Xenoliths amphiboles

Olafsson M, Eggler DH (1983) Phase relations of amphibole, amphibole-carbonate, and phlogopite-carbonate peridohte petrologic constraints on the asthenosphere. Earth Planet Sci Lett 64 305-315 Olson P, Schubert G, Anderson C, Goldman P (1988) Plume formahon and lithosphere erosion a comparison of laboratory and numerical experiments. J Geophys Res 93 15065-15084 Pearson DG, Shirey SB, Carlson RW, Boyd FR, Nixon PH (1995) Stabilisahon of Archean lithospheric manhe A Re-Os isotope isotope study of peridohte xenoliths. Earth Planet Sci Lett 134 341-357... 

Wilshire H. G., Nielson J. E., Pike J. E. N., Meyer C. E., and Schwarzman E. C. (1980) Amphibole-rich veins in Iherzolite xenoliths, Dish Hill and Deadman Lake, CaUfomia. Am. 

In an extensive review of the geochemistry of volatile-bearing minerals in mantle xenoliths, Ionov et al (1997) have pointed out that although minerals such as mica, amphibole, and apatite are often referred to as hydrous, in many cases they have very low H2O contents (Boettcher and O Neill, 1980). In such cases, these minerals may have significant amounts of fluorine, chlorine and CO2. Mica, amphibole, and apatite, together with the oxide phases, are important hosts for titanium, potassium, rubidium, strontium, barium, and niobium (Table 9). 

Amphiboles show a wide variety of REE patterns that cannot be uniformly related to modal abundance. Ionov et al. (1997) suggest that where the mineral has equilibrated extensively with silicate melt, LREE-enrichment and convex REE patterns occur. Hence, vein amphiboles from noncratonic xenoliths are enriched in LREE and MREE over HREE and thus have convex REE patterns (e.g.. Figure 28). Disseminated amphiboles have lower MREE/HREE in some instances but those in xenoliths from the W. Eifel region are much more LREE-enriched than amphibole from other locations. Disseminated amphibole can be either LREE enriched or depleted relative to PUM (Figure 28). 

Figure 28 Primitive mantle normalized REE and multi-element patterns for amphiboles from mantle xenoliths. Vein amphibole and disseminated ampbibole data from off-craton spinel Iberzolite xenolitbs (Ionov and Hofmann, 1995 Ionov et al, 1997). MARID ampbibole data from Gregoire et al. (2002). 

In summary, amphibole, along with mica, is the dominant silicate host for niobium in peridotitic xenoliths. In mica-absent assemblages amphibole also dominates the barium and tantalum budgets (Ionov et al, 1997 Eggins et al, 1998) and its presence strongly alfects the bulk rock Zr/Nb ratio. 

One of the first studies to show this was performed on Kilboume Hole spinel Uierzolites (Jagoutz et al, 1980). Equihbrated neodymium isotopes in orthopyroxene and diopside defined essentially zero age isochrons, consistent with the very recent eruption age of the host volcanic rocks, while strontium isotopes were un-equilibrated. Stolz and Davies (1988) found varying degrees of equihbration between amphibole, clinopyroxene and apatite in peridotite xenoliths from S.E. Australia. Several samples contained coexisting amphibole and clinopyroxene and had almost reached isotopic equilibrium for strontium but displayed disequilibrium relations for lead and neodymium isotopes. This was taken to indicate more rapid diffusion of strontium than lead and neodymium. Some peridotite and eclogite... 

Compared with neodymium and strontium, there are relatively few studies of the lead isotopic compositions of mantle xenoliths and the systematics are probably biased towards samples that show some degree of patent metasomatism in the form of introduction of amphibole and/or mica. Much of the data come from noncratonic metasomatized peridotites (e.g., Stolz and Davies, 1988) and cratonic MARID xenoliths. Some type I xenoliths that do not have patent metasomatism, from cratonic and noncratonic settings (Kramers, 1977 Galer and O Nions, 1989 Walker et al., 1989 Lee et al., 1996) together with various... 

Hydrogen isotope data for mantle xenoliths is usually acquired on hydrous minerals such as amphibole and mica. One problem is that early studies did not texturally characterize mica occurrences and so the information is of limited value. Perhaps the best-documented study is that... 

Dawson J. B. and Smith J. V. (1977) The MARID (mica-amphibole-rutile-ilmenite-diopside) suite of xenoliths in kimberlite. Geochim. Cosmochim. Acta 41, 309-323. 

Francis D. (1976) The origin of amphibole in Uierzolite xenoliths from Nunivak Island, Alaska. J. Petrol. 17, 357-78. 

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. 

For ease of presentation, we consider spinel-facies samples first, then the deeper gamet-facies samples. The key points to keep in mind for the purpose of this chapter are (i) amphibole and mica are the products of melt or fluid interaction with the host mantle rocks, and (ii) once introduced, both amphibole and mica are stable at the pres sure-temperature conditions from which the xenoliths were sampled hence they are viable hosts for volatiles at (upper) mantle conditions. 

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). 

The Al-augite series occurs either as discrete xenoliths, or as veins cross-cutting Cr-diopside series peridotite. They are usually interpreted to be high-pressure segregations from hydrous mafic melts. At many locations, the Al-augite series characteristically contains a kaersutitic amphibole and phlogopite (see review by Kempton, 1987). 

Potential insight into the fate of a chlorinebearing fluid came from the study of Andersen et al. (1984) of xenoliths from Bullenmerri and Gnotuk maars in southwestern Australia that contained abundant C02-rich fluid inclusions and vugs up to 1.5 cm in diameter. They found the trapped fluids had reacted with the host minerals to produce secondary carbonates and amphiboles, such that the original composition of the fluid was inferred to be a chlorine- and sulfurbearing CO2-H2O fluid. The evidence for chlorine was the presence of a chlorine peak in the energy-dispersive spectmm of the amphibole unfortunately, no quantitative analyses were possible on these amphiboles. This does pose the possibility that this sort of reaction is common, and that the normal host for chlorine in the mantle is a mineral phase, such as apatite, amphibole, and mica. 

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