Contributions to Mineralogy and Petrology

, Volume 152, Issue 4, pp 523–540 | Cite as

Rhyodacite magma storage conditions prior to the 3430 yBP caldera-forming eruption of Aniakchak volcano, Alaska

  • Jessica F. LarsenEmail author
Original Paper


This study presents a pre-eruptive magma storage model for the rhyodacite and andesite magmas erupted during the 3430 yBP caldera-forming eruption of Aniakchak volcano, Alaska, derived from phase equilibria experiments and petrological data. The compositions of Fe–Ti oxide pairs from the early erupted Plinian rhyodacite pumice yield core temperatures of 871–900°C, with rims up to ∼942°C, and fO2 from  −10.6 to  −11.8 log units. Melt inclusions entrapped in plagioclase phenocrysts have H2O contents between 3 and 5 wt%, estimated by FTIR and electron microprobe volatiles by difference methods, with no detectable CO2. Assuming water saturation, this corresponds to entrapment pressures between ∼65 and 150 MPa. Phase equilibria results reproduce the natural phase assemblages at \(P_{\text H_{2}\text O}\) of 95–150 MPa at 870–880°C, assuming water saturation. A mismatch in experimental versus natural glass SiO2 and Al2O3, and MELTS models for H2O-undersaturated conditions indicate that the rhyodacite may not have been H2O saturated. MELTS models with \( X_{\text{H}_2\text{O}} =0.8\) and P total of 125–150 MPa at 870–880°C reproduce the natural groundmass glass Al2O3 composition best, indicating the magma may have been slightly H2O undersaturated. Those pressures correspond to storage at 4.5–5.4 km depth in the crust. MELTS models and VBD estimates from melt inclusions in titanomagnetite grains from the andesite indicate pre-eruptive conditions of ∼1,000°C and > 110 MPa, corresponding to a minimum residence depth of ∼4.1 km assuming water saturation or greater if the magma was H2O undersaturated. Previous geochemical studies indicate separate histories of the two magmas, though they retain some evidence that they are ultimately related through fractional crystallization processes. Analogous to the 1912 Novarupta magmas, the rhyodacite and andesite presumably originated within the same crystal mush zone beneath the edifice, yet were separated laterally and underwent different degrees of crustal assimilation. The andesite must have resided in close proximity, with ascent occurring in response to movement of the rhyodacite, and resulting in extensive syn-eruptive mingling.


Plagioclase Phenocryst Pumice Clast Experimental Glass Groundmass Glass Natural Glass 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.



This work was supported by the Alaska Volcano Observatory and National Science Foundation grant EAR 0106658. K. Severin provided assistance with the electron microprobe and J. Lowenstern provided the use of the Nicolet FTIR spectrometer at the USGS (Menlo Park). S. Dreher, J. Eichelberger, J. Gardner, and T. Neal contributed discussions, figures, and access to data that improved this work. The author thanks C. Bacon and M. Coombs for comments that improved the manuscript prior to submission. The author thanks J. Hammer and E. Cottrell for reviews that improved this study.

Supplementary material


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Copyright information

© Springer-Verlag 2006

Authors and Affiliations

  1. 1.Alaska Volcano Observatory, Geophysical InstituteUniversity of AlaskaFairbanksUSA

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