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Home , Lherzolite, Peridotite, Pyrolite, Upper mantle (Earth)

Magmatic Processes: Physicochemical Principles © The Geochemical Society, Special Publication No. I, 1987 Editor B. O. Mysen

Mantle-derived -roles of variable source and variable C-H-O fluid compositions

D. H. GREEN,T. J. FALLOONand W. R. TAYLOR Department, University of Tasmania, GPO Box 252C, Hobart, Australia 7001

Abstract-The system -nepheline-quartz is a useful simple system analogue of relations in upper peridotite. The liquidus phase fields at 28 kbar differ from those at low pressure by expansion of the field at the expense of forsterite. The system illustrates a large field of liquid compositions, from model basanites to model quartz tholeiites, which can be derived from one peridotite source. More refractory source compositions permit a greater compositional range of derivative liquid compositions than more fertile compositions and in particular are required as source or parent compositions for enstatite-rich liquids.

The effects of C-H-O volatiles on melting relationships have been explored with H20, CH4 and COr vapour saturated experiments. The effect of water is to expand the field and depress liquidus temperatures by 350-400°C, but liquids at low degrees of melting of a model peridotite remain nepheline-normative. The effect of CO2 is most marked with liquids moving to increasingly undersaturated compositions. Methane saturation produces a similar liquidus depression but results in OH- solution, low carbon solubility and a reduced melt structure, i.e., Si:O < 1:2. The studies in the simple Fo-Ne-Qz system are matched by melting studies of several peridotite compositions and by liquidus studies on a variety of magnesian primary magmas from different tectonic settings. Mid-ocean ridge are most commonly deriviative from picritic parents at 15-20 kbar although some low-olivine to quartz tholeiite liquids are probably primary from ap- proximately 8 kbar pressure. However, other primary magmas such as high- quartz tho- leiites and olivine-poor tholeiites and the very siliceous, low-calcium liquids, are derived from much more refractory source rocks than MORB and require two-stage or multistage melting processes. The role of C-H-O fluids in fluxing such multi-stage melting on convergent margins is

very important. An additional source of water, accompanied by lowf02 conditions (i.e., H20> CH4 fluids) is identified in the redox-interaction of oxidized with CH4 > H2 fluids degassing from the deep .

INTRODUCfION It is possible that the mantle source regions for primary magmas may be relatively homogeneous IN SEEKINGto unravel the complexities of in major and compatible elements but quite widely genesis in the earth's , experimental variable in incompatible elements, i.e., those which petrologists have successfully demonstrated the di- are perceived as mobile because of high solubility versity of basaltic magmas which can arise from the in mantle fluid phase(s) or in small, volatile-rich same source composition by variation of pressure melt phases. This view finds support from studies (depth of magma segregation) and temperature of mantle samples which provide evidence (degree of ) (see BASALTICVOLCA- for multievent histories including late stage en- NISMSTUDYPROJECT,1981,and referencestherein). richment in incompatible elements (BASALTIC In addressing the same problem, geochem- VOLCANISMSTUDYPROJECT,1981) and from the ists have demonstrated that mantle-derived mag- recognition of mantle-derived which are mas have formed from isotopically different sources extremely depleted in incompatible elements yet or reservoirs which have remained isolated and with retain major element chemistry with large 'basaltic considerably different ratios of radiogenic elements component' (CaO, Ah03 > 3 weight percent; FREY for long periods of time (BASALTICVOLCANISM and GREEN, 1974; MENZIES,1983; KURATet al., STUDYPROJECT, 1981, and references therein). 1980; FREYet al., 1985). Some studies of trace element have In this paper our major concern is to demonstrate helped to link these two approaches (e.g., FREYet that the mantle source regions for basaltic magmas al., 1978), particularly by emphasizing the role are inhomogeneous in terms of major elements and played by large differences in partition coefficients that the inhomogeneity arises principally from between residual crystals and liquids for different multistage melting of more lher- elements and have introduced concepts of distinc- zolite. Our further purpose is to elaborate the role tive behaviour for 'compatible' vs 'incompatible' ofC-H-O fluids in controlling the presence or ab- elements, 'LIL-element', 'HFS' elements, 'light' vs sence of melting and to present arguments for re-

'heavy' rare earth elements etc. dox-interactions between oxidized (H20 + CO2) 139 140 D. H. Green,T. J. Falloonand W. R. Taylor

lithosphere and reduced (C~ + H2) deeper mantle, within the tectonic framework of large-scale sub- duction and transform faulting along convergent margins of lithospheric plates. We will firstly present data on a simple system forsterite (Fo)-nepheline (Ne)-silica (Qz), as a con- 28kbar venient means of illustrating the principles involved in the study of the multicomponent natural system.

A SIMPLE SYSTEM ANALOGUE FOR GENESIS (Fo-Ne-Qz)

YODERand TiLLEY(1962) introduced the con- cept that the broad family of basalts, from olivine , through basanites, alkali olivine ba- Fo salts, picrites, olivine tholeiites to quartz tholeiites FIG. I. LiquidussurfaceofthesystemNe-Fo-Qz [weight and basaltic , could be pictured as a con- percent]at 1bar (dashedlines)and 28 kbar (solidlines) tinuum of compositions within the 4-component (seetextforsourcereferences).Liquidsformedbymelting tetrahedron with apices represented by olivine, of a jadeite-enstatite-forsterite assemblagevary from quartz, cinopyroxene and feldspathoids. Since that compositionK alongthe olivine-enstatitecotecticuntil time experimental petrologists have devoted much enstatite or olivineis eliminatedfrom the residue.The heavilyshadedareaincludesall liquidcompositionswhich effort to defining, as functions of pressure and vol- can be derivedfrom model peridotiteA by singlestage atile content, the liquidus fields and cotectics, ther- meltingat 28kbarto 1bar.Themorerefractoryperidotite mal divides and liquid evolution paths within the compositionB can yieldliquidswithinthe sameareabut tetrahedron. Parallel approaches to the problem also withinthe lightlyshadedarea. have used natural basaltic compositions and simple system analogues. The system forsterite-nepheline-quartz (Fo-Ne- (A) (enriched in the low-melting components), and Qz) serves as a simplified analogue of basalt/peri- a relatively 'refractory' peridotite (B). It is clear from dotite in that it contains low melting liquids en- Figure 1,that although the initial melt composition riched in sodium aluminosilicate and large liquidus in each peridotite is the same (at the Fo-En-Jd in- fieldsfor olivine and enstatite. The system has been variant point) the more refractory peridotite will studied at 28 kbar under dry conditions by WINDOM traverse further along the olivine-enstatite cotectic and BOETTCHER(1981) and GUPTAet al. (1986). before enstatite is eliminated as a residual phase. A pressure of 28 kbar approximates that near the The liquid then moves to 100% melting along the top of the Earth's Low Velocity Zone beneath olivine control line passing through the bulk com- oceanic lithosphere, i.e., depths of 80-90 km. In positions A or B. The positions of the liquidus phase terms of mantle magma genesis,the most important boundaries at 1 bar pressure (SCHAIRERand boundary in Fo-Ne-Qz is the cotectic between ol- YODER, 1961) are also illustrated (boundaries to ivine and enstatite which defines the range of liquids shaded fields of Figures 1 and 2). Liquids formed formed by increasing degrees of melting of mantle at 28 kbar along the olivine-enstatite cotectic will peridotite in the presence of residual olivine and crystallize olivine at lower pressure and move to- enstatite. wards the 1 bar olivine-enstatite reaction boundary Initial melts (Figure 1) of a model mantle, i.e., or olivine-albite cotectic as appropriate. The shaded olivine + enstatite + jadeite, are 'basanitic' with area in Figure 1 illustrates the range of derivative similar normative nepheline and albite contents. liquids which could be formed by anhydrous melt- The partial melting involves the reaction Forsterite ing of both compositions A and B at 28 kbar, fol- (Fo) + Jadeite (Jd) -+ Enstatite (En) + Liquid as lowed by crystal fractionation at lower pressure. The the enstatite liquidus surface crossesthe Fo-Jdjoin. area marked by lighter shading includes derivative With increasing temperature, liquids traverse the liquid compositions which could not be formed base of the simplified basalt tetrahedron passing from peridotite A by anhydrous melting followed from nepheline-normative compositions into hy- by crystal fractionation at lower pressure but could persthene-normative compositions. be formed from the more refractory composition We wish to consider melting of two model pe- B. In a later section we use this simple system an- ridotite compositions, a relatively 'fertile' peridotite alogue approach to demonstrate that very different Variable source and fluid compositions 141

