The oldest dated solids in our Solar System, CAIs, contain isotopic anomalies in a whole suite of elements relative to later formed Solar System materials. Previous work has reported differences in the proportions of nucleosynthetic components between CAIs and terrestrial rocks as a function of mass. However, the nucleosynthetic fingerprint of the CAI-forming region is still lacking significant data in the heavier mass range (A > 154). Erbium (Er) and ytterbium (Yb) isotopic data along with hafnium (Hf) isotopic compositions yield insights into in a wide variety of CAIs derived from a variety of CV and CK chondrites. Relative to terrestrial rock standards, CAIs - regardless of host rock, petrologic or chemical classification - have uniform and resolvable Er, Yb, and Hf isotopic compositions. The CAI isotopic patterns correspond to r-process deficits (or s-process excesses) relative to terrestrial values of 9 ppm for Er, 18 ppm for Yb, and 17 ppm for Hf. This new Er, Yb, and Hf data help complete the nucleosynthetic fingerprint of the CAI-forming region, further highlighting the systematic difference between the CAIs and later formed bulk planetary bodies. Such a systematic difference between CAIs and terrestrial rocks cannot be caused by different amounts of any known single presolar phase but is likely the result of a well-mixed reservoir made of diverse stellar sources.
Igneous zircon is not only a robust geochronometer but also reflects the trace element content of its source magma and incorporates mineral inclusions inherited from the magma during growth. All of these clues help constrain the origins of detrital and other out-of-context zircon, with applications ranging from tectonic reconstructions to studies of Earth’s earliest crust. The mineral inclusion assemblage of igneous zircons is underdeveloped as a tracer for zircon origins, despite potentially promising applications of oxide and phosphate inclusions for determining the composition of detrital zircon source granitoids. Although apatite-poor zircons appear to mainly derive from highly silicic melts, apatite-rich zircons appear to derive from a variety of sources. Relative mineral inclusion abundances in igneous accessory minerals should be affected by 1) crystallization sequence, 2) proximity to other crystallizing phases, and 3) overall magmatic composition. We have selected several plutonic units in the Peninsular Ranges Batholith (PRB) of southern California with zircons rich in apatite (>20% of inclusions) in which whole rock petrology, zircon trace element chemistry, and several robust accessory minerals in a clear crystallization sequence elucidate the relative importance of these three factors. In these PRB granitoids, earlier crystallizing phases have proportionally larger contents of apatite inclusions and lower contents of late-crystallizing phases (i.e., quartz, K-feldspar, muscovite). Ilmenite inclusion assemblages are shifted toward higher contents of mafic minerals relative to zircon and hornblende. Contents of apatite and late-crystallizing phases in inclusion assemblages appear to be more correlated with the first two factors than more directly with melt composition. Within one zoned pluton (La Posta pluton), melt compositional evolution as shown by zircon trace element chemistry correlates more clearly with apatite and late phase inclusion contents than whole rock SiO2 and other compositional factors such as A/CNK.
Differentiated planetary bodies exhibit small but discernible elevations in heavy/light stable isotope ratios among rock forming elements. Preferential evaporation of light isotopes from melt exposed to space is a possible mechanism. A theoretical framework has been developed for evaluating the isotopic effects of evaporation as a function of size of the body and the nature of any enveloping gas. We are testing this framework in the laboratory by laser heating aerodynamically levitated samples in gases of controlled compositions. Our data bear on the effects of variable saturation on evaporative isotope fractionation, providing tests of the theoretical predictions. Results show the 56Fe/54Fe the vapor/melt isotope fractionation factors defined by the experimental products are closer to unity than evaporation to vacuum and define Fe gas saturation values of ~0.7, corresponding to effective total pressures of 0.1 to 0.2 bar in the boundary layer of the flowing gas. Results like these constrain models for evaporative fractionation under ranges in relevant conditions.