Not too long ago in a country not too far away from here I was nothing but an insignificant graduate student at a little known university in Cambridge, Massachusetts. I don't really want to admit it, but it took me almost five years until I got my Ph.D. I even was a G4+ (more than four years of graduate school) for a while. I was an integral part of the chaotic office in the Geological Museum #209 - just beneath the nice gem stone collection. I used to be a member of the Geochemistry group and I was working with Roberta Rudnick, Bill McDonough, Cin-Ty Lee and Debbie Hassler.
Well, somehow I managed to finish my glorious Ph.D. project:
Eclogites, bi-mineralic rocks composed of garnet and omphacite, are minor but ubiquitous constituents in mantle xenolith suites brought up by kimberlites in Archean cratons. They have been variously interpreted as cumulates from mantle-derived magmas or as fragments of recycled oceanic crust. Recently, some eclogites have been interpreted as residues from partial melting of oceanic crust to produce felsic magmas of tonalite-trondhjemite-granodiorite (TTG) composition, which make up large portions of the crust in Archean cratons. If true, this has important implications: it implies that subduction was operative in the Archean and that one reason the continental crust is more evolved than basalt is because Archean crustal growth was accomplished, at least in part, by direct addition of felsic magmas from the mantle.
Although the major and trace element data are suggestive of a link between xenolithic eclogites and TTGs, no systematic study of these rocks from a single region has yet been performed in order to test this hypothesis.
The aim of our research was to
test the hypothetical connection between eclogites in mantle roots and Archean TTGs in the overlying crust,
produce a comprehensive chemical and isotopic database with which to evaluate the origin of eclogite xenoliths,
make comparisons of inclusions in diamonds from eclogites with minerals in the host rock in order to evaluate the importance of mantle metasomatism in producing the observed eclogite compositions and,
place better constraints on the processes of Archean crustal growth and evolution.
This was accomplished through a detailed geochemical study of a suite of well-characterized eclogite xenoliths and TTGs from a single Archean craton, the Man shield in Sierra Leone, west Africa.
We have measured the trace element compositions of garnets, omphacites, and oxides and the oxygen isotopic composition of garnets in xenolithic eclogites from the Mesozoic Koidu kimberlite pipe by laser ablation ICP-MS.
systematic difference between high MgO and low MgO eclogites apparent in trace elements and oxygen isotopes:
1) The oxygen isotopic compositions of garnet are within the mantle range in the high MgO eclogites but extend
beyond this (4.68 to 6.78 o/oo) in the low MgO eclogites.
2) All low MgO eclogites have depleted REE patterns while the high MgO eclogites have enriched to depleted REE patterns.
high MgO are probably olivine gabbros or pyroxenites
low MgO = subducted altered Archean oceanic crust that probably underwent an episode of partial melting during subduction
rutile and ilmenite control the HFSE in eclogites
eclogites = missing Nb reservoir ?
A flip book of a diamondiferious eclogite
Rudnick, R.L., Barth, M., Horn, I., and McDonough, W.F. (2000) Rutile-bearing refractory eclogites: the missing link between continents and depleted mantle. Science, 287, 278-281.
Barth, M.G., McDonough, W.F., and Rudnick, R.L. (2000) Tracking the budget of Nb and Ta in the continental crust. Chem. Geol., 165, 197-213.
Barth M.G., Rudnick R.L., Horn I., McDonough W.F., Spicuzza M.J., Valley J.W., and Haggerty S.E. (2001) Geochemistry of xenolithic eclogites from West Africa, Part I: A link between low MgO eclogites and Archean crust formation. Geochim. Cosmochim. Acta 65, 1499-1527.
Barth M.G., Rudnick R.L., Horn I., McDonough W.F., Spicuzza M.J., Valley J.W., and Haggerty S.E. (2002) Geochemistry of xenolithic eclogites from West Africa, Part II: Origins of the high MgO eclogites. Geochim. Cosmochim. Acta 66, 4325-4345.
Barth M.G., Rudnick R.L., Carlson R.W., Horn I., and McDonough W.F. (2002) Re-Os and U-Pb geochronological constraints on the eclogite-tonalite connection in the Archean Man Shield, West Africa. Precambrian Res. 118, 267-283.
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© 2004 by Matthias Barth.