Under CH4-saturated conditions, liquidus tem- peratures are depressed by 80-90°C at 28 kbar (see also EGGLER and BAKER, 1982). Electron micro- probe, gas chromatographic analyses and infrared spectroscopic studies show carbon solubilities up to

0.2% carbon (i.e., -0.25% CH4 or -0.7% CO2 equivalent) and OH solution equivalent to 2-3 28kbar weight percent H20. The effect of methane-satu- ration is to expand the field of olivine at the expense of enstatite (Figure 2) but to a lesser extent than

H20-saturation (see also EGGLER and BAKER, 1982). The infrared spectroscopic study of quenched glasses shows changes in the absorption bands attributed to the aluminosilicate network

Fo which suggest an increase in the Si:O ratio (i.e., > 1:2) of the network (TAYLOR, 1985; TAYLOR and FIG. 2. As for Figure 1 but with the positions of the vapour-saturated cotectic between olivine and enstatite GREEN, 1987). Thus, under methane-saturated shown for H20, CH. and CO2 saturated conditions at 28 conditions, melts may become reduced and contain kbar. The effect of C-H-O volatiles is to expand the field small dissolved carbon and larger OH- contents. of possible liquids from peridotites A and B but the re- On decompression, without redox-interaction with striction of liquids lying in the lightlyshaded areas to source wall-rocks, such melts will exsolve CH H . It is compositions such as B, remains valid. 4 + 2 noteworthy that initial melts near the low temper- ature minimum melting for jadeite-enstatite-for- sterite, remain nepheline normative in all three cases source peridotite compositions are required for illustrated in Figure 2. However, the degree of un- MORB and boninite parent magmas. dersaturation is sensitively dependent on the fluid phase present. The effect of C-H-O fluids on melting In considering melting in the upper mantle, the evidence from mantle-derived liquids and xeno- in the Fo-Ne-Qz system liths, from the presence of graphite and In Figure 2, the position of the forsterite-enstatite and fluid inclusions within mantle samples, argue cotectic at 28 kbar in the vapour-absent Fo-Ne- that mantle fluids are dominated by the C-H-O Qz system is compared with the positions of the system. In considering melting in the upper mantle, same cotectic under vapour-saturated conditions, it is necessary to consider possible variations on the three specific cases illustrated being H20-sat- fluid phase compositions within the C-H-O system urated, COz-saturated and CH4-saturated (EGG- and whether particular source regions may be char- LER, 1978; TAYLOR, 1985; GREEN et al., 1986). The acterized by particular fluid phase characteristics. effect of water is to expand the olivine field at the The simple system analogue illustrated in Figure 2 expense of enstatite (KUSHIRO, 1972) but liquids shows that variation in fluid phase composition will at low degrees of partial melting remain strongly not invalidate one of the major conclusions from silica-undersaturated and nepheline-normative Figure 1, that is, that some specific liquids and po- (EGGLER, 1975). Water enters the melt as OH-, tential source peridotite compositions cannot be causing depolymerization by hydrolysis of Al-O- related to each other because of limitations imposed Si and Si-O-Si bonds and depression of the liquidus by their major element compositions and phase re- by 350-400°C. In contrast, CO2 enters the melt at lations. C03- in the form of metallocarbonate complexes resulting in melt polymerization and marked ex- FLUID-CONTROLLED MELTING pansion of the liquidus field of enstatite at the ex- IN THE UPPER MANTLE pense of forsterite (see earlier discussions by EGG- LER, 1978; BREY and GREEN, 1975, 1976; MYSEN The importance of water in determining the and BOETTCHER, 1975; MYSEN et al., 1980, and pressure-temperature conditions and form of the others on related systems). Liquids at low degrees mantle solidus is well determined from studies on of melting are nepheline-rich (Ne > Ab) and ap- peridotite compositions (KUSHIRO, 1968; GREEN, proach simple system analogues of olivine neph- 1973; MYSEN and BOETTCHER, 1975). The model elinite. '' composition studied (GREEN, 1973) may 142 D. H. Green, T. J. Falloon and W. R. Taylor

1981; Figure 9.5.7, p. 1180). The presence or ab- 2000 sence of a region of partial melting along geotherms B, C and D and the depth to such a region is clearly sensitively controlled by the role of water, specifi- cally by the activity of water. Recognising that carbon, as well as water, is a component of mantle fluids adds further complexity ..~ 20 to the determination of the mantle solidus. It is .x:.0 necessary to determine the activity of water in the W C-H-O system under mantle conditions. TAYLOR a: :> ~ (1985, 1986) has presented a thermodynamic model OI+Ga+Px+V-- w'" a:'" of this system appropriate to mantle pressures and 0.. temperatures. Results may be presented, following 40 FROST (1979) as isobaric, isothermal diagrams plotting logfo2 vs Xc· defined as the mole fraction of carbon relative to hydrogen in the bulk fluid. In Figure 4 and Table I we illustrate results at 30 kbar, 1327°C showing the carbon-saturation

