L. Grossman’s research while affiliated with University of Chicago and other places

It is widely accepted that the building blocks of the planets and other inner solar system objects have been vaporized and homogenized at the birth of the solar system. According to this view, solids condensed as the solar nebula cooled, with composition evolving from more refractory phases rich in Ca, Al, and Ti to more volatile phases rich in K and Na. This view has been supported by the observation of the depletions in both the least (Eu, Yb) and most refractory (Gd-Er, Lu) rare earth elements (REEs) in group II calcium-aluminum-rich inclusions (CAIs) [1]. The formation of group II CAIs were thought to represent a snapshot of the condensation sequence, the partial condensate in equilibrium with the solar nebula as it cooled [2,3,4]. However, this view of group II CAI formation was never tested other than through thermodynamic calculations that suffer from large uncertainties due to the paucity of thermodynamic data. To test this paradigm of equilibrium condensation of solar gas, we measured the isotopic compositions of Ce, Nd, Sm, Eu, Gd, Dy, Er, and Yb in 8 CAIs (7 with group II REE patterns). Contrary to expectations for equilibrium condensation, we found significant negative isotopic compositions for the most refractory REEs and more subdued isotopic variations for the least refractory REEs, indicative of disequilibrium formation pathways.

Equilibrium condensation of solar gas is often invoked to explain the abundance of refractory elements in planets and meteorites. This is partly motivated, by the observation that the depletions in both the least and most refractory rare earth elements (REEs) in meteoritic group II calcium-aluminum–rich inclusions (CAIs) can be reproduced by thermodynamic models of solar nebula condensation. We measured the isotopic compositions of Ce, Nd, Sm, Eu, Gd, Dy, Er, and Yb in eight CAIs to test this scenario. Contrary to expectation for equilibrium condensation, we find light isotope enrichment for the most refractory REEs and more subdued isotopic variations for the least refractory REEs. This suggests that group II CAIs formed by a two-stage process involving fast evaporation of preexisting materials, followed by near-equilibrium recondensation. The calculated time scales are consistent with heating in events akin to FU Orionis– or EX Lupi–type outbursts of eruptive pre–main-sequence stars.

TS34 is a Type B1 Ca-Al-rich inclusion (CAI) from the Allende CV3 chondrite, consisting of spinel, melilite, Ti-Al-rich clinopyroxene (fassaite) and minor anorthite in an igneous texture. Oxygen and magnesium isotopic compositions were measured by secondary ion mass spectrometry in spots of known chemical composition in all major minerals in TS34. Using the sequence of formation from dynamic crystallization experiments and from chemical compositions of melilite and fassaite, the oxygen isotopic evolution of the CAI melt was established. Oxygen isotopic compositions of the constituent minerals plot along the carbonaceous chondrite anhydrous mineral line. The spinel grains are uniformly ¹⁶O-rich (Δ¹⁷O = −22.7 ± 1.7 ‰, 2SD), while the melilite grains are uniformly ¹⁶O-poor (Δ¹⁷O = −2.8 ± 1.8 ‰) irrespective of their åkermanite content and thus their relative time of crystallization. The fassaite crystals exhibit growth zoning overprinting poorly-developed sector zoning; they generally grow from Ti-rich to Ti-poor compositions. The fassaite crystals also show continuous variations in Δ¹⁷O along the inferred directions of crystal growth, from ¹⁶O-poor (Δ¹⁷O ∼ −3 ‰) to ¹⁶O-rich (Δ¹⁷O ∼ −23 ‰), covering the full range of oxygen isotopic compositions observed in TS34. The early-crystallized ¹⁶O-poor fassaite and the melilite are in oxygen isotope equilibrium and formed simultaneously. The correlation of oxygen isotopic compositions with Ti content in the fassaite imply that the oxygen isotopic composition of the CAI melt evolved from ¹⁶O-poor to ¹⁶O-rich during fassaite crystallization, presumably due to oxygen isotope exchange with a surrounding ¹⁶O-rich nebular gas. Formation of spinel, the liquidus phase in melts of this composition, predates crystallization of all other phases, so its ¹⁶O-rich composition is a relic of an earlier stage. Anorthite exhibits oxygen isotopic compositions between Δ¹⁷O ∼ −2 ‰ and −9 ‰, within the range of those of fassaite, indicating co-crystallization of these two minerals during the earliest to intermediate stage of fassaite growth. The melilite and fassaite yield an ²⁶Al−²⁶Mg mineral isochron with an initial value of (²⁶Al/²⁷Al)0 = (5.003 ± 0.075) × 10⁻⁵, corresponding to a relative age of 0.05 ± 0.02 Myr from the canonical Al−Mg age of CAIs. These data demonstrate that both ¹⁶O-rich and ¹⁶O-poor reservoirs existed in the solar nebula at least ∼0.05 Myr after the birth of the Solar System.

