Olivine phenocrysts in some Cameroonian Basalts—implications for primary magma composition

Summary The chemical composition of olivine phenocrysts was determined for 13 basaltic samples taken from the central part of the Cameroon Volcanic line to estimate the compositions of the primary olivine phenocrysts and the primary magmas. The Mg/(Mg + Fe) ratios of the olivines attain 0.91, though many of the magnesian olivines are identified on a textural basis as xenocrysts from peridotite inclusions. Most magnesian olivine phenocrysts have Mg/(Mg + Fe) ratios of 0.87–0.88 and NiO contents of 0.32–0.35 wt %. The NiO versus Mg/(Mg + Fe) ratios of the olivines can be used to estimate the compositions of the primary phenocrysts in a range of Mg/(Mg + Fe) = 0.88–0.90. Assuming the Fe(3)/(total Fe) ratios of the magmas are 0.17–0.25, which is estimated from the Fe-Mg partitioning between plagioclase and groundmass, the compositions of the primary magmas were obtained from the Fe²⁺-Mg partition equilibrium with the primary olivine phenocrysts. The primary magmas of the Cameroonian basalts have 13.5 ± 4.0 wt MgO, which is similar to the compositions of basalts with highest MgO contents reported so far from the Cameroon volcanic line.

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Olivin Kristalle in einigen Basalten aus Kamerun: Hinweise auf die Zusammensetzung des primitiven Magmas

Zusammenfassung Die chemische Zusammensetzung von Olivin Phenokristallen wurde in 13 Basalt-Proben, die vom Zentralteil .des Kamerun-Vulkangürtels stammen, bestimmt, um Hinweise auf die Zusammensetzung der primären Olivineinsprenglinge and des primären Magmas zu erlangen. Die Mg/(Mg + Fe) Verhältnisse der Olivine reichen bis 0.91, obwohl viefle dieser Mg-Olivine auf Grund textureller Kriterien als aus Peridotit-Einschlüssen stammende Xenokristalle zu identifizieren sind. Die Mg/(Mg + Fe) Verhältnsse der Hauptmasse der Olivin-Phenokristalle liegen im Bereich von 0.87 bis 0.88, die Ni0 Gehalte zwischen 0.32 und 0.35 Gew %. Die Ni0 gegen Mg/(Mg + Fe) Verhältnisse eignen sich zur Abschätzung der Zusammensetzung der primären Phenokristalle, solange die Mg/(Mg + Fe) Verhältnisse im Bereich von 0.88 bis 0.90 liegen. Die Zusammensetzung des Primärmagmas wurde mittels der Fe²⁺-Mg-Verteilung zwischen dem Magma und den sich mit dem Magma im Gleichgewicht befindlichen primären Olivinkristallen unter der Annahme eines Fe³⁺/Fe total Verhältnisses von 0.17–0.25, welches sich aus der Mg-Fe Verteilung zwischen Plagioklas und Grundmasse ergibt, bestimmt. Demnach ergibt sich für das primäre Magma der Kamerun Basalte ein MgO Gehalt von 13.5 + 4.0 Gew%. Dieser Wert ist mit der Zusammensetzung von Basalten mit höchsten MgO-Gehalten, wie sie bisher vom Kamerun-Vulkangürtel berichtet worden sind, vergleichbar.

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With 6 Figures     

Application of Convergence Confinement Analysis to the study of preceding displacement of a squeezing rock tunnel

Summary This paper deals with a study on the geomechanical behavior of the rock mass around the tunnel. The Convergence Confinement Analysis is one of the most appropriate way to estimate this behavior. Elasto-plastic models proposed by Hoek and Brown, Egger and Kastner were adopted in this study.Measurements of the preceding displacement were performed in a road tunnel in Japan, the geology of which is Neogene squeezing mudstone. The measured data was compared with the results of Convergence Confinement Analysis.The preceding displacement was about 40% against the upper bench face converged displacement and 17% against the final converged displacement. Meanwhile, it increased drastically in lower bench excavation step.Back analysis was also executed to calculate the extension of the plastic zone of the rock mass, which was half of that estimated by Convergence Confinement Analysis in upper bench excavation step. But, in lower bench excavation step, the results of the two analyses were very similar. The support pressure and support stiffness were also calculated.     

Temporal change in chemistry of magma source under Central Kyushu,Southwest Japan: progressive contamination of mantle wedge

