The mantle metasomatism: diversity and impact What the mantle xenoliths tell us?

Transcription

1 The mantle metasomatism: diversity and impact What the mantle xenoliths tell us?

2 Mantle metasomatism Physical and chemical processes that are implemented during the flow of magmas and / or fluids within the upper mantle. The main changes affecting the mantle wall rocks (peridotites: lherzolites, harzburgites and dunites): changes in texture, recrystallization, formations of new minerals and chemical exchanges leading to the enrichment (rarely depletion) of mantle peridotites in incompatible trace elements (REE, HFSE and LILE: large-ion lithophile element). Flow: general meaning (both pervasive at grain boundaries and focused in dykes or veins)

3 Spinel harzburgite xenolith in basalt (Lihir Island, Papua New Guinea) 12x8 cm METASOMATISM OF THE PHANEROZOIC MANTLE MOSTLY SPINEL PERIDOTITES: Ol-Opx-Cpx-Sp

4 Metasomatism of the upper mantle in asthenospheric upwelling zones (Plumes, rifting zones, Asthenospheric windows )

5 cpx phl sp am ol opx Mineralogical changes Dunite wall rock sp Amp (phl-ilm) vein ol phl

6 Spinel peridotites Cpx: asthenospheric upwelling contexts 2 Clinopyroxenes (Oman, Kerguelen, Cameroun, Patagonie) Cr 2 O 3 1,6 1,2 0,8 0,4 Na 2 O 0 0,5 1 1,5 2 2,5 3 Chemical changes

7 sample/standard sample/standard sample/standard Cpx: asthenospheric upwelling contexts 100 Kerguelen clinopyroxenes Cameroon clinopyroxenes 0,1 10 0,01 Rb Ba Th U Nb Ta La Ce Sr Nd Zr Hf Sm Eu Ti Gd Dy Ho Er Yb Lu ,1 Oman clinopyroxenes 100 0,01 Rb Ba Th U Nb Ta La Ce Sr Nd Zr Hf Sm Eu Ti Gd Dy Ho Er Yb Y Lu 10 Chemical changes 1 0,1 0,01 Rb Ba Th U Nb Ta La Ce Sr Nd Zr Hf Sm Eu Ti Gd Dy Ho Er Yb Y Lu

8 Clinopyroxenes Poikilic harzburgites vs Kerguelen alkaline magmas Poikilitic harzburgites Pyroxenites: Alkaline mantle cumulates Megacrysts in Alkaline lavas

9 Metasomatism in zones of plate convergence (dehydration and sometimes melting of slabs: effects on the mantle wedge)

10 ol oxydes opx laser Mineralogical changes ol opx cpx

11 Al (cations) Lihir : Mantle wedge Ca (cations) Chemical changes

12 sample/standa sample/standa Pyroxenes from veins: 10 1 Orthopyroxene Monglo Lihir : Mantle wedge 0,1 Lihir Chemical changes 0,01 10 Rb Ba Th U Nb La Ce Sr Nd Zr Sm Eu Ti Gd Dy Ho Er Yb Lu Lihir clinopyroxenes in veins 1 0,1 0,01 Rb Ba Th U Nb Ta La Ce Sr Nd Zr Hf Sm Eu Ti Gd Dy Ho Er Yb Lu

13 Chemical changes Lihir : Mantle wedge Pyroxenes from peridotitic wall rocks

14 Main metasomatic agents Context of asthenospheric uplift: mostly intraplaque settings Mafic alkaline and high alkaline silicated melts (more or less rich in CO 2 - carbonatitic, Fe-Ti basaltic, tholeiitic, ultramafic melts ) Context of subduction: Hydrated Liquids (fluids) commonly SiO 2 rich Related to the deshydratation process of the slab (adakites, basalts )

15 MANTLE WEDGE ZONES Mineralogy : Opx-Am-Cpx-Ol-Phl-Sp-Su-Mt Geochemistry : Majors Opx2 and cpx2 poorer in Al than cpx1 et opx1 Phlogopite low in Ti Amphibole mostly less sodic at a fixed mg# Traces U/Th high Amphibole and phlogopite Low in Nb and Ta ASTHENOSPHERIC UPWELLING ZONES Mineralogy : Cpx-Phl-Am KNaFs-Pl-Ol-Sp-Ru-Ap-Ilm-Arm Cb-Gl-Su Geochemistry : Majors Cpx: Commonly enrichment in Cr and Na, sometimes only Na or Na and Ti Traces U/Th variable (commonly U/Th<1) Amphibole and phlogopite are Nb and Ta-rich

