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Journal of Petrology | Volume 45 | Number 3 | Pages 485-506 | 2004
Journal of Petrology 45(3) © Oxford University Press 2004; all rights reserved.
A Hornblende Basalt from Western Mexico: Water-saturated Phase Relations Constrain a Pressure–Temperature Window of Eruptibility
DEPARTMENT OF EARTH AND PLANETARY SCIENCE, UNIVERSITY OF CALIFORNIA, BERKELEY, CA 94720-4767, USA
RECEIVED AUGUST 25, 2001; ACCEPTED JULY 31, 2003
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONEXPERIMENTAL PROCEDURESANALYTICAL TECHNIQUESRESULTSDISCUSSIONREFERENCES |
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Trachybasalt scoria from a cinder cone near the Mexican volcanic front contain phenocrysts of olivine with chromite inclusions, apatite, augite and hornblende, with microphenocrysts of plagioclase. The water-saturated phase relations reproduce the phenocryst assemblage between 1040°C and 970°C with water contents of between 2·5 and 4·5% (50–150 MPa). The absence of biotite phenocrysts in the scoria places a tight constraint on the pressure–temperature conditions of phenocryst equilibration, as there is only a small zone where biotite does not accompany hornblende in the experiments. Diluting the fluid phase with CO2 changes the composition of the olivine, indicating that CO2 was only a minor component of the fluid of the scoria. Hornblende is stable to 1040°C at oxygen fugacities of NNO + 2 (where NNO is the nickel–nickel oxide buffer), but at lower oxygen fugacities, the upper limit is 990°C. There is a progressive increase in crystallinity in experimental runs as both pressure and temperature decrease. Isobaric plots of crystallinity show that the onset of hornblende crystallization involves a reaction relation, and also results in a marked
15–40 vol. % increase in crystallinity. Ascending hydrous magmas intersecting the cooler crust could be trapped there by the large increase in crystallinity accompanying the isobaric crystallization of hornblende. KEY WORDS: crystallization; eruptibility limit; experiments; hornblende trachybasalt; Mexico
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESANALYTICAL TECHNIQUESRESULTSDISCUSSIONREFERENCES |
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Water has a profound influence not only on the temperature and depth of melting of arc-related magmas (e.g. Kawamoto & Holloway, 1997
; Prouteau et al., 2001) but also on melt dynamics, influencing such processes as mixing, assimilation and differentiation (e.g. Gaetani et al., 1993; Sisson & Grove 1993; Thomas & Tait, 1997; Hort, 1998), and ultimately controls the eruptive behaviour of the magma on the Earth's surface (e.g. Sparks et al., 1994; Bower & Woods, 1997). Volcanic arcs associated with subduction, especially beneath a continental plate, are the sites of the most diverse range of magmatism found on Earth, exemplified by variations in composition and eruptive style.
Estimating the water content of subduction-related magmas is therefore a first-order problem in the petrogenesis of volcanic arcs. Estimates have been obtained from the composition of melt either trapped in phenocrysts within lavas or preserved in quenched glasses from submarine eruptions (Anderson, 1974, 1979; Sisson & Layne, 1993), or by using experimental phase equilibria to reproduce the phenocryst assemblage, the phenocryst composition and the phenocryst abundance of a lava (e.g. Rutherford et al., 1985; Blatter & Carmichael, 1998, 2001; Moore & Carmichael, 1998). Such studies have unequivocally demonstrated the importance of water in controlling many aspects of subduction-related magmas. Despite this, there are few examples of high-H2O basalts that are erupted containing primary hydrous phases (the mineralogical evidence for high water content), with extraordinarily few occurrences documented worldwide [Bogoslof, Alaska, Arculus et al. (1977); Kick 'em Jenny, Lesser Antilles, Sigurdsson & Shepherd (1974); Cerro la Pilita, Western Mexico, Luhr & Carmichael (1985); Mount Lamington, Papua New Guinea, Arculus et al. (1983)]. This is in spite of the almost ubiquitous presence of amphibole in high-H2O subduction-related magmas as a whole. This would suggest that either (1) high H2O content magmas are largely confined to more evolved magmas or (2) there is a correlation between high H2O content and magma eruptibility in mafic magmas.
