Sonderdrucke aus der Albert-Ludwigs-Universität Freiburg
KURT BUCHER Transfer of mantle fluids to the lower continental crust Constraints from mantle mineralogy and Moho temperature Originalbeitrag erschienen in: Chemical geology 83 (1990), S. 249-261
Chemical Geology, 83 (1990) 249-261 249Elsevier Science Publishers B.V., Amsterdam — Printed in The Netherlands
Transfer of mantle fluids to the lower continental crust:Constraints from mantle mineralogy and Moho temperature*
Kurt Bucher-NurminenDepartment of Geology, University of Oslo, N-0316 Oslo 3 (Norway)
(Accepted for publication February 2, 1990)
ABSTRACT
Bucher-Nurminen, K., 1990. Transfer of mantle fluids to the lower continental crust: Constraints from mantle mineralogyand Moho temperature. In: B.K. Nelson and Ph. Vidal (Guest-Editors), Development of Continental Crust throughGeological Time. Chem. Geol., 83: 249-261.
The lower continental crust may receive mantle-derived CO 2—H 20 fluids if the Moho temperature is > 660°C. Belowthis temperature a harzburgitic sub-continental mantle is devoid of a free fluid phase. A free fluid phase may be presentonly locally along faults, thrust and shear zones in the upper mantle beneath normal continental areas (e.g., shields, Her-cynian crust of Central Europe). In such zones the orthopyroxene+olivine assemblage is replaced by various assemblagesinvolving hydrates (serpentine, talc) and/or magnesite. The temperature limit of 660°C for fluid-present conditions in aharzburgitic mantle is independent of Moho depth (pressure).
At temperatures of > 670°C the Moho is generally permeable for mantle fluids. The composition of this fluid is effec-tively controlled by the assemblage orthopyroxene+olivine in the upper mantle under fluid-present conditions. The max-imum CO2 content of a fluid phase which can be transferred from the mantle to the lower crust depends on both Mohotemperature and depth (pressure). Generally, maximum CO2 content increases with T and decreases with P (depth). At700°C and — 30-km Moho depth (8 kbar), a fluid transferred from the mantle may contain max. 40 mole% CO 2. This isnot sufficiently CO 2-rich to explain the formation of low-temperature granulites by a process of dehydration of the lowercrust as a result of the interaction with mantle-derived fluids.
1. Introduction
Mantle-derived fluids may be exported to thecrust across the interface separating the uppermantle from the lower continental crust (crust—mantle interface) essentially by two differentmechanisms: (1) transfer of volatile compo-nents dissolved in a melt phase by magmatransport to the crust (e.g., Touret, 1971, 1985;Frost and Frost, 1987 ); and (2) transfer byflow of a fluid phase (e.g., Newton et al., 1980;Harris et al., 1982; Newton, 1985) . The termMoho will be used synonymous with the crust—mantle interface throughout this paper for
"*Contribution No. 69 of the Norwegian Lithosphere Pro-lgramme (ILP).
simplicity. However, it is clear that the observ-able seismic discontinuity (Moho) not neces-sarily coincides with the crust—mantle bound-ary. Depending on the geological situation theMoho may represent a seismic feature in themantle, in the crust or at the crust—mantleboundary.
The transfer of mantle volatiles to the crustvia a melt phase is probably an efficient pro-cess. On the other hand, mantle fluid release inthe crust may be restricted to local areas suchas pipes, sills or dykes, or it may be limited toareas around cooling intrusions of variousshapes. This type of mantle fluid transfer is notpervasive and will presumably not affect largeareas of regional dimensions or the entire lowercrust. Large volumes of basaltic liquids may,
(0009-2541/90/$03.50 © 1990 Elsevier Science Publishers B.V.
250 K. BUCHER-NURMINEN
on the other hand, be placed along the Mohoby a process referred to as crustal underplat-ing. The melts may give off large volumes ofvolatiles upon cooling. However, the finaltransfer of the volatiles to the lower crust willtake place by pervasive flow of a fluid phase.The mechanism bypasses the buffer restric-tions imposed on mantle fluid transferred bythe mineralogy of upper-mantle rocks.
Production of volatiles in the upper part ofthe subcontinental mantle can be conceived bya process similar to regional crustal metamor-phism. Volatiles temporarily stored in themantle in carbonates and hydrates may be re-leased into a fluid phase by adding heat to themantle. Pervasive flow of a fluid phase whichtransports volatiles in the mantle and across theMoho may be accomplished exclusively by hy-drofracturing (Brenan and Watson, 1988 ).CO2–H2O fluids in olivine-rich mantle mate-rials have wetting characteristics which re-duces pore connectivity to zero, thus prevent-ing porous flow (Watson and Brenan, 1987 ) .Potentially large volumes of mantle volatilesmay be produced and may stream into thelower crust. The flooding of the lower conti-nental crust by mantle-derived volatiles mayhave important consequences for the natureand evolution of the lower crust and for thethermal structure and rheology of the conti-nental lithosphere (e.g., Newton et al., 1980) .Therefore, it appears important to examine therestrictions on fluid flow and fluid composi-tion imposed by the mineralogy of the mantlerocks and by the pressure–temperature regimealong the Moho.
