Masters of Science student, Karissa Vermillion, from New Mexico State University received an award for her proposal which is summarized below. Karissa will be mentored by Dr. Jake Ross.
The New Mexico Geochronology Research Laboratory (NMGRL) is a participant in the “Awards for Geochronology Student Research” program (AGeS2 ). AGeS2 grants are funded by the National Science Foundation Earthscope program, in conjunction with the Geological Society of America, and are designed to link students with geochronology laboratories to facilitate in depth student understanding of geochronology methods with hands on experience ultimately leading to publication of new data.
In southern New Mexico, a flare-up of voluminous caldera volcanism from ~38-23 Ma  generated the Mogollon-Datil (MDVF) and Boot Heel (BHVF) volcanic fields. Despite excellent exposure of calderas and volcanic deposits (Fig. 1A, B), minimal modern 40Ar/39Ar geochronology has made it difficult to elucidate spatial and temporal variations in volcanism. Importantly, this silicic caldera volcanism is thought to be related to foundering and rollback of the Farallon plate and associated asthenospheric upwelling [e.g., 8, and references therein]. Primary and detrital sanidine geochronology of distal volcanic deposits in the MDVF and BHVF, correlated to source caldera tuffs by high-precision single crystal geochronology, will be used to establish spatial and temporal variations of volcanism in southern New Mexico. Using this 40Ar/39Ar geochronology, this study will test the following hypotheses: 1) volcanism in southern New Mexico migrated westward over time in response to tearing and foundering of the Farallon slab [e.g., 8], and 2) major volcanic eruptions are periodic. Results from this work have implications for tectonic and crustal processes controlling voluminous silicic volcanism.
The voluminous volcanics of the Mogollon-Datil (MDVF) and Boot Heel volcanic fields (BHVF) in southern New Mexico (Fig. 1A) have been the focus of studies seeking to constrain the timescales [1- 7] and geochemistry [1,3,5,6] of volcanism; however, there are minimal modern age constraints. Caldera volcanism in southern New Mexico is thought to have initiated and migrated westward in response to tearing and foundering of the Farallon slab; initial foundering may be responsible for the large volumes of silicic magma generated in such a short time-span [e.g., 8]. The repose interval between eruptions should also relate to magma flux and crustal processing. Past data (although antiquated), combined with some recent high-precision age dates (e.g., Organ , Doña Ana , and Schoolhouse Mountain  calderas, Fig. 1A) suggests that volcanism in the MDVF is episodic, with major eruptions every ~0.5 Ma. Yet these important questions of spatial and temporal variations are difficult to answer without modern high-precision 40Ar/39Ar geochronology and robust correlations between volcanic deposits and source calderas. Geochemistry could be used to link volcanic deposits and source calderas, however major element geochemistry of the tuffs are virtually indistinguishable (e.g., ) and even whole rock 87Sr/86Sr is uniform in tuffs from 36-27 Ma (Fig. 2); thus geochemistry of the deposits alone can’t be used to distinguish between different eruptions. Modern geochronology (i.e., sanidine age precision of <20 ka (1 sigma)), of both tuffs at source calderas and regional volcanic deposits (Fig. 1A, B) is thus necessary to link the two and to establish temporal and spatial variations in volcanism. High-precision magmatic and detrital sanidine 40Ar/39Ar geochronology of deposits and calderas ~38-33 Ma, coupled with recent high-precision dates on three calderas in the MDVF [2,7,11], will allow for robust assessment of the spatial-temporal evolution of the MDVF. For example, changes in the direction and rate of migration of volcanism should reflect the geometry and rate of removal of the foundering Farallon slab at depth (e.g., ). Furthermore, these data will reveal whether eruptions were episodic or clustered. If episodic, this would imply that the rate of mantle upwelling and crustal processing to generate the rhyolites is ~constant over time. These results will impact our understanding of tectonic and crustal processes driving silicic volcanism in complex regimes.
Continuous, exposed stratigraphy of the lower Bell Top Formation (BTF, Fig. 1B), comprised of numerous tuffs and pyroclastic fall deposits of undetermined origin, is found central to several calderas thought to be active ~37-33 Ma (Fig. 1A). Deposits of the lower BTF are bracketed by new high-precision dates on the underlying Palm Park formation (40-45 Ma; ), and ongoing work on upper BTF. Pyroclastic fall deposits include discrete ash and pumice layers and reworked volcaniclastics (Fig. 3); detailed stratigraphy will resolve the number of eruptions preserved. Although some of these units (Fig. 1B) were previously dated by 40Ar/39Ar geochronology [4,5,6], the majority of the units in this section are undated and major technical advances in mass spectrometry have occurred at New Mexico Tech since 2000. Thus, new ages will be far more robust than relying on antiquated literature data. Additionally, because of wellexposed stratigraphic relationships, the accuracy of ultra-precise dates can be evaluated based on superposition.
To test the hypotheses of this study, high-precision sanidine 40Ar/39Ar geochronology is required on deposits proximal to the calderas and on distal deposits. Samples to be dated include tuffs and pyroclastic fall sequences of the lower BTF and tuffs from calderas in the region that lack recent high-precision dates (Fig. 1A). Geochronology will be completed at the New Mexico Geochronology Research Laboratory (NMGRL). High-precision single crystal (sanidine) laser fusion techniques will be used to date tuffs and fall layers and detrital sanidine geochronology will be used for reworked volcaniclastic units. The detrital sanidine technique utilizes a large sample size and population of youngest ages to provide a maximum depositional age. The various populations in the distribution of sanidine ages for a volcaniclastic unit is used to identify potential volcanic sources. The geochronology of the lower BTF and source caldera tuffs will be used to establish correlations between the two, thus constraining the ~40-33 Ma window of volcanism, and enabling assessment of the direction and rate of migration of volcanism, and tempo of volcanism, over time. If volcanism migrated westward at an approximately constant rate, that would imply a steady, westward rollback of the Farallon slab. If eruptions are found to be episodic, this indicates an approximately steady flux of upwelling mantle and similar crustal processing to make the rhyolites.
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