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Journal of Petrology 45(6) © Oxford University Press 2004; all rights reserved
Geochemical Constraints on Possible Subduction Components in Lavas of Mayon and Taal Volcanoes, Southern Luzon, Philippines
1 SCRIPPS INSTITUTION OF OCEANOGRAPHY, UNIVERSITY OF CALIFORNIA, SAN DIEGO, LA JOLLA, CA 92093-0212, USA
2 USGS, MAILSTOP 351310, UNIVERSITY OF WASHINGTON, SEATTLE, WA 98195-1310, USA
RECEIVED AUGUST 1, 2002; ACCEPTED NOVEMBER 14, 2003
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTING AND SAMPLESANALYTICAL PROCEDURESRESULTSDISCUSSIONIMPLICATIONS FOR THE REGIONAL...SUMMARY AND CONCLUSIONSREFERENCES |
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Mayon is the most active volcano along the east margin of southern Luzon, Philippines. Petrographic and major element data indicate that Mayon has produced a basaltic to andesitic lava series by fractional crystallization and magma mixing. Trace element data indicate that the parental basalts came from a heterogeneous mantle source. The unmodified composition of the mantle wedge is similar to that beneath the Indian Ocean. To this mantle was added a subduction component consisting of melt from subducted pelagic sediment and aqueous fluid dehydrated from the subducted basaltic crust. Lavas from the highly active Taal Volcano on the west margin of southern Luzon are compositionally more variable than Mayon lavas. Taal lavas also originated from a mantle wedge metasomatized by aqueous fluid dehydrated from the subducted basaltic crust and melt plus fluid derived from the subducted terrigenous sediment. More sediment is involved in the generation of Taal lavas. Lead isotopes argue against crustal contamination. Some heterogeneity of the unmodified mantle wedge and differences in whether the sediment signature is transferred into the lava source through an aqueous fluid or melt phase are needed to explain the regional compositional variation of Philippine arc lavas.
KEY WORDS: Mayon Volcano; Philippines; sediment melt; subduction component; Taal Volcano
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND SAMPLESANALYTICAL PROCEDURESRESULTSDISCUSSIONIMPLICATIONS FOR THE REGIONAL...SUMMARY AND CONCLUSIONSREFERENCES |
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Most magmas generated along convergent margins come from a mantle wedge that has been enriched by a subduction component consisting primarily of volatiles (e.g. H2O, CO2) and large ion lithophile elements (LILE; e.g. Cs, Ba, Rb, Sr) (Kay, 1980
; Gill, 1981; Hawkesworth et al., 1991). The subduction component originates from the subducted oceanic lithosphere and overlying sediments, but the type, composition, and proportion of these source materials are highly variable. Thus, the degree of geochemical enrichment exhibited by volcanic arc lavas is also highly variable. Constraining the nature and specific source of the subduction component in volcanic arcs is a prime objective of many Earth scientists, for it provides the key to a better understanding of mass balance problems and other magmatic processes along convergent margins, as well as formation of mantle heterogeneities and sources of intraplate magmas (e.g. Kogiso et al., 1997b; Kamber & Collerson, 2000). Progress in constraining the nature and source of the subduction component is advancing (e.g. Elliot et al., 1997; Plank & Langmuir, 1998; Class et al., 2000; Defant & Kepezhinskas, 2001; Hochstaedter et al., 2001), but there is still a lack of consensus.
In this paper, we present a detailed major element, trace element, and Sr–Nd–Pb isotopic investigation of Mayon Volcano, a highly active arc volcanic center along the east margin of the southern portion of Luzon, Philippines (Fig. 1). The primary objective of our study is to constrain the nature of the subduction component in Mayon lavas. We extend our investigation to constrain the nature of the subduction component in the lavas from Taal Volcano, the most active center along the west margin of southern Luzon (Fig. 1). Previous studies (e.g. Knittel & Defant, 1988; Defant et al., 1989; Forster et al., 1990; Mukasa et al., 1994) have indicated that a subduction component was recently added to the mantle wedge beneath Taal Volcano. Using the combined results for the two most highly active volcanoes along the east and west margins of southern Luzon, we attempt to provide a better understanding of magma generation along oceanic convergent margins.
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) and other volcanoes (
) along the east and west margins of the archipelago. |
GEOLOGICAL SETTING AND SAMPLES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND SAMPLESANALYTICAL PROCEDURESRESULTSDISCUSSIONIMPLICATIONS FOR THE REGIONAL...SUMMARY AND CONCLUSIONSREFERENCES |
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The east margin of the Philippine archipelago is lined with volcanic centers that are associated with the Philippine Trench where the Philippine Sea Plate, with a modest cover of pelagic sediment, is being subducted toward the west (Fig. 1a). These volcanic centers belong to the east Philippine arc system and are grouped regionally into the Bicol Arc at the northern end of the Philippine Trench (e.g. Newhall, 1979
; Knittel et al., 1988), volcanoes on Leyte Island in the center (Sajona et al., 1994), and the East Mindanao Arc at the southern end (Sajona et al., 1997). Mayon is a highly active stratovolcano that belongs to the Bicol Arc on the east margin of southern Luzon (Fig. 1b). Although its first recorded activity was in AD 1616, Mayon has probably been erupting since the Pliocene and has continued to erupt intermittently up to the present. The Bicol Arc is underlain by Tertiary–Quaternary sedimentary and volcanic rocks and pre-Tertiary schists, gneisses, and ultramafics.
