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.     

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.     

Seasonality and duration in extreme value distributions of significant wave height

This paper presents a statistical model to characterize the long-term extreme value distribution of significant wave height, conditioning to the duration of the storm and accounting for seasonality. A time-dependent version of the peak over threshold (POT) approach is used to build the model, which is then applied to specific reanalysis time series and NOAA buoy records. The model considers the annual and semiannual cycles which are parameterized in terms of harmonic functions. The inclusion of seasonal variabilities substantially reduces the residuals of the fitted model. The information obtained in this study can be useful to design maritime works, because (a) the model improves the understanding of the variability of extreme wave climate along a year and (b) the model accounts for the duration of the storm, which is a key parameter in several formulations for rubble mound breakwater design.     

The oxygen isotope composition of water masses within the North Sea

Based on measurements of the ¹⁸O isotope composition of 247 samples collected over a 3-year period we have assessed the oxygen isotope composition of water masses in the North Sea. This is the first δ¹⁸O data set that covers the entire North Sea basin. The waters lie on a mixing line: δ¹⁸O (‰_VSMOW) = −9.300 + 0.274(S) with North Atlantic sub-polar mode water (SPMW) and surface waters, and Baltic Sea water representing the saline and freshwater end members respectively. Patterns exhibited in surface and bottom water δ¹⁸O distributions are representative of the general circulation of the North Sea. Oxygen-18 enriched waters from the North Atlantic enter the North Sea between Scotland and Norway and to a lesser extent through the English Channel. In contrast, oxygen-18 depleted waters mainly inflow from the Baltic Sea, the rivers Rhine and Elbe, and to a lesser degree, the Norwegian Fjords and other river sources. Locally the δ¹⁸O–salinity relationship will be controlled by the isotopic composition of the freshwater inputs. However, the range of local freshwater compositions around the North Sea basin is too narrow to characterise the relative contributions of individual sources to the overall seawater composition. This dataset provides important information for a number of related disciplines including biogeochemical research and oceanographic studies.