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Experimental constraints on phlogopite chemistry in lamproites; 2, Effect of pressure-temperature variations

S. F. Foley
Experimental constraints on phlogopite chemistry in lamproites; 2, Effect of pressure-temperature variations
European Journal of Mineralogy (June 1990) 2 (3): 327-341

Abstract

The chemistry of micas from high-pressure (5-40 kbar) experiments on leucite lamproite and olivine lamproite show much less variation as a function of pressure and temperature at fixed fluid composition (H (sub 2) O > CH (sub 4) ; results reported here) in comparison to the variation due to fluid composition (H (sub 2) O/CH (sub 4) and H (sub 2) O/CO (sub 2) ; results given in Part 1). Micas at high pressures tend to have higher Al (sub 2) O (sub 3) , and lower TiO (sub 2) , K/Al and Cr (sub 2) O (sub 3) . Decreasing temperature produces decreasing Mg/(Mg+Fe) and Al (sub 2) O (sub 3) , and increasing SiO (sub 2) , K (sub 2) O and F in micas, but these trends may be a function of the degree of crystallization and may differ in natural lamproites where fractional crystallization occurs. Mica has higher Ti0 (sub 2) where it coexists with titanate minerals. Since lamproites are thought to oxidize considerably during emplacement, the difference in composition between micas crystallized in these experiments and those crystallized under lower-pressure, more oxidizing conditions shows that most lamproite mica phenocrysts are not typical of high-pressure crystallization and can therefore not be used to model high-pressure fractionation. The strong decrease in Al (sub 2) O (sub 3) contents during differentiation of natural lamproites appears to be due to fractionation of mica in water-rich conditions, in which micas have very high Al contents. High-TiO (sub 2) mica phenocrysts must have been crystallized under low-pressure conditions at high temperatures. Natural phlogopites may contain appreciable Ti (super 3+) if crystallized at fO (sub 2) nearly equal iron-wiistite, but not at 2 log units fO (sub 2) higher. Strongly olivine-normative lamproitic melts are likely agents of mantle enrichment in the sub-continental mantle at depths of 100-200 km. The sub-liquidus experimental mica compositions can be used to constrain models of vein chemistry in inhomogeneous mantle compositions.


ISSN: 0935-1221
Serial Title: European Journal of Mineralogy
Serial Volume: 2
Serial Issue: 3
Title: Experimental constraints on phlogopite chemistry in lamproites; 2, Effect of pressure-temperature variations
Author(s): Foley, S. F.
Affiliation: Max-Planck-Inst. Chem., Abt. Kosmoschem., D-6500 Mainz, Federal Republic of Germany
Pages: 327-341
Published: 199006
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 48
Accession Number: 1993-008898
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 5 tables, 43 anals.
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from Geoline, Bundesanstalt fur Geowissenschaften und Rohstoffe, Hanover, Germany
Update Code: 1993
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