Ratio correlations and major element mobility in altered basalts and komatiites

Attempts have been made in the recent literature to constrain major element mobility in altered basalts and komatiites using the molecular proportion ratio (MPR) diagrams developed by T.H. Pearce (1970). The method involves ratioing the molecular proportions of two oxides against a third oxide and presenting the results as an oxide-oxide ratio plot. The transformation appears to clean up scatter plots with low correlations and produces highly correlated ratio plots. These plots have been taken to be indicative of igneous fractionation trends and thus of minimal bulk chemical change during alteration. There are however inherent statistical problems in ratio correlation which have received much attention in the mathematical literature. This paper explains the implications of enhanced correlations for work on altered rocks using ratio plots and suggests that extreme caution must be exercised in the interpretation of MPR diagrams.     

Dolomitization and the genesis of the Woodcutters lead-zinc prospect,Northern Territory,Australia

Conclusion Roberts' mechanism of ore genesis in this paper is plausible and is worthy of consideration. It could well apply to situations such as the Missouri-type deposits, although Gerdemann and Myers (1972) might disagree. One cannot deny that dolomitization and diagenetic processes have occurred at Woodcutters, but in view of the folding and the time-relationships exhibited by the rocks, it would appear that these are not the ultimate ore-forming processes.It is possible that the mechanism of ore genesis proposed by Roberts might apply more fully to the Bulman deposits east of Woodcutters (Patterson, 1965) where the resemblance to a Missouritype situation is stronger.     

The effects of temperature,pressure, and oxygen on copper and iron sulphides synthesised in aqueous solution

Experiments with both iron sulphide and copper sulphide precipitates in aqueous solution show that oxygen is significant in determining the mineral phases resulting after heating up to 180 °C. Tetragonal FeS converts to FeS₂ and Fe₃O₄ in the presence of oxygen, while in the absence of oxygen under the same conditions it converts to hexagonal FeS. Suspensions of covellite and of chalcocite under the same conditions convert to digenite in the presence of oxygen, but remain unchanged when oxygen is absent. Experimental evidence is advanced to support structure determinations that CuS contains disulphide ions.

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Zusammenfassung Experimente mit in wässrigen Lösungen ausgefällten Eisen- und Kupfersulfiden dokumentieren die außerordentliche Bedeutung des Sauerstoffes auf die Bildungs- und Verhaltensweise von Mineralphasen, beobachtet nach dem Aufheizen auf 180 °C. So bildet sich aus ursprünglich tetragonalem FeS letzten Endes Pyrit und Magnetit, wenn Sauerstoff zugegen war, während bei dessen Abwesenheit, aber sonst gleichen Versuchsbedingungen, die Umwandlung in hexagonales FeS erfolgte. Suspensionen von Covellin und Chalkosin wurden, unter analogen Versuchsbedingungen und in Gegenwart von Sauerstoff, in Digenit überführt, während sonst, wenn keine Sauerstoffeinwirkung stattfand, die Mineralphasen keinerlei Veränderungen erkennen ließen. Mittels solcher experimenteller Methoden lassen sich bei der Strukturbestimmung des CuS offenbar Merkmale des Vorhandenseins von Disulfidionen erkennen.

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Published by permission of the Director, Bureau of Mineral Resources, Canberra A. C. T., Australia.

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Temporary Address: Mineralog.-Petrograph. Inst. Univ. Heidelberg, Berliner Str. 19, 69 Heidelberg, Germany     

Numerical models of convection in the earth's mantle

Calculations on convection in Newtonian fluids at large Rayleigh numbers show behaviour in general agreement with geophysical observations. The calculated values of surface velocities, regional gravity and topographic anomalies all lie within the range of values found at the earth's surface. The sign of the gravity anomaly over rising fluid is positive because the gravitational effect of the surface deformation is greater than that of the density deficit. This result appears to agree with geophysical observations and should permit the flow within the mantle to be followed in some detail.     

A comparison of natural and experimental strain patterns around fold hinge zones

Patterns of finite strain around fold hinges in incompetent layers in buckled interlayered materials have been investigated in a series of experimental tests conducted with a view to comparing the results with a recently described anomalous slaty cleavage pattern from northern Norway. As well as compressing rubber, putty and plasticene models, a photoelastic strain analysis of buckled rubber strips in gelatine has been carried out. Strain patterns from the various tests are strikingly similar to the natural cleavage patterns, and a discussion of the factors favouring their development is presented.     

Pioneers of Photogrammetry Commemorated In Antarctic Place-Names

Unique place-names are essential for scientific fieldwork in Antarctica. This shorter contribution describes how duplication of place-names in British Antarctic Territory has been avoided by the systematic naming of geographically related features according to groups of associated ideas. One such theme, commemorating pioneers in the development of photogrammetry, recognizes the important contribution of photogrammetry to Antarctic mapping and research.     

Autumn MIST

The annual one-day meeting of the MIST (Magnetosphere, Ionosphere and Solar–Terrestrial) community was held on 23 November 2001 at the Geological Society, Burlington House. Neil Arnold reports.