Journal of Petrology Advance Access originally published online on June 13, 2005
Journal of Petrology 2005 46(11):2313-2336; doi:10.1093/petrology/egi056
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Similar V/Sc Systematics in MORB and Arc Basalts: Implications for the Oxygen Fugacities of their Mantle Source Regions
1 DEPARTMENT OF EARTH SCIENCES, MS-126, RICE UNIVERSITY, 6100 MAIN STREET, HOUSTON, TX 77005, USA
2 SCHOOL OF EARTH AND OCEAN SCIENCES, UNIVERSITY OF VICTORIA, PETCH BUILDING, ROOM 280, 3800 FINNERTY ROAD, VICTORIA, BC, CANADA V8W 3P6
RECEIVED JULY 15, 2004; ACCEPTED MAY 3, 2005
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONTHEORY: TRACKING MANTLE fO2...LITERATURE DATASETS AND...CASE STUDIESDISCUSSIONCONCLUSIONSAPPENDIXREFERENCES |
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V/Sc systematics in peridotites, mid-ocean ridge basalts and arc basalts are investigated to constrain the variation of fO2 in the asthenospheric mantle. V/Sc ratios are used here to ‘see through’ those processes that can modify barometric fO2 determinations in mantle rocks and/or magmas: early fractional crystallization, degassing, crustal assimilation and mantle metasomatism. Melting models are combined here with a literature database on peridotites, arc lavas and mid-ocean ridge basalts, along with new, more precise data on peridotites and selected arc lavas. V/Sc ratios in primitive arc lavas from the Cascades magmatic arc are correlated with fluid-mobile elements (e.g. Ba and K), indicating that fluids may subtly influence fO2 during melting. However, for the most part, the average V/Sc-inferred fO2s of arc basalts, MORB and peridotites are remarkably similar (–1·25 to +0·5 log units from the FMQ buffer) and disagree with the observation that the barometric fO2s of arc lavas are several orders of magnitude higher. These observations suggest that the upper part of the Earth's mantle may be strongly buffered in terms of fO2. The higher barometric fO2s of arc lavas and some arc-related xenoliths may be due respectively to magmatic differentiation processes and to exposure to large, time-integrated fluid fluxes incurred during the long-term stability of the lithospheric mantle.
KEY WORDS: vanadium; scandium; oxygen fugacity; mantle; arcs
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONTHEORY: TRACKING MANTLE fO2...LITERATURE DATASETS AND...CASE STUDIESDISCUSSIONCONCLUSIONSAPPENDIXREFERENCES |
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The purpose of this study is to constrain the oxygen fugacity (fO2) of the asthenosphere—that part of the upper mantle that is mobile enough to undergo adiabatic decompression and is the dominant source region for most juvenile magmas in arcs and in mid-ocean ridges. Oxygen fugacity is defined as the activity (or roughly the partial pressure) of O2 within a system (Carmichael, 1991
; Frost, 1991; Kress & Carmichael, 1991) and represents an intensive variable (such as pressure and temperature) that provides a measure of the system's redox potential at equilibrium. Our interest in fO2 is motivated by the fact that fO2 controls the valence state, and hence speciation, of redox-sensitive elements. For example, fO2 controls the mobility of redox-sensitive trace metals (e.g. first series transition metals and the platinum group elements) and the species composition of volcanic gases emitted to the ocean–atmosphere system. Understanding how fO2 varies in space and time therefore has important implications for studies of ore genesis, atmospheric evolution and trace-element partitioning.
In practice, fO2 is determined indirectly by O2 thermobarometry wherein the activities of the different valence states of a redox-sensitive element in minerals and/or glasses are measured. In homogeneous (single phase) systems, e.g. a melt, experimentally calibrated relationships between fO2 and the activities of Fe3+ and Fe2+ in a glass are typically used to obtain the fO2 of a magma just prior to quenching (Christie et al., 1986; Kress & Carmichael, 1991). In a heterogeneous (e.g. multiphase) system, the fO2 of last equilibration is recorded by the distribution of Fe3+ in different system phases. For example, pertinent to peridotitic mantle lithologies, the reaction
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; Wood & Virgo, 1989; Ballhaus et al., 1990, 1991), provides an indirect measure of the fO2 of a peridotite just before it cooled through its closure temperature. These methods for estimating the fO2 of last equilibration are referred to here as ‘barometric fO2’.
The barometric method of inferring fO2 has now been widely applied to mantle xenoliths and lavas. Mid-ocean ridge basalts (MORB) appear to have barometric fO2s systematically lower than that of arc lavas: relative to the fayalite–magnetite–quartz (FMQ) buffer, MORB have fO2s between –2 and 0 log units from the FMQ buffer (herein referred to as FMQ–2 and FMQ), whereas arc lavas have fO2s ranging between FMQ and FMQ+6 (Christie et al., 1986; Carmichael, 1991). If these magma fO2s directly reflect that of their mantle source regions, it must be concluded that sub-arc mantle is more oxidized than ambient mantle. That sub-arc asthenosphere is oxidized is, arguably, a widely accepted view, and has led to the paradigm that oxidation of the mantle is related, in some manner, to the availability of oxidized fluids or sediments originating from the subducting slab (Carmichael, 1991). Possibilities include overprinting by hydrous fluids characterized by high fO2 (Carmichael, 1991) and/or addition of silicic magmas derived from melting of hot subducting and oxidized oceanic crust (Mungall, 2002). The problem, however, is that true primary arc magmas are extremely rare. This begs the question of whether the fO2s of arc magmas are really representative of their magma source regions. In fact, a number of magmatic differentiation processes can lead to oxidation of a magma. For example, early fractional crystallization of reduced minerals, interaction with oxidizing fluids or wall rock derived from continental crust, or degassing of reduced species of C, H and S (Mathez, 1984) may lead to an increase in fO2. In addition, reaction of H2O with reduced species (e.g. Fe2+) in conjunction with preferential loss of H2 to the atmosphere can result in auto-oxidation of the magma without any open-system exchange other than H2 loss (Sato & Wright, 1996; Holloway, 2004). Given these possibilities for increasing oxidation, the range in barometric fO2 of arc lavas is a maximum bound on their source regions, e.g. arc asthenosphere.
