intraplate mantle oxidation by volatile-rich silicic magmasintraplate mantle oxidation by...

Lithos 292–293 (2017) 320–333

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Intraplate mantle oxidation by volatile-rich silicic magmas

Audrey M. Martin a,⁎, Etienne Médard a, Kevin Righter b, Antonio Lanzirotti c

a Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston, TX 77058, USAb NASA Johnson Space Center, Mailcode XI2, 2101 NASA Parkway, Houston, TX 77058, USAc University of Chicago, GSECARS, 5640 S. Ellis Avenue, Chicago, IL 60637, USA

⁎ Corresponding author.E-mail address: [email protected] (A.M. Marti

http://dx.doi.org/10.1016/j.lithos.2017.09.0020024-4937/© 2017 Elsevier B.V. All rights reserved.

a b s t r a c t
a r t i c l e i n f o
Article history:Received 4 January 2017Accepted 2 September 2017Available online 12 September 2017

The upper subcontinental lithosphericmantle below the FrenchMassif Central ismore oxidized than the averagecontinental lithosphere, although the origin of this anomaly remains unknown. Using iron oxidation analysis inclinopyroxene, oxybarometry, and melt inclusions in mantle xenoliths, we show that widespread infiltration ofvolatile (HCSO)-rich silicic melts played a major role in this oxidation. We propose the first comprehensivemodel of magmatism and mantle oxidation at an intraplate setting. Two oxidizing events occurred: (1) a 365–286 Ma old magmatic episode that produced alkaline vaugnerites, potassic lamprophyres, and K-rich calc-alkaline granitoids, related to the N–S Rhenohercynian subduction, and (2) b30 Ma old magmatism related toW–E extension, producing carbonatites and hydrous potassic trachytes. These melts were capable of locallyincreasing the subcontinental lithospheric mantle fO2 to FMQ + 2.4. Both events originate from the melting ofa metasomatized lithosphere containing carbonate + phlogopite ± amphibole. The persistence of this volatile-rich lithospheric source implies the potential for new episodes of volatile-rich magmatism. Similarities withworldwide magmatism also show that the importance of volatiles and the oxidation of the mantle in intraplateregions is underestimated.

© 2017 Elsevier B.V. All rights reserved.

Keywords:Intraplate magmatismCarbonatitesPotassic trachytesMantle oxidationXANES

1. Introduction

The shallower part of the subcontinental lithosphericmantle (SCLM)beneath the FrenchMassif Central (FMC), a currently intraplate volcanicregion, has a higher oxygen fugacity (FMQ-0.47 bΔlogfO2 b FMQ+1.66with a mean value of FMQ + 0.68; Uenver-Thiele et al., 2014) thanthe average intraplate continental lithosphere (FMQ-4.25 b ΔlogfO2

b FMQ + 1, mean value FMQ-0.68; Foley, 2011) (FMQ representingthe Fayalite = Magnetite + Quartz buffer). Uenver-Thiele et al.(2014) suggested that this anomaly can be explained by the infiltrationof volatile-richmelt and/or gas, which have the ability to oxidize iron inmantle silicates and oxides (Malaspina et al., 2010; Martin and Righter,2013; McGuire et al., 1991). Such infiltration has also been proposed byWilson and Downes (1991) and Yoshikawa et al. (2010), to explain thepresence of hydrousminerals (phlogopite and amphibole) andCO2fluidinclusions in mantle xenoliths from the FMC and Central Europe.However, the composition of these melt and/or gas, and their impacton the FMC mantle, has not yet been constrained.

Rare Variscan calc-alkaline igneous rocks, formed bymantle meltingduring the Southward Lizard–Rhenohercynian subduction, are presentin the FMC (e.g., Limousin Tonalitic Line; basalts, andesites and dacites

n).

from the Morvan arc; 360–365 Ma; Pin and Paquette, 2002; Fig. 1).Their parental magmas are volatile-rich (Sisson and Grove, 1993);nevertheless, they are localized in the Northern part of the FMC andtherefore, cannot explain the high mantle fO2 of the entire region.Volatile-rich aluminous magmatic suites (355–295 Ma), which are fre-quent in the FMC (e.g., Millevaches leucogranite, Guéret Al-granite,Margeride Al-granite, and Velay migmatites and Al-granite; Ledruet al., 2001), formed by crustal melting and, therefore, cannot explainthe high mantle oxidation state. Alkali basalts that abundantly infiltrat-ed the lithospheric mantle during the last 65 Ma (Michon and Merle,2001) are volatile-poor; therefore, they cannot explain the highoxidation state of the upper lithosphere. However, volatile-rich alkalineigneous rocks of unusual composition are also present at variouslocations in the FMC (Fig. 1). For example, carbonatites have beenfound in the Velay Province (8.5 Ma old; Chazot et al., 2003), andcarbonate-bearing peperites in the Limagne Basin (15–23 Ma old;Chazot and Mergoil-Daniel, 2012). Hydrous potassic lamprophyres(kersantite, spessartite, and minette; 286–306 Ma; Perini et al., 2004)and plutonic equivalents (vaugnerites; 302–360 Ma; Couzinié et al.,2016; Ledru et al., 2001; Michon, 1987; von Raumer et al., 2014) alsoappear in various places, particularly in the Eastern half of the FMC. Inaddition, K-rich calc-alkaline granitoids (KCG), derived fromvaugnerites, are abundant in the FMC (Fig. 1; Moyen et al., 2017).These temporally and spatially scattered HCSO-rich magmatic events,and lower degrees of melting of the same source(s), may be responsible

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Fig. 1. Location of carbonatitic and potassic igneous rocks in the FrenchMassif Central (FMC) and oxidation state in theUpper LithosphericMantle. Carbonatite/Peperite (volcanic rocks): 1.Chabrières carbonatite mingled with trachytic tuff and cumulates (Chazot et al., 2003) ~8.5 Ma; 2. Gergovie peperite (Chazot and Mergoil-Daniel, 2012) N15 Ma; 3. Pileyre peperite(Chazot and Mergoil-Daniel, 2012) 23–15 Ma. Potassic lamprophyres (dykes): 4. Oradour sur Glane minette lamprophyre (Perini et al., 2004) ~300 Ma; 5. Ambert spessartitelamprophyres (Perini et al., 2004) ~300 Ma; 6. Livradois kersantite and minette lamprophyres (Aubert et al., 1982); 7. Les Pelletiers basaltic trachyandesite (Delfour et al., 1995; Periniet al., 2004) ~300 Ma; 8. Morvan minette lamprophyres (Turpin et al., 1988); 9. Mont Lozère lamprophyres (Brichau et al., 2008) ~306 Ma; 10. Aigoual-St Guiral-Liron lamprophyricmagmas (Brichau et al., 2008) ~306 Ma; Mazel kersantite and minette lamprophyres (Faure et al., 2009) ~286 Ma. Vaugnerites (plutonic rocks): 11. Mayres vaugnerites (Ledru et al.,2001) ~314–313 Ma; 12. Rocles vaugnerites (Michon, 1987) ~302 Ma; 13. Tournon vaugnerites (Ledru et al., 2001; Michon, 1987) ~337 Ma; 14. Margeride vaugnerites (Couturier,1977; Michon, 1987) ~334 Ma; 15. Guéret low-K vaugnerites (Galàn et al., 1997) 350–340 Ma; 16. Livradois vaugnerites (Gardien et al., 2011) b360 ± 4 Ma; 17. Chambon-sur-Dolorevaugneritic diorite (Kornprobst, 1984); 18. Vivarais vaugnerites (Ledru et al., 2001; Michon, 1987); 19. Lyonnais vaugnerites (Michon, 1987). Variscan K-rich calc-alkaline granitoids(KCG) plutons are redrawn from the 1/1,000,000 scale geological map of France (Chantraine et al., 1996); Variscan calc-alkaline igneous rocks are from Pin and Paquette (2002). Oxygenfugacity calculations are based on the composition of minerals from xenoliths sampled all around the FMC (green dots) from Uenver-Thiele et al. (2014), except the value at FMQ+ 2.35(this study). Structural data were added for comparison (Averbuch and Piromallo, 2012; Michon and Merle, 2001).

