Geophysical Research Letters

Birck, Jean‐Louis; Allègre, Claude J.

doi: 10.1029/GL011i010p00943pmid: N/A

Measurements of both 53Cr/52Cr and 54Cr/52Cr ratios with a precision of 10−4 in Allende inclusions display widespread chromium isotopic anomalies which are in the range of 5• 10−4. Instrumental mass fractionation precludes specific identification of the affected isotopes, but there are strong arguments favoring excesses in 54Cr and a hint of a very small deficit in 53Cr (≃ 10−4). These samples also display excesses in 50Ti and no significant effect in nickel isotopes.

Mathez, E. A.; Blacic, J. D.; Beery, J.; Maggiore, C.; Hollander, M.

doi: 10.1029/GL011i010p00947pmid: N/A

Profiles of C concentration versus depth were determined in a spinel megacryst and two olivine crystals of a mantle xenolith from Kilbourne Hole, New Mexico, using the 12C(d,p)13C nuclear reaction technique. The 0‐1400 Å layers of all crystals are characterized by C concentrations of ∼1000 wt. ppm. This C is interpreted to be a contaminant acquired during sample preparation or ion bombardment. Below the surface, measurable C concentrations were not found in the spinel and one of the olivine crystals, but a bulk C content of 425 ppm was measured for a second crystal. The latter is interpreted to be due to the presence of discrete C‐rich phase(s) in submicroscopic inclusions or microcracks rather than C dissolved in the crystal lattice. The detection limit potentially obtainable by the (d,p) reaction technique for bulk C in minerals is several 10's of ppm.

Abbott, Dallas; Lyle, Mitchell

doi: 10.1029/GL011i010p00951pmid: N/A

The age of the subducting plate as it enters the trench controls the maximum depth of volatile transport by the downgoing plate. As the slab descends and heats up, decarbonation and dehydration reactions cause alteration minerals and sediments to release volatiles. Our calculations show that subducting oceanic plates <11 m.y. old in oceanic arcs and <34 m.y. old in continental arcs heat up so rapidly that no H2O or CO2 can return to the asthenosphere. Instead, these volatiles rise into the over‐riding lithospheric plate. CO2 and H2O are released differently during subduction. A thickly‐sedimented plate subducting beneath an oceanic arc will return H2O to the asthenosphere only if the subducting plate is older than 11 m.y. and CO2 only if it is older than 25 m.y. If Archaean oceanic lithosphere had a maximum age of 30‐50 m.y. and an average age of 10‐18 m.y., then the amount of volatile recycling to the asthenosphere could have been much lower than at present, despite a greater total consumption rate.

Bina, Craig R.; Wood, Bernard J.

doi: 10.1029/GL011i010p00955pmid: N/A

We have found that the forms of the experimentally determined phase diagrams for pyroxene‐garnet transformations require the volume changes for these reactions to be much smaller at 130 kbar (400 km) than at 1 atm. Using these volume data we calculated the width of the eclogite‐garnetite transition in natural quartz tholeiite and alkali olivine basalt compositions taking account of mixing in multicomponent pyroxene and garnet phases. We find that the eclogite‐garnetite transition does not exhibit any discontinuity in bulk sound velocity in the pressure range 110 to 230 kbar. In addition, this velocity, when plotted as a function of pressure, possesses a curvature which is in the opposite sense to that given by the Preliminary Reference Earth Model (PREM). This phase transition, therefore, does not appear to be a satisfactory model for the 400 km seismic discontinuity.

Lambeck, Kurt; Nakiboglu, S. Mete

doi: 10.1029/GL011i010p00959pmid: N/A

Tide gauge records indicate that a global rise in sealevel has occurred over the past 80 years at a rate of about 1.5 mm yr−1. Because of the poor geographical distribution of the tide gauges, this rise may be partly a consequence of a redistribution of water in the oceans without there being an increase in volume of the oceans. A principal contribution to this redistribution arises from the ongoing rebound of the crust to the melting of the Pleistocene ice sheets, a contribution that is of global significance even far from the limits of the original ice sheets. Model calculations indicate that this contribution may explain between 30 and 50% of the published estimates of the secular rise in sealevel.

