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?
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
Zr/Sm
Garnetamphibolite
Foley, Tiepolo & Vannucci (2002) Nature 417, 837-840
Explanation of Nb/Ta in rutile
Ta Nb
Optimal ionic radius 58 pm
Rutile must have DNb / DTa approx 0.6-0.7
Results in melts with high Nb/Ta
D Ru/Lq
Ionic radius
Calcic amphibolesOptimal ionic radius 62 - 66 pmHighest where low Mg# and high DTi
Nb = 64 pmTa = 0.625 pm
Mantle eclogite xenoliths
Mantle eclogite xenoliths:
the best samples of Archaean oceanic crust?
D. E. Jacob
1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4
18O (‰)
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
Garnet
Cpx
Reconstructed Bulk
0.01
0.1
1
10
100
La Ce Nd Sm Eu Gd Tb Dy Ho Er LuPr Tm Yb
sample/chondrite
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)
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 crustWhere 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, 249-252
Near-surface origin of cratonic peridotites as depleted uppermost mantle
1. Eclogites of near-surface origin entrained2. Subcalcic garnets in garnet harzburgites require Cr-rich harzburgite - formed at MOR3. Mounting textural evidence for re-enrichment4. Trace elements show harzburgitic precursors