Eclogite

Barth MG, Foley SF, Horn 1 (2002) Partial melting in Archean subduction zones constraints from experimentally determined trace element partition coefficients between eclogitic minerals and tonahtic melts under upper mantle conditions. Precamb Res 113 323-340... 

Figure 1. Solidus as a function of pressure and temperature for diy peridotite, wet peridotite, pyroxenite and eclogite. This diagram illustrates the range in solidus temperatures potentially found in the intraplate tectonic setting where compositional differences and differences in volatile contents are expected compared with the mid-ocean ridge (dry peridotite).

At shallow pressure (less than 1 GPa), Th becomes slightly more compatible than U in clinopyroxene (Landwehr et al. 2001) and garnet is not stable even in pyroxenite or eclogite. On the other hand, it is believed (mostly based on observations) that Pa remains more incompatible than U through the entire length of the melt column. Thus, melting at intermediate pressure should yield small °Th or U excesses, but significant Pa excess. This has been observed in the lithospheric mantle in the Colorado plateau by... 

Assuming a Rayleigh distillation process and bulk eclogite/fluid D s for 3 GPa without rutile from Table 2... 

Bourdon et al. (2003) with composition of 3 GPa eclogite fluid from Table 3 also plotted. Inset shows interpretation for lava arrays not directed towards the fluid (see text for explanation and discussion). Symbols and data sources as in Figure 1. 

At temperatures germane to melting and erystallizrng mantle magmas, Li isotopes do not show permil-level mass fractionation (Fig. 4 Tomascak et al. 1999b). This has since been corroborated by examination of bulk rocks and olivine separates from basaltic lavas, which yield consonant isotopic values (Chan and Frey 2003). Also, whole rocks and omphacite mineral separates from alpine eclogite with metamorphic peak temperatures approximately 650°C (Zack et al. 2003) show no consistent Li isotopic difference. 

Figure 8. Plot of Li isotopic composition vs. Na O content of eclogites from Trescolmen, Swiss Alps (Zack et al. 2003). Data from pairs of bulk rock ( ) and omphacite mineral separate (O) samples, indicating that bulk rocks generally preserve isotopic systematics of these rocks. Well-preserved, high pressure metamorphosed basaltic rocks were interpreted to retain much of their elemental character, and as such were good recorders of the residue remaining after subduction dehydration. The data indicated that originally isotopically heavy altered sea floor basalt could be transformed during subduction into some of the isotopically lightest materials in the Earth system.

Heinrich CA (1986) Eclogite facies regional metamorphism of hydrous mafic rocks in the Central alpine Adula nappe. J Petrol 27 123-154... 

Rudnick RL, McDonough WF, Tomascak PB, Zack T (2003) Lithium isotopic composition of eclogites implications for subduction zone processes. Eighth Int Kimberlite Conf Abst Ryan JG, Langmuir CH (1987) The systematics of lithium abundances in young volcanic rocks. Geochim Cosmochim Acta 51 1727-1741... 

Zack T, Tomascak PB, Rudnick RL, Dalpe C, McDonough WF (2003) Extremely light Li in orogenic eclogites The role of isotope fractionation during dehydration in subducted oceanic crust. Earth Planet Sci Lett 208 279-290... 

Figure 15. 6 Fe of garnet versus that of coexisting clinopyroxene for eclogites, garnet peridotites, and garnet pyroxenite. Six of the seven samples scatter about a A FCq,j.g, of +0.3%o over a wide range in 6 Fe values measured in the minerals. Error bars are 1-SD of the mean as determined by two or more replicate analyses of the mineral. Modified from Beard and Johnson (2004).

Eclogites (alm-pir), gneisses, metamorphic schists, granites, pegmatites, granulites (mineral index)... 

Basic and ultrabasic rocks (peridotites, serpentinites), kimberlitic xenolites, rodingites, eclogites... 

Formula Nax(Mg,Mn,Fe)7 YAlY(Alx+Y,Si8-x-Y)(OH>F,Cl)2 X + Y > 1.00 Occurrence amphibolites, eclogites, cordierite-bearing and garnet-bearing gneisses... 

Formula Na2(Mg,Fe)3Al2Si8022(0H,F,a)2 Occurrence glaucophanites, eclogites, prasinites... 

Banno S. (1970). Classiheation of eclogites in terms of physical conditions of their origin. Phys. Earth Planet Int, 3 405-421. 

Ghent E. D., Stout M. Z., Black P. M., and Brothers R. N. (1987). Chloritoid bearing rocks associated with blueschists and eclogites, northern New California. J. Met. GeoL, 5 239-254. 

Raheim A. (1976). Petrology of eclogites and surrounding schists from the Lyell Highway-Collingwood River area, W. Tansmania. Geol Soc. Austr. Jour., 23 313-327. 

Raheim A. and Green D. H. (1975). P, T paths of natural eclogites during metamorphism, a record of subduction. Lithos, 8 317-328. 

Ungaretti L., Smith D. C. and Rossi G. (1981). Crystal-chemistry by X-ray strncture refinement and electron microprobe analysis of a series of sodic calcic to alkali-amphiboles from the Nybo eclogite pod, Norway. Bull Mineral, 104 400-412. 

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