Evolution of Quaternary intraplate mafic lavas detailed using 3He surface exposure and 40Ar/39Ar dating, and elemental and He, Sr, Nd and Pb isotopic signatures: Potrillo volcanic field, New Mexico, U.S.A. and San Quintin volcanic field, Baja California Norte, Mexico
Part I: Potrillo volcanic field, New Mexico, U.S.A. The Pleistocene Potrillo volcanic field (PVF) resides within the southern axis of the Rio Grande Rift, New Mexico, U.S.A., near the eastern extent of the Basin and Range Province. Its alkalic mafic volcanism has resulted in several hundred cones, flows and maars distributed over approximately 4,600 km2. Three of the five maars have brought peridotitic and lower to upper crustal xenoliths to the surface. This area has been active since 1 Ma ago to as recently as 20 ka ago. The Kilbourne Hole maar was dated at 28 ka. ^ Elemental and isotopic signatures indicate source heterogeneity coupled with varying degrees of partial melting and polybaric crystal fractionation events. Melts underwent early clinopyroxene fractionation within the mantle, but then the easternmost volcanic complexes experienced a second, shallow-level olivine fractionation history. Two isotopic groups are observed. At least three mantle reservoirs may have contributed to the melts. These observations are consistent with progressive partial melting of spinel lherzolite at unusually shallow mantle levels. ^ A complex magmatic history for the Potrillo volcanic field is now elucidated from combining improved Quaternary dating methods with detailed geochemical studies and structural information. Five phases of volcanism are recognized. Changes in magma input rates and/or differential stress fields are invoked. These findings for the Potrillo volcanic field are all causes for concern with respect to prediction of future activity trends within small mafic fields in intraplate extensional terranes. ^ Part II: San Quintín volcanic field, Baja California Norte, México. We have now gathered helium data for the San Quintín field in México to determine not only the timing of eruption events, but also to evaluate this dating method for samples collected from both aa and pahoehoe surfaces located near sea level. Multiple lava flows from 9 of the 10 exposed complexes were sampled for cosmogenic helium dating, yielding eruption dates from 165 ± 13 ka to 22 ± 5 ka (1 σ). The data provide quantitative geochronologic evidence for multiple eruptions from a single mafic volcanic center. Comparison of 3He to 40Ar/39Ar dating for the field yields agreement within the uncertainties of the these dating methods. An interesting petrologic insight is provided by the helium trapped in the olivine and clinopyroxene mineral separates. Lavas from those two centers and the other 7 centers we analyzed yield 3He/4He values ranging from 4.9 ± 0.9 R/Ra to 7.4 ± 1.1 R/Ra. However, the San Quintín trapped component determinations do overlap within analytical error. The trapped component composition may represent the low end of the range reported by Graham and others (1992) for MORB rather than a crustal signature. (Abstract shortened by UMI.) ^
Williams, Wendi Joan Whitehead, "Evolution of Quaternary intraplate mafic lavas detailed using 3He surface exposure and 40Ar/39Ar dating, and elemental and He, Sr, Nd and Pb isotopic signatures: Potrillo volcanic field, New Mexico, U.S.A. and San Quintin volcanic field, Baja California Norte, Mexico" (1999). ETD Collection for University of Texas, El Paso. AAI9944017.