The samples analyzed in this study are a subset of Mayon lavas previously analyzed for petrography, mineralogy, and bulk major element chemistry by Newhall (1977, 1979) plus two newer samples from the 1984 and 1993 eruptions. The samples are historic lavas erupted from the main vent except for two older, primitive basalts (Lignon1 and Pac8) from two parasitic cones near the base of the volcano. All samples were analyzed for their major and trace element contents. Strontium, Nd and Pb isotopic ratios were determined on a smaller number of samples selected on the basis of petrographic data of Newhall (1977, 1979) and new major and trace element results.
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ANALYTICAL PROCEDURES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND SAMPLESANALYTICAL PROCEDURESRESULTSDISCUSSIONIMPLICATIONS FOR THE REGIONAL...SUMMARY AND CONCLUSIONSREFERENCES |
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Major oxide and Ba, Co, Cr, Nb, Ni, Rb, Sr, V, Y, and Zr contents were determined by X-ray fluorescence (XRF) on a wavelength-dispersive Phillips instrument at the Scripps Institution of Oceanography (SIO). Determination of major element oxides was conducted on fused disks (0·5 g sample to 2·5 g LiBO2 flux) following the method of Norrish & Hutton (1969)
. Trace elements were measured using pressed powder pellets (3 g sample to 1 g methyl cellulose) following the method of Norrish & Chappell (1977). Rare earth element (REE), U, Th and Pb determinations were performed by inductively coupled plasma mass spectrometry (ICP-MS) on a Finnigan Element 2 high-resolution ICP-MS instrument. Some of the Taal samples previously analyzed for Sr, Nd and Pb isotope ratios by Mukasa et al. (1994) were analyzed for REE, U, Th, and Pb concentrations. For ICP-MS analyses, rock powders were digested using the procedure described by Janney & Castillo (1996) and then diluted in a 2·5% HNO3 solution containing 1 ppb 115In as an internal standard. The accuracy and precision of the major and trace element analyses were monitored by repeated analysis of known rock standards and are reported as notes under Table 1.
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Almost all the Sr, Nd, and Pb isotope measurements by thermal ionization mass spectrometry (TIMS) were carried out at the Department of Terrestrial Magnetism (DTM) of the Carnegie Institution of Washington using well-established procedures (e.g. Walker et al., 1989
; Castillo et al., 1991). About 200 mg of rock powders were dissolved in Teflon beakers and then passed through small HBr ion exchange columns to collect Pb. The residues from Pb extraction were collected and dried, and less than half (
75 mg) of each was then passed through primary cation exchange columns to collect Sr and the REE. Finally, Nd was separated from the rest of the REE by passing the REE cuts through small EDTA ion exchange columns. Pb and Sr isotope measurements were made on a five-collector VG 354 thermal ionization mass spectrometer. Nd isotope measurement was carried out in oxide form using a home-built, 15 inch radius mass spectrometer. A few isotope measurements were carried out at SIO. The sample preparation procedure used at SIO is similar to that described by Lugmair & Galer (1992) and Janney & Castillo (1996). Fractionation corrections used in both laboratories are similar. Details of fractionation corrections and analytical uncertainties are presented as notes under Table 2.
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RESULTS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND SAMPLESANALYTICAL PROCEDURESRESULTSDISCUSSIONIMPLICATIONS FOR THE REGIONAL...SUMMARY AND CONCLUSIONSREFERENCES |
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Major element chemistry
Mayon lavas range from basalt to andesite, but most are basaltic andesites (Table 1 and Fig. 2). The lavas belong to the medium-K, calc-alkaline arc series except for one primitive basalt (Pac8) that is in the high-K series. Mayon lavas overlap with those from Leyte and East Mindanao segments of the east Philippines arc system to the south of Mayon Volcano. Some of the andesites and dacites from the southern arc segments belong to the low-K lava series and were thought by Sajona et al. (1994
, 1997) to be ‘adakites’ formed by partial melting of the subducted Philippine Sea basaltic crust (PSBC) during initiation of subduction along the eastern margin of the Philippines. As a whole, Mayon lavas are less alkalic than lavas from Taal Volcano, which belongs to the west Philippine arc at the west margin of southern Luzon (Fig. 1).
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Except for the two primitive basalts, Mayon lavas fall within a narrow compositional range of 54–58 wt % SiO2 (Fig. 2). This narrow bulk compositional range is reflected in a relatively constant mineral assemblage. Mayon lavas are all porphyritic with 25–50% phenocrysts (mostly 35–45%) typically consisting of plagioclase, augite, hypersthene, olivine, titaniferous magnetite, and sparse hornblende set in a matrix of the same minerals plus glass and accessory apatite. Despite the narrow compositional range and the fact that the lavas are porphyritic, and thus were not erupted at near-liquidus conditions, the major element oxides of Mayon lavas define linear or curvilinear trends in Harker diagrams with little scatter (Fig. 3).
