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McIntosh, W C (2004): Summary of 40Ar/39Ar results from sediment core CRP-2A (Table 1). PANGAEA,, Supplement to: McIntosh, WC (2000): 40Ar/39Ar geochronology of tephra and volcanic clasts in CRP-2A, Victoria Land Basin, Antarctica. Terra Antartica, 7(4), 621-630, hdl:10013/epic.28247.d001

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40Ar/39Ar analyses of tephra and clasts of volcanic rock provide age constraints for upper parts of the CRP-2A core. Single-crystal laser-fusion analyses of anorthoclase phenocrysts from three tephra-bearing layers yielded the most precise age constraints for CRP-2A. The dated tephra layers are: 1) a 2.7-m-thick interval of pumice and ash layers between 111.5 and 114.2 meters below sea floor (mbsf) (weighted mean age = 21.44 ± 0.05 Ma, +2.2); 2) a concentration of pumice near 193.4 mbsf (23.98 ± 0.13 Ma): and 3) a concentration of pumice near 280 mbsf (24,22 ± 0.03 Ma) (all ages are calibrated relative to Fish Canyon Tuff sanidine at 27.84 Ma). The 111 to 114 mbsf tephra is almost entirely non-reworked, and the 193 mbsf and 280 mbsf tephra concentrations are interpreted as being reworked and redeposited soon after eruption. All three of the tephra ages are therefore considered to be equivalent to depositional ages. The variation in precision of these three age determinations is largely a function of phenocryst size and abundance. The accuracy of these ages is equal to the accuracy of the current calibration of the 40Ar/39Ar methode (about ± 1 %). 40Ar/39Ar results from volcanic clasts provide three additional maximum age constraints for the CRP-2A core. Single-crystal laser-fusion of sanidine phenocrysts from a rhyolitic clast from 294 mbsf yielded a precise maximum depositional age of 24.98 ± 0.08 Ma, and plateau ages of groundmass concentrates from basaltic clasts near 36.02 mbsf and 125.92 mbsf yielded maximum depositional ages of 19.18 ± 0.12 Ma, and 22.56 ± 0.14 Ma, respectively. The 40Ar/39Ar data, in association with biostratigraphic, paleomagnetic, and isotopic age constraints for CRP-2A, confirm interpretation for rapid sedimentation rates in the 36 to 280 mbsf interval, particularly in the 193 to 280 mbsf interval where they support interpretations for sedimentation cycles spanning 100 k.y. intervals. In addition to the 19 to 25 Ma ages measured from thephra layers and clasts, provenance-related ages ranging from 150 to 450 Ma were determined from clasts and individual detrital or xenocrystic crystals from CRP-2A.
Latitude: -77.005980 * Longitude: 163.719450
Date/Time Start: 1998-10-16T07:30:00 * Date/Time End: 1998-11-25T14:20:00
Minimum DEPTH, sediment/rock: 36.02 m * Maximum DEPTH, sediment/rock: 453.58 m
CRP-2A (14.2 km at 096° true from Cape Roberts) * Latitude: -77.005980 * Longitude: 163.719450 * Date/Time Start: 1998-10-16T07:30:00 * Date/Time End: 1998-11-25T14:20:00 * Elevation: -177.9 m * Recovery: 624 m * Location: off Cape Roberts, Ross Sea, Antarctica * Campaign: CRP-2 * Basis: Sampling/drilling from ice * Method/Device: Core wireline system (CWS) * Comment: 921 m at 284° from CRP-1. Sea-ice thickness: 2.0 m (1 Oct) to 2.2 m (23 Nov). Sea riser embedded to 13.03 mbsf. Lateral ice movement: 9.87 m to east from 17 Oct to 23 Nov. HQ core to 199.31 mbsf. NQ core to 624.15 mbsf. 13.03 to 45.97 mbsf, 11.29 m (34%) partially following CRP-2. 45.07 to 624.15 mbsf, 548.67 m (95%) new hole. Deepest core-lithology: hard sandy siltstone. Deepest core-age: earliest Oligocene (ca. 33 Ma on diatoms, nannofossils and dinoflagellates)
#NameShort NameUnitPrincipal InvestigatorMethod/DeviceComment
1DEPTH, sediment/rockDepth sedmGeocode
2Sample IDSample IDMcIntosh, W C
3Sample commentSample commentMcIntosh, W C
4CommentCommentMcIntosh, W CIrradiation: 6 seperate in vacuo 7-hr irradiations, D-3 position
5Age, dated materialDated materialMcIntosh, W C
6Sample methodSample methodMcIntosh, W CAnalysis
7NumberNoMcIntosh, W CNumber of crystals in weighted mean or heating steps in plateau
8Argon-3939Ar%McIntosh, W CIsotope ratio mass spectrometryPercent of 39 Ar released within the plateau age interval
9Calcium/Potassium ratioCa/KMcIntosh, W C
10Age, datedAge datedkaMcIntosh, W CIsotope ratio mass spectrometryMolar ratio calculated from reactor produced 37 Ar Ca and 39 Ar K.
11Age, errorAge e±McIntosh, W Csee commentCalculated using inverse radiance weighting of Samson and Alexander (1987)
103 data points

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