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Journal of Petrology | Volume 43 | Number 4 | Pages 705-723 | 2002
© Oxford University Press 2002
Ancient and Modern Subduction Zone Contributions to the Mantle Sources of Lavas from the Lassen Region of California Inferred from Lu–Hf Isotopic Systematics
1INSTITUTE OF METEORITICS, UNIVERSITY OF NEW MEXICO, ALBUQUERQUE, NM 87131, USA
2ÉCOLE NORMALE SUPÉRIEURE DE LYON, 69364, LYON CEDEX 7, FRANCE
3US GEOLOGICAL SURVEY, 345 MIDDLEFIELD RD, MS 910, MENLO PARK, CA 94025, USA
Received February 25, 2001; Revised typescript accepted October 29, 2001
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONREVIEW OF THE GEOCHEMISTRY...RESULTSPROCESSES THAT MODIFY Hf...ISOTOPIC EVOLUTION OF THE...A PETROGENETIC MODEL FOR...CONCLUSIONREFERENCES |
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Hafnium isotopic compositions have been determined on a suite of calc-alkaline and high-alumina-olivine tholeiitic lavas from the Lassen region of California and are used, in conjunction with previously published mineralogical, geochemical, and isotopic data, to constrain their petrogenesis. Positive correlation between
Hf values and geochemical indices of the modern subduction component indicates that the isotopic compositions of the calc-alkaline lavas record addition of radiogenic Hf from the subducted slab. However, the addition of the modern subduction component increases the Hf values of most calc-alkaline lavas by <0·5 units over estimates of non-subduction enriched peridotites of the mantle wedge. The Lu–Hf isotopic systematics of the Lassen lavas suggest that the calc-alkaline magmas have equilibrated with garnet at some point in their history, whereas the tholeiitic magmas have not. These observations require the two lava types to be derived from different sources. The isotopic variability of the Lassen lavas cannot be produced by mixing mantle sources inferred to be present in the eastern–central Pacific and western USA with a modern subduction component. Instead, the isotopic variability is consistent with mixing of a depleted mantle source, a more fertile mantle source enriched by an ancient subduction component, and a modern subduction component. KEY WORDS: Hf isotopes; Cascade arc; subduction zone; calc-alkaline; tholeiitic
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONREVIEW OF THE GEOCHEMISTRY...RESULTSPROCESSES THAT MODIFY Hf...ISOTOPIC EVOLUTION OF THE...A PETROGENETIC MODEL FOR...CONCLUSIONREFERENCES |
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The geochemical and isotopic variability of mantle wedge peridotites beneath continental volcanic arcs is poorly constrained. This is a result of the fact that most of what is known about the geochemistry of the mantle wedge is inferred from the composition of basaltic arc lavas, which, although derived from the wedge, are affected by variable contributions from the subducting slab and continental crust (e.g. Gill, 1981
; Tatsumi et al., 1986; Hawkesworth et al., 1993, 1994). Despite these limitations there is significant evidence to suggest that mantle wedge peridotites are heterogeneous in composition and mineralogy. For example, it has been postulated that the compositions of mantle peridotites beneath the Cascade arc vary from compositions that are similar to sources for mid-ocean ridge basalt (MORB), Hawaiian ocean-island basalt (OIB), and high-alumina-olivine tholeiites (HAOT) from the Basin and Range province (Hughes, 1990; Leeman et al., 1990; Bacon et al., 1997; Borg et al., 1997; Clynne & Borg, 1997). The restite mineralogy of the source regions for Cascade magmas is poorly constrained as well. For example, experiments conducted by Baker et al. (1994) found that calc-alkaline basalts from the Mt. Shasta region of California did not have garnet on their liquidus, and Reiners et al. (2000) were able to model trace element variations in basaltic lavas from the central Cascades without garnet in their source. In contrast, Guffanti et al. (1990) noted that the depth of the slab beneath the southernmost Cascades was in the garnet peridotite stability field. In addition, several studies of calc-alkaline lavas from the Cascade range have suggested the presence of garnet in their source regions based on rare earth element (REE) geochemistry of the lavas (Borg et al., 1997; Conrey et al., 1997). This paper assesses the heterogeneity, mineralogy, and evolution of the sub-arc mantle in the southernmost Cascades by examining the Hf isotopic systematics of a suite of calc-alkaline and tholeiitic lavas. This can be done because, although the modern subduction component is found to contribute radiogenic Hf to the mantle wedge, the Hf isotopic composition of many Lassen lavas primarily reflects the isotopic composition of their mantle source regions before the addition of the modern subduction component. Before the Hf isotopic systematics of the Lassen lavas are discussed a review of the pertinent geologic, petrologic, and geochemical observations is presented below.
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REVIEW OF THE GEOCHEMISTRY AND PETROLOGY OF PRIMITIVE LAVAS FROM THE LASSEN REGION OF THE CASCADE RANGE |
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TOPABSTRACTINTRODUCTIONREVIEW OF THE GEOCHEMISTRY...RESULTSPROCESSES THAT MODIFY Hf...ISOTOPIC EVOLUTION OF THE...A PETROGENETIC MODEL FOR...CONCLUSIONREFERENCES |
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The Lassen region is the southernmost segment of the active Cascade arc (Guffanti & Weaver, 1988
) and is located in northeastern California between the Klamath terrane and the northern Sierra Nevada. Volcanism in the Lassen area can be characterized on two scales. Between the large long-lived volcanic centers that produce evolved rocks, regional volcanism has built a broad platform of small to intermediate-sized volcanoes that have short lifetimes. Typically, these volcanoes are monogenetic or are active over a relatively short period of time, from a few years to a few thousand years. The overall result is construction of a broad platform of volcanic rocks that in the vicinity of the active arc is 
4 km thick.
The extensional Basin and Range province is expanding westward and impinging upon the active Cascade arc in northern California (Guffanti et al., 1990), with the result that normal faults are prominent in the Lassen area. Linear arrays of small cinder cones and lava flows that trend parallel to the normal faults demonstrate that magmas often exploit these weaknesses in the crust. A small subset of lavas erupted from monogenetic vents are the most primitive in the area, and further discussion is limited to them.
