The effect of manganese on olivine-quartz-orthopyroxene stability

The effect of manganese on the stability of ferrosilite relative to fayalite + quartz has been experimentally determined to assess its importance to orthopyroxene barometry. Reaction reversals in a piston-cylinder apparatus were obtained to within 0.1-kbar intervals indicating instability of Fs₉₅Rh₅ below 10.3, 10.9, 11.4, 12.2, 12.9, 13.7 kbar and Fs₉₀Rh₁₀ below 9.8, 10.4, 10.9, 11.6, 12.4 and 13.2 kbar at 750, 800, 850, 900, 950 and 1000°C, respectively. Each mole % MnSiO₃ extends the pyroxene stability by approximately 0.12 kbar relative to FeSiO₃. Electron microprobe analyses of run products indicate a small preference of Mn for pyroxene over olivine withK_DMn-Fe^opx-oliv = 1.2?1.5, similar to values observed for natural pairs. Mössbauer spectra are consistent with a random distribution of Mn between the M1 and M2 sites in the orthopyroxene. These experimental data allow downward revision of pressure estimates based on the orthopyroxene barometer in areas where Mn is a significant component in orthopyroxene.     

Dissolved free amino acids in interstitial waters of Georgia salt marsh soils

The patterns of concentration of dissolved free amino acids (DFAA) are different and more variable in pore water of soils from a Georgia salt marsh than in most surface waters. Elevated concentrations of DFAA (up to 8850 nmoles liter^?1) and high relative levels of alanine and glutamic acid suggest that bacteria may produce these amino acids. In addition to the common protein amino acids, an unidentified compound was present and was the dominant free amino acid in some sections. This compound appeared to be specific to pore waters as it was not observed in surface waters or in hydrolysates of sediments, bacteria or plants from the marsk.     

An experimental determination of rare earth partition coefficients between a chloride containing vapor phase and silicate melts

The partitioning behavior of cerium, europium, gadolinium and ytterbium between an aqueous “vapor” phase and water saturated silicate melt have been experimentally examined using a new experimental approach employing radioactive tracers and a double-capsule technique. Equilibrium was established by reversing the partition coefficient¹ and by betatrack autoradiography. Aqueous solution compositions were varied by adding different amounts of chloride and in some cases fluoride or carbon dioxide. The H₂O contents of the Spruce Pine pegmatite melts were varied by conducting experiments at 4.0 kb, 800°C and at 1.25 kb, 800°C. A jadeite-nepheline composition (75 wt% Jadeite) also was employed at 4.0 kb, 800°C.The chloride experiments (Spruce Pine 4 kb, 800°C) show a linear relationship between the cube of the chloride molality and the partition coefficients of the trivalent rare earths. Europium, under the experimental fO₂ conditions (quartz-fayalite-magnetite buffer), varied linearly as the fifth power of the chloride molality. At the chloride molalities examined (<1.1 mC1), all the rare earths partitioned preferentially into the melt phase (K_P^RE <1). Relative to pure water, the presence of chloride and fluoride fon increased the partitioning of the individual rare earths into the vapor phase, while carbon dioxide did not. Europium anomalies were recorded 1n all experiments, particularly those involving the Spruce P1ne melt at 4.0 kb and 800°C which displayed a large positive europium anomaly at all chloride molalities. Furthermore, a relative fractionation of the trivalent rare earths was also observed in these experiments, such that K_P^Ce>K_P^Gd>K_P^Yb. The smaller ytterbium ion was consistently concentrated in the melt phase relative to the other rare earths in all experiments on the Spruce Pine composition. Experiments on the jadeite-nepheline composition showed no relative fractionation and a positive europium anomaly. The 1.25 kb experiment on the Spruce Pine composition showed a negative europium anomaly in plots of K_p^RE vs. REE.The overall rare earth partitioning at a constant chloride molality (mCl = .914) was such that K_P^SP(1.25 kb) > K_P^SP(4.0 kb) > K_P^Jd-Ne(4.0 kb), where SP = Spruce Pine, Jd-Ne = jadeitenepheli Using the model of Burnnam (1975), It is suggested that the trivalent rare earth partitioning is related to the cube of the melt octahedral site concentration; a property which 1n hydrous melts 1s dependent on melt composition and hydroxyl molality. Excellent agreement was found for the Spruce Pine melt, whereas the jadeite-nepheline melt gave apparent hydroxyl molalities which were too high for the measured partition coefficient. Additional octahedral sites are proposed for this unusual composition perhaps due to some aluminum in 6-fold coordination. The apparent compositional variation of europium partitioning at a constant oxygen fugacity is believed to be related to both the octahedral melt site concentration for trlvalent europium and an 8-coordinated site concentration for divalent europium. Any parameter which affects the numbers of these sites (_PH₂O, melt composition) will affect the rare earth partitioning. The observed dependency of the partition coefficient on the structural state of the melt could be as significant as its dependency on crystalline structural constraints. Furthermore, since _PH₂O can drastically effect the melt structural state, its effects could be reflected in melt/crystal partition coefficients.     