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Trace elements
Trace element abundances are listed in Table 1 together with major element data and are shown graphically in Figs 3–5. The trace element contents of Mayon lavas are typical of most calc-alkaline lavas in that they have enriched incompatible trace element patterns with negative high field strength element (HFSE) anomalies (Fig. 4). Specifically, Mayon lavas have high concentrations (20–130 times primitive mantle; Sun & McDonough, 1989
) of LILE (such as Rb, Ba, K) but have low contents (4–8 times primitive mantle) of moderately incompatible trace elements (Y, Er, Yb, Lu). The HFSEs Nb and Ti are depleted (3–14 times primitive mantle) relative to adjacent elements. In many respects, Mayon lavas overlap with Leyte lavas and to a lesser extent, with East Mindanao Arc lavas, which extend to lower total ranges of incompatible trace elements (see Sajona et al., 1994, 1997). Contents of compatible elements Ni, Cr, and V are fairly low in Mayon lavas, except in the two primitive basalts. The aforementioned East Mindanao adakites have the lowest contents of moderately incompatible as well as compatible elements among east Philippine arc lavas.
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In detail, the behavior of individual trace elements in Mayon lavas is fairly complex. The concentration of incompatible trace elements shows more scatter than that of major elements with increasing SiO2 content (Fig. 3). This behavior is also demonstrated by their REE concentrations (Fig. 5). Mayon lavas show a light REE (LREE)-enriched pattern (La/SmN >2·0) similar to other Philippine mafic arc lavas (e.g. Defant et al., 1988
, 1989; Miklius et al., 1991). Their REE concentration pattern overlaps with that of Taal lavas that belong to the west Philippine arc system. However, unlike typical arc lavas, the REE contents of Mayon lavas do not exhibit a continuous enrichment from basalts to andesites.
Sr, Nd, and Pb isotopic ratios
The isotopic ratios of representative Mayon lavas are presented in Table 3 and plotted in Fig. 6a and b. The most outstanding isotopic feature of Mayon lavas is that the total range is relatively small (87Sr/86Sr = 0·70370–0·70383; 143Nd/144Nd = 0·51292–0·51300; 206Pb/204Pb = 18·54–18·57). The two primitive basalts, which have higher LREE contents than the rest of Mayon lavas, are isotopically indistinguishable from the other Mayon lavas. Also shown in Fig. 6 are fields of data for Leyte and East Mindanao Arc lavas and from several arc volcanic localities along the west Philippines arc. As pointed out by Castillo (1996), there appears to be a systematic decrease in 206Pb/204Pb and to a lesser extent an increase in 87Sr/86Sr from Mayon to Leyte to east Mindanao (i.e. from north to south along the east Philippine arc). Mayon lavas have lower 87Sr/86Sr, higher 143Nd/144Nd, and lower 207Pb/204Pb and 208Pb/204Pb for given 206Pb/204Pb than lavas from Taal, Laguna de Bay, and Arayat volcanoes in the central and southern Luzon segment of the west Philippine arc system (Mukasa et al., 1994). Compared with the west arc volcanic lavas of the Batanes Islands in northern Philippines (McDermott et al., 1993; Castillo, 1996; Yang et al., 1996), Mayon lavas have higher 206Pb/204Pb but lower 207Pb/204Pb and 208Pb/204Pb, although they overlap with some of these lavas with low 87Sr/86Sr and high 143Nd/144Nd.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND SAMPLESANALYTICAL PROCEDURESRESULTSDISCUSSIONIMPLICATIONS FOR THE REGIONAL...SUMMARY AND CONCLUSIONSREFERENCES |
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Compositional variation of Mayon lavas
Newhall (1977
, 1979) showed that most of the observed petrographic and major element variation of Mayon lavas can be ascribed to crystal fractionation and magma mixing. Compositions of Mayon lavas showing repeated cycles of basaltic andesite and andesite result from fractional crystallization in a shallow magma chamber, punctuated by periodic influxes of basaltic magma from depth that mix with the differentiating magma. Most of the melt passes through the chamber, but a small portion bypasses it, forming parasitic cones (e.g. Lignon1 and Pac8). New batches of parental melt mix with fractionating magma and probably trigger eruptions.
The trace element concentrations of Mayon lavas, however, are variable and do not show systematic behavior with increasing SiO2 content (Figs 3 and 5). Our attempts to model the trace element concentration of individual lavas through simple crystal fractionation or fractional crystallization plus mixing with primitive basalts were unsuccessful. It is possible that the trace element variation could have resulted from assimilation and fractional crystallization (AFC) processes (e.g. DePaolo, 1981; Reagan et al., 1987). We have no samples of the basement of the Bicol Arc, but only minimal assimilation of the basement is allowed by the limited range of major element and isotopic compositions as well as by the lack of correlation between isotopic ratios and major and trace element concentrations. Thus, some trace element variations of Mayon lavas are probably intrinsic to a common, compositionally heterogeneous source of the lavas. The distinct, though small variation in Sr, Nd, and Pb isotopic ratios is consistent with this interpretation.
