Kristine Thrane

Extremely Brief Formation Interval for Refractory Inclusions and Uniform Distribution of 26Al in the Early Solar System

Calcium-aluminum-rich inclusions (CAIs) are millimeter-sized refractory objects commonly found in chondritic meteorites and are the oldest solids formed in our solar system. Primary CAI formation may have occurred through condensation and/or evaporation processes near the proto-Sun or, alternatively, during localized events in the asteroid belt. As such, these objects provide us with a unique window into the earliest development of the Sun and into the evolution of the protoplanetary disk. Here we report a 26Al-26Mg isochron for bulk CAIs from four CV carbonaceous chondrites, which yields an initial 26Al/27Al of (5.85 ± 0.05) × 10-5, suggesting that primary formation of the CV CAIs may have occurred within an interval as brief as 20,000 years. This timescale is inconsistent with the secular evolution of T Tauri stars but may be consistent with CAI formation during the infall stage of the protostellar evolution of the Sun. High-precision Mg isotope measurements of samples from the Earth, Moon, Mars, and bulk chondrite meteorites show that these have identically elevated 26Mg abundances compared to the initial 26Mg abundance (δ26Mg* = -0.0317‰ ± 0.0038‰) defined by the CAI isochron. This observation unequivocally demonstrates the homogeneous distribution of 26Al within the accretion region of the terrestrial planets. However, the initial 26Mg abundance of CAIs implies a brief history of elevated Al/Mg in CAI precursor material, which may represent primary condensation of refractory silicates and oxides from the solar nebula.

Late Mesoproterozoic to early Neoproterozoic history of the East Greenland Caledonides: evidence for Grenvillian orogenesis?

Detrital zircons from high-grade metasedimentary rocks (Krummedal supracrustal sequence) in the East Greenland Caledonian orogen yield ion-microprobe U–Pb ages mainly in the range 1100–1800 Ma but with a few grains of c. 1000 Ma, different from zircon ages (mainly 1800–2800 Ma) obtained from the crystalline basement that underlies the metasedimentary rocks. These results indicate that original deposition of the Krummedal sequence took place after 1000–1100 Ma ago, and that the sediment was not derived from the underlying crystalline basement, but from younger, at present unknown sources. High-grade metamorphism of the Krummedal sequence and formation of anatectic granites took place around 930 Ma ago. Caledonian granites are also present in the region, but cannot be distinguished on visual criteria in the field from the older granites, unless emplaced into a younger (900–600 Ma) sequence of sedimentary rocks, the Eleonore Bay Supergroup. It is not yet certain whether the high-grade metamorphism and granite formation at c. 930 Ma are related to a ‘Grenvillian’ or slightly younger collisional event, or to an episode of rifting and crustal thinning. If present at all, a ‘Grenvillian’ orogen in East Greenland would be of very different character than that in North America and southern Scandinavia.

Evidence for a Late Supernova Injection of 60Fe into the Protoplanetary Disk

High-precision 60Fe-60Ni isotope data show that most meteorites originating from differentiated planetesimals that accreted within 1 million years of the solar systems formation have 60Ni/58Ni ratios that are ∼25 parts per million lower than samples from Earth, Mars, and chondrite parent bodies. This difference indicates that the oldest solar system planetesimals formed in the absence of 60Fe. Evidence for live 60Fe in younger objects suggests that 60Fe was injected into the protoplanetary disk ∼1 million years after solar system formation, when 26Al was already homogeneously distributed. Decoupling the first appearance of 26Al and 60Fe constrains the environment where the Suns formation could have taken place, indicating that it occurred in a dense stellar cluster in association with numerous massive stars.

