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Ion microprobe uranium‐thorium‐lead dating of Shergotty phosphates
Author(s) -
Sano Yuji,
Terada Kentaro,
Takeno Setsuo,
Taylor Lawrence a.,
Mcsween Harry y.
Publication year - 2000
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.2000.tb01779.x
Subject(s) - isochron , geology , microprobe , geochemistry , radiometric dating , uranium , thorium , closure temperature , isochron dating , whitlockite , mineralogy , apatite , metamorphic rock , materials science , metallurgy
— We report ion microprobe U‐Th‐Pb dating of Shergotty phosphates by means of the sensitive high‐resolution ion microprobe (SHRIMP) recently installed at Hiroshima University, Japan. ten analyses of whitlockite (merrillite) and three analyses of apatite indicate a 238 u/ 206 pb isochron age of 225 ± 200 ma and a tera‐wasserburg concordia‐constrained linear three‐dimensional isochron age of 217 ± 110 ma in the 238 u/ 206 pb‐ 207 pb/ 206 pb 204 pb/ 206 pb diagram. These ages agree well with the 232Th‐208pb age of 189 ± 83 Ma, which suggests that primary crystallization or a shock metamorphic event defined the formation age of the phosphate minerals. The average of the later two ages, 204 ± 68 Ma, is consistent with the previously published Rb‐Sr age of 165 ± 11 Ma and U‐Th‐Pb age of ∼200 Ma. These show marginal agreement with the 40 Ar‐ 39 Ar age of 254 ± 10 Ma but are significantly different from the Sm‐Nd age of 360 ± 16 Ma. Taking into account the closure temperature of the U‐Pb system in apatite, we suggest the time that Shergotty last experienced a temperature of ∼900 °C was 204 ± 68 Ma.