Trace elements and the formation of early crust (1) Trace elements: what can we learn from them? (2) TTG do not have the “subduction signature” (whatever that is) (3) Mineral assemblages, not tectonic settings, control trace elements (4) Igneous and metamorphic trace element partitioning patterns differ A few collected, but not necessarily connected, thoughts: (5) Eclogite xenoliths: what were they? (6) What happens at subduction if the ocean crust is not basalt? (7) Ultradepleted early Archaean uppermost mantle?

Subduction setting for Archaean TTG gneisses Tarney (1976)

Cont crust TE Low Ti, Nb, Ta is well known to be typical of subduction zone volcanics…. But we have to look closer

TTG are melts of garnet amphibolite Garnet amphibolite Foley, Tiepolo & Vannucci (2002) Nature 417,

Explanation of Nb/Ta in rutile Ta Nb Optimal ionic radius 58 pm Rutile must have D Nb / D Ta approx Results in melts with high Nb/Ta D Ru/Lq Ionic radius Calcic amphiboles Optimal ionic radius pm Highest where low Mg# and high D Ti Nb = 64 pm Ta = pm

Mantle eclogite xenoliths Mantle eclogite xenoliths: the best samples of Archaean oceanic crust? D. E. Jacob

 18 O (‰) Macquarie Island Ophiolite Samail Ophiolite Koidu (Sierra Leone, ca. 3.4 Ga) Udachnaya (Yakutia, ca. 2.7 Ga) Mir (Yakutia) Orapa (Botswana) Bellsbank (South Africa) Roberts Victor (South Africa, ca. 2.7Ga) Kuruman (South Africa) Rietfontein (South Africa) Blaauwbosch (South Africa) Newlands (South Africa) Frank Smith (South Africa) Sloan (USA) Grib (Arkhangelsk) Ekati (Canada) Oxygen isotopes D.E.Jacob

Metamorphic trace element partitioning in eclogites Eclogites melted as garnet amphibolites, then transformed to eclogites in the subsolidus (high Nb/Ta rutiles must be metamorphic) D.E. Jacob (2004)

Growth curves and eclogites Eclogites and continental crustal growth

Growth curves and eclogites Continental crustal growth and survival early formation Crustal recycling

Melting of basalt (MORB) Melting curves and stabilities of minerals well defined by experiments Basaltic crust can melt as garnet amphibolite on Geotherm 2 Early Archaean (pre-3600 Ma?) If much hotter conditions: Widespread melting of amphibolite in early Archean should have produced enormous quantities of silicic crust Where is it? If Geotherm 1 applied, then trace elements of melts would not resemble TTG

Melting of picritic- ultramafic crust Picritic to ultramafic parts of the crust will not form amphibolites during metamorphism, but varieties of pyroxenite instead. If pyroxenites melt, they form nephelinitic to basaltic melts, not intermediate to silicic melts like TTG: Production of silicic crust is stalled Foley, Buhre & Jacob (2003): Nature 421,

Formation of TTG Foley, Buhre & Jacob (2003) Nature 421, Supposing subduction:

Near-surface origin of cratonic peridotites as depleted uppermost mantle 1. Eclogites of near-surface origin entrained 2. Subcalcic garnets in garnet harzburgites require Cr-rich harzburgite - formed at MOR 3. Mounting textural evidence for re-enrichment 4. Trace elements show harzburgitic precursors

Philosophy for the Archaean “Where nobody knows anything, there is no point in changing your mind.” Bertrand Russell (1952) “When a man tells you he knows the exact truth about anything, you are safe in inferring that he is an inexact man.” Bertrand Russell (1931)