The unmodified composition of the Mayon sub-arc mantle
The isotopic composition of Mayon lavas is closer to the isotopic composition of normal mid-ocean ridge basalt (N-MORB) than most Philippine arc lavas, except for some of those in the northernmost segment of the west Philippine arc (Mukasa et al., 1987, 1994; McDermott et al., 1993; Castillo, 1996; Yang et al., 1996). In detail, the isotopic ratios of Mayon lavas are more akin to PSBC, which has an Indian MORB-like isotopic signature (Fig. 6; Hickey-Vargas, 1991, 1998; Spadea et al., 1996). This implies that the main source of Mayon lavas has a history of long-term (approximately billion years) depletion relative to bulk Earth values of incompatible trace elements (i.e. low Rb/Sr, Nd/Sm, U/Pb, and Th/Pb ratios). The Mayon sub-arc mantle, however, is apparently unlike those beneath many other subduction zones, which are ultra-depleted in incompatible elements (see Woodhead et al., 1993; Elliot et al., 1997). This is shown by the generally low and constant Zr/Nb ratios (average of 16; Fig. 7a) in Mayon lavas, which are much lower than the Zr/Nb ratio of the average, incompatible element-depleted, N-MORB (Zr/Nb = 60; Sun & MacDonough, 1989). The incompatible HFSE ratios such as Zr/Nb are regarded as good indicators of the unmodified sub-arc mantle composition because they generally are thought to be not modified by dehydration fluids (e.g. Woodhead et al., 1993; Pearce & Peate, 1995; Pearce et al., 1999). The Mayon Zr/Nb values are also close to the average value for the PSBC subducting beneath Mayon Volcano (Zr/Nb = 13; Hickey-Vargas, 1998). Thus, assuming that the mantle source of PSBC has not been affected by a prior history of subduction and/or that HFSE have not been added to the Mayon sub-arc mantle by subduction in a significant way, then both isotopic and HFSE ratios indicate that the unmodified Mayon sub-arc mantle probably has an Indian MORB-like composition.
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It is also tempting to attribute the isotopic and Zr/Nb ratios of Mayon lavas to direct melting of the subducted PSBC. Indeed, Sajona et al. (1993
, 2000) have proposed that melts coming from the basaltic portion of subducted slabs play a significant role in generating many Philippine arc magmas. Along the eastern Mindanao segment of the east Philippine arc, Sajona et al. (1993, 1994, 1997) pointed out that some silicic (>56 wt % SiO2) lavas are pure PSBC-derived melts because of their high Sr/Y (>100) and La/Yb (>7) ratios and low Y (<10) and Yb (<1) values. These chemical characteristics are also possessed by lavas generated by melting of underplated basalt or lower mafic crust (e.g. Atherton & Petford, 1993; Xu et al., 2002), and some of these lavas have already been proposed to occur along the west Philippine arc (e.g. Yumul et al., 1999). Mayon lavas and particularly the primitive basalts, however, do not possess these and other criteria for melts coming from a basaltic source (see Rapp et al., 1999). It should be noted that in Mayon lavas, the primitive basalts have the highest La/Yb (15) and among the highest Sr/Y (40) ratios, so we infer that processes other than melting of the subducted basaltic crust can produce high Sr/Y and La/Yb ratios in arc lavas (see also Atherton & Petford, 1993; Kay & Kay, 1993; Castillo et al., 1999; Yumul et al., 1999; Xu et al., 2002).
The subduction component beneath the southern Luzon east Philippine arc
Although the Mayon sub-arc mantle has an Indian MORB-like composition, Mayon lavas are enriched in LILE and depleted in Nb (Fig. 4). The enrichment in LILE indicates that a subduction component, derived from subducted sediment and basaltic crust, is being added to the mantle wedge source of the arc lavas (e.g. Gill, 1981; Hawkesworth et al., 1991, and references therein). Interestingly, the absolute concentrations and normalized patterns of the highly incompatible trace elements of the Mayon lavas are similar to those of the bulk sediments (Fig. 4). In fact, the key Zr/Nb ratios of Mayon lavas are close to the average of the bulk pelagic sediments overlying the subducting PSBC (bulk sediment Zr/Nb = 18; Fig. 7a). The major questions then are: (1) how is a pelagic sediment compositional signature physically being transferred as a subduction component? (2) Is pelagic sediment the only source of subduction component in the Mayon sub-arc mantle?
To constrain the possible sources of subduction component in the Mayon sub-arc mantle, we compare the Th/Nb ratios of our Mayon data with those for lavas of the Mariana Arc. The ratio of Th/Nb is a good tracer of the source of arc lavas because both Nb and, to a certain extent, Th are relatively immobile in aqueous fluids (Brenan et al., 1995b; Elliot et al., 1997); a recent experimental study showed that Th can be efficiently transferred from subducted sediments to the mantle source of arc lavas through melting (Johnson & Plank, 1999). Mayon lavas have a wide range of Th/Nb ratios (Fig. 7a–d). Similar to Mariana arc lavas, there is little or no correlation between Th/Nb ratios and SiO2 contents of Mayon lavas. In Mariana Arc lavas, Th/Nb ratios show systematic correlations with other indices of geochemical enrichment (e.g. La/Sm, U/Nb, La/Nb, Zr/Nb). The same general relationships are shown by Mayon lavas (Fig. 7b–d). Elliot et al. (1997) proposed that the wide range of Th/Nb ratios in Mariana arc lavas results from a bimodal source of the subduction component. The main subduction component involved in the production of lavas that have the highest Th/Nb ratios (i.e. lavas from Agrigan Island) is melted pelagic sediment. The subduction component responsible for lavas that have the lowest Th/Nb ratios (i.e. lavas from Guguan Island) was postulated by Elliot et al. (1997) to be aqueous fluid dehydrated from the subducted basaltic crust. In the Izu–Bonin arc volcanic front, a relatively more geochemically enriched aqueous fluid component from the subducting basaltic crust and sediment is being added to the mantle wedge whereas in the backarc, an aqueous fluid dehydrated further from the residual slab is being added to a relatively more enriched mantle (Hochstaedster et al., 2001). The wide range of Th/Nb ratios of Mayon lavas, therefore, by analogy suggests that both melt and aqueous fluid phases may be involved in transferring the subduction component into the Mayon sub-arc mantle.