We define primitive lavas as those meeting a combination of physical and geochemical criteria generally considered to characterize lavas that are little evolved since being separated from their mantle source [e.g. Basaltic Volcanism Study Project (BVSP), 1981]. Primitive lavas are sparsely porphyritic and have simple mineral assemblages (usually singly saturated) of unzoned phenocrysts with compositions in equilibrium with mantle peridotites. Bulk-rock compositions have high contents of compatible elements and are also in equilibrium with mantle peridotite. Two mineralogically and geochemically distinct types of primitive lavas are present in the Lassen area; HAOT [equivalent to low-potassium olivine tholeiites of Bacon et al. (1997)] and compositionally diverse calc-alkaline basalts–andesites (CAB). The andesitic lavas meet these primitive criteria, but have higher silica content (53–57% SiO2) and thus are andesites. These are similar to rocks found in other arcs and are often referred to as high-magnesium andesites or sanukitoids (e.g. Tatsumi, 1981; Tatsumi & Ishizaka, 1982; Yogodzinski et al., 1994).
Primitive HAOT lavas contain up to a few percent olivine phenocrysts. The olivines typically include chromian spinel, which is also occasionally present in the groundmass. The olivine and spinel are unzoned and have magnesian compositions with high Ni (Fo86–88, Ni 
2000). These compositions are in equilibrium with the bulk composition of the lava and potentially in equilibrium with fertile peridotite (e.g. BVSP, 1981). Chromian spinels have magnesian compositions and fall within the mantle array of Arai (1987). Olivine–spinel equilibria indicate crystallization at temperatures approaching 1300°C and elevated pressures (Clynne & Borg, 1997). Sparse calcic plagioclase (An70) is present in a few primitive HAOT, but clinopyroxene is absent.
The most striking feature of HAOT from the Lassen region is the limited range of major- and trace-element variability displayed by these lavas (Clynne, 1993; Bacon et al., 1997). Distinctive features of HAOT from the Lassen region are low SiO2 (48 wt %), FeO*/MgO of 0·9–1·0, high Al2O3 (17–18 wt %), and low alkali content, especially K2O (0·2 wt % or less). HAOT have high contents of compatible elements (MgO 9 wt %, Ni 100 ppm and Cr 200 ppm). In comparison with CAB, HAOT have lower concentrations of large ion lithophile elements (LILE), higher concentrations of heavy REE (HREE), lower LILE/high field strength element (HFSE) values, and lower light REE (LREE)/HREE (Fig. 1a). In general, their trace element characteristics are E-MORB-like. HAOT display weak enrichments of Ba, Sr, and Pb relative to LILE or LREE. Sr and Nd isotopic systematics of HAOT are distinct from those of CAB, primarily in having slightly higher 144Nd/143Nd ratios at equivalent 87Sr/86Sr ratios (Bacon et al., 1997; Borg et al., 1997).
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) to high (Sr/P)N represented by LC86-1009 (•). (b) Source mixing–partial melting models from Borg et al. (1997)
, 5% non-modal partial melt of a source with ol:opx:cpx:spl:grt:amph = 56:12:6:4:7:15 melting in the proportions ol:opx:cpx:spl:grt:amph = 5:10:45:1:4:35. The trace element composition of this source is a mixture of 99·31% MORB-source + 0·6% slab-derived fluid + 0·09% Pacific sediment. Composition of MORB source, fluid, and sediment for all models from Borg et al. (1997)
, 8% non-modal partial melt of a source with ol:opx:cpx:spl:grt:phl = 51:19:19:5:4:12 melting in the proportions ol:opx:cpx:spl:grt:phl = 11:17:55:1:1:15. The trace element composition of this source is a mixture of 99·6% OIB-source + 0·4% sediment. The models are compared with typical low (Sr/P)N CAB (LM87-1384) and high (Sr/P)N CAB (LC86-1009). (c) Sample LMM81-279 (
) has an incompatible element pattern that is typical of most HAOT (Clynne, 1993
) has higher LILE abundances and is representative of only a small fraction of HAOT. 
, a mixture of 75% HAOT (LMM-279) and 25% CAB (LC88-1314). {dtri}, results of 8% partial melting of a peridotite with ol:opx:cpx:spl = 55:15:25:5 in the proportions ol:opx:cpx:spl = 15:25:58:2. The incompatible element composition of the source is assumed to be represented by a mixture of 97% depleted mantle and 3% slab-derived fluid. It should be noted that the models are consistent with production of the high-LILE HAOT lavas from the low-LILE HAOT lavas both by the addition of a modern subduction component and by magma mixing with CAB. The low-LILE HAOT lavas cannot be derived from depleted mantle sources through the addition of a modern subduction component.
Bartels et al. (1991) have experimentally equilibrated a magnesian HAOT composition with a spinel lherzolite assemblage at a temperature of 1290°C and pressure of 13–15 kbar, consistent with the mineral geothermometry cited above. Clynne (1993) suggested that HAOT were derived by 10% partial melting of depleted subcontinental lithospheric peridotite that had been subjected to Fe and Al enrichment by addition of pyroxene through metasomatism. Baker et al. (1994) concluded that HAOT at Mt Shasta represents a nearly anhydrous 6–10% partial melt of subcontinental mantle that last equilibrated near the base of the crust. Bacon et al. (1997) attributed their weak subduction-related geochemical signature (elevated Sr, Ba, Pb, and Sr/P) to passage of arc magmas through their mantle source region in the Mesozoic era.
Primitive CAB are sparsely porphyritic and have simple mineral assemblages. They contain up to 5% unzoned olivine phenocrysts, most with included chromian spinel. Olivine has a magnesian composition and high Ni (Fo88; Ni 2000 ppm) in equilibrium with the bulk composition of the lava and potentially in equilibrium with mantle peridotite (e.g. BVSP, 1981). Chromian spinels have magnesian compositions and fall on, or near, the mantle array of Arai (1987). Olivine–spinel equilibria indicate crystallization at 1250°C and elevated pressures (Clynne & Borg, 1997). A few magnesian andesites also contain sparse chromian diopside with 0·5 wt % Cr2O3 and mg-numbers 90 (Clynne, 1993). Primitive CAB lack plagioclase except as fine groundmass crystals. Primitive CAB contain high contents of compatible elements (MgO 8 wt %, Ni 100 ppm and Cr 200 ppm, and whole-rock FeO*/MgO = 0·7-1·1).
