ALBERT

Notes: The hardening of soils containing little clay content has often been related to the cementing of skeleton grains. In sandy-loam soils from the southern plain of the Chad basin the skeleton grains of the hard horizon are coated and bonded by wall-shaped bridges. We have studied the bonding agent by X-ray diffraction on extracted fractions (〈 0·5 pm) and by quantitative in situ electron microprobe analysis on thin sections. The coatings and the wall-shaped bridges are composed mainly of SO2 (mean = 60%), Al2O3 (29%) and FezO3 (7%). The TiO2 and K2O contents were small (1–2%), and the MgO, Na3O and CaO contents were less than 1%. The composition of the coatings and the wall-shaped bridges is almost homogeneous, even if the Si/Al atomic ratio varies weakly with the morphology of the bridges. The bonding agent constituting the coatings and the wall-shaped bridges seems to be a mixture of predominantly Al-Fe beidellite with small contents of kaolinite, illite and quartz. A small content of fine clay minerals plays the major role of bonding agent and is responsible for hardness in sandy or sandy-loam soils.

Notes: Summary The results of geochemical investigations in the Arakapas Fault Belt (AFB) in the Cyprus ophiolite are discussed to provide a comprehensive model for the genesis of the ophiolitic intrusive and extrusive rocks which occur in a single but evolutive tectonic environment. On the basis of field and petrographic criteria, five lava types are defined, one in the Axis Sequence (AS) and four in the Upper Pillow Lavas (UPL). The comparison between observed and calculated partial melting and fractional crystallization trends favors high DCr and DMg and low Dv consistent with high fO2. Partial melting occurred in a depleted mantle after a prior melting event. The extent of mantle depletion varies with space and increases with time. Different crystallization orders in lavas reflect distinct parental magma compositions and the extent of pre-eruption fractionation dominated by wehrlitic crystallization in closed magma chambers. Long-lived magma chambers disappear at the end of the AS. Small magma chambers occurred during the formation of the UPI, and were destroyed at various degrees of evolution.

Notes: Abstract Ultramafic rocks, gabbros, plagiogranites, dolerites and basaltic dikes and pillows lavas of the ophiolite slices of eastern Corsica have been metamorphosed in both oceanic and orogenic environments. The trace element content of the metabasaltic rocks which exhibit a tholeiitic differentiation trend towards a ferrobasaltic composition, is similar to that of oceanic basalts. The cumulate sequence is interpreted to have been formed by fractional crystallization from an olivine-tholeiite magma. The troctolites representing cumulates derived from melts representing liquid fractions in the range F=1-0.85, gabbros from melts with F=0.85-0.45, and ferrogabbros and plagiogranites from melts with F〈0.45. An oceanic environment with moderate spreading rates and magmatic processes similar to present-day normal oceanic ridge segments is considered the more probable original setting of the Corsican ophiolites.