321A.M. Martin et al. / Lithos 292–293 (2017) 320–333

for the high oxygen fugacity of the upper subcontinental lithosphericmantle beneath the French Massif Central.

At small scale, volatile-rich melt migration in the mantle(i.e., metasomatism) can produce veins enriched in one or severaloxidized minerals (e.g., carbonates, phlogopite, amphibole, oxides,Fe3+-rich pyroxenes), a process known as modal metasomatism. Suchveins have been observed in xenoliths (e.g., McGuire et al., 1991) andperidotite massifs (e.g., Le Roux et al., 2007; Zanetti et al., 1999). Ele-mental diffusion can alsomodify the chemistry of the surroundingman-tle (“cryptic” metasomatism), including its oxidation state (McGuireet al., 1991). Over time, the average fO2 of the mantle can substantiallyincrease, in particular, in highlymetasomatizedmantle regions like sub-duction zones (up to FMQ+ 3 according to Malaspina et al., 2010; seealso Brounce et al., 2014; McInnes and Cameron, 1994; Parkinson andArculus, 1999). Similar oxidation can be expected in the mantle underfrequently active volcanic zones, in particular, because volatiles areincompatible, and thus concentrate in melts. However, the amount of

oxidation caused by magmatic or metasomatic processes at intraplatesettings has never been estimated.

To constrain the origin of the high mantle fO2 below the FrenchMassif Central, we sampled mantle xenoliths at Puy Beaunit, a maar lo-cated in the Chaîne des Puys (Boivin et al., 2009; Downes et al., 2003)(Fig. 1). Most samples are rounded lherzolite/harzburgite xenoliths ofup to 15 cm width (Supplementary Material 1), containing olivine(ol), orthopyroxene (opx), clinopyroxene (cpx) and spinel (sp) (sam-plesMLhA,MLhB, HzE, PHz3, ALh1, Lh2; Figs. 2 & 3; Table 1). Other min-erals are occasionally present, e.g. phlogopite (phl) and apatite (ap) inPHz3, amphibole (amp) in ALh1, and sulfide (sulf) in MLhA and PHz3.Some samples (MLhA, MLhB) contain olivine websterite veins that areremnants of previous melt infiltration in or above the mantle sourceregion of the magma that transported the xenoliths to the surface(Fig. 2). In addition, we found few samples of pure olivine websterite(WbC). Ol websterite can be considered as “cumulate” from ametasomatic / magmatic melt (Downes, 2007; Pilet et al., 2011). The

Image of Fig. 1

Fig. 2. Images of Beaunitmantle xenoliths. (a) Photograph of an olivinewebsterite vein-bearing samples (MLhA) with the location of the cpx crystals selected for the Fe oxidation analyses.(b) Back-Scattered Electron (BSE) image ofMLhA showing the vein limits (red), the lines of equal oxidation state of iron (white), and the lines of equal fO2 (blue). (c) Chemicalmap of the olwebsterite vein (MLhA) (Ca = red, Si = green, Fe = blue). (d) Chemical map of the phlogopite-bearing harzburgite (PHz3) (Si = red, Al = green, Fe = blue). (e) BSE image of meltinclusions inMLhA. (f) BSE image of melt pools in PHz3.

322 A.M. Martin et al. / Lithos 292–293 (2017) 320–333

oxidation state of iron in cpx from these websteritic samples, therefore,should reflect the temporary conditions that affected themantle duringthe metasomatic/magmatic event.

2. Methods

2.1. Major elements analysis

All xenoliths were mounted in epoxy resin and cut using a sawequipped with a diamond-covered Cu blade. Samples containing cpx-rich veins (MLhA and MLhB) were cut perpendicular to the veins. Allsamples were then ground with SiC-covered paper and polished usingAl2O3 powder and ethanol just before XANES analysis to avoid surfaceoxidation. Back-Scattered Electron images, chemical maps (Fig. 2) and

chemical analyses (Figs. 3 & 4) were acquired using an electron micro-probe (Cameca SX100, Johnson Space Center). Mineral modes were cal-culated using chemical maps (Adobe Photoshop CS5/ImageJ) (Table 1).Chemical analyses were run using analytical conditions of 15 kV–15 nAfor silicate minerals and 15 kV–10 nA for silicate glass. The electronbeam was also defocused to 5 μm for the glass, amphibole and phlogo-pite analyses. The standards used for these analyses were diopside forSi, Ca and Mg, oligoclase for Al and Na, NiO for Ni, rutile for Ti, chromitefor Cr, orthoclase for K, rhodochrosite forMn, olivine for Fe,fluorite for F,tugtupite for Cl, and apatite for P. The hydrogen content of phlogopite,amphibole, and apatite was calculated by stoichiometry. The H2O con-tent in the melts was calculated considering phlogopite as the solesource— the K2O, F, and Cl fractions in phlogopite thus being the limit-ing parameters. The CO2 content in the melts was determined by

Image of Fig. 2

Fig. 3. Mineralogy and composition of Beaunit xenoliths. Left: Olivine–orthopyroxene–clinopyroxene ternary diagram showing the mineral mode of the investigated samples: the olwebsterite vein-bearing lherzolites (MLhA and MLhB) and their ol websterite veins, the pure ol websterite (WbC), the lherzolite (Lh2) and amp-bearing lherzolite (ALh1), theharzburgite (HzE) and the phl-bearing harzburgite (PHz3). The shaded area indicates the field of the French Massif Central mantle xenoliths (lherzolites and harzburgites) from theliterature (Brown et al., 1980; Werling and Altherr, 1997). PBN compositions are ol-websterites and websterites (Femenias et al., 2003). Other samples from the literature arerepresented by white circles (Yoshikawa et al., 2010) and squares (Wilson and Downes, 1991). Right: Cr# (Cr/(Cr + Al) in at. %) in spinel vs. Mg# (Mg/(Mg + Fe) in at.%) in olivinecompared to the Olivine–Spinel Mantle Array (OSMA; Arai, 1994).


difference to 100 wt.%. Sulfides were analyzed using a 15 kV–20 nAbeam. The standards used are troilite for Fe and S, pentlandite for Ni,cuprite for Cu, and cobaltite for Co.

2.2. Iron oxidation analysis

Fe oxidation state in clinopyroxenes was acquired using Fe K-edgemicro-X-ray Absorption Near-Edge Structure (XANES) analyses at theAdvanced Photon Source, Argonne National Lab (GSECARS 13-ID-E)(Bajt et al., 1994; Martin and Righter, 2013; Righter et al., 2013). Amonochromatic X-ray beam from a Si(311) monochromator was fo-cused on the sample and the FeKα fluorescent X-ray yield was plottedas a function of incident X-ray energy. Energy was calibrated on thefirst derivative peak of a Fe metal foil, set at 7110.75 eV. Spectra were

Table 1Mineralogy of the xenolith samples from Puy Beaunit.

Sample Rock type ol opx cpx phl amp sp sulf Melta

HzE Harzburgite 88 8 3 – – 1 – –Lh2 Lherzolite 58 23 16 – – 2 – 4PHz3 phl-harzburgite 64 30 – 5 – 1 Traces –ALh1 amp-lherzolite 52 32 9 – 4 3 – –MLhA Metasomatized

lherzolite74 18 7 – – 1 – –

MLhAvein

Metasomatizedlherzolite vein

15 38 46 – – – Traces –

MLhB Metasomatizedlherzolite

76 18 5 – – 1 – –

MLhBvein

Metasomatizedlherzolite vein

26 43 30 – – Traces – –

WbC ol websterite 39 21 38 – – – – 2BE11 Lherzolite 67.5 26.8 4.6 – – 1.1 – –BE41 Lherzolite 57.3 27.6 8.7 1.1 – 4.4 – 0.9BE1 ol websterite 10.3 39.9 44.9 – – 3.3 – –PBN98-09

ol websterite 35.2 23.9 40.9 – – - – –

PBN98-39

ol websterite 5.2 73.1 21.7 – – – – –

PBN98-01

Websterite 0.0 18.8 81.2 – – – – –

PBN86-19

Websterite 1.1 82.9 16.0 – – – – –

Mineral modes were calculated using image analysis of chemical maps. Representativedata from the literature are also reported for comparison. BE samples are from Yoshikawaet al. (2010), and PBN samples from Femenias et al. (2003). The error on themineralmodecalculations is ±2%.

a Thismelt is present as interstitial veins connected to the outer lava crust. Primarymeltinclusions are not taken into account here.

collected from 7062 to 7356 eV in three sections with a 2 s dwell foreach energy step: 5 eV steps from 7062 to 7107 eV, 0.1 eV steps acrossthe pre-edge peak from 7107 to 7137 eV, and 2.5 eV steps from 7137to 7356 eV. Spectra were normalized to the incident photon flux mea-sured using an ion chamber upstream of the sample, and dead-timecorrected (dead-times are usually ~30%). Possible drifts in monochro-mator energy were monitored using a homogeneous natural magnetitecrystal from Balmat (New York, USA) that has a composition extremelyclose to the pure Fe2+Fe3+2O4 end-member. No significant drift wasrecorded during the session.

