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

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)

Growth curves and eclogitesEclogites and continental crustal growth

Growth curves and eclogitesContinental crustal growth and survival

early formation

Crustalrecycling

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

Formation of TTG

Foley, Buhre & Jacob (2003) Nature 421, 249-252

Supposing subduction:

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

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)