 whole-rock samples illustrating the similarity in chemical composition and diversity in isotopic composition despite >276 ky between eruption times Sample (Pre-1982) 9 (1982) 5a SiO2 57·8 55·7 Al2O3 18·2 18·2 Fe2O3 6·23 6·37 MgO 2·21 2·21 CaO 7·04 7·92 Na2O 4·07 4·09 K2O 2·73 2·72 TiO2 0·64 0·65 MnO 0·17 0·17 P2O5 0·32 0·35 SO3 0·02 0·65 Total 99·50 99·15 Sr (ppm) 1200 1200 Ba (ppm) 800 800 87 GEOLOGIC SETTING NUMBER 9 Sr/86Sr 0·70439 0·70406 Sample 9 is from a lava dome or flow in the wall of the summit crater and dated by K–Ar yielding a whole-rock age of 0·27 ± 0·006 Ma (Duffield et al., 1984). Sample 5a is a bread-crust bomb on top of a pyroclastic flow southwest of t (...truncated)