Standards used for these analyses were large natural homogeneousclinopyroxene crystals: hedenbergite (Ca1.00Mg0.14Mn0.11 Fe0.742+

Fe3+0.01Na0.05) (Si1.99Fe3+0.01)O6 from Huanggang Deposit (China),augite (Ca0.86Mg0.67 Fe0.132+ Fe3+0.11 Al0.09Ti0.08Na0.05) (Si1.67Al0.33)O6

from Montbuzat (France), and aegirine (Ca0.03Mn0.02Fe0.042+ Fe3+0.86

Al0.01Ti0.07Na0.93)Si2.00O6 from Mount Malosa (Malawi). Typical erroron XANES measurements on anisotropic minerals like pyroxenes isknown to be around 10–15% absolute due to the crystallographic orien-tation effect (Dyar et al., 2002). For this reason, the standards were ori-ented, reducing the error down to 1–5% absolute (McCanta et al., 2004).Crystal faces were indexed and the crystals oriented using their ele-ments of symmetry. They were then embedded in epoxy resin, andcut so that their Y optical axes (identical to their b crystallographicaxes) are parallel to the polarization direction of the X-ray beam. To ori-ent unknowns, we ran fast (b2min) X-ray spectra on a large number ofcrystals in each sample. Crystals with orientations close to the Y opticalaxis were recognized from the characteristic peak shapes above theedge (Dyar et al., 2002; McCanta et al., 2004) and selected for longerand precise analyses.

Spectra where normalized in Athena (IFEFFIT package) (Newville,2001) and fitted using the OriginPro® 8 software (SupplementaryMaterials 2 & 3, Fig. 5). The baseline is a combination of anunderdamped (Z b 1) harmonic oscillator function and a linear function(Cottrell et al., 2009), with six adjustment parameters (Z, x0, C1, C2, aand b), following the equation:

f xð Þ ¼ e−Z x0−xð Þ C1 cos x0−xð Þffiffiffiffiffiffiffiffiffiffiffiffiffi1−Z2

p� �þ C2 sin x0−xð Þ

ffiffiffiffiffiffiffiffiffiffiffiffiffi1−Z2

p� �� þ axþ b

ð1Þ

Four pre-edge peaks (at 7111.0, 7112.0, 7113.2 and 7114.5 forMLhA; Supplementary Material 3) were then fitted using Gaussianfunctions. The error on the centroid position is derived from the

Image of Fig. 3


standard error on each peak area (Ai), using fixed peak positions (Pi),following the equation:

δC ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiP4

i¼1 ∑ j≠i Pi−Pj� �

Aj� �2δAi

2q

∑4i¼1 Ai

� �2 ð2Þ

The Fe3+ content in clinopyroxenewas then determined using a lin-ear calibration. The error on this calibration is 2.4% absolute. The stan-dard error on the calculated Fe3+ value is thus a combination of thestandard error on the spectra fitting (0.8–1.8% absolute) and calibrationerror. We estimate that this error is 3.2 to 4.2% absolute.

3. Mineralogy and chemistry of the mantle xenoliths

Most harzburgite and lherzolite samples from Puy Beaunit are locat-edwithin the Olivine–SpinelMantle Array (OSMA) defined by the Cr# insp and the Mg# in ol (Arai, 1994; Fig. 3), and within the mineralogicalfield of French Massif Central xenoliths (Fig. 3). The most depleted sam-ple (HzE) is a massive and homogeneous ol-rich harzburgite (~88% ol).Samples Lh2 and ALh1 are less-depleted lherzolites. The latter contains4% amphibole, and is particularly enriched in Al-rich spinel (3%),confirming that this sample underwent no or very little melting. Thephl-bearing sample (PHz3, Fig. 2d) is deformed andhas no cpx, as sampleLi-35 from Limagne (Brown et al., 1980). PHz3 composition showshigh degree of metasomatism, while the phl-bearing sample BE4(Yoshikawa et al., 2010) is within the OSMA domain. Ol websterite sam-ples appear either in the form of veins (b1 cm width) inside lherzolite(MLhA, MLhB; Fig. 2a, b & c), or as the dominant widespread phase inthe rock (WbC). The composition of the pure websterite samples (WbCand BE1 from Yoshikawa et al.) is in agreement with high degree offractional crystallization. The equigranular texture of these samples,and the fact that they contain melt inclusions and no sp, confirms thatthey result from melt infiltration. Their mineralogy is very close toother samples from the FMC (Wilson and Downes, 1991; Yoshikawaet al., 2010). With our new samples, trends appear in the ol websteritefield of the ol–opx–cpx diagram (Fig. 3). They may be interpreted asmixing between cumulates and residual melt.

Chemical variations appear between the veins and the surroundinglherzolite in MLhA and MLhB (Supplementary Materials 4 & 5). Ol hasa slightly lower Mg# outside the veins (0.89–0.90) than inside (0.90–0.91). This can be explained by the high fO2 of the metasomatic vein,and the very low potential for Fe3+ incorporation in olivine (Corteset al., 2006). Opx has a relatively constant Mg# (0.90–0.91), althoughthe opx from the vein of sample MLhA has slightly more FeO than out-side the vein. In addition, opx from the veins contain more MnO andCr2O3 than outside the veins. Some chemical differences are also ob-served between the two vein-bearing samples. Opx and cpx fromMLhA have lower SiO2 and higher Al2O3 contents than in MLhB. Cpxalsohas lower TiO2 content,while sp ismoreAl2O3-rich andhas a higherMg#. MLhA and MLhB may thus have undergone different degree ofmetasomatism/re-equilibration. In the pure ol websterite (WbC), oland opx are more Fe- and Mn-rich than in the vein-bearing samples(MLhA and MLhB). Cpx from WbC is also more TiO2-rich, while sp. ismore Mn-rich. This might reflect a higher degree of metasomatism orfractional crystallization. However, the presence of multiple basaltveins connected to the outer crust inside WbC may indicate a late con-tamination by the lava that brought this xenolith to the surface.

4. Metasomatic melt composition

We constrained the nature of the percolating melt using primarymelt inclusions trapped in minerals from an ol websterite vein (MLhA,Fig. 2e), melt pools from a phl-bearing peridotite (PHz3, Fig. 2f), andCO2 gas inclusions observed in xenoliths from the same outcrop

(Yoshikawa et al., 2010). Melt inclusions from the ol websteritevein are potassic (4.5–5.5 wt.% K2O) with high silica content(trachyandesite/trachyte; 56–61 wt.% SiO2) (Fig. 4, SupplementaryMaterial 6). They also contain 2.77 wt.% Na2O, 1.64 wt.% H2O,0.95 wt.% CO2, 0.39 wt.% F, and 0.18 wt.% Cl. Their Mg# (defined asMg/(Mg+ Fe) in at. %) is similar to that of melt inclusions in magmaticcrystals from Puy Beaunit (Jannot et al., 2005; Type I inclusions: alkalibasalts with anMg#=0.62–0.68; Fig. 4), implying that they are equally“primary”. However, our melts have a higher silica content, similar toType II inclusions from mantle xenoliths (Jannot et al., 2005; SchianoandClocchiatti, 1994), butwithmore potassium. Ourmelt compositionsare also close to melts observed in phl-bearing xenoliths from PuyBeaunit (Wilson and Downes, 1991; Yoshikawa et al., 2010). Trachytemelt pools present in our phl-bearing xenolith (PHz3) have a lowerMg# (0.56–0.60), but a SiO2 content similar to melt inclusions foundin the veins (60.8 ± 1.4 wt.%). The high K2O (4–8 wt% in PHz3) contentof all those silica-rich melts indicates that they formed in a mantlewhere phlogopite was present. The ol websterites were, therefore,produced by infiltration of a potassic hydrous silicic melt that formeddeeper in the lithospheric mantle, by low degree melting of aphlogopite-bearing lherzolite or harzburgite (Condamine and Médard,2014). Thermobarometry indicates that the temperature of thismetaso-matic melt was ~1115 °C, while the surrounding mantle temperaturewas 950–1060 °C (Table 2).