In the Lassen region, CAB define a compositional and isotopic continuum (Figs 1 and 2). The ends of the continuum can be defined by the degree to which Sr is enriched over other similarly incompatible elements. This definition has petrogenetic significance because most of the geochemical and isotopic characteristics of Lassen basalts correlate with Sr enrichment (Fig. 2). To make use of a considerable dataset for major elements and Sr, Borg et al. (1997) used the ratio (Sr/P)N as an analog to the commonly used LILE/LREE (e.g. Sr/Nd) to indicate the relative enrichment of Sr over LREE in arc rocks (Gill, 1981). The Sr enrichment is defined as normalized (Sr/P)N > 1 in which the data are normalized to primitive mantle values of Sun & McDonough (1989). Good correlation between P and Nd (r 2 = 0·90) and between (Sr/P)N and (Sr/Nd)N (r 2 = 0·82) demonstrates that these ratios are analogous. It is important to note that the variations in Sr/P are not simply the result of depletions in Sr and P associated with fractional crystallization of plagioclase and apatite. Lack of plagioclase phenocrysts and Eu anomalies in the whole-rock REE patterns suggests that variations in the abundances of Sr observed in the lavas do not reflect plagioclase fractionation. Likewise, a lack of apatite phenocrysts, as well as relatively constant P/Zr ratios in the whole rocks indicate that P is unlikely to be controlled by apatite fractionation. Instead, the variation in Sr/P ratios is the result of simultaneous increase in the abundance of Sr and accompanying depletion in incompatible elements (including P and LREE). Except for a few evolved samples, there are good correlations between (Sr/P)N and some other commonly used indicators of arc enrichment, such as Ba/Ta (Fig. 2a), illustrating the usefulness of (Sr/P)N as a geochemical indicator.
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Primitive CAB display a range of incompatible element patterns (Fig. 1a). At one end of the spectrum are lavas with low contents of incompatible elements, high ratios of LILE to HFSE abundances (e.g. Sr/P), high LREE/HREE and Sr and Nd isotopic values approaching those of MORB (Fig. 3). The most extreme lavas in this group are primitive magnesian andesites. These samples have the largest proportion of slab-derived component (Clynne, 1993; Bacon et al., 1997; Borg et al., 1997). The other end of the spectrum is characterized by lavas with higher abundances of incompatible elements, lower LILE/HFSE, lower LREE/HREE, and Sr, Nd, and Pb isotopic ratios that are like those of OIB. These features and lower relative enrichment of Sr over LREE and P led Borg et al. (1997) to propose that these samples are the least influenced by a slab component. We use (Sr/P)N = 3·3 to distinguish low and high (Sr/P)N samples, but emphasize that the CAB display a continuum of (Sr/P)N and that few of the samples analyzed for Hf have intermediate (Sr/P)N.
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Clynne (1993) and Borg et al. (1997) discussed across-arc variation in the geochemistry of primitive lavas in the southernmost Cascade range. Low (Sr/P)N basalts characterize the arc axis and backarc, and are sparsely present in the forearc, whereas high (Sr/P)N basalts and magnesian andesites characterize the forearc, and are extremely rare in the arc axis and backarc.
The Os isotopic compositions of some of the same CAB samples reported here were determined by Borg et al. (2000). They found that the proportion of slab component, as measured by (Sr/P)N, correlates with the Os isotopic composition of CAB (Fig. 2d). This correlation is interpreted to be the result of mixing between a radiogenic Os component derived from the slab and mantle Os derived from the mantle wedge. However, the low Os abundances of the high (Sr/P)N CAB make them particularly susceptible to contamination. Thus, Borg et al. (2000) suggested a second possibility: that high (Sr/P)N CAB were contaminated by small amounts of radiogenic Os from the crust. Other geochemical characteristics of the rocks are not affected by this small amount of crustal contamination. The Os isotopic compositions of three HAOT lavas from the lassen region were not reported by Borg et al. (2000). The Os isotopic compositions of these samples are similar to those of HAOT to the north of the study area (Table 1), which have been interpreted to be derived from the mantle lithosphere (Hart et al., 1997).
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Four conclusions derived from these studies provide a framework for a model in which to interpret and test the origin of the Hf isotopic systematics. First, unradiogenic Pb isotopic compositions and low 187Os/188Os ratios of many CAB and low 187Os/188Os ratios of HAOT suggest that crustal contamination was minimal (Borg et al., 1997, 2000). This is supported by the primitive nature of CAB and HAOT.
Second, the origin of the geochemical variation of primitive CAB in the Lassen region can be explained by source mixing between a relatively enriched component with a primitive mantle or Bulk Earth-like isotopic composition (OIB-like) and a slab-derived fluid component with an arc geochemical signature and near MORB-like isotopic composition (Borg et al., 1997). Low Cs/Rb ratios (<0·064) in all CAB and low Pb isotopic compositions in high (Sr/P)N CAB indicate that subducted sediment plays at most a minor role in their origin. The relative proportion of the slab-derived component in the Lassen magmas, as measured by (Sr/P)N, decreases from the forearc to the backarc.
Third, Clynne & Borg (1997) showed that systematic compositional differences between phenocrysts in HAOT and CAB and correlations between the composition of spinels and the bulk composition of Lassen basalts reflect bulk chemical variability and relative fertility of their mantle sources. They concluded that the relative source fertility decreases in the order HAOT—low (Sr/P)N CAB—high (Sr/P)N CAB. Of the high (Sr/P)N CAB, the magnesian andesites are derived from the least fertile mantle sources. Thus, the relative fertility of the CAB magma source decreases from the backarc to the forearc.
Finally, Clynne (1993) proposed that HAOT and CAB have fundamentally different source regions and are not linked by a common source or process, a conclusion that was shared by Baker et al. (1994) and Bacon et al. (1997). Thus, the origins of HAOT and CAB magmas and their Hf systematics are considered separately below. Rare basalt lavas have geochemical characteristics that are intermediate between HAOT and the CAB spectrum. The origin of these rocks is unclear, but probably involves source or magma mixing between HAOT and CAB before phenocryst crystallization. Four of these samples, LC88-1398, LC86-951, LC88-1312, and LC88-1305, were analyzed here for Hf isotopic composition.