60~----~------~----~---L~~~~-" surface bounding the metastable carbon-oversa- FIG. 3. Solidi for mantle peridotite (,Hawaiian' pyrolite) turated fluid field (TAYLOR 1985, 1986). The strong (GREEN,1973): (I) Anhydrous (2) 0 < H20 < 0.4 weight dependence of fluid composition on oxygen fugacity percent (Amphibole dehydration solidus) (3) H20> 0.5 is very clear in Figure 4 and Table 1 and it is note- weight percent = water-saturated solidus. Also shown are worthy that graphite coexists with fluid approaching geothermal gradients for mature oceanic (B), 'shield' pure water in composition atfo2 = IW + 1 to 2 log regions (C, D) and near-mid ocean spreading centres (A) (BASALTICVOLCANISMPROJECT,1981; Figure 9.5.7).01, units. olivine; Px, ; Ga, ; V, vapour. The composition ofC-H-O fluids present along geothermal gradients in Archean shield and in regions (taken from Figure 9.5.7, p. be referred to as 'Hawaiian pyrolite' composition 1180, BASALTIC STUDY PROJECT, as the 'melt' component in the 'melt + residue' 1981) can be calculated from this thermodynamic model used to calculate the model 'pyrolite' com- position (RINGWOOD, 1966), was based on Hawaiian

olivine tholeiite liquids, i.e., enriched in Ti02, K20 and incompatible elements relative to MORB. In Figure 3, the solidi for three distinctive water con- tents are illustrated: (1) Anhydrous solidus §9 g; (2) Dehydration solidus; pargasitic hornblende is 111 present on the solidus below 28-29 kbar but the GRAPHITE + FLUID water content is less than approximately 0.4 weight 13 percent. 15 (3) Water-saturated solidus; pargasitic horn- blende is present on the solidus but water content c.co 0.9 1.0 exceeds approximately 0.4 weight percent so that water is present in excess and sub-solidus assem- blages coexist with a water-rich fluid. FIG. 4. Log/02 vs Xc [=mol fraction (C ~ HJ diagram

In Figure 3 we illustrate geothermal gradients at 1600 K (I 327°C), 30 kbar showing the graphite appropriate to thermal upwelling beneath spreading saturation surface, stable fluid-only pseudo-divariant re- centres (A), beneath 'old' oceanic crust and litho- gion (blank) and Xc-independent traces of IW and MW sphere (B) and beneath ancient Archean crust and oxygen buffers. Arrows indicate the off-scale positions of the six C-O-H volatile species, filled circles ('a' to 'f") on lithosphere (C, D) (BASALTICVOLCANISMPROJECT, the graphite saturation surface correspond to fluid com- positions given in Table 1. The maximum mol fraction of H20 on the graphite saturation curve is reached at • Xc = Xco + XC02 + 1/3XCI4 + %XC2!4' point C. Variable source and fluid compositions 143

Table 1. Fluid compositions at selected points in Figure 6; T = 1600 K, P = 30 kbar

EQUILIBRIUM FLUIDS Mol percent species*

-Iogfa, mix 3 Xc Xo H2O CO2 CO H2 CH. C2H6 V cm /mol

a** 6.15 1.000 1.000 0 97.1 2.9 0 0 0 30.27 b 6.40 0.510 0.671 48.5 49.3 1.6 0.4 0.1 0 24.21 ct 7.40 0.032 0.339 94.1 2.3 0.2 1.5 1.9 0 17.90 dt 9.30 0.220 0.091 26.9 0.1 0.1 7.8 63.3 1.8 25.60 et 9.30 0.100 0.192 57.4 0 0 12.7 29.7 0.2 21.05 f 13.60 0.306 0.001 0.2 0 0 8.7 87.7 3.4 28.69 g 13.60 0.100 0.004 0.2 0 0 68.8 29.9 0 19.83 -- ** fa, = GCO, t fo, = GW(max.xH,O), tfo, = IW. * round-off error = ±0.1 mol percent.

model provided that we assume that graphite or quences of this redox interaction, coupled with the diamond is a stable accessory mantle mineral. The sensitivity of the mantle solidus to aH,O, are ex- C-H-O fluid composition at any pressure and tem- plored more fully elsewhere (GREENet al., 1986) perature is only fixed iffo, is also known--in Figure but are presented in summary form in the following 5 (GREENet al., 1986) we calculate fluid compo- section and Figures 6 and 7 which also build upon sitions for the two geothermal gradients assuming ideas canvassed by others including WYLLIE(1978, fo, = FMQ, IW + 2 log units and IW, respectively. 1979, 1980), WOERMANNand ROSENHAUER The results in Figures 4 and 5 demonstrate that if (1985), EGGLER(1978), EOOLERand BAKER(1982), mantle fo, conditions are near IW to IW + 2 or 3 RYABCHIKovet al. (1981) and GREEN(1973). log units, then mantle fluids are dominated by water or water + methane. In the presence of a mixed Mantle degassing and magma genesis: fluid, (H 0 C~), pargasitic hornblende will reach 2 + Oceanic lithosphere a maximum thermal stability near aH,O - 0.5 (see study by HOLLOWAY(1973) on pargasite + CO2 We adopt the view that the earth's upper mantle + H20)--an interpretation supported by studies in beneath the is at fo, - IW ± 1 log progress in which subsolidus amphibole + garnet unit and that this region contains C-H-O fluids, lherzolite crystallized at 25 kbar, 1100°C in the possibly remnant from primordial entrapped vol- presence of graphite and anfo,-buffered fluid phase atiles, and minor carbon (diamond) contents. (In ofCH4:H20 - 1:1. adopting this view we recognize that alternative There is evidence from measurements of intrinsic views of higher oxygen fugacity (QFM to MW) are oxygen fugacity of mantle minerals, of deep-seated also current and our purpose is to explore the 're- igneous intrusions and from identification of re- duced mantle' model.) The fluids will be dominated

duced gases(CH4) evolving from MORB glassesand by CH4 > H20> H2, C2H6 (cf GOLDand SOTER, from submarine volcanic centres (ARCULUSet al. 1980, 1983). The transition from these reduced 1984; SATO and VALENZA,1980; WELHANand conditions towards more oxidized lithosphere will

CRAIG,1979, 1983) that the earth's mantle in the result in fluid change with increase in H20:CH4 source regions for basaltic magmas is reduced with (Figures 4 and 5) but the oceanic geothermal gra- fo, - IW to IW + 1 to 2 log units. These interpre- dient will lie within a field of partial melting at P tations are not unchallenged and others, for ex- > 28-29 kbar (Figure 3) due to instability of par- ample, EGGLER(1983) and MATTIOLIand WOOD gasitic hornblende and the role of water in depress- (1986) infer higher mantle fo, conditions. At near- ing the mantle solidus. Provided that fluid concen- surface and shallow crustal conditions, mineral as- trations are low, this region of partial melting will semblages of igneous rocks indicate fo, ;;::FMQ be a fluid-absent region in which a minor melt (HAGGERTY,1978; SATO,1978). Thus, the earth's phase can accommodate continuously variable fo, lithosphere may be seen broadly as a via melt structure redox changes (SiO variation) or boundary layer between deeper mantle withfo, with solubility of OH- and minor carbon, and at - IW and mantle fluids dominated by CH4, H20 higher fo" by increasing solubility of C03-. At + H2 and near-surface conditions withfo, - FMQ depths < 90 km, the stability of pargasitic horn- and fluids dominated by H20, CO2. The conse- blende atfo, - IW + 1 to 3 log units creates a lid 144 D. H. Green, T. J. Falloon and W. R. Taylor

