Submitted to the Faculty of the Graduate School of Vanderbilt University

This primary objective of this study was to assess the history and evolution of the PST system using textures and compositions of accessory minerals (sphene, zircon, allanite, chevkinite) and glass in pumice clasts and fiamme from different areas of the PST outflow and. New detailed mapping of the area around Oatman, AZ (southern Black Mountains) by Pearthree et al. Known as the Silver Creek caldera (Ferguson 2008), this is now believed to be the source of the PST.

The primary objective of this work was to study the history and evolution of the magmatic PST system as recorded by the relatively abundant accessory mineral phases present.

Figure 1. Areal extent of the Peach Spring Tuff. Red dots indicate sample locations. White circle indicates the approximate location of the source caldera (Silver Creek Caldera)

Samples

No attempt was made to distinguish between chevkinite and perrierite, which can only be done by X-ray diffraction (Macdonald & Belkin 2002), and we refer to it as chevkinite for convenience. Kingman Kingman Kingman Kingman Kingman Grasshopper Junction Grasshopper Junction Grasshopper Junction Piute Mountains Warm Springs West Warm Springs West Warm Springs West Warm Springs West Warm Springs West Warm Springs West Warm Springs West Warm Springs West Warm Springs West Cathedral Rock.

Analytical Methods

Bulk Density Determinations, Thin Section Documentation and Crystal Separation Bulk densities were determined using an immersion technique based on Archimedes’

Kingman Kingman Kingman Kingman Kingman Grasshopper Junction Grasshopper Junction Grasshopper Junction Piute Mountains Hot Springs West Hot Springs West Hot Springs West Hot Springs West Hot Springs West Hot Springs West Hot Springs West Hot Springs West Hot Springs West Cathedral Rock. thu: distal outflow po: proximal outflow ic: intracaldera. pc: pumice f: fiamme me: mafic enclave s: SHRIMP-RG. Kingman Kingman Kingman Kingman Kingman Grasshopper Junction Grasshopper Junction Grasshopper Junction Piute Mountains Hot Springs West Hot Springs West Hot Springs West Hot Springs West Hot Springs West Hot Springs West Hot Springs West Hot Springs West Hot Springs West Cathedral Rock. do do do do do do do po po po po po po po po po ic Location inside. pc pc pc pc pc pc f f f f Example description. Differential Absorption X-ray Tomography (DAT) and Crystal Size Distributions (CSD) , clustering, shapes) can be documented (e.g.

Differential absorption X-ray tomography (Gualda et al., in press) has been introduced as a more sophisticated method for studying minerals rich in high-Z elements, especially Zr and rare earth elements (REE).

Run Letter

All tomographic imaging was performed on the GeoSoilEnvironCARS bending magnet beam at the Advanced Photon Source (APS) at Argonne National Laboratory (Chicago, IL) (Rivers et al. 1999, Sutton et al. 2002, Gualda & Rivers 2006). As a result, the voxel (volume element) size in the tomograms above and below the edge is twice that of the tomogram taken at ideal energy in each linear dimension.

Cylinder Size (mm)

Resolution (µm/voxel)

Typically, crystals in pumice clusters are independent of each other (not touching) and can be automatically measured by Blob3D. This problem can be avoided during image processing using tools available in Blob3D that allow for manual separation of contact crystals so that they can be measured individually. Even though important qualitative information on sphene can be obtained from Ce maps, Ce content in PST sphene crystals was low enough that sphene crystals could not be quantitatively measured using Ce maps, and ideal energy tomograms were used instead , Magnetite results were also determined from ideal energy tomograms.

Unfortunately, this latter approach requires significantly more processing time (Gualda & . Rivers 2006), and due to time constraints, sphene and magnetite data were obtained for only two of the five Kingman samples.

Scanning Electron Microscope (SEM)

Accessory minerals in PSTs, however, tend to cluster, and automatically measuring these clusters can skew the resulting size distribution. The decision to use two samples as representative was supported by the fact that the Kingman samples are quite similar to each other in various respects, including zircon and allanite + chevkinite size distributions and geochemistry. Finally, it should also be noted that the density of the highly compressed PSTG01C fiam was too high to be successfully imaged at the lowest resolution used (A Run), but zircon data were collected from low-resolution ideal energy tomograms (X Run) of this sample .