An alternative approach for determining the fO2 of arc asthenospheric mantle is to study arc mantle xenoliths. Indeed, peridotite xenoliths from arc-related environments, such as those from Simcoe near the Washington Cascades, Japan, and the Solomon Islands (Brandon & Draper, 1996; Parkinson & Arculus, 1999), appear to record higher barometric fO2s (up to FMQ+2), but these values still do not match the higher values seen in some arc lavas. Similarly, the majority of continental peridotite xenoliths, some of which may have originated in or been influenced by arc magmas or fluids, have barometric fO2s ranging between FMQ–2 and FMQ+1 (Mattioli & Wood, 1986; Wood & Virgo, 1989; Ballhaus et al., 1990, 1991), overlapping the barometric fO2s of MORB, and lower than that of most arcs. Only the unusual amphibole-bearing mini-xenoliths from western Mexico record higher barometric fO2s (up to FMQ+4; Blatter & Carmichael, 1998). If these arc-related mantle xenoliths represent the source regions of arc lavas, we are faced with the paradox that their fO2s are just not as high as those of arc lavas. However, it is possible that barometric fO2s of mantle xenoliths have nothing to do with the fO2s of arc magma source regions. This is because nearly all peridotite xenoliths at present derive from the lithospheric mantle, even though at some point they were part of the asthenosphere, and have undergone some amount of partial melting (see Fig. 1). This is evidenced by the fact that the last equilibration temperatures and pressures of most mantle xenoliths are typically subsolidus. What this means is that, given the long-term stability of the lithospheric mantle (Jordan, 1975; Pearson et al., 1995; Griffin et al., 1999; Lee et al., 2001a; O'Reilly et al., 2001), its barometric fO2 is not likely to retain any information about the fO2 of the asthenospheric mantle from which the lithosphere was derived. For example, lithospheric fO2 is subject to overprinting by subsequent metasomatic events. In fact, most metasomatizing processes are likely to be oxidizing (McGuire et al., 1991; McCammon et al., 2001). This suggests that even the barometric fO2s of arc-related mantle xenoliths are likely to be maximum bounds on the fO2 of the asthenospheric mantle from which they have originally derived.
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We are left with the question of what is the true fO2 of arc asthenosphere, i.e. the source region to arc lavas, given that barometric fO2s of arc lavas and xenoliths are maximum estimates. This uncertainty stems from the fact that barometric fO2 records only the last equilibrium state and hence bears no memory of past equilibrium states. Thus, it is impossible to estimate the fO2 of the asthenosphere by applying barometric fO2 determinations to magmas that have already left their source regions, or to mantle xenoliths that have already cooled below their solidus temperatures and experienced subsequent metasomatic overprinting. What is needed is a proxy for fO2 that retains a memory of the original fO2 conditions attending melting in the asthenosphere. This original fO2 is referred to here as the primary mantle or primary magma fO2 (compare with the term primitive magma fO2 to describe magmas that have experienced only minor amounts of differentiation since leaving their melt source regions). A requirement of this fO2 proxy is that it must be able to ‘see through’ early differentiation processes in a magma and metasomatic processes in a mantle peridotite, allowing it to retain a memory of the original melting conditions.
Here, we develop the use of V/Sc ratio systematics in peridotites and magmas as a technique for retrieving primary fO2. We choose V and Sc for the following reasons. The behaviors of V and Sc during mantle melting are more similar to each other than to any other elements, as evidenced by their similar enrichments (Fig. 2) in continental crust, arc magmas and MORB relative to primitive mantle (Sun & McDonough, 1989; McDonough & Sun, 1995; Rudnick & Fountain, 1995): they are both mildly incompatible during the formation of MORB and arc lavas, and they are not mobile in fluids. However, superimposed on their overall geochemical similarities is the fact that, in detail, the speciation and thus the partitioning of V is redox-sensitive, whereas that of Sc is not (Fig. 3). V concentrations of primitive melts and residual peridotites are predicted to depend on the fO2 conditions attending melting (Canil & Fedortchouk, 2000; Canil, 2002). The use of V/Sc ratios rather than V alone helps to reduce the effects of magmatic differentiation processes that may dilute V and Sc concentrations, but not significantly modify their relative proportions. This is because early fractional crystallization of olivine will not affect the V/Sc ratio of the magma, as both elements are highly incompatible in olivine. Finally, two additional properties of V and Sc make them ideal for constraining mantle fO2 during melting: (1) in peridotites, these elements are not significantly affected by ‘cryptic’ metasomatism because they are only mildly incompatible (Canil, 2004), and (2) magmatic systems undergoing degassing remain closed to V and Sc exchange. Thus, V/Sc systematics in peridotites and primitive basalts may provide a simple but powerful tool for ‘seeing through’ processes of mantle metasomatism, magmatic degassing and early magmatic differentiation, allowing us to estimate the primary fO2 during mantle melting.
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FMQ) from the fayalite–magnetite–quartz buffer (FMQ); these partition coefficients use the parameterizations given by Canil (2002) |
THEORY: TRACKING MANTLE fO2 USING V/Sc SYSTEMATICS |
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TOPABSTRACTINTRODUCTIONTHEORY: TRACKING MANTLE fO2...LITERATURE DATASETS AND...CASE STUDIESDISCUSSIONCONCLUSIONSAPPENDIXREFERENCES |
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In this section, we develop the basis for using V/Sc systematics in residual peridotites and primitive lavas. We define residual peridotites as those having experienced melt extraction. We ignore those that represent cumulates or have significant melt–rock reaction products. Residual peridotites examined here have Mg-numbers [molar Mg/(Mg + Fe)]
0·88 and Al2O3 contents 
4·3 wt %. Primitive lavas are defined here as those magmas that have experienced only minimal amounts of fractional crystallization. We thus consider only basaltic lavas having SiO2 contents < 52 wt % and MgO contents > 6 wt % to be primitive. Such magmas are likely to have crystallized only olivine and minor amounts of spinel and clinopyroxene.
Modelling the fO2 dependence of V/Sc during melting
Melting model
To illustrate the fO2 dependence of V/Sc on partial melting, we begin with the following simplifying assumptions.
(1) Partial melting takes place at isothermal (1410°C) and isobaric (1·5 GPa) conditions, hence, the effects of changing pressure and temperature on mineral partition coefficients and melting stoichiometry are ignored.
(2) MORB and arc basalts largely originate from the spinel stability field, so we adopt the melting stoichiometry of fertile spinel peridotite (olivine + orthopyroxene + clinopyroxene + spinel) at 1·5 GPa, as determined using the pMELTS thermodynamic algorithm (Ghiorso et al., 2002) and used by Lee et al. (2003).
(3) The V/Sc ratio of fertile convecting mantle is uniform.
Assumption (1) has been made because the exact P–T paths during decompressional melting beneath mid-ocean ridges and in the arc mantle wedge are not known. The errors introduced with this assumption are unlikely to be serious because the pressure dependence of V and Sc partitioning is small, and the range of temperature over which partial melting takes place is not very large (<200°C). Assumption (2) should not lead to large errors. The lack of strong garnet signatures in MORB and most arc basalts indicates that melting in the garnet stability field, although it does occur, is probably small. Nevertheless, in a later section, we will address the effects of garnet melting.
