Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.v13a2101f&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #V13A-2101
Other
1115 Radioisotope Geochronology, 1160 Planetary And Lunar Geochronology, 1194 Instruments And Techniques, 3662 Meteorite Mineralogy And Petrology (1028, 6240), 6240 Meteorites And Tektites (1028, 3662)
Scientific paper
Measurement of a 40Ar/39Ar age in a meteorite requires proper estimation of a 40Ar/36Ar "initial" ratio. For a primordial one, it is reported to be an order of 10-4, suggesting that one can ignore it in age calculation. However, there is no basis for applying this value as individual meteorites have different histories. In terms of 40Ar/39Ar studies on various meteorites, very few studies discuss how the ratios were estimated. Using terrestrial value of 295.5 is invalid. The only case in which the ratio does not affect on the results is when an age of a meteorite is in an order of billion years, resulting in a large raw 40Ar/36Ar ratio. A proper approach to estimate the ratio is application of isochron analyses. When a small amount of sample (a few micrograms) is studied in laserprobe analysis, this often becomes difficult. A shocked meteorite Y-75097 has developed maskelynite veins, which were identified in optical microscope, electron microprobe and thermoluminescence studies. We have applied 40Ar/39Ar dating method using both stepwise heating by a continuous laser and pinpoint dating by a pulsed laser. Stepwise heating results of several fragments (about 50 micrograms) of the meteorite yielded various old and young ages. Most pinpoint analyses gave the raw 40Ar/36Ar ratio less than the terrestrial value with sporadically and anomalously old ages. Among the data, three points isochron from a maskelynite vein yielded about 300 Ma with "initial" ratio of 180. To confirm this result, plagioclase and olivine data near the vein were plotted in a correlation diagram, but an isochron is not well defined due to relatively large analytical errors. A new method to analyze these data is proposed. Once 40Ar/39Ar ratio is determined from an experiment, the only factor which determines the age is the "initial" ratio in the 40Ar/39Ar age equation. Instead of using a known fixed ratio, we set the ratio (y-axis) and the age (x-axis) as variables. In this diagram, a single data point defines a curve. If the data belong to the same group with the same ratio and age, they share a single common crossing point. This method helps identifying different groups visually. In fact, crossing points of three curves from the maskelynite vein created a relatively tight group. Furthermore, plagioclase and olivine data formed a loose group of 300 Ma age and ratio value of zero. These results suggest that the different minerals in a few millimeters recorded the same shocked age with different "initial" ratio. Examples of several other studies on shocked meteorites are compared and a possible origin of anomalous old ages will be discussed.
Fujimoto Hirofumi
Hyodo Hironobu
Ninagawa Kiyotaka
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