Advances in <i>in-situ</i> Rb-Sr dating using LA-ICP-MS/MS: applications to igneous rocks of all ages and to the identification of unrecognized metamorphic events

Abstract

Age determinations of rocks by in-situ Rb-Sr dating of minerals has the advantage of providing spatially resolved analysis without time-consuming preparation, but is hampered by isobaric and polyatomic interferences on Sr isotopes, especially Rb-87. LA-ICP-MS/MS using N2O as a reaction gas allows online chemical separation of Rb-87 from 87Sr by measuring SrO+. Here, the method for in-situ Rb-Sr is optimized and applicability demonstrated for geological samples ranging from late Archean to 20 Ma in age.Rb-Sr isotopes of multiple reference materials and geological samples have been analyzed by LA-ICP-MS/MS to evaluate the method for dating rocks throughout the geological record. The quality of Sr isotopic data can be greatly improved by external calibration of mass bias. For internal calibration, better accuracy is obtained with the addition of H-2 as a trace molecular gas. The elemental fractionation of Rb and Sr during LA-ICP-MS/MS analysis varies for different reference materials. We find that the matrix of NIST SRM 610 renders it unsuit-able for correction of 87Rb/86Sr. The Mica-Mg reference material is preferred for calibration of Rb-87/Sr-86 of micas despite showing significant Rb-Sr fractionation.In-situ Rb-Sr ages were determined for igneous samples ranging in age from 20 Ma to Mesoproterozoic, confirming accurate application of the method to samples of widely differing ages. The most accurate isochron ages were obtained with 87Rb/86Sr calibrated against Mica-Mg. Application to two metamorphic gneisses with late Archean protoliths from Labrador and Finland reveal thermal events at 1.66-1.70 Ga when they were part of a large Paleoproterozic active margin along the southern edge of Laurentia-Baltica. Data collected for minerals with differing closure temperatures for the Rb-Sr system allow detection of thermal events not recorded by other geochronological systems, indicating a huge potential of this technique not only for accurately dating samples, but also for providing previously inaccessible information on metamorphic histories.