Melting of sediment
The high Th/Nb end of the Mayon trace element arrays overlaps with or plots close to the bulk sediment value (Fig. 7a–d). Because Nb is immobile in hydrothermal fluid (e.g. Brenan et al., 1995b; You et al., 1996) and there is no compelling evidence in Mayon lavas that Th was mobilized by hydrothermal fluid (e.g. absence of positive Th concentration anomaly; Fig. 4), it is unlikely that the bulk sediment Th/Nb signature was imparted into Mayon lavas through an aqueous fluid phase. On the other hand, although experimental results show that partial melts of sediments have different trace element compositions and ratios, Th and Nb are both effectively mobilized from sediment above the solidus (Johnson & Plank, 1999). Hence, the Mayon data support the idea that melt is more effective than aqueous fluid in transferring the bulk sediment signature of the subducted pelagic sediment into the source of Mayon Arc lavas.
To further constrain the source of the subduction component in the Mayon sub-arc mantle, we plot Th/Nb against Ba/La and La/Sm against Ba/La ratios of the Mayon lavas (Fig. 8a and b). Barium is only slightly more incompatible than La. Hence, during the normal range of partial melting of the mantle (10–20%), the Ba/La ratio of the resultant magma is similar to that of its source material and only becomes slightly higher at extremely small degree of melting (
1%). On the other hand, experimental results show that Ba is extremely mobile in aqueous fluids during hydrothermal dehydration of subducted sediments (You et al., 1996) and altered basalts (e.g. Brenan et al., 1995b; Kogiso et al., 1997a). Thus a high Ba/La ratio is an excellent index of an aqueous fluid contribution from the subducted sediment and basaltic crust. West Pacific chalk has extremely high Ba/La ratio (Lin, 1991), but no such sediments have been found in the subducting oceanic crust beneath Mayon Volcano (Solidum, 2002). Figure 8a shows an overall negative correlation between Ba/La and Th/Nb ratios of Mayon lavas. This is opposite to what is expected if Th is mobile in aqueous fluid and requires that Th be more incompatible in aqueous fluid than Ba during slab dehydration (e.g. Elliot et al., 1997; Johnson & Plank, 1999). Finally, La/Sm is opposite Ba/La ratio to a certain extent because La is more incompatible than Sm during partial melting and La is only slightly more mobile than Sm in hydrothermal fluid (e.g. You et al., 1996). The La/Sm ratio therefore should become more elevated during partial melting than during dehydration of materials from a subducted slab. This means that a subduction component carried by an aqueous fluid should have the highest Ba/La and lowest La/Sm ratios whereas that carried by a melt phase should have lowest Ba/La and highest La/Sm ratios. Figure 8b shows an overall negative correlation between Ba/La and La/SmN ratios of Mayon lavas, with the low Ba/La–high La/SmN end of the array overlapping with the bulk sediment. This indicates that the Ba/La and La/SmN ratios of the subducted sediments probably are being transferred to the Mayon sub-arc mantle through a melt phase.
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Dehydration of basaltic crust
The low Th/Nb end of the Mayon trace element arrays trends toward PSBC (Fig. 7a–d), the isotopic composition of which is presumed to be that of the Mayon sub-arc mantle. Because PSBC is evidently not being melted, the low Th/Nb signature of Mayon lavas could be coming directly from the sub-arc mantle, or the PSBC signature is being transferred to the mantle through aqueous fluid, or both. The low Th/Nb end of the array, however, has a high Ba/La ratio, a signature of slab-derived aqueous fluid (Fig. 8a). Thus it is most likely that the low Th/Nb ratio of some of the Mayon lavas is coming from fluids dehydrated from PSBC.
Finally, we plot Ce/Pb against 207Pb/204Pb ratio (Fig. 8c; Miller et al., 1994). It has been shown experimentally that Pb is highly mobile whereas Ce is only slightly mobile in hydrothermal fluids (e.g. Brenan et al., 1995a; You et al., 1996). The lead isotopic signature is not affected by dehydration or melting. Mayon lavas have the same Pb isotopic signature as the PSBC, indicating that their Pb isotopic composition is not coming from the sediment, but from either the mantle wedge or subducted PSBC (Figs 6b and 8c). Indeed, Pb isotopic data indicate only a small bulk sediment contribution of 1%, although Sr and Nd isotopic ratios indicate a slightly higher sediment contribution of 5% to the Indian MORB-like sub-arc mantle (Fig. 6a and b). Mayon lavas, however, have much lower Ce/Pb ratios than the PSBC and mantle wedge, so the bulk of their Pb content must have been added through hydrothermal fluids coming from PSBC.
A possible reason why the Pb dehydrated from the Pb-enriched West Philippine Sea sediment was not transferred into the mantle source of Mayon lavas is its high mobility in hydrothermal fluids. The bulk of the Pb content of the subducted sediment may have been lost early at shallower depths in the subduction zone (You et al., 1996). The fact that the Pb content of Mayon lavas is much lower than that of sediments (Fig. 4) is consistent with this interpretation. This also implies that the result of mass balance calculation based on Pb isotopic compositions and concentrations in sediments and mantle wedge (Fig. 6b) should be considered a minimum, as Pb is not a conservative element during sediment subduction.