This paper explores the necessary condi-tions for fluid transport across the crust–man-tle interface. In addition, it investigates the re-strictions imposed on the composition ofmantle-derived CO2–H2O fluids by the mantlemineralogy. It will be shown that the condi-tions for fluid flow across the Moho and thecomposition of this fluid largely depend on thepressure and temperature at the crust–mantleboundary. Wyllie (1978) presented an analy-
sis and discussion of mantle fluid composi-tions controlled by carbonate-, mica- and am-phibole-involving equilibria in lherzolites.' Ikehas been mainly interested in undepleted per-.idotites, near-solidus conditions, melt forma-tion and deeper parts of the upper mantle. Thepresent study is concerned with the uppermantle directly below the Moho and with ther-mal regimes typical for the continentallithosphere.
2. The mantle material
The upper mantle beneath the continentalcrust is dominated by harzburgitic rock com-positions. Mineralogicaly upper-mantle peri-dotites are essentially composed of a forsterite-rich olivine + orthopyroxene ± spinel. Most ofthe meta-peridotite fragments occurring in, forexample, the Scandinavian Caledonides intypical continental associations are either oli-vine–orthopyroxene rocks or hydrated/car-bonated versions of such meta-harzburgites(Bucher-Nurminen, 1988 ). The subcontinen-tal upper mantle constitutes a very large, inex-haustible reservoir of harzburgite ( depletedperidotite) which owns the capacity to controlthe composition of a fluid phase containedwithin it very efficiently. In the following, theequilibrium constraints imposed on the com-position of a fluid phase in mantle rocks willbe explored by considering a binary orthopy-roxene–olivine system.
The presence of substantial amounts of cli-nopyroxene together with olivine and ortho-pyroxene (lherzolites) will only have minoreffects on the calculated fluid compositions.
3. The fluid phase in the upper mantle
Mantle fluids are probably rich in CO 2. Theevidence comes from several fluid inclusionstudies on mantle xenolith material (e.g.,Amundsen, 1987 ) . Carbonates ( dolomite andmagnesite) were also reported from mantlerocks (McGetchin and Besancon, 1973;
TRANSFER OF MANTLE FLUIDS TO THE LOWER CONTINENTAL CRUST
251
Amundsen, 1987) which testifies the impor-tance of CO2 as an important volatile speciespresent in the mantle. Navon et al. (1988) re-ported molecular N 2-, H2O- and CO2-rich mi-cro-inclusions in diamond. The inclusions alsocontain carbonate minerals and hydrous sheetsilicates. The presence of amphibole and micain some mantle peridotites is strong evidencethat H2O is present at least in some parts of themantle (e.g., Dawson and Powell, 1969;Amundsen, 1987; Dawson and Smith, 1988).Much of the volatiles stored in the upper man-tle are probably of crustal origin. Crustal CO2and H2O can be recycled to the mantle by sub-duction of crustal material or by fluid flowalong deep faults and shear zones penetratingthe Moho. Although other volatile compo-nents such as He, H2, N2 and others may bepresent, the two components CO2 and H2O arevolumetrically dominant in the uppermostmantle. In the following discussion a binaryCO2—H20 fluid will be considered. The twovolatile components may form a discrete bi-nary fluid phase in the mantle. This fluid phase,if present, occupies isolated pores and poreconnectivity is very low or absent for CO2-richfluids. Considering harzburgite as the domi-nant rock type in the subcontinental mantle re-stricts potential storage minerals for the twovolatiles to magnesite for CO 2, and talc, antho-phyllite and antigorite for H2O.
4. Consequences of mantle fluid flow for thelower continental crust
There are four different scenarios for flow ofa CO2—H20 fluid from the mantle to the crust:
(1) The crust—mantle boundary is im-permeable for mantle fluid flow. In this case,the lower crust receives only heat from themantle. Volatile transfer to the crust is relatedto magmatic or tectonic activity.
(2) The Moho is permeable for mantle fluidsbut the amount of fluids transferred is too lowto cause any appreciable effects in the crust, orthe high flow may be strongly focussed and will,
therefore, only affect local crustal domains.(3) The flow can be high and pervasive and
the fluid composition is H 2O-rich. If the tem-perature of the lower crust is high enough, thisfluid could induce partial melting of materialof suitable composition.