One way to better understand processes related to chondrite metamorphism is to evaluate changes in chondrite features as a function of petrologic type. Toward this end the valence and coordination of Ti in olivine and pyroxene in suites of ordinary (H, L, and LL) and enstatite (EH and EL) chondrites of types 3 through 6 have been determined with XANES spectroscopy. Trivalent Ti, typically 10-40% of the Ti in the analytical volumes, was found in ordinary chondrites of all types, despite the stability of oxidized iron in the samples. Average valences and the proportions of Ti that are in tetrahedral coordination generally decrease with increasing grade between types 3.0 and 3.5, increase from 3.5 to 4, and then level off. These trends are consistent with previous studies of chondrite oxidation states using other methods, except here the onset of oxidation is observed at a lower type, ∼3.5, than previously indicated (4). These results are also consistent with previous suggestions that oxidation of higher-grade ordinary chondrite samples involved exposure to aqueous fluids from melting of accreted ice. In the enstatite chondrites, typically 20-90% of the Ti is trivalent Ti, so it is reduced compared to Ti in the ordinary chondrites. Valence decreases slightly from petrologic type 3 to 4 and increases from 4 to 6, but no increases in tetrahedral coordination with petrologic type are observed, indicating a redox environment or process distinct from that of ordinary chondrite metamorphism. The presence of Ti4+ in the E chondrites supports previous suggestions that they formed from oxidized precursors that underwent reduction. Unlike ordinary chondrites, enstatite chondrites are thought to have been derived from a body or bodies that did not accrete ice, which could account for their different valence-coordination-petrologic type relationships. The hypothesis, based on observations of unmetamorphosed chondrules and supported by laboratory experiments, that equilibration of Ti valence is sluggish compared to that of Fe could account for the coexistence of reduced Ti and oxidized Fe seen in chondrites of all petrologic types.

Oxygen isotopic variations across the outer margins and Wark-Lovering (WL) rims of a diverse suite of six coarse-grained Types A and B refractory inclusions from both oxidized and reduced CV3 chondrites suggest that CAIs originated from a 16O-rich protosolar gas reservoir and were later exposed to both relatively 17,18O-rich and 16O-rich reservoirs. The O-isotope profiles of CAIs can be explained by changes in the composition of gas near the protoSun or the migration of CAIs through a heterogeneous nebula. Variability within the inclusion interiors appears to have been set prior to WL rim growth. Modeling the isotopic zoning profiles as diffusion gradients between inclusion interiors and edges establishes a range of permissible time-temperature combinations for their exposure in the nebula. At mean temperatures of 1400 K, models that match the isotope gradients in the inclusions yield timescales ranging from 5×103 to 3×105 years. Assuming CAIs originated with a relatively 16O-rich (protosolar) isotopic composition, differences among the melilite interiors and the isotopic gradients in their margins imply the existence of a number of isotopically distinct reservoirs. Evidence at the edges of some CAIs for subsequent isotopic exchange may relate to the beginning of rim formation. In the WL rim layers surrounding the interiors, spinel is relatively 16O-rich but subtly distinct among different CAIs. Melilite is often relatively 16O-poor, but rare relatively 16O-rich grains also exist. Pyroxene generally exhibits intermediate O-isotope compositions and isotopic zoning. Olivine in both WL and accretionary rims, when present, is isotopically heterogeneous. The extreme isotopic heterogeneity among and within individual WL rim layers and in particular, the observed trends of outward 16O-enrichments, suggest that rims surrounding CAIs contained in CV3 chondrites, like the inclusions themselves, formed from a number of isotopically distinct gas reservoirs. Collectively, these results support numerical protoplanetary disk models in which CAIs were transported between several distinct nebular reservoirs multiple times prior to accretion onto a parent body.

High temperature condensates found in meteorites display uranium isotopic variations (235U/238U) that complicate dating of the formation of the Solar System and whose origin remains mysterious. It is possible that these variations are due to decay of the short-lived radionuclide 247Cm (t1/2=15.6 Myr) into 235U but they could also be due to uranium kinetic isotopic fractionation during condensation. We report uranium isotope measurements of meteoritic refractory inclusions that reveal excesses of 235U reaching ~+6 % relative to average solar system composition, which can only be due to decay of 247Cm. This allows us to constrain the 247Cm/235U ratio at Solar System formation to (7.0 +- 1.6) x 10-5. This value provides new clues on the universality of nucleosynthetic r-process of rapid neutron capture.

High-temperature condensates found in meteorites display uranium isotopic variations ((235)U/(238)U), which complicate dating the solar system's formation and whose origin remains mysterious. It is possible that these variations are due to the decay of the short-lived radionuclide (247)Cm (t 1/2 = 15.6 My) into (235)U, but they could also be due to uranium kinetic isotopic fractionation during condensation. We report uranium isotope measurements of meteoritic refractory inclusions that reveal excesses of (235)U reaching ~+6% relative to average solar system composition, which can only be due to the decay of (247)Cm. This allows us to constrain the (247)Cm/(235)U ratio at solar system formation to (1.1 ± 0.3) × 10(-4). This value provides new clues on the universality of the nucleosynthetic r-process of rapid neutron capture.

A coordinated, electron-backscatter-diffraction (EBSD) and transmission electron microscope (TEM) study was undertaken to obtain information on the origin of rims on refractory inclusions in the Allende and Axtell CV3 chondrites. These measurements were supported by theoretical modeling using density functional theory. Crystal-orientation analysis of Wark-Lovering rims via EBSD revealed pyroxene grains with similar crystallographic orientations to one another in both inclusions. An epitaxial relationship between grains within the diopside and anorthite rim layers was observed in Allende. TEM examination of the rims of both samples also revealed oriented crystals at depth. The microstructural data on the rims suggest that grain clusters grew in the form of three-dimensional islands. Density functional theory calculations confirm that formation of oriented grain islands is the result of energy minimization at high temperature. The results point toward condensation as the mode of origin for the rims studied here.