Spherical Fe–Ni sulfide drops are associated with silicate melt in-clusions in PHz3 and in the vein of MLhA. These sulfides crystallizedfrom a sulfur-rich melt formed by immiscibility during the silicatemelt cooling. Additionally, CO2 fluid inclusions, co-genetic with thesilica-rich melt inclusions (Schiano and Clocchiatti, 1994), were re-ported in ol from Beaunit xenoliths (Yoshikawa et al., 2010). Few ex-amples of carbon-rich lavas are known in the FMC, always associatedwith silicate melts. Silicate glasses found in carbonate-bearingpeperites from the Limagne Basin (Chazot and Mergoil-Daniel,2012) have compositions similar to the typical alkali basalts pro-duced by differentiation of asthenospheric melt (Fig. 4). On theother hand, the composition of the silicate glasses associated withcarbonatites from Chabrières, Velay Province (Chazot et al., 2003), isvery close to the melts found in the olivine websterite veins MLhA andin the phl-bearing harzburgite PHz3 (Figs. 1 & 4), but with lower Mg#and higher SiO2 content. The source of Chabrières carbonatites may,therefore, be similar to the source of the olivine websterite veins foundin Beaunit xenoliths (i.e., a carbonate- and phlogopite-rich lithosphericmantle). The trachyandesite/trachyte melts and carbonatites may havebeenmiscible at high depth, but rapidly undergone olivine and pyroxenecrystallization, degassing and/or immiscibility (Dasgupta et al., 2006;Martin et al., 2012; Martin and Righter, 2013). Low pressure differentia-tion of melts with volatile-rich silicic compositions similar to Beaunit in-clusions may also explain some of the trachytes and phonolites eruptedduring the W–E extension (30 Ma–present) in the FMC (e.g., in theVelay Province; Fig. 4).

Strong similarities also appear between thepotassic trachyandesite/tra-chyte melt inclusions and pools from Beaunit xenoliths, and some of the365–286 Ma old plutonic (vaugnerites) and volcanic (lamprophyres) po-tassic melts from the FMC (Fig. 4). These melts likely formed from thesamemantle source, i.e., a phl-bearing lherzolite/harzburgite at lithospher-ic pressure. No evidence for carbonatitic magmatism of Variscan age hasbeen found, which may be attributed to erosion.

Due to their high viscosity, silica-rich magmas do not easily migrate inthe mantle; however, the presence of volatiles significantly increases theirmobility potential. The mobility of the silicate melt present as inclusionsand pools in Beaunit xenoliths was calculated considering the presence of1.64 wt.% H2O (average concentration of the melt inclusions in cpx fromsample MLhA). At 1115 °C (Table 2), the viscosity (Giordano et al., 2008)of these melts is 800 Pa.s (log η = 2.90), while their density (Lange andCarmichael, 1990) is ~2550 kg/m3. Assuming that these melts formed at~0.7 ± 0.4 GPa (i.e., a maximum pressure of 1.1 GPa; Table 2), and a

Fig. 4.


Image of Fig. 4

Fig. 4.Models of crystallization-degassing of alkaline magmas in the French Massif Central based on the composition of our melts and of all other alkaline igneous rocks from the region.b30Ma old lavas from the Chaîne des Puys, Limagne, Velay, Monts Dore, Cantal, Cézallier, Chaîne de la Sioule and Forez/Montbrison are reported, as well as 365–286Ma old hydrous po-tassic igneous rocks (vaugnerites, lamprophyres and KCG). We also indicated the composition of melts obtained experimentally using various starting material: phlogopite-bearinglherzolite, phlogopite-bearing harzburgite, phlogopite + carbonate peridotite, CHO-bearing garnet lherzolite and CO2-bearing eclogites for comparison. Supplementary References arein Supplementary Material 6. All alkaline magmas can be explained by the crystallization and degassing of low-degree melts from three peridotitic sources at 1 GPa (Lp1), 1.5 GPa(Lp2) and 3 GPa (Ap1). Some magma compositions might also have been influenced by an asthenospheric eclogitic source at 3 GPa (Ae1) (e.g., Velay area), but this influence was notmodeled in our study.


crust thicknessof 27kmbeneathPuyBeaunit (Boivinet al., 2009), themax-imum distance that these melts should have traveled in the mantle toreach the base of the crust is ~11.7 km. The mantle extraction time for amelt fraction of 20% (see below) is, therefore, of 88,000 years (McKenzie,1989), with an extraction rate of ~13 cm/year, which is only 3.8 timesslower than basalt (minimum 50 cm/year). This high rate enhances theireruption potential, and advocates for a co-genetic link between thesemantle melt inclusions and volcanic/plutonic rocks (vaugnerites,lamprophyres, KCG, trachytes, phonolites, and carbonatites).

5. Newmodel of alkaline magmatism in the French Massif Central

Alkali basalts and basanites erupted during the last 65 Ma haveMg#s of 0.55–0.70, similar to the potassic trachyandesites/trachytesfound in Beaunit xenoliths and to the 365–286 Ma vaugnerites and po-tassic lamprophyres, but with lower SiO2 contents (Fig. 4). Consideringtheir major elements composition, the source of the alkali basalts/basanites could be a HCSO-bearing asthenospheric lherzolite/harzburgite (~3GPa) and their parental melts, olivine melilitites or

Fig. 5. Iron oxidation analyses and fO2 calculations forMLhA. In thefigures on the left, the spectrawere shifted along the Y axis to show the peak differences at the pre-edge energies. Thefigureson the right report the evolution of the centroid position, Fe3+/ΣFe in cpx and calculated fO2 as a function of the distance to the center of the vein, fitted using a linear and an error function.


nephelinites (Baasner et al., 2016; Brey, 1978). Using olivine melilitites /nephelinites as references, we propose that the Mg# of all parentalalkaline magmas of the FMC is between 0.68 and 0.82 (gray area onFig. 4), and is independent of the depth of themantle source (lithosphereor asthenosphere). This range is consistent with experiments on anhy-drous and hydrous peridotite (e.g., Balta et al., 2011; Hirose andKushiro, 1993; Kushiro, 1996; Supplementary Material 6), phlogopite-bearing peridotite (Condamine and Médard, 2014), and phlogopite-and carbonate-bearing peridotite (Thibault et al., 1992).

A new equilibrium crystallization model based on this initial Mg#range, including low-degree ofmelting andH–C–Ovolatiles, can explainall alkaline magmatism in the FMC from 365 Ma to present. The equa-tion used in this model calculates the fraction of oxides (X) in the melt(Mi) during the crystallization of various minerals (Ci):

XMi ¼XM0−R

PXCi

1−Rð3Þ

where R is the crystallization fraction, andM0 is the parentalmelt. The na-ture and proportions of crystallizing phases were optimized by iterationto fit the observed natural differentiation trends. The presence of CO2 in-clusions in olivine insidemantle xenoliths fromPuyBeaunit indicates thatdegassing occurred during the metasomatism. Volatile (CO2 + H2O)degassingwas thereforemodeled simultaneously to themelts crystalliza-tion, using constant degassing rates (SupplementaryMaterial 7). In orderto explain all b365 Ma old alkaline magmas from the FMC, independentdifferentiation trends from the melting of three sources are required(Fig. 4, Supplementary Material 7):

(1) a HCSO-rich asthenospheric peridotite source at ~3 GPa (Ap1)that produced the basaltic suite. Low-degree melts from thissource are very close to olivine melilitite, with 11.0 wt.% CO2

+H2O. The firstminerals to crystallize from this melt are olivine,clinopyroxene, spinel, melilite (mel) and calcite (cc). Calcite,melilite and olivine disappear during differentiation, and are

Image of Fig. 5


replaced by plagioclase (pl), biotite (bt), alkali feldspar (alk fds)and titanite (titan).