(4) High, pervasive flow of a CO 2-rich man-tle fluid across the Moho. This may cause thedehydration of the lower continental crust andthe formation of granulite-facies terrains.
The term "carbonic metamorphism" hasbeen coined by Touret (1970) for this generalprocess.
5. Granulites and mantle carbon dioxide
Granulite-facies terrains have been meta-morphosed in the pressure range 7-12 kbar.Typical granulites were heated to tempera-tures of 800-900°C. Granulites reported byLamb et al. (1987) from the AdirondackMountains, New York, U.S.A., equilibrated attemperatures of — 650-750°C (low- T granu-lites) . The transition from amphibolite faciesto granulite facies can be described by a gen-eral dehydration process, where H 2O stored inhydrous minerals such as amphibole and micais released into a fluid phase (or a melt phaseduring a partial melting process), leaving be-hind an assemblage of anhydrous minerals.This transition occurs at temperatures ofN 850 ± 50°C in the presence of an H2O-richfluid phase. The transition can be modelled bya simple reaction in the pure MgO— Si02—H20-CO2 system (MS—HC system) :
Mg7Si8022 ( OH ) 2 7MgSiO3 + Si02 + H20 (1)
anthophyllite enstatite quartz fluid
The reaction describes the formation of the as-semblage orthopyroxene (enstatite) + quartzwhich is diagnostic for the granulite facies fromthe decomposition of a hydrous phase (am-phibole) . The equilibrium conditions for re-
252 K. BUCHER-NURMINEN
1000 Any fluid given off by the mantle at this tem-perature would contain max. 15 mole% CO2.Therefore, it would stabilize the amphibolite-facies assemblage.
6. Pressure and temperature at the Moho
U^w 800
a-I-<a_W 600a-2w
//(2///0/4//,4000 0.6 0.8
XCO2
Fig. 1. T-X diagram at 10 kbar (39-km depth) showingthe equilibrium conditions for the model reaction (1)separating the amphibolite facies (hatched) from thegranulite facies.
action ( 1 ) are shown in Fig. 1*. This figureshows that at 10-kbar pressure which corre-sponds to - 38-km depth, the granulite-faciesassemblage requires ti 800° C in the presenceof a water-rich fluid phase. The figure alsoshows that the anhydrous assemblage could beproduced at 700° C by introducing a fluid withXco2 > 0.65 to the amphibolite-facies assem-blage and exhausting its buffer capacity (at600°C, Xc02 must be > 0.85) . This mecha-nism of granulite formation could, at least inprinciple, explain the occurrence of low-tem-perature granulites. It has been a subject of in-tense discussion in the recent years for whattypes of geological scenarios this mechanism isfeasible (e.g., Touret, 1970, 1971; Lamb et al.,1987) . Newton et al. (1980) proposed that theexternal CO 2-rich fluid required for the dehy-dration of the lower crust could be obtainedfrom the mantle. It will be shown that the man-tle harzburgite reservoir buffers the fluid com-position to Xc02 < 0.15 (700 °C and 10 kbar) .
*Fig. 1 and all other phase diagrams have been calculatedusing the code PTX-system by Perkins et al. (1986) withsome Oslo specific modifications. Thermodynamic datafor solids are those given by Berman (1988 ). Gas data arefrom Kerrick and Jacobs (1981 ) . Standard state for allcomponents is unit activity for the pure compound at givenP and T. All abbreviations of mineral names after Kretz(1983).
The question of fluid transfer across theMoho is closely associated with the prevailingpressure and temperature conditions along thecrust-mantle boundary. In Fig. 2 some mod-ern geothermal gradients of typical continentalareas are shown together with the position ofthe Moho along the different gradients. Thetwo gradients from the European Alps aretaken from Rybach et al. (1977) and representthe gradient in the Northern Alpine forelandand at the position of the maximum crustalthickness of the Alpine root zone along the"Swiss Geotraverse, Basel-Chiasso." The Al-pine geotherms show that at the same depth thetemperature in the root zone is - 100°C lowercompared to the foreland. The maximum crus-tal temperatures in the root zone are N 900°C,but the pressure is also very high (an averagecrustal density of 2.6 g cm- 3 has been used toconvert distances to pressures) . The two other
16
14
2300 400 500 600 700
TEMPERATURE (°C)
Fig. 2. Four characteristic continental geotherms (shield,Hercynian crust, Central Europe, Alpine root zone) withposition of Moho (M in hexagon). Dash-point line = po-sition of the Moho along the Norway-Danmark profile(distance - 250 km). Superimposed: phase relationshipsfor harzburgite rock composition in the presence of pureH2O fluid.
