40Ar/39Ar age constraints for the Jaramillo Normal Subchron and the Matuyama‐Brunhes geomagnetic boundary

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Abstract
Our mid‐Pleistocene 40Ar/39 Ar age recalibration of the geomagnetic polarity timescale is nearly in accord with the oxygen isotope, climate record calibration of the astronomical timescale proposed by Johnson (1982) and Shackleton et al. (1990). 40Ar/39Ar ages of a normally magnetized rhyolite dome in the Valles caldera, northern Mexico, yielded a weighted‐mean age of 1.004±0.019 Ma. A K‐Ar age of 0.909±0.019 Ma for this rock by Doell and Dalrymple (1966) was the linchpin for the recognition and calibration of the Jaramillo Normal Subchron (JNS). Other 40Ar/39Ar ages from the Valles caldera and 40Ar/39Ar ages of Ivory Coast tektites indicate that the JNS began at about 1.11 Ma and ended before 0.92 Ma, probably near 0.97 Ma. The Matuyama‐Brunhes boundary occurred between 0.79 Ma and 0.76 Ma on the basis of 40Ar/39Ar sanidine ages from (1) three reversely magnetized rhyolite domes of the Valles caldera (0.793±0.018 Ma, 0.794±0.007 Ma, and 0.812±0.023 Ma) and pumice (0.789±0.006 Ma) from the reversely magnetized Oldest Toba Tuff of Sumatra and (2) pumice (0.764±0.005 Ma and 0.757±0.009 Ma) from the lower and upper units of the normally magnetized Bishop Tuff. The age of the boundary may be close to 0.77 Ma as deduced from rates of sedimentation in ancient Lake Bonneville, Utah.