Magmas phlogopite

The Cr-diopside series is the most abundant type of xenolith found in alkali basalts. Amphibole is uncommon in samples of this series, but rare examples have been found from locations across the world (see review by Kempton, 1987). The amphibole is typically a chromium-rich pargasite and has been observed to constitute up to 6% of the mode. Commonly, these amphiboles have partially broken down, a process interpreted to be a response to the incorporation of the xenolith into the ascending host magma. Phlogopite seems to be less commonly observed in spinel peridotites, but is present along with amphibole in some suites (Kempton, 1987 and references therein). In other suites, phlogopite is the only hydrous phase present (Francis, 1987 Canil and Scarfe, 1989). 

Wendlandt RF, Eggler DH (1980) The origins of potassic magmas 2. Stability of phlogopite in natural spinel Iherzolite and in the system KalSi04-Mg0-H20-C02 at high pressures and high temperatures. Am J Sci 280 421-458... 

In volcanic rocks formed by the solidification of magma near the Earth s surface, small cavities of millimeter to centimeter order are often present, in which idiomorphic crystals of phlogopite and hematite occur. All these crystals grown from the vapor phase show typical spiral patterns. However, they grow in a much-reduced free space as compared to the case of pegmatite, and this characteristic is well represented on their surface microtopographs. 

The presence of volatile-bearing phases such as phlogopite, apatite, and carbonates in kimberhtes testify to the volatile-rich nature of the parental magma (e.g., Mitchell, 1986). The ubiquitous serpentization present in kimberlites cannot be used as evidence of magmatic water, with the exception of groundmass serpentine that is interpreted to be primary in nature. As discussed by Mitchell (1986), there are hmited stable isotopic data consistent with a meteoric origin for some of the water in the serpentine. However, it is unclear if these results could be attributed to postemplacement exchange of deuteric serpentine with meteoric fluids. 

Although shallow-mantle xenoliths, hosted in alkali basalts, commonly contain C02-rich fluid inclusions (see below), there have been no reports, to the author s knowledge, of H20-rich fluid inclusions in these samples. The C02-rich fluid inclusions are commonly attributed to late, possibly magma-derived, metasomatism of the samples. If such metasomatism was produced by silicate- or carbonate-rich melts, ascent of such a melt could produce saturation in a C02-rich vapor, but H2O would partition strongly into either residual melt or hydrous phases such as phlogopite or amphibole. Thus, the absence of H2O in the fluid inclusions in these samples cannot be taken as evidence that the metasomatic agent was anhydrous. 

Sudo A. and Tatsumi Y. (1990) Phlogopite and K-amphibole in the upper mantle implication for magma genesis in subduction zones. Geophys. Res. Lett. 17, 29-32. 

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