Depletion and mineral modes Mineral modes of peridotite are 48% Oli - - PDF document

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Lecture III: Mantle garnets as lithosphere proxies – a major-element and temporal perspective Herman Grütter UBC Lectures - Mantle Indicators in modern use 30 October 2013

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Depletion and mineral modes

Mineral modes of peridotite are related to extent of melt extraction (% broadly basaltic melt removed)

48% Oli 32% Opx 18% Cpx 2% Spl

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Peridotite solidus and degree of melting

Variation of assemblage and melt fraction at 15 kbar for peridotite PHN-1611 Kushiro (1994) J Geol Soc Japan 100: 103-110

Cpx-Out ~20% melt Ts +75°C Opx-Out ~40% melt Ts +190°C Ts 1235°C LHERZ HARZ DUN Ol-Out 100% melt 1890 °C Ts +655°C

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Mantle peridotite whole rock data Major-element depletion trend

20% 30% 40%

Depletion => basaltic melt extracted Residual depleted mantle has

• Higher Mg/Fe
• Low Al/Cr
• Low Ca
• Low Ti

% depletion almost linear

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Why choose garnet ?

Garnet is the perfect proxy for mantle rocks

• Occurs in a very wide range of mantle rock types (PER, PRX, ECL, MEG)
• Wide P-T stability, including across graphite / diamond
• Can retrieve major-element data and T (±P) at low unit cost

Garnet Formula: X3Y2Si3O12 X: Mg/(Mg+Fe) ⇒ ⇒ ⇒ ⇒ reflects Olivine or WR Mg# X: Ca/(Ca+Mg+Fe) ⇒ ⇒ ⇒ ⇒ reflects Opx and/or Cpx presence X: Minor Mn, Ni ⇒ ⇒ ⇒ ⇒ mantle thermometry Y: Cr/(Cr+Al) ⇒ ⇒ ⇒ ⇒ reflects Chromite or WR Cr#

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4 8 12 16 3 6 9

CaO Cr2O3

Near-fertile mantle (0 to 5% melting)

Alkali basalt sources Thin, hot lithosphere Phanerozoic No diamonds

Pali-Aike, Antarctic, Vitim, China, Mongolia

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Marginal kimberlites < 150 km lithosphere Mid Proterozoic No diamonds

5 – 12% melt extracted (weak depletion)

4 8 12 16 3 6 9

CaO Cr2O3

Gibeon Also in Proterozoic Angola-Brazil, Kundelungu

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5 – 20% melt extraction (Cpx still present)

4 8 12 16 3 6 9

CaO Cr2O3

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Kimberlites Up to 220 km lithosphere Late Archean / Early Proterozoic L-dominant Some diamonds – mostly E ?

Sloan-Nix Also in Finland, Prairie Creek, Majghawan, Kundelungu

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5 – 30% melt extraction (“Cpx-Out” at 20% melting)

4 8 12 16 3 6 9

CaO Cr2O3

Kimberlites Up to 200 km lithosphere Archean H significant P ± E diamonds

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Shandong, China Also in Botswana, Kuruman, Mbuji Mayi, Mali, South Slave

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5 – 50% melt extraction (“Opx-Out” at 40% melting ?)

4 8 12 16 3 6 9

CaO Cr2O3

Kimberlites Up to 200 km lithosphere Early – Mid Archean H strong, D present P ± E diamonds

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Grütter, H. S., Apter D. B. and Kong J. (1999) Crust-Mantle coupling: Evidence from mantle- derived xenocrystic garnets. Proc. Seventh International Kimberlite Conference Vol 1, 307-313.

Aikhal + Udachnaya, with added eclogite Also in West KV, Venetia, central Slave

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Extent of depletion through time

20% 30% 40%

Depleted residues Highest MgO (komatiite) melts

Francis (2003) Lithos 71: 135-152 Slide 12

Continents constant Growing continents 150K cooling since 3 Ga 240K cooling since 3 Ga

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Interpret the unknowns…….

Sources (ABAS, KIM etc) Lithosphere depth Extent of depletion Possible lithosphere age Diamond associations

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Barometry and layering of depleted mantle……

Grütter, H., Latti, D. and Menzies, A. (2006) Cr-saturation arrays in concentrate garnet compositions from kimberlite and their use in mantle barometry. Journal of Petrology 47: 801-820.

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Whole-rock Mg/(Mg+Fe) during melt extraction

Garnet and Olivine in Kaapvaal & Lesotho peridotite xenoliths (n = 888)

Eaton et al (2009) The elusive lithosphere-asthenosphere boundary (LAB) beneath cratons. Lithos 109:1-22. 86 88 90 92 94 96 0.0 1.0 2.0 GAR TiO2 (wt%) FO OLIVINE 86 88 90 92 94 96 1 2 3 4 5 6 GAR CA_INT (wt%) FO OLIVINE G10 G9 86 88 90 92 94 96 0.0 1.0 2.0 GAR TiO2 (wt%) FO OLIVINE 86 88 90 92 94 96 1 2 3 4 5 6 GAR CA_INT (wt%) FO OLIVINE G10 G9

+ Ti, Fe & Ca melt-related metasomatism

Questions ?