To model magma compositions, we assume that the erupted magmas represent aggregates of fractional melts and hence model the magmas as batch melts. The residual peridotite, on the other hand, is modelled as a fractional residue by assuming incremental extraction of small-degree batch melts. We used the pMELTS thermodynamic algorithm (Ghiorso et al., 2002) to model the melting stoichiometry at 1·5 GPa (Lee et al., 2003).
Initial V and Sc concentrations in the source
The initial V and Sc concentrations (Vo and Sco) of the mantle prior to melt extraction are assumed to be equivalent to that of ‘primitive mantle’, also known as ‘bulk silicate Earth’ (BSE). These values were determined as follows. Sco was estimated by multiplying the estimated Al content of BSE by the chondritic Sc/Al ratio under the assumption that Sc and Al are both refractory lithophile elements and therefore should remain in chondritic proportions in the BSE. This yields Sco of 16·5 ppm (McDonough & Sun, 1995). Unlike Sc and Al, V is moderately volatile and moderately siderophile; hence, some fraction may have been volatilized or lost to the core. Because V cannot be directly inferred from chondrites, we estimate Vo by considering peridotite melt depletion trends where V/Sc is plotted against MgO, Al2O3 and Sc, all of whose BSE concentrations have been previously estimated (Fig. 4; McDonough & Sun, 1995). Extrapolations based on an internally consistent database, which will be discussed below, result in a Vo/Sco of 4·9 (Fig. 4). This yields a Vo of 83 ppm [compare Vo 
82 from McDonough & Sun (1995)]. We note that the mantle today is clearly not ‘primitive’ because it already has seen extraction of highly incompatible elements by the formation of continental crust. However, the net effect of the continental crust formation on major elements and moderately incompatible elements in the residual mantle is nearly negligible because continental crust makes up only 0·6% of the total mass of the BSE. Thus, for all intents and purposes, the V and Sc concentrations of the so-called ‘depleted upper mantle’ are considered here to be nearly identical to that of BSE models. We will return to the issue of mantle heterogeneities in the Discussion section.
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Partition coefficients
We used the following partition coefficients for V and Sc (see Fig. 3). Parametrizations of V partitioning (DV) for olivine, orthopyroxene, clinopyroxene and spinel as a function of fO2 were summarized by Canil (2002)
. Experiments performed on spinels having different Cr-number [Cr/(Cr + Al)] indicate that for a given fO2, V is more compatible in spinels with higher Cr content. The Cr-number of spinels increases with increasing melting degree. The effects are most pronounced at high degrees of melting (>20%) when Cr-numbers approach 0·7 as compared with 0·3 in peridotites having more fertile major element compositions. Some arc magmas probably represent higher-degree melts (or derive from more refractory sources) than MORB (Smith & Leeman, 2005), which makes it difficult to compare directly the V/Sc compositions of primitive MORB and arc lavas if they form by different degrees of melting. However, many arc lavas and MORB overlap in Cr/Al–MgO and TiO2–MgO systematics (Fig. 5), suggesting that the majority of primitive arc lavas and MORB derive from similar degrees of melting. We thus use the same partition coefficients for spinel in our treatment of MORB and arc lavas. Temperature effects on V partitioning were not considered here, as Canil showed that the effects of variable fO2 were much greater than the effects of temperature.
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For Sc, we used published empirical parameterizations of DSc with respect to DMgO for olivine and orthopyroxene (Beattie et al., 1991
; Jones, 1995), where DMgO was constrained by the temperature at which melting was assumed to occur in our models (1410°C; Sugawara, 2000). For clinopyroxene, we used a DSc of 1·2, which represents the average of partition coefficients measured at 1380, 1405 and 1430°C (Hart & Dunn, 1993; Hauri et al., 1994). The partitioning behavior of Sc in spinel is poorly known, but it is likely that Sc is highly incompatible in spinel. We thus assumed that DSc 0 for spinel. We note that the low modal abundance of spinel, and its very low Sc partition coefficient, make the predicted Sc concentrations in the magma and solid residue insensitive to large uncertainties in the partition coefficient.
Application to melts and melt residues
In Fig. 6a, V/Sc ratios of 1·5 GPa melts are plotted as a function of F at given constant fO2s. It can be seen that fO2 and the degree of melting (F) both control the V/Sc in the melt. At low fO2, V is more compatible than Sc, so the V/Sc ratio of the melt is low. At high fO2, V becomes more incompatible than Sc, leading to high V/Sc ratios in the melt. The separations between V/Sc–fO2 melting contours are greatest at low F but the contours converge at very high F due to mass balance effects. Strictly speaking, the fO2 of the melt source region of primitive lavas can only be derived if the degree of melting required to produce the lava is known. However, it can be seen from Fig. 6a that V/Sc ratios are not very sensitive to F at intermediate degrees of melting (F 10%).
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It is generally agreed that MORB represents aggregate melt fractions of
10% based on relative enrichments of highly incompatible elements in MORB (Hofmann, 1988; Langmuir et al., 1992). The average melting degree required to produce primitive arc lavas is more difficult to constrain by this approach because their source composition may be variably metasomatized by the passage of fluids. A more robust approach is to use Ti concentrations. Ti is a moderately incompatible element. Its abundance in a primitive magma, including primitive arc lavas, can be used as a qualitative inverse indicator of F because it is fluid-immobile, and hence its concentrations in the arc source are minimally affected by fluid metasomatism. Using this approach, tholeiitic-type arc lavas have been estimated to represent 6–10 wt % melts of fertile (in the major-element sense) spinel lherzolite (Baker et al., 1994; Bacon et al., 1997; Grove et al., 2002), and are thus similar to MORB. In Fig. 5b, we compare MgO and TiO2 for global arc lavas and MORB. We can see that MORB and primitive arc basalts (MgO > 8 wt %) have very similar TiO2 contents. Only a few arc basalts have lower TiO2 contents, which would indicate higher degrees of melting or partial melting of a refractory source. For these reasons, we assume in all ensuing discussions that primitive arc lavas and MORB generally originate from 10% melting.
