We obtained single-grain (U-Th)/He ages from 14 merrillite and five chlorapatite aggregates from St. Séverin to constrain its low-T thermal history. In addition, we performed 3He/4He stepped heating diffusion experiments for merrillite and chlorapatite crystals from Guareña chondrite to better constrain He diffusion properties in chondritic phosphates in general. Among the 19 phosphate grains from St. Séverin, the five oldest merrillites and four oldest chlorapatites yielded weighted mean ages of 4412±75Ma (1σ: MSWD=0.34) and 4152±70Ma (MSWD=0.48), respectively. These weighted mean ages overlap with the peaks of the corresponding probability density plots, thus likely represent the most pristine (U-Th)/He ages of St. Séverin. The radiogenic 4He and proton-induced 3He diffusion experiments on Guareña chondrite resulted in two well-defined linear trends in Arrhenius plot for merrillite (radius=∼59μm) and chlorapatite (∼43μm) grains. Error-weighted linear regressions of the 3He data yielded following diffusion parameters: Ea=135.8±4.8kJ/mol, and ln(Do/a2)=5.83±0.66ln(s-1) for merrillite; and Ea=109.3±9.7kJ/mol, and ln(Do/a2)=8.15±1.93 ln(s-1) for chlorapatite. Assuming the analyzed fragments approximate the diffusion domain, logDo was calculated as (-1.93±0.29) log(cm2/s) and (-1.19±0.84) log(cm2/s) for merrillite and chlorapatite, respectively. These results indicate higher closure T for merrillite than chlorapatite. Assuming these results also apply to St. Séverin and that crystal dimensions define the limiting diffusive length-scale, the closure temperatures of merrillite and chlorapatite in St. Séverin are estimated to be 94-112°C (for cooling rates of 0.3-2.6°C/Ma) and 18°C (for 0.3°C/Ma), respectively. The new single-grain (U-Th)/He ages and 3He/4He stepped heating diffusion results, combined with previously reported Pb/Pb and 40Ar/39Ar (with updated λ-tFCs pairs) data indicate that St. Séverin experienced relatively rapid cooling from 477°C down to ∼100°C at a cooling rate of 2.6°C/Ma. The identified thermal history is generally consistent with the reported cooling path from Pu fission track data. These results suggest that St. Séverin experienced relatively slow cooling compared to H chondrites, which may have resulted from its parent body size being larger than that of the H chondrites.
ASJC Scopus subject areas
- Geochemistry and Petrology