Trace Element Analysis of Quartz Grains in the Wah Wah Springs Tuff and Granodiorite Intrusion
Faculty Mentor: Dr. Eric Christiansen, Department of Geological Sciences
The Wah Wah Springs tuff and cogenetic granodiorite were part of the creation of the Indian Peak-Caliente Caldera Complex, which lies on the Nevada-Utah border (Skidmore, 2013). The complex formed during the middle Cenozoic (36-18 Ma) during an episode of explosive silicic activity (Best et al., 2013). Rollback of the subducting lithosphere likely caused the silicic activity, evidenced by the migration of magmatism southward (Best et al., 2013). The dehydration of the subducting oceanic lithosphere caused mafic magmas high in water to rise and fractionate in continental crust before eruption (Skidmore, 2013).
Over twenty-four different ignimbrites have been found in the Indian Peak complex, with the largest ash flow spreading 150 km from the complex (Best et al., 2013). The eruptions include the Lund Tuff (29.0 Ma), the Cottonwood Wash Tuff (30.9 Ma), the Wah Wah Springs (30.0 Ma) and many others (Best et al., 2013). The Wah Wah Springs eruption expelled an estimated 5,900 cubic kilometers of material, making it the largest of these eruptions and exceeding the size of the largest Yellowstone eruptions (Best et. al. 2013).
The cause of this massive eruption was likely influenced by the paleo-temperatures and paleo-pressures of the rising melt. The tuff and co-genetic granodiorite are primarily composed of feldspar, hornblende, biotite, Fe-Ti oxides, and quartz (Skidmore 2013). Finding conditions pre-eruption may be possible by studying phenocrysts, specifically quartz. As quartz crystals grow, they add new layers of SiO2 along with differing amounts of trace elements depending on the conditions of the magma chamber. These layers can act as a relative time capsule for geologists to open. The layers are visible as lighter and darker areas under cathodoluminescence (CL) imaging. This disequilibrium texture is known as zonation. Some samples of the granodiorite have quartz crystals that are bimodal in size, with the smaller grains appearing much brighter in CL.
It has been thought that titanium in quartz tends to luminesce when bombarded by electrons in a scanning electron microscope, giving the areas with more ppm titanium a brighter appearance. Once measured, the amount of Ti can be placed into an equation developed experimentally by Huang and Audetat (2012) that functions as a geothermometer or geobarometer for the magmas.
Dr. Christiansen and I specifically wanted to test whether the Wah Wah Springs magma was decreasing in temperature or pressure directly before eruption. Studies by Bachmann et al., (2003) on the Fish Canyon Tuff indicate that the temperature of that magma body increased significantly before eruption. This hypothesis has been widely applied to other silicic eruptions. Its similarity in geologic setting and time period makes the Fish Canyon a good reference point for Wah Wah Springs. In an earlier study, Woolf (2008) and Skidmore (2013) concluded that before eruption the magma was cooling and crystallizing. This process was followed by decompression of the magma intruded into the caldera at a shallow level. We planned to test this hypothesis with careful analysis of the amount of Ti in the quartz. We wanted to determine (1) Does a brighter area on CL images indicate higher titanium in quartz, (2) what were conditions like in the magma chamber pre-eruption, and (3) the significance of the small bright grains of quartz in the granodiorite.
1) Five polished thin sections were prepared: four from the intrusive granodiorite and one from a cogenetic, extrusive tuff. These were manufactured by Micheal Jensen at Wagner Petrographic. The sections were cut at 200 microns thick to withstand the intensity of laser ablation.
2) Around 10 larger quartz phenocrysts were picked from each sample using reflected and transmitted light on a petrographic microscope. Two samples, MIN-0312 and ATCH-5012, were the only samples containing small quartz large enough to ablate.
3) Cathodoluminecence images were taken of the quartz grains using a scanning electron microscope at Brigham Young University. The images were then marked with specific points for laser ablation.
4) Laser ablation technology (ICPMS- inductively coupled plasma mass spectrometry) at the University of Utah was used to carefully measure the abundance of Ti and other trace elements such as Al, Na, and Fe. On the larger grains, the ablation spots were in different zones of the quartz ranging from core to intermediate to rim. The diameters of the smaller, bright quartz grains were similar to the diameter of the laser spot, making it impossible to analyze separate zones within them. The laser settings were Rep.=15 Hz, Shots=480, 100% Energy, 7.1 J/cm3, on a laser spot of 65 microns.
