40AR/39AR MULTI-COLLECTOR REVOLUTION AND AGE OF THE MATUYAMA-BRUNHES BOUNDARY

The Matuyama-Brunhes (MB) boundary marks the base of the Middle Pleistocene Subseries, thus it is global tie point for records of the transition from obliquity- to eccentricity-dominated climate states. Constraints on its age come from marine sediments including ODP cores and on-land sections at Chiba, Japan, and Montalbano Jonico, Italy. ODP cores through North Atlantic sediments from which O isotope-based astrochronologic age models have been generated indicate that the MB reversal occurred at 773±0.8 ka (2s). At Chiba an astrochronologic age model is anchored by a ²³⁸U/²⁰⁶Pb zircon date obtained by ion microprobe from an ash bed 110 cm below the MB boundary, indicating that the reversal occurred 771.7±7.3 ka. At Montalbano Jonico, an astrochonology for a ¹⁰Be/⁹Be proxy of magnetic field intensity is anchored by the ⁴⁰Ar/³⁹Ar sandine age of 773.9±1.3 ka of an ash bed suggesting that the MB reversal occurred 776-772 ka. These estimates have called into question the ⁴⁰Ar/³⁹Ar age of the reversal determined more than a decade ago by dating transitionally-magnetized lava flows from Haleakala volcano, Maui, using a single collector mass spectrometer and pre-EarthTime standards and lab procedures. To address this issue, and test whether lacustrine and marine sediments at Sulmona, and Valle de Manche, Italy, record the MB reversal at 786 ka, we re-determined the ⁴⁰Ar/³⁹Ar age of the Haleakala lava sequence by measuring groundmass using an ion-counting 5 collector mass spectrometer characterized by low argon background, high mass resolving power, and 100x the sensitivity of single-collector instruments (Jicha et al., 2016, Chem Geol). Experiments were monitored with Alder Creek sanidine calibrated to 1.1864±0.0006 Ma relative to 28.201 Ma Fish Canyon sanidine, in agreement with recent results from several labs. Experiments on 19 subsamples from 5 lava flows spanning the base to top of the Haleakala sequence give plateau dates indistinguishable from one another. Their weighted mean age of 772.2±2.0 ka confirms at the millenial (±1 to 2 kyr) level the accuracy of the astrochronology derived from the ODP cores, Chiba section, and Montalbano Jonico. Our findings do not support claims that the MB reversal occurred prior to 780 ka, nor are any of these marine sediment records consistent with a duration of the MB reversal shorter than 3 kyr.