In summary, the combined isotopic and trace element data suggest that the mantle wedge beneath Mayon Volcano is enriched by a subduction component. The subduction component comes from partial melting of the subducted pelagic sediment and from dehydration of the subducted PSBC (see also Plank & Langmuir, 1998). Surprising in this light, a preliminary 10Be result for a Mayon lava failed to detect the sediment input (Morris & Tera, 1989).
Implication for HFSE contents of arc lavas
Another interesting implication of our results concerns the suggested role of a titanate phase in creating negative HFSE anomalies in arc lavas (e.g. Green, 1981; Morris et al., 1990). Lavas generated from the Mayon sub-arc mantle mixed with bulk sediment-derived melt do not need residual titanate phase in the mantle wedge, as the sediment itself has negative HFSE anomalies. A similar conclusion was arrived at by Elliot et al. (1997) regarding some of the Mariana Arc lavas. When the proportion of sediment contribution decreases and the amount of PSBC-hydrated aqueous fluid increases, the negative HFSE anomaly diminishes. Such positive correlation between sediment contribution and HFSE depletion clearly indicates that, at least in the present case, a titanate phase is not necessary to create negative HFSE anomalies.
The subduction component beneath Taal Volcano, revisited
Taal Volcano erupts a large volume of lavas in the southern Luzon segment of the west Philippine arc (Fig. 1) and is also one of the better-studied arc volcanic systems in the Philippines (e.g. Miklius et al., 1991; Mukasa et al., 1994; Knittel & Yang, 1998). The modern Taal Volcano is a remnant of an eruptive complex occupying a massive volcano-tectonic depression, now occupied by Taal Lake, resulting from multiple phases of collapse (Miklius et al., 1991). The lava flows and pyroclastic materials from several of the eruption centers produced during different phases of volcanic activity in the complex as well as from the older NW margin of the volcanic depression range from basalt to dacite. Despite the wider range in composition (Fig. 2), the Taal lava series are similar to Mayon lavas in that they fall along trends defined by crystal fractionation and magma mixing (Miklius et al., 1991).
Could Taal lavas be affected by crustal contamination? SiO2 and MgO are not correlated with isotopic ratios, particularly the Pb isotopes, which show the most systematic variation (Fig. 6b). Moreover, petrological investigations along the Luzon Arc show that lavas containing the largest subduction components (i.e. those with the highest 87Sr/86Sr ratios and the most enriched in LILE) are those that are most primitive (i.e. have the highest MgO, Cr, and Ni contents, and hence are the least contaminated). Examples include several rock suites in the Taiwan segment of the Luzon Arc (McDermott et al., 1993) and the Mt. Pinatubo rock suite (Castillo & Punongbayan, 1996). Karig (1983) found no evidence that the Luzon Arc is underlain by continental crust, and the nearby Palawan continental block (Cardwell et al., 1980; Defant et al., 1988) has 206Pb/204Pb and 207Pb/204Pb ratios (Tu et al., 1992) that are too low to be the appropriate high 207Pb/204Pb contaminant for the Taal lavas. Finally, the high 3He/4He ratios reported for Taal geothermal gases (7·5 Ra; Poreda & Craig, 1989) do not support significant crustal contributions of helium to the Taal lava sources. Thus, the highly active magma supply to Taal does not appear to melt and assimilate crust through which it rises.
Instead, we think the compositional heterogeneity of Taal lavas is due to a common, although heterogeneous mantle source (Miklius et al., 1991; Mukasa et al., 1994; Knittel & Yang, 1998). Strontium and Nd isotopic ratios indicate that the Taal sub-arc mantle has had long-term depletion in incompatible trace elements, but the lavas are enriched in incompatible trace elements (i.e. high Rb/Sr and Nd/Sm ratios; Mukasa et al., 1994). This decoupling between isotopic and elemental ratios was interpreted as due to recent addition to the mantle wedge of subduction component, consisting mainly of materials from subducted sediments (see also Knittel et al., 1988).
One of the best arguments for the sedimentary origin of the subduction component in the Taal sub-arc mantle is Pb isotopic data. Taal lavas have variable 207Pb/204Pb and 208Pb/204Pb for given 206Pb/204Pb ratios, forming linear arrays that trend from the low 207Pb/204Pb and 208Pb/204Pb values of lithospheric mantle toward the high 207Pb/204Pb and 208Pb/204Pb signatures of oceanic sediment (Fig. 6b). Taal lavas also have lower 143Nd/144Nd but higher 87Sr/86Sr than Mayon lavas, closer to the sediment Nd and Sr isotopic signature (Fig. 6a). A significant sediment contribution to the genesis of Taal lavas is consistent with the thick terrigenous sediment cover of the South China Sea basaltic crust and absence of sedimentary wedge along the Manila Trench (Silver & Rangin, 1991).
