Overview: The injection of mafic magma into a cooler crustal reservoir is often proposed as a trigger for volcanic eruptions. If the arrival of hot recharge magma does provide the immediate trigger for an eruption, then mineral compositions should reflect a final heating event immediately prior to eruption. Recent work at the Chaos Crags member of the Lassen Volcanic Center, California, identifies evidence for pronounced post-recharge cooling and crystallization of mafic enclaves, suggesting that time must have elapsed between the arrival of mafic recharge magma and eruption (Scruggs and Putirka, 2018). We test if the arrival of mafic recharge magma can explain the timing of a series of eruptions at Brokeoff Volcano, a Pleistocene aged stratovolcano in the Lassen Volcano Center, by measuring mantle-to-rim compositional transects and calculating thermal profiles for 92 orthopyroxene, clinopyroxene, and plagioclase phenocrysts from ten andesite flows at Brokeoff Mountain. Seventy-four (81%) of the analyzed thermal profiles record the arrival of high-temperature recharge magma followed by post-recharge cooling back toward pre-recharge temperatures, while only seventeen (19%) thermal profiles record heating immediately prior to eruption. Most mineral thermal profiles from nine of the andesitic flows exhibit evidence for post-recharge cooling, while only one flow primarily preserves evidence for eruption immediately following recharge. This analysis suggests that while the arrival of recharge magma may be necessary to produce an eruptible magma, the increase in volume and heat flux associated with recharge may be insufficient to directly trigger some volcanic eruptions.

Ongoing Work: I'm in the process of drafting the manuscript for publication. I also plan to measure additional compositional transects to improve the representation of some mineral phases, particularly CPX, in the dataset. I am also collaborating with a researcher to employ OH-in-plagioclase hygrometry to improve the equilibrium melt water content estimates used in the feldspar thermometry calculations.

Overview: A long-standing issue in Cordilleran geology involves the nature of Basin and Range (western USA) volcanism, and whether such magmatism provides a trigger for extensional deformation, or if volcanic activity is a passive response to extension. We use space-time-composition patterns across the central and southern Basin and Range, and where appropriate, reconstructed latitudes and longitudes of volcanic rocks, to show that volcanism is fundamentally a passive process. Our analysis suggests that Basin and Range volcanism is initiated by the transition from a subduction to a transform boundary (now manifest as the San Andreas fault), which causes a slab window to open, as the subducting Farallon plate falls away. Accordingly, volcanic activity follows the northward-migrating Mendocino Triple Junction (MTJ). In the wake of the MTJ, continental mantle lithosphere is heated over a time scale of 10–12 Ma; it then rapidly degrades (or is removed) 17–20 Ma after MTJ arrival at any given latitude, and is replaced by asthenosphere. In the central Basin and Range, MTJ migration triggers the well-documented structural migration of the Sierra Nevada away from the Colorado Plateau. Not only is volcanism triggered by the tectonic transition, but in the central Basin and Range volcanism also migrates west, following the initiation of upper crustal extensional faulting; the lag between the onset of extension and initiation of volcanism is a remarkably consistent 2 Ma. Within this framework, the initial stage of eastward-concentrated volcanism is dominated by felsic magmatism, which then gradually degrades to a more mafic composition. This pattern, and temporal relations indicate that once lithospheric extension begins, between 2 and 5 Ma are needed to develop conduits through which nonviscous magmas can transit the crust, and that especially high amounts of extensional strain favor the eruption of K2O-rich, low-degree partial melts. Our observations indicate that it is unlikely that mantle plume processes initiated Basin and Range volcanism within the study area, and that decreases in SiO2 and increases in incompatible element abundances to the east within the Cordillera are best explained by continental mantle lithosphere that thickens to the east, which reduces average melt fractions.

My role: I developed an ArcGIS-based tectonic reconstruction model based on Snow and Wernicke (2000), which I used to calculate the eruptive locations of volcanic rocks from the NAVDAT and GEOROC databases. I was then able to calculate the age of each sample relative to the onset of extension, the arrival of the Mendocino Tripple Junction (MTJ), and analyze sample time-space-compositional trends.

Results: Preliminary results presented at the American Geophysical Union fall meeting (Platt and Putirka, 2011) and published in the journal Geosphere (Putirka and Platt, 2012).

Overview: The Kern Plateau, a low relief and high elevation region located in the southeastern Sierran Microplate, contains four rhyolitic and several basaltic Pleistocene aged volcanic centers, which is atypical for the nominally stable tectonic setting. As of the time of this study, only preliminary work had been performed to characterize the geochemistry and petrography of the rhyolite flows, which ranged in composition from the porphyritic high-silica rhyolite at Long Canyon Dome to the almandine-fayalite rhyolite of Monache Mountain (Bacon and Duffield, 1981). The objective of this study was to map the extent of flows, measure major and trace element compositions, and develop a petrogenetic model to explain the occurrence of rhyolitic volcanism within the nominally stable microplate.

My role: I collected hand samples from Monache Mountain and Long Canyon Dome for geochemical and petrographic characterization. I also collected both oriented and unoriented hand samples from Long Canyon dome. Oriented hand samples were re-oriented, drilled, and demagnetized in the paleomagnetism laboratory at CSU Fresno. Thin sections were cut from representative samples from each flow. Each sample was ground into powder to be analyzed with the XRF and XRD at CSU Fresno.

Results: Mapping the extent of flows at Monache and Long Canyon Domes was complicated by the limited extent of in place outcrop at Monache Mountain (limited to a single outcrop) and a prior pumice mining operation at Long Canyon Dome. Compositional variations were identified in thin sections between the one in-place and several float samples collected at Monache Mountain, however, the XRF broke before geochemical characterization of all samples could be completed. An analysis of paleomagnetic data from oriented hand samples collected at Long Canyon Dome suggests that there was a minimum of two eruptions from that volcanic center, which were consistent with two tephra samples collected in the Manix Basin that were previously postulated to have been derived from Long Canyon Dome.

This project was my original Senior Thesis topic, however following the failure of the CSU Fresno XRF I had to discontinue this study and began working on a new project to assess the relationship between the onset of extension and volcanism in the Basin and Range.