FIRE and MUD Contents

Petrology and Sr, Nd, and Pb Isotopic Geochemistry of Mount Pinatubo Volcanic Rocks

By Paterno R. Castillo¹ and Raymundo S. Punongbayan²

² Philippine Institute of Volcanology and Seismology.

ABSTRACT

Dacitic ash and pumice and andesitic scoria erupted during the June 1991 Mount Pinatubo volcanic activity are geochemically similar to the calc-alkaline volcanic rocks typical of island-arc settings. Their isotopic signature overlaps with that of other volcanic rocks from the Luzon arc, particularly those from the northern segment. In detail, the dacite is slightly less enriched in incompatible trace elements, ⁸⁷Sr/⁸⁶Sr, and ²⁰⁶Pb/²⁰⁴Pb and is slightly higher in ¹⁴³Nd/¹⁴⁴Nd than the andesite. These data clearly indicate that the dacite and andesite have different magma sources--probably variable mixtures of depleted basalts of the Zambales ophiolite complex (or their mantle source), and an enriched component from subducted sediments. Direct sediment involvement in the generation of the Mount Pinatubo volcanic rocks was minimal; the presence of ¹⁰Be, high ³⁴S, and recent light element enrichment were most probably acquired through metasomatism of the ophiolitic basement basalts, or their mantle source, by fluids derived from the subducted slabs.

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INTRODUCTION

Mount Pinatubo is located approximately 90 km northwest of Manila in a chain of mostly inactive volcanoes on the west side of the central part of Luzon Island, Philippines (fig. 1). This volcanic chain is the Bataan segment (Defant and others, 1989) of the Luzon arc, which extends from Taiwan in the north to Mindoro Island in the south. Volcanism along the Luzon arc is the result of the eastward subduction of the South China Sea plate under northern Luzon and the Philippine Sea plate, and it is generally believed that sediments above the subducting slab are involved in the generation of Luzon arc magmas (Knittel and others, 1988; Defant and others, 1988; McDermott and others, 1993; Mukasa and others, 1994; Bernard and others, this volume). The types and actual mechanisms of sediment involvement in the generation of the magmas, however, are still a subject of intense scrutiny (Chen and others, 1990; Maury and others, 1992; McDermott and others, 1993).

Figure 1. Locations of Mount Pinatubo and other volcanic centers in the northern Philippines, western Pacific.

After about 500 years of dormancy (Newhall and others, this volume), Mount Pinatubo erupted about 5 km³ of magma during its June 1991 eruptive activity (W.E. Scott, and others, this volume). Most of the rock erupted was dacitic ash and pumice, of which two general types were recognized (Pallister and others, 1992). One group is phenocryst-rich and has large (>1 mm) irregular vesicles, whereas the other group is phenocryst-poor and has smaller (<1 mm) spherical vesicles. Despite their textural differences, however, both pumice types are mineralogically and chemically very similar. In addition to the pumice and ash, a lesser amount of andesitic scoria was erupted during the early, less-intense stages of the volcanic activity, from June 12 to June 14 (Hoblitt, Wolfe, and others, this volume). Pallister and others (1992) believe that the andesite is a mixture of crystal-rich dacite and an olivine-hornblende basalt and that the mixing event occurred when a mafic magma intruded the Mount Pinatubo chamber containing silicic magma; the injection of the mafic magma may have also triggered the eruptions.

In this paper, we present the major element, rare earth element (REE), and strontium (Sr), neodymium (Nd), and lead (Pb) isotopic compositions of andesite blocks from the small lava dome built in the summit of the volcano during the period June 7 to 11 (Hoblitt, Wolfe, and others, 1991; Wolfe and Hoblitt, this volume) and of representative pumice, ash, and basement rocks. The main objective of our study is to use these analytical data to constrain the petrogenesis and source composition of the June 1991 Mount Pinatubo eruptive products.

SAMPLES AND METHODS

The analyzed samples consist of ash from the northeastern slope of the volcano, a medium- to coarse-grained (>1 mm) crystal-rich pumice, two medium-grained (<5 mm) crystal-rich pumices, and two fine-grained (<1 mm) crystal-poor pumices from near Clark Air Base, and two samples of the June 1991 andesite dome. Two samples of magnesian basalts from the basement terrane northwest of Pinatubo were also analyzed. Major element contents of these samples were analyzed by the X-ray fluorescence method, REE contents by inductively-coupled plasma mass spectrometry, and Sr, Nd, and Pb isotopic compositions by thermal ionization mass spectrometry. Analytical results and precision of the analyses are presented in table 1.

