Tara Albertson Allen

The Isom formation is a package of distinctive ignimbrites found in an Oligocene caldera complex along the Utah-Nevada border (Best and Christiansen, 1991). Similar high-alkali rocks are also found in other caldera complexes and imply their generation is a fundamental part of the evolution of large volcanic systems (Dorais, Whitney, and Stormer, 1991). Best and Christiansen (1989) hypothesized that the Isom magma is a result of fractional crystallization of an andesite. In contrast, other large volume ignimbrites appear to involve significant mixing and hybridization, but compositional analyses of the rocks found in this caldera complex do not match these trends.

This hypothesis has been tested by using compositions of andesites in the vicinity of the Isom tuffs and modeling their fractional crystallization paths using MELTS (Ghiorso, 1995). Different environmental parameters were examined to see if those paths lead to a distinctive Isom-type composition.

Initial compositions of parent andesites were taken from two different samples: CHUCK-1A and BAN-10. Generally, for CHUCK-1A to be the parent of an Isom-type magma, pressures would have to be about 8 kbar with 2% water and an fO2 of QFM+1. This matches the compositional trends for SiO2, TiO2, CaO, Al2O3, and MgO, but is a poor match for the alkalis and Fe2O3.

Tests using BAN-10 as the parent andesite produced Isom-like fractional crystallization trends at 4 kbar. Compositional variations were best for SiO2, TiO2, CaO, Al2O3, and MgO, but are still low for alkalis and Fe2O3.

There is not enough evidence to confirm or disprove the hypothesis, and the correct combination of initial magma composition and environmental parameters has not yet been determined.

The ORCA grant has allowed me to continue this research, to publish it in the form of an Honors thesis, and the present it at a number of forums. Additionally, I have been able to continue research after the publication of my preliminary findings in my thesis. The funding allows me to use equipment that I would otherwise not have access to because of cost.

I have started studying thin sections of the physical rocks to identify and compare mineral assemblages, and thanks to the ORCA grant, will be able to perform microprobe analyses on these samples. Included in the study will be samples from both the Isom formation and the possible parent andesites. This will allow me to compare actual and specific chemical compositions of certain minerals, such as plagioclase or clinopyroxene, and also provide insight regarding the robustness of MELTS and further verify whether fractional crystallization is in fact a valid method to explain the composition of the Isom formation.

Overall, my ORCA project has been a rewarding experience for me and the department. Although there is still more work to be done, the research I have conducted has already provided thorough results regarding the method I used, and specifically the accuracy and robustness of MELTS. The research has also helped to further justify the original hypothesis by more clearly disproving alternate methods of formation.