J. D. Gilmour

Ar-Ar chronology of the Martian meteorite ALH84001: evidence for the timing of the early bombardment of Mars.

ALH84001, a cataclastic cumulate orthopyroxenite meteorite from Mars, has been dated by Ar-Ar stepped heating and laser probe methods. Both methods give ages close to 3,900 Ma. The age calculated is dependent on assumptions made about 39Ar recoil effects and on whether significant quantities of 40Ar from the Martian atmosphere are trapped in the meteorite. If, as suggested by xenon and nitrogen isotope studies, Martian atmospheric argon is present, then it must reside predominantly in the K-rich phase maskelynite. Independently determined 129Xe abundances in the maskelynite can be used to place limits on the concentration of the atmospheric 40Ar. These indicate a reduction of around 80 Ma to ages calculated on the assumption that no Martian atmosphere is present. After this correction, the nominal ages obtained are: 3940 +/- 50, 3870 +/- 80, and 3970 +/- 100 Ma. by stepped heating, and 3900 +/- 90 Ma by laser probe (1 sigma statistical errors), giving a weighted mean value of 3,920 Ma. Ambiguities in the interpretation of 39Ar recoil effects and in the contribution of Martian atmospheric 40Ar lead to uncertainties in the Ar-Ar age which are difficult to quantify, but we suggest that the true value lies somewhere between 4,050 and 3,800 Ma. This age probably dates a period of annealing of the meteorite subsequent to the shock event which gave it its cataclastic texture. The experiments provide the first evidence of an event occurring on Mars coincident with the time of the late heavy bombardment of the Moon and may reflect a similar period of bombardment in the Southern Highlands of Mars. Whether the age determined bears any relationship to the time of carbonate deposition in ALH84001 is not known. Such a link depends on whether the temperature associated with the metasomatic activity was sufficient to cause argon loss from the maskelynite and/or whether the metasomatism and metamorphism were linked in time through a common heat source.

Disentangling xenon components in Nakhla: Martian atmosphere, spallation and Martian interior

Abstract A powdered sample of Nakhla was separated into 3 subsamples. One was left otherwise untreated, one was washed in water and one etched with HNO3 removing 6% of the original mass. We report results of isotopic analysis of xenon released by laser step heating on aliquots of each of these subsamples; some aliquots were neutron irradiated before isotopic analysis (to allow determination of I, Ba and U as daughter xenon isotopes) and some were not. There is evidence that water soluble phases contain both martian atmospheric xenon and a component with low 129Xe/132Xe, either martian interior xenon or terrestrial atmosphere. Higher temperature data from unirradiated aliquots of the water and acid treated samples reveal two-component mixing. One is a trapped xenon component with 129Xe/132Xe = 2.350 ± 0.026, isotopically identical to the martian atmosphere as measured in shock glass from shergottites. It is associated with leachable iodine, suggesting it is trapped close to grain boundaries. It may be a result of shock incorporation of adsorbed atmospheric gas. The second component is best explained as an intimate mixture of martian interior xenon and spallation xenon. The martian interior component is present at a concentration of ∼10−12 cm3 STP g−1 132Xe, around 40 times lower than that observed in Chassigny. Its association with spallation xenon (produced from Ba and light rare earth elements) suggests it is in the feldspathic mesostasis. We propose that it was trapped during crystallisation and reflects the mantle source of the parental magma.

A resonance ionization mass spectrometer for xenon

A time-of-flight resonance ionization mass spectrometer is described which has been used to measure isotopic ratios of the nine naturally occurring xenon isotopes, with a precision of less than 0.5% for the major isotopes, in samples of 10-12 cm3 stp of xenon. The precision is essentially limited by counting statistics. A linear mass discrimination up to 0.4% per u, favouring the light isotopes, is exhibited and may be a velocity effect at the detector. No resonance effects nonlinear in mass are seen. The sensitivity of the instrument, which has a detection limit of 3*104 atoms for a particular isotope of xenon, agrees well with theoretical predictions and is comparable to that of the best electron bombardment instruments. Use of lasers with a higher duty cycle and/or a cold finger to concentrate the sample in the ionizing volume could increase the sensitivity by up to two or three orders of magnitude. Interferences due to the non-resonant ionization of hydrocarbons can be measured by tuning off resonance but are negligible in normal operation.