Continental Geotherm/FMQ Buffer Oceanic Geotherm/FMQ Buffer a a H2O I- Igraphitel / diamond ~ HTI-g-ra-Phl'" tel diamond (i) ., not ., /C02 I not stable " stable at >19 kb .~-1 .~-1 I~Okb .,Q. / Q. /- c: c:" 0 0 :;:; / C0 I t / 2/ .go / .g ., ., / I '0 / '0 5-2 5-2 I / I .Q'" I .Q'" / I / I -3+---+---.---~~~~---+ -3 I (oj I I I 10 20 30 40 50 60 70 10 20 30 40 50 60 70 Pressure(kbar) Pressure(kbar) Continental Geotherm/IW+2 Buffer Oceanic Geotherm/IW+2 Buffer

I I I I OtT" " (i) .s" I ., o .~-1 \ ~-1 \CH 4 Q. \ \ \CH4 c" '"c: \ o \ ... \ ...... ~ ... t; ... o ... .g ... .g ... '- .... , .... ., ...... , '0 '0 " 5-2 ...... 5-2 '" / <, ------..::-:....-...,.. / H2 ~ /''X""' ------":::,- ~ ,/ I \ .....- H2 / \ I/ I I / \ CZH6 -3 I 10 20 30 40 50 60 70 20 30 40 50 60 70 Pressure(kbar) Pressure(kbar) Continental Geotherm/IW Buffer Oceanic Geotherm/IW Buffer a _ a+------CH:------CH4

~., ~ .~ o .~o .,~-1 H20 ~-1 c: c:" o H2 _-H1. _ :;:; ~ ..... _- o o ~.:?_ot::.:::_:.- - / / -" ...... _._._._._._._._._._._ . .g ,'/ -._ .g -.__ H e /~/ (2 6 - I /~....C2H6 '0 '0" 5-2 5-2 / // // / I .Q'" / / .Q'" / / / .l // / -3+L_L~--.---.-_,r__.--_+ -3+---.---.---.--..--.---+ 10 20 30 40 50 60 70 10 20 30 40 50 60 70 Pressure(kbar) Pressure(kbar) Temperature (Oe) Temperature 1°C) 600 100 900 1XIO ,,00 1200 1)(1(1 1350 I i 4~ I 6~ ec:o '01?0 119~ '~ lJqO ':Z ••• 100 1000 1200 1400 1600 1600 1000 1200 1400 «eo 1600 Temperature (K) Temperature (K) FIG. 5. F1uid compositions along model 'oceanic' and 'shield' geotherms (BASALTIC VOLCANISM STUDY PROJECf, 1981; Figure 9.5.7, p. 1180) assuming fluid coexists with graphite/diamond and with specific fo,'s corresponding to IW, IW + 2 log units and FMQ buffers. Variable source and fluid compositions 145

"ANTLE

'HAWAIIAN' 'YAOLlTE (enriched) fLUID ..... eNT

'IIIORII' ManUe "'lhana "Ium,'

'Hoi .pol.O ••• pot_w,t,"ol' ) ~)

FIG. 6. Schematic model of the oceanic lithosphere and asthenosphere. The lithosphere is variable in composition due to melt extraction and fluid enrichment events and is envisaged as variably oxidized from FMQ (shaded areas) to -IW + 1 to 2 log units (blank areas of lithosphere). The asthenosphere is a fluid-absent region in which C-H-O volatiles are dissolved in a small melt fraction (1-2% melt) which migrates towards the top of the asthenosphere or base of the lithosphere. Source regions for Hawaiian-type island chains (hot-spots) are in the upper part of the asthenosphere or base of the lithosphere. Source regions for MORB are in the lower part of the asthenosphere. The mantle is degassing reduced fluids (CH. + H2) and the asthenosphere is a fluid-absent region with intrinsic oxygen content and fugacity varying from - IW to IW + 2 to 3 log units-this variation is accommodated in change of melt structure from reduced (Si:O > 1:2) to contain OH- and COj- (/0, - IW + 2 to 3 log units). The patterned symbol on the oceanic lithosphere signifiesfo, - FMQ, i.e., 'oxidized' lithosphere. In the convergent margin or plate boundary scenario, the oxidized lithosphere is portrayed as penetrated along fracture zones by Co. + H2 fluids from subjacent mantle. The redox-interactions produce H20-rich fluids which strongly influence melting relations above or within the subducted lithosphere. Thus, even at depths below those at which hydrated

minerals of the oceanic crust and lithosphere have been lost, the 'slab' may be a source of H20-rich fluids. Abbreviations for primary magmas developed within the schematic diapirs are: M.O.R. = mid-ocean ridge; LA.T. = island arc tholeiite; B.A.B. = back-arc basin. Seetext for further discussion, including the genesis ofboninite parent magmas.

to the partially molten layer because any interstitial WYLLIE, 1978) and most importantly, a zone of melt migrating upwards from the asthenosphere will chemical differentiation rather than homogeniza- crystallize to pargasite-bearing lherzolite containing tion due to the migration of an interstitial melt phase graphite or dolomite ± graphite (GREEN, 1973; towards the top of the asthenosphere. The character EGGLER, 1978; WYLLIE, 1978; GREEN et aI., 1986). of the interstitial melt is extremely undersaturated In Figure 6, the asthenosphere beneath oceanic olivine melilitite to olivine (GREEN, crust is a fluid-absent region separating methane- 1971; GREEN and LIEBERMANN, 1976; BREY and rich fluids in the deep mantle from H20-rich and GREEN, 1976; WYLLIE, 1979; EGGLER, 1978;

H20 + CO2 fluids in the lithosphere. It is a zone of WENDLANDT and MYSEN, 1980) and apart from decoupling between lithospheric plates and under- enriching the upper part and depleting the lower lying mantle, a zone of increased seismic attenua- part of the asthenosphere in incompatible elements, tion and decreased seismic velocity (GREEN and the high Cal AI ratio of olivine melilitite will decrease LIEBERMANN, 1976; GREEN, 1973; EGGLER, 1978; the Cal Al ratio of the lower part of the astheno- 146 D. H. Green, T. J. Falloon and W. R. Taylor

UPPER MANTLE A. OXIDIZED (.,,) AT OE"TH •. IIEDUCED {/WJ AT DE,.rH oxidized subducted lithospheric slab with reduced f", .. .,W'NUTHOS,.HEIIE , .... " .. T""OUOI'! U'Z mantle below the asthenosphere (Figure 6). It is generally accepted that the peridotite of the lithosphere is variable in composition from lher- zolite to , probably as a result of prior extraction of basaltic to picritic melts. Local, more extreme chemical variation results from trapped liquids, high-pressure accumulates or metasomatic enrichment from migratory fluid phases. Two ex-