In the above modelling, the assumption is that partial melting takes place at constant fO2, although there is no obvious reason why this should be the case. Constant fO2 melting means that either the system is open to O2 exchange and controlled by an external buffer, or that the change in phase proportions and distribution of Fe3+ and Fe2+ among the phases is such that constant fO2 is fortuitously maintained. We assume constant fO2 only because we do not know how to predict exactly how fO2 varies during partial melting. It is often pointed out that because Fe3+ is more incompatible than Fe2+, the bulk Fe3+ content of the residual mantle should decrease. This statement is often casually used to argue that the fO2 of the residual mantle must decrease with progressive melt extraction. Although the first statement is indeed true, the latter statement is not only an over-simplification, but also represents a misunderstanding of what governs fO2. Unlike a magma, which can be considered a homogeneous system, peridotites are heterogeneous systems and therefore the fO2 of a peridotite is not recorded by the bulk Fe3+/Fe2+ ratio, but rather by the proportioning of Fe3+ among the different system phases. As spinel is the dominant phase that incorporates Fe3+, it is important to note that in a peridotite system assumed to be closed to O2 exchange, the fO2 of this system will be controlled largely by the activity of Fe3+ in spinel, which itself must be controlled by the modal proportion of spinel, because spinel is the only phase that takes in appreciable amounts of Fe3+. Thus, if the spinel mode decreases, as it does during partial melting, the effect of decreasing bulk Fe3+ contents is counteracted by the concentrating of Fe3+ into smaller amounts of spinel. The end result is that the change in mantle fO2 during melting may be small. Consistent with this reasoning is the fact that the V systematics of peridotites have previously been shown to follow near constant fO2 melting paths (Canil, 2002; Lee et al., 2003). In any case, the fO2 isopleths shown in our model results can be used to qualitatively assess the effect of changing fO2 during melting.
Effects of fractional crystallization
In the above modelling, we have assumed from the outset that early fractional crystallization of olivine will not affect V/Sc ratios because DV and DSc in olivine are very small [e.g. the exponent in the fractional crystallization equation 
is nearly zero; where the subscript m denotes the melt and the subscript 0 denotes the original melt composition]. The effects of olivine fractionation are illustrated in Fig. 7a, where it can be seen that even over six orders of magnitude in fO2, the effect of olivine crystallization on the magma V/Sc ratio is negligible. We can also consider the effects of spinel co-crystallization with olivine. V is compatible in spinel under most relevant fO2 conditions, whereas Sc is incompatible (Fig. 3). As a consequence, significant spinel fractionation will result in a decrease in V/Sc ratio of the magma. In Fig. 7b and c, we show the effects of co-crystallizing spinel and olivine for the case in which the proportion of spinel in the crystallizing solids is 1 and 5%. It can be seen that for these small amounts of spinel crystallization, the effect on V/Sc in the magma is actually fairly small relative to the large differences predicted in V/Sc at differing fO2s. We note that although the total amount of spinel crystallization in basaltic magmas is difficult to constrain accurately, it is probably less than 5% based on petrographic observations. We thus conclude that for primitive magmas in which only olivine and spinel are on the liquidus, the effects of fractional crystallization on V/Sc ratios can be largely ignored, and hence the measured V/Sc may be taken as representative of the primary V/Sc ratio.
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On the other hand, we note that if clinopyroxene is on the liquidus, the effect on the V/Sc ratio of the magma may be large, particularly at higher fO2s. This can be seen in Fig. 7a–c, where we have co-crystallized clinopyroxene and olivine at MgO < 8 wt %. Crystallization of clinopyroxene at fO2s near FMQ or higher result in an increase in V/Sc in the melt. We also note that clinopyroxene crystallization results in a decrease in Sc concentration (DSccpx > 1), which is not typically seen in MORB and arc basalts with MgO contents greater than
8 wt % MgO.
Effect of garnet during melting
Although melting in the garnet stability field is probably minor for MORB and most arc basalts, we can nevertheless address the role of garnet. Canil has shown that DV for garnet under most terrestrial fO2 conditions is likely to be less than one (Canil, 2002, 2004). Because DSc for garnet in basaltic systems is 2·6 (Hauri et al., 1994), melting of garnet lherzolites yields higher V/Sc than melts of spinel lherzolites for a given fO2. In Fig. 8, we have explicitly modelled melting of lherzolite in the garnet stability field using experimentally determined melting stoichiometries at 3 GPa (Walter, 1998).
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LITERATURE DATASETS AND INTERNALLY CONSISTENT DATA |
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TOPABSTRACTINTRODUCTIONTHEORY: TRACKING MANTLE fO2...LITERATURE DATASETS AND...CASE STUDIESDISCUSSIONCONCLUSIONSAPPENDIXREFERENCES |
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The development of comprehensive geochemical databases (e.g. GEOROC and RidgePetDB) has provided an unparalleled opportunity to investigate global geochemical correlations that might otherwise go unnoticed with small regional datasets. However, certain precautions must be considered when using literature compilations. V and Sc concentrations reported in the literature are often determined using different analytical techniques (XRF, ICP-MS, ICP-AES, INAA) and external standards. Thus, literature compilations may have an inherent level of uncertainty associated with inter-laboratory biases, as previously discussed by Lee et al. (2003)
. Theoretically, these biases can be corrected for but, unfortunately, this is seldom possible because the normalization values of external standards are not always published.
We can quantify how much of the scatter in the literature database is due to inter-laboratory bias by comparing the literature data with an internally consistent dataset for peridotites collected from different localities and tectonic settings. New V and Sc data were determined by inductively coupled plasma mass spectrometry using the USGS basalt standard BHVO-1 as an external standard. Normalization values for V and Sc in BHVO-1 are from Eggins et al. (1997). The peridotites analyzed include spinel peridotite xenoliths from various localities in western North America [Simcoe in the Washington Cascades (Brandon & Draper, 1996), Dish Hill and Cima in California, Vulcan's Throne in Arizona, Lunar Crater in Nevada, Kilbourne Hole in New Mexico], ophiolitic terranes in the western Sierra Nevada foothills (Feather River Ophiolite) and ophiolites and orogenic massifs in the North American Cordillera in British Columbia, Yukon and Alaska. The Simcoe xenoliths are believed to be related to arc mantle and have previously been shown to have barometric fO2s up to FMQ+1·8 (Brandon & Draper, 1996). More details of our samples are presented in the Appendix.
The new V/Sc and Sc data are shown in Fig. 9a compared with the literature database of spinel peridotites. The literature data were not filtered for data quality and, hence, may not be internally consistent due to inter-laboratory bias. It can be seen that the literature V/Sc data are highly scattered. However, considering only the new, internally consistent peridotite data, there is much less scatter and there appears to be a subtle positive correlation between V/Sc and Sc, which is completely obscured using unfiltered literature data. The reduced scatter in the new dataset is remarkable, given that the chosen peridotites come from different localities and tectonic settings, and have different metasomatic histories. This supports the suggestion that the scatter seen in the literature database is probably due to inter-laboratory bias. It follows that the development of internally consistent V and Sc databases can potentially reveal additional structure in V/Sc systematics that may otherwise go unnoticed when using literature compilations.
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Our discussion below is based on a combination of new data and literature-compiled data. Inter-laboratory bias and analytical uncertainties are likely to be largest for peridotites because their V and Sc contents often approach the detection limits of various analytical techniques (e.g. XRF). Thus, for peridotites, our interpretations are based only on new high-precision ICP-MS data. For MORB and arc lavas, analytical uncertainties are likely to be less problematic. Our discussion on lavas begins with the literature dataset for which there is an abundance of data. We then present a subset of V and Sc data for arc lavas, which we have independently confirmed (see the Appendix). In all ensuing discussion, we explicitly denote when new data (this study) and literature data are used.