There is no information regarding sediment subducting directly beneath Taal Volcano, but there are limited data available for sediments currently subducting beneath the northern segment of the west Philippine arc (Fig. 1; McDermott et al., 1993). These South China Sea sediments are mainly derived from continental Eurasia and are similar to Sulu Sea sediments sampled during Ocean Drilling Program Leg 124 (e.g. Brass et al., 1991; Solidum & Castillo, 2001; Solidum, 2002). The composition of the South China Sea terrigenous sediments is distinct from that of the pelagic sediments in the Philippine Sea being subducted beneath Mayon Volcano. This is clearly shown by the relatively higher Rb and Th but lower Sr contents of the South China Sea than Philippine Sea sediments, which are respectively reflected by Taal and Mayon lava compositions (Figs 4 and 9). Thus, it appears that the main compositional difference between Mayon and Taal lavas can be traced to the different types of sediments being subducted on either side of southern Luzon.
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We proposed in the previous section that the sediment compositional signature is being transferred to the source of Mayon lavas mainly through a melt phase. On the other hand, Knittel & Yang (1998)
proposed that an aqueous fluid phase is responsible for the transfer of sediment component into the Taal sub-arc mantle. One clue to the composition of the unmodified Taal sub-arc mantle is that in the sediment discrimination diagrams (Fig. 9a–d), Taal lava compositional trends point toward PSBC. Previous studies have also shown that the Taal sub-arc mantle, like the Mayon sub-arc mantle, is not as depleted in highly incompatible trace elements as many other sub-arc mantles (e.g. Miklius et al., 1991; Knittel & Yang, 1998). Thus, we assume that both Taal and Mayon sub-arc mantles have similar, Indian MORB-like compositions.
There are no available Nb data for Taal arc lavas to directly compare their Th/Nb ratios with those of Mayon and Mariana arc lavas. Nevertheless, Taal lavas form linear arrays in both Ba/La vs La/SmN and Ce/Pb vs 207Pb/204Pb plots (Fig. 8b and c), indicating that, similar to the Mayon sub-arc mantle, the subduction component in the Taal sub-arc mantle is also coming from subducted sediment and basaltic crust. Sediment contribution is more significant in Taal sub-arc mantle than in Mayon sub-arc mantle. This is clearly shown in the subvertical array of Taal lavas in the Ce/Pb vs 207Pb/204Pb plot (Fig. 8c). The high 207Pb/204Pb end of the array points to the high 207Pb/204Pb signature of terrigenous sediment whereas the low 207Pb/204Pb end points to the aqueous fluid signature of the subducted basaltic crust (see Miller et al., 1994; Brenan et al., 1995a). In the Ba/La vs La/SmN plot (Fig. 8b), Taal lavas have a fairly limited and slightly higher Ba/La ratio than both the PSBC and West Philippine Sea bulk terrigenous sediment. This is consistent with the derivation of the low La/SmN–high Ba/La end of the array from fluids derived from the subducted basaltic crust. The high La/SmN–high Ba/La end of the array, on the other hand, indicates that the Ba/La signature of subducted sediments is being transferred to Taal lavas either through a small-degree melt or an aqueous fluid phase, or both. Further evaluation of whether the terrigenous sediment signature is being transferred to Taal lavas through either melt or aqueous fluid phase, however, will have to wait until analyses of terrigenous sediments subducting beneath Taal Volcano have become available.
In summary, as at Mayon, the mantle wedge beneath Taal Volcano is being enriched by a subduction component. At both volcanoes, the subduction components come from subducted sediment and basaltic crust. It appears that in both cases the basaltic crust component is being transferred in an aqueous fluid phase. The sediment component in Mayon is being transferred through a melt phase whereas at Taal it is being transferred either as a small-degree melt or through an aqueous fluid, or both. The main difference between the two settings is the difference in the composition of the pelagic and terrigenous sediments being subducted beneath the east and west Philippine arcs (Karig et al., 1975; Rangin et al., 1990; Solidum, 2002). More sediment is also involved in the generation of the subduction component beneath Taal Volcano than beneath Mayon Volcano. The South China Sea Basin being subducted eastward along the Manila Trench has moderate to thick cover of terrigenous sediments (Cardwell et al., 1980; Taylor & Hayes, 1983; Rangin et al., 1990; Silver & Rangin, 1991; Solidum, 2002) and delivers a relatively large volume of terrigenous sedimentary component into the mantle beneath Taal Volcano. In contrast, the Philippine Sea Plate being subducted westward along the Philippine Trench has a thin cover of pelagic sediments (Karig et al., 1975; Cardwell et al., 1980; Silver & Rangin, 1991; Solidum, 2002). This thin sediment cover of the subducting slab limits the amount of pelagic sediment in the mantle beneath the east margin of southern Luzon.