(2) a HCSO-rich lithospheric peridotite source at ~1.5 GPa (Lp2). Thissource explains the less silicic vaugnerites (Couzinié et al., 2016),and potentially the trachytes/phonolites from the Velay Province.This trend is also concordant with crystallization trends of“durbachites” (vaugnerites equivalents) determined experimen-tally (Parat et al., 2010). Low-degree melts from this source have8.7 wt.% CO2 + H2O. Olivine, clinopyroxene and spinel are thefirst minerals to appear during the magmas ascent. Spinel andolivine disappear during crystallization, while kaersutiticamphibole, alkali feldspar and titanite appear. The same sourcecan explain the K-rich calc-alkaline granitoids (KCG) with a Mg#of 0.35–0.5 (Fig. 4; Moyen et al., 2017). Using a lower degassingrate after the SiO2 in the melt reaches ~57 wt.%, plagioclase andpotassic feldspar become the main minerals to crystallize, withminor olivine, clinopyroxene, biotite, titanite and magnetite. TheCO2 + H2O content in the resulting melts is ~1 wt.%, similar toKCG. Two different magmatic suites can thus be produced fromthis source. They separate at a “crystallization divide” correspond-ing to a change in crystallizing assembly and degassing rate.

(3) a HCSO-rich lithospheric peridotite source containing phlogopiteat ~1 GPa (Lp1) leading to our melts composition (MLhA andPHz3), some vaugnerites and possibly some melts fromChabrières, Velay Province (Chazot et al., 2003). The CO2 + H2Ocontent in the low-degree melts from this source is 6.9 wt.%.Olivine, clinopyroxene, phlogopite, spinel and titanite are thefirst minerals to crystallize. Olivine, phlogopite and titanite disap-pear at lower depths, while kaersutitic amphibole and alkali feld-spar crystallize. This source can also explain the KCG with a Mg#of 0.50–0.65, if we consider a lower degassing rate when themelt reaches ~61 wt.% SiO2 (and a second crystallization divide).Plagioclase and potassic feldspar then are the main phases to

Table 2Oxythermobarometry on Beaunit mantle xenoliths.

Olivine–orthopyroxene–spinel

Sample P (GPa)cpx-opx

T (°C)cpx-opx

Δlog fO2/FMQCTserver (OFM Research)

Δlog fO2/FMQBallhaus et al. (1

HzE 0.79 947 +0.28/+0.20 ± 0.07 (n = 3) +1.15 ± 0.3/+1MLhA 0.67 998 +0.19/+0.16 ± 0.05 (n = 3) +0.74 ± 0.3/+0MLhA vein 0.70 1014 – –MLhB 0.80 1002 +0.28/+0.27 ± 0.24 (n = 3) +1.28 ± 0.3/+1MLhB vein 0.50 998 +0.34/+0.32 ± 0.18 (n = 3) +1.26 ± 0.3/+1WbC 0.88 951 – –PHz3 1.55 1056 +0.23/+0.21 ± 0.13 (n = 3) +1.10 ± 0.3/+1Lh2 0.99 922 +0.25/+0.28 ± 0.09 (n = 3) +0.52 ± 0.3/+0ALh1 0.86 961 +0.42/+0.37 ± 0.17 (n = 5) +0.62 ± 0.3/+0

Thermometers (T in °C) Melt only

Reference Putirka (2008)

Equation Eq. (15)

Standard error estimate 46MLhA vein (melt inclusions in ol) 1111MLhA vein (melt inclusions in cpx) 1119PHz3 (melt pools) 1133

Pressures and temperatures were calculated using thermometers and barometers compiled(Standard Error Estimate = 0.37 GPa). Temperature was calculated from cpx-opx thermometemperatures were also calculated with melt only, olivine/melt, and cpx/melt thermometersand olivine/melt thermometers, since the value given by the cpx/melt thermometer is very difOxygen fugacity was calculated using two different oxybarometers. For the olivine-orthopoxybarometer from OFM research is available at http://ctserver.ofm-research.org/Olv_Spn_Opquartz) reference buffer (Huebner, 1971; O'Neill, 1987). For all the methods using the ol–opx–using average compositions of ol, opx and sp., and the second is an average of calculations bassamplesMLhA and WbC, Δlog fO2/FMQ was also calculated using in-situ Fe3+ analyses (XANEdiscussed in the text.

crystallize, with minor clinopyroxene, biotite, titanite and magne-tite. The CO2 + H2O content in the resulting melts is ~1 wt.% likeKCG.

The location of the crystallization divides on the trends from litho-spheric sources (Lp1 and Lp2), and of the corresponding appearance ofplagioclase and alkali feldspar with the asthenospheric source (Ap1), isconcordant with the beginning of the Daly gap, a range of silica contentthat is rarely encountered in igneous rocks (Bonnefoi et al., 1995). In ad-dition, the SiO2 content in the melts that form the limits of the Daly gapincreases when the pressure of the source and the volatile content inthe parentalmelts decrease.We thus propose that the Daly gap is directlylinked to the degassing of volatiles during magmatic crystallization, andtherefore, to the path of the magma from the source to the surface.

The difference in SiO2 content between the parental lithospheric andasthenosphericmelts arises from themelting reactions (olivine is eitherconsumed or produced; Kinzler, 1997). The global evolution of the pri-mary melt composition with pressure appears to be linear in the majorelements vs. silica space (gray area on Fig. 4). When pressure decreases,an increase in SiO2 (38.2 wt.% at 3 GPa to 54.3 wt.% at 1 GPa), Al2O3

(9.5 wt.% at 3 GPa to 17.2 wt.% at 1 GPa), K2O (0.7 wt.% at 3 GPa to4.3wt.% at 1 GPa) and Na2O (1.9wt.% at 3 GPa to 2.4wt.% at 1 GPa) con-tents is observed, while FeO (10.7 wt.% at 3 GPa to 3.2 wt.% at 1 GPa),MgO (15.4 wt.% at 3 GPa to 4.0 wt.% at 1 GPa), CaO (11.0 wt.% at 3 GPato 6.0 wt.% at 1 GPa), and H2O + CO2 (11.0 wt.% at 3 GPa to 6.9 wt.%at 1 GPa) contents decrease. The evolution of TiO2 ismore difficult to as-sess (likely because of analytical errors) but is probably decreasing orconstant (~1.5 wt.%). Using these models, a linear evolution of the com-position of the parental melts with pressure can be calculated, e.g., forSiO2 (Supplementary Material 8):

SiO2 ðwt:%Þ ¼ −7:603 ∗ P ðGPaÞ þ 60:602 ð4Þ

cpx alone

991)Δlog fO2/FMQBryndzia and Wood (1990)

Δlog fO2/FMQXANES analyses+ Cortes et al. (2006)

.08 ± 0.12 (n = 3) +1.47 ± 0.3/+1.37 ± 0.12 (n = 3) –

.72 ± 0.04 (n = 3) +1.07 ± 0.3/+1.03 ± 0.04 (n = 3) +0.33– +2.35

.27 ± 0.26 (n = 3) +1.40 ± 0.3/+1.39 ± 0.26 (n = 3) –

.31 ± 0.24 (n = 3) +1.50 ± 0.3/+1.55 ± 0.12 (n = 3) –– +4.78

.15 ± 0.09 (n = 3) +1.05 ± 0.3/+1.01 ± 0.13 (n = 3) –

.56 ± 0.08 (n = 3) +0.99 ± 0.3/+1.03 ± 0.10 (n = 3) –

.57 ± 0.19 (n = 3) +1.08 ± 0.3/+0.99 ± 0.20 (n = 5) –

Olivine/melt cpx/melt

Beattie (1993) in Putirka (2008) Putirka (2008)

Eq. (19) Eq. (33)