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RESULTS |
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TOPABSTRACTINTRODUCTIONREVIEW OF THE GEOCHEMISTRY...RESULTSPROCESSES THAT MODIFY Hf...ISOTOPIC EVOLUTION OF THE...A PETROGENETIC MODEL FOR...CONCLUSIONREFERENCES |
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The samples used for this investigation represent a subset of samples for which mineral, major element, trace element, and isotopic compositions were determined (Clynne, 1993
; Borg, 1995; Bacon et al., 1997; Borg et al., 1997, 2000; Clynne & Borg, 1997) and were chosen because they encompass the range of mineralogical and geochemical variability observed in the southernmost Cascades. The Hf isotopic data for CAB and HAOT lavas are reported in Table 1 along with analytical techniques and standards. The Hf isotopic compositions of the Lassen rocks range from 176Hf/177Hf of 0·282920 (Hf = +5·2) to 0·283118 (Hf = +12·2). In comparison, Nd values range from +1·3 to +6·5, and 87Sr/86Sr ratios range from 0·7030 to 0·7045. The Hf–Nd isotopic systematics of the Lassen lavas are compared with lavas from the Pacific oceanic basin, the western USA, and island arcs in Fig. 3a and b. The Lassen lavas have Hf–Nd isotopic systematics that are similar to worldwide island-arc volcanics (White & Patchett, 1984; Salters & Hart, 1991; Pearce et al., 1999) and fall in the Hf–Nd mantle array defined by oceanic basalts. In fact, with the exception of a few samples, all of the Lassen lavas lie in the Hf–Nd field defined by Hawaiian OIB, and are also similar to some Basin and Range lavas from the western Great Basin. The western Great Basin lavas are identified from other Basin and Range lavas by elevated Sr contents and 87Sr/86Sr ratios (Leeman, 1970; Beard & Johnson, 1997). Other Basin and Range lavas, characterized by lower Sr contents and 87Sr/86Sr ratios, have significantly lower Hf isotopic compositions for a given Nd isotopic composition than the Lassen lavas (Beard & Johnson, 1997). Lavas from the Sonoma–Tolay volcanic field in northwestern California are the only mafic lavas analyzed from the western USA that have Hf and Nd isotopic compositions that are more radiogenic than the Lassen lavas (Beard & Johnson, 1997).
The Hf isotopic composition of the lavas correlates inversely with the many incompatible element abundances, such as LREE (Fig. 4a), but not with SiO2 (Fig. 4b) or abundances of highly compatible elements such as Ni (Fig. 5). The Hf isotopic composition of CAB also varies with geochemical indices of the subduction component, such as (Sr/P)N, so that lavas with low (Sr/P)N also have low Hf values (Fig. 2f).
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PROCESSES THAT MODIFY Hf AND Nd ISOTOPIC COMPOSITIONS OF LASSEN MAGMAS |
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TOPABSTRACTINTRODUCTIONREVIEW OF THE GEOCHEMISTRY...RESULTSPROCESSES THAT MODIFY Hf...ISOTOPIC EVOLUTION OF THE...A PETROGENETIC MODEL FOR...CONCLUSIONREFERENCES |
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The Hf and Nd isotopic compositions of the Lassen lavas may result from (1) interaction of primitive magmas with continental crust, (2) mixing between components derived from the mantle wedge and the subducting slab, and (3) the long-term isotopic variability of the mantle source regions. In this section, the effect that crustal differentiation and the addition of the subduction component to the mantle wedge has on the Hf–Nd isotopic systematics of the Lassen lavas is discussed.
Assimilation–fractional crystallization
As discussed above, the lavas analyzed for Hf isotopic compositions in this study were chosen because they are the most primitive lavas observed in the Lassen region. As a result, they are expected to have undergone a minimal amount of differentiation and thus are likely to have had only minimal interaction with the continental crust. This conclusion is supported by the observation that many CAB and HAOT have mantle or near mantle-like 187Os/188Os isotopic ratios (Table 1; Hart et al., 1997; Borg et al., 2000; Borg et al., unpublished data, 2000). Furthermore, there is no correlation between Hf isotopic composition and SiO2 (Fig. 4b) or between Hf isotopic composition and compatible element abundances such as Ni within the Lassen suite (Fig. 5). Assimilation–fractional crystallization (AFC) models presented in Fig. 5 demonstrate that Ni abundances decrease dramatically with a limited amount of differentiation, whereas Hf isotopic compositions require significantly more differentiation before they are affected. Thus, the Hf isotopic compositions of Lassen lavas with high Ni contents are unlikely to be strongly affected by AFC. It should be noted that several high (Sr/P)N samples with low Ni abundances have among the most radiogenic Hf isotopic compositions, suggesting that they have evolved by fractionation of small amounts of olivine with no measurable contamination (Fig. 5).
The Hf isotopic composition of two samples, LB91-125 and LC88-1305, may reflect significant crustal contamination. These samples have the lowest Hf and Nd isotopic compositions and the most Fe-rich olivines (Fo78–82) analyzed in the suite (Borg, 1995; Clynne & Borg, 1997). These samples are not used to model the isotopic composition of mantle source regions.
Addition of the subduction component to the mantle wedge
Borg et al. (1997) argued that much of the geochemical variability observed in the Lassen CAB suite results from the variable contribution of the subduction component to the mantle source region. HAOT also have a weak subduction-related geochemical signature (Bacon et al., 1997). White & Patchett (1984) noted that the Hf isotopic compositions of many arc rocks are less radiogenic than MORB and concluded that this reflected contributions from a subduction component dominated by pelagic sediment. Thus, the Hf and Nd isotopic compositions of the Lassen lavas could be influenced by variable contributions from the subducted slab. Below we discuss the effect that variable contributions of the modern subduction component has on the compositions of CAB and HAOT.
Effect of the subduction component on the Hf and Nd isotopic composition of CAB
The trace element and isotopic signature of the subduction component depends on several factors including: (1) the initial composition of the subducted oceanic basalt and sediment; (2) the basalt/sediment ratio contributing to the subduction component; (3) the composition and proportion of fluids or melts released from the slab before reaching the forearc; (4) the phases that are present in the slab during dehydration–melting beneath the forearc. In the discussion presented below, the subduction component is modeled to be a mixture of fluids derived from dehydration of oceanic basalt and Pacific sediments. The composition of the fluid component used here is that estimated by Stolper & Newman (1994), whereas the sediment component is modeled simply as bulk Pacific basin sediment (Ben Othman et al., 1989). The model results are not strongly dependent on the type of sediment (terrigeneous vs pelagic) because both types of sediment have similar abundances of Hf and Nd (Vervoort et al., 1999).
The slab-derived fluids calculated for Mariana trough basalts by Stolper & Newman (1994) have elevated Sr/Nd and (Sr/P)N ratios and are strongly enriched in incompatible elements such as Sr (4681 ppm), Pb (17 ppm), Nd (147 ppm), and Zr (985 ppm). Although Stolper & Newman (1994) did not estimate the abundance of Hf, slab-derived fluids from the southern Cascades arc are estimated to be as high as 56 ppm (Grove et al., 2002). These calculations belie a misconception that slab-derived fluids are depleted in HFSE. Rather, slab-derived fluids have high LILE/HFSE ratios relative to most mantle and crustal rocks because they have very high LILE abundances. The presence of the subduction component in the source region of CAB can therefore potentially affect their Sr, Pb, Nd, and Hf isotopic compositions.
