Discovery of metamorphic diamonds in central China: an indication of a > 4000-km-long zone of deep subduction resulting from multiple continental collisions

The Central Orogenic Belt (COB) of China is a major continental collision zone that contains extensive outcrops of deeply subducted and exhumed rocks at both the eastern and the western end of the belt. Here we report discovery of microdiamonds from both eclogites and felsic gneisses in the North Qinling zone in the central portion of the COB. This discovery demonstrates that the country rocks of continental affinity shared in the ultra‐high‐pressure metamorphic (UHPM) event and provides a bridge connecting the two previously recognized UHPM terranes, thereby establishing the existence of a UHPM belt extending more than 4000 km. Geochronological dating yields Early Palaeozoic ages in the west and Early Mesozoic ages in the east, recording two separate continental collisions overprinted within the COB. Occurrence of UHP metamorphism during recurrent continental collision here and in the Alps suggests that deep subduction of continental material during such collisions is probably common rather than exceptional, with significant implications for processes of plate tectonic reorganization and mantle mixing over time.     

Mössbauer milliprobe studies of small mineral samples with a silicon drift detector

Analysis of ⁵⁷Fe transmission Mössbauer spectra collected on a system where the proportional counter has been replaced with a silicon drift detector (SDD) to test milliprobing of mineral samples is described. In the region of the 14.4 keV Mössbauer line the detector has about 70% efficiency and is capable of delivering spectroscopic information with a high energy resolution and high counting rate. Satisfactory results are obtained from a phase analysis of mixtures of olivine and ilmenite in the proportion 97:3, 99:1 wt%, where in the latter case 2.4 μg of Fe³⁺ in the form of hematite was found in the ilmenite. New perovskite-type minerals (Pb_1.33Ba_0.67Fe₂O₅, Pb_1.33Sr_0.67Fe₂O₅ and Pb_1.33Ba_0.33Sr_0.33Fe₂O₅), synthesised by a combustion method, were studied by X-ray diffraction and Mössbauer spectroscopy as well. The advantage of the system with SDD compared to a conventional Mössbauer spectrometer equipped with a proportional counter as a detector is demonstrated for the perovskite samples. The Mössbauer set-up with the silicon drift detector may be successfully used for a wide range of materials containing a negligible amount of iron.     

Digital simulation of the scale effect in hydraulic conductivity

 Measured hydraulic conductivity increases with the scale of testing, but the reason for this increase is not clear. Possibly, high-conductivity heterogeneities are more effective in raising hydraulic conductivity over the regional scale than at the local scale. Alternatively, borehole skin and storage effects, among others, can systematically bias the results of small-scale tests; thus, the increase may simply be an artifact of the test method. Radial-flow tests were simulated at various scales in digital models of fractured double-porosity media. The mean hydraulic conductivity increases until the radius of influence in the test exceeds the fracture spacing. Therefore, under radial flow, hydraulic conductivity is indeed dependent on measurement scale. The increase in hydraulic conductivity with scale is a natural consequence of heterogeneity. Over short distances, water converging toward a borehole must generally flow across heterogeneities. Therefore, small-scale tests tend to measure a weighted harmonic mean of the hydraulic-conductivity field. Over a large area, however, flow is primarily along high-conductivity heterogeneities. Therefore, large-scale tests approach a weighted arithmetic mean where high-conductivity heterogeneities have a greater influence. Received, April 1997 Revised, January 1998 Accepted, December 1997     

A direct recording ocean bottom seismometer

We have designed a simple, cheap and reliable ocean-bottom seismometer. Signals from three-component geophones are recorded directly on magnetic tape running continuously at a speed of 1 mm s¹. Time reference is derived from a temperature-compensated quartz crystal oscillator and encoded on a fourth channel as an amplitude modulation of a 20 Hz carrier. A bipolar square-root signal-compression scheme doubles the tape dynamic range to 80 db, and the available bandwidth is 2 to 100 Hz. Tape and batteries are capable of 500-hr operation, and the unique magnetic release comes close to being a fail-safe system. A heavy, high-drag concrete anchor shaped like a flower-pot provides easy launching, fast stable descent and good coupling to the ocean floor. We have had numerous successful field emplacements which have yielded good earthquake and shot-refraction data.     

Subglacial to proglacial sediment transition in a shallow ice-contact lake

A complete subglacial to glacial-lacustrine facies transition is described from a temporary exposure in the Lake Michigan bluffs of southeastern Wisconsin. This transition occurs where a Late Woodfordian terminal moraine intersects the bluff line and grades from basal meltout till to chaotic diamicton to rhythmites over an abrupt 90 m lateral distance. The boundary of the subglacial meltout deposits is marked by an abrupt increase in pebbles and cobbles, which cluster at specific horizons, producing an incipient stratification. Thereafter, the diamicton develops contorted flow structures with progressive segregation into coarse and fine-grained fractions, ultimately into well-stratified rhythmites.