Geothermobarometry in Four-phase Lherzolites I. Experimental Results from 10 to 60 kb

Experiments were carried out on primitive natural lherzoliticcompositions from 10 to 60 kb and to 1400?C. Mineral compositionswere reversed by using mixes of finely ground (<20µm)minerals from natural rock samples with different initial compositions.The minerals were mixed in proportions to give a primitive uppermantle composition. Further starting materials were a syntheticmineral mix and a sintered oxide mix. Equilibrium was thus approachedfrom different chemical directions, and equilibrium mineralcompositions were inferred from the overlap of microprobe analyses.Fe-loss problems were avoided by using single-crystal olivinesas sample containers. Samples were placed into holes drilledinto cylindrically shaped olivine crystals, which in turn werewelded into Pt capsules. A further advantage of this methodis that oxygen fugacities seem to be buffered by the Ni precipitationcurve. This curve lies very close to the iron-w?stite (IW) buffercurve for mantle olivines. Oxygen fugacities calculated fromcompositions of experimentally produced spinels give valuesslightly above the IW buffer curve. Therefore, practically noFe³⁺ should be present in our experimental charges. Ortho- and clinopyroxene compositions were strictly reversedwith respect to Al Cr, and Na (only cpx) contents and theirmutual amounts of enstatite and diopside components. The datashow the fundamental influence of Na in cpx on two-pyroxenethermometry. Garnet compositions were reversed with respectto Ca, Al, Cr, Ti, and Mn. Other than in CMAS, Ca in garnetis mostly dependent on temperature in the natural system andnot so much on pressure. This behaviour of Ca will most stronglyinfluence barometers based on the Al content of orthopyroxenecoexisting with garnet. The mg-numbers of coexisting phaseswere not strictly reversed. The homogeneity of olivine compositionsand internal consistency of the data set give us confidencethat equilibrium values of mg-number have been reached in almostall cases. The experimental results presented in this paperoffer the unique possibility of simultaneously testing manydifferent thermobarometers which are based on dfferent exchangereactions and which were calibrated in different laboratories.     

EXPLORING THE INTERIOR OF SALT DOMES FROM BOREHOLES*

Presently, salt deposits are used for storage of energy from liquid or gaseous carrier materials. Another application being considered is the storage of radioactive material. In order to avoid possible future environmental risk, extremely high security has to be guaranteed in advance for a long period. Therefore, a very precise exploration is absolutely necessary. Exploration methods already known—which are applicable during and after drilling of test holes—help in the recognition of structures in the salt only in the close vicinity of the holes. Radiowave electromagnetic methods developed for routine measurements in salt mines provide a penetration of some hundred meters. Appropriate well-logging equipment has been designed here for measurements from surface boreholes. Discontinuities around one borehole in the salt can be detected through the electromagnetic reflection method. When measuring between two boreholes, additional reflecting zones can be detected, and attenuation of directly pathed waves can point out material with greater absorption properties between the holes. Additional attempts to use acoustic waves of nearly the same wavelength proved that it is possible to recognize the direction of signals arriving at the receiver and allow separation of P- and S-waves. Instrumentation as well as recent results are reported.     

Alluvial-fan-carbonate facies with evaporites, Eocene Guarga Formation, Southern Pyrenees, Spain

Continental carbonates of the basal part of the Guarga Formation, Upper Eocene, have been studied in the field and in the laboratory. Both geographic pattern and vertical sequences served as a basis for sedimentary models. The pattern reveals that carbonate deposition was greater in the western part of the Tremp-Graus Basin than in the eastern part. Studies of slabs and thin sections suggest high energy deposition in the eastern part of the basin and low energy deposition in the western part. Gross lithology, petrography, and faunal/floral associations indicate sedimentary environments which include calcimorphic palaeosoils, caliche nodule zones, solid caliche nodule zones, caliche crusts, fluvial carbonates, paludal carbonates, and pond/lake carbonates. Distribution pattern of the carbonate bodies and lithologies seem to be controlled by alluvial-fan processes. They are typical fining-upward sequences in the sense of Collinson (1978) which furthermore can be assigned to a channel-bound group and a succession independent of fluvial channels. Referring to fan-physiography the sequences can be subdivided into fluvial proximal interchannel, fluvial distal interchannel, lacustrine distal interchannel, nearshore lacustrine-paludine, and lacustrine offshore carbonate facies. The two last categories are distal alluvial plain and/or fan-edge associations. The presence of especially charophyte oogonia adjacent to pseudomorphs of lenticular gypsum in the lacustrine carbonates of the western Tremp-Graus Basin indicates that their depositional environment was that of shallow lakes with alternating salinities. Diagenetic alterations consist of three major phenomena: (a) general cementation of the carbonates by low-Mg calcite, (b) pseudomorphs of lenticular gypsum, and (c) silification. The relationship of gypsum and silica diagenesis to the regional distribution of lacustrine carbonates suggests early diagenetic processes for their formation. Gypsum pseudomorphs are small and are found to be replaced by Fe-calcite throughout the whole area, or are large and replaced preferentially by silica in the west only. There is experimental evidence (Cody, 1979) that distribution of gypsum morphology and crystal sizes can be related to increased salinities and increased organic production in the west. Chert-gypsum fabrics from that area resemble the ‘silex nectique’ of Cayeux (1929). The occurrence of this noteworthy type of chert together with length-slow chalcedony is an indication of relatively early silification of the gypsum. It is suggested that the sediments containing the silicified lenticular gypsum hosted brines of partly marine origin which alternately became brackish or fresh. An attempt was made to check the influence of evaporation during carbonate deposition by determining the stable isotope composition of the carbonates and of the chert. The data show no distinct variations with respect to the presumed palaeoenvironments. The ranges in the carbonates (δ¹³C ? 5,0 to ?7,9; δ¹⁸ O ?4,5 to ?7,7) fit among the data given in the literature. The values derived from chert (δ¹⁸O 26,9 to 31 SMOW) support a concept of alternating salinities. Distribution of carbonate lithofacies indicate a systematic change in sedimentation from fluvial/ephemeral to permanent lacustrine from east to west. Diagenetic alterations suggest an increase of salinities of pore waters in the same direction. In addition the lacustrine limestones in the west were due to frequent changes in pore-water salinity. Similar sequences lacking subtidal and/or lagoonal fossil associations but including pseudomorphs of carbonate and chert after lenticular gypsum were described from the Lower Purbeckian of Dorset (West, 1964, 1975, 1979), and from the Tertiary of the Paris Basin (Fontes, 1968). Guarga limestone environments of especially the western Tremp-Graus Basin probably resembled the modern semi-arid ephemeral saline lakes in Australia (Burne, Bauld & De Dekker, 1980). Gypsum precipitation in these environments can be compared to the one in modern sabkhas from the Mediterranean coast of Egypt (West, Ali & Hilmy, 1979; Ali West & Hilmy, 1980).