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CASE STUDIES |
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TOPABSTRACTINTRODUCTIONTHEORY: TRACKING MANTLE fO2...LITERATURE DATASETS AND...CASE STUDIESDISCUSSIONCONCLUSIONSAPPENDIXREFERENCES |
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Peridotites (this study)
In Fig. 9b, we combine our model predictions with the new peridotite V and Sc data. These data represent a range of tectonic environments, including continental lithospheric mantle xenoliths from western North America, an orogenic massif and obducted ophiolites. There is a slight correlation between V/Sc and Sc. The range in Sc (from low values of
5 up to values of 17) reflects progressive depletion in Sc during melt extraction. The decrease in V/Sc with degree of melt extraction indicates that overall V is slightly more incompatible than Sc. When compared with the model predictions, most of the V/Sc–Sc data appear to closely follow the FMQ–1 melting curve; the total variation in fO2 seems to be less than ±0·5 log units. The V/Sc-predicted fO2s fall within the range of barometric fO2 recorded for abyssal peridotites and MORB glasses (Christie et al., 1986; Wood & Virgo, 1989; Wood et al., 1990). These peridotites come from oceanic, arc and continental settings where barometric fO2s in lavas range from –2 to +6 log units from the FMQ buffer. Considering the sensitivity of V/Sc ratio to fO2, the uniformity in V/Sc (and, by inference, primary mantle fO2) is remarkable.
MORB (RidgePetDB)
Here, we examine MORB data from the RidgePetDB database (Lehnert et al., 2000). The MORB data comprise whole-rock and glass analyses from the Mid-Atlantic, Juan de Fuca, Indian, and East Pacific Rise ridges. The MORB data are plotted along with arc data in Fig. 10a in terms of V/Sc versus MgO. V/Sc ratios appear to increase slightly with degree of magmatic differentiation (decreasing MgO content) at MgO < 8 wt %. As shown in Fig. 7, the increase in V/Sc is probably due to the onset of clinopyroxene crystallization. However, if we consider only those samples with MgO contents between 8 and 12 wt %, we capture those samples that have primarily experienced only olivine (and minor spinel) crystallization, which has little influence on melt V/Sc ratio because both elements are highly incompatible in olivine. Thus, V/Sc ratios of basalts with MgO contents between 8 and 12 wt % probably come close to representing that of their magmatic parent, and provide a window to mantle fO2. The MgO-filtered V/Sc data are shown as a frequency distribution in Fig. 10b. The range in MgO-filtered V/Sc ratios decreases substantially, yielding an average V/Sc ratio of 6·7 ± 1·1 (1
).
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Superimposed on Fig. 10a and b are the calculated V/Sc ratios for given fO2s at F = 10 wt %. If the assumptions in our melting model are reasonable, then the narrow range in V/Sc implies that the fO2 of the MORB source is constrained roughly between FMQ–1 and FMQ+0·2. The model predictions fall within the range of fO2 estimated from the Fe2+/Fe3+ ratios of minerals and glasses in abyssal peridotites and MORB, respectively [FMQ–2·5 to FMQ+0·5; (Christie et al., 1986
; Wood et al., 1990)]. The fO2s predicted for MORB by the V/Sc redox method are also self-consistent with the redox conditions predicted by the same method for peridotites.
Arc lavas (literature data and this study)
Global literature database (GEOROC)
We now examine the V/Sc systematics of arc magmas, beginning with the GEOROC database. In the next section, we examine the Cascades magmatic series in more detail. Pre-compiled datasets for the following arcs were used: Aleutian, Andean, Cascades, Central American, Izu–Bonin, Kamchatka, Luzon, Marianas, Mexican and Tonga arcs. The Cascades, Central American and Mexican arcs correspond to regions where young, hot oceanic lithosphere is being subducted. In contrast, the western Pacific arcs (Izu–Bonin, Kamchatka, Luzon, Marianas and Tonga) correspond to regions where old, cold lithosphere is being subducted. The pre-compiled files were filtered to eliminate any non-magmatic rocks (e.g. sediments, xenoliths, cumulates) that were accidentally incorporated into the database. We also eliminated any samples (e.g. fore-arc, oceanic crust, seamounts, etc.) that do not specifically represent arc lavas.
Figure 10a contrasts all arc and MORB data in terms of V/Sc versus MgO. The arc lavas can be subdivided into three parts. For MgO > 8 wt %, V/Sc ratios appear to be roughly constant over a range in MgO. Between 3 and 8 wt % MgO, the scatter in V/Sc ratios increases substantially and the average V/Sc ratio increases. Finally, below 3% MgO, V/Sc plummets. These systematics can be rationalized as follows. At MgO > 8 wt %, the main crystallizing phase is olivine, which does not significantly change the V/Sc ratio of the melt, for reasons discussed above. At MgO between 3 and 8 wt %, most of the rise in V/Sc is attributed largely to the appearance of clinopyroxene as a crystallizing phase. For MgO < 4 wt %, the rapid decrease in V/Sc reflects the appearance of Fe–Ti oxide as a significant crystallizing phase, which strongly prefers V over Sc. The MgO > 8 wt % window thus represents the best estimate of the V/Sc ratio of the parent arc basalt (7·09 ± 2·5, 1; Fig. 10b). The V/Sc ratios of these high-MgO basalts can be compared with the V/Sc–fO2 melting contours, as we did for MORB. As discussed previously, we treat primary arc lavas as if they are 10% melts, like MORB. This is consistent with similar Ti contents (inverse proxy for degree of melting, F) in primitive MORB and most arc lavas (Fig. 5b). Although some high-Mg-number [Mg/(Mg + Fe)] basaltic andesites and boninites may represent much higher-degree melts (up to 30%), these are volumetrically minor in most arcs (see Fig. 5a and b).
Figure 10a shows that basalts with the highest V/Sc ratios tend to be from arcs. However, independent of model assumptions, an unequivocal feature of Fig. 10a and b is that for MgO > 8 wt %, most of the V/Sc ratios of arc lavas overlap extensively with MORB. These results are consistent with a recent investigation, which shows that Ti/V, previously used to distinguish the tectonic affinities of magmatic rocks (Shervais, 1982), may not be adequate for distinguishing arc basalts and MORB (Vasconcelos-F. et al., 2001). Like the peridotite perspective, the remarkable similarity in V/Sc ratios between arc and mid-ocean ridge basalts is surprising in light of the wide range of barometric fO2s seen in arc lavas. If the V/Sc ratios of the arc basalts are representative of their mantle source regions, this implies that the primary mantle fO2 beneath arcs is not very different from that beneath mid-ocean ridges (e.g. identical to within ±0·5 log units). Even if we consider some of the arc basalts with higher V/Sc ratios, the estimated V/Sc-inferred fO2s are nowhere near as high as the corresponding barometric fO2s of the arc lavas.
