Michael Jensen and Faculty Mentor: Dr. Eric Christiansen PhD, Department of Geological Sciences

INTRODUCTION

The Wah Wah Springs Tuff and the Wah Wah Springs Intrusive Granodiorite are both part of the Indian Peak caldera complex in southwest Utah, an area of intense volcanic activity 30 million years ago. This time period is known for explosive silicic activity due to the subduction of the Farallon Plate along North America’s western edge. In the eruption connected with the Wah Wah Springs an estimated 5,900 km³ of magma came out of the Earth, making it one of the largest known explosive eruptions in our planet’s history. This massive eruption was partially dependent upon the changing temperatures and pressures of the magma body as it rose through he crust. These changes are recorded in the rocks through a disequilibrium texture known as zoning. Zoning is found in microscopic crystals of igneous rocks and can reveal the history of a magma body. In the case of the Wah Wah Springs Granodiorite, it is thought that variable abundances of titanium in the mineral quartz are responsible for dark and bright zones seen within individual grains when viewed in a scanning electron microscope using a cathodoluminescence detector (Figure 1). These small changes in the abundance of Ti can be used to calculate paleo temperatures and pressures thanks to recent experiments of Huang and Audetat¹. H&A developed an equation that can be used as a geothermometer and geobarometer given the abundance of Ti measured in ppm in quartz.

According to their conclusions, a bright zone should indicate a higher abundance of Ti and an increase in temperature or a decrease in pressure. Dr. Christiansen and myself wanted to test the hypothesis that the Wah Wah Springs was decreasing in temperature and pressure around the time of eruption. This is significant because studies on similar volcanic rocks such as the Fish Canyon Tuff², which erupted around the same time and under the same tectonic setting, indicate a temperature increase right before the eruption. However a previous study by Skidmore³ shows that the Wah Wah Springs magma body decompressed as it rose through the crust, rather than heating. By measuring and then comparing the Ti concentrations in the bright and dark zones, we can have an idea of the conditions around the time of eruption.

METHODS

Three polished thin sections of the intrusive granodiorite were prepared at a 200-micron thickness at Wagner Petrographic in Lindon Utah. Because I work at WP I have specialized training in thin section preparation so was able to prepare my own samples, something that is usually outsourced. Once completed, cathodoluminescence images of selected quartz grains were taken using a scanning electron microscope at Brigham Young University in order to see the bright and dark zones in the quartz grains. Ten grains were selected from each sample for further work based on their size and how well the zoning was developed. 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 within the different zones of the quartz grains. For most grains, three spots were selected, one in the center of the grain, one along the rim and one intermediate spot. I reduced the data and made it useable with the computer program Iolite.

RESULTS

CL proved to be a reliable method for visualizing the zoning. In samples ATCH and MIN, at least two clearly distinct zones can be seen and in some cases, as with ATCH_4 (fig 1), three zones can be seen. 4 grains in sample MIN had both bright cores and rims with a dark intermediate zone.

All 10 of the grains in sample PINTO lacked any obvious zonation though some grains were brighter than others. The lack of any zoning in PINTO and the very fine ground mass around the quartz grains indicates this part of the intrusion cooled rapidly and little new quartz formed and is evidence that temperatures were not increasing within the magma body. LA results of PINTO show average Ti ppm was higher in the cores than the rims but this contrast produces no difference in pressure or depth of crystallization based on the TitaniQ equation (Table 1).

CL for sample MIN revealed very thin bright rims on quartz that were hard to analyze accurately with LA. In three grains, the laser beam captured the composition of the surrounding groundmass and contaminated the analysis. The remaining 7 quartz grains produced more accurate data with some grains having Ti-enriched rims, others Tienriched cores, or the Ti abundance was very similar. Rims were caught in the act, so to speak, in their growth when temperatures were too low to continue crystal growth. The mixed results are also evident in the calculated pressures and depths from the TitaniQ equation and show essentially no difference between early and late growth (Table 1). Sample ATCH best matched our predictions for both CL and Ti abundance. 5 grains have bright rims and dark cores and higher Ti in the rims relative to the cores. 2 grains showed the opposite pattern though the Ti numbers are close. From these 7 grains, the rim averaged a Ti abundance of 141 and the core averaged 132 (Table 1). The calculated average pressures decrease from core to rim. 3 quartz grains did not yield reliable data from LA.

CONCLUSIONS

Cathodoluminescence imaging and laser ablation analysis can be used to determine the abundances of elements in quarts but the results did not always agree with our hypothesis that CL-bright zones would be correlated with higher Ti. Some brightly rimmed grains had higher Ti ppm in the darker core than the brighter rim. It could be that other elements such as Al or Fe affected the zoning pattern. The TitaniQ equation from H&A predictably calculated greater pressures for lower Ti abundance and lower pressures for higher Ti abundance at a given temperature. ATCH shows an average 0.4 kbar difference from core to grain which indicates the magma body rose in the crust about 1.4 km before the quartz rims formed. The thin, bright rims in MIN, and the fine groundmass and lack of zoning in PINTO are evidence that the magma body was cooling too fast for thick rims to form on the quartz, which would be the case if the magma was rising through the crust.

Figure 1: CL image of a quartz grain from sample ATCH as seen with a SEM. Three distinct zones are visible that formed during growth like tree rings. The red dots are the locations of the laser spots for Ti analysis and are labeled with the measured Ti in ppm. Notice how the bright rim is enriched in Ti which I interpret to be the result of lower pressure on the magma.

Table 1: Comparison of the average pressures and depths from core to rim by sample.

1. Huang, R., Audetat, A., 2012, The titanium-in-quartz (titaniQ) thermobarometer: A critical examination and re-calibration: Geochimica Et Cosmochimica Acta, v. 84, p. 75-89, doi: 10.1013/j.gca.2012.01.009.
2. Bachmann, O., Dungan M. A., Lipman, P.W., 2002, The Fish Canyon Magma Body, San Juan Volcanic Field, Colorado: Rejuvination and Eruption of an Upper-Crustal Batholith. Journal of Petrology, v. 43, p. 1469-1503, doi: 10.1093/petrology/43.8.1469.
3. Skidmore, C., 2013, Exploring Connections between a Very Large Volume Ignimbrite and an Intracaldera Pluton: Intrusions Related to the Oligocene Wah Wah Springs Tuff, Western US [M.S. thesis]: Provo Utah, Brigham Young University, p. 1-74.