The detailed experimental investigation of the role of P–T–H2O on mineral stabilities and crystallinities in hydrous basaltic magmas currently provides the only way to investigate this relationship in detail. The unique information provided on phenocryst assemblage, composition and abundance provides information of relevance not only to the P–T path of the magma but also to its likely rheological evolution, a factor that can directly control the probability of eruption.
Unfortunately, complete experimental studies of hydrous mafic magmas are comparatively rare.
In this paper we explore the conditions (P–T) of water-saturated phase equilibria for one of the rare hornblende-bearing basaltic magmas (the Cerro la Pilita trachybasalt) and also the effect of CO2–H2O on the phase assemblages. By plotting the modal abundance of crystals (referred to subsequently as the crystallinity) on the phase diagram we investigate more fully the role that water played in both magma ascent and eruption. In doing so we also attempt to address broader questions relating to magma ascent and the changing crystallinity and rheology that will ultimately control the eruption of hydrous mafic magmas in subduction zones.
The Cerro la Pilita scoria cone and lava flow
Throughout the last 3 Myr, and particularly in the Quaternary, eruptions of basalt (SiO2 <52%) have been infrequent in the western Mexican volcanic belt, and of the varieties that do occur (Luhr & Carmichael 1981, sample 22E), those that contain hornblende, and thus display mineralogical evidence of the host magma being hydrous, are very rare. The trachybasalt of a small Quaternary cone (Cerro la Pilita) in the southern part of the Michoacán–Guanajuato Volcanic Field, located in the western portion of the Mexican Volcanic Belt, is the one example found to date. It consists of a scoria cone and lava flow pre-dating the historical (1759–1774) basaltic andesites of Volcan Jorullo, Michoacán (Luhr & Carmichael 1985). Scoria of ne-normative trachybasalt (Jor 46) are found high on the cinder cone and are conspicuous for their phenocryst assemblage of hornblende without reaction rims, apatite, olivine and augite, with scattered plagioclase microphenocrysts. On the lower slopes of the Cerro la Pilita cone are small blocks of lava (MAS 2003a) that contain phenocrysts of biotite in addition to the phenocryst phases listed above (but with less augite), and hornblende and plagioclase as unambiguous phenocrysts. The lava emitted from the cone is more siliceous than the scoria, and hornblende, without biotite, occurs therein as residual cores to extensive oxide rims.
Trachybasaltic magmas (LeMaitre et al., 1989) are commonly associated with ocean island basalts and intraplate magmatism, but where they do occur in volcanic arcs, hornblende-bearing trachybasalts are comparatively common, accounting for most of the occurrences of hornblende-bearing basalts as a whole. They also tend to be associated with more K-rich shoshonitic magmas, as in the Atenguillo graben of western Mexico (Wallace & Carmichael, 1992), Papua New Guinea (Ruxton, 1966; Arculus et al., 1983), and the Hellenic Arc (Wyers & Barton, 1986; Barton & Wyers, 1991; Pe-Piper & Piper, 1992). The trachybasalt lava from Jorullo is distinctive because of its local association with basaltic andesite and its very high P2O5 content (Table 1), a feature it shares with minettes (lavas with phenocrysts of biotite, apatite and olivine) from western Mexico; however, it is less rich in MgO and Ni than the absarokite lavas associated with minettes (Wallace & Carmichael, 1992; Carmichael et al., 1996).
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This paper represents the first attempt to explore the conditions of water-saturated phase equilibria for an erupted basaltic magma that clearly contains hornblende as part of its stable mineral assemblage. Although such a composition may not readily be considered to be representative of an ‘average’ subduction-related mafic magma, it is necessary to elucidate those conditions under which eruption of hornblende-bearing basalt is possible in order to understand why such magmas are largely absent at the Earth's surface.