The Cascades: a closer look
In this section, we specifically focus on the Cascades magmatic arc to see if there are any petrogenetically significant systematics ‘hidden’ within the scatter of the global literature database. We chose a suite of primitive basalts from the Cascades volcanic arc in southern Washington (Smith & Leeman, 2005), southern Oregon and northern California (Bacon et al., 1997; Grove et al., 2002). The data represent samples from Mount Shasta and the Lassen volcanic field (northern California), Crater Lake (Oregon) and our own data from southern Washington. We have independently confirmed a few of Bacon et al.'s and Grove et al.'s data by re-analyzing some of their samples (see the Appendix); hence, we believe that this subset of data is internally consistent with ours.
The Cascades primitive magmatic series includes low-potassium tholeiitic to alkalic basalts of intraplate affinity, as well as calcalkalic basalts (Leeman et al., 2005). The former are characterized by lower K2O and Ba/Yb values than the calcalkalic basalts. In Fig. 11a, Ba/Yb ratios and K2O contents for primitive basaltic magmas from the Cascades (SiO2 < 52 wt %, MgO > 6 wt %) are positively correlated. Importantly, there appears to be no correlation between Ba/Yb and K2O with MgO (not shown) content or TiO2 content (Fig. 11b). Variations in MgO content are in part due to fractional crystallization, whereas variations in TiO2 content in the most primitive lavas are due to differences in the degree of melting. The lack of correlations with MgO and TiO2 thus indicate that Ba/Yb and K2O reflect source compositions. It is widely assumed that Ba is a fluid-mobile element, and that high Ba/Yb ratios indicate the influence of fluids (Yb is not fluid-mobile). The variations in Ba/Yb and K2O are believed to reflect different slab contributions from the sources for arc lavas (Leeman et al., 2005).
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An important feature is that there is a distinct correlation between V/Sc and Ba/Yb ratio (and K2O; Fig. 11a) in primitive basalts (Fig. 12a). V/Sc ratios range from
5 in the tholeiitic end-member (low Ba/Yb) to up to 10–11 in the more calcalkalic end-members (high Ba/Yb). When the data are taken as a whole, there is no correlation between V/Sc and MgO content (Fig. 12b). Because it is unlikely that the V/Sc ratio of the mantle wedge has been significantly modified by fluid metasomatism, the variations in V/Sc for a given MgO must derive from partial melting processes and not variations in the V/Sc of the source. The two melting parameters that can affect V/Sc in the melt are variations in fO2 and degree of melting. There appears to be no correlation between V/Sc and TiO2 (not shown, but this can be inferred from Fig. 11b) in the Cascades magmas, so the latter is not considered here to be a significant effect. This suggests fO2 may be the remaining parameter. If we now use the F = 10 wt % V/Sc–fO2 contours, we find that the observed range in V/Sc ratio corresponds to an apparent fO2 of melting between FMQ–1·25 and FMQ+0·5. This range coincides closely with that of MORB mantle, as inferred from V/Sc systematics (Fig. 8). Thus, whereas the apparent correlation between V/Sc and Ba/Yb in the Cascades magmatic series suggests that fluids indeed influence fO2 during melting, the overall effect is small. Importantly, the range in V/Sc ratios of primitive Cascade basalts spans most of the range seen in the global database of primitive arc basalts, reinforcing the suggestion in the previous section that arc lavas appear, on average, to derive from mantle of similar fO2 to MORB mantle. However, of equal importance is the fact that upon closer inspection, subtle variations in arc source fO2 may exist as a consequence of fluid metasomatism.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONTHEORY: TRACKING MANTLE fO2...LITERATURE DATASETS AND...CASE STUDIESDISCUSSIONCONCLUSIONSAPPENDIXREFERENCES |
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On the similarities in V/Sc ratios of arc lavas and MORB
The above case studies show that although there may be some correlation between primary mantle fO2 and the amount of subduction-related fluids in arc environments, the overall range in primary mantle fO2 inferred from V/Sc systematics is much smaller than the range in barometric fO2 of the lavas. The most obvious question is whether this similarity may be an artifact of some process that tends to decrease the V/Sc ratio of arc lavas before or very early in the differentiation process. A number of factors come to mind. These are discussed in sequence below.
Crustal contamination in ‘primitive’ arc lavas
Continental crust is characterized by V/Sc ratios in the range of 4–6 (Rudnick & Fountain, 1995), hence its incorporation into a hypothetical arc basalt with a high V/Sc ratio (and hence originating from a mantle with high fO2) would decrease the basalt's V/Sc ratio. However, because the concentrations of V and Sc in continental crust are not significantly different from that in primitive basalts, unreasonable amounts (>50 wt %) of crustal contamination are needed to significantly decrease the V/Sc ratio of the basalt. Addition of such large amounts of crust would be manifested in distinct major element changes, such as a large decrease in MgO (Figs 10a and 12b) or an increase in SiO2 content. Such mixing trajectories are not seen in arc lavas. Similarly, it can also be shown that the correlation between Ba/Yb and V/Sc seen in the Cascades magmas (Fig. 12a) cannot be explained by crustal contamination.
Garnet peridotite melting for arc lavas
The effect of melting in the garnet lherzolite stability field also cannot explain the low V/Sc ratios of arc lavas. As discussed earlier, melting of a garnet lherzolite yields a higher V/Sc ratio in the melt (Fig. 8).
Higher melting degrees for arc lavas
One of the assumptions in comparing the V/Sc ratios of primitive arc lavas and MORB is that their primary/parental magmas represent similar degrees of melting (e.g. 10%). At high degrees of melting, the fO2 isopleths converge such that the sensitivity of the V/Sc ratio to fO2 decreases. For example, if arc lavas actually represent 20% melts, their expected V/Sc ratios at fO2s of FMQ+3 would be 12, which itself would correspond to an fO2 of only FMQ+1 if arc lavas were erroneously assumed to be 10% melts. Although this scenario may explain our observations, the problem is that arc lavas, in general, are probably not significantly higher-degree melts than MORB. As shown in Fig. 5, the similar Cr and Ti systematics of primitive arc and MORB lavas suggest that, on average, arc lavas and MORB derive from similar degrees of melting.
High-pressure crystallization
Another possibility is that arc lavas begin differentiating at greater depths than MORB due to the thick pre-existing lithosphere through which they must pass. At high pressure, the olivine phase field contracts significantly so that other phases, such as clinopyroxene, will be more likely to crystallize. Clinopyroxene crystallization, however, would lead to an increase in V/Sc ratio, especially for fO2s greater than FMQ.