53 451113 –– 11501175 1077

by Putirka (2008). Pressure calculations use the cpx-opx equilibrium from his Eq. (38)try using his Eq. (36) (Standard Error Estimate = 45 °C). For the glass-bearing xenoliths,. The average T for the melts in MLhA vein (1115 °C) was calculated using the melt onlyferent.yroxene-spinel oxybarometer, we compared three different methods. The ol–opx–spx/index.php. Oxygen fugacity was recalculated relative to the FMQ (fayalite–magnetite–sp oxybarometer, two values of Δlog fO2/FMQ are reported: the first value was calculateded on analyses of contiguous ol, opx and sp ± 1 sigma (n = number of calculations). ForS) in cpx and the oxybarometer from Cortes et al. (2006). Errors on these calculations are

http://ctserver.ofm-research.org/Olv_Spn_Opx/index.php


At low pressure (b1.5 GPa), SiO2 variations are concordant with previ-ous studies on anhydrous and hydrous systems, even though they in-cluded samples with a higher degree of melting (Hirose and Kushiro,1993; Wood and Turner, 2009). Above 1.5 GPa, the SiO2 content in theparental melt is much lower than in these previous studies (38 wt.%against 46 wt.%). This difference may arise from the presence of bothhydrogen and carbon in the mantle source, which produces melilititicparental melt compositions (Baasner et al., 2016). As suggested byWood and Turner, our data points can also be correlated using alogarithmic function:

SiO2 ðwt:%Þ ¼ −14:49 ∗ ln ½P ðGPaÞ� þ 53:929 ð5Þ

At the base of the crust (~0.7 GPa), this equation would give a percent-age of silica in the parental melt of ~60 wt.%.

This parental melt evolution with pressure is close to the alkaline–sub-alkaline division from Miyashiro (1978) in the total alkalis (Na2O+ K2O) vs. silica diagram (TAS, Fig. 4). This boundary was explained bythe presence of a thermal divide formed by three phases: olivine–clinopyroxene–plagioclase. Our data show that pl is not one of the firstminerals to crystallize, even if the pressure–temperature conditions arein the pl stability field (MLhA, PHz3, Table 2). This can be explained bythe presence of carbonate, which stabilizes spinel against pl (Martinet al., 2012). The presence of H2O in the source also promotes the earlycrystallization of sp (Sisson and Grove, 1993). Our results show that thehigher the pressure of the source, the higher the volatiles content in theprimary melt, and the higher the iron oxidation state in the first spinelthat crystallizes (Fe3+/ΣFe = 0 in Lp1, 88% in Lp2, and 95% in Ap1). Theglobal compositional evolution of the primary melt (gray area on Fig. 4)is, therefore, due to simultaneous variations of pressure, volatiles content,and oxidation. When traveling upward, magmas undergo CO2 + H2Odegassing, allowing pl to become stable again. The first parts of thesethree differentiation trends evolve relatively similarly, because of thecrystallization of similar minerals (ol, cpx, and sp). The trends then differdue to the asynchronous crystallization of various minerals (pl, alk fds,amp, bt, phl, titan, mel, cc). Carbonatites likely form by immiscibilityfrom relatively primary melts along these trends (before completedegassing of CO2).

6. Redox evolution of the mantle during HCSO-richlithospheric magmatism

The olivine–orthopyroxene–spinel oxybarometer of Bryndzia andWood (1990) and Wood et al. (1990) was used by Uenver-Thiele et al.(2014) to constrain the fO2 in the mantle beneath the French MassifCentral (FMQ-0.47 to FMQ + 1.66, mean FMQ + 0.68). Thisoxybarometer gives consistent fO2 values (FMQ + 1.05 ± 0.04) onmost of our peridotite samples from Puy Beaunit (MLhA, Lh2, PHz3and ALh1; Table 2). These values are close to that determined byUenver-Thiele et al. onfive harzburgite xenoliths from the sameoutcrop(FMQ + 0.87–1.15). However, calculations on our harzburgite sampleHzE and metasomatized lherzolite sample MLhB using the sameoxybarometer give a higher oxidation state (FMQ + 1.44 ± 0.03). Theol–opx–sp oxybarometer of Ballhaus et al. (1991) gives lower fO2

values, of FMQ + 0.63 ± 0.11 for MLhA, Lh2, and ALh1, and of FMQ+ 1.19 ± 0.08 for HzE, MLhB, and PHz3. The ol–opx–sp oxybarometerfrom the CTserver (OFM Research; Sack and Ghiorso, 1991) gives evenlower but more consistent fO2 values (FMQ + 0.29 ± 0.08) on all ourlherzolite and harzburgite samples. An oxybarometer solely based onthe ferric iron fraction in clinopyroxene has also been developed byCortes et al. (2006). Using this cpx oxybarometer, and XANES analysesof the Fe3+ content in oriented cpx from the lherzolite sample MLhA,we obtained a fO2 value of FMQ + 0.33, consistent with the results ofthe ol–opx–sp oxybarometer from the CTserver. An average fO2 ofFMQ + 0.3 for the mantle below Puy Beaunit is thus more likely,

although the difference of 0.3 to 0.7 log unit compared to Ballhauset al. (1991) and Bryndzia and Wood (1990) oxybarometers, respec-tively, is unexplained. If a 0.7 log unit correction was applied to all fO2

values determined using Bryndzia andWood's method on similar sam-ples, the whole range of oxygen fugacity of the FMC mantle calculatedby Uenver-Thiele et al. (2014) would be shifted downward by 0.7 logunit fO2. The new fO2 range of the FMC mantle (FMQ − 1.17 toFMQ+ 0.96, average FMQ− 0.02) would still be 0.66 log unit higherthan the oxygen fugacity in normal continental lithosphere (FMQ −4.25 to FMQ + 1, average FMQ − 0.68; Foley, 2011).

Considering the rarity of calc-alkaline magmas, and the earlydegassing of asthenospheric volatile-rich alkaline melts, only theHCSO-rich alkaline silicic magmas formed by the melting of ametasomatized lithospheric peridotite source can explain the high oxi-dation state of the subcontinental lithospheric mantle beneath the FMC.The veins resulting from the percolation of thesemelts can thus be usedto constrain the fO2 evolution of the Upper Lithospheric Mantle. Unfor-tunately, the texture and chemistry of the minerals from the olivinewebsterite vein in MLhB strongly suggest late dissolution and re-equilibration of this vein, leading to fO2 values close to the lherzoliteand harzburgite samples. The vein in MLhA is more preserved, butdoes not contain spinel; therefore, the olivine–orthopyroxene–spineloxybarometers cannot be used. Similarly, the pure olivine websteritesample WbC does not contain spinel; however, it is very contaminatedby the basalt that brought this sample to the surface. The oxidationinduced by melt infiltration in these olivine websterite samples canalso be constrained using the oxybarometer based on clinopyroxenealone from Cortes et al. (2006). XANES analysis on well-orientedclinopyroxenes from WbC shows that the ferric iron fraction (Fe3+/ΣFe) is extremely high (up to 51%), most likely because of the late inter-actions with the basalt. In the lherzolite part of MLhA, the minimum(Fe3+/ΣFe) is ~13%, within the range of “normal” values for cpx fromlherzolites. In the cpx from the olivine websterite vein, Fe3+/ΣFereaches 29% (at the vein center). The iron redox map of MLhA (Fig. 2)was built by combining various point analyses of the Fe oxidationstate in well-oriented clinopyroxenes from the vein and surroundingperidotite (Supplementary Materials 2 & 3). A gradual oxidation ofiron in the cpx appears on a 18 mm distance from the center of the7.4 mm wide vein. This gradient can be fitted using a linear function(Fig. 5):

Fe3þ

ΣFein cpx ¼ 0:872 � Distance=vein axisþ 29:001 ð6Þ

An error function is usually more representative of chemical diffu-sion in crystals. Assuming that the vein was completely crystallizedwhen diffusion occurred, the Eq. (2.15) from Crank (1975) (Eq. (8))fits our data with an initial Fe3+/ΣFe of 11.5% in the peridotite, an initialFe3+/ΣFe in the vein of 59%, and a product of the diffusion coefficientand time (Dt) of 3.4 × 10−5 m2:

Fe3þ

ΣFein cpx ¼ 11:5

þ23:75 erf3:7−Distance=vein axis

11:66

� �þ erf

3:7þ Distance=vein axis11:66

� �

ð7Þ

The high theoretical initial value of Fe3+/ΣFe in cpx at the center ofthe vein (59%) is likely not representative of the oxidation state of theinfiltrating melt, but rather expresses the continuous reinjection ofmelt in the vein, which maintained a high oxidation state (Fe3+/ΣFeprobably ~29% in the cpx) for an extended period of time.

