Rb-Sr Systematics of the Ungrouped Achondrites

Initial Sr chronometry was proposed by [1] and based on different condensation temperature of Rb (≈800K) and Sr (≈1465K) [2].It allows estimating the time of separation of the low-Rb/Sr achondrite precursors from high-Rb/Sr solar nebula [3] by comparing initial 87 Sr/ 86 Sr of CAIs [4] and achondrites.We analysed Rb-Sr system of eight achondrites: Ibitira, Asuka 881394, NWA 11119, eucrite NWA 8661, NWA 6704, NWA 4587, NWA 7325, and ureilite ALM-A.Two plagioclase fractions from each meteorite (for NWA 4587 whole-rock fractions) were ultrasonically cleaned, then one was leached in cold 0.5M HNO3 and hot 6M HCL with dissolution in the mixture of HF and HNO3, while the second fraction was dissolved without acid leaching.Sr was separated using cation exchange chromatography with Sr Spec resin and measured by three-line multidynamic method on the Triton Plus at RSES, ANU.For our set of achondrites, we have got following initial 87 Sr/ 86 Sr ratios: 0.698961±0.000016for Ibitira, 0.698972±0.000008for Asuka 881394, 0.698934±0.000026for NWA 11119, 0.699004±0.000004for NWA 7325, 0.698982±0.000011for NWA 8661, 0.698995±0.000025for NWA 6704, and 0.6990242±0.0000076for NWA 4587.ALM-A has high measured 87 Sr/ 86 Sr of 0.703521±0.000007and therefore gives an imprecise initial value of 0.699050±0.000118.Our results show that, while the initial 87 Sr/ 86 Sr ratios of other achondrites are higher than the CAI value [4], and are apparently suitable for evaluation of the accretion timing, the initial 87 Sr/ 86 Sr ratios of Ibitira and Asuka 881394 are indistinguishable from the CAIs value, and NWA 11119 has the lowest initial 87 Sr/ 86 Sr measured in achondrites so far that is ~0.6 ε-units below the CAI value.These data and recent findings of CAIs with variable initial 87 Sr/ 86 Sr ratios [5] suggest heterogeneous distribution of 87 Sr/ 86 Sr in the solar nebula and complicate age detetmination with the initial 87 Sr/ 86 Sr chronometry.