The mantle metasomatism: diversity and impact What the mantle xenoliths tell us?
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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<1)
Amphibole and phlogopite
are Nb and Ta-rich
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
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