The cpx oxybarometer from Cortes et al. (2006) applied to theXANES analyses on sample WbC gives an average fO2 of FMQ + 4.8(max. FMQ + 6.3), a value that has never been recorded in any mantlesample (Foley, 2011). It is most likely due to late oxidation by the basaltduring eruption (Cortes et al., 2006) or by post-eruption interaction


with air. However, using the measurements on the better-preservedMLhA olivine websterite vein, we calculated a fO2 value of FMQ + 2.35(Figs. 2 & 5, Supplementary Materials 2 & 3), close to the maximumvalue of mantle wedges (~FMQ + 3, Foley, 2011; Malaspina et al.,2010). A comparison with the non-metasomatized (lherzolite) partsof MLhA shows that the percolating volatile-rich silicic melt was ableto locally increase the mantle fO2 by 2.03 log units, from FMQ + 0.32to FMQ+2.35. The infiltration of thismelt can thus explain the high av-erage fO2 of the FMC mantle (FMQ+ 0.68; Uenver-Thiele et al., 2014).Consequently,we propose an extension of the FMCmantle fO2 range de-termined by Uenver-Thiele et al. to include our olivine websterite veinfrom sample MLhA (FMQ − 0.47 b ΔlogfO2 b FMQ + 2.35). Accordingto Cortes et al. (2006), the error on the fO2 calculations using thisoxybarometer is ±2 (2σ). This error is due to their use of stoichiometry(Lindsley, 1983) to calculate Fe3+/ΣFe in the cpx from their experimen-tal dataset, and to temperature and composition variations. The increaseof 2.03 log units fO2 between the lherzolite and the vein in sampleMLhAis slightly larger than the error reported by Cortes et al. (2006). In addi-tion, this increase cannot be an artifact caused by cpx composition var-iations since the cpx from the vein and outside the vein have similarcompositions. It can also not be an artifact caused by temperature vari-ations, which are only 16 °C between the lherzolite and the vein. There-fore, only a fO2 variation can explain this increase in Fe3+ content in thecpx.

A fO2 gradient in the peridotite can thus be calculated in the 18 mmwidth zone on each side of the vein center (from FMQ + 0.33 to FMQ+ 2.35), even if this material did not undergo any mineralogicalmodification:

Δ log f O2

� �=FMQ ¼ 0:108 � Distance=vein axisþ 2:287 ð8Þ

Fig. 6.Volatiles cycle andmantle redox evolution beneath the FrenchMassif Central.Melt compmagmas during the Variscan episodewas not represented, even though they may have been erupper subcontinental lithospheric mantle during the twomainmelting/metasomatism events,(Foley, 2011; Uenver-Thiele et al., 2014).

Cryptic oxidation thus extends ~4.9 times farther thanmodal oxida-tion, implying that the intercrystalline diffusion of O, Fe3+, and/or of asecondary HCSO-rich gas is efficient. The oxidation zone measuredaround the MLhA vein in our Beaunit sample is smaller than thatobserved in a metasomatized spinel peridotite xenolith from Dish Hill,California (McGuire et al., 1991). In this sample, an amphibole+ phlog-opite+apatite vein of at least 5mmwidthwas observed. It is a remnantof metasomatism by a H2O- and K2O-rich silicate melt. Fe3+/Fe in thecpx from the vein is very similar to our sample MLhA (~33%). Fromthe center of their vein, an oxidation distance of 55 ± 5 mmwas mea-sured, which is much larger than in MLhA (18 mm). A longer durationof melt percolation in the sample from Dish Hill may explain thisdifference.

Based on our sample alone, we can calculate the maximum fractionof metasomatic melt that infiltrated the mantle beneath Puy Beaunit.Considering 7.4 mm wide veins overprinting a total width of 36 mm(=2 ∗ 18) each by modal and cryptic oxidation, the redox state of theentire mantle volume can be overprinted by 20% (=7.4 ∗ 100/36) ofveins. The residual presence of non-metasomatized (reduced) portionsof mantle thus requires that the fraction of metasomatic melt that infil-trated the mantle was lower than 20%.

Therefore, recent (b30 Ma) melting of carbonates- ± phlogopite-bearing portions of the lithospheric mantle is at least partly responsiblefor the anomalously high oxidation state of the FMC mantle. Melting ofamphibole may have also played a role, although it is difficult to evalu-ate given the lack of mineralogical and major elements geochemicalmarkers for melts produced from amp-bearing peridotite.

In addition, the widespread 365–286 Ma old hydrous potassicmagmatism (vaugnerites, lamprophyres, K-rich calc-alkaline granit-oids) likely contributed to the global lithospheric mantle oxidation,by similar processes. The only possible source for the volatiles involved

ositions are all attested by field observations; therefore, the potential formation of CO2-richased by erosion. The drawings on the right show the evolution of the oxidation state in thebased on the record by our mantle xenoliths from Puy Beaunit and data from the literature

Image of Fig. 6


in the recent magmatic events is the N–S Lizard–Rhenohercynian sub-duction; carbon was thus already present in the mantle source ofthese hydrous potassicmagmas, althoughwe did notfindany carbonatein our samples. Considering our lherzolite and harzburgite samples(HzE, MLhA, MLhB, Lh2, PHz3, ALh1) as representative of the UpperLithospheric Mantle just after the Variscan magmatic/metasomaticepisode, the average fO2 was at least FMQ + 0.3 between 286 and30 Ma.

A two-stage model of oxidation of the Upper Lithospheric Mantlebelow the Puy Beaunit can be built using our data. Before 365 Ma, theaverage fO2 was probably close to “normal” continental lithosphere,i.e., FMQ − 0.68 down to FMQ − 2.5 (with potential local values aslow as FMQ− 4.25; Foley, 2011). Between 365 and 286Ma, the averageUpper Lithospheric Mantle fO2 progressively increased to at least FMQ+ 0.3, because of the infiltration of HCSO-fluids from the N–S subduc-tion and subsequent volatile-rich magmatism. This fO2 then stayed rel-atively stable until the W–E extension started ~30 Ma ago, inducingnew melting of carbonates, phlogopite, and amphibole, and the migra-tion of thesemelts throughout theUpper LithosphericMantle. The aver-age fO2 then likely increased again, with local values up to FMQ+ 2.35as observed in our olivine websterite MLhA vein.

7. Comparison to worldwide intraplate magmatism

Numerous occurrences of similar volatile-rich alkaline magmatismand/or volatile-richmantle samples have been reported in intraplate re-gions worldwide. Themantle oxidation of all these regions is thus likelyunderestimated, and very high fO2 values (similar to mantle wedges)may be locally measured. In addition, our crystallization-degassingmodels could apply to these magmas, although the pressure of thesource may vary.

