Quaternary kimberlitic volcanism on the Tanzania Craton

The tuffs and lava of the Igwisi Hills are the youngest magmatic rocks on the Tanzania Craton. They consist of small xenoliths of phlogopite-calcite-garnet lherzolite in an olivine-calcite-dolomite-apatite-perovskite-spinel-serpentine matrix. Their mineralogy, major element chemistry and isotope chemistry indicate strong affinities with kimberlite, particularly the calcite-rich kimberlites of the Benfontein Sills, South Africa. As such, the Igwisi Hills are the youngest example of kimberlite magmatism. The magma formed by small degrees of melting of a carbonate-apatite-bearing peridotite source at depths in excess of 110 km.     

Geology of the platanares geothermal area, Departamento de Copán, Honduras

The Platanares geothermal area, Departamento de Copán, Honduras, is located within a graben that is complexly faulted. The graben is bounded on the north by a highland composed of Paleozoic (?) metamorphic rocks in contact with Cretaceous - Tertiary redbeds of unknown thickness. These are unconformably overlain by Tertiary andesitic lavas, rhyolitic ignimbrites, and associated sedimentary rocks. The volcanic rocks are mostly older than 14 Ma, and thus are too old to represent the surface expression of an active crustal magma body. Thermal fluids that discharge in the area are heated during deep circulation of meteoric water along faults in a region of somewhat elevated heat flow. Geothermometry based upon the chemical composition of thermal fluids from hot springs and from geothermal gradient coreholes suggests that the reservoir equilibrated at temperatures as high as 225 to 240°C, within the Cretaceous redbed sequence. Three continuously cored geothermal gradient holes have been drilled; fluids of about 165°C have been produced from two drilled along a NW-trending fault zone, from depths of 250 to 680 m. A conductive thermal gradient of 139°C/km, at a depth of 400 m, was determined from the third well, drilled 0.6 km west of that fault zone. These data indicate that the Platanares geothermal area holds considerable promise for electrical generation by moderate- to hightemperature geothermal fluids.     

Mica in the upper mantle

Despite several lines of indirect evidence, there has hitherto been little unambiguous evidence of a volatile bearing phase in the upper mantle. Mica has been found as a primary phase in several specimens of peridotite and one specimen of garnet lherzolite from the Lashaine volcano, northern Tanzania.     

Na,K, P and Ti in garnet,pyroxene and olivine from peridotite and eclogite xenoliths from African kimberlites

Electron microprobe analyses sensitive to 20ppmw (2σ) were made for Na, P, K and Ti in garnet, pyroxenes and olivine from peridotite and eclogite xenoliths from African kimberlites and volcanic rocks in Tanzania. Average concentrations (ppmw) in peridotite (mostly garnet lherzolite) are: Na₂O gt 340 ol 90 opx 1070 cpx 2.1 (wt.%); P₂O₅ gt 460 ol 130 opx 50 cpx 350; K₂O gt <20 ol <20 opx 30 cpx 170; TiO₂ gt 1470 ol 130 opx 480 cpx 1630. For eclogites and a cpx megacryst with gt inclusions: Na₂O gt 610 cpx 4.3 (wt.%); P₂O₅ gt 530 cpx 300; K₂O gt <20 cpx 370; TiO₂ gt 1990 cpx 1980.In garnet, Na can be explained by coupled substitution with P and Ti, and there is no need to invoke six-coordinated silicon. The Na distribution between garnet and clinopyroxene correlates with the Fe/Mg distribution for both eclogites and peridotites, and for the peridotites correlates with estimates of pressure and temperature from pyroxene composition. When calibrated experimentally, the Na distribution may be a useful indicator of physical conditions at depths for which the Fe/Mg distribution is insensitive; furthermore the Na distribution may be less sensitive to oxidation state.     