Table 1. Chemical and isotopic analyses of representative Mount Pinatubo samples.

[Major elements are measured in weight percent; REE's are measured in parts per million. Analytical errors based on repeated analyses of standards are better than 5% for the major and rare earth elements, +-0.00002 for ⁸⁷Sr/⁸⁶Sr, +-0.000016 for ¹⁴³Nd/¹⁴⁴Nd, +-0.012 for ²⁰⁶Pb/²⁰⁴Pb, +-0.013 for ²⁰⁷Pb/²⁰⁴Pb, and +-0.030 for ²⁰⁸Pb/²⁰⁴Pb. Data are reported relative to standards values: ⁸⁷Sr/⁸⁶Sr = 0.71025 for NBS 987 Sr and ¹⁴³Nd/¹⁴⁴Nd = 0.51186 for La Jolla Nd; Pb isotopic compositions were fractionation corrected by using the NBS 981 Pb values of Todt and others (1983)]

RESULTS

As has been shown previously (Bernard and others, 1991; Pallister and others, 1992), the different pumice types and ash are all dacites (~64 wt% SiO₂) that have very similar, if not identical, major element compositions. The variation in their REE contents is barely outside analytical error, with the coarse-grained pumice generally showing the highest REE content. The REE concentration pattern of dacite of Mount Pinatubo overlaps with the fairly wide field for other Luzon arc volcanic rocks (fig. 2). The pattern is different from that of Mount Mayon volcanic rocks (P.R. Castillo and C.G. Newhall, unpub. data, 1993), which were generated as a result of subduction of the Philippine Sea plate along the eastern margin of southern Luzon (fig. 1).

Figure 2. Chondrite-normalized REE concentration patterns of the 1991 Mount Pinatubo eruptive products and basement samples. Fields for volcanic rocks for Mount Mayon in the Bicol arc (P.R. Castillo and C.G. Newhall, unpub. data, 1993), the Luzon arc from the literature (Knittel and others, 1988; Defant and others, 1988, 1989, and 1990; McDermott and others, 1993), and some of the variably depleted basalts from the Zambales ophiolite complex (Evans and others, 1991; F. Florendo and J. Hawkins, unpub. data, 1993) are shown for comparison. See figure 1 for location of samples.

The two samples of the dome are andesites (~60 wt% SiO₂) and have identical major element compositions. They are more enriched in light and heavy REE than the dacites (fig. 2). Bernard and others (this volume) report that the incompatible element contents of the olivine-hornblende basalt inclusions within the andesite dome are also as enriched, if not more enriched than those of the dacite. In general, both the dacite and the andesite are calc-alkaline rocks that are typical of island arcs, and, in particular, are typical of the nearby volcanoes along the Bataan segment (Defant and others, 1988, 1990; Knittel and others, 1988).

The magnesian basalts represent part of the basement of Mount Pinatubo. They have high (>10 wt%) MgO contents and are very depleted in light REE, very much like some of the Eocene volcanic rocks from the Zambales ophiolite complex located north and west of Mount Pinatubo (fig. 2). Basalts of the Zambales ophiolite complex were derived from variably depleted mantle sources including a source like that for mid-ocean ridge basalt (Evans and others, 1991; F. Florendo and J. Hawkins, unpub. data, 1993). On the basis of their REE concentrations and model calculations, the dacites and andesites could not have been derived from these basalts through simple crystal fractionation. This notion is strongly supported by the isotopic data, which clearly suggest that the three rock groups have compositionally distinct mantle sources.