16 Precambrian mantle with a focus on the Archean mantle from South Africa

17 Mostly garnet peridotites in kimberlites MARID Coarse grained Garnet lherzolites PIC High-T Sheared (near the LAB)

18 The melts responsible for the modal metasomatism of the group 1 and group 2 garnet lherzolites have similarities with the melts responsible for the formation of the PIC and MARID rocks, respectively. These melts are high alkaline mafic silicate melts. A genetic link between PIC rocks and Group I kimberlites and between MARID rocks and Group II kimberlites. Genetic relationships between the two mantle metasomatic agents affecting garnet lherzolites from the Kaapvaal Craton and the two groups of kimberlite magmas. Two types of garnet lherzolites have been distinguished Important modifications of the cratonic upper mantle related to the circulation of kimberlitic melts Coarse grained (relatively shallow) and high T- Sheared (deep, LAB) garnet lherzolites: same metasomatic history

19 100Mg/(Mg+Fe) Kaapvaal Craton: ancient metasomatism ol Melting with fluid Felsic melt or fluid addition opx Off craton xenoliths Abyssal Kaapvaal low-t Kimberley low-t Wiedemannfjord Norway WGR Izu Bonin Forearc Iwanai-dake BFT olivine-rich 88 melting (Mg+Fe)/Si - atomic

20 Opx-rich vein

21 To summarize in the archean mantle beneath South Africa there is also two main types of metasomatic agents: - high alkaline melts (kimberlites, orangeites and associated fluids (CO2, H2O) and differentiates) mostly linked to asthenospheric upwellings - Intermediate to Si-rich silicate melts and associated fluids and differentiates probably mostly linked to suprasubduction zones

22 Conclusions from mantle xenoliths studies: Even if there is a lot of different kind of chemical and mineral changes associated to mantle metasomatism we may propose two main large types of mantle metasomatism both for old (Archean) and young mantle: Asthenospheric upwellings-related metasomatism: mostly linked to the associated magmatic events leading at the end to the uplift of the xenoliths such as by alkaline and kimberlitic lavas (orangeites, lamproites) Subduction-related metasomatism: It could be see directly without any later imprints of the previous type if xenoliths were uplifted by calc-alkaline lavas(but rare ex: Kamchatka) More often you have to be lucky and find the good samples within a lot of samples showing evidences of the first type if your collection was uplifted by alkaline and kimberlites.

23 The processes of mantle enrichment by metasomatism could have commonly affected the upper mantle and more than one time through the upper mantle history making difficult to decipher that history (even more complicate if you add mantle depletion processes such as PM). They imply changes in the petrological, mineralogical and chemical characteristics of the upper mantle and therefore imply changes in the physical properties of this upper mantle (density, porosity, anisotropy ). Finally when pieces of upper mantle are recycled (delamination processes) within the convective mantle they imply changes in the composition of this convective mantle

24 DAVE BELL PREDICTED CONSEQUENCES OF MANTLE UPWELLING BENEATH CONTINENTAL LITHOSPHERE Hot mantle emplaced Transient thermal pulse Transient density decrease Transient uplift Thermal erosion of lithosphere Melting Melt migration Melt-rock reaction Plume chemical anomaly Elevation of geotherm Radiogenic heat Metasomatism of lithosphere Permanent Density decrease Permanent Density increase Permanent Uplift Delami nation? Subsidence

25 THANKS FOR YOUR ATTENTION

26 Mantle metasomatism could lead to mantle depletion and not only to mantle enrichment Spinel and plagioclase harzburgite Spinel harzburgite Websterite vein Spinel harzburgite Mantle section of the Trinity Ophiolite (California) REE vs distance from the vein Rarely melt/fluid circulation could result in an impoverishment in incompatible TE Here circulation of Subduction-related «boninitic like melts»

27 sample/standard 10 Peridotitic Wall rocks 1 Pyroxenitic veins 0,1 0,01 La Ce Pr Nd Sm Eu Gd Dy Ho Er Yb Lu

28 Transformation of peridotites by an influx of sub-alkaline magmas forming websteritic channels where the volume of magma was the higher Bodinier et al. (2008)

29 Circulation of subalkaline magmas within the Patagonian upper mantle Dantas et al., 2009