Figure F5. Age plateau and isochron diagrams for 40Ar-39Ar incremental heating experiments, performed at Oregon State University, for ODP Hole 1138A ash layers. Plateau ages are the weighted means of consecutive, concordant step ages constituting at least 50% of the total gas released during incremental heating experiments; isochron ages are determined from least-squares regression of collinear step compositions. The 40Ar-39Ar intercept value gives the Ar composition in the mineral/glass at the time of crystallization. A. Sample 183-1138A-12R-3, 5-7 cm (glass). B. Sample 183-1138A-13R-1, 22-24 cm (biotite). C. Sample 183-1138A-15R-1, 73-75 cm (glass). D. Sample 183-1138A-19R-1, 110-112 cm (glass). E. Sample 183-1138A-19R-1, 110-112 cm (biotite). Calculations and plots were produced using ArArCALC (version 21) by Koppers (2002).

Figure F5. Age plateau and isochron diagrams for ⁴⁰Ar-³⁹Ar incremental heating experiments, performed at Oregon State University, for ODP Hole 1138A ash layers. Plateau ages are the weighted means of consecutive, concordant step ages constituting at least 50% of the total gas released during incremental heating experiments; isochron ages are determined from least-squares regression of collinear step compositions. The ⁴⁰Ar-³⁹Ar intercept value gives the Ar composition in the mineral/glass at the time of crystallization. A. Sample 183-1138A-12R-3, 5-7 cm (glass). B. Sample 183-1138A-13R-1, 22-24 cm (biotite). C. Sample 183-1138A-15R-1, 73-75 cm (glass). D. Sample 183-1138A-19R-1, 110-112 cm (glass). E. Sample 183-1138A-19R-1, 110-112 cm (biotite). Calculations and plots were produced using ArArCALC (version 21) by Koppers (2002).