Volcanism related to subduction of the Philippine Sea (PHS) plate began in Central Kyushu at 5 Ma, after a pause of igneous activity lasting about 10 m.y. It formed a large volcano-tectonic depression, the Hohi volcanic zone (HVZ), and has continued to the present at a decreasing eruption rate. The products are largely andesite and dacite, which became enriched in K with time. The proportion of tholeiitic to calc alkalic rocks also increases with time. Calc-alkalic high-Mg basaltic andesites (YbBs) were erupted in the early stage of the HVZ activity (5–3 Ma), and high-alumina basalts (KjBs) were erupted in the later stage (2–0 Ma). In contrast to the basalts in the HVZ, Northwest Kyushu basalts (NWKBs) have been erupted on the backarc side of the HVZ since 11 Ma, and hence are not related to the PHS plate subduction. They are mainly high-alkali tholeiitic to alkali basalt that shows no notable chemical change with time. NWKB, YbB, and KjB have MORB-normalized incompatible-element spectra that differ from each other, as is well expressed in both Nb and Sr anomalies. The patterns of KjB and NWKB are typical of those for island-arc basalt (IAB) and ocean-island basalt (OIB), respectively. YbB shows a pattern intermediate between the two. We suggest that the magma source beneath the HVZ changed in composition from an OIB-type mantle to an IAB-type mantle as the subduction of PHS plate advanced. However, the magma source remained fertile under Northwest Kyushu. In order to explain the temporal change of source mantle beneath the HVZ, we propose a model for progressive contamination of the mantle wedge, in which three processes (contamination by a slab-derived component, subtraction of magma from the mantle, and mixing of the mantle residue and slab-derived component) are repeated as subduction continues. As long as the progressive contamination of mantle wedge proceeds, its trace-element composition converges at a steady-state value for a short period. This value does not depend on the initial composition of the mantle wedge but instead on the composition of the slab-derived component. The trace-element composition of the magma produced in such a mantle wedge approaches that of the slab-derived component with time, but the major-element composition is determined by the phase relations of mantle peridotite. The slab-derived component may be basaltic liquid that is partially melted from rutile-bearing eclogite.     

Ultrasonic measurements of Vp and Qp: relaxation spectrum of complex modulus on basalt melts

Ultrasonic compressional wave velocity V_p and quality factor Q_p have been measured in alkali basalt, olivine basalt and basic andesite melts in the frequency range of 3.4–22 MHz and in the temperature range of 1100–1400°C. Velocity and attenuation of the melts depend on frequency and temperature, showing that there are relaxation mechanisms in the melts. Complex moduli are calculated from the ultrasonic data. The results fit well a complex modulus of Arrhenius temperature dependence with log-normal Gaussian distribution in relaxation times of attenuation. The analysis yields average relaxation time, its activation energy, relaxed modulus, unrelaxed modulus and width of Gaussian distribution in relaxation times. Relaxed modulus is smaller (17.5 GPa) for basic andesite melt of high silica and high alumina contents than for the other two basalt melts (18.1–18.4 GPa). The most probable relaxation times decrease from ～ 3 × 10^?10 s for basic andesite to ～ 10^?11 s for alkali basalt at 1400°C. Activation energies of attenuation, ranging from 270 to 340 kJ mol^?1 in the three melts, are highest in basic andesite. Longitudinal viscosity values and their temperature dependences are also calculated from V_p and Q_p data. The volume viscosity values are estimated from the data using the shear viscosity values. Longitudinal, volume and shear viscosities and their activation energies are highest in the basic andesite melt of the most polymerized structure.     

Anomalous hydrogen emissions from the San Andreas fault observed at the Cienega Winery,central California

We began continuous monitoring of H₂ concentration in soil along the San Andreas and Calaveras faults in central California in December 1980, using small H₂/O₂ fuel-cell sensors. Ten monitoring stations deployed to date have shown that anomalous H₂ emissions take place occasionally in addition to diurnal changes. Among the ten sites, the Cienega Winery site has produced data that are characterized by very small diurnal changes, a stable baseline, and remarkably distinct spike-like H₂ anomalies since its installation in July 1982. A major peak appeared on 1–10 November 1982, and another on 3 April 1983, and a medium peak on 1 November 1983. The occurrences of these peaks coincided with periods of very low seismicity within a radius of 50 km from the site. In order to methodically assess how these peaks are related to earthquakes, three H₂ degassing models were examined. A plausible correlational pattern was obtained by using a model that (1) adopts a hemicircular spreading pattern of H₂ along an incipient fracture plane from the hypocenter of an earthquake, (2) relies on the FeO–H₂O reaction for H₂ generation, and (3) relates the accumulated amount of H₂ to the mass of serpentinization of underlying ophiolitic rocks; the mass was tentatively assumed to be proportional to the seismic energy of the earthquake.     

Effect of oxygen partial pressure on the dislocation recovery in olivine: a new constraint on creep mechanisms

The dislocation annihilation rate in experimentally deformed olivine single crystals was measured as a function of oxygen partial pressure (P_O2). It was shown that the dislocation annihilation rate decreased with increasing P_O2. This result is inconsistent with the reported P_O2 dependence of creep rate () in single olivine crystals, thus indicating that the creep in single olivine crystals is not rate-controlled by recovery, under the experimentally investigated conditions.     

Rockfall hazard in the Daisekkei Valley, the northern Japanese Alps, on 11 August 2005

This paper describes a rockfall event in the Daisekkei Valley of Mount Shirouma-dake (2,932 m), the northern Japanese Alps. The rockfall occurred on a steep cliff comprising well-jointed felsites and produced debris of ≥8,000 m³. Most debris was deposited on an elongated snowpatch located immediately beneath the cliff, and it caused casualties among people who were trekking along a trail on the snowpatch. Additionally, a large rock block slipped 1 km on the snowpatch. The rockfall could have been due to the differential retreat of the rockwall, which contains areas of high- and low-density joints. Seasonal and diurnal freeze–thaw activities and snow avalanches and wash appear to be important factors responsible for the retreat. Although some rock blocks that can collapse further remain on the rockwall, the position of the mountain trail in the Daisekkei Valley is fixed. Fundamental reform of tourism systems for climbers, including education on natural hazards, is required.