1i!iOkm----- Gau..rlollt •• amples of chemical variation are illustrated in Fig- Klmb.rltle Men with dl •• olvad C-H-O ure 8, showing chemical trends in lherzolite inclusions from W. Victoria (NICKEL and GREEN, 1984) and from garnet and spinellherzolites within FIG. 7. Schematic model for the continental lithosphere the Ronda high-temperature, high-pressure peri- beneath Archean/Proterozoic shield regions. See text for dotite intrusion (FREY et al., 1985). Even more re- detailed discussion. The deep mantle is degassing (CH. fractory harzburgite compositions are seen in some + H2) fluids that interact with oxidized lithosphere. (A) suggestsa deep, thin asthenospheric layer (partial melting) complexes. These are commonly partially in which the fa, change from mantle or lithosphere is ac- or completely serpentinized and during serpentin- commodated within the fluid-absent melt. (B) suggests a ization, became highly oxidized with abundant mechanism of 'redox' melting (see text) in which diamond- development. bearing garnet harzburgite is left as a residue from oxidation The evidence from deep focus sug- of CH. + H2 to H20 + C with extraction of water-rich kimberlitic melt. (C) suggests a role for deep lithosphere gests that subducted lithospheric slabs retain their fractures in localizing mantle fluid release and interaction identity to depths of at least 600-700 km. Dehy- ofthese fluidswith oxidized crustal fluidsat shallow depths. dration reactions in subducted basaltic crust and serpentinized peridotite may lead to migration of

H20-rich or H20 + CO2 fluids from the subducted slab into the overlying peridotite wedge, instigating sphere and increase the Cal Al ratio of the upper- melting on the water-saturated peridotite solidus most part. The extent of chemical differentiation (solidus 3 of Figure 3, KUSHIRO 1968; GREEN, will be related to: (a) the age of the lithosphere, (b) 1973). Melts formed in this way will have geochem- the extent to which volumes of old lithosphere/as- ical characteristics reflecting the partially residual thenosphere are preserved near transform / character of the remobilized lithosphere together spreading centre intersections or at spreading axes with the imprint of components partitioned into themselves, and (c) depth within the asthenosphere. The base of the lithosphere will also become en- riched in incompatible elements, hosted particularly in pargasitic hornblende, and apatite as minor and accessory phases. In Figure 6, the lower part of the asthenosphere is depicted as a fluid- ,.,Ii absent region at low f02 (IW to IW + 1 log unit) ..•~ i. {(. .:.:l; I..:,J. which has the incompatible element contents ap- propriate to MORB source (abbreviated as 'MORB'

o MORB pyrolite) whereas the upper part of the astheno- " H••• n sphere has fo, near IW + 2 to 3 log units and in- re c Tlnlqullkl Fo compatible element contents appropriate to source FIG. 8. Two examples of peridotite suites of variable regions for tholeiites of oceanic island chains such composition from lherzolite to harzburgite, reflecting prior as Hawaii ('hot spot' source regions, abbreviated as extraction of basaltic to picritic melts, plotted in the nor- 'Hawaiian pyrolite'). mative system Fo-Jd + Ca'Ts-Qz-Di, projected from Di onto the plane Fo-Jd + CaTs-Qz, (a) Ronda, high tem- The relationships between magmatism at spread- perature, high pressure peridotite intrusion FREYet aI., ing centres, at 'hot spots' or migrating tension frac- 1985; (b) Spinel lherzolite inclusions from Lake Bullen- tures (see Figure 6) and the chemically zoned as- merri, W. Victoria, NICKELand GREEN,1984. Alsoplotted thenosphere has been fully explored elsewhere are 'MORB' pyrolite, 'Hawaiian' pyrolite and Tinaquillo lherzolite compositions, which have been studied experi- (GREEN, 1971, 1972; GREEN and LIEBERMANN, mentally under high pressures and temperatures (JAQUES 1976; GREEN et al., 1986) and in this paper we ad- and GREEN, 1980; FALLOONand GREEN, unpublished dress the possibilities created by interaction of an data). Variable source and fluid compositions 147

and transported by the fluid phase migrating from neath this plate and reflects a 'fixed' mantle or a the subducted slab. A related model suggests that deep mantle in which the convective flow is much temperatures on the upper part of the subducted slower than and decoupled from the lithospheric slab (the subducted basaltic/eclogitic crust partic- plate motions (MORGAN, 1972; DUNCAN, 1981). If ularly) may be high enough for fluid-saturated or large slabs of subducted lithosphere are carried to dehydration melting and for melt to migrate from depths> 600 km and embedded within the return the subducted slab to modify and melt the overlying flow or deeper mantle pattern (HOFF- peridotite mantle wedge. The models have been ex- MAN and WHITE, 1982; RINGWOOD, 1986), then

plored and evaluated in many previous publications they may act to oxidize CH4 + H2 fluids passing (see RINGWOOD (1974) and GILL (1981) for review through them to CH4 + H20. This effect could feed and references). However the concept of redox- in- CH4 + H20 plumes into overlying asthenosphere teraction between deep mantle degassing CH4 + lithosphere and trigger 'hot spot volcanism' as + Hj-rich fluids and lithospheric slab with mineral depicted in Figure 6. For example we suggest that assemblages reflecting foz near FMQ to MW con- the great frequency of mounts and island chain ditions adds a further dimension to subduction. It volcanoes in the West Pacific may be due to the is possible for dehydrated, very refractory peridotite Pacific plate over-riding relict subducted slabs or of the subducted slab to generate H20- embedded in the 150-650 km depth interval of the rich fluid by reaction of anhydrous oxidized mineral upper mantle and reflecting material originally assemblages (Fe3+ in silicates, , carbonates) subducted on Palaeozoic-Mesozoic subduction with primary CH4 + H2 fluids. The increased zones along the East Asian margin. aHzo will cause melting, even of refractory harz- burgite (KUSHIRO, 1972; MYSEN and BOETTCHER, Mantle degassing and magma genesis: 1975; GREEN, 1973). A second, important effect Continental lithosphere will be that any redox front bounding a residual slab of subducted lithosphere in contact with normal In Figure 7 tcf GREEN et al., 1986) we present MORB pyrolite will be a locus for high aH,o-the a summary diagram illustrating mantle degassing mechanical effect of this will be to concentrate stress of CH4+ H2 beneath a very old, thick lithosphere release ( activity) at such redox fronts. characterized by very low Lower temperatures within the subducted slab will (Shield geotherms C and D of Figure 3). The litho- generally inhibit melting of the slab itself but this sphere has a zone of amphibole stability in lher- may be counteracted by high aH,O contrasting with zolitic compositions at P < 28-29 kbar but at deeper high aCH, and thus high solidus temperatures in levels subsolidus assemblages contain phlogopite surrounding mantle. A particularly favourable lo- and may contain other hydrous phases such as ti- cation for redox interaction coupled with higher tanoclinohumite (McGETCHIN et al., 1970). Carbon temperatures at the edges of a subducted slab will will be present as diamond or graphite, or at depths be at trench/transform intersections, e.g., the south > 150 km, as if fo, > FMQ (Figure 5; end of the Marianas Trench and the northern end EGGLER 1975, 1978; GREEN et al., 1986; BREYet of the Tonga Trench, where the subducted slab al., 1983; WYLLIE, 1978, 1979). Samples of deep abuts, across a transform fault zone, against normal continental lithosphere occur as in kim- oceanic lithosphere/asthenosphere to deep levels. berlite pipes and illustrate heterogeneity and vari- The movement zone will probably act as a channel- ations from possible trapped liquids or relict sub- way for enhanced mantle degassing and by provid- ducted oceanic crust (eclogite, grosspydite, lherzolite ing a temperature contrast across the transform fault to highly refractory harzburgite, and extremely K- zone, will act as a heat input to the subducted slab. and Ti-rich phlogopite rocks). In Figure 6, we suggest that the immediate source If mantle temperatures approach the solidus 3 peridotite for Hawaiian or 'hot spot' volcanism lies (Figure 3), then redox melting of lithosphere may

