The December 4, 1983 to February 18, 1984 eruption of Piton de la Fournaise (La Reunion, Indian Ocean): Description and interpretation

On December 4, 1983 an eruption started at vents located 1.5 km southwest of the summit of Piton de la Fournaise at the base of the central cone. After 31 months of quiescence this was one of the longest repose period in the last fifty years. The eruption had two phases: December 4 to January 18 and January 18 to February 18. Phase 1 produced about 8 × 10⁶ m³ of lava and Phase II about 9 × 10⁶ m³. The erupted lava is an aphyric basalt whose mineralogical and geochemical composition is close to that of other lavas emitted since 1977.The precursors of the December 4 outbreak were limited to two-week shallow (1.5–3 km) seismic crisis of fewer than 50 events. No long-term increase was noted in the local seismicity which is very quiet during repose periods and no long-term ground inflation preceded the eruption. Outbreaks of Phases I and II were preceded by short (2.5 hours and 1.5 hours) seismic swarms corresponding to the rise of magma toward the surface from a shallow reservoir. Large ground deformation explained by the emplacement of the shallow intrusions, was recorded during the seismic swarms. A summit inflation was observed in early January, before the phase II outbreak, while the phase I eruption was still continuing.Piton de la Fournaise volcanological observatory was installed in 1980. Seismic and ground deformation data now available for a period of 4 years including the 1981 and the 1983–1984 eruptions, allow us to describe the physical behavior of the volcano during this period. These observations lead us to propose that the magma transfer from deep levels to the shallow magma reservoir is not a continuous process but a periodic one and that the shallow magma reservoir was not resupplied before the 1981 and 1983–1984 eruptions. Considerations on the eruptive history and the composition of recent lavas indicate that the reservoir was refilled in 1977.     

The partitioning of Fe and Mg between olivine and carbonate and the stability of carbonate under mantle conditions

We have investigated the effect of Fe on the stabilities of carbonate (carb) in lherzolite assemblages by determining the partitioning of Fe and Mg between silicate (olivine; ol) and carbonates (magnesite, dolomite, magnesian calcite) at high pressures and temperatures. Fe enters olivine preferentially relative to magnesite and ordered dolomite, but Fe and Mg partition almost equally between disordered calcic carbonate and olivine. Measurement of K _d (X _Fe ^carb X _Mg ^ol /X _Fe ^ol X _Mg ^carb ) as a function of Fe/ Mg ratio indicates that Fe–Mg carbonates deviate only slightly from ideality. Using the regular solution parameter for olivine W _FeMg ^ol of 3.7±0.8 kJ/mol (Wiser and Wood 1991) we obtain for (FeMg)CO₃ a W _FeMg ^carb of 3.05±1.50 kJ/mol. The effect of Ca–Mg–Fe disordering is to raise K _d substantially enabling us to calculate W _CaMg ^carb -W _CaFe ^carb of 5.3±2.2 kJ/mol. The activity-composition relationships and partitioning data have been used to calculate the effect of Fe/Mg ratio on mantle decarbonation and exchange reactions. We find that carbonate (dolomite and magnesian calcite) is stable to slightly lower pressures (by 1 kbar) in mantle lherzolitic assemblages than in the CaO–MgO–SiO₂(CMS)–CO₂ system. The high pressure breakdown of dolomite + orthopyroxene to magnesite + clinopyroxene is displaced to higher pressures (by 2 kbar) in natural compositions relative to CMS. CO₂. We also find a stability field of magnesian calcite in lherzolite at 15–25 kbar and 750–1000°C.     

The “geo” of United States national strategy

The paper discusses the geo dimensions of the new world order, with particular emphasis on the increasing role of geoeconomics, and its impact on US national strategy. While the paper uses the US experience to illustrate the growing importance of geoeconomic considerations, the issues raised have direct bearing on many other nations throughout the world.US post Cold War strategy is determined by its three geo challenges. The most important is the geoeconomic challenge caused by the tri-polar division of the world along trading bloc lines, instant global communication and other technologies overcoming the constraints of physical geography, transnational corporations (TNCS) that are becoming supranational in character, and the emergence of a well educated global labor force. The geopolitical challenge is characterized by America's declining relative economic power and its traditional military allies having become economic competitors, while religious, ethnic, and regional tensions threaten its global interests. The third challenge is the military geography issue of effectively projecting power over distance, within the constraints of greatly reduced budgets and loss of overseas bases.     

Influence of bottom topography roughness on the jet and inertial recirculation of a mid-latitude gyre

Several numerical experiments are conducted to examine the influence of mesoscale, bottom topography roughness on the inertial circulation of a wind-driven, mid-latitude ocean gyre. The ocean model is based on the quasi-geostrophic formulation, and is eddy-resolving as it features high vertical and horizontal resolutions (six layers and a 10 km grid). An antisymmetrical double-gyre wind stress curl forces the baroclinic modes and generates a strong surface jet. In the case of a flat bottom, inertia and inverse energy cascade force the barotropic mode, and the resulting circulation features strong, barotropic, inertial gyres. The sea-floor roughness inhibits the inertial circulation in the deep layers; the barotropic component of the flow is then forced by eddy-topography interactions, and its energy concentrates at the scales of the topography. As a result, the baroclinicity of the flow is intesified: the barotropic mode is reduced with regard to the baroclinic modes, and the bottom flow (constrained by the mesoscale sea-floor roughness) is decoupled from the surface flow (forced by the gyre-scale wind). Rectified, mesoscale bottom circulation induces an interfacial form stress at the thermocline, which enhances horizontal shear instability and opposes the eastward penetration of the jet. The mean jet is consequently shortened, but the instantaneous jet remains very turbulent, with meanders of large meridional extent. The sea-floor roughness modifies the energy pathways, and the eddies have an even more important role in the establishment of the mean circulation: below the thermocline, rectification processes are dominant, and eddies transfer energy toward permanent mesoscale circulations strongly correlated with topography, whereas above the thermocline mean flow and eddy generation are influenced by the mean bottom circulation through interfacial stress. The topography modifies the vorticity of the barotropic and highest baroclinic modes. Vorticity accumulates at the small topographic scales, and the vorticity content of the highest modes, which is very weak in the flat-bottom case, increases significantly. Few changes occur in surface-intensified modes. In the deep layers of the model, the inverse correlation between relative vorticity and topography at small scales ensures the homogenization of the potential vorticity, which mainly retains the largest scales of the bottom flow and the scale of β.     

Volatile fluxes from volcanoes

Volatile fluxes from Mid Ocean Ridge (MOR) and subaerial volcanism have been estimated or re-evaluated using several natural tracers-³He, ²¹⁰Po, SO₂-and chemical ratios of volatile species in lavas and volcanic gases.
These estimates confirm the net predominance of anthropogenic fluxes over volcanic fluxes for CO₂, SO₂ and trace metals.
They also suggest that, while most of the volatiles transferred during MOR volcanism come from the mantle, volatiles stored at the surface of the Earth supply an appreciable fraction of subaerial fluxes and can be the dominant source for some of them.
The surface inventory of volatile species cannot result from steady-state degassing with constant rate and needs much greater fluxes in the past or other volatile supply processes. This inventory is the result of several of the following processes: capture of the solar nebula and its subsequent partial escape, impact degassing of accreting bodies, and, from Archean to present mantle, degasssing through volcanism and associated phenomena, with recycling into the mantle through subduction.