Stolper & Newman (1994) demonstrated that the fluid component in Mariana trough magmas has unradiogenic 87Sr/86Sr (0·7026) and Pb isotopic compositions (e.g. 207Pb/204Pb = 15·56) and a radiogenic 143Nd/144Nd (0·5129) isotopic composition. Inverse correlations of (Sr/P)N with 87Sr/86Sr, 207Pb/204Pb, 143Nd/144Nd, and 176Hf/177Hf isotopic ratios of CAB (Fig. 2) indicate that the subduction component in the southernmost Cascade arc is also characterized by unradiogenic Sr and Pb and radiogenic Nd (and Hf). Mixing models demonstrate that the Sr and Pb isotopic variability observed in CAB can be produced by mixing between peridotite with the isotopic composition of the low (Sr/P)N lavas and a subduction component that is isotopically MORB-like (Bacon et al., 1997; Borg et al., 1997; Grove et al., 2002; Fig. 3c and d). The fact that lavas with relatively large subduction signatures have isotopic compositions that approach MORB suggests that the contribution of either terrigeneous or pelagic sediment to the isotopic composition of the lavas is either minimal and/or masked by a proportionally large contribution of dehydration products of the subducted oceanic basalt.
The mixing models presented in Fig. 3c and d also demonstrate that simple binary mixing between a single mantle source and a slab-derived fluid will not reproduce all the Sr, Pb, and Hf isotopic variability observed in CAB. To reproduce all of the isotopic variability observed in CAB at least three sources are required. This stems from the fact that at a particular Sr or Pb isotopic composition there is a range of Hf (and Nd) isotopic compositions. This suggests that CAB are derived from multiple mantle source regions with variable Hf (and Nd) isotopic compositions. It is therefore probable that the Hf and Nd isotopic variations observed in the CAB suite reflect the isotopic compositions of the mantle wedge to a large extent. To investigate this possibility, the contribution of slab-derived Hf and Nd is estimated using the results of source mixing–partial melting models presented by Borg et al. (1997) and in Fig. 1.
The contribution of Hf and Nd from the modern subduction component was assessed by Borg et al. (1997). They modeled the incompatible element compositions of CAB assuming that they were derived from peridotites with MORB to OIB-source incompatible element compositions that were enriched through variable input from the subducted slab (Fig. 3b). The subduction component was modeled as a mixture of slab-derived fluids with the composition based on the calculations of Stolper & Newman (1994) for Mariana arc basalts and Pacific basin sediments measured by Ben Othman et al. (1989). From these models the proportions of Hf and Nd derived from the mantle wedge peridotite and fluids from subducted oceanic crust and sediments were determined.
The proportion of Hf derived from sediment was estimated to range from 1 to 2% and the proportion of Nd was estimated to range from 2 to 6% for the various models. The relatively small sediment contribution to the modern subduction component means that sediment will not significantly contribute to the Hf and Nd isotopic compositions of the source regions. For example, the Nd isotopic composition of a MORB-like source with an Nd value of +9 will only be lowered to +8·4 by the addition of the maximum amount of average Pacific sediment (Nd = -6; Ben Othman et al., 1989) allowed by the models. A similar calculation for Hf demonstrates that the Hf isotopic composition of a MORB-like source will only be lowered by 1·3 epsilon units by the addition of sediment with an Hf of -49 [corresponding to the most unradiogenic sediment analyzed by Vervoort et al. (1999)]. The contribution of sediment to the Hf–Nd isotopic composition of the Lassen source region is therefore minimal and will be ignored in the following calculations that estimate the isotopic effect that the addition of the subduction component has on the mantle wedge.
The proportion of Hf and Nd derived from fluids from the slab is proportional to the Sr/P ratios of the lavas (Borg et al., 1997). The low (Sr/P)N lava source was calculated to inherit 1–2% of its Hf and Nd from the slab-derived fluids, whereas the highest (Sr/P)N lava source was calculated to have up to 45% of its Hf and 54% of its Nd inherited from slab-derived fluids (Borg et al., 1997). These represent maximum values and are used to present an extreme scenario for subduction control of the Hf and Nd isotopic compositions of CAB. The Hf and Nd isotopic composition of the CAB source region before fluxing with the subduction component is estimated by subtracting the proportions of Hf and Nd inherited from slab-derived fluids from the isotopic composition of the lavas. The slab-derived fluid is assumed to have a 143Nd/144Nd ratio of average Juan de Fuca MORB (0·51316; Hegner & Tatsumoto, 1987; White et al., 1987). The 176Hf/177Hf ratio of the fluid component is estimated to be 0·28316 using a regression of the Hf–Nd isotopic compositions of primitive CAB and the 143Nd/144Nd ratio of average Juan de Fuca MORB.
The difference between the isotopic compositions calculated for the part of the mantle wedge that has not been enriched through the addition of the modern subduction component and the measured Hf and Nd isotopic compositions of the Lassen lavas is presented in Fig. 6. The calculations indicate that the Hf isotopic composition of all but two of the lavas is within 2 epsilon units of the non-subduction enriched peridotites, and that all but two of the low (Sr/P)N lavas are within 0·5 epsilon units. This suggests that most of the 6·5 epsilon unit variation observed in the Hf isotopic composition of CAB is not the result of variable contributions from the subduction component. In contrast, the Nd isotopic compositions of the highest (Sr/P)N lava is estimated to be 6 epsilon units higher than its non-subduction enriched peridotite source, whereas most low (Sr/P)N lavas are within 2 epsilon units. Taken together, these results suggest that the Hf and Nd isotopic compositions of the low (Sr/P)N lavas are not strongly affected by the addition of the modern subduction component to the mantle wedge, whereas the Hf and Nd isotopic compositions of the high (Sr/P)N lavas are affected to some extent. As a result, the isotopic ratios of the low (Sr/P)N lavas are used in the following section to constrain the geochemical evolution of the mantle wedge beneath the southernmost Cascades.
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Effect of the subduction component on the Hf and Nd isotopic composition of HAOT
The majority of Lassen HAOT, such as LMM81-297, have relatively flat incompatible element patterns with variable enrichments in Ba, Sr, and Pb relative to other equally incompatible elements (Clynne, 1993), indicative of a weak but variable arc geochemical signature (Bacon et al., 1997). Some HAOT, such as LC88-1398 and LC86-951, have significantly higher incompatible element abundances that are suggestive of a larger calc-alkaline input. These samples are considered to be HAOT lavas because they have appropriate mineral compositions (Clynne, 1993; Clynne & Borg, 1997) but have been classified as calc-alkaline by Bacon et al. (1997) based on incompatible element compositions.