in the uppermost part of the asthenosphere or al- occur by interaction of reduced CH4-rich fluids ternatively within the base of the lithosphere, where from deeper levels with oxidized lherzolite, or harz- this has been enriched by migration, reaction and burgite. GREEN et al. (1986) have suggested that the crystallization of an interstitial melt. The success distinctive mineral inclusion suite within South Af- of the 'fixed hot spot' frame of reference in analy- rican which is characterized by extremely zying relative plate movement over the past 100 magnesian olivine, refractory low-AI enstatite and million years shows that the cause or trigger for extremely low calcium, Cr-rich garnet, is a island chain or 'hot spot trace' volcanism does not consequence of redox melting (see Figure 7, sce- lie within the moving lithospheric plate but lies be- nario B). 148 D. H. Green,T. J. Falloonand W. R. Taylor

Although Archaean and Proterozoic pre- naquillo being the most refractory composition serve a remarkably long history of stability as re- (JAQUESand GREEN,1980; GREENet al., 1979). corded by cover of undeformed Proterozoic sedi- The trends in Figure 8 defined by the mantle ments, they are frequently cut by dyke samples (which could be matched by other swarms. In the post-Permian breakup of Gond- suites or high-temperature, high-pressure intrusive wanaland, stable cratons with thick underlying peridotite suites) are considered to result from par- lithosphere, have rifted and drifted apart so that tial melting and incomplete melt extraction (FREY regions of old, thick lithosphere are now juxtaposed et al., 1985; NICKELand GREEN, 1984) and thus against young lithosphere of oceanic type. An initial we infer that mantle source compositions, prior to situation of rapid temperature change at the base melt extraction are at least as 'fertile' as the most of the lithosphere ofa newly rifted (cf BOYD -rich and -rich limits of the 1973;BoYDand NIXON,1975;NICKELand GREEN, trends shown in Figure 8. Ifthe processes outlined 1985) will be followed by heating of the sub-craton in previous sections, which lead to one or more lithosphere so that the solidus of phlogopite-bearing remelting events, particularly under the fluxing ef- lherzolite and harzburgite will be exceeded. Small fects of C-H-O fluids, are operative in the earth, melt fractions of olivine to leucite lam- then the range of potential source peridotite com- proite composition (FOLEY, 1986; FOLEYet al., positions covers at least the range of the samples 1986) reflect phlogopite harzburgite source rocks plotted, i.e., trending from lherzolite towards harz- and depths of segregation from 150 km to -70 km burgite with 80 weight percent 01, 20 weight per- respectively. Source rocks of phlogopite-bearing cent En. garnet lherzolite will produce potassic olivine ne- phelinites to basanites (WEDEPOHL,1985) with the Source for mid-ocean ridge basalts particular liquid composition being determined by CO2:H20 as well as by source rocks and depth of Compositions of (>70) glasses from dredged magma segregation (FREY et al., 1978; EGGLER, ocean floor basalts are plotted in Figures 9 and 10-- 1978; GREEN, 1976; BREY and GREEN, 1976; all have 100 Mg/(Mg + Fe2+) > 68, i.e., they are WYLLIE,1979). If upwelling and diapirism occurs primitive in the sense of suitability as melts in equi- from within the heated former sub-cratonic litho- librium with olivine> Mgg7• It may be noted that sphere, then magmas produced at lower pressures will retain trace element and isotopic compositions reflecting their source compositions but the magmas will be picritic to tholeiitic or alkali olivine basaltic in character, reflecting higher degreesof melting and shallower depths of magma segregation.

MAGMA SOURCE COMPOSITIONS

Mantle-derived peridotite suites Compositions of two suites ranging from lher- zolite to harzburgite and with high pressure mineral assemblages requiring crystallization at pressures in excessof 10 kbar are projected in Figure 8. The Ronda peridotite suite (FREYet al., 1985) defines a linear trend consistent with extraction of magma lying within the olivine-hypersthene-plagioclase- diopside volume, i.e., an olivine tholeiite or tholei- itic picrite liquid (FREYet al., 1985). The Bullen- FIG.9. Equilibriumpartialmeltsof MORBpyroliteat merii suite shows more scatter but again a trend 8, 10 and 20 kbar (FALLOONand GREEN,unpublished data) and 'Hawaiian' pyroliteat 10 kbar (JAQUESand implying an olivine tholeiite or tholeiitic picrite ex- GREEN,1980)comparedwithprimitiveMORBglassesin traction (cf NICKEL and GREEN, 1984).• Model the normativesystemOl-Jd + Ca'Ts-Qz-Di. Composi- Hawaiian pyrolite, MORB pyrolite and Tinaquillo tions are projectedfrom plagioclase(An + Ab) onto the lherzolite compositions are also plotted in Figure planeOl-Di-Qz, Cotecticsareasfollows:(1)stippledline, 01 + opx + cpx + L cotectic.(2) dashed line, 01 + opx 8. All three have CaO and Al 0 contents> 3-4 2 3 + L cotectic.(3)solidline,01 + Lcotectic.(T) istheMORB weight percent but the suites differ in Na20, Ti02, glassDSDP3-18-7-1,studiedbyGREENet aI., (1979)and K20, and incompatible element contents with Ti- (\1) isthelow-olivineparentcompositionofBRYAN(1979). Variablesourceand fluidcompositions 149

