Summary
We present an empirical technique for estimating crystallization pressure and H2O content of MORB and BABB glasses using glass major element compositions alone. This technique is based on models describing 1 atm mineral — basaltic melt equilibria for olivine (ol), plagioclase (pl), and clinopyroxene (cp) which allow calculation of liquidus temperatures for each mineral on the basis of melt composition alone. A necessary requirement for using the proposed technique is cosaturation of glasses with of andpl for estimation of H2O contents, and withol andcp for estimation of crystallization pressures. Mineralogical criteria can be used to assess cosaturation.
The following logic is employed:
Calculated temperatures ofol, pl andcp liquidus must be the same for a glass which represents a quenched melt which crystallized these minerals at 1 atm. The effect ofpl, forcingcp to appear earlier on the liquidus. For higher-pressure crystallization, calculated 1 atm temperature forcp must be lower than that forol due to stronger dependence ofcp crystallization temperature on pressure. Thus the difference between calculated 1 atm liquidus temperatures forol andcp must reflect crystallization pressure. It is well known that H2O can depress crystallization temperature ofpl compared to those ofol andcp. Thus forol-pl saturated H2O-bearing melts, calculated 1 atm anhydrous temperatures forpl must be higher than those forol, and the difference between calculated anhydrous 1 atm liquidus temperatures forpl andol must reflect melt H2O content. p ]As the results of this technique are entirely dependent on the quality of glass major element analyses, we emphasize that application of this method to a single glass analysis can lead to erroneous results. A preferable approach is to obtain a statistically meaningful number of glass analyses for each suite and use averages. We also emphasize the danger of extrapolation of our technique to compositions beyond the MORB — BABB compositional range.
Zusammenfassung
Wir präsentieren eine empirische Methode zur Abschätzung des Kristallisationsdruckes und des H2O-Gehaltes von MORB und BABB Gläsern, wobei auschließlich die Hauptelementzusammensetzung der Gläser benützt wird. Diese Methode beruht auf Modellen, die die Schmelzgleichgewichte zwischen Olivin (ol), Plagioklas (pl) und Klinopyroxen (cp) bei 1 atm beschreiben und die die Berechnung der Liquidus- Temperaturen für jedes dieser Minerale auf Basis der Schmelzzusammensetzung erlauben. Eine notwendige Voraussetzung zur Anwendung dieser Technik für die Abschätzung des H2O-Gehaltes ist die Sättigung der Gläser bezüglich of undpl bzw. für die Abschätzung des Kristallisationsdruckes bezüglichol undcp. Mineralogische Kriterien können zur Abschätzung der Sättigung herangezogen werden.
Die folgende Logik wird angewendet:
Die berechneten Liquidus-Temperaturen vonol, pl undcp müssen gleich sein wie für ein Glas, das abgeschreckte Schmelze repräsentiert aus dem diese Minerale bei 1 atm kristallisiert sind. Der Einfluß des Druckes resultiert hauptsächlich in einer Erhöhung dercp Kristallisations-Temperatur, im Vergleich zu der vonol undpl, wodurchcp gezwungen wird früher am Liquidus zu erscheinen. Für Kristallisation unter höherem Druck muß, als Folge der stärkeren Abhängigkeit dercp Kristallisationstemperatur vom Druck, die für 1 atm berechnete Temperatur voncp daher niedriger sein als die fürol. Die Differenz zwischen den bei 1 atm berechneten Liquidus-Temperaturen fürol undcp müssen daher den Kristallisationsdruck widerspiegeln. Es ist bekannt, daß H2O die Kristallisationstemperatur vonpl im Vergleich zuol undcp herabsetzt. Fürol-cp gesättigte H2O-führende Schmelzen muß daher die bei 1 atm und unter wasserfreien Bedingungen berechnete Temperatur vonpl höher sein als die fürol. Die Differenz zwischen diesen berechneten Temperaturen fürpl undol ergibt daher den H2O-Gehalt der Schmelze. p ]Da die Ergebnisse dieser Methode ausschließlich von der Qualität der Hauptelementanalysen der Gläser abhängen, weisen wir darauf hin, daß die Verwendung einer einzigen Glasanalyse zu falschen Ergebnissen führen kann. Die Verwendung von Mittelwerten, berechnet aus einer statistisch gesicherten Anzahl von Glasanalysen, wird daher bevorzugt. Wir weisen außerdem auf die Gefahr der Extrapolation dieser Technik auf Zusammensetzungen außerhalb des MORB—BABB Bereiches hin.
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Danyushevskyl, L.V., Sobolevz, A.V. & Dmitrievz, L.V. Estimation of the pressure of crystallization and H2O content of MORB and BABB glasses: calibration of an empirical technique. Mineralogy and Petrology 57, 185–204 (1996). https://doi.org/10.1007/BF01162358
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DOI: https://doi.org/10.1007/BF01162358