800
900
TRANSFER OF MANTLE FLUIDS TO THE LOWER CONTINENTAL CRUST 253
gradients are taken from Balling (1985) andshow the P,T distribution in the lithospherebeneath Danmark and southern Norway. TheMoho depth under the two areas is similar (30and 35 km) . However, the modelled tempera-ture field shows large isobaric temperaturegradients along the Moho. the temperature dif-ference of 350°C (Fig. 2) at the Moho depthfor the two areas occurs along a profile of '' 170km. This results in an average temperaturegradient of - 2 °C km- 1 along the Moho andcauses Moho-parallel heat transport. Along theprofile between Norway and Danmark inter-mediate gradients link the two extreme P,Tdistributions and the Moho temperature var-ies as indicated along the point-dashed line inFig. 2. Note that the Alpine foreland gradientand the Danmark gradient are nearly identicalalthough the Moho depth is slightly different.The geotherms are characteristic for the Her-cynian crust in Central Europe.
7. Carbon dioxide-absent environments
The calculated phase relationships in theMSH system in the presence of pure H2O as afluid phase are superimposed in Fig. 2. Onlyphase boundaries relevant for harzburgiticcompositions are shown. Within the P,T win-dow of Fig. 2 there are only five different two-phase assemblages stable. Fig. 2 shows thatharzburgite (enstatite + forsterite) is stable inthe presence of pure H 2O at least down to700°C and pure H2O could pass the Moho attemperatures as low as 650 °C. At lower tem-peratures H2O will be consumed by heat-pro-ducing hydration reactions involving harzbur-gite. The product assemblage depends on Mohodepth. The upper mantle in the Alpine root areais in the En + Fo field; however, in the Alpineforeland the Moho is in the Tic+Fo field. Thismeans that mantle in this area consists oftalc + forsterite (in the presence of excessH20) . Because of the large amount of harzbur-gite present in the upper mantle, it is probablethat all H2O supplied to the mantle will be
quantitatively consumed by the reaction:
5MgSiO3+H2O#
enstatite fluid
Mg3Si4O 10 (OH) 2 + Mg2SiO4(2a)talc forsterite
All H2O reaching the mantle region immedi-ately below the Moho either through deep faultsand shear zones from the crust or by transportfrom deeper areas of the mantle will be storedin a hydrous silicate [talc, anthophyllite, anti-gorite ( serpentine) depending on Mohodepth ] . The two reactions which produce am-phibole or serpentine are:
9MgSiO 3 + H20
enstatite fluid
Mg2SiO4 + Mg7Si8O22 ( OH ) 2forsterite anthophyllite
1 4Mg2SiO4 + 20MgSiO3 + 31 H20 ,=%.
forsterite enstatite fluid
Mg48Si34085 (OH ) 62
antigorite
These areas of the mantle will be fluid-ab-sent and in the case of the Alpine foreland thestable fluid-absent assemblage will beTlc + Fo + En. Fluid saturation will probably berestricted to faults and shear zones where fullyhydrated meta-harzburgites may be present (cf.Warner and McGeary, 1987).
The Moho along the profile between Nor-way and Danmark (Balling, 1985) passesthrough four of the divariant fields shown inFig. 2. The Moho beneath Danmark is trans-parent for pure H 2O; however, it will be "waterproof" immediately north of the coast of Dan-mark and towards Norway. Beneath Norwayand the Scandinavian Shield in general theMoho is in the stability field for brucite andantigorite. This is remarkable and has some in-teresting consequences. Harzburgitic uppermantle becomes impermeable for pure H2Ofluids at 650-700°C if enstatite (OPX) is not
(2b)
(2c)
254 K. BUCHER-NURMINEN
exhausted in the hydration reactions. This willnormally be the case resulting from the largeharzburgite reservoir. If the upper mantlewould consist of pure dunite ( olivine rock) theMoho could stay open for fluid transfer to thecrust to temperatures as low as 430°C (at 33-km Moho depth) (Fig. 2 ) . In shield areas theupper mantle is probably characterized byfluid-absent assemblages with the exception oflocalized areas related to faults and shear zoneswhich may contain high modal concentrationsof serpentine, talc or even brucite, and locallypossibly also a hydrous fluid. Some of thebright, horizontal and dipping reflectors in theupper mantle beneath the continental crust ofGreat Britain (Smythe et al., 1982; Brewer etal., 1983; McGeary and Warner, 1985) couldbe seismic signals of such hydrated upper-mantle shear ones (Warner and McGeary,1987 ) . Recently, Boillot et al. (1988) directlyobserved large volumes of serpentinized uppermantle beneath the continental margin of Gal-icia, Spain.