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EXPERIMENTAL PROCEDURES |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESANALYTICAL TECHNIQUESRESULTSDISCUSSIONREFERENCES |
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Synthesis experiments were performed using finely powdered samples of Cerro la Pilita basalt [sample Jor 46 of Luhr & Carmichael (1985)
]. Experiments were conducted between 40 and 300 MPa using an internally heated pressure vessel (IHPV) with a vertical rapid quench design (Holloway et al., 1992) with modifications as described by Moore & Carmichael (1998). Experiments were designed to explore all the possible variables that could control phase equilibria and were thus conducted in sealed Ag70Pd30 capsules both under water-saturated conditions and with the fugacity of water reduced by half in the vapour phase. For concordance with the natural assemblage, experiments were largely conducted at nickel–nickel oxide (NNO) + 2 but a few runs were also performed at oxygen fugacities (fO2) equivalent to NNO + 0·5 to investigate the effect of fO2 on the composition of the ferromagnesian phases, particularly the stability of hornblende. Individual run conditions and results from these three sets of experiments are summarized in Tables 1–3. Additionally, four atmospheric pressure experiments were performed in a Deltech VT-31-DT gas-mixing furnace by Mo and Carmichael (Table 4, previously unpublished results) at fO2 equivalent to the NNO buffer.
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The sample assembly for a run in the IHPV consisted of a melt capsule and an oxygen sensor capsule connected together and hung from Pt wire. Up to 50 mg of powder was sealed into a capsule of Ag70Pd30 with sufficient distilled water to achieve H2O saturation (water-saturated experiments) or with sufficient crystalline oxalic acid dihydrate to achieve 0·5H2O and 0·5CO2 in the vapour phase during the experiments (H2O–CO2 saturated experiments). The Ag70Pd30 capsules were welded at both ends and samples were carefully weighed before and after each experiment and then weighed again following puncturing and drying (
100°C) for 5 min to confirm the presence of a fluid phase at the end of the run. This alloy has been consistently shown to minimize iron loss in H2O-rich experiments (Sisson & Grove, 1993; Grove et al., 1997; Blatter & Carmichael, 1998; Moore & Carmichael, 1998) and our analysis of capsule material after the runs confirms that iron loss to the capsule was below the limit of detection (500 ppm). Pressure was measured with a calibrated Heise gauge accurate to ±5 bars and temperature was monitored using three (one opposite the melt, one at the bottom of the melt capsule, one opposite the base of the sensor capsule) ‘K’ type thermocouples calibrated to ±5°C of the melting point of gold. The temperature gradient was generally less than 10°C across the melt capsule; if the gradient was greater, then this is noted in Table 1. The fO2 was buffered at approximately NNO + 2 by using 0·1 vol. % H2 in the argon pressurizing medium, and monitored using a H2O-saturated, compressed 50–50 mol % Ni–Pd pellet, isolated from the Ag70Pd30 capsule within an alumina sleeve. The fO2 values were calculated based on the mole fraction of Ni in the sensor capsule corrected for any differences in melt and sensor temperature (usually Tmelt ± 30°C) using the calibration of Pownceby & O'Neill (1994). The estimated error on reported fO2 values as the result of uncertainty in temperature, inhomogeneity of Ni–Pd powder, and analytical error is ±0·2 log10 units fO2. |
ANALYTICAL TECHNIQUES |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESANALYTICAL TECHNIQUESRESULTSDISCUSSIONREFERENCES |
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Experimental glasses and minerals were analysed on the Cameca SX-50 microprobe at UC Berkeley. All mineral phases were analysed using an accelerating voltage of 15 kV, and a beam current of 20 nA. A focused beam was used for olivine, augite, oxides and apatite, and a slightly defocused beam (5 µm) was used for biotite, hornblende and plagioclase, to minimize alkali loss. Standards used were oxides of Mg, Si, Al, Ti, Mn, V, Cr, synthetic fayalite (Fe), diopside (Ca), nepheline (Na), orthoclase (K), strontium titanate (Sr), fluor-phlogopite (F) and chlor-apatite (Cl and P). MAN corrections (Donovan & Tingle, 1996
) and 10 s counting times were used on major elements (Si, Al, Mg, Fe, Ca, Na, K). Longer count times and off-peak corrections were applied to less abundant elements (Ti, Mn, Sr, P, Cl, F) as appropriate. A natural kaersutite sample, analysed by wet chemistry (Brown & Carmichael, 1969), was used as an internal secondary standard throughout each analytical session. Tabulated analyses are the result of multiple analyses of rims and cores of several different crystals unless otherwise stated.
