USE OF U-PB LASER ABLATION MULTI-COLLECTOR ICPMS ZIRCON DATING IN CONSTRAINING SEDIMENTARY FORMATION AGES: A COMPARISON WITH 40AR/39AR LASER PROBE AGES

Jurassic ash beds from the Morrison, Carmel, and Temple Cap Formations have been dated by both single-crystal ⁴⁰Ar/³⁹Ar laser fusion methods on sanidine (except SR-5-28-1 where biotite was used) and U-Pb laser ablation multi-collector inductively coupled plasma mass spectrometry on zircons.  The sample ages and associated errors are shown below.  One sample, DQW-21, from the Morrison Formation was analyzed in two different U-Pb analytical sessions using different grains. Although this is a fairly small data set, there are still some interesting trends.  In general, the ages determined by both techniques overlap at the 2σ level with the exception of the first set of analyses of sample DQW-21.  The other feature seen in the data is an apparent systematic shift in ages to younger values by as much as 2 or 3 m.y. using the U-Pb system. This is in contrast to other U-Pb to ⁴⁰Ar/³⁹Ar intercalibrations indicating that TIMS zircon ages are typically 1-2% older than the corresponding sanidine age (e.g., Min et al., 2000).

Sample	Formation	U-Pb Age Ma	+/- 2σ	40Ar/39Ar age	± 2σ
DQW-21	Morrison	145.6	+2.3/-3.1	148.97	± 0.84
DQW-21	Morrison	146.5	+2.6/-3.0	148.97	± 0.84
LCM-1	Morrison	147.4	+2.5/-2.2	150.18	± 1.00
SR-5-28-1	Carmel	166.9	+0.9/-2.1	168.6	± 1.2
GUN-B	Carmel	167.6	+3.4/-1.9	167.95	± 1.02
MWCB-14	Temple Cap	169.4	+4.2/-3.4	170.25	± 1.10