1. Carbonates in mantle xenoliths from the French Massif Central:
inference for carbonatite-related metasomatism
Christiane WAGNER1
Etienne DELOULE2
1 Sorbonne Universités, UPMC, Univ Paris 06, CNRS, ISTeP, F-75005, Paris, France.
2 CRPG, UMR-CNRS 5873 – Univ de Lorraine, F Vandoeuvre-lès-Nancy, France
Vienne – 2016
The interpretation of carbonate - and particularly - carbonate “globules” in mantle xenoliths has been much debated since the 80’/90’s in relation to the origin of carbonate melt and mantle metasomatism.
In regard to this debate, we discuss here two representative types of carbonated lherzolites from Mont Coupet (Devès, French Massif Central). Both samples show no infiltration of the host magma. Sample MC2 locally shows traces of late oxidation.
Mt Coupet Cc
Carbonate occurrences
Veins and Vesicles in reaction zones
BSE image
pg2
+ gl ol
spongy
cpx
OPX 1
Reaction zones with calcite (MgO <1 wt%) vesicles and Al- and Ti-rich cpx2, Ca-rich ol2, Cr-rich sp2, plagio2 and glass.
Sometimes the CC veins cut former spongy cpx.
Pockets
Sample MC9
Sample MC2
Less developed reaction zones commonly located around resorbed sp grains.
ol2
SP 1
pg2 + gl
sp2
CPX 1
Mg-rich fibrous Cc
Mg-poor Cc
Mg-poor and
Mg-rich Cc
The carbonate vesicles differentiated into Mg-rich (3.3-5 wt% MgO) and Mg-poor ( wt% MgO) calcite due to pressure decrease during the magmatic ascent and eruption.
Carbonate composition
In both samples the calcite have low REE abundances. In sample MC9, the REE abundances are mostly below the detection limit except for La and Ce.
The calcite (Mg-poor) from MC9 contains higher P2O5 and lower Sr and Ba than the two calcite types from MC2.
CPX composition
core
rim
cpx 2
MC2
MC9
reaction OPX 1 in contact with host bas
spongy cpx
host basalt
- cpx2 show variable and higher Al and Ti than CPX 1.
They differ from cpx in the host, or in reaction at OPX 1 / host contact, or spongy cpx.
- They equilibrated at higher pressure than igneous cpx
- They are thus unrelated to any influence of the host magma
Glass composition
In both samples, glasses vary from andesite to trachyte.
They are silica-oversaturated, and differ from the host basanite.
Glasses straddle the boundary for mantle glasses related to carbonatite/Na-silicate metasomatism.
Na-silicate*
Carbonatite*
K-silicate*
*Coltorti et al 2000
host
Spongy cpx (in MC9) have lost REE compared to CPX 1 and the glass is enriched in REE.
Introduction C
Spongy cpx are formed by reaction of the primary peridotite with an infiltrating melt.
This first metasomatic event occurs prior to the formation of the cross-cutting carbonate veins.
This points to a metasomatic co-precipitation of carbonate and associated secondary minerals.
As inferred from pressure estimated for cpx2, this occurred near the base of the crust.
This suggests the carbonate being related with magmatic fractionation of an alkali and /or alkali carbonate mantle-derived melt, that was injected shortly before the eruption to preserve the calcite crystals.
Interpretation
The carbonate is always associated with secondary minerals and silicate glasses sometimes reduced to a thin rim at the margin of veins or pockets
Secondary ol2 and cpx2 that indent the carbonate (red arrows in C) and may be included the carbonate crystals.
sharp meniscus A
Although apparent immiscibility features (A), the tie-line glass-carbonate do not intersect experimentally determined immiscibility (B).
The carbonate represents thus crystal cumulates and not quenched carbonate melts or immiscible liquids.
The carbonate have high XCa, low alkalis and low REE.
CaO+MgO+FeO
SiO2+Al2O3
Na2O+K2O
0.5 GPa
1.5 GPa B
MC9 - MC2
(Brooker & Kjarsgaard, J Petrology, 52, 2011) 1 2 3
MC southern domain
sp facies
pl facies
gar facies
90 mWm-2
60 mWm-2
400
800
1200
0.5
1.0
1.5
2.0
2.5
P GPa
T °C
CPX 1
cpx2
Moho D