The Ni–Pd alloy compositions of the oxygen sensors were analysed using a focused beam and a sample current of 40 nA. The reported alloy compositions are the average of 5–10 individual grain analyses, with 1
<3 mol % Ni.
Hydrous glasses were analysed using a 5 nA, 20 µm beam to minimize sodium loss. A less defocused beam and a slightly stronger beam current using an on-line Volatile Element Correction procedure (where Na, K and Si counts were monitored every 2 s) was also used, and it was shown that although the precision of these measurements was greater, the former method gave preferred results. As the result of volatile element loss, we estimate the error in Na is 10%. Using such a broad beam presented some difficulties in analysing melt pools in crystal-rich experiments. However, with the large sample sizes, this was possible in all but a few of the 50 MPa experiments. In some cases, only single analyses are presented as the number of melt pools >20 µm was limited. The analytical totals of the hydrous glasses are systematically low (Table 5).
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Modal calculations
Mineral modes were calculated using a least-squares model (XTALFRAC, Stormer & Nicholls, 1981
), assuming that the initial bulk composition of Jor 46 remained constant, which therefore represented the sum of the crystalline phases and the quenched glass. Although more practicable and accurate than point counting, this method is affected by the uncertainty in the analyses of the experimental glass and phenocryst phases. This is evident in the more crystalline experimental samples where the largest array of phenocryst phases are present, most of which have some degree of compositional zoning. To illustrate this, we compared least-squares calculations where we used: (1) the average crystal compositions; (2) average composition ±1 for each of the experimental runs. This produced a maximum variation of 10% in the total crystallinity (much less for samples with <20 wt % crystals), although the relative uncertainty for individual phenocryst modes varied between 0 and 25%. We regard our estimates of crystallinity to have a maximum uncertainty of ±10% but we found that the relative error associated with the abundance of individual phases can be greater, especially for the less abundant phenocryst phases. The calculated modes are given in Table 5.
Mineral nomenclature
The synthesized and natural calcic amphibole has nearly fully occupied A-sites and can be classified as pargasite (Leake et al., 1997). However, in keeping with earlier studies of amphibole stability in basaltic magmas (Helz, 1973; Sisson & Grove, 1993), and lacking Fe3+ determinations, this amphibole is more simply described as hornblende. Following Luhr & Carmichael (1985), we refer to the high Ca-pyroxene synthesized in the experiments as augite. Although a range of iron–titanium oxides (chrome spinel to titanomagnetite) has been synthesized, depending on the P, T and fO2, we refer to the accessory oxide phase as spinel. Using the mica classification of Deer et al. (1997), whereby molar Mg:Fetotal <2:1, the synthesized mica is referred to as biotite.
