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The Cadmium-Lined Outer-Core Irradiation Tube (CLOCIT) is a new irradiation facility for 40Ar/39Ar geochronology at the Oregon State University TRIGA® reactor. We report fluence parameters from the first four CLOCIT irradiations and compare them to the existing Cadmium-Lined Inner-Core Irradiation Tube (CLICIT). CLOCIT provides an average neutron flux equivalent of 1.45–1.53 ×10-4 J/h; about 55% of CLICIT. Radial fluence gradients are on the order of 0.2–2.4 %/cm. A planar fit of J-values results in residuals in the range of uncertainty in the J-value, but systematic deviations resolve a non-planar component of the neutron flux field which has also been observed in CLICIT. Axial neutron fluence gradients are 0.6–1 %/cm, compared to 0.7–1.6 %/cm for the CLICIT. Production rate ratios of interfering reactions are (40Ar/39Ar)K = (4±6) ×10-4 and (38Ar/39Ar)K = (1.208±0.002) ×10-2, (36Ar/37Ar)Ca = (2.649±0.014) ×10-4, (38Ar/37Ar)Ca = (3.33±0.12) ×10-5, and (39Ar/37Ar)Ca = (9.1±0.28) ×10-4, similar to the CLICIT values.

Terra Nova, 24, 339–350, 2012 Abstract Studies of regional plate-scale processes that may influence the exhumation of ultra-high pressure (UHP) metamorphic rocks are few despite their potential importance. Here, we review and summarize available age constraints along with the Qinling-Dabie Shan orogen, which separates the North China Block (NCB) from the South China Block (SCB). We find that collision-related events in the west and extension-related events in the east, where the large Dabie Shan UHP terrane is located, were contemporaneous. Available palaeomagnetic data indicate clockwise rotation of the SCB relative to the NCB during this time. Previous work indicated that the collision propagated from east to west where indentation of the SCB into the Qinling-Dabie Shan orogen was active in the Hannan Dome area at 165 ± 3 Ma as constrained by new 40Ar/39Ar dates on synkinematic sericite in an indentation-related mylonitic shear zone. While this contraction was occurring in the west, exhumation of the UHP terrane was occurring in the Dabie Shan in the east. We propose that the collision of the SCB with the NCB occurred not only in a scissor-like fashion, as previously suggested, but also involved a later rotation about a pole west of the Dabie Shan during the closure of the Mianlue Ocean. As a consequence of this rotation, extension occurred between the South China Block and North China Block east of the pole of rotation leading to extensional exhumation of the Dabie Shan orogen. This microcontinent-scale rotation of the SCB may have provided an important component in the exhumation of the largest UHP terrane in the world.

The potassium-argon (K-Ar) dating method has been widely used over the past 40 years to provide radioisotopic age control of hominid/hominoid evolutionary time scales. The wide appeal of the technique to paleoanthropology and archeology has been, in part, a result of its broad time range of applicability, from materials as young as a few thousand years old to an essentially unbounded upper age limit. Another reason for its appeal is the many geological circumstances in which datable materials are found. Beginning about two decades ago and accelerating into this decade, however, the conventional K-Ar technique has given way to 40Ar/39Ar dating as the method of preference. This technique is not only more precise and accurate when dating ideal materials, but also permits excellent ages to be obtained from situations that often stymie the conventional K-Ar technique, such as dating of contaminated tuffs and altered rocks. © 1998 Wiley-Liss, Inc.

The phylogenetic relationship between Australopithecus anamensis and Australopithecus afar- ensis has been hypothesized as ancestor-descendant. However, the weakest part of this hypothesis has been the absence of fossil samples between 3.6 and 3.9 million years ago. Here we describe new fossil specimens from the Woranso-Mille site in Ethiopia that are directly rele- vant to this issue. They derive from sediments chrono- metrically dated to 3.57-3.8 million years ago. The new fossil specimens are largely isolated teeth, partial mandi- bles, and maxillae, and some postcranial fragments. However, they shed some light on the relationships between Au. anamensis and Au. afarensis. The dental morphology shows closer affinity with Au. anamensis from Allia Bay/Kanapoi (Kenya) and Asa Issie (Ethiopia) than with Au. afarensis from Hadar (Ethiopia). However, they are intermediate in dental and mandibular mor- phology between Au. anamensis and the older Au. afar- ensis material from Laetoli. The new fossils lend strong support to the hypothesized ancestor-descendant rela- tionship between these two early Australopithecus spe- cies. The Woranso-Mille hominids cannot be unequivo- cally assigned to either taxon due to their dental mor- phological intermediacy. This could be an indication that the Kanapoi, Allia Bay, and Asa Issie Au. anamensis is the primitive form of Au. afarensis at Hadar with the Laetoli and Woranso-Mille populations sampling a mosaic of morphological features from both ends. It is particularly difficult to draw a line between Au. anamen- sis and Au. afarensis in light of the new discoveries from Woranso-Mille. The morphology provides no evidence that Au. afarensis and Au. anamensis represent distinct taxa. Am J Phys Anthropol 141:406-417, 2010. V C 2009