Granny Smith diopside megacrysts from the kimberlites of the Kimberley area and Jagersfontein,South Africa

Calcic diopside megacrysts, called Granny Smith nodules, in the Kimberley area and Jagersfontein kimberlites are sheared, commonly contain lenticles of ilmenite and intergrowths of phlogopite, and have a distinctive apple-green color. These diopsides have $\text{Ca}\text{(Ca + Mg)}\text{> 0.45,}\text{Mg}\text{(Mg + Fe)}\text{> 0.90, 0.2?0.4}$ wt% TiO₂ and 0.5–3 wt% Cr₂O₃. They have Na > (Al + Cr), in contrast to diopsides in peridotite xenoliths and those that form subcalcic discrete nodules, but in this respect are similar to diopsides in amphibole-bearing MARID nodules and mica-rich glimmerites. Granny Smith megacrysts are not cognate (Kramers, 1979); their parental magmas (in an igneous or metasomatic sense) may have been parts of the spectrum between kimberlites and lamproites.     

Advances in kimberlite geology

Over the past fifteen years the original data on the kimberlites of South Africa have been supplemented with a wealth of information on the kimberlites of the U.S.S.R., the U.S.A., and various parts of Africa. From their distribution pattern, it can now be shown that kimberlites are virtually confined to the old Precambrian cratons and that the diamond-bearing kimberlites exist on the older cratonic nuclei that have not been deformed for the past 2,500 m years. The kimberlite dykes infill major, deep-seated fractures that cross the cratons on a geometrical pattern. The kimberlite that infilled these fractures was a hot fluid that penetrated to within 2–3 km of the surface before there was explosive breakthrough to the surface with subsequent formation and infilling of the high-level diatremes by a gas-solid fluidisation process. The kimberlites can vary widely texturally, from the fragmental variety found in the diatremes, to massive varieties found within the hypabyssal dykes or skills.     

A pyroxene-ilmenite intergrowth from the Monastery Mine,South Africa

A clinopyroxene-ilmenite intergrowth in a xenolith from the kimberlite Monastery Mine, South Africa, may be derived by exsolution from a single phase at depth.     

Analysis of long-period events recorded at Mount Etna (Italy) in 1992, and their relationship to eruptive activity

Seismic activity recorded at Mount Etna during 1992 was characterized by long-period (LP) events and tremor with fluctuating amplitudes. These signals were associated with the evolution of the eruptive activity that began on December 14, 1991. Following the occurrence of numerous volcano-tectonic earthquakes at the onset of the eruption, LP events dominated the overall seismicity starting in January, 1992. The LP activity occurred primarily in swarms, which were temporally correlated with episodic collapses of the crater floor in the Northeast Crater. Source depths determined for selected LP events suggest a source region located slightly east of Northeast Crater and extending from the surface to a depth of 2000 m. Based on the characteristic signatures of the time series, four families of LP events are identified. Each family shares common spectral peaks independent of azimuth and distance to the source. These spectral features are used to develop a fluid-filled crack model of the source. We hypothesize that the locus of the LP events represents a segment of the magma feeding system connecting a depressurizing magma body with a dike extending in the SSE direction along the western wall of Valle del Bove, toward the site of the Mount Etna eruption. We surmise that magma withdrawal from the source volume beneath Northeast Crater may have caused repeated collapses of the crater floor. Some collapse events may have produced pressure transients in the subjacent dike which acted as seismic wave sources for LP events.     

Coastal sedimentation associated with the December 12th, 1992 tsunami in Flores,Indonesia

This paper presents the result of a detailed granulometric investigation of sediments deposited by a modern tsunami, the 1992 tsunami in Flores, Indonesia. Eyewitness accounts indicate that sediments were deposited upon coastal lowlands over wide areas as a result of the tsunami inundation. Distinctive vertical and lateral variations in particle size composition are characteristic features of the tsunami deposits and these are intimately related to sedimentary processes associated with flood inundation. The geomorphological and sedimentary evidence is used here to establish a preliminary model of tsunami sedimentation. This information is believed to be of great value in understanding sedimentary processes associated with tsunami flooding and in the interpretation of palaeo-tsunami deposits.