The mantle metasomatism: diversity and impact

What the mantle xenoliths tell us?

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)

12x8 cm

Spinel harzburgite xenolith

in basalt (Lihir Island, Papua New Guinea)

METASOMATISM OF THE

PHANEROZOIC MANTLE

MOSTLY SPINEL PERIDOTITES: Ol-Opx-Cpx-Sp

Metasomatism of the upper mantle in asthenospheric upwelling zones

(Plumes, rifting zones, Asthenospheric windows…)

cpx

sp

ol

sp

phl

ol

am

phl

opx

Dunite

wall rock

Amp (phl-ilm)

vein

Mineralogical changes

0,4

0,8

1,2

1,6

2

0 0,5 1 1,5 2 2,5 3

Cr2O3

Na2O

Clinopyroxenes (Oman, Kerguelen, Cameroun, Patagonie)

Cpx: asthenospheric upwelling contexts

Spinel peridotites

Chemical changes

0,01

0,1

1

10

100

Rb Ba Th U Nb Ta La Ce Sr Nd Zr Hf Sm Eu Ti Gd Dy Ho Er Yb Lu

sa

mp

le/s

tan

da

rd

Kerguelen

clinopyroxenes

0,01

0,1

1

10

100

Rb Ba Th U Nb Ta La Ce Sr Nd Zr Hf Sm Eu Ti Gd Dy Ho Er Yb Y Lu

sa

mp

le/s

tan

da

rd

Cameroon

clinopyroxenes

0,01

0,1

1

10

100

1000

Rb Ba Th U Nb Ta La Ce Sr Nd Zr Hf Sm Eu Ti Gd Dy Ho Er Yb Y Lu

sa

mp

le/s

tan

da

rd

Oman clinopyroxenes

Cpx: asthenospheric upwelling contexts

Chemical changes

Clinopyroxenes

Poikilic harzburgites vs

Kerguelen alkaline magmas

Poikilitic harzburgites

Pyroxenites:

Alkaline mantle cumulates

Megacrysts in

Alkaline lavas

• Metasomatism in zones of plate convergence

(dehydration and sometimes melting of slabs:

effects on the mantle wedge)

ol

opx

cpx

laser

ol

opx

oxydes

Mineralogical changes

Ca (cations)

Al

(cat

ion

s)

Chemical changes

Lihir : Mantle wedge

0,01

0,1

1

10

Rb Ba Th U Nb La Ce Sr Nd Zr Sm Eu Ti Gd Dy Ho Er Yb Lu

sam

ple

/sta

nd

ard

Lihir

Monglo

Orthopyroxene

0,01

0,1

1

10

Rb Ba Th U Nb Ta La Ce Sr Nd Zr Hf Sm Eu Ti Gd Dy Ho Er Yb Lu

sa

mp

le/s

tan

da

rd

Lihir

clinopyroxenes in veins

Pyroxenes from veins:

Chemical changes

Lihir : Mantle wedge

Lihir : Mantle wedge

Pyroxenes from peridotitic wall rocks

Chemical changes

Main metasomatic agents

• Context of asthenospheric uplift: mostly intraplaque settings

Mafic alkaline and high alkaline silicated melts (more or less rich in CO2 - carbonatitic, Fe-Ti basaltic, tholeiitic, ultramafic melts…)

• Context of subduction:

Hydrated Liquids (fluids) commonly SiO2 –rich

Related to the deshydratation process of the slab (adakites, basalts…)

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

Precambrian mantle with a focus on the Archean mantle from

South Africa

Garnet lherzolites

MARID

PIC High-T Sheared (near the LAB)

Coarse

grained

Mostly garnet peridotites in kimberlites

Two types of garnet

lherzolites have been

distinguished

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.

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

Kaapvaal Craton: ancient metasomatism

84

86

88

90

92

94

96

98

0.60.81.01.21.41.61.82.02.2

(Mg+Fe)/Si - atomic

100M

g/(

Mg+

Fe)

Off craton xenoliths

Abyssal

Kaapvaal low-T

Kimberley low-T

Wiedemannfjord

Norway WGR

Izu Bonin Forearc

Iwanai-dake

BFT olivine-rich

ol opx

melting

Felsic melt

or fluid addition

Melting with fluid

Opx-rich vein

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

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.

• 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

PREDICTED CONSEQUENCES OF MANTLE UPWELLING

BENEATH CONTINENTAL LITHOSPHERE

DAVE BELL

Transient

uplift

Hot mantle

emplaced

Thermal erosion

of lithosphere

Elevation of

geotherm

Transient

thermal pulse

Transient

density

decrease

Permanent

Density

decrease

Permanent

Uplift

Melting

Melt migration

Melt-rock reaction

Metasomatism

of lithosphere

Plume

chemical anomaly

Permanent

Density

increase

Subsidence

Delami nation?

Radiogenic

heat

THANKS FOR YOUR ATTENTION

Spinel

harzburgite Spinel

harzburgite

Spinel and plagioclase

harzburgite

Websterite

vein

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 »

Mantle metasomatism could lead to mantle depletion and not only to mantle enrichment

0,01

0,1

1

10

La Ce Pr Nd Sm Eu Gd Dy Ho Er Yb Lu

sa

mp

le/s

tan

da

rd

Pyroxenitic

veins

Peridotitic

Wall rocks

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)

Circulation of subalkaline magmas within the Patagonian upper mantle

Dantas et al., 2009