8. Transfer of CO2-H20 fluids to the crust
Hydration reaction (2a), or equivalent onesinvolving antigorite (2c) or anthophyllite (2b )instead of talc, limits the harzburgite assem-blage in H2O-rich fluids. The limiting mineralis orthopyroxene (enstatite) . The crucial min-eral reaction which controls the compositionof a CO2-rich fluid phase in a harzburgiticmantle is:
Mg2SiO4 +CO2 ,MgSiO3 +MgCO3(3 )
forsterite fluid enstatite magnesite
The limiting mineral is olivine ( forsterite ).The equilibrium conditions for reaction (3 )are shown in Fig. 3 in terms of the variablespressures and fluid composition for a Mohotemperature of 800°C. At this high Moho tem-perature, harzburgite can be in equilibriumwith CO2-H20 fluids of any composition pro-vided the continental crust does not exceed athickness of 35 km. With increasing Moho
14 . ^^_ T = 800 °C MGS+QTZ
Fig. 3. P-X diagram showing the phase relationships inthe MSHC system at a Moho temperature of 800°C(hatched area = harzburgite window) .
depth, harzburgite will buffer the fluid com-position to progressively more H2O-rich com-positions. At the base of a 60-km-thick crust, afluid phase with a maximum of 10 mole% CO2could be transferred from the mantle to thecrust. At 800°C the Moho is permeable forfluids irrespective of Moho depths. At P> 9kbar, the composition of this fluid depends onMoho depth and is controlled by reaction (3 )(Fig. 3 ) . The conditions for the presence of aCO2-H20 fluid in harzburgitic rocks could betermed "harzburgite window" for fluid trans-fer to the crust. The harzburgite window is theruled field in Fig. 3.
At a more moderate Moho temperature of700°C (e.g., under Danmark; Fig. 2) the harz-burgite windows is much smaller (Fig. 4 ) . Afluid with a maximum XCO2 of 0.5 could betransferred to the crust at a Moho depth of 30km ( 8 kbar) . With increasing Moho depththe maximum CO2 content of a fluid "to be"transferred from the mantle to the crust de-creases very rapid and is < 3 mole% CO 2 at 60km. The buffer capacity of the mantle assem-blage may be exhausted locally along shearzones and fractures and the correspondingmagnesite-bearing assemblages may be pres-ent along veins. Please note that the harzbur-gite window also shows a limit on the H2O-richside at low pressures.
T .700°C P •11 kbar depth - 42 kmb EN+ IGIGS
d Q TZ+MGSTLC+MGSc
010m 20 0/065^ole% CO2
FO EN
a
TRANSFER OF MANTLE FLUIDS TO THE LOWER CONTINENTAL CRUST
255;
Fig. 4. P-X diagram showing the phase relationships inthe MSHC system at a Moho temperature of 700°C(hatched area. harzburgite window).
The phase relationship at 700°C and a Mohodepth of 42 km are shown in Fig. 5. The diag-onally hatched triangles represent fluid-pres-ent assemblages. Harzburgite does not tolerate> 10 mole% CO 2 under these conditions. Athigher CO2 concentrations three differentmagnesite-involving assemblages ( Fig. 5b–d )are stable. Therefore, the upper-mantle harz-burgite assemblage can coexist with a veryH2O-rich fluid phase or it may contain mag-nesite as an additional phase to orthopyroxeneand olivine in the absence of a free fluid phase(horizontally ruled triangle in Fig. 5 ). Locally,forsterite-absent assemblages, such asMgs + En + fluid or Mgs + En + Tlc may occur
near transport channels for CO 2-rich fluidsfrom deeper parts of the mantle.
The phase relationships for a Moho temperature of 675°C are shown in Fig. 6. Because ofthe enlargement of the stability fields for an-thophyllite and talc in forsterite-saturatedrocks, the harzburgite window is limited alsoon the H 2O-rich side of possible fluid compo-sitions down to 38 km (10 kbar) Moho depth.A Moho at 30 km (8 kbar) can be penetrated.by a fluid which contains > 10% but <25mole% CO 2 . The harzburgite window has be-come very narrow at this temperature whichhas some important consequences for the for-mation of low-temperature granulites. High-CO2 fluids cannot be transferred to the crust,even if the crust is very thin. The harzburgiticmantle beneath thick continents (> 40 km )restrict the composition of a free fluid phase toextremely H 2O-rich compositions.