Back-Scattered Electron (BSE) images were used to select areas for standard-free quantitative analysis of minerals and glasses, which was performed using the INCA software and the EDS detector.

SEM Operating Conditions

Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA-ICPMS)

Attempts were made to select spot locations at a distance from other spots to avoid analysis of material previously sputtered. The GLITTER data reduction package (GEMOC, Australia) was used for data processing (Griffin et al. 2008). Measurements by NIST612 show that measured values are within 10% of the predicted values by Pearce et al.

Table 4. LA-ICPMS operating conditions (modified from Colombini 2009)

Reverse Geometry Senstive High Resolution Ion Microprobe (SHRIMP-RG)

An important caveat when applying these thermometers to real systems is that the activity values of TiO2, SiO2, and, in the case of sphene, ZrSiO4 must be limited. In the case of the PST, which is rutile undersaturated but contains sphene, a value of 7. The best estimates of the crystallization pressures are in the range of 200-250 MPa for PST samples (see Carley, 2010), and thus P = 0.25 GPa was used here in the temperature calculations.

In the case of sphene, a ±0.05 GPa change in pressure leads to a temperature difference of ~5-10 ºC.

Whole Rock Geochemistry

RESULTS

Bulk Density Determinations

Sample name Sample Density (g/cm 3 )

Pumice clasts and fiamme vary considerably in whole-rock composition, from high silica content rhyolite to trachyte (Carley, 2010).

Phenocryst Assemblages

CRW differs from all other samples in that it contains a population of abundant apatite, not found in other samples.

Trace Element Compositions

Glass and Whole Rock Compositions and Trends

All plots are relative to Sr, which acts as an indicator of fractionation in this feldspathic system. Glasses in the mafic enclave (WSW1) are also enriched in Sr, Ba, Hf, Gd, La and Nd relative to other samples, but show no difference in Yb content. Whole rock Ba, Hf, Gd, La, Nd and Yb are almost all depleted relative to glass - a glass analysis is more depleted in Yb relative to whole rock.

Whole rocks Ba, Hf, Gd, La, Nd, and Sr in WSW1 are enriched relative to Kingman but depleted relative to CRW.

Figure 2. SiO 2 vs. Sr in CRW, PSTG01C, WSW1, WSW2A, WSW2B and Kingman. SiO 2

Zircon and Sphene Compositions

Ba, Hf, Gd, La, and Nd contents of both glasses and whole-rock samples of the outflow and intracaldera are positively correlated with Sr, so the Kingman samples appear to be the most developed. The Yb content of the glasses in both samples shows no correlation with Sr, but the Yb content of the whole rocks is negatively correlated, so that the CRW is the most depleted in Yb.

Rare Earth Elements

The selected sphene grain shows depletion in HREE relative to MREE and LREE, with much higher REE concentrations than zircon, especially for MREE and LREE. In general, the rims of KPST01A and WSW2A zircon crystals are depleted in MREE compared to normalizing zircon, and the WSW2A cores are enriched in LREE (except Ce). The KPST01A and WSW2A cores are generally enriched in MREE relative to the normalizing sphene, while the edges are similar to it.

KPST01A (a) and WSW2A (b) are depleted in MREE relative to the normalizing zircon, while WSW2B (c), PSTG01C (d) and CRW (e) are similar to the normalizing zircon.

Table 7. REE compositions of zircon and sphene crystals used for normalization.

Trace element variations

The higher Gd contents of the zircon cores of these samples indicate that zircon growth may have started before the sphene. The higher Nd content of the zircon cores of these samples also suggests that zircon crystallization may have started before allanite + chevkinite crystallization. Nd is positively correlated with Gd in (a) and (b), so rims are depleted relative to cores, indicating fractionation by allanite + chevkinite crystallization.

WSW2B (c) and PSTG01C (d) show the opposite trend which, similar to zircon trends, supports a heating event that occurred between core and edge crystallization causing resorption of allanite + chevkinite.

