Xenoliths carbonates

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... 

Moecher DP, Valley JW, Essene EJ, (1994) Exhaction and carbon isotope analysis of COj from scapolite in deep crustal granulites and xenoliths. Geochim Cosmochim Acta 58 959-967 Mojzsis SJ, Harrison TM, Pidgeon RT (2001) Oxygen-isotope evidence from ancient zircons for liquid water at the Earth s surface 4,300 Myr ago. Nature 409 178-181 Muehlenbachs K, Clayton RN (1976) Oxygen isotope composition of the oceanic crust and its bearing on seawater. J Geophys Res 81 4365-4369... 

Figure 10 shows the percent total carbon and percent volatile matter plotted vs. the distance across the xenolith. The total carbon (daf), ranging from 85 to 93%, increases toward the center of the xenolith whereas the volatile matter, ranging from 6.5 to 16%, decreases. This may be explained by the movement of volatiles outward to the top and bottom of the xenolith. 

Figure 10. Carbon and volatile matter distribution across xenolith of natural coke...

Heating progressing inward may have coked the outer edges prior to the center and the volatiles from the central portion were condensed or trapped by the outer coke structure. Thin sections show that translucent material is present in some vesicles and microfissures of the outer zones of coke. This also explains the increase in reflectance as being real in this case even though coupled with an increase in volatile matter. Figure 11 shows carbon and ash plotted vs. the distance across the xenolith. As expected, the ash decreases as the distance from the contacts increases. The ash content is extremely high, from 18.6 to 37.6%. 

This eliminated enough of the carbon to get readable patterns. The major minerals determined were quartz, calcite, kaolinite, and chlorite. The most obvious and abundant mineral, quartz, decreased in relative amounts toward the center of the xenolith, and this trend was apparent in all the other minerals. Diffraction patterns of the ash from the whole coke, in general, showed the same mineral decrease trend except, of course, no calcite at all was detectable. Calcination or emission of carbon dioxide from calcite occurs at 898°C. (7), significantly below the ashing temperature of 950°C. 

Several ejected blocks are found in the Somma-Vesuvio pyroclastics. These include lavas, sedimentary carbonate rocks, skams, and mafic and ultramafic xenoliths (e.g. Joron et al. 1987). The latter have been suggested... 

Trull, T., Nadeau, S., Pineau, F., Polve, M., Javoy, M. (1993) C-He systematics in hotspot xenoliths Implications for mantle carbon contents and carbon recycling. Earth Planet. Sci. Lett., 118, 43-64. 

Figure 27 Covariation of Rb/Sr versus Sr for mantle xenolith minerals. Primitive mantle (PUM) line for Rb/Sr marked. Data sources as in Figure 23 (Carb. = carbonate).

Figure 30 Primitive mantle normalized multielement patterns for carbonates from mantle xenoliths, modified from Ionov and Harmer (2002). Data shown as points are from Mongolia peridotite xenoliths, compared to carbonates in xenoliths from Kerguelen, Tanzania, and Patagonia. See Ionov (1998) and Ionov and Harmer (2002) for data sources.

Canil D. (1990) Experimental study bearing on the absence of carbonate in mantle-derived xenoliths. Geology 18, 1011-1013. 

Ionov D. (1998) Trace element composition of mantle-derived carbonates and coexisting phases in peridotite xenoliths from alkali basalts. J. Petrol. 39, 1931 — 1941. 

Ionov D. A. and Harmer R. E. (2002) Trace element distribution in calcite-dolomite carbonatites from Spitskop inferences for differentiation of carbonatite magmas and the origin of carbonates in mantle xenoliths. Earth Planet. Sci. Lett. 198, 495-510. 

Ionov D. A., Dupuy C., O Reilly S., Kopylova M. G., and Genshaft Y. S. (1993b) Carbonated peridotite xenoliths from Spitsbergen implications for trace element signature of mantle carbonate metasomatism. Earth. Planet. Sci. Lett. 119, 283 -297. 

Lee C. T., Rudnick R. L., McDonough W. F., and Horn I. (2000b) Petrologic and geochemical investigation of carbonates in peridotite xenoliths from northeastern Tanzania. Contrib. Mineral. Petrol. 139, 470-484. 

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. 

The presence of diamond and graphite in mantle-derived samples such as kimberlites and the xenoliths they host is prima facie evidence that neutral carbon is stable in the Earth s mantle. Outstanding questions remain, however, concerning the stability of neutral carbon in areas other than those beneath continental cratons, and concerning the mechanism by which diamond forms. To a large extent, the latter question revolves around the unresolved problem of the oxidation state of the mantle, and how—and if— the oxidation state is controlled. 

Carbonate is rarely found in mantle-derived xenoliths. It has been found in mantle-derived garnets (McGetchin and Besangon, 1975 Smith, 1987), clinopyroxenes (Hervig and Smith, 1981) and in rare xenoliths (e.g., Ionov et al., 1993a, 1996 Lee et al., 2000 Laurora et al., 2001). 

Ionov et al. (1993a, 1996) found carbonate in spinel Iherzolite xenoliths as interstitial crystals and as aggregates with calcium-rich olivine and aluminum- and titanium-rich clinopyroxene. They interpreted the former to be primary and the latter as evidence for metasomatism by a carbonate-rich melt. Subsequently, Ionov (1998) measured trace-element abundances in the carbonates and coexisting phases, and proposed the aggregate carbonates were formed by crystal fractionation from a carbonate melt. That these carbonates represent crystallized cumulates,... 

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. 

Dautria J. M., Dupuy C., Takherist D., and Dostal J. (1992) Carbonate metasomatism in the lithospheric mantle the peridotitic xenoliths from a melilititic district of the Sahara Basin. Contrib. Mineral. Petrol. Ill, 37-52. 

Eaurora A., MazzuccheUi M., Rivalenti G., Vannucci R., Zanetti A., Barbieri M. A., and Cingolani C. A. (2001) Metasomatism and melting in carbonated peridotite xenoliths from the mantle wedge the Gobemador Gregores case (southern Patagonia). J. Petrol. 42, 69—87. 

Schiano P., Clocchiatti R., Shimizu N., Weis D., and Mattielh N. (1994) Cogenetic sihca-rich and carbonate-rich melts trapped in mantle minerals in Kerguelen ultramafic xenoliths implications for metasomatism in the oceanic upper mantle. Earth Planet. Sci. Lett. 123, 167—178. 

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