Fig. 7 shows that the harzburgite window isclosed at 650 °C. No fluid of any compositioncan be transferred from the mantle to the crustunder these conditions which correspond tothose at the base of the crust in the Alpineforeland for example (Fig. 2 ) . Fluid-presentconditions in the mantle require En- or/andFo-absent assemblages. At the base of a typical35-km crust (9 kbar ), Fo + Tlc permits thepresence of an H 2O-rich fluid phase whereasMgs + Tlc is consistent with a fluid of inter-
Fig. 5. Phase relationships at 700°C and 11 kbar (- 42-km depth) as a function of fluid composition (diagonal pat-tern. fluid-present assemblages; horizontal pattern.- fluid-absent, harzburgite-excess assemblage) .a. Fluid present in harzburgite at CO 2 < 10 mole%. b-d. Harzburgite is fluid-absent.
2
256 K. SUCHER-NURMINEN
Fig. 6. P-X diagram showing the phase relationships inthe MSHC system at a Moho temperature of 675°C(hatched area= harzburgite window). Note, the isother-mal invariant assemblage Fo + En + Mgs+ Tlc is at P» 16kbar at this T (intersection of the equilibria FoEn + Mgs and En Mgs + Tlc, respectively }.
0.2
0.4
0.6
08
xCO2
Fig. 7. P-X diagram showing the phase relationships inthe MSHC system at a Moho temperature of 650°C(hatched area= harzburgite window) . The Moho is im-permeable for mantle fluids at this T. Note, the isother-mal invariant assemblage Fo + En + MgS+ Tlc is at -6kbar (23 km) which corresponds to the maximum depthof harzburgite window at this T (there is a very smallAtg+Mgs field separating the Atg+Fo and Mgs+Tlcfields, respectively, which has been omitted here ).
mediate composition (Fig. 7 ) . The dramaticchange in the topology of phase relationshipsbetween 675° (Fig. 6) and 650°C (Fig. 7) isa consequence of a very steep dP/dT slope forthe assemblage Fo + En + Mgs + Tlc in P, Tspace at P> 5 kbar. The assemblage corre-sponds to the invariant assemblage of the high-pressure limit of the harzburgite window in Fig.
7. The consequence of this is that the Mohobecomes locked for pervasive fluid transfer byflow for any mantle CO 2—H2O fluids at a tem-perature of 660°C irrespective of Mohodepth.
At a temperature of 550°C the assemblageorthopyroxene (En) + olivine (Fo) is re-stricted to very low pressures (> 2 kbar) andCO2-rich fluid compositions (Fig. 8 ) . Notethat a Moho temperature of 550°C is still rel-atively high, — 150-200 higher than in shieldareas (e.g., Fig. 3 ) . A harzburgitic mantle isunable to transmit CO 2—H 2O fluids at thistemperature; however, it will resorb all crustalfluids which may reach the upper part of themantle through faults and shear zones for "lateruse". This also applies to fluids released fromcrustal rocks in subduction zones. H 2O will bestored in serpentine (antigorite) and CO 2 willbe stored in magnesite. With "later use" ismeant here that the sub-Moho mantle can po-tentially act as an important, temporary stor-age device for large amounts of volatiles whichmay be released during subsequent periods ofincreased heat supply to this volume of the up-per mantle.
Fig. 8 clearly suggests the possibility of man-tle-derived serpentinites and soapstones. Ifsuch rocks are found exposed on the surface (in
16
14
12
10
8
6
4
2
T :550°C
0 0.2
0.4
0.6
0.8
cot
Fig. 8. P-X diagram showing the phase relationships inthe MSHC system at a Moho temperature of 550°C. Theharzburgite window is restricted to P< -2 kbar. Note, theantigorite assemblages in the Moho depth range, with H 20-rich fluids.
x
0 4
XCO 2
02 06 08
TRANSFER OF MANTLE FLUIDS TO THE LOWER CONTINENTAL CRUST 257
continental associations) they are normallyconsidered of crustal origin. The stable-iso-tope signatures of mantle serpentinites andsoapstones are unlikely very different fromthose of crustal origin if the volatiles are ofcrustal origin and have been introduced to themantle along faults and thrust [ e.g., such as theones seen on deep seismic reflection profiles(Warner and McGeary, 1987) ] .
8.1. Non-binary fluids
The assemblage olivine–orthopyroxene–spi-nel buffers foe to values which, together withthe CO2 buffer of reaction (3 ), are close to orwithin graphite saturation. If graphite is pres-ent in the rocks, reaction (3) fully controls thecomposition of a fluid phase in the C-0–Hsystem and CO2 is the dominant carbon-bear-ing fluid species under the consideredconditions.