Linking the Palaeoproterozoic Nagssugtoqidian and Rinkian orogens through the Disko Bugt region of West Greenland

The parallel Trans-Hudson-aged Nagssugtoqidian Orogen and Rinkian Fold Belt of West Greenland have long been considered distinct orogenic belts separated by a zone of low or no Palaeoproterozoic strain in the intervening Disko Bugt region. New U–Pb geochronology confirms the continuity of Palaeoproterozoic deformation and metamorphism through this region, thereby directly linking the Nagssugtoqidian Orogen and Rinkian Fold Belt. Furthermore, two newly identified Archaean blocks in this region are interpreted to represent the southern and northern extensions of the Rae and North Atlantic cratons, respectively. Their boundary comprises a 15 km wide belt of distributed, NW-vergent ductile thrusts that deform ultramafic pods and is interpreted to represent the main collisional suture of a single, variably exhumed, >1100 km wide Nagssugtoqidian–Rinkian orogenic system. We conclude that Palaeoproterozoic convergence of a northern Archaean craton carried a south-facing passive margin sedimentary sequence (Karrat Group) towards a south-dipping subduction zone along the North Atlantic Craton. Terminal collision created a high-grade core centred on the magmatic arc of the central Nagssugtoqidian Orogen and a NW-vergent fold and thrust belt in the Rinkian Fold Belt.

Discovery of a New FUN CAI from a CV Carbonaceous Chondrite: Evidence for Multistage Thermal Processing in the Protoplanetary Disk

We report the mineralogy, petrography, as well as oxygen and magnesium isotope data of a newly identified FUN inclusion from the CV carbonaceous chondrite NWA 779. Variability in the texture of the mineral phases coupled with oxygen isotope data provides evidence for multistage evolution of this inclusion under distinct thermal regimes: slow crystallization of 16O-rich melt accompanied by evaporation, and subsequent remelting in an 16O-poor reservoir during transient heating events, possibly associated with the formation of CV chondrules. The inferred oxygen isotope composition of the precursor material of this inclusion (δ17,18O = − 48.4+ 2.5−3.0‰) is consistent with that observed for CAIs and amoeboid olivine aggregates from least metamorphosed carbonaceous chondrites, suggesting that both FUN and normal CAIs formed in an16O-rich reservoir with oxygen isotope composition similar to that inferred for the Sun. However, in contrast to normal CAIs, most FUN inclusions show no evidence for live 26Al at the time of their formation. Based on these observations, we propose that the protosolar molecular cloud was polluted with stellar-derived 26Al prior to its collapse. Thus, FUN CAIs formed at a time when dust inherited from the molecular cloud—including the carrier of 26Al—was still poorly homogenized in the protoplanetary disk.

Rapid determination of Pb isotopes to define Precambrian allochthonous domains: An example from West Greenland

Lacking provincial fauna, establishing autochthoneity or allochthoneity of domains in Precambrian orogens typically requires comparison of precollisional attributes across suspect terrane boundaries. We have measured the Pb isotopic compositions of K-feldspar grains in epoxy mounts and rock slabs by a rapid laser ablation–inductively coupled plasma mass spectrometry method with sufficient precision and accuracy to define two large Pb isotopic domains in the Paleoproterozoic Nagssugtoqidian-Rinkian orogen of West Greenland. These domains are interpreted to correlate with and help delineate the Rae and North Atlantic cratons. Their boundary coincides with a recently proposed location of the main collisional suture of this orogen, thus supporting this interpretation. This study validates a rapid analytical Pb isotopic method that will be useful in addressing a range of problems that requires knowledge of crustal affinity in Precambrian terrains.

ORIGIN OF EXCESS 176Hf IN METEORITES

After considerable controversy regarding the 176Lu decay constant (λ176Lu), there is now widespread agreement that (1.867 ± 0.008) × 10–11 yr–1 as confirmed by various terrestrial objects and a 4557 Myr meteorite is correct. This leaves the 176Hf excesses that are correlated with Lu/Hf elemental ratios in meteorites older than ~4.56 Ga meteorites unresolved. We attribute 176Hf excess in older meteorites to an accelerated decay of 176Lu caused by excitation of the long-lived 176Lu ground state to a short-lived 176m Lu isomer. The energy needed to cause this transition is ascribed to a post-crystallization spray of cosmic rays accelerated by nearby supernova(e) that occurred after 4564.5 Ma. The majority of these cosmic rays are estimated to penetrate accreted material down to 10-20 m, whereas a small fraction penetrate as deep as 100-200 m, predicting decreased excesses of 176Hf with depth of burial at the time of the irradiation event.