It can be seen that all of the above complicating factors cannot explain the remarkable similarity in V/Sc ratios between primitive arc lavas and MORB. Crustal contamination is an ineffective means of decreasing the V/Sc ratio, whereas all of the other factors only serve to increase V/Sc. We thus conclude that the similarity in V/Sc ratios reflects a similarity in the primary fO2s of the mantle source regions of MORB and primitive arc lavas.
Why do arc lavas and some arc xenoliths have higher barometric fO2s than their primary fO2s as inferred from V/Sc systematics?
Based on the above discussion, our preferred interpretation is that the V/Sc-inferred fO2s of the mantle source regions to arc lavas are significantly lower than their corresponding barometric fO2s. Similarly, some arc-related peridotite xenoliths appear to have higher barometric fO2s than MORB-source mantle (though not as high as arc lavas), even though their V/Sc systematics imply MORB-like fO2s. Below, we first discuss why arc lavas have barometric fO2s that tend to be higher than the barometric fO2s of arc xenoliths and the V/Sc-inferred fO2s of the source regions to arc lavas. We leave the smaller discrepancy between V/Sc-inferred and barometric fO2s of arc xenoliths to the end.
On the origin of high barometric fO2 in arc lavas
The large discrepancy between the barometric fO2s of arc lavas and the V/Sc-inferred fO2s of their source regions suggests that the high fO2s of arc lavas might be due to the evolution of magmatic fO2 during ascent, emplacement and/or magmatic differentiation. Contamination by crustally derived fluids, dissociation of volatiles, fractional crystallization and hydrothermal alteration are all processes that could potentially modify fO2 in the magma. Holloway (2004) recently hypothesized that rapidly crystallized magmas may undergo an auto-oxidation process in which ferrous iron is oxidized by water to generate magnetite and H2, the latter of which is free to leave the system as a fluid:
 | (2) |
). If this auto-oxidation hypothesis is correct, it provides an attractive explanation for the higher fO2s of arc lavas (which are invariably sampled in crystalline form) compared with their magma source regions. As auto-oxidation occurs in a closed system (except for loss of H2), there would be no noticeable effects on bulk-rock chemistry (e.g. no changes in V and Sc contents). This leaves us with the question of how much of the high barometric fO2s seen in arc lavas is due to differentiation and degassing processes and how much is due to true fO2 variations in their source regions. Our investigation of the Cascades arc magmas suggests that there may be fO2 variations in the source regions by 1–1·5 orders of magnitude, but much larger source variations are not likely considering the global similarity in V/Sc systematics of arc lavas and MORB.
On the origin of high barometric fO2 in mantle xenoliths
We now discuss why barometric fO2s of mantle xenoliths tend to be more variable and reach higher values than those implied by their remarkably similar V–Sc systematics. As pointed out earlier, mantle xenoliths undoubtedly derive from the lithospheric mantle, as evidenced by the fact that nearly all mantle xenoliths have last equilibrated at subsolidus conditions. Because the lithospheric mantle is less mobile than the asthenospheric mantle, it remains isolated from the convecting mantle for much longer periods of time. For these reasons, the trace-element composition of the lithospheric mantle often represents the time-integrated product of numerous metasomatic events. Given that most metasomatizing fluids are oxidizing (McGuire et al., 1991; McCammon et al., 2001), it seems reasonable to suggest that the fO2 of lithospheric mantle could eventually be increased by the passage of numerous oxidizing, metasomatizing fluids. Because arc lithospheric mantle has a much longer residence time above a dehydrating slab than any given parcel of asthenospheric mantle wedge material, it is likely that arc lithosphere has interacted more with oxidizing fluids than the asthenospheric mantle itself. It is thus possible that arc lithosphere could become slightly more oxidized than the asthenospheric mantle. We suggest that the slightly high barometric fO2s of some arc xenoliths may occur due to metasomatic resetting during long-term residence in the lithospheric mantle, whereas their low V/Sc-inferred fO2s may reflect the original fO2s attending partial melting of these xenoliths when they were part of the asthenosphere and not yet incorporated into the lithosphere.
If continental lithospheric mantle has been subsequently oxidized, it follows that melts of lithospheric mantle should yield higher V/Sc ratios, all other parameters being constant (e.g. initial V and Sc concentrations and average degree of melting). In this context, the positive correlation between V/Sc and Ba/Yb in primitive Cascades arc lavas could occur due to melting fluid-metasomatized asthenospheric mantle or melting fluid-metasomatized lithospheric mantle. Indeed, Leeman et al. (2005) have suggested that some arc basalts may acquire their compositions from melting or interaction with lithospheric mantle.
Is the asthenosphere buffered in fO2?
Assuming that the fO2 of dehydrating slab fluids is high (due to oxidation at the Earth's surface) and that fluids are probably more abundant in arc environments than beneath mid-ocean ridges, the low fO2s of asthenospheric arc mantle, as inferred from V/Sc systematics, suggest that the fO2 of the upper mantle may be globally buffered (e.g. the volume of oxidizing fluids is insufficient to overprint the asthenospheric mantle fO2) and the oxidizing potential of slab fluids is not as high as widely perceived. It has been pointed out before that the oxidizing potential of water in the mantle is in fact very low, so other oxidizing agents are required (Frost & Ballhaus, 1998). Fe-bearing hydrous melts have been proposed by Mungall (2002) as more effective oxidizing agents, but it is unclear how pervasive such fluids are in subduction zone environments. According to Mungall, these fluids are likely to be released only when temperatures high enough to melt oceanic crust are achieved; such conditions are met only in the rare cases in which hot and young oceanic lithosphere is being subducted. Thus, if Mungall's hypothesis is correct, it means that, generally, the amount and oxidizing power of subduction-related fluids released into the sub-arc mantle wedge may be insufficient to exceed the buffering capacity of the asthenospheric mantle.
These observations suggest that the upper convecting mantle may be strongly buffered, supporting previous proposals of this concept (Blundy et al., 1991). A buffered mantle is also in line with observations that the fO2 of the mantle has also not changed significantly with time, as evidenced from V systematics of Archean komatiites, basalts and peridotites (Canil, 2002; Lee et al., 2003; Li & Lee, 2004) and Cr systematics in Archean basalts and komatiites (Delano, 2001). Although it is beyond the scope of this paper to discuss in detail the different types of possible buffering assemblages, we note that assemblages involving H, C and S fluid species or Fe species equilibria have all been proposed. Which particular system controls fO2 is, so far, equivocal given the uncertain abundances of some of these elements, and, thus, buffering capacity, in the mantle (Canil et al., 1994). On a final note, we point out that continental lithospheric mantle, by contrast, may not be so well buffered, owing to the likelihood that the total flux of oxidizing metasomatic fluids through continental lithospheric mantle is much larger than that in the convecting mantle itself.
