摘要:

We present a record of the Matuyama‐Brunhes (0.73 Ma) polarity transition from a southern hemisphere deepsea sediment core (V16‐58; 46°S, 30°E). The transition is recorded across at least 30 cm of section and is defined by a nearly 180° change in directions which occurs during a low in the relative intensities. An increase in the10Be/9Be ratios associated with the intensity low suggests that the relative intensities may document a decrease in the strength of the geomagnetic field. The virtual geomagnetic pole (VGP) path is neither clearly near‐ or far‐sided, but tracks approximately 120° west of the site longitude. Considered with other Matuyama‐Brunhes transition records, this southern hemisphere record supports previous interpretations that the Matuyama‐Brunhes transitional field was not dipolar. Instead, the V16‐58 transition together with other records of this reversal recently obtained from this longitudinal sector suggests that nonzonal terms which are symmetric about the equator were important during this t

ISSN:0094-8276    

DOI:10.1029/90GL02714

年代:1991

数据来源: WILEY

摘要:

Partitioning of transition metals, V, Cr, Mn, Fe, Co, and Ni between Mg‐perovskite, magnesiowustite and metallic iron has been studied at 27 GPa and 1700–1900°C. V, Cr and Mn are strongly enriched in Mg‐perovskite and magnesiowustite, whereas Fe, Co, and Ni are strongly concentrated in metallic iron under the present experimental condition. Using these data on partitioning, we tested models of core‐mantle separation in the primordial earth.Abundances of V, Cr, and Fe in the upper mantle are consistent with a model of the upper mantle formed by olivine accumulation and/or majorite fractionation in the primordial earth after the core segregation under the core‐mantle equilibrium. However, depletion of Mn, and overabundances of Co and Ni in the mantle remain unexplained by this model. Other processes, such as vaporization of Mn before or during accretion and the equilibrium partitioning of Co and Ni between minerals and iron in the deep lower mantle may account for the observed abundances of these elements in

ISSN:0094-8276    

DOI:10.1029/90GL02717

年代:1991

数据来源: WILEY

摘要:

Fine‐grained powder samples of Mg2SiO4forsterite (α phase) have been partially transformed to the beta phase in a 2000‐ton uniaxial split‐sphere apparatus (USSA‐2000) at P = 15 GPa and T = 1000 °C in runs of 5 minutes duration. This technique allowed us to catch the first steps of the α ‐‐>β transformation in Mg2SiO4. A TEM study of the run products showed that a structurally disordered phase is an intermediate of the reaction. The oxygen sublattice of this intermediate phase is well ordered, but a very large density of planar faults on (010) planes destroys the long range order in the cation sublattices. A two‐steps mechanism for the α ‐‐>β transformation is therefore probable, involving first the nucleation of the structurally disordered phase, then the ordering of this phase. These two steps may be associated with low energy activation barriers and could thus explain the fast kinetics of th

ISSN:0094-8276    

DOI:10.1029/90GL02719

年代:1991

数据来源: WILEY

摘要:

Ultrasonic measurements ofQp−1andVpfor An and Di.64An.36melts are reported in the frequency range 3 – 20 MHz and in the temperature range from melting to 1934K and 1831K5 respectively. WhenQp−1and shear viscosity are normalized to their respective liquidus temperature values, they have the same linear dependence on reciprocal homologous temperature. From the Arrhenius temperature dependence ofQp−1, the activation energies calculated for An and Di.64An.36melts are on the order of 216 and 302 kJ/mol, respectively. Although there is insufficient data for An, the activation energy ofQp−1for Di.64An.36is in excellent agreement with its activation energy of viscosity.The relaxed adiabatic bulk modulus, derived from relaxed velocities, for An at 1893K is 20.4 ± 0.2 GPa and for Di.64An.36at 1677K is 23.0 ± 0.1 GPa. These values fall into a general trend of decreasing bulk modulus with incre

ISSN:0094-8276    

DOI:10.1029/90GL01711

年代:1991

数据来源: WILEY

摘要:

Compositions of primary magmas were estimated for Cenozoic Mg‐rich basalts from the NE China region based on the olivine maximum fractionation model. These data together with available experimental data gave an estimate for magma segregation depth for each basalt magma, which is inversely correlated with87Sr/86Sr isotopic ratio and positively correlated with143Nd/144Nd isotopic composition. This observation may provide convincing evidence that the enriched upper mantle is underlain by more depleted asthenosphere mantle reservoi

ISSN:0094-8276    

DOI:10.1029/90GL02716

年代:1991

数据来源: WILEY

摘要:

Application of the principles of elastic plate bending to subsurface geometric information and outcrop in northwestern Canada suggests that Proterozoic crust was loaded and deflected during orogenic activity between about 1.6 and 1.1 Ga. Exposed Hudsonian (ca. 1.8–2.0 Ga) basement separating younger but coeval stratigraphic sequences in the Coppermine Homocline on the west and Bathurst Inlet on the east can be explained as a forebulge associated with the loadin

ISSN:0094-8276    

DOI:10.1029/90GL02715

年代:1991

数据来源: WILEY

摘要:

Two‐station measurements of Rayleigh wave group velocity between Midway Atoll and O'ahu provide a useful constraint on lithospheric thickness along the Hawai'ian Swell. Comparison of the observed dispersion curve with age‐dependent, regionalized dispersion curves suggests that the swell acts as 50–110 m.y. old lithosphere, and not as 20–50 m.y. old lithosphere as is predicted by the currently favored hypothesis for the swell's formation. A preliminary isotropic shear velocity model for the swell suggests that the lithosphere may be as thick as 100 km. This value is difficult to reconcile with the lithosphere being substantially thinned during its passage over the Hawai'ian

ISSN:0094-8276    

DOI:10.1029/90GL02718

年代:1991

数据来源: WILEY

摘要:

Interstitial waters from boreholes in the reef conglomerate of Tikehau atoll (S.W. Pacific) contain positive anomalous concentrations of dissolved inorganic nutrients compared to adjacent oceanic and lagoonal waters. These anomalies have been interpreted by geothermal circulation of deep oceanic waters penetrating the porous reef carbonates and ascending through the atoll flanks by thermo‐convective advection as already proposed for other atolls. We present here a new strong evidence of this geothermal circulation inside atoll reefs from the record of helium‐3 anomalies in borehole waters of Tikehau atoll. These results bear directly on three controversial aspects of reef history: the efficiency of thermal energy for circulation of reef pore waters, the sources of nutrients to support the net productivity of reef ecosystems, the early diagenesis of reef foundation carbona

ISSN:0094-8276    

DOI:10.1029/90GL02720

年代:1991

数据来源: WILEY

摘要:

Gases and a water condensate have been sampled at White Island volcano in two selected fumaroles (100 and 495°C). They have been analysed for major, minor and trace elements. Both the chemical composition of gases and thermodynamic calculations suggest that the fluids feeding high and low temperature fumaroles have the same origin, but that they follow different evolutions while ascending to the surface. Very low CO/CO2ratios with respect to White Island previous results suggest that White Island is now in stage of reduced activity. According to the very low solubility of CO in silicate melts, an increase in activity due to a magmatic pulse should cause a significant increase of CO in the released gas phase.Long term fluxes of gases and metals have been estimated on the basis of COSPEC SO2flux measurements performed during a medium activity stage. Our data concerning some heavy metals are similar to previous data, suggesting a constancy of the emissions during quiescent periods

ISSN:0094-8276    

DOI:10.1029/90GL01477

年代:1991

数据来源: WILEY

ISSN:0094-8276    

DOI:10.1029/90GL02728

年代:1991

数据来源: WILEY