If the fluid phase contains substantialamounts of NaC1 (KC1), additional compli-cations may arise. It has been pointed out byTrommsdorff and Skippen (1986) and Skip-pen and Trommsdorff (1986) that it is impos-sible to predict a unique fluid composition inthe ternary NaCI–H 2O–0O2 system on the ba-sis of mineral assemblages alone. They alsoshowed that two different fluid phases may co-exist with a buffer assemblage such as forster-ite–enstatite–magnesite [reaction (3) ] at agiven pressure and temperature. The CO2buffer assemblage controls the activity of CO2(rather than the composition of the fluid) andthe activity–composition relationships may becomplex. Unfortunately, experimental data onmulticomponent fluid systems at the condi-tions of interest in this paper are lacking.Trommsdorff and Skippen (1987) presentedgeneral topologies of the ternary fluid systemat high pressures and temperatures. From thesetopologies, it can be concluded that the pre-dicted binary H 2O–0O2 mole fractions will bevery similar in ternary NaCl-rich fluids (idealsolutions) . At low temperatures (Fig. 8 ), fluid
unmixing in a NaCI-rich system may evolve.However, the unmixed fluid would be moreH2O-rich compared to the fluid calculated inthe binary system.
8.2. Fluids buffered by lherzolites
The presence of clinopyroxene in mantlerocks has the consequence that two new bufferreactions control the composition of the fluidphase:
2Mg2SiO4 + CaMgSi2O6 + 2CO2forsterite diopside fluid
4MgSiO3 + CaMg (CO 3 ) 2 (4 )enstatite dolomite
6Mg2 SiO4 + 13CaMgSi206 + 10002 +4H2Oforsterite diopside fluid fluid
4Ca2Mg5 Si8O22 (OH ) 2 + 5CaMg ( CO 3 ) 2 (5 )tremolite dolomite
Reaction (4) consumes CO2 and replacesclinopyroxene by dolomite. Reaction (5) con-sumes both CO 2 and H2O. The equilibriumconditions for both reactions are on the H 20-rich side of reaction (3) in Fig. 9 for all iso-thermal sections. This means that the lherzo-lite assemblage forsterite + diopside (+ ortho-pyroxene) buffers mantle fluids to more H20-
Fig. 9. P-X diagram showing the equilibrium conditionsof the reaction Mgs+En Fo+CO2 at a series of Mohotemperatures between 650° and 900°C.
258 K. BUCHER-NURMINEN
rich compositions than the harzburgite assem-blage forsterite (+ orthopyroxene) . However,the magnitude of this effect is small. At 700°Ca maximum difference inXc02 of 0.07 has beencalculated at 8 kbar (harzburgite = 0.35;lherzolite = 0.28) .
In rocks containing calcic pyroxene (lher-zolite ), the first carbonate will be dolomite ifthe rock experiences reaction with CO2-richfluids. If calcic pyroxene is absent, magnesitewill form from reaction (3) . Because calcic py-roxene occurs in subordinate modal amountsin typical lherzolites (5-10 vol.%) it may becompletely consumed by reaction (4) or (5) .If this happens, the composition of the fluidwill eventually, become controlled by reaction(3).
9. Summary and conclusions
Fig. 9 summarizes the equilibrium condi-tions of reaction (3) in the temperature range650-900°C. This reaction constrains the max-imum CO2 content of a fluid phase in equilib-rium with orthopyroxene and olivine (theharzburgite assemblage) . Therefore, it also re-stricts the composition of a fluid phase whichcan be transferred from the mantle to the lowercontinental crust. Below — 660°C the Moho isimpermeable for pervasive fluid flow across theMoho. At slightly higher temperature(' 670°C ) the Moho becomes transparent forH2O-rich fluids. This temperature is indepen-dent of the position of the Moho. With in-creasing Moho temperature the maximum CO2content of the fluid which is permitted by theharzburgite assemblage increases for a givenposition of the Moho. A 30-km-thick crustneeds "to be" at least at 770°C as its base inorder to be permeable for pure CO 2 . In active(or young) collision belts the Moho often isunusually hot. However, the temperature ef-fect on the composition of a fluid is compen-sated by the increased thickness of continentsin collision areas. Fig. 2 shows the geotherm atthe position of maximum crustal thickness in
the Central Alps. The temperature at the Mohois near 900°C but the crust is — 60 km thick(— 16 kbar), which restricts fluids in the up-per mantle to Xc02 < 0.4.