Low temperature alteration processes affecting ultramafic bodies

At low temperatures, in the presence of an aqueous solution, olivine and orthopyroxene are not stable relative to the hydrous phases brucite, serpentine and talc. Alteration of dunite and peridotite to serpentine or steatite bodies must therefore proceed via non-equilibrium processes. The compositions of natural solutions emanating from dunites and peridotites demonstrate that the dissolution of forsterite and/or enstatite is rapid compared with the precipitation of the hydrous phases; consequently, dissolution of anhydrous minerals controls the chemistry of such solutions. In the presence of an aqueous phase, precipitation of hydrous minerals is the rate-controlling step.Brucite-bearing and -deficient serpentinites alter at low temperature by non-equilibrium processes, as evidenced by the composition of natural solutions from these bodies. The solutions approach equilibrium with the least stable hydrous phase and, as a consequence, are supersaturated with other hydrous phases. Dissolution of the least stable phase is rapid compared to precipitation of other phases, so that the dissolving mineral controls the solution chemistry.Non-equilibrium alteration of anhydrous ultramafic bodies continues until at least one anhydrous phase equilibrates with brucite, chrysotile or talc. The lowest temperature (at a given pressure) at which this happens is defined by the reaction:

$\text{3H}{}_2\text{O + 2Mg}{}_2\text{SiO}{}_4\text{? Mg}{}_3\text{Si}{}_2\text{O}{}_5\text{(OH)}{}_4\text{+ Mg(OH)}{}_2$

(Johannes, 1968, Contrib. Mineral. Petrol. 19, 309–315) so that non-equilibrium alteration may occur well into greenschist facies metamorphic conditions.     

Demethylated hopanes in crude oils and their applications in petroleum geochemistry

Analyses of some Australian crude oils show that many contain varying concentrations of A/ B-ring demethylated hopanes. These range from C₂₆ to C₃₄ and have been identified from their retention times and mass spectral data as 17α(H)-25-norhopanes. Comparison of hopane and demethylated hopane concentrations and distributions in source-related, biodegraded oils suggests that demethylated hopanes are biotransformation products of the hopanes. Further, it appears that the process occurs at a late stage of biodegradation, after partial degradation of steranes has occurred. Demethylated hopanes are proposed as biomarkers for this stage of severe biodegradation. The presence of these compounds in apparently undegraded crude oils is thought to be due to the presence of biodegraded crude oil residues which have been dissolved by the undegraded crude oil during accumulation in the reservoir sands. The timing of hopane demethylation, relative to the degradation of other compounds, has been assessed and the progressive changes in crude oil composition with increasing extent of biodegradation have been identified. The use of demethylated hopanes as maturity parameters for severely biodegraded crude oils, and the applicability of established biomarker maturity parameters to such oils, are also discussed.     

Australian Rock Coasts: review and prospects

Studies of Australian rock coasts (except carbonate reefs) are reviewed and considered in view of recent process and morphological studies. The unique nature of the Australian coast, its geographical distribution and relative stability mean that it is a productive environment in which to research fundamental questions concerning rock coasts. Future research directions are identified, specifically in the areas of processes, morphology and modelling.     

A Water Balance Model for a Subarctic Sedge Fen and its Application to Climatic Change

A model to calculate the water balance of a hummocky sedge fen in the northern Hudson Bay Lowland is presented. The model develops the potential latent heat flux (evaporation) as a function of net radiation and atmospheric temperature. It is about equally sensitive to a 2% change in net radiation and a 1°C change in temperature. The modelled potential evaporation agrees well with the Priestley–Taylor formulation of evaporation under conditions of a non-limiting water supply. The actual evaporative heat flux is modelled by expressing actual/potential evaporation as a function of potential accumulated water deficit. Model evaporation agrees well with energy balance calculations using 7 years of measured data including wet and dry extremes. Water deficit is defined as the depth of water below reservoir capacity. Modelled water table changes concur with measurements taken over a 4 year period. When net radiation, temperature and precipitation measurements are available the water balance can be projected to longer time periods. Over a 30 year interval (1965–1994) the water balance of the sedge fen showed the following. During the growing season, there was an increase in precipitation, no change in temperature and a decrease in net radiation, evapotranspiration and water deficit. There was also a decrease in winter snow depths. The fen was brought back to reservoir capacity during final snowmelt every year but one. Summer rainfall was the most important single factor affecting the water balance and the ratio actual/potential evaporation emerged as a linear function of rainfall amount. A 2 × CO₂ climate warming scenario with an annual temperature increase of 4°C and no precipitation change indicates lesser snow amounts and a shorter snow cover period. A greater summer water deficit, triggered mainly by greater evaporation during the month of May, is partially alleviated by lesser evaporation magnitudes in July. The greater water deficit would be counterbalanced by a 23% increase in summer rainfall. On average, the fen's water reservoir would still be recharged after winter snowmelt but the ground would remain at reservoir capacity for a shorter time. The warming scenario with a 10% decline in summer rainfall would create a large increase in the longevity and severity of the water deficit and this would be particularly evident during drier years. The carbon budget and peat accumulation and breakdown rates are strongly affected by changes in the water balance. Some evidence implies that greater water deficits lead to an increase in net carbon emissions. This implies that the sedge peatland could lose biomass under such conditions. An example is given where increased water deficit results in large decreases in local wetland streamflow.