In situ calcite U−Pb geochronology by high-sensitivity single-collector LA-SF-ICP-MS

Science China Earth Sciences - U?Pb geochronology of calcite using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is an emerging method, with potential applications...     

Continental growth and reworking on the edge of the Columbia and Rodinia supercontinents; 1.86–0.9 Ga accretionary orogeny in southwest Fennoscandia

Geological history from the late Palaeoproterozoic to early Neoproterozoic is dominated by the formation of the supercontinent Columbia, and its break-up and re-amalgamation into the next supercontinent, Rodinia. On a global scale, major orogenic events have been tied to the formation of either of these supercontinents, and records of extension are commonly linked to break-up events. Presented here is a synopsis of the geological evolution of southwest Fennoscandia during the ca. 1.9–0.9 Ga period. This region records a protracted history of continental growth and reworking in a long-lived accretionary orogen. Three major periods of continental growth are defined by the Transscandinavian Igneous Belt (1.86–1.66 Ga), Gothian (1.66–1.52 Ga), and Telemarkian (1.52–1.48 Ga) domains. The 1.47–1.38 Ga Hallandian–Danopolonian period featured reorganization of the subduction zone and over-riding plates, with limited evidence for continental collision. During the subsequent 1.38–1.15 Ga interval, the region is interpreted as being located inboard of a convergent margin that is not preserved today and hosted magmatism and sedimentation related to inboard extensional events. The 1.15–0.9 Ga period is host to Sveconorwegian orogenesis that marks the end of this long-lived accretionary orogen and features significant crustal deformation, metamorphism, and magmatism. Collision of an indenter, typically Amazonia, is commonly inferred for the cause of widespread Sveconorwegian orogenesis, but this remains inconclusive. An alternative is that orogenesis merely represents subduction, terrane accretion, crustal thickening, and burial and exhumation of continental crust, along an accretionary margin. During the Mesoproterozoic, southwest Fennoscandia was part of a much larger accretionary orogen that grew on the edge of the Columbia supercontinent and included Laurentia and Amazonia amongst other cratons. The chain of convergent margins along the western Pacific is the best analogue for this setting of Proterozoic crustal growth and tectonism.     

Rapid colorimetric determination of phosphorus in geochemical survey samples

A colorimetric method for the determination of phosphorus in batches of geochemical survey samples is described. The powdered material is digested with a nitric-perchloric acid mixture, excess of acid is evaporated off and the residue is leached with water. Aliquots of the leachate are treated with vanadomolybdate reagent and the yellow phosphorus complex is extracted into isobutyl methyl ketone. The colour in the organic phase, which is stable for many days, is matched against a series of similarly prepared standards. Iron, other coloured ions, and titanium do not interfere. Standard deviations of ±0.02% about a mean value of 0.20% P₂ O₅ and ±0.4% about a mean of 11.6% P₂ O₅ were obtained. At least 40 powdered samples may be analysed per working day per person.     

Blocky craters: Implications about the lunar megaregolith

Radar, infrared, and photogeologic properties of lunar craters have been studied to determine whether there is a systematic difference in blocky craters between the maria and terrae and whether this difference may be due to a deep megaregolith of pulverized material forming the terra surface, as opposed to a layer of semi-coherent basalt flows forming the mare surface. Some 1310 craters from about 4 to 100 km diameter have been catalogued as radar and/or infrared anomalies. In addition, a study of Apollo Orbital Photography confirmed that the radar and infrared anomalies are correlated with blocky rubble around the crater.Analysis of the radar and infrared data indicated systematic terra—mare differences. Fresh terra craters smaller than 12 km were less likely to be infrared and radar anomalies than comparable mare craters: but terra and mare craters larger than 12 km had similar infrared and radar signatures. Also, there are many terra craters which are radar bright but not infrared anomalies.Our interpretation of these data is that while the maria are rock layers (basaltic flow units) where craters eject boulder fields, the terrae are covered by relatively pulverized megaregolith at least 2 km deep, where craters eject less rocky rubble. Blocky rubble, either in the form of actual rocks or partly consolidated blocks, contributes to the radar and infrared signatures of the crater. However, aging by impacts rapidly destroys these effects, possibly through burial by secondary debris or by disintegration of the blocks themselves, especially in terra regions.PSI Contribution No. 110.     

Use of the frozen flux approximation in the interpretation of archaeomagnetic and palaeomagnetic data

Summary. The frozen flux approximation of Roberts & Scott is a constraint on the core field that can be used to aid interpretation of the very sparse datasets that palaeomagnetism and archaeomagnetism provide. It gives bounds on the size of the components of the magnetic field at a point, of the Gauss coefficients, and, if valid over such long time periods, limits the shape of the field during transitions between normal and reversed polarities. The maximum intensity at a point, consistent with the present flux, is 281 μT or 4 times the maximum field observed today. The present dipole is about 50 per cent of its upper bound. Polarity reversal is impossible if the transition field is purely axisymmetric. None of the measurements we consider violate the frozen flux approximation.     

Wave propagation in a magnetic cylinder

The nature of oscillations in a magnetic cylinder embedded in a magnetic environment is investigated. It is shown that the standard slender flux tube analysis of a kink mode in a cylinder excludes the possibility of a second mode, which arises under photospheric conditions. Under coronal conditions, two widely separated classes of oscillation can be freely sustained, one on an acoustic time-scale and the other on an Alfvénic time-scale. The acoustic-type oscillations are always present, but the much shorter period, Alfvénic-type, oscillations arise only in high density (strictly, low Alfvén velocity) loops. An application to waves in fibrils is also given, and suggests (following Wentzel, 1979) that they are fast kink waves propagating in a density enhancement.