Isotopic compositions have been measured mass spectrometrically for xenon fractions released from the carbonaceous chondrite Murray in stepwise heating experiments. The isotopic ratios varied quite considerably; for example, the 136Xe/132Xe ratio in the 1000°C fraction was almost identical to the atmospheric ratio (0.330), whereas the ratio in the 1300°C fraction agreed with that in Sucor (0.305). It appears that these variations can best be explained as being due to the fact that reservoirs of two isotopically distinct gases (solar and planetary) exist in the meteorite and that mixtures of these gases in various proportions are being released at different temperatures. The major difference in the isotopic compositions of solar and planetary xenon can be attributed to a mass‐dependent fractionation process, which must have occurred during the early stages of the history of the solar system. The abundance ratios of the xenon isotopes are also altered by the cosmic ray irradiation and neutron capture processes. According to this interpretation, it is unnecessary to assume the existence of the so‐called carbonaceous chondrite fission component. The decay products of extinct radionuclides 129I and 244Pu did not alter the xenon isotopic ratios significantly in the case of the carbonaceous chondrite Murray. The light isotopes 124Xe and 126Xe in the earth's atmosphere appear to be partially of cosmic ray origin.
The activities of 210Pb, 210Bi, and 210Po were measured in 17 rain samples that were collected at Fayetteville, Arkansas, during the months of April–June 1973, and 210Pb and 210Po only were measured in 52 additional rain samples collected during the months of September‐December 1973 and January‐April 1974. In approximately 70% of the cases (12 out of 17 rains) the residence times calculated from the 210Bi/210Pb and 210Po/210Pb ratios were either concordant or near concordant. In about 30% of the cases (5 out of 17 rains) the 210Bi/210Pb ratios yielded mean residence time values of 3 to 6 days, whereas the values calculated from the 210Po/210Pb ratios varied from 16 to 40 days. The discordant residence times can be shown to result from the mixing of two different air masses: one represented by a short and the other by a much longer residence time of the radon daughters. In order to test this idea the radionuclides 210Pb and 210Po were sequentially sampled from three rainstorms that occurred at Fayetteville, Arkansas, on October 6, 1973, February 18, and April 18, 1974. In the case of the October 6, 1973, rain the 210Po/210Pb ratio was found to vary markedly within a single rainfall, and the data indicated that a mixing of two air masses occurred during the rainstorm: one air mass was represented by a residence time of about 30 days and the other by roughly 300 days. These values are suggestive of the mean residence times of aerosols for the entire troposphere (about 30 days) and for the entire stratosphere (about 1 year) frequently mentioned by previous investigators. In the case of the April 18, 1974, rain the mean residence time calculated from the 210Po/210Pb ratios remained fairly constant at about 30 days, whereas the February 18, 1974, rain was an intermediate case, and the mean residence time varied between 39 and 135 days.
The isotopic compositions have been measured mass spectrometrically for xenon fractions released from the carbonaceous chondrite Murchison in stepwise heating experiments. Variation of the isotopic ratios was found to be relatively small: for example, the 136Xe/132Xe ratio in the 600°C fraction was 0.328 (the atmospheric ratio is 0.330), and the ratio in the 1500°C fraction was 0.310 (the Sucor ratio is 0.301), whereas the ratios observed in other temperature fractions had intermediate values. It appears that these variations can best be explained as being due to the fact that reservoirs of two isotopically distinct gases (solar and planetary) exist in the meteorite and that mixtures of these gases in various proportions are being released at different temperatures. The major difference in the isotopic compositions of solar and planetary xenon can be attributed to a mass‐dependent fractionation process, but the solar xenon contains excesses of xenon isotopes at mass numbers 128, 130, 131, and 132 that appear to have been produced by neutron capture processes that took place in the sun during its deuterium‐burning stage. The solar xenon must have been transported from the sun to the meteorites in the form of solar wind. The relative abundances of the light isotopes of xenon in carbonaceous chondrites are appreciably altered by the cosmic ray irradiation process. The decay products of 129I and 244Pu further modify the isotopic compositions of xenon fractions released from the carbonaceous chondrites. According to this interpretation, it is unnecessary to assume the existence of the so‐called Renazzo‐type fission xenon component (CCF or xenon X) in the carbonaceous chondrites.
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