Megacrystals

An alternative description of iUite—smectite mixed-layer clays begins with megacrystals of smectite that incorporate smaller packets of iUite (163). These constituents are observed as mixed-layer minerals in x-ray analysis. Diagenesis increases the percentage of iUite layer and with increasing alteration the mixed-layer mineral takes on the characteristics of an iUite dominated iUite—smectite. 

Irving, A. J. Frey, F. A. (1984). Trace-element abundances in megacrysts and their host basalts Constraints on partition coefficients and megacryst genesis. Geochim. Cosmochim. Acta, 48, 1201-21. 

Sulfur occurs in a variety of forms in the mantle, the major sulfur phase is monosulfide solid solution between Fe, Ni, and Cu. Recent ion microprobe measurements on sulfide inclusions from megacrysts and pyroxenite xenohths from alkali basalts and kimberlites and in diamonds gave 5 " S-values from — 11 to - - 14%c (Chaussidon et al. 1987, 1989 Eldridge et al. 1991). Sulfur isotope variations within diamonds exhibit the same characteristics as previously described for carbon i.e., eclogitic diamonds are much more variable than peridotitic diamonds. 

The largest intrusion is represented by the Monte Capanne monzogran-itic stock (about 10 km in diameter), located in the western side of the island. It exhibit a porphyritic texture with centimetre- to decimetre-sized euhedral megacrysts of K-feldspar that are set in a medium- to coarsegrained matrix formed by variable amounts of plagioclase, quartz, K-feldspar and biotite with accessory apatite, zircon, monazite, ilmenite, and tourmaline. The intrusion contains abundant microgranular calc-alkaline... 

Lee D.-C. and Halliday A. N. (2000b) Hf-W isotopic systematics of ordinary chondrites and the initial i82Hf/i80Hf of the solar system. Chem. Geol. 169, 35-43. Lee D.-C., Halliday A. N., Davies G. R., Essene E. L, Eitton J. G., and Temdjim R. (1996) Melt enrichment of shallow depleted mantle a detailed petrological, trace element and isotopic study of mantle derived xenohths and megacrysts from the Cameroon line. J. Petrol. 37, 415-441. 

Cr-poor variety widespread, locally abundant (e.g.. Monastery). Garnets, clino- and orthopyroxenes, phlogopite and ihnenite most common, zircon and olivine rarer. Debatable whether phlogopite and olivine are members of Cr-poor suite. Wide range in chemistry but Cr-poor, Fe-Ti-rich relative to type I (low-Z) peridotite minerals. Mineral chemistry and estimated equilibration P/Ts overlap those of type V (high-Z) Iherzolites. Some Slave craton Cr-poor megacrysts show mineral chemistry links to type II megacrystalline pyroxenite xenoliths. See review of Schulze (1987). 

Miscellaneous mostly garnets and pyroxenes with no clear paragenetic association or links to other megacryst suites. May represent disrupted peridotites/eclogites/pyroxenites. 

Polymict aggregates of peridotite, eclogite and megacrysts, of variable grain size, some containing quenched melt. Mineral assemblages not in elemental or isotopic equilibrium. 

N. Lesotho (Nixon and Boyd 1973b) Monastery (Gurney et al, 1979), Jagersfontein (Hops et al, 1992), Kaapvaal craton The Malaita megacryst suite (Nixon and Boyd, 1979), occurs in an ocean plateau alnoite, but has many similarities with the kimberhtic low-Cr suite... 

Wo < 35%) occur in cratonic suites as megacrysts or discrete nodules, and indicate very high T of equilibration, perhaps in equilibrium with melt (Boyd, 1969, 1970 Boyd and Nixon, 1978). 

The systematics of sulfur isotopes in mantle xenoliths have been reviewed by Kyser (1990). Most sulfide data have so far been obtained by in situ analyses using SIMS or laser probe and this is less prone to alteration effects than whole-rock analyses. Chaussidon et al. (1989) found that considerable sulfur isotope variation exits in mantle minerals (S S —5 to 8 per mil), which they attributed to fractionation between residual sulfide and the melt during melting. However, it is megacrysts that show most variation in their data set, possibly due to magmatic processes while sulfide from the garnet peridotites has a much more restricted range, of between —1 per mil and - -4 per mil, typical of mantle values. Wilson et al. (1996) found elevated in peridotites from Dish Hill, which they proposed was due to metasomatic introduction of subducted cmstal sulfur. 

Boyd F. R., Dawson J. B., and Smith J. V. (1984a) Granny Smith diopside megacrysts from the kimberlites of the Kimberley area and Jagersfontein, South Africa. Geochim. Cosmochim. Acta 48, 381-384. 

Gurney J. J., Jacob W. R. O., and Dawson J. B. (1979) Megacrysts from the Monastery kimberlite pipe, South Africa. In Kimberlites, Diatremes and Diamonds Their Geology, Petrology and Geochemistry (eds. F. R. Boyd and H. O. A. Meyer). American Geophysical Union, Washington, DC, vol. 1, pp. 222-243. 

Hops J., Gurney J. J., and Harte B. (1992) The Jagersfontein Cr-poor megacryst suite-towards a model for megacryst petrogenesis. J. Volcanol. Geotherm. Res. 50, 143-160. 

Moore R. O., Griffin W. L., Gurney J. J., Ryan C. G., Cousens D. R., Shee S. H., and Suter G. F. (1992) Trace element geochemistry of ilmenite megacrysts from the Monastery kimberlite. South Africa. Lithos 29, 1-16. 

Schulze D. J. (1987) Megacrysts from alkahc volcanic rocks. In Mantle Xenoliths (ed. P. H. Nixon). Wiley, Chichester, pp. 433-451. 

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