Jd+CaTs Oz normative hypersthene contents, before clinopy-

PR~~I~~VE roxene disappears. Examination of Figure 10 alone . .../~.L"SSES would suggestthat MORB glassesmay be explained as primary olivine tholeiite melts separating from ~~:~ OJ + Opx + Cpx residues at approximately 8 kbar to 13 kbar pressure. Limited olivine fractionation and variation in percent melting could possibly ac- Hy count for the observed compositional variation of Figure 10. However, it is insufficient to examine only one projection or plot of the many chemical variables involved. From Figure 9, it is clear .that ..",.om,l PYROLITE .\ I i'It'" almost all MORB glasses are too rich in normative diopside to have formed as primary liquids on 01 + Opx + Cpx cotectics. However, a few exceptions W01 lie close to 01 + Opx + Cpx cotectics at - 8 kbar and these distinctive MORB glasses have low nor- FIG.10.Equilibriumpartialmeltsof'MORB' pyrolite at 8, 10and 20 kbar and 'Hawaiian' pyroliteat 10kbar mative olivine contents or are quartz normative. comparedwiththe fieldofprimitiveMORBglassesin the They are similar in composition to the low-olivine normativesystemOl-Jd + CaTs-Qz-Di, Compositions parent composition of BRYAN(1979) and are con- are projectedfrom Di onto the plane Jd + CaTs-Qz-OI. sistent with being close to primary liquids or derived Cotecticsand symbolsas for Figure 9. For purposesof claritythe primitiveMORBglassesare not plotted indi- from -8 kbar primary olivine tholeiite magmas viduallyas in Figure9 but outlinedby the dotted line. via limited olivine fractionation. Examination of Figure 9 suggests that liquids formed by small degrees of melting at 10-12 kbar, might fractionate small amounts of olivine to reach olivine tholeiite liquids dominate the group but they MORB glasscompositions. In fact, this mechanism do spread from quartz-normative compositions (5 does not work as liquids at small degrees of melting

points) to nepheline-normative compositions (2 have Ti02 contents, Na/Ca ratio, K20 contents and points). The glass DSDP 3-18-7-1, studied exper- PzOs contents that are too high for MORBs. imentally by GREENet al., (1979) as a primitive The data, as illustrated in Figures 9 and 10, but MORB lies centrally within the MORB field. confirmed by other oxide, element ratio and nor- GREENet al. (1979, Table 4, column D) calcu- mative plots, show that most MORB glasses are lated a model source composition for DSDP 3-18- derivative by olivine factionation from primary pi- 7-1 and we have studied the anhydrous melting critic magmas (i.e., liquids with >20% normative relationships of this composition (MORB pyrolite) olivine), segregating from residual olivine, ortho- from 8-20 kbar. A 'sandwich' technique (STOLPER, pyroxene and clinopyroxene at pressures of 15-20 1980) was used in which a layer of DSDP 3-18-7- kbar. Melting conditions are essentially anhydrous 1 glass is placed between two layers of MORB py_ although small quantities of reduced and oxidised rolite. The bulk compositions of these mixtures lie volatiles may be present (BYERS et aI., 1983; in the join between these two compositions (Figures MATHEZ,1984). It is inferred from Figure 10 par- 9 and 10). This approach avoids the bulk compo- ticularly, that we do not see liquids formed by very sitional changes inherent in equilibrating diverse high degrees of melting along the olivine + ortho- liquids within unrelated peridotite layers (fuJII and pyroxene cotectic and we believe that this is unlikely SCARFE,1985; TAKAHASHIand KUSHIRO,1983; in a single stage melting process as such liquids STOLPER,1980)although these studies provide use- would require >30-35% melting and melt segre- ful comparative data for our purposes. gation apparently occurs prior to this (GREENand The results are summarized in Figures 9 and 10, RINGWOOD,1967; MYSENand KUSHIRO,1977; showing 3-phase cotectics (01 + Opx + Cpx), fol- JAQUESand GREEN1980). lowed by (01 + Opx) cotectics until the olivine- Liquids at 10 kbar from the Hawaiian pyrolite control line is reached as the temperature increases. melting study (JAQUESand GREEN,1980) are also Attention is particularly drawn to the compositional plotted in Figure 10.The pattern ofliquid variation range of liquids along the 3-phase cotectic. For ex- with progressively higher degrees of melting is sim- ample, at 20 kbar liquids in equilibrium with ol- ilar to that for MORB pyrolite but the 01 + Opx ivine, orthopyroxene and clinopyroxene range from + Cpx cotectic is at much higher normative diopside nepheline-normative picrites to picrites with low contents. Although the cotectic overlaps the MORB 150 D. H. Green, T. J. Falloon and W. R. Taylor field in this projection the liquids have > 3 weight rental lava composition with derivative glasses and percent Ti02, >0.5 weight percent K20 and Na/ aphyric basalts to compare with the area of MORB Ca ratios unlike MORB. The Hawaiian pyrolite glasses and the range of liquids derived from MORB composition cannot produce MORB liquids in pyrolite. The Troodos Group III parental liquid lies terms of major and minor elements and vice versa- marginally outside (i.e., is more refractory than) the different source compositions are required. olivine-control line from MORB pyrolite. In terms of phase relations a liquid close to Troodos Group Source for Troodos Upper Pillow Lavas III parental magma could be formed by melting of MORB pyrolite at the point of elimination of or- CAMERON (1985) has subdivided the Troodos thopyroxene from the residual phases, i.e., a Upper Pillow Lavas into three chemical groups with residue. However, minor and trace element contents trace element and isotopic abundances requiring of the MORB-pyrolite derived melt do not match chemical differences between the source composi- these of Troodos Upper Pillow Lava--a relation tions for each group. DUNCAN and GREEN (1986) readily apparent in the first plagioclase to crystallize studied a parental Group III composition (Arakapas (- An91 in Troodos UPL and - An78 in DSDP 3- area) experimentally with particular attention to 18-7-1). matching of observed phenocryst assemblages with Ifwe compare the melting relations ofTinaquillo phases crystallized under controlled pressure and lherzolite (Figure 11), its more refractory character temperature conditions. Conditions of magma seg- means that the olivine/orthopyroxene cotectic does regation from residual harzburgite were determined not end at but continues through Troodos UPL as 1350°C, 8-9 kbar and the magma contained composition (cf Figure 1). In phase relations, trace -0.5-1 percent weight H20. element and minor element abundances, the source In Figure 11, we plot the Troodos Group III pa- peridotite for Troodos UPL must be more refractory than MORB pyrolite--we infer that it is a residual lherzolite composition in which melting is enhanced Jd+CaTs An+Ab Qz by addition of a very small amount of water (0.5- PRIMITIve MORB GLASSES 1 percent weight H20 in the melt fraction) sufficient TROODOS GP III ! UPPER PILLOW LAYAS to depress the melting temperature by about 30-50°C.