The dacites are very similar, if not identical, in their Sr, Nd, and Pb isotopic compositions, and so are the andesites (fig. 3) (S.B. Mukasa analyzed a split of our ash sample and his Sr, Nd, and Pb isotopic ratios are very similar to ours--written commun., 1993). That both the ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios of the dacites and andesites plot above the bulk earth values suggests that their source materials have a long-term depletion of Rb with respect to Sr and Nd with respect to Sm. This is inconsistent with their REE concentration patterns, which show high Nd/Sm and, by inference, Rb/Sr (fig. 2). In detail, the isotopic signatures of the dacites and the andesites are distinct from one another, with the dacites having higher ¹⁴³Nd/¹⁴⁴Nd and lower ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb ratios. Interestingly, the ¹⁴³Nd/¹⁴⁴Nd and ²⁰⁶Pb/²⁰⁴Pb of the dacites, respectively, are among the highest and lowest values reported for volcanic rocks from the Bataan and southern segments of the Luzon arc (Bernard and others, this volume; Mukasa and others, 1994). Both the dacites and andesites, however, plot within the field for Batanes Islands and very close to the field for Lutao and Lanhsu Islands in the ⁸⁷Sr/⁸⁶Sr against ²⁰⁶Pb/²⁰⁴Pb diagram (fig. 4).

Figure 3. ¹⁴³Nd/¹⁴⁴Nd against ⁸⁷Sr/⁸⁶Sr (upper panel) and ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb against ²⁰⁶Pb/²⁰⁴Pb (lower two panels) diagrams for the samples analyzed. Only volcanic rocks from Luzon, Batanes, Lutao, and Lanhsu with Sr, Nd and Pb isotopic ratios measured on the same samples are shown for clarity of presentation. Data are from P.R. Castillo and others (unpub. data, 1993); McDermott and others (1993); Mukasa and others (1994). Additional Pb isotopic data from Sun (1980) for Lanhsu are included. Inset in the top diagram shows isotopic data for Mount Pinatubo samples in greater detail. Arrows indicate linear mixing relationship among the magnesian basalt (o), dacite (), and andesite (). NHRL, Northern Hemisphere Regression Line (see text).

Figure 4. ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd against ²⁰⁶Pb/²⁰⁴Pb diagrams for the samples analyzed. Solid arrows and dashed lines illustrate binary mixing curves between the magnesian basalt and sediments necessary to produce the Mount Pinatubo volcanic rocks. Data suggest that the magnesian basalt and the sediments that were mixed to produce the Mount Pinatubo samples do not have identical Sr/Pb and Nd/Pb concentration ratios. Sources of data as in figures 2 and 3.

The magnesian basalts also have identical ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios. They have the highest ¹⁴³Nd/¹⁴⁴Nd and lowest ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb ratios among the samples analyzed, and, altogether, the analyses define consistent linear arrays in all permutations of isotopic ratio diagrams (figs. 3 and 4). The magnesian basalt isotopic signature overlaps with those of the volcanic and ophiolitic rocks of the Taiwan Coastal Range (Sun, 1980; Chen and others, 1990; P.R. Castillo and T. Lee, unpub. data, 1993). Their ¹⁴³Nd/¹⁴⁴Nd also overlap with, though are generally lower than, those of the volcanic rocks from the Zambales ophiolite complex (Evans and others, 1991). It is important to note, however, that the ¹⁴³Nd/¹⁴⁴Nd of the ophiolitic volcanic rocks generally decrease toward the south, and the magnesian basalts are located south of the ophiolitic samples. Moreover, the Pb isotopic ratios of a dolerite from the Zambales ophiolite complex are fairly unradiogenic and close to the composition of the magnesian basalts (²⁰⁶Pb/²⁰⁴Pb=18.19--Hamelin and others, 1984). (The few reported ⁸⁷Sr/⁸⁶Sr ratios for volcanic rocks of the Zambales ophiolite complex clearly have been affected by alteration--Evans and others, 1991.) The similarity of the Nd and Pb isotopic ratios and REE concentration pattern of the magnesian basalts of Mount Pinatubo and volcanic rocks of the Zambales ophiolite complex, in addition to their geographical proximity, strongly suggest that they are petrogenetically related.

Another interesting isotopic feature shared by the dacite and andesite is that their Pb isotopic ratios plot above the Northern Hemisphere regression line (NHRL of fig. 3), which is a line fitted through Pb isotopic data for mid-ocean ridge basalts from the Pacific and North Atlantic and some oceanic islands (Hart, 1984). This Pb isotopic signature, together with their ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios and calc-alkaline affinity, appears to be a characteristic feature of many Philippine volcanic rocks (McDermott and others, 1993; Mukasa and others, 1987; 1994).

