Cristina de Ignacio

Mineralogical and geochemical constraints for the origin and evolution of magmas in Sierra Chichinautzin, Central Mexican Volcanic Belt

Abstract The Sierra Chichinautzin Volcanic Field is a key area for understanding the origin of the controversial Mexican Volcanic Belt (MVB). This is due to its recent volcanic activity, which is located at the front of the Central MVB. Although a direct genetic relationship between the volcanism of the MVB and the subduction of the Cocos plate is accepted by most authors, geological, geophysical and geochemical key features of the MVB depart from those of typical magmatic arcs. This fact has promoted a debate on the nature of the complex mantle beneath the MVB: (1) slab-induced convection in the mantle wedge that causes advection of asthenospheric mantle and (2) anomalously hot mantle related to the eastward channeling of plume-related material. This is a petrological study of the Sierra Chichinautzin volcanism, using mineralogical data from selected representative samples, together with their geochemistry, to refine the existing models of magmatic processes. The integrated petrological analyses have been used to define four main groups of rocks in Sierra Chichinautzin: (1) Ol-mafic rocks, (2) Opx-intermediate rocks, (3) Opx-felsic rocks, and (4) Qtz-intermediate/felsic rocks. Moreover, two different types of mantle-derived primitive mafic magmas have been described based on their mineralogical and geochemical features: (1) OIB-like mafic magmas (presence of olivine+plagioclase phenocrysts; higher incompatible element contents with no HFSE/LILE negative anomalies), and (2) mafic magmas derived from a metasomatized mantle (no plagioclase phenocrysts and more forsteritic olivine; HFSE/LILE negative anomalies). We prove that the diversity of magmas (basalts to dacites) in Sierra Chichinautzin is un-related to fractional crystallization processes from the mafic magmas. Rather, magma mixing phenomena between mafic magmas and two different crustal-derived felsic magma types (Opx-dacite and Qtz+Bt+Amp+Pl+Cpx+Opx-dacite) explain the textural, mineralogical, and geochemical signatures of most of the Sierra Chichinautzin rocks. There is a strong genetic relationship between the mafic and felsic end members of each mixing trend. The relatively anhydrous OIB-like mafic magmas promote partial melting of a basaltic lower crust under low water fugacity conditions, generating Opx-bearing felsic rocks. On the contrary, hydrous mafic magmas derived from a metasomatized mantle, promote basaltic crustal partial melting under high water fugacities, yielding magmas with a complex hydrous phenocryst assemblage. Several important geodynamical implications for the origin of the MVB magmatism can be extracted from this petrological study: (1) the existence of two different primitive mafic magmas implies the presence of an heterogeneous mantle source beneath Sierra Chichinautzin; (2) present data do not offer enough evidence to constrain the nature of the metasomatizing fluids beneath Sierra Chichinautzin; and (3) the negative Nb anomaly of most of the Sierra Chichinautzin rocks (andesites and dacites) is due to magma mixing, and therefore is not a signature of the role of fluids from the subducting Cocos plate. These geochemical features, together with the geophysical and tectonic scenario of the Sierra Chichinautzin, indicate that volcanism in this area is mainly related to the upwelling of an anomalously hot OIB-like mantle, and cast further doubts on a significant role of the subducting Cocos plate in the origin of this magmatism.

El Chichón Volcano (Chiapas Volcanic Belt, Mexico)Transitional Calc-Alkaline to Adakitic-Like Magmatism:Petrologic and Tectonic Implications

The rocks of the 1982 eruption of El Chichón volcano (Chiapas, Mexico) display a series of geochemical and mineralogical features that make them a special case within the NW-trending Chiapas volcanic belt. The rocks are transitional between normal arc and adakitic-like trends. They are anhydrite-rich, and were derived from a water-rich, highly oxidized sulfur-rich magma, thus very much resembling adakitic magmas (e.g., the 1991 Pinatubo eruption). We propose that these rocks were generated within a complex plate tectonic scenario involving a torn Cocos plate (Tehuantepec fracture zone) and the ascent of hot asthenospheric mantle. The latter is supported by an outstanding negative S-wave anomaly widely extending beneath the zone, from 70 to 200 km in depth. The adakitic-like trend would be derived from the direct melting of subducting Cocos plate, whereas the transitional rocks would have resulted from the mixing of two poles, one reflecting a mantle source, and the other, the already mentioned adakitic melts. The basaltic source would also account for the high sulfur content and δ34S values of the El Chichón system (about +5.8), as result of a contribution of SO2 in fluids released from an underlying mafic magma.

Extremely fast supercooling of water in the anti-Jovian hemisphere of Europa: A speculative model for the fracturing pattern and ascent of brines along cracks

The anti-Jovian hemisphere of Europa (the hemisphere opposite the one facing Jupiter) displays a complex array of fractures, including roughly concentric and arcuate ones. The geometry of this intriguing pattern on the icy surface of Europa resembles the textural features displayed by perlitic volcanic glass. We argue that fast cooling of a rhyolitic magma may serve as an analogue for fast supercooling of water leading to formation of amorphous ice (glassy water). We envisage two possible scenarios for ice melting and subsequent extremely fast supercooling: (1) massive subglacial volcanic activity or (2) a large impact on the anti-Jovian hemisphere. We suggest that extremely fast formation of amorphous ice on Europa would result in a fracture pattern geometrically equivalent to that observed in perlitic glass. Fast supercooling of water will initially lead to brine retention in the ice phase; however, after some time these brines would be laterally rejected toward fractures. Furthermore, as solidification progresses downward, a shrinking ocean would increase brine concentration. These brines would eventually escape to the surface via the fracture network. Finally, high-pressure crystallization of salts within fractures may provide an efficient mechanism for differential plate separation, horizontal movement, and formation of new fractures.