EpisodicHolocene eruption of theSaltonButtes rhyolites,California, from paleomagnetic,U‐Th, andAr/Ar dating

Abstract

In the Salton Trough, CA, five rhyolite domes form the Salton Buttes: Mullet Island, Obsidian Butte, Rock Hill, North and South Red Hill, from oldest to youngest. Results presented here include 40Ar/39Ar anorthoclase ages, 238U‐230Th zircon crystallization ages, and comparison of remanent paleomagnetic directions with the secular variation curve, which indicate that all domes are Holocene. 238U‐230Th zircon crystallization ages are more precise than but within uncertainty of 40Ar/39Ar anorthoclase ages, suggesting that zircon crystallization proceeded until shortly before eruption in all cases except one. Remanent paleomagnetic directions require three eruption periods: (1) Mullet Island, (2) Obsidian Butte, and (3) Rock Hill, North Red Hill, and South Red Hill. Borehole cuttings logs document up to two shallow tephra layers. North and South Red Hills likely erupted within 100 years of each other, with a combined 238U‐230Th zircon isochron age of: 2.83 ± 0.60 ka (2 sigma); paleomagnetic evidence suggests this age predates eruption by hundreds of years (1800 cal BP). Rock Hill erupted closely in time to these eruptions. The Obsidian Butte 238U‐230Th isochron age (2.86 ± 0.96 ka) is nearly identical to the combined Red Hill age, but its Virtual Geomagnetic Pole position suggests a slightly older age. The age of aphyric Mullet Island dome is the least well constrained: zircon crystals are resorbed and the paleomagnetic direction is most distinct; possible Mullet Island ages include ca. 2300, 5900, 6900, and 7700 cal BP. Our results constrain the duration of Salton Buttes volcanism to between ca. 5900 and 500 years.

Age of theLavaCreek supereruption and magma chamber assembly at Yellowstone based on40Ar/39Ar andU‐Pb dating of sanidine and zircon crystals

“…Geochemistry, Geophysics, Geosystems (Table 2), at the 95% confidence level including systematic uncertainties. The U-Pb dates from the zircon crystal faces are expected to reflect crystallization over centuries or millennia before eruption [e.g., Schmitt et al, 2011;Wright et al, 2015], whereas the 40 Ar/ 39 Ar sanidine date is reasonably expected to reflect cooling at the time of eruption and thus be the youngest of these two dates. Several scenarios may account for contrary relations between U-Pb and 40 Ar/ 39 Ar dates, which we briefly explore.…”

“…With account for a reasonable magnitude of melt ( 230 Th)/( 238 U) disequilibrium during zircon crystallization, there is agreement between the weighted mean 206 Pb/ 238 U date of 626.5 ± 5.8 ka for the zircon crystal faces (Table ) and the preferred or weighted mean 40 Ar/ 39 Ar isochron dates of 631.3 ± 4.3 ka or 631.6 ± 2.6 ka, respectively (Table ), at the 95% confidence level including systematic uncertainties. The U‐Pb dates from the zircon crystal faces are expected to reflect crystallization over centuries or millennia before eruption [e.g., Schmitt et al ., ; Wright et al ., ], whereas the 40 Ar/ 39 Ar sanidine date is reasonably expected to reflect cooling at the time of eruption and thus be the youngest of these two dates. Several scenarios may account for contrary relations between U‐Pb and 40 Ar/ 39 Ar dates, which we briefly explore.…”

“…By analyzing the crystal faces of unpolished zircon crystals, the several micrometer‐depth resolution typical of SIMS analysis can be used to sample the last increment of crystallization represented by the faces and outermost (few micrometers) zones of single zircon crystals. This technique has proved useful for effectively resolving near‐eruption crystallization [e.g., Vazquez and Lidzbarski , ; Wright et al ., ], and resolves the presence of xenocrysts [e.g., Storm et al ., ]. In this study, the term rim refers to the outermost few micrometers of crystal growth sampled by analyses of unpolished crystal faces, intermediate zone refers to near‐rim growth zoning (outer 50% of a crystal) sampled in sectioned crystals, and core refers to the centers of sectioned crystals.…”

Mantle Degassing Through Continental Crust Triggered by Active Faults: The Case of the Baja California Peninsula, Mexico

“…Recent volcanism has occurred at the intersection of transtensional fault domains and intervening dextral-slip transform faults. Felsic volcanism has been reported in the Salton Buttes (Robinson et al, 1976;Schmitt et al, 2013;Wright et al, 2015), Cerro Prieto (García-Sánchez et al, 2017;Schmitt et al, 2013), Roca Consag (Schmitt et al, 2013), and Delfín basins (Martín-Barajas et al, 2008). Mafic intrusive rocks and basaltic xenoliths have been found at the Cerro Prieto and Salton basin, indicating basaltic parent magma at depth (Herzig, 1990;Herzig & Elders, 1988;Schmitt et al, 2013).…”

“…The highest percentages of mantle contribution are observed at Cerro Prieto geothermal field (~87%), San Luis Gonzaga (~82%), Salton Sea geothermal field (~79%), and Las Tres Vírgenes geothermal field (~68%), which are located in zones of recent volcanic activity (Figure 7c, García-Sánchez et al, 2017;Robinson et al, 1976;Schmitt et al, 2012Schmitt et al, , 2013Wright et al, 2015).…”