Mn-Cr relative sensitivity factor in ferromagnesian olivines defined for SIMS measurements with a Cameca ims-1280 ion microprobe: Implications for dating secondary fayalite

The short-lived radionuclide Mn-53, which decays to Cr-53 with a half-life of similar to 3.7 Myr, is useful for sequencing objects that formed within the first 20 Myr of Solar System evolution. Mn-53-Cr-53 relative chronology enables aqueously formed secondary minerals such as fayalite and various carbonates in ordinary and carbonaceous chondrites to be dated, thereby providing chronological constraints on aqueous alteration processes. In situ measurements of Mn-Cr isotope systematics in fayalite by secondary ion mass spectrometry (SIMS) require consideration of the relative sensitivities of the Mn-55(+) and Cr-52(+) ions, for which a relative sensitivity factor [RSF = (Mn-55(+)/Cr-52(+))(SIMS)/(Mn-55/Cr-52)(true)] is defined using appropriate standards. In the past, San Carlos olivine (Fa(similar to 10)) was commonly used for this purpose, but a growing body of evidence suggests that it is an unsuitable standard for meteoritic fayalite (Fa(>90)). Natural fayalite also cannot be used as a standard because it contains only trace amounts of chromium, which makes determining a true Mn-55/Cr-52 ratio and its degree of heterogeneity very difficult. To investigate the dependence of the Mn-Cr RSF on ferromagnesian olivine compositions, we synthesized a suite of compositionally homogeneous Mn, Cr-bearing liquidus-phase ferromagnesian olivines (Fa(31-99)). Manganese-chromium isotopic measurements of San Carlos olivine and synthesized ferromagnesian olivines using the University of Hawai'i Cameca ims-1280 SIMS show that the RSF for Fa(10) is similar to 0.9; it increases rapidly between Fa(10) and Fa(31) and reaches a plateau value of similar to 1.5 +/- 0.1 for Fa(>34). The RSF is time-dependent: it increases during the measurements of olivines with fayalite content <30 and decreases during the measurements of olivines with fayalite content >50. The RSF measured on ferroan olivine (Fa(>90)) is influenced by pit shape, whereas the RSF measured on magnesian olivine (Fa(10)) is less sensitive to changes in pit shape. For these reasons, Mn-53-Cr-53 systematics of chondritic fayalite (Fa(>90)) should be determined using standards of similar composition that are measured under the same analytical conditions as the "unknown". The Mn-53-Cr-53 ages of secondary fayalites (Fa(90-100)) in the Elephant Moraine (EET) 90161 (L3.05), Vicencia (LL3.2), Asuka 881317 (CV3) and MacAlpine Hills (MAC) 88107 (C3) chondrites (2.4(-1.3)(+1.8), 4.0(-1.1)(+1.4), 4.2(-0.7)(+0.8) and 5.1(-0.4)(+0.5) Myrs after CV CAIs, respectively) are similar to 3 Myr older when using an RSF measured on a matrix-matched (Fa(99)) standard, rather than on a San Carlos olivine. The inferred Mn-53-Cr-53 ages of fayalite formation are consistent with the ages reported for calcites in CM chondrites measured with similarly matrix-matched standards, suggesting an early onset of aqueous alteration on the ordinary and carbonaceous chondrite parent bodies heated by decay of Al-26. (c) 2015 Elsevier Ltd. All rights reserved.