Caveats, concerns and continuing work
Several issues have not been addressed in great detail in this study. The first issue is that we considered a homogeneous source. Although we believe that, in general, this is a reasonable assumption, there are undoubtedly pockets of major-element heterogeneities in the mantle. For example, some xenolith suites contain ample evidence for metasomatic veins and dikes. Unlike cryptic metasomatism, which involves only changes in trace-element composition, veins and dikes reflect modal metasomatism. Not only might these veins or dikes be locally oxidizing, but they are also likely to have V and Sc contents differing from the primitive mantle and to have melting stoichiometries differing substantially from typical lherzolite. Some of these veins or dikes are rich in hydrous minerals, such as amphibole or phlogopite. The V/Sc ratios of such magmas will be difficult to interpret because of the added free parameters in the modelling (e.g. initial V and Sc content, melting stoichiometry and partitioning of V and Sc in hydrous phases are at present ill-constrained). Given all these free parameters and the remarkable similarity in V/Sc systematics of most arc basalts and MORB, it seems highly unlikely that melting of unusual metasomatic veins plays a significant role in the generation of MORB and arc lavas. However, unusually hydrous lavas in arc and intraplate settings might indeed originate from melting of such veins. We note that intraplate lavas appear to have much greater variation in V/Sc ratios, including values much greater (>9) than those seen in MORB and most arc basalts. These higher values could imply higher fO2s during melting, lower degrees of melting, the presence of residual garnet, and/or different V and Sc contents of the source. For such magmas, V and Sc systematics should be carefully examined in conjunction with other trace and major elements in order to unravel these complexities. If these complexities can be dealt with, V/Sc studies of intraplate lavas derived from lithospheric melting may help to constrain the fO2 of the lithospheric mantle, which would be especially helpful in those regions in which mantle xenoliths are absent.
Finally, a methodical study of V/Sc systematics in conjunction with barometric determinations of fO2 in a continuous magmatic differentiation series would go far in unraveling the evolution of fO2 during magmatic differentiation. This approach would entail not only measuring bulk-rock V/Sc ratios, but also estimating empirically the partition coefficients of phenocryst–lava pairs at different stages of the magma differentiation history. These studies could also be combined with other redox-sensitive elements, e.g. Cr and Eu.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONTHEORY: TRACKING MANTLE fO2...LITERATURE DATASETS AND...CASE STUDIESDISCUSSIONCONCLUSIONSAPPENDIXREFERENCES |
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An in-depth examination of V/Sc systematics in primitive arc lavas shows that V/Sc is correlated with fluid-mobile element enrichment (K, Ba), indicating that mantle fO2 may be subtly influenced by the addition of fluids to the mantle wedge. Nevertheless, it appears that, on average, MORB and many arc magmas have indistinguishable V/Sc systematics within error, resulting in similar fO2s (FMQ–1·25 to FMQ+0·5) predicted for their source regions. The V/Sc systematics of peridotites from different localities and tectonic environments, with varying degrees of metasomatic alteration, also show remarkably similar V/Sc systematics. The peridotite V/Sc data correspond to predicted fO2s ranging from FMQ–1·25 to FMQ, completely overlapping with the range seen in MORB and arc lavas. Thus, despite a small influence of fluids on fO2 in subduction zones, the fO2 of the asthenospheric mantle is, on average, surprisingly homogeneous. In the case of MORB, the V/Sc fO2s coincide with barometric fO2s for abyssal peridotites and MORB glasses. In the case of arc magmas, the V/Sc fO2s are lower than the high barometric fO2s seen in arc lavas and some arc-related mantle xenoliths. These observations suggest that the upper mantle is largely buffered in terms of fO2. High barometric fO2s in mantle xenoliths and arc lavas can be explained if the former receive a much larger time-integrated fluid flux due to their long residence times in the lithospheric mantle, and if the latter have become oxidized during magma ascent and differentiation. A buffered mantle implies that secular changes in mantle fO2 are likely to be small.
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APPENDIX |
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TOPABSTRACTINTRODUCTIONTHEORY: TRACKING MANTLE fO2...LITERATURE DATASETS AND...CASE STUDIESDISCUSSIONCONCLUSIONSAPPENDIXREFERENCES |
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The new ICP-MS measurements are given in Tables A1 and A2. Table A1 consists of xenolithic and ophiolitic peridotites from scattered localities extending from the southwestern United States to the Canadian Cordillera and Alaska. Some of the xenolith localities have been previously described in the literature (Wilshire et al., 1988
; Mukasa & Wilshire, 1997). Nearly all samples are spinel peridotites, but a few contain both spinel and garnet (Big Creek, California; Lee et al., 2000; Lee et al., 2001b). Some ophiolitic samples (e.g. Feather River Ophiolite, California) have been variably serpentinized. These samples are included to show that V and Sc systematics are not significantly disturbed during serpentinization. The data in Table A1 were analyzed in three different ICP-MS laboratories and are referenced in Table A1 as (1) Rice University; (2) University of Victoria; (3) Australia National University. All measurements were determined by external standard normalization to an international rock standard. The data from Australia National University were previously published by Lee et al. (2003). However, during the course of this study, we discovered that the preferred V value for the external standard (United States Geological Survey Hawaiian Basalt standard BHVO-1) used in those measurements was lower (281 ppm) than that used in the laboratories at Rice University and University of Victoria (321 ppm; Eggins et al., 1997). We have thus made a small correction to these numbers. The normalization value of BHVO-1 for Sc was taken to be 31·8 (Eggins et al., 1997).
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Table A2 consists of data for basaltic lavas from Mount Shasta in northern California and the southern Washington Cascades. We re-analyzed some of the samples from Bacon et al. (1997)
and Grove et al. (2002). Their data are shown for comparison. In general, the agreement is very good. |
ACKNOWLEDGEMENTS |
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Support from NSF grants (EAR 0309121, 0440033, 0003612 and 0409423) to Lee and Leeman and NSERC of Canada and Yukon Geology Program grants to Canil are acknowledged. M. Hu is thanked for help with data compilation. Kevin Righter and an anonymous reviewer are thanked for thoughtful comments. Discussions with Chip Lesher, Lara Heister, Terry Plank, Mark Little, Yongsheng Liu and Alan Brandon are greatly appreciated. Alan Brandon and Marc Norman are also thanked for contributing the data analyzed at Australia National University. We also thank Tim Grove for providing splits of lavas from the Northern California Cascades used to confirm inter-laboratory agreement.
* Corresponding author. Telephone: 1-713-348-5084. E-mail: ctlee{at}rice.edu
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