Partial melting models are unable to reproduce the compositions of typical HAOT lavas (e.g. LMM81-279) from an amphibole- and garnet-free depleted mantle source that has been enriched through the addition of a modern subduction component similar to that observed in CAB (Fig. 1c). Instead, a component is required that, on one hand, has elevated (Sr/P)N, La/Ta, and Ba/Ta ratios like the modern subduction component, but, on the other hand, has lower Rb/Ta, K/Ta, and LREE/HREE ratios. The fact that HAOT lavas have 207Pb/204Pb isotopic ratios that are significantly elevated above both modern subducted sediments (Church & Tilton, 1973; Church, 1976) and the northern hemisphere reference line (Table 1) indicates that this component could not be added to their mantle source region in the modern subduction regime. Instead, elevated 207Pb/204Pb isotopic ratios of HAOT suggest that their source has been enriched in a subduction component containing a relatively large proportion of ancient sediment. Bacon et al. (1997) have argued that this component was added to the lithospheric mantle by calc-alkaline magmas, possibly during the Mesozoic era. Thus, the weak subduction signature observed in most HAOT is not a result of the presence of the modern subduction component. Therefore, the modern subduction component is expected to have little influence on the Hf and Nd isotopic compositions of these HAOT.
The LILE enrichment observed in a few of the HAOT lavas, such as LC88-1398, can be produced by mixing between LILE-depleted HAOT magmas and typical CAB magmas (Fig. 1c). These lavas are likely to be produced by mixing of HAOT and CAB magmas before the onset of phenocryst crystallization (Clynne, 1993). These hybrid HAOT lavas are not used to distinguish petrogenetic differences between HAOT and CAB and are not discussed further.
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ISOTOPIC EVOLUTION OF THE MANTLE BENEATH THE LASSEN REGION |
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Because the Hf–Nd isotopic compositions of the low (Sr/P)N CAB and HAOT lavas are least affected by contributions from the modern subduction component, they are used to constrain the isotopic composition of the mantle wedge before enrichment by the modern subduction component.
Unique sources for CAB and HAOT magmas
Evidence for garnet
The Lassen CAB have LREE-enriched patterns indicated by chondrite-normalized La/Lu ratios that are as high as nine, whereas most HAOT lavas that lack calc-alkaline trace element signatures (see above) have chondrite-normalized La/Lu ratios of one or less. Clynne (1993) suggested that the elevated LREE/HREE ratios observed in CAB required them to have equilibrated with garnet at some point early in their history. Furthermore, Borg et al. (1997) could only model REE patterns of CAB if garnet was present in their source regions. In contrast, the lower La/Lu ratios of the HAOT lavas do not require garnet to be present.
Simple batch partial melting models are consistent with the presence of garnet in the source region of CAB, and its absence in the source region of HAOT (Fig. 7). These models assume that the sources for CAB and HAOT have similar abundances of Lu and Hf. However, variations in the composition of chromian spinels in HAOT and CAB indicate that the source regions of these lavas are different (Borg & Clynne, 1997). The presence of garnet in the source regions of the low (Sr/P)N CAB and HAOT can also be assessed using their Hf isotopic compositions. The advantage of this approach is that the composition of the source regions is calculated based on the Hf isotopic compositions of the lavas. The underlying assumption of these models is that the Lu/Hf ratios and Hf isotopic compositions of the lavas reflect their parental melt compositions, and that their Hf isotopic compositions record the long-term Hf isotopic composition of their source regions. In other words, the Hf isotopic compositions of the lavas are not strongly affected by either AFC or the recent addition of Hf from the modern subduction component. These assumptions seem reasonable given the primitive nature of the lavas analyzed in this study and the Hf isotopic systematics calculated for the subduction component.
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The 176Lu/177Hf ratio of CAB and HAOT source regions is calculated based on the measured Hf isotopic compositions of the lavas in Fig. 8. The calculated 176Lu/177Hf ratio depends on the age of the mantle reservoir and its initial Hf isotopic composition (Beard & Johnson, 1993). As a result, calculations are made for sources having ages from 0·5 to 2 Ga and ranging from primitive (chondritic) mantle to depleted mantle compositions. Superimposed on this plot are non-modal batch partial melting models of Beard & Johnson (1993) that distinguish between garnet- and spinel-bearing source regions. The location of the boundaries of the garnet and the spinel peridotite fields depends on the amount of melting required to produce the magmas, which is assumed to range from 1 to 20%. Thus, the amount of melting required to produce any given magma increases from left to right in Fig. 8.
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From Fig. 8 it is apparent that even 2 Ga peridotites are required to contain garnet to produce the measured 176Lu/177Hf ratios of CAB from their calculated sources. This is true even if the peridotite has a depleted mantle composition before further depletion at 2 Ga (Fig. 8). CAB with a large contribution of the subduction component, i.e. the high (Sr/P)N lavas, fall very near the low (Sr/P)N lavas, further indicating that the model results do not depend on the size of the modern subduction signature.
In general, the addition of a subduction-derived component to the mantle wedge will have only a limited effect on the results of these models. This stems from the fact that, to affect the outcome of the models, the Hf isotopic composition and Lu/Hf ratios of the mantle wedge must be altered dramatically by the addition of the modern subduction component. For example, for a typical Lassen CAB to fall in the spinel peridotite field in Fig. 8, its Hf isotopic composition must be 9 epsilon units lower than the measured value. Furthermore, the addition of the subduction component to the mantle wedge is unlikely to significantly change its Lu/Hf ratio because bulk sediment and MORB, as well as fluids derived from these components, are expected to have Lu/Hf ratios that are roughly similar to the mantle sources (e.g. Stolper & Newman, 1994; Vervoort et al., 1999).
In contrast to CAB, HAOT have significantly higher 176Lu/177Hf ratios and therefore generally fall in the spinel peridotite field. In fact, the source region for HAOT must be melted >20% and be <0·5 Ga to contain garnet. It is therefore likely that HAOT are derived from spinel-bearing peridotites, whereas the ultimate source of CAB is garnet bearing.
