a b s t r a c tNew 40 Ar/ 39 Ar and U/Th ages provide insight into the youngest eruptions at the Valles caldera and also reveal previously unknown pulses of magmatism. The youngest eruptive units, collectively termed the East Fork Member of the Valles Rhyolite, include the El Cajete pyroclastic beds and co-erupted Battleship Rock ignimbrite, and the disconformably overlying Banco Bonito lava flow. Previous attempts to date these units using a variety of techniques yielded ages ranging from b 40 ka to N 1 Ma. New 40 Ar/ 39 Ar ages were generated using the highsensitivity, multi-collector ARGUS VI mass spectrometer, which provides more than an order of magnitude increase in precision compared to most single-detector mass spectrometers. 40 Ar/ 39 Ar dating of single crystals yields a range of ages, of which the youngest populations are interpreted to represent the eruption age. Sanidine ages indicate that the El Cajete pyroclastic beds and Battleship Rock ignimbrite erupted at 74.4 ± 1.3 ka, whereas the Banco Bonito lava erupted at 68.3 ± 1.5 ka. Populations of older crystals represent variably degassed xenocrysts, explaining why previous 40 Ar/ 39 Ar bulk step-heating analyses yielded spuriously old and irreproducible results. U/Th dating of unpolished zircon surfaces also yield multiple age populations, which range from the eruption age to N350 ka, indicating protracted magmatism during the 453-ka-long eruptive hiatus prior to the eruption of the East Fork Member. 40 Ar/ 39 Ar and U/Th ages indicate that the East Fork Member represents a short (6.1 ± 2.8 ka) eruptive cycle, from a longer-lived magmatic system beneath the southern caldera. Equally short repose periods, similar to the interval between the El Cajete-Battleship Rock and Banco Bonito eruptions, are possible during future volcanism at the Valles caldera. Results demonstrate that detailed geochronology using single-crystal and in-situ techniques is necessary for understanding the eruptive history and magmatic evolution at some young volcanic systems.Published by Elsevier B.V.
High-precision 40 Ar/ 39 Ar ages of late-Quaternary monogenetic basaltic volcanoes in the Raton-Clayton volcanic field, northeastern New Mexico characterize time-space patterns of eruptions and allow assessment of volcanic hazards. Determining eruption periodicity and durations of eruptive cycles are fundamental for volcanic hazard assessment. A previous 40 Ar/ 39 Ar study concluded that volcanism in the field began at ~9 Ma and ended at ~56 ka with the eruption of the well-known Capulin volcano. However, this chronology is not sufficiently precise or complete for a comprehensive volcanic hazard assessment. High-precision ages for this study were determined using a low-volume, high-sensitivity, multi-collector ARGUS VI mass spectrometer, which yields about an order of magnitude more precise isotopic measurements compared to older generation, single-detector mass spectrometers. New 40 Ar/ 39 Ar ages indicate 1) sparse volcanic activity between 200 and 500 ka, 2) eruptions from two centers at ~100 ka, and 3) a significant pulse of volcanism between ~32 and 55 ka including as many as three eruptions that are younger than Capulin volcano. Eruptions prior to 200 ka include the 426 ± 8 ka Las Maetas South center and 240 ± 7 ka Horseshoe Crater, which are both located to the south of Capulin volcano. The Trinchera Pass volcano, located near the New Mexico-Colorado border, erupted at 113 ± 7 ka. Volcanism shifted back to the southern part of the field where numerous lava flows erupted from The Crater cinder cone. A stratigraphically low lava flow at The Crater yielded an age of 97 ± 3 ka, whereas a lava flow on the flanks of The Crater yielded an indistinguishable age of 103 ± 5 ka indicating the volcano is monogenetic. Finally, volcanic activity migrated to the vicinity of Capulin volcano. Our new age for Capulin volcano is 55 ± 2 ka. This new age is indistinguishable from, but more precise than the previously determined age of 56 ± 8 ka. Baby Capulin is located ~4 km north of Capulin volcano. Multiple analyses of lavas from this center yielded an age of 46 ± 4 ka. East of Baby Capulin is Twin Mountain volcano, which yielded an age of 40 ± 4 ka. The youngest dated center in the field is Purvine Hills, three east-west aligned vents likely representing a fissure eruption. The 40 Ar/ 39 Ar age of Purvine Hills is 32 ± 5 ka. Electron microprobe analyses of spinel inclusions in olivine were conducted to assess the source of the 46 ± 15 ka Folsom Falls lava flow. Subtle variation in Cr 2 O 3 and MgO/FeO ratios suggest that the Folsom Falls lava is the distal flow of the Baby Capulin lava flow. Ages and associated uncertainties indicate recurrence rates of 0 ka to as much as 17 ka for this youngest phase of volcanism in the Raton-Clayton field. New 40 Ar/ 39 Ar ages demonstrate the unprecedented capabilities of the ARGUS VI mass spectrometer, which is currently being used to assess temporal-spatial patterns at all late Quaternary volcanic centers in the Rio Grande rift and along the Jemez lineament.
U/Th and Ar/Ar dating was conducted to determine the age of the youngest eruptions at the Valles caldera. The youngest series of eruptions, collectively known as the East Fork Member of the Valles Rhyolite, includes the co-erupted El Cajete pumice and Battleship Rock ignimbrite, and overlying Banco Bonito lava flow. Despite the importance for hazard assessment, previous geochronology investigations have not produced conclusive eruption ages for these units. Ages range from less than 30 ka to more than 1 Ma. Ar/Ar dating was conducted using the high-sensitivity ARGUS VI mass spectrometer, which is capable of dating late Quaternary samples with the single crystal method. Ar/Ar ages indicate that the El Cajete pumice and Battleship Rock ignimbrite erupted at 74.7 ± 1.3 ka, whereas the Banco Bonito lava erupted at 68.3 ± 1.5 ka. To test the reproducibility of these eruption ages, as well as to investigate the crystallization of the East Fork Member, the U/Th method was used to date zircon. Unpolished zircon surfaces were dated using the high-spatial resolving power of the SHRIMP-RG. By analyzing the surface of zircon crystals we are able to determine the timing of crystal growth immediately prior to eruption. U/Th ages indicate that the youngest phase of crystal growth occurred at 70 ± 4 ka for the El Cajete pumice and Battleship Rock ignimbrite, and 68 ± 4 ka for the Banco Bonito lava flow. These ages agree with the Ar/Ar eruption ages. In addition to zircon that crystallized near the time of eruption, U/Th ages indicate an abundance of inherited crystals. Most antecrystic or xenocrystic zircon yield ages ranging from near the eruption age to 97 ± 11 ka, and from 149 ± 16 ka to 160 ± 20 ka. These older zircon populations reveal a previously unknown period of protracted magmatism prior to the eruption of East Fork Member.
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