Resolution Ion Microprobe Research Articles

Bulk chemical control on metamorphic monazite growth in pelitic schists and implications for U–Pb age data

AbstractSeveral petrographic studies have linked accessory monazite growth in pelitic schist to metamorphic reactions involving major rock‐forming minerals, but little attention has been paid to the control that bulk composition might have on these reactions. In this study we use chemographic projections and pseudosections to argue that discrepant monazite ages from the Mount Barren Group of the Albany–Fraser Orogen, Western Australia, reflect differing bulk compositions. A new Sensitive High‐mass Resolution Ion Microprobe (SHRIMP) U–Pb monazite age of 1027 ± 8 Ma for pelitic schist from the Mount Barren Group contrasts markedly with previously published SHRIMP U–Pb monazite and xenotime ages of c. 1200 Ma for the same area. All dated samples experienced identical metamorphic conditions, but preserve different mineral assemblages due to variable bulk composition. Monazite grains dated at c. 1200 Ma are from relatively magnesian rocks dominated by biotite, kyanite and/or staurolite, whilst c. 1027 Ma grains are from a ferroan rock dominated by garnet and staurolite. The latter monazite population is likely to have grown when staurolite was produced at the expense of garnet and chlorite, but this reaction was not intersected by more magnesian compositions, which are instead dominated by monazite that grew during an earlier, greenschist facies metamorphic event. These results imply that monazite ages from pelitic schist can vary depending on the bulk composition of the host rock. Samples containing both garnet and staurolite are the most likely to yield monazite ages that approximate the timing of peak metamorphism in amphibolite facies terranes. Samples too magnesian to ever grow garnet, or too iron‐rich to undergo garnet breakdown, are likely to yield older monazite, and the age difference can be significant in terranes with a polymetamorphic history.

Micron-Scale Isotopic Zoning in Minerals: A Record of Large-Scale Geologic Processes

Isotopic microanalysis can reveal details of mineralizing processes that would otherwise remain undiscovered, because minerals often exhibit micron-scale isotopic zonations recording the large-scale geologic events which have affected them. The SHRIMP (sensitive hi~,h resolution ion microprobe) performs in-situ 3 S/32S analyses of minerals, utilizing a sampling volume that is only 15-25 Ixm wide and 5 txm deep. A negativelycharged oxygen primary beam is used to sputter secondary sulphur ions for analysis in the mass spectrometer sector of the instrument. The precision and accuracy of SHRIMP 834S values are + 2~ (2or) (Eldridge et al., 1987, 1989). Ion microprobe analyses of S isotopic variations with the SHRIMP have documented numerous examples where individual minerals or mineral intergrowths exhibit micron-scale 834S variations that approach the known range of conventional bulk S isotopic compositions in nature. Moreover, the magnitudes and directions of micron-scale S isotopic zoning are often diagnostic of specific geochemical processes which have caused mineralization.