DISCUSSION

Our data show that the 1991 Mount Pinatubo eruptive products are similar to some Luzon arc volcanic rocks. Another important point detailed by the data is that the dacite, andesite, and magnesian basalt are all isotopically collinear, which strongly suggests a mixing relationship among them. If the andesite is already a mixture of the dacite and the olivine-hornblende basalt inclusion in the andesite (Pallister and others, 1992; this volume), then the inclusion should plot at the end of the mixing line, opposite the magnesian basalt. Indeed, Pb and Sr isotopic ratios for the basaltic inclusion (U. Knittel and others, unpub. data, 1993) are the highest among those for the 1991 Mount Pinatubo eruptive products and plot at the end of the mixing array away from the magnesian basalt.

Available data also clearly show that the dacite magma is not a mantle source end-component because it is not a direct melt from the mantle and its isotopic composition trends toward the more extreme isotopic composition of the magnesian basalt. The dacite is most likely a crystal fractionation product of a more mafic magma whose source is a product of mixing between the mantle source of the geochemically depleted and isotopically nonradiogenic basement magnesian basalt and an isotopically radiogenic and geochemically enriched end-component (figs. 3 and 4). It is also possible that the dacite magma was produced by melting of the basaltic basement due to the influx of isotopically and geochemically enriched melts generated above the subducting slab. Of course, there are other possible scenarios to generate the dacite magma, but it is rather premature to constrain the best possible model(s) for its generation at this stage of the project because the available data are limited. For simplicity, the remainder of the discussion focuses on the isotopically and geochemically enriched end-component of the mixing proposed by the isotopic data (figs. 3 and 4) and on the most probable mechanism of mixing this component with the source (or melt) of the depleted magnesian basalt. A more detailed discussion of the mixing process is beyond the scope of this paper.

Mantle materials that are enriched in incompatible trace elements but have ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios that record a time-integrated history of depletion of these elements are the source of many Luzon Island volcanic rocks; these are most probably generated by recent incorporation by the depleted upper mantle of sediment-derived components coming from the subducted slabs (Knittel and others, 1988; Defant and others, 1990; Mukasa and others, 1994). Such sediment-derived components most probably also comprise the enriched endmember that was mixed with the basement magnesian basalt source to produce the parental magma of the dacite of Mount Pinatubo. This hypothesis is strongly supported by the presence of excess ¹⁰Be and ³⁴S in the dacite (Knittel and others, 1992; Bernard and others, this volume). This hypothesis is also consistent with the Pb isotopic compositions of the South China Sea and other western Pacific sediments (Sun, 1980; McDermott and others, 1993; P.R. Castillo and T. Lee, unpub. data, 1993) because the Pb isotopic ratios of the dacite and andesite of Mount Pinatubo indeed line up between the sediments and the basement magnesian basalt (fig. 3). Diagrams of ²⁰⁶Pb/²⁰⁴Pb against ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios are also consistent with such a mixing relationship (fig. 4).

A direct sediment involvement, however, is most probably minimal. Sediments contain a lot more Pb than the ophiolitic basalts (Sun, 1980), so even a small amount of sediment mixed with the basalt source will produce a large shift in the isotopic ratios of the resultant product toward the Pb isotopic composition of the sediments. Yet data show that the Pb isotopic composition of the dacites is only roughly halfway between those of the sediments and the magnesian basalt (fig. 3). The same is true for the ¹⁴³Nd/¹⁴⁴Nd isotopes of these samples because the South China Sea sediments contain much more Nd than the ophiolitic basalts (Chen and others, 1990). In fact, the ¹⁴³Nd/¹⁴⁴Nd compositions of the dacite, andesite, and magnesian basalt argue against sediment involvement. Figure 5 is a diagram of ¹⁴³Nd/¹⁴⁴Nd against 1/Nd in which mixing would be represented by a straight line, as is shown again by the alignment of the magnesian basalt, dacite, and andesite (the olivine-hornblende basalt inclusion is not plotted in the diagram because data are lacking). This Mount Pinatubo mixing line, however, does not pass through the sediment field, so it is suggested that sediments are not directly involved in the mixing process.

Figure 5. ¹⁴³Nd/¹⁴⁴Nd against 1/Nd diagram for the samples analyzed. Note that the sediment field is not collinear with the Pinatubo samples and (or) any of the Philippine volcanic rocks. Sources of data as in figures 2 and 3.

