Mathematics – Probability
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p14c..04m&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P14C-04
Mathematics
Probability
[1115] Geochronology / Radioisotope Geochronology, [1140] Geochronology / Thermochronology, [1160] Geochronology / Planetary And Lunar Geochronology, [6240] Planetary Sciences: Solar System Objects / Meteorites And Tektites
Scientific paper
Thermal evolution of chondrites provides valuable information on the heat budget, internal structure and dimensions of their parent bodies once existed before disruption. St. Severin LL6 ordinary chondrite is known to have experienced relatively slow cooling compared to H chondrites. The timings of primary cooling and subsequent thermal metamorphism were constrained by U/Pb (4.55 Ga), Sm/Nd (4.55 Ga), Rb/Sr (4.51 Ga) and K/Ar (4.4 Ga) systems. However, cooling history after the thermal metamorphism in a low temperature range (<200 °C) is poorly understood. In order to constrain the low-T thermal history of this meteorite, we performed (1) single-grain (U-Th)/He dating for five chlorapatite and fourteen merrillite aggregates from St. Severin, (2) examination of textural and chemical features of the phosphate aggregates using a scanning electron microscope (SEM), and (3) proton-irradiation followed by 4He and 3He diffusion experiments for single grains of chlorapatite and merrillite from Guarena meteorite, for general characterization of He diffusivity in these major U-Th reservoirs in meteorites. The α-recoil-uncorrected ages from St. Severin are distributed in a wide range of 333 ± 6 Ma and 4620 ± 1307 Ma. The probability density plot of these data shows a typical younging-skewed age distribution with a prominent peak at ~ 4.3 Ga. The weighted mean of the nine oldest samples is 4.284 ± 0.130 Ga, which is consistent with the peak of the probability plot. The linear dimensions of the phosphates are generally in the range of ~50 µm to 200 µm. The α recoil correction factor (FT) based on the morphology of the phosphate yields improbably old ages (>4.6 Ga), suggesting that within the sample aggregates, significant amounts of the α particles ejected from phosphates were implanted into the adjacent phases and therefore that this correction may not be appropriate in this case. The minimum FT value of 0.95 is calculated based on the peak (U-Th)/He age and 40Ar/39Ar data which provide the upper limit of the α-recoil-corrected (U-Th)/He ages. From these data, we conclude that the St. Severin cooled through the closure temperatures of chlorapatite and merrillite during ~4.3 - 4.4 Ga. The radiogenic 4He and proton-induced 3He diffusion experiments yield two well-defined linear trends in Arrhenius plot for chlorapatite (r = 43 µm) and merrillite (r = 59 µm) grains. The linear regression of 3He data for chlorapatite yields Ea = 128.1 ± 2.4 kJ/mol, and ln(Do/a2) = 11.6 ± 0.5 ln(s-1) which are generally consistent with the terrestrial Durango apatite and meteoritic Acapulco apatite. Linear regression to the merrillite data corresponds to Ea = 135.1 ± 2.5 kJ/mol, and ln(Do/a2) = 5.73 ± 0.37 ln(s-1). The new data indicate that diffusive retentivity of He within merrillite is significantly higher than that of chlorapatite, which has implications for quantitative interpretation of He ages measured in meteoritic phosphates.
Min Kyoung
Reiners Peter W.
Shuster David L.
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