Source for north Tongan arc high magnesium lavas

In 1984, the Japanese research vessel 'Natsu- shima' dredged an extraordinary variety of high MORB PYROLITE. magnesium lavas from water depths in excess of TlNAOUILL.O LHERZOLITE 0 .0' I 2000 m at the northern termination of the Tongan arc (FALLOON, 1985; FALLOONand GREEN, 1986a). In detail, the samples show evidence for decoupling of incompatible elements from major plus com- FIG. 11. Comparison of primitive MORB glasses and patible elements. For example, rocks with similar Troodos Group III upper pillow lavas (CAMERON,1985) major element chemistry have REE patterns varying with equilibrium partial melts from 'MORB' pyrolite at from strongly LREE enriched to strongly LREE 8, 10 and 20 kbar and Tinaquillo lherzolite at 5 kbar depleted. There is also unequivocal evidence for (JAQUESand GREEN, 1980; GREENet aI., unpublished data), in the normative system Ol-Jd + CaTs-Qz-Di, magma mixing involving several magnesian end- projected from Di onto the plane Jd + CaTs-Qz-OI. Co- member liquids, differing principally in their degree tectics same as for Figure 9. Symbols are as follows: (.) of depletion in the more incompatible elements and calculated parental composition for the Troodos Group in ratios such as Na/Ca. One sample contained F094 III upper pillow lavas studied by DUNCANand GREEN olivine phenocrysts and within these unusually (1986). (T) refractory primitive magma composition identified from glassinclusions in magnesian (F09.) olivine magnesian olivine crystals occur large glass inclu- phenocrysts from Tonga (FALLOONand GREEN,1986b). sions with CaO/Na20 > 20 and magnesian quartz (.) Marianas fore-arc lava (JENNER,1983). (0) Cape Vo- tholeiite compositions (FALLOON and GREEN, gel, boninite parental magma composition calculated by 1986b). Correction for quench outgrowth on the WALKERand CAMERON(1983). (\7) primitive MORB glass DSDP3-18-7-1 studied experimentally by GREENet al. olivine host enabled calculation ofthe original liquid (1979). (Figure 11). In other samples from the same and Variable source and fluid compositions 151

other dredge sites, extremely calcic plagioclase (up chemistry and petrology of suggests vol- to AnlOo)megacrysts occur and are compatible with atile-induced (H20-rich) melting of very refractory crystallization from a liquid matching the compo- residual harzburgite. There is evidence for signifi- sition of the glass inclusions in olivine. The role of cant addition of incompatible elements either with water in depressing the liquidus and fluxing melting or prior to the addition of H20-rich fluid. of refractory lherzolite composition, is as yet un- certain. However, the composition of the glass in- CONCLUSIONS clusion is such that the original magma is a possible partial melt of very refractory lherzolite composi- The genesis of primary magmas in the earth's tions, such as Tinaquillo lherzolite, with magma mantle is controlled by segregation at - 5 kbar (anhydrous) or a slightly a) Pressure, temperature variation, i.e., mantle higher pressure if Off" solution expands the olivine geotherms, which differ in different tectonic settings; phase volume. b) The presence and composition of a fluid phase, principally within the system C-H-O; and Source for boninite or high magnesia andesites c) The bulk compositions of source peridotite. In young West Pacific Arcs, distinctive very high Experimental studies at high pressure and tem- Si02 (>56 weight percent, high magnesium, low perature constrain the effects of C-H-O fluid em- calcium lavas characterized by rare olivine and phasizing the importance of water-rich and water- abundant enstatite or multiply twinned clinoensta- methane fluids at oxygen fugacities of IW to IW tite (inverted from protoenstatite) have been found + 1 to 3 log units. The experimental studies also both outcropping in emergent islands and dredged show that different source peridotite compositions from the sea floor (DALLWITZet al., 1966;KURODA are required for primary magmas from different et al., 1978; CRAWFORDet al., 1981; UMINO,1985; tectonic settings. These different source composi- DIETRICHet al., 1978). Strongly porphyritic char- tions can be simply related within a framework in acter, accumulation of phenocrysts and growth of which the source compositions for Hawaiian (Hot- quench phases makes determination ofliquid com- spot) and Mid-Ocean Ridge tholeiitic picrites to positions very difficult. Experimental studies (TA- olivine tholeiites are regarded as 'enriched' and 'de- TSUMI, 1981; JENNERand GREEN, unpublished) pleted' variants of the mantle composition within have established conditions of multiple olivine and the asthenosphere. The process of enrichment and orthopyroxene saturation and the latter studies lead depletion is attributed to upward migration of a us to a preferred petrogenetic model in which liquids very small (-1 %) melt fraction of olivine melilitite contain - 2 weight percent water and segregatefrom or olivine nephelinite character within the astheno- residual olivine and orthopyroxene at -2-5 kbar, sphere. T - 1250-1300°C. Source rocks for back-arc basin tholeiites, island In Figure 11, plots of possible parental magma arc tholeiites, high-Mg tholeiites such as Troodos compositions lie on the refractory (olivine-enstatite) Upper Pillow Lavas or from the Tongan fore-arc, side of the olivine-control line for peridotite such and particularly for boninite parent magmas, are as MORB pyrolite or Tinaquillo lherzolite. Liquids highly refractory to . The such as boninites cannot be derived from Iherzolites roles of such compositions as source rocks for mag- as calcic as those and require very refractory harz- mas requires second and probably third stage melt- burgite source rocks (cJ Figure 1). Potential source ing, in which residues from extraction of earlier rocks in terms of (Jd + CaTs)-OI-Qz-Di include magmatic events are re-melted in different P, T the more refractory compositions of the lherzolite- and tectonic settings. harzburgite suites of Figure 8. However, examina- In such multiple melting episodes, a key role is tion of trace element and minor element contents played by C-H-O fluids in fluxing the melting, and ofboninites reveals great variability (JENNER1981, in the case of boninite liquids, in introducing in- HICKEYand FREY 1982, CAMERONet al., 1983) compatible elements to the source rock prior to or from incompatible elements enriched to V-shaped during the melting process. Although the subducted REE patterns suggesting addition of a minor en- slab has long been regarded as a source for H20- riched component to a previously strongly depleted rich fluids by dehydration of minerals within the peridotite. A similar mechanism is required to ex- upper parts of the slab, in this paper we suggest that plain the non-refractory character of the plagioclase an additional source of water, particularly at deeper (which appears very late in the crystallization se- levels of the subduction process, comes from redox quence and lacks a high Ca/Na ratio). The geo- interaction of reduced CH4 + H2 fluids from the 152 D. H. Green, T. J. Falloon and W. R. Taylor deep mantle with the oxidized (FMQ) mineral as- DALLWITZ W. B., GREEN D. H. and THOMPSON J. E. semblages of the subducted slab. (1966) Clinoenstatite in a from the Cape Vogel area, Papua. J. Petrol. 7, 375-403. Particularly favourable environments for mul- DIETRICH V., EMMERMAN R., OBERHANSLI R. and tistage melting, even of extremely refractory harz- PuCHELT H. 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