One may argue that the dacite and andesite are both differentiated rocks and, thus, their 1/Nd values must not be used in the ¹⁴³Nd/¹⁴⁴Nd against 1/Nd diagram. These differentiated rocks have to be corrected for crystal fractionation first, and then the higher 1/Nd values of their more mafic parental magmas must be used instead. In other words, the 1/Nd values of the dacite and andesite parental magmas may be high enough for them to plot between the magnesian basalts and sediments in figure 5. However, this scenario implies that the dacite and andesite, which have ~25 to 30 ppm Nd, must have fractionally crystallized from parental magmas that have only ~2 ppm, and this is very unlikely. Fractional crystallization, though, potentially can generate the high-Nd dacite and andesite from low-Nd parental magmas if accompanied by assimilation of the country rocks. The presence of a third, high ¹⁴³Nd/¹⁴⁴Nd end-component with high Nd content may also explain the nonlinear arrangement of the magnesian basalt, dacite, andesite, and sediments. Unfortunately, these alternative explanations neither can be supported nor ruled out by the few available data; the one thing clearly suggested by figure 5 is that direct sediment involvement in the generation of the dacite and andesite is minimal.

On the basis of these results, we propose the following tentative model for the origin of Mount Pinatubo eruptive products. The source of the dacite magma, or, more appropriately, of the parental magma of the dacite, represents a mixture of the variably depleted mantle source (or melt) of volcanic rocks of the Zambales ophiolite complex and metasomatic fluids coming from the dehydrating slab being subducted. This fluid carries with it the geochemical and isotopic signature of the sediments in proportion to the differential mobility of incompatible elements in hydrous fluids (Tatsumi and others, 1986; Spivack and others, 1992; You and others, 1992). As such, a "sediment component" in the elemental and isotopic compositions is more readily seen in S and Be than in Sr, in Sr more than in Pb, and particularly more than in Nd. Subsequent small degrees of partial melting of the mixed source will further enrich the incompatible trace element concentrations in the resultant melt but will no longer affect the isotopic composition. Further enrichment of the incompatible elements can also be accomplished during percolation of melt through the mantle (Navon and Stolper, 1987) and finally by crystal fractionation. As Pallister and others (1992) proposed earlier, the dacitic magma most probably had been crystallizing in a magma chamber for some period of time (>400 years) when an olivine-hornblende basalt magma intruded. Hybrid andesite was formed and the 1991 eruptions began. The mantle source of the olivine-hornblende basalt is similar to that of the dacite, except that the olivine-hornblende basalt magma apparently has more sediment component than the dacite magma.

SUMMARY AND CONCLUSIONS

Dacitic ash and pumice, as well as andesitic scoria extruded during the 1991 Mount Pinatubo eruptive activity, and ophiolitic magnesian basalt from the volcano basement, have been analyzed for major element, REE and Sr, Nd, and Pb isotopic compositions. Not surprisingly, the samples analyzed are similar to the lavas along the Luzon volcanic arc, to which Mount Pinatubo belongs.

Data indicate that the samples analyzed could have been generated by a simple mixing of the variably depleted mantle source of the ophiolitic basement basalt and an enriched, metasomatic fluid from the subducting slab. The metasomatic fluid carries with it some of the elemental and isotopic signatures of the sediments being subducted. The elemental, though not the isotopic, composition of the melt generated from the mixed mantle source most probably can be further enriched by small degrees of partial melting plus subsequent elemental enrichment processes during ascent of the melt through the mantle and the crust. Mixing of melts inside the shallow magma chamber, as evidently shown by the dacite-andesite-olivine-hornblende basalt melt mixing relationship, can further complicate the composition of the volcanic rocks.

ACKNOWLEDGMENT

We thank PHIVOLCS, USGS, and MGB (Mines and Geosciences Bureau) members of the Pinatubo Volcano Observatory Team for helping us collect samples; P. Janney, F. Florendo, C. MacIsaac, and B. Hanan for assistance during the analyses; and J. Hawkins, C.-F. You, U. Knittel, and M. Defant for reviews of the manuscript. This work is supported by grant NSF INT91-96017 and NSF EAR93-13571 to P.R. Castillo.

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