Figure 7. Zircon-normalized Gd vs. Hf in zircon crystals for (a) KPST01A, (b) WSW2A, (c) WSW2B, (d) PSTG01C and (e) CRW

Ti-in-Zircon and Zr-in-Sphene Thermometry

In both zircon and sphene, the same core-to-edge trends (Figs. 11a,b, 12a,b) are found for each sample: KPST01A and WSW2A show decreasing temperatures, while WSW2B, PSTG01C and CRW show increasing temperatures. CRW edge CRW core PSTG01C edge PSTG01C core WSW2B edge WSW2B core WSW2A edge WSW2A core KPST01A edge KPST01A core. CRW edge CRW core PSTG01C edge PSTG01C core WSW2B edge WSW2B core WSW2A edge WSW2A core KPST01A edge KPST01A core.

Sphene temperature trends are similar to those in zircon but record a smaller range of temperatures, suggesting that sphene had a shorter history of crystallization than zircon.

Figure 12. Ti-in-zircon temperatures. Reference temperatures are calculated using TiO 2 and SiO 2

Textures

Crystal Size Distributions

Zircon size distributions follow a linear trend, which is consistent with a simple nucleation and growth model. The zircon size distributions (ZSD) of the outcrop tuff (fig. 14a) can best be described as following a simple linear trend showing enrichment in small crystals and a compr. All allanite+chevkinite size distributions (ACSD) from the outcrop pumice (fig. 14c) show concave patterns that can be reasonably described by power-law (or fractal) functions;

Magnetite size distributions (MSD) of outflow pumice groups show both linear and kinked shapes (fig. 14d).

Figure 14. Crystal size distributions of (a) zircon, (b) sphene, (c) allanite+chevkinite and (d) magnetite

Qualitative textural features of minerals in the PST

The reversal occurs in the 105 µm bin in the WSW distributions and occurs at smaller sizes (52.5 µm) in KPST01B. Reconstructions of sphene and magnetite crystals (yellow and green, respectively) show the tendency to accumulate accessory minerals. Zircon crystals found in thin section and separated from PSTG01C and CRW are generally rounded (fig. 15f).

Note that when sphene (yellow) and magnetite (green) crystals are obscured, it becomes clear that most of the zircon in the sample is concentrated in clusters in and around sphene and magnetite crystals (see Gualda et al., in press).

Figure 15. Phenocryst textures in thin section and crystal separates. Textures of phenocrysts are considerably different between outflow pumice clasts (a, c, d) and intracaldera fiamme (b, e)

DISCUSSION

Cooling, crystallization, and decompression in the Peach Spring magma system Textures and compositional variations in accessory minerals from the rhyolites show a

A closer look at trace elements also suggests differences in the onset of crystallization of the various additional mineral phases. Crystal size distributions also provide interesting insight into the crystallization history in the PST and have implications for the onset of the PST eruption. Some of the size distributions here, especially for allanite+chevkinite, are concave or fractal in shape.

Similarly, the kinked sphene and magnetite size distributions seen in the PST rhyolite may record a similar decompression event in the PST system, indicating the initiation of the PST eruption.

Heating and zoning in the Peach Spring Tuff

The presence of mafic magmatic enclaves in the PST supports an argument for mafic input to the system, which may have provided the heat necessary to raise the temperature to the observed levels. The lack of significant chemical or textural evidence for mixing (absence of reaction rims or widespread xenocrysts) indicates that chemical interaction between the mafic and felsic magmas was limited, suggesting that this event may have occurred close to the time of the eruption and may even to have acted as a trigger for the start of the explosive process. These results suggest that the warming event did not affect the entire PST system to the same extent.

Again, this suggests that the heat anomaly did not reach all areas of the magma chamber, further implying that the heating event occurred close to the time of the eruption, as there was not enough time for the heat to spread throughout the system.

CONCLUSIONS

APPENDIX A

Whole Rock Geochemistry of the Peach Spring Tuff

APPENDIX B

LA-ICPMS Analyses From the Peach Spring Tuff

GJPST1B GJPST1B GJPST1B GJPST1B Spot #

CRWPST CRWPST CRWPST CRWPST CRWPST CRWPST CRWPST CRWPST CRWPST CRWPST

CRWPST CRWPST CRWPST CRWPST Spot #

PSTG01C PSTG01C PSTG01C

WSWPST4D WSWPST4D WSWPST4D

APPENDIX C

Trace Element Compositions From SHRIMP-RG of Zircon Grains From the Peach Spring Tuff and Cathodoluminescence Images of Analyzed Zircon Crystals