5) The mass spectrometry data were imported into Iolite and reduced using NIST610 as a reference material.
6) Analyzing the data: Excel was used extensively to graph data in terms of spot location, trace element concentrations, and bright vs. dark. Images of the grains were marked with spots indicating Ti concentration and beam size for every ablation spot.
Results and Conclusion
According to the studies of Huang and Audetat (2012), higher Ti concentration correlates with higher temperature (T) or lower pressure (P). This relationship can be seen in H&A’s equation:
In three of the samples, (PINTO-0612, MIN-0412, and GOUGE-3P13) there is a strong correlation between the location on the grain, and the concentration of Ti. In these cases, the ppm Ti decreases from core to rim. This is evidenced in the darker rims of many of the grains in these samples. We have hypothesized that this decrease in Ti concentration, correlates to decreasing temperatures within the magma chamber. PINTO-0612 and MIN-0412 are both intrusive granodiorites with a very fine matrix. SEM analysis suggest that the matrix contains bright quartz and some plagioclaise. The brightness indicates higher levels of Ti in the fine quartz, which may have formed concurrently with the Wah Wah Springs eruption. This eruption would have dramatically decreased the pressure, which is inversely related to Ti concentration. PINTO-0612 contains a thin white rim that may have formed in the same regime. The GOUGE does not have the fine quartz, likely due to the fact that is was ejected and became a tuff during the eruption. The ejected material was not able to form small quartz in a pressure-reduced environment.
Quartz in the intrusive samples MIN-0312 and ATCH-0512 suggest three environments of crystal growth. These can be seen in the zonation of the grains. As imaged by CL, all of the grains generally have bright cores, followed by darker intermediate regions, and then bright rims. We hypothesize that the brighter core and darker intermediate zone formed in similar environments to the tuff sample. We conclude that the Ti-rich cores formed at a higher temperature than the intermediate zone. Although, many of the cores
from ATCH-0512 and MIN-0312 showed signs of much faster crystal growth in the form of dendritic extensions on the corners of the crystals (Fig. 12). The bright ring around the grains in these samples may be due to a decrease in pressure as the magma rose to be emplaced as a shallow intrusion. We suggest that this is more likely explanation than an increase in temperature. It seems unlikely that quartz would grow during heating (it should melt) or that the magma would have become hotter as it rose to a shallow level. The thickness of the bright rims may be directly related to the amount of time conditions in the melt were sustaining the growth of the quartz in the low pressure environment. For example, the sample MIN-0412 may have cooled too quickly to form a thick, bright rim.
The small quartz in the intrusive samples MIN-0312 and ATCH-0512 were large enough to analyze, and generally showed a bimodal distribution of Ti concentrations. Some Ti concentrations were similar to those found in the larger quartz grains, while others were much more concentrated with Ti. We hypothesize that the depressurization and degassing caused many quartz phenocrysts to shatter, thus creating smaller quartz with similar Ti amounts to the larger grains. We suggest the quartz extremely high in Ti formed as the magma rose from 8km to 2km, drastically decreasing the pressure and lowering the temperature. This event would have spiked the nucleation rate of quartz before solidification, thus forming the fine matrix.
In summary, we have formulated hypothesies for the three questions determined at the beginning of our study: (1) Ti is generally a good indicator of relative Ti concentrations within quartz grains. (2) Conditions in the magma chamber were likely decreasing in both temperature and pressure. (3) The small quartz seem to come from two backgrounds; One was formed from the depressurization and fracturing of quarts and the other from increased nucleation closer to the surface.
For the full report, please contact Dr. Eric Christiansen in the Department of Geological Sciences
Best, M.G., Christiansen, E.H., Deino, A.L., Gromme,S.,Hart, G.L., andTingey, D.G., 2013, The 36-18 Ma Indian Peak-Caliente ignimbrite field and calderas, southeastern Great Basin, USA: Multicyclic super-eruptions: Geosphere, in press.
Skidmore, C.N., Christiansen, E.H., and Best, M.G., 2012, Exploring the connections between very large volume ignimbrites and intracaldera plutons: Intrusions related to the Oligocene Wah Wah Springs tuff, Great Basin, USA: Geological Society of America Abstracts with Programs, v. 44, no. 7.
Woolf,K.S., 2008, Pre-eruptive conditions of the Oligocene Wah Wah Springs Tuff, southeastern Great Basin ignimbrite province: M.S. Thesis, Brigham Young University, Provo,Utah, 77 p.