Experimental hydrogen isotope studies III: Diffusion of hydrogen in hydrous minerals,and stable isotope exchange in metamorphic rocks

Diffusion parameters for hydrogen diffusion in epidote-group minerals and micas have been measured under hydrothermal conditions, or calculated from existing experimental data, for bulk hydrogen isotope exchange experiments between hydrous minerals and water. Activation energies in the range 14 to 31 kcals/g-atom H are comparable to those derived by application of kinetic theory to experimental hydrogen isotope exchange data, and to those for oxygen diffusion in minerals under hydrothermal conditions. Diffusion of hydrogen in epidote is about four orders of magnitude faster than in muscovite, and about two orders of magnitude faster than in zoisite. Hydrogen diffusion in micas is about five orders of magnitude faster than oxygen diffusion, and hydrogen transport occurs dominantly parallel to the layers rather than parallel to the c-axis as for oxygen.Rapid hydrogen transport in minerals may proceed by hydrolysis of Si-O and Al-O bonds, followed by exchange of hydrolyzed oxygens with slower-diffusing (OH) or H₂O. Water appears to be essential for stable isotope exchange between minerals in slowly cooling metamorphic rocks.Stable isotope data for regional metamorphic mineral assemblages suggests that water is usually present in small amounts during cooling of prograde regional metamorphic systems, and estimated closure temperatures for cessation of stable isotope exchange are often more comparable to those calculated from diffusion data than to likely temperatures of metamorphism.Alpine deformation of the Hercynian Monte Rose Granite (Frey et al. 1976) permitted access of water and initiated stable isotope exchange amongst coexisting minerals. The diffusional behaviour of species in relict Hercynian muscovites is consistent with available experimental diffusion data.     

Amino acid racemization dating of fossil bones,I. inter-laboratory comparison of racemization measurements

Enantiomeric measurements for aspartic acid, glutamic acid, and alanine in twenty-one different fossil bone samples have been carried out by three different laboratories using different analytical methods. These inter-laboratory comparisons demonstrate that D/L aspartic acid measurements are highly reproducible, whereas the enantiomeric measurements for the other amino acids show a wide variation between the three laboratories. At present, aspartic acid measurements are the most suitable for racemization dating of bone because of their superior analytical precision.     

Experimental hydrogen isotope studies,II. Fractionations in the systems epidote-NaCl-H2O,epidote-CaCl2-H2O and epidote-seawater,and the hydrogen isotope composition of natural epidotes

The hydrogen isotope fractionation factors between epidote and aqueous 1 M and 4 M NaCl, 1 M CaCl₂ solutions, and between epidote and seawater, have been measured over the temperature range 250–550°C over which the degree of dissociation of dissolved species varies significantly. Measured fractionations at 350°C are decreased by up to 12‰, 9‰ and 7‰ relative to pure water in seawater, 1 M CaCl₂ and 1 M NaCl respectively, while above 500°C fractionations are not measurably dependent on fluid composition. Water—solution fractionation factors are derived which are generally applicable to the correction of mineral—water hydrogen isotope fractionations for the composition of the fluid phase.The hydrogen isotope compositions of natural epidotes are interpreted in the light of experimental fractionation data for situations where temperature, δD (fluid), and, in some cases, fluid chemistry, are independently known. Epidotes from active geothermal systems have hydrogen isotope quench temperatures consistent with or close to measured well temperatures unless the measured temperature has declined substantially since epidote formation or there is uncertainty in the D/H ratio of the water associated with the epidote because of isotopic heterogeneity in the well waters. Hydrothermal and metamorphic epidotes show closure temperatures of 175–225°C and 200–250°C. There is no evidence that retrograde metamorphic fluids, if present, are isotopically different from prograde fluids.The water-solution fractionations indicate strong solute-solvent interactions between 250 and 450°C and imply that both dissociated and associated species contribute to the fractionation effects through modification of the orientations and structure of the water molecules. Solute-solvent interactions become negligible at temperatures around 550°C.     