Isotopic differences between CAB and HAOT sources
The fact that the Hf and Nd isotopic compositions of low (Sr/P)N CAB and HAOT are least affected by contributions from the subducting slab permits comparisons between the isotopic compositions of their mantle sources. Furthermore, because the subduction component in the Lassen region has unradiogenic Sr and Pb isotopic compositions, the Sr and Pb isotopic compositions of low (Sr/P)N samples can be used to constrain the minimum Sr and Pb isotopic compositions of the mantle wedge before modern subduction zone enrichment.
Evidence for the presence of garnet in the source region, as well as the depth of the subducted slab beneath the southern Cascade arc (Guffanti et al., 1990), suggests that the CAB source region is in the asthenosphere. In contrast, the lack of evidence for the presence of garnet in the source region, combined with experimental evidence for equilibrium of HAOT at 13–15 kbar (Bartels et al., 1991), suggests that the HAOT source region is in the lithosphere. The isotopic composition of the CAB source region is best represented by the range of isotopic compositions of low (Sr/P)N CAB with small subduction geochemical signatures [i.e. (Sr/P)N < 2; excluding LB91-125] and is: 87Sr/86Sr = 0·7038–0·7043; 208Pb/204Pb = 38·51–38·72; 207Pb/204Pb = 15·60–15·64; 206Pb/204Pb = 18·84–19·00; Nd = +2·4 to +4·5; Hf = +7·5 to +10·1. The isotopic composition of the lithospheric portion of the mantle beneath the Lassen region is best represented by the range of isotopic composition of HAOT (excluding hybrid HAOT-CAB samples) and is: 87Sr/86Sr = 0·7036–0·7040; 208Pb/204Pb = 38·44–38·64; 207Pb/204Pb = 15·58–15·65; 206Pb/204Pb = 18·81–19·18; Nd = +4·4 to +6·2; Hf = +9·7 to +12·2.
The Pb isotopic compositions of CAB and HAOT are indistinguishable. This suggests that the Pb isotopic compositions of the asthenospheric mantle wedge may not be much higher than the compositions of the low (Sr/P)N CAB. If this is true, then the Pb isotopic composition of the mantle beneath the Lassen region is fairly constant. However, the Hf and Nd isotopic compositions of low (Sr/P)N CAB are less radiogenic than the Hf and Nd isotopic composition of HAOT. The Sr isotopic composition of the average low (Sr/P)N CAB may also be slightly higher than HAOT. Thus, despite similar Pb isotopic compositions CAB and HAOT appear to be derived from different mantle source regions with different Hf, Nd, and Sr isotopic compositions.
Constraints on mantle sources
Comparison with mantle sources present in the region
A central issue is whether the mantle sources beneath the southernmost Cascades are similar to other mantle sources inferred to be present in the eastern Pacific and the western USA and whether mixing of these mantle sources can account for the inferred isotopic variability of the mantle source beneath the southernmost Cascades. The Pb isotopic compositions of the low (Sr/P)N CAB and HAOT are more radiogenic than lavas from oceanic environments in the eastern–central Pacific region. More radiogenic Pb in the Lassen lavas relative to MORB and Hawaiian OIB cannot reflect input of the modern subduction component because this component has unradiogenic Pb isotopic compositions and is minimal in the source regions of these lavas (Borg et al., 1997). The Sonoma–Tolay lavas lie on the MORB end of the Lassen Hf–Nd isotopic array and may be representative of a depleted mantle component in northwestern California. Although these lavas are characterized by radiogenic Pb isotopic compositions (average 206Pb/204Pb = 18·75; 207Pb/204Pb = 15·60), they have lower 206Pb/204Pb ratios for a given 207Pb/204Pb ratio than the Lassen lavas (Table 1) and therefore are not representative of the Lassen mantle source. Continental basaltic lavas also have inappropriate isotopic compositions. For example, Basin and Range lavas have Nd isotopic compositions that are too radiogenic (Fig. 3b), whereas the western Great Basin lavas have Sr isotopic compositions that are too radiogenic (87Sr/86Sr = 0·704–0·708; Farmer et al., 1989; Beard & Glazner, 1995). Thus, the isotopic compositions of the Lassen lavas are unlike those of other lavas observed in the eastern–central Pacific or the western USA, suggesting the mantle source region beneath the southernmost Cascades is isotopically unique.
Evidence of the addition of an ancient subduction component to mantle source regions
The fact that low (Sr/P)N CAB and HAOT have very small modern subduction geochemical signatures, yet have fairly radiogenic 207Pb/204Pb ratios, suggests that their mantle source regions have been enriched by the addition of an ancient subduction component. This enrichment could also account for the fact that both low (Sr/P)N CAB and HAOT have Hf and Nd isotopic compositions that are lower than depleted mantle values. If this scenario is correct, then the isotopic systematics of the ancient subduction component must be distinct from the modern subduction component, being characterized by low Hf and Nd and high 207Pb/204Pb ratios. Differences in the isotopic composition of the ancient and modern subduction component are consistent with a greater contribution of sediment in the ancient subduction component relative to the modern subduction component. The western USA has been the site of convergent plate tectonics since the Paleozoic, at least, so that the ancient subduction component could have been added to the mantle at almost any time (e.g. Hooper et al., 1995; Rogers et al., 1995). However, there are intrusive calc-alkaline rocks near the Lassen region emplaced during Sierran magmatic events in the late Mesozoic era (Gromme et al., 1967) and during Antler magmatic events in the Devonian period (Hietanen, 1976). It is therefore probable that ancient subduction zone enrichment of the mantle could have occurred at these times.
We model the Hf, Nd, and Pb isotopic composition of low (Sr/P)N CAB and HAOT sources to determine if their isotopic compositions are consistent with mixing between depleted mantle sources and a sediment-enriched subduction component at 0·1, 0·4, and 1 Ga. These models also illustrate the potential relationship between the low (Sr/P)N CAB and HAOT sources. The Hf and Nd isotopic compositions of the low (Sr/P)N CAB and HAOT source regions are calculated at various ages using the present-day isotopic compositions of the lavas and estimates of 176Lu/177Hf and 147Sm/144Nd ratios of their source regions (see Fig. 9 caption). The mixing models demonstrate that low (Sr/P)N CAB and HAOT sources have isotopic compositions that are consistent with mixing between a MORB-source and a subduction component characterized by low Hf and Nd and high 207Pb/204Pb (Fig. 9a). It should be noted that as the age of subduction zone enrichment of the mantle source regions increases, the estimated Nd isotopic compositions of the sources spread away from the mixing line. This suggests that the addition of the subduction component to the mantle was relatively recent. Alternatively, the inferred range of Hf and Nd isotopic compositions of the CAB and HAOT sources could reflect mixing between several depleted mantle sources and multiple subduction components with variable isotopic compositions. Although these models do not define the age of subduction zone enrichment conclusively, the models in which the subduction component was added to the mantle most recently seem to best fit the data.