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IMPLICATIONS FOR THE REGIONAL VARIATION OF PHILIPPINE ARC LAVAS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND SAMPLESANALYTICAL PROCEDURESRESULTSDISCUSSIONIMPLICATIONS FOR THE REGIONAL...SUMMARY AND CONCLUSIONSREFERENCES |
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Lavas erupted in various segments of the east and west Philippine arcs are diverse in their chemical and isotopic compositions (e.g. Mukasa et al., 1987
, 1994; McDermott et al., 1993; Castillo, 1996), and the origin of the variation is controversial. A few investigators call for variations in the amount of melt from subducted basaltic crust to explain some of the compositional variabilities (Sajona et al., 2000), but others believe that the entire Philippine arc setting is underlain by a common, geochemically depleted mantle source and that the arc compositional variation results from variable type and proportion of sediments being subducted beneath the different segments of Philippine arcs (e.g. Defant et al., 1989; McDermott et al., 1993). For example, the Sr isotopic ratios along the central Luzon segment of the west Philippine arc display latitudinal variation starting from a low, almost MORB-like value (0·7030) in the north to a high of 0·7047 in Taal Volcano in the south (Figs 1 and 6). Defant et al. (1989) suggested that such variation is due to increasing amounts of continental sediment subducting from north to south along the Manila Trench. Subvertical trends in 207Pb/204Pb and 208Pb/204Pb vs 206Pb/204Pb plots are also a characteristic feature of lavas from different segments of the Philippine arc systems, and McDermott et al. (1993) have proposed that this feature is also the result of addition of sediments with variable isotopic compositions to what might otherwise be an isotopically homogeneous mantle wedge.
Still other investigators have proposed that the compositional variation in the lavas is a combined effect of the variable composition of the unmodified mantle wedge and later added components from subducted slab (e.g. Chen et al., 1990; Castillo, 1996). The Philippine archipelago is an amalgamation of different tectonic terranes from different locations (e.g. Hamilton, 1979; Hall, 1996), and hence it is possible that these terranes carried with them portions of the lithospheric mantle and deep mafic lower crust from previous locations. Along the east Philippine arc, for example, there is also distinct latitudinal variation in Sr and Pb isotopic compositions although there is neither continental sediment nor demonstrated compositional variation in the sediment being subducted along the Philippine Trench (Castillo, 1996).
Our results show that sediments play a major role in generating the overall arc compositional signature of Mayon and Taal arc lavas. Thus, the results favor variation in the type and amount of subducting sediments as the major factor behind the regional variation of Philippine arc lavas. However, we do not yet know whether the sediment signature is transferred through melt in all Philippine arcs. Neither do we know whether dehydration or melting of the basaltic crust plays a major role in generating compositional variation (e.g. White & Dupre, 1986; McDermott et al., 1993; Miller et al., 1994; Elliot et al., 1997). Long-lived, but small-scale, heterogeneities in the mantle wedge (e.g. Morris & Hart, 1983; Wallace & Carmichael, 1999; Castillo et al., 2002) might also add to regional compositional variability. Finally, the subparallel, subvertical trends in 207Pb/204Pb and 208Pb/204Pb vs 206Pb/204Pb plots are inconsistent with a homogeneous mantle source for these lavas. These trends must be due to fairly recent addition of Pb from subduction components to a mantle wedge originally variable in 206Pb/204Pb (Castillo, 1996).
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SUMMARY AND CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND SAMPLESANALYTICAL PROCEDURESRESULTSDISCUSSIONIMPLICATIONS FOR THE REGIONAL...SUMMARY AND CONCLUSIONSREFERENCES |
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Lavas from Mayon Volcano in southeastern Luzon, Philippines, are compositionally variable. A part of the variation can be ascribed to shallow-level fractional crystallization and magma mixing, but the other part is due to compositional variability of the source. Combined trace element and isotopic data show that the composition of the unmodified mantle wedge beneath Mayon Volcano has an Indian MORB-like composition. To this mantle wedge is added a small amount of subduction component consisting of a few percent partial melt of the subducted pelagic sediment and aqueous fluid dehydrated from the subducted PSBC. Interestingly, the depletion of HFSE in Mayon lavas could be inherited from the sediment HFSE depletion, and hence Mayon lavas do not require the formation of a residual titanate phase in their source.
Previous studies have also shown that the composition of lavas erupted from Taal Volcano on the western side of southern Luzon resulted from sediment addition to its mantle wedge source. Compared with Mayon lavas, Taal lavas have a much wider range of composition. The wide compositional range is most probably due to the larger amount of subduction component being added to the mantle beneath Taal. Combined trace element and isotopic data suggest that the subduction component in the Taal sub-arc mantle is similar to that in the Mayon sub-arc mantle, coming from both the subducted basaltic crust and sediment. The contribution from basaltic crust is transferred in the form of an aqueous fluid phase; sediment contribution is transferred in either a melt or a fluid phase, or in both.
Our results suggest that sediment input exerts a major control on the composition of arc lavas on both sides of southern Luzon. This brings into question the origin of the observed regional variation of arc lavas in the entire Philippine arc systems. Our results favor the idea that the type and amount of sediment can account for most observed variation. However, factors such as the composition of the unmodified mantle wedge and the processes by which the sediment signature is being transferred to the lava source (i.e. whether through an aqueous fluid or melt phase) may also add to generating compositional variations of arc lavas.
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ACKNOWLEDGEMENTS |
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We are grateful to R. Solidum for his help in the analytical work, comments, and suggestions; to M. Flower for providing the Taal samples; and to C. MacIsaac, the DTM staff, and the SIO Analytical Facility for the use of analytical laboratory and instruments. We also much appreciate the thorough reviews of W. Hildreth, J. Ryan, T. Sisson, and two anonymous reviewers and editorial handling of D. Geist, which significantly improved the manuscript. This work is supported by NSF grant EAR00-01212 and Carnegie Institution of Washington Postdoctoral Fellowship to P.C.
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FOOTNOTES |
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* Corresponding author. Telephone: (858) 534-0383. Fax: (858) 822-4945. E-mail: pcastillo{at}ucsd.edu
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