The isotherms shown in Fig. 9 also havesome implications regarding the formation oflow-temperature granulites (e.g., the Adiron-dack example mentioned on p. 251). Fig. 1shows that at 700°C (an average T for the Ad-irondack granulites; Lamb et al., 1987) granu-lite-facies assemblages (OPX + Qtz) can beformed by flushing the lower crust with an ex-ternally derived fluid with a minimum Xc02 of> 0.65 if one accepts reaction (1) as a reason-able description of the amphibolite—granulitefacies transition. The pressure estimate for theAdirondack granulites is 7-8 kbar (27-31 km) .At this pressure the equilibrium temperaturefor reaction (1) is slightly higher (— 10°C )than at 10 kbar. Granulite formation willtherefore, requireXco 2 > 0.7. If we assume thatthe Moho was close to 8 kbar the maximumXCp2 of a mantle-derived fluid which couldhave flooded the lower crust is — 0.35. Such afluid would stabilize the amphibolite-facies as-semblage rather than dehydrate the lower crust.It seems, therefore, impossible to generate low-temperature granulites by flooding the lowercrust with mantle-derived CO2—H2O fluids.This is simply because a fluid-present harzbur-gitic upper mantle is only possible if this fluidis H2O-rich. Normal granulites (T> 800°C )will also form in the presence of an H2O-richfluid (Fig. 1) and do not necessarily requirefluid dilution with CO 2 . The origin and signif-icance of CO 2-rich fluid inclusions present inmost granulite-facies rocks is disputed (Kreu-len, 1987; Hollister, 1988 ).
Lastly, it should be mentioned that carbon-ate-bearing ultramafic rocks are far moreabundant in the Scandinavian Caledonidesthan carbonate-free ones (Qvale and Stigh,1984; Bucher-Nurminen, 1988). Most of theAlpine-type peridotites along the coastal areaof central Norway are characterized by the as-semblage Fo + En + Mgs (or En + Mgs
range of possible fluid compositions
TRANSFER OF MANTLE FLUIDS TO THE LOWER CONTINENTAL CRUST 259
sagvandite, Fig. 9 ) . These rocks can be used asmodel rocks for partially carbonated parts ofthe mantle. If such rocks are heated in the pres-ence of a CO 2—H20 fluid (e.g., because of iso-baric temperature gradients along the Moho;fig. 2) reaction (3) would run to the left-handside producing CO2 . The fluid would be re-moved from the site of production by hydro-fracturing which represents the only mecha-nism to transport CO 2—H20 fluids in themantle (Brenan and Watson, 1988) in addi-tion to transport in a melt phase.
9.1. An example from the Central Alps
Rybach et al. (1977) presented geophysicaldata on the temperature field along a NW—SE-running profile across the Central Alps ("Geo-traverse") together with the position of theMoho along the same profile ( from Kahle etal., 1976 ) . In Fig. 2 the two extreme geothermshave been shown ( foreland and root) . Fig. 10shows the Moho temperature and Moho pres-sure along the profile using the same datasource. The pressure axis has been inverted inorder to get a visual representation of thethickened crust in the root area. The presentcontinental crust in the northern foreland area
is .— 30 km thick, in the root zone (beneath theNorth Pennine nappes of the Ticino area, It-aly) the crust is nearly twice as thick (60 km) .As noted by Rybach et al. (1977) the Moho isclearly not isothermal. The data can be used toestimate the range of possible fluid composi-tions for mantle-derived fluids crossing theMoho along the profile (Fig. 10 ) . The Mohobeneath both, the northern and southern fore-land areas are impermeable for mantle fluids.From the Molasse basin on southwards to thearea near the Insubric Line the Moho stays po-tentially open for transfer of mantle fluids tothe crust. The possible fluid compositions arehatched in Fig. 10. It is remarkable that themost CO2-rich fluids could be transferred in theexternal zones of the Molasse basin and alongthe Northern Alpine front. Beneath the win-dows of the Hercynian basement ) "Aar andGotthard massifs") only very H 20-rich fluidsmay be able to penetrate the Moho which couldinduce partial melting of the crust above theMoho. Maximum XCO2 of fluids crossing theMoho in upward direction in the area of max-imum temperature and crustal thickness is
0.4-0.5, which would not be sufficient toprevent excessive crustal melting under fluid-present conditions.
1000900
800700600
8
1416
-10
-1273
^̂
=
MOHO PRESSURE
-MOHO TEMPERATURE
SE
Foreland / Molasse Basin / Helvetic /Windows /Penninic/Insubric Line
CO2 / 1.00.8 x
-0.6 00.4N'
0.2
050
100
150
200km
Fig. 10. A. Moho pressure and temperature along a NW-SE profile ("geotraverse") through the Central Alps.B. Range of possible fluid compositions in equilibrium with harzburgite at Moho P and T along the profile.
H20(B)
260 K. BUCHER-NURMINEN
Acknowledgements
The support from the Norwegian ScienceFoundation (NAVF grant No. D.40.31.109) isgratefully acknowledged. Thorough, carefuland constructive reviews by Jacques Touretand Jacques Kornprobst are gratefully ac-knowledged. Special thanks go to Philipe Vidalfor being a patient Editor (with impatient au-thors) of this special issue.
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