40Ar/39Ar dating of Pleistocene tuffs; an accurate age for the Matuyama-Brunhes geomagnetic reversal (MBGR)

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http://abstractsearch.agu.org/meetings/2012/FM/V21E-07.html
Author(s): Mark, D. F.; Renne, P. R.; Morgan, Leah E.; Deino, A.; Smith, Victoria C.; Ellis, Ben S.; Pearce, N. J.
Author Affiliation(s): Primary:
Scottish Universities Environmental Research Centre (SUERC), East Kilbride, United Kingdom
Other:
Berkeley Geochronology Center (BGC), United States
University of Oxford, United Kingdom
ETH Zurich, Switzerland
University of Aberystwyth, United Kingdom
Volume Title: AGU 2012 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2012; American Geophysical Union 2012 fall meeting, San Francisco, CA, Dec. 3-7, 2012. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: Recent recalibrations of the 40Ar/39Ar system reveal inconsistencies with some previous ages inferred for the MBGR. An Ar/Ar age for the Bishop Tuff (BT) (which post-dates the MBGR by at least 15.3±2.2 ka) recalculated yields an age of 778.0±3.8 ka (1σ, full systematic uncertainty). The age is c. 10 ka older than the BT zircon ID-TIMS U-Pb age and places the MBGR at c. 793 ka, c. 13 and 20 ka older than astronomical ages for the MBGR of 780 ka and 773 ka, respectively. To determine an accurate age for the MBGR, we have made a series of 40Ar/39Ar age determinations for Pleistocene tuffs from both Indonesia and North America that have direct relationships to the MBGR. Blind analyses were conducted at SUERC and BGC. We observed excellent inter-laboratory agreement and no systematic offset in data. Ar/Ar ages are reported (1σ, full systematic uncertainty). Drill cores from ODP Site 758 show the precise location of the MBGR. Below the MBGR are two distal tephra horizons that we have identified as products of two temporally distinct Old Toba Tuff (OTT) eruptions (layer d OTT1 and layer D OTT2). Continuous sedimentation between OTT1 (802.8±0.7 ka, n=100, MSWD 1.2) and OTT2 (796.2±0.8 ka, n=62, MSWD 1.3) allows for calculation of an accurate sedimentation rate and for extrapolation of an age from OTT2 to the MBGR. Data define an age for the MBGR of 795.2±0.9 ka. Using tephra above the MBGR boundary, the Middle Toba Tuff (layer C) and Young Toba Tuff (layer A), extrapolation down core supports a MBGR age of c. 795 ka. Recent age data for BT sanidine reported relative to FCs at 28.172 Ma (767.4±1.1 Ma) oddly yielded an Ar/Ar age that was indistinguishable from the BT zircon U-Pb age, which is consistent with previous 40Ar/39Ar age measurements made relative to FCs at 28.02 Ma. Thus we made a series of 40Ar/39Ar measurements on the exact same sample as used by Rivera et al. and observed potential for age biasing owing to small grain size and the requirement to run large crystal populations rather than single crystals. To obtain a reliable BT age we have made new 40Ar/39Ar measurements on a different (coarser) BT sample. Single crystal sanidine data define a robust age of 776.4±0.9 ka (n=100, MSWD 0.6), which is indistinguishable from previous age data, but c. 6 ka older than the BT age, placing the minimum age for the MBGR at 791.7±2.4 ka. 40Ar/39Ar data from the Indonesian and North American tuffs yield ages for the MBGR that are indistinguishable at 68% confidence. The offset between the MBGR and the ages for the Pleistocene tuffs is calibration-independent and as such both Crowley et al. and Channell et al. cannot be right with respect to their proposed ages for the BT and the MBGR, respectively--at least(!) one must be wrong beyond their stated uncertainties.
Year of Publication: 2012
Research Program: ODP Ocean Drilling Program
Key Words: 24 Surficial Geology, Quaternary Geology; Brunhes Chron; Cenozoic; Igneous rocks; Indian Ocean; Leg 121; Magnetic field; Marine sediments; Matuyama Chron; Ninetyeast Ridge; ODP Site 758; Ocean Drilling Program; Paleomagnetism; Pleistocene; Pyroclastics; Quaternary; Reversals; Sediments; Tuff; Upper Cenozoic; Upper Quaternary; Volcanic rocks
Coordinates: N052302 N052303 E0902141 E0902140
Record ID: 2014070754
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