Wyatt, F.; Bilham, R.; Beavan, J.; Sylvester, A. G.; Owen, T.; Harvey, A.; Macdonald, C.; Jackson, D. D.; Agnew, D. C.

doi: 10.1029/GL011i010p00963pmid: 11541998

A collection of high‐precision tiltmeters is being operated at Piñon Flat Observatory, southern California, both to compare instruments and to measure tectonic deformation. We report on 1.2 years of data from four of these: two Michelson‐Gale long fluid tiltmeters, one long center‐pressure tiltmeter, and a shallow borehole tiltmeter. The three long‐base instruments are all located on the same baseline, with a precise leveling line running between their end‐monuments. At nontidal frequencies, only the two Michelson‐Gale instruments show some coherence (γ² = .3 for periods of 2 to 4 days), while the center‐pressure instrument is correlated with air temperature at periods from a few days to a few weeks. The most stable tilt record shows a secular rate of 0.28 µrad/a, which may be real. Over much longer times, leveling to specially stabilized benchmarks should confirm this. Comparing instruments has identified more and less successful measurement techniques; it appears that low‐noise data will most probably be produced only by relatively complex and expensive instruments, though even for these, the operating costs over any reasonable lifetime will exceed the capital cost. Even the best existing sensors must be improved to measure continuous tectonic motions.

Webb, Spahr; Cox, Charles S.

doi: 10.1029/GL011i010p00967pmid: N/A

Pressure and electric field fluctuations are suitable for detecting seismic disturbances on the seafloor in the frequency range of .02 to 2 Hz. The background noise has two maxima in this range. From .01 to .03 Hz the pressure spectrum decreases because hydrodynamic filtering increasingly removes the direct influence of long surface gravity waves. The electric spectrum decreases because of increasing absorption of ionospheric disturbances. From 0.09 to 0.11 Hz both spectra increase sharply to a series of peaks where the non‐linear interference of opposed surface wave trains generates microseisms. At 0.09 Hz the inferred vertical ground motion on the seafloor approaches that of quiet continental sites.

Dorbath, C.; Dorbath, L.; Gaulon, R.; George, T.; Mourgue, P.; Ramdani, M.; Robineau, B.; Tadili, B.

doi: 10.1029/GL011i010p00971pmid: N/A

Field geological and seismological investigations conducted after the Guinean earthquake on December 22, 1983 show a shallow dextral strike‐slip fault with a large normal component. The NW‐SE horizontal compressive stress deduced from this event is consistent with measurements made at other places in this region. Old fissures, some of them reactivated during this earthquake, show that this intraplate area constitutes a localized weakness zone in the crust.

Kranz, Robert L.; Blacic, James D.

doi: 10.1029/GL011i010p00975pmid: N/A

Permeability was measured during creep deformation of a tight sandstone and a granite at upper crustal conditions. With water at 100°C as the pore fluid, permeability first decreased then increased during loading to peak stress. Thereafter, permeability decreased as much as 50% over time, even though sample volumetric strain indicated continuous dilatant creep. A silica‐rich precipitate was found at the ends of the specimens and etch pits found on quartz grain surfaces. From these observations, we infer that permeability changes during creep are a result of the competing processes of dissolution, precipitation and dilatant microcracking. Depending upon deformation rates, this could have ramifications which are beneficial for waste isolation, but deleterious for geothermal energy extraction schemes.

McCabe, Chad; Van der Voo, Rob; Ballard, Martha M.

doi: 10.1029/GL011i010p00979pmid: N/A

Samples of the Middle Ordovician Trenton Limestone were collected from 8 sites in Quebec, Ontario, and New York State for paleomagnetic study. Stepwise thermal demagnetization reveals a very well defined characteristic magnetization that has a corresponding paleomagnetic pole at 53.0° N, 126.9° E (K=97.7, A‐95=2.7°). This pole is not at all close to other Ordovician poles from cratonic North America but is near Late Carboniferous and Early Permian ones, indicating that the Trenton was remagnetized in the Late Paleozoic. An earlier paleomagnetic investigation of the Trenton gave a pole that did not appear to be Late Paleozoic in age. However, we demonstrate that this earlier study failed to isolate the ancient characteristic magnetization. Rock magnetic experiments and analyses of magnetic extracts show that the carrier of magnetization is Fe3O4 that does not contain Ti or other elements in solid solution that would indicate a detrital source. The magnetite and its associated remanence appear to have a diagenetic origin.