Search

The ~5 km³, 4.54–4.09 Ma Caspana ignimbrite of the Altiplano-Puna volcanic complex (APVC) of the Central Andes records the eruption of an andesite and two distinct rhyolitic magmas. It provides a unique opportunity to investigate the production of silicic magmas in a continental arc flare-up, where small volumes of magma rarely survive homogenization into the regional magmatic system that is dominated by supereruptions of monotonous dacitic ignimbrites. The fall deposit and thin flow unit that record the first stage of the eruption (Phase 1) tapped a crystal-poor peraluminous rhyolite. The petrological and geochemical characteristics of Phase 1 are best explained by partial melting of or reheating and melt extraction from a granodioritic intrusion. Phase 2 of the eruption records the emplacement of a more extensive flow unit with a crystal-poor, fayalite-bearing rhyolite and a porphyritic to glomeroporphyritic andesite containing abundant plagioclase-orthopyroxene-Fe-Ti oxide (norite) glomerocrysts. The isotopic composition of Phase 2 is significantly more “crustal” than Phase 1, indicating a separate petrogenetic path. The mineral assemblage of the noritic glomerocrysts and the observed trend between andesite and Phase 2 rhyolite are reproduced by rhyolite-MELTS–based models. Pressure-temperature-water (P-T-H2O) estimates indicate that the main (Phase 2) reservoir resided between 400 and 200 MPa, with the andesite recording the deeper pressures and a temperature range of 920–1060°C. Rhyolite phase equilibria predict an estimated temperature of ~775 °C and ~5 wt% H2O. Pressures derived from phase equilibria indicate that the rhyolite was extracted directly from the noritic cumulate at ~340 MPa and stored at slightly shallower pressures (200–300 MPa) prior to eruption. The rhyolite-MELTS models reveal that latent-heat buffering during the extraction and storage process results in a shallow liquidus during the extensive crystallization that produced a noritic cumulate in equilibrium with a rhyodacitic residual liquid. Spikes in latent heat facilitated the segregation of the residual liquid, creating the pre-eruptive compositional gap of ~16 wt% SiO2 between the andesite and the Phase 2 rhyolite. Unlike typical Altiplano-Puna volcanic complex (APVC) magmas, low ƒO2 conditions in the andesite promoted co-crystallization of orthopyroxene and ilmenite in lieu of clinopyroxene and magnetite. This resulted in relatively high Fe concentrations in the rhyodacite and Phase 2 rhyolite. Combined with the co-crystallization of plagioclase, this low oxidation state forced high Fe2+/Mg and Fe/Ca in the Phase 2 rhyolite, which promoted fayalite stability. The dominance of low Fe3+/FeTot and Fe-Ti oxide equilibria indicates low ƒO2 (ΔFMQ 0 − ΔFMQ − 1) conditions in the rhyolite were inherited from the andesite. We propose that the serendipitous location on the periphery of the regional thermal anomaly of the Altiplano-Puna magma body (APMB) permitted the small-volume magma reservoir that fed the Caspana ignimbrite eruption to retain its heterogeneous character. This resulted in the record of rhyolitic liquids with disparate origins that evaded assimilation into the large dacite supereruption-feeding APMB. As such, the Caspana ignimbrite provides a unique window into the multiscale processes that build long-lived continental silicic magma systems. [ABSTRACT FROM AUTHOR]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.06.031 Byline: Kathleen Mandt (a)(b), Shanaka de Silva (c), James Zimbelman (d), Danielle Wyrick (e) Keywords: Mars, Surface; Mars, Climate Abstract: The form of erosional remnants of the Medusae Fossae Formation (MFF) on Mars provide evidence of their development progression and implicate two spatially distinct environments in the equatorial regions of Mars. Ubiquitous yardangs are clearly the product of strong unidirectional winds acting over time on variably indurated deposits. Yardang orientation is used as a proxy to map regional and local wind direction at meso-scale resolution. In other, more limited areas not subjected to strong unidirectional winds, randomly oriented kilometer-scale mesas and buttes are found to be remnants of progressive cliff recession through mass wasting as support is lost from within the MFF lithology at its margins. The differing processes that dominate the formation of the distinctive landforms have implications for meso-scale variations in climate that remain unresolved by current modeling efforts. Additionally, the variability of erosional forms within the deposit emphasizes the overall complexity of this extensive formation. Author Affiliation: (a) University of North Dakota, Department of Space Studies, Grand Forks, ND 58202, United States (b) Space Science and Engineering Department, Southwest Research Institute, San Antonio, TX 78238, United States (c) Department of Geosciences, Oregon State University, Corvallis, OR 97331, United States (d) CEPS/NASM MRC 315, Smithsonian Institution, Washington, DC 20013, United States (e) Geosciences and Engineering Division, Southwest Research Institute, San Antonio, TX 78238, United States Article History: Received 17 January 2009; Revised 10 June 2009; Accepted 16 June 2009

Recharge by mafic magma is established as a trigger of major explosive volcanic eruptions. However, recharge by silicic magma can also induce perched resident magma to erupt. The evidence of such recharge may be more cryptic than for mafic recharge and requires detailed petrologic and geochemical study to identify. We present a study of plagioclase petrology supported by other data that implicates silicic-silicic magma interaction during the eruption and evolution of the A.D. 1600 eruption of Huaynaputina, one of the largest historical eruptions. Recharge by silicic magma is probably more common than recognized and should be considered an important eruption trigger. Keywords: silicic recharge, crystal mush, magma mingling, eruption trigger, Huaynaputina.