Crystallization of chromite from nickel-iron sulphide melts

An experiment, in which an iron-nickel-copper sulphide melt was heated with synthetic chromite and then cooled, showed that substantial quantities of chromite had dissolved in the melt and had then recrystallized as euhedral crystals rimmed with magnetite. This experiment suggests that the unusual chromite (low in Mg and Al) which is associated with the sulphide phase in Western Australian nickel ores may have formed in a similar way.J.M.R. carried out the experimental part of this investigation as a post-graduate student at Flinders University, South Australia, working under a CSIRO extramural grant.     

Lunar sample 15405: Remnant of a KREEP basalt-granite differentiated pluton

Large, coarse-grained fragments of granite, containing plagioclase, a silica polymorph, potash feldspar, and exsolved pyroxene, with minor ilmenite, a phosphate, Fe-metal, and troilite, occur in sample 15405. A similar coarse-grained clast type (KREEP-rich quartz-monzodiorite) has a similar mineralogy but contains more ilmenite, large phosphates, less silica, and lacks troilite. One unusual KREEPy olivine vitrophyre fragment is also present. All the other fragments in 15405 are of Apollo 15-type KREEP basalt; ANT-suite and breccia fragments are conspicuously absent. The groundmass of 15405, of a KREEP basalt composition, is vesicular with a variolitic texture and is interpreted as an impact melt. Except for the olivine vitrophyre, the fragments are believed to be the remnants of a shallow-level KREEP basalt-granite differentiated pluton, in which granite was produced as the residual liquid without involvement of immiscibility effects.The large amount of melt required to produce the pluton, and the retention of the pluton's integrity from crystallization until the formation of the source boulder of 15405 suggest that KREEP basalt magma is not ancient (～4.3 b.y.), but was produced by the partial melting of the interior of the moon at around 3.90–3.95 b.y.; this conclusion is supported by the presence of KREEP basalt in soil breccia 15205, to the exclusion of other highland rock types. If this interpretation is correct, the source of Apollo 15-type KREEP basalt had a Rb/Sr ratio higher than anorthositic norite, commonly proposed as the source rock.     

Vein arrays, hydraulic fractures and pressure-solution structures in a deformed flysch sequence S.W. England

Vein arrays and pressure-solution cleavages are common in the sandstone units of the deformed flysch. Both structures were established prior to the folding of the sediments, and they continued to evolve during this folding. Tensile fractures and conjugate sets of shear fractures (of wrench fault type) were formed. En-echelon arrays of veins are closely associated with the formation of these principal veins. The geometry and relations of the veins are described in detail and their relation to the principal stresses at the time of formation is discussed.

Forking at vein terminations, branch fractures and offset structures in veins are described. The analogies between these and structures produced in rock deformation experiments and found in magmatic dykes are discussed. The branch fracture provides a record of the orientation of the maximum principal stress at the time of its formation.

The veins are infilled with quartz and siderite displaying drusy growth fabrics, indicating that crystal growth occurred into a fluid-filled cavity. It is suggested that the veins originated as hydraulic fractures in a flysch sequence that had developed high pore-fluid pressures during sedimentation. The material in the veins was derived by pressure-solution activity in the sandstone units, which produced a spaced pressure-solution cleavage throughout the region. The relations between veins and pressure-solution cleavages are described. Both small- and large-scale solution transfer of material was involved.     

A rationale for the origins of massif anorthosites

The origin of massif anorthosites cannot be simply explained by a single magma type. Two of the commonly proposed parents for anorthosites are andesites (quartz-diorites) and high-alumina basalts. It is proposed here that these two magmas are the parents for two groups of anorthosites which include all anorthosite massifs, and that the parents for any given anorthosite massif can be determined by the rock sequence associated with the massif.Evidence from experiments and from phenocrysts in volcanics, suggests that andesites crystallizing in the granulite facies would produce plagioclase cumulates (anorthosites) at the base, followed by dioritic and acidic material, whereas high-alumina basalts would produce gabbros followed by anorthosite with very little succeeding acidic material. All massif anorthosites for which relevant data is available have one or the other of these stratigraphic sequences. Grouped according to these sequences, they coincide with two previously proposed groups, i.e. Andesine-type and Labradorite-type, whose characteristics are shown to be compatible with derivation from andesite and high-alumina basalt, respectively.