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) at 0·1, 0·4, and 1 Ga are plotted. The Hf and Nd isotopic compositions of the low (Sr/P)N CAB and HAOT source regions are calculated at various ages using the present-day isotopic compositions of the lavas, and estimated 176Lu/177Hf and 147Sm/144Nd ratios of their source regions. The 176Lu/177Hf and 147Sm/144Nd ratios of the sources are in turn calculated from the compositions of the lavas using simple batch partial melting models and the partition coefficients in Table
It should be noted that in most of the mixing models in Fig. 9, the low (Sr/P)N CAB and HAOT sources fall along a common mixing line. This suggests that the isotopic differences between CAB and HAOT sources are a result of an increased proportion of ancient subduction component in the CAB source. However, the compositions of chromian spinels in CAB and HAOT indicate that the two source regions are distinct (Clynne & Borg, 1997). These differences might reflect the style of metasomatism in the lithosphere and asthenosphere. In the asthenospheric mantle, the subduction component might be added to the wedge by fluids coming off the slab, whereas in the lithospheric mantle, the subduction component might be added by magmas rising from the asthenosphere. Thus, this lithospheric mantle might, on average, be more fertile than the asthenospheric mantle. This model is consistent with the conclusion of Bacon et al. (1997) that the geochemical enrichment observed in HAOT in the southern Cascades reflects the passage of a calc-alkaline component to the mantle lithosphere.
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A PETROGENETIC MODEL FOR THE SOUTHERNMOST CASCADES |
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Several observations constrain the petrogenetic model of the Lassen lava suite including the following: (1) the mantle underwent metasomatism through the addition of an ancient subduction component, possibly during Sierran magmatism; (2) the fertility of the mantle increases towards the backarc and the sources of the Lassen lavas decrease in fertility from HAOT, low (Sr/P)N CAB, to high (Sr/P)N CAB; (3) garnet is a residual phase in the CAB source, and absent in the HAOT source; (4) the modern subduction signature is present in CAB and mostly absent in HAOT and decreases from forearc to back arc.
The petrogenetic model for the southernmost Cascades is summarized in Fig. 10. Before the onset of recent subduction, an ancient component, characterized by unradiogenic Hf and Nd, and radiogenic Pb, is added to the mantle now beneath the Lassen region by subduction. Similar 207Pb/204Pb isotopic ratios in low (Sr/P)N CAB and HAOT are consistent with this component being added to both the lithospheric and asthenospheric mantle. The Nd isotopic systematics of low (Sr/P)N CAB and HAOT suggest that this ancient subduction component was added to the mantle source regions during Sierran magmatism (Fig. 9).
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HAOT are associated with the recent impingement of the Basin and Range Province on the Lassen region (Guffanti et al., 1990), which has caused melting of the lithosphere beneath the region. The isotopic composition of the HAOT source reflects interaction between lithospheric mantle and an ancient subduction component. Mixing models in Fig. 9 suggest that the HAOT mantle source contains a proportionally smaller amount of this ancient subduction component than the CAB source. Thus, the mineralogical and geochemical differences between CAB and HAOT reflect a variety of factors including: (1) the addition of the modern subduction component to the CAB source region; (2) differences in the depth of magma generation and corresponding presence or absence of garnet; (3) derivation from isotopically distinct source regions probably resulting from variable addition of an ancient subduction component to the mantle; (4) variable fertility of their peridotite sources. Occasionally, CAB and HAOT magmas mix forming hybrid magmas with mineralogical and geochemical characteristics that are intermediate.
Like the HAOT source, the mantle source of CAB has been enriched through the addition of an ancient subduction component. The range of CAB is produced by variations in (1) the proportion of modern subduction component in the source region, and (2) changes in the relative fertility of the source region. A decrease of the subduction signature is observed from west to east across the arc, suggesting that the arc signature reflects slab-dehydration in the current tectonic setting (Borg et al., 1997). Geochemical indices of the modern subduction component and mineralogical indices of source fertility correlate, suggesting that larger amounts of subduction fluids are present in more refractory mantle sources in the forearc. We suggest that the bulk mantle wedge becomes increasingly refractory as melts are extracted from peridotites as they move from east to west beneath the arc. An increasing amount of the fluid-rich subduction component is therefore required to induce melting of a progressively more refractory mantle wedge.
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CONCLUSION |
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The modern subduction component in the southernmost Cascade arc is characterized by radiogenic Hf isotopic compositions. This is consistent with previous studies that concluded that the subduction component in the Cascade, Aleutian, and Mariana arcs have unradiogenic Sr and Pb and radiogenic Nd (Stolper & Newman, 1994
; Yogodzinski et al., 1994; Borg et al., 1997; Grove et al., 2002). Nevertheless, the Hf isotopic compositions of low (Sr/P)N CAB and HAOT are not strongly affected by the addition of components derived from the modern subducting slab. Partial melting models based on Hf isotopic compositions indicate that CAB equilibrated with garnet at some point in their history, whereas HAOT lavas do not show evidence for equilibration with garnet. This is consistent with differences in the Hf and Nd isotopic systematics of CAB and HAOT and with differences in spinel compositions of CAB and HAOT that are interpreted to reflect derivation from unique source regions with different major element abundances. Most mantle source regions inferred to be present in the eastern–central Pacific and western USA, including the sources for Hawaiian OIB, Basin and Range lavas, and lavas from the Sonoma–Tolay volcanic field have inappropriate isotopic compositions for the mantle source region of the Lassen lavas. Instead, variations in Hf isotopic compositions in the CAB and HAOT lavas are consistent with mixing between a MORB source, a more fertile source enriched by an ancient subduction component, and a modern subduction component.
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ACKNOWLEDGEMENTS |
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The authors are grateful for editorial assistance by Dennis Geist and reviews by W. K. Hart and an anonymous reviewer, and for reviews of a previous version of the manuscript by J. P. Patchett, T. L. Grove, V. J. M. Salters, C. M. Johnson, and C. Thornber. We thank Philippe Télouk for assistance with the P54. J. B.-T. acknowledges funding by the Institut National des Sciences de l’Univers through the program Dynamique des Transferts Terrestres.
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FOOTNOTES |
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*Corresponding author. Telephone: (505) 277-3842. Fax: (505) 277-3577. E-mail: lborg{at}unm.edu.
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