Such magmatism appears in many other regions of the EuropeanVariscan Belt affected by lithospheric extension and rifting (Wilsonand Downes, 2006). Volatile-rich alkaline magmatism in the RhineGraben, Rhenish Massif, Bohemian Massif, and Iberian Massif is also re-lated to the Alps formation. For example, b700 Ka old carbonatites,basanites, nephelinites, leucitites, andphonolites from the Eifel, RhenishMassif, are believed to have formed from low-degree melting ofa phlogopite-, amphibole-, and CO2-bearing mantle (Mertes andSchmincke, 1985). Mantle xenoliths with carbonate-rich spherules,phlogopite, amphibole, and volatile-rich silicic melt inclusions havebeen recovered in the West Eifel (O'Connor et al., 1996; Schiano andClocchiatti, 1994). Carbonatites, melilites, phonolites, and olivine-nephelinites also erupted at the Kaiserstuhl stratovolcano, RhineGraben (19–16 Ma; Wang et al., 2014). Amphibole- and garnet-bearing peridotite xenoliths were found at this location. Basanites(21–24 Ma) from the Ohře/Eger rift, Bohemian Massif, contain mantlexenoliths with carbonate-bearing silicate melt pools (Ackerman et al.,2013). Recent carbonatites, melilites, and leucitites are also present inthe Iberian Massif, Spain (e.g., Calatrava, Olot, Cartagena). These lavassampled mantle xenoliths containing phlogopite and amphibole veins,as well as high pressure carbonates (aragonite) included in olivine(Humphreys et al., 2010). Volatile-rich alkaline magmatism of Variscanage is also observed in those regions. For example, 338–335 Ma old po-tassic magmas (durbachites) from the Třebíč pluton, Bohemian Massif,are believed to result from the melting of phlogopite-bearing mantle(Parat et al., 2010). Similar volcanism is observed in other regions affect-ed by the Variscan orogeny, but not by the Alps. For example,carbonatites and strongly undersaturated rocks have been recognizedin the Appalachians (240-90 Ma; Eby, 1987). Mantle xenoliths withphlogopite and amphibole from the Tianshan area, China, witness theinfiltration of hydrous carbonatites and potassic melt in the lithosphericmantle (Zheng et al., 2006).

In addition, similar magmatism is observed in many continental in-traplate regions that were not influenced by the Variscan orogeny.They all underwent mantle metasomatism, followed by lithospheric

thinning caused by tectonic extension, collisional/post-collisional con-ditions, or/and the presence of a hot spot. For example, alkaline and po-tassic lavas (b17 Ma) have been discovered in the Pannonian Basin,Central Europe. They were produced by the melting of a mantlemetasomatized from the Alpine subduction (Wilson and Downes,2006). Carbonatites and melilitites containing mantle xenocrysts of ol-ivine, pyroxene and phlogopite, have been observed at Monte Vulture(~133 Ka), San Venanzo (~265 Ka), Polino (~246 Ka), and Cupaello(b639 Ka), Italy (Stoppa andWoolley, 1997). Alkaline lavas are presentin Sardinia (b5Ma), and Sicily (6.5–4Ma), where inclusions of volatile-rich silicic melt have also been found in mantle xenoliths (Schiano andClocchiatti, 1994). These Sicilianmagmas result from the recentmeltingof a mantle metasomatized during an early subduction (32–13 Ma;Wilson and Downes, 2006). Volatile- and potassium-rich magmashave also been found in North America. In particular, 28–19 Ma oldpotassic volcanic rocks occur on the Colorado Plateau area (e.g., phllamprophyres, Shiprock, New Mexico) and in the transition zone tothe Basin and range province (e.g., potassic lamprophyres, BuellPark, Arizona; Esperança and Holloway, 1987). They are formed by re-melting of metasomatized lithosphere from a Mesozoic subductionalong the west coast of North America. Additionally, Cambrian–Ordovician (664–427 Ma) alkaline rocks and carbonatites are knownin Colorado (e.g., Iron Hill, Gem Park) and New Mexico (e.g., Lobo Hill;McMillan and McLemore, 2004). Inclusions of volatile-rich silicicmelts have been discovered in mantle xenoliths from New Mexico(Kilbourne Hole) and Arizona (San Carlos; Schiano and Clocchiatti,1994). Phlogopite lamprophyres thought to derive from low-degreemelting of a metasomatized phl-bearing peridotite have also beenfound in the western Mexican Volcanic Belt (Carmichael et al., 1996;Righter and Carmichael, 1996). In addition, chemical analysis of xeno-liths from North Tanzanian rifts region, where carbonatites and phono-lites are frequent, reveals the presence of phlogopite formed by volatile-rich metasomatism in the lithosphere during the 650 Ma old Pan-African orogeny (Koornneef et al., 2009). Alkaline magmas from aphlogopite- and/or amphibole-bearing mantle source have been ob-served in theWest Antarctic rift system (Rocchi et al., 2002). Inclusionsof volatile-rich silicic melts have also been discovered in mantle xeno-liths sampled by alkaline lavas from many other continental intraplatesettings worldwide, e.g., in Mongolia, Russia, Vietnam (Schiano andClocchiatti, 1994).

Alkaline lavas with compositions resembling those found in theFrench Massif Central and other continental intraplate provinces arealso observed in oceanic intraplate settings. Alkaline lavas (basalts,melilites, nephelinites) from Hawaii are thought to derive from low de-gree of melting of amantle source with residual phlogopite or amphibole(Yang et al., 2003). The presence of alkaline lavas, carbonatites andvolatile-rich silicicmelts inclusions inmantle xenoliths has been reportedin theKerguelen (Gregoire et al., 2000; Schiano andClocchiatti, 1994) andCanary Islands (Schiano and Clocchiatti, 1994). Carbonatites have alsobeen observed on the CapeVerde Islands (De Ignacio et al., 2012), and ev-idence of mantlemetasomatism by volatile-rich silicic melts in the Socie-ty Islands and Comoros (Schiano and Clocchiatti, 1994).

8. Conclusions

The only possible origin for the high contents of K2O, H2O and CO2 inthe magmas from the French Massif Central is the subduction ofpotassium-rich (e.g., K-feldspar, K-mica, glauconite) and carbon-rich (car-bonates) sediment during the second stageof theVariscan subduction (re-versed subduction, 365–360 Ma; Averbuch and Piromallo, 2012) (Fig. 6).Upon heating, volatile-rich melts were produced. They metasomatizedthe mantle wedge, crystallizing phlogopite, amphibole and carbonates.Subsequent collision induced the melting of the metasomatized mantle,forming potassic lamprophyres, vaugnerites, K-rich calc-alkaline granit-oids and possibly carbonatites, and oxidizing theUpper LithosphericMan-tle. Slab break-off has been suggested to explain themantlemelting at this


stage (Ledru et al., 2001). Around 30 Ma, the FMC entered a W–E exten-sion period related to the alpine subduction (Wilson and Downes,2006). Subsequent mantle decompression triggered a new magmatic/metasomatic episode, particularly marked by the eruption of CO2-richlavas (carbonatites, peperites) in the Limagne Basin and its SE extension,accompanied by potassic silicate melts (Chazot et al., 2003; Chazot andMergoil-Daniel, 2012). The melt inclusions from Beaunit xenoliths werelikely trapped at the same period. This activity extended the oxidationrange of the mantle toward higher values, also increasing its mean fO2.The persistence of carbonate, phlogopite and amphibole in the subconti-nental lithospheric mantle beneath the FMC implies the potential fornewevents of volatile-richmantlemelting and subsequentHCSO-rich vol-canism (e.g., carbonatites, hydrous potassic trachyandesite and trachytes),if mantle temperature conditions are met.

The presence of volatiles in the mantle of numerous intraplateregions worldwide is related to the occurrence of numeroussubductions – and subsequent mantle metasomatism – since theonset of plate tectonics. Melting of preserved metasomatized litho-spheric mantle at intraplate conditions produces volatile-rich magmasthat have the potential to increase the upper lithospheric mantle fO2

to very high values. Considering the worldwide abundance of HCSO-rich alkaline magmatism similar to that occurring in the FMC, the im-portance of volatiles in continental (and locally oceanic) intraplate set-tings is likely underestimated, as well as their mantle oxidation.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.lithos.2017.09.002.

Acknowledgments

Anne Peslier and Loan Le are gratefully acknowledged for theirassistance with the electron microprobe analyses, and MatthewNewville for his supportwith theXANES analyses. A portion of thisworkwas performed at GeoSoilEnviroCARS (TheUniversity of Chicago, Sector13), Advanced Photon Source (APS), Argonne National Laboratory.GeoSoilEnviroCARS is supported by the National Science Foundation —Earth Sciences (EAR-1128799) and Department of Energy —GeoSciences (DE-FG02-94ER14466). This research used resources ofthe Advanced Photon Source, a U.S. Department of Energy (DOE) Officeof Science User Facility operated for the DOE Office of Science byArgonne National Laboratory under Contract No. DE-AC02-06CH11357. L.P.I. Contribution # 2058.

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