Notes: Abstract  Interstitial glasses and glasses in small inclusions in Mg-rich phenocrysts of 14 boninites from New Caledonia, the Mariana Trench, Cape Vogel and Chichi-jima were analysed by electron microprobe and the water contents measured in situ by ion microprobe. The glasses are remarkably fresh and abundant (∼30–90 vol.%), and the phenocrysts are often skeletal with glass inclusions. Broad-beam analyses (∼1030) of interstitial glasses and ∼180 point analyses of glass inclusions were carried out, as well as ∼100 hydrogen analyses. Most glasses have low water-free totals, high water contents, very low MgO, and low total iron; they are almost entirely quartzofeldspathic and with few exceptions (Q+or+ab+ an+C) lies in the range 83–96. The interstitial glasses from New Caledonia, the Marianas and most of the glasses from Chichi-jima are dacitic, those from Cape Vogel straddle dacitic and andesitic compositions, whereas the glasses in a highly glassy sample from Chichi-jima are high-Mg andesitic or boninitic with up ∼9 wt% MgO, and are, with the exception of a few high-Ca boninites from Tonga, the most Mg-rich interstitial glasses so far described in boninites. Glasses included in orthopyroxene, olivine or clinoenstatite are boninitic or high-Mg andesitic in the highly glassy rock and dacitic to high-silica dacitic in the others. They are in general slightly more differentiated than the interstitial glasses, because of more-extensive crystallization on the host crystal in small inclusions. The interstitial glass compositions show a direct relationship between silica and Al2O3 and, for most glasses, alkalis, and inverse relationships between silica and CaO, FeO and MgO; alkalis and TiO2 show, however, a broad spread in values in glasses from the Marianas and New Caledonia. Included glasses show similar variations. Water contents in interstitial glasses are ∼2 wt% for the highly glassy high-Mg andesitic glasses from Chichi-jima, ∼5.4 wt% for the more differentiated andesitic to dacitic glasses from Cape Vogel, and ∼6.7–7.0 wt% in the most differentiated dacitic ones from the Marianas and New Caledonia. Water contents in glass inclusions in olivine, orthopyroxene and clinoenstatite are in the range ∼1.9–3.3 wt%. The interstitial glasses are black and not vesicular, showing that the liquids did not reach supersaturation after eruption on or intrusion near the sea floor, or were insufficiently so to allow nucleation of water vapour bubbles. The water is inferred to be primary and to increase strongly with crystallization in the residual liquid down to the glass-transition T.

Notes: Abstract Within the Vourinos ophiolite evidence of two magmatic series has been preserved in cognate cumulates and in effusive and hypabyssal rocks, which constitute the earlier Krapa sequence and the younger Asprokambo sequence. The Asprokambo dyke basic magmas which are poor in incompatible elements and relatively Ni and Cr rich, bear some resemblance to very low Ti basalts (transitional to boninites) found in subduction related arcs or interarc basins. Krapa series magmas from sills, massive and pillow lavas are best equated with low-K tholeiites of island arc suites. Compositions of Al- and Ti- poor Cpx in lavas from both series are comparable to those in island arc basalts, the Asprokambo Cpx being richer in Ca and Cr than those from Krapa. The large volume of cumulates from the Krapa sequence displays the following crystallization order: Ol±Sp, Cpx, Pl±Opx, Mt. Periodic influx of fresh magma batches into the magma chamber occurred mainly during the formation of the lower cumulates (wehrlite, Ol-clinopyroxenite and melagabbro). The upper cumulates, gabbronorite and leucogabbronorite with minor Mt-bearing gabbronorite, crystallized in the upper levels of a magma chamber which became progressively smaller with time. In the Asprokambo sequence, Ol+Sp, Opx, Cpx, PI and Amph are the successively crystallizing phases. The ortho to heteradcumulates consist of websterite, Pl-websterite, gabbronorite, amphibole bearing leuconorite, diorite and granophyre. In cumulates, especially in the lower Krapa sequence, significant subsolidus reaction was probably induced by the persistence of high geothermal gradients linked to continuous magmatism. Petrological features indicate that the evolution of the Krapa series is more compatible with an intermediate fractional/equilibrium crystallization history in an initially open system, whereas nearly perfect fractional crystallization in closed system may have occurred in the small Asprokambo magma chambers. Chemical variations in the lavas of both series can be explained in terms of crystallization of the observed cumulates. Significantly, the Asprokambo intrusives have igneous Mg-hornblende and vanadium bearing, chromian, aluminous titaniferous magnetite, crystallization of which is responsible for the calcalkaline evolutionary trend of these rocks. Major and trace element modelling necessitates a two stage model for the petrogenesis of the Vourinos parental melts, involving high-degree remelting of previously depleted mantle sources favoured by the influx of subduction derived hydrous fluids. The primary magmas parental to the Krapa and Asprokambo series could have been derived respectively by 20 and 30% equilibrium partial fusion of variably depleted lherzolitic sources, leaving residua having a harzburgitic to dunitic composition.