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RESULTS |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESANALYTICAL TECHNIQUESRESULTSDISCUSSIONREFERENCES |
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Attainment of equilibrium
The majority of experiments in this study are direct synthesis runs in which crystals have grown from melt. Where sufficient melt pools are available to make multiple analyses, the experimental glasses are homogeneous in composition (Table 5), with synthesized crystals generally being euhedral and equant or tabular, and in the less crystalline experiments they tend to reach sizes of >100 µm (see Fig. 5, below). In general, we note that runs with <15 wt % crystals tend to have very homogeneous crystal populations, and our calculated mineral–liquid partition coefficients (Table 1) for olivine and glass are similar to those reported by Sisson & Grove (1993)
; however, some zonation is inevitable in more densely crystalline experiments, and an increase in both inter- and intra-grain compositional variation is observed, as also noted by Sisson & Grove (1993) and Moore & Carmichael (1998).
To test equilibrium in these cases, we ran three experiments where the T or P was initially held at higher values, followed by 48 h at the recorded run conditions (Table 1; Jor46.32, Jor46.29 and Jor46.28). We noted no difference between either the phenocryst assemblage or their compositions within these experiments (Tables 6 –11) and the direct synthesis experiments, although some olivine cores within the 50 MPa experiments are clearly relict from the initial higher T conditions. However, to test this further, we also conducted one experiment at 48 MPa and 1000°C (Jor46.46) using a homogeneous glass from the 105 Pa experiments. Here, although hornblende was stable, no plagioclase was found to be present, conflicting slightly with the findings for Jor46.28. Both these experiments are very close to the stability limit of both plagioclase and hornblende, reflecting the difficulty of plagioclase nucleation when using glassy starting materials (Weber & Pichavant, 1986). Phase compositions are otherwise similar and this discrepancy was observed only in these crystal-rich experiments. A few relict grains were detected in the experiments, other than the presence of occasional chromite cores (<<1 vol. %) within some spinels, which may have originated from inclusions within the natural olivines. Although equilibrium cannot be unequivocally demonstrated in all experimental samples, we nevertheless believe that a close approach to equilibrium has been attained, sufficient to interpret several aspects of the evolution of the Cerro la Pilita magma.
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Water-saturated phase equilibria
The phase stability fields for the trachybasalt have been determined for PH2O up to 300 MPa and are plotted in Fig. 1 for experiments equilibrated at NNO +
2 (Table 1). Olivine is the liquidus phase at 105 Pa (Table 4), but our apparatus is unable to reach liquidus temperatures below 300 MPa, and thus the liquidus is poorly known. Olivine is followed in succession by spinel, apatite and augite at 200 MPa, and varies slightly in composition (Fo89–86) above the hornblende stability field, but becomes more Fa rich at lower temperatures. The temperatures at which spinel, augite and apatite crystallize are separated by only a few tens of degrees. Hornblende without biotite occurs only over a very small temperature interval (Fig. 1), and is joined by biotite at higher pressures. Plagioclase (An43–34) (Table 10) is stable at PH2O <75 MPa for T > 970°C in the absence of biotite (Fig. 1).
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, water-saturated liquid composition. Boundary curves are continuous lines, and are labelled for the phases that crystallize on the low-temperature side. Dashed lines are water isopleths (wt %) calculated from Moore et al. (1998)Exploratory experiments at fO2 values equivalent to NNO + 0·5 (Table 3) were performed to establish the effect of reducing fO2 on the Cerro la Pilita magma (Fig. 2). It can be seen in Fig. 2 that the upper limit of hornblende stability is
990°C, substantially less than its stability limit (1040°C) in experiments at NNO + 2; olivine compositions are richer in the Fa component at the lower fO2. Curiously, hornblende without biotite is stable to 200 MPa at NNO + 0·5 whereas it is stable only to 150 MPa at NNO + 2 (Fig. 1). Amphibole compositions are more Fe rich (Table 8), but otherwise similar to those found in the more oxidizing experimental runs.
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Water-undersaturated phase equilibria
Although solid–liquid experiments with a pure water fluid phase are simple to execute, it seems likely that magmas may have other volatile components dissolved in the liquid, and both CO2 and S are plausible additions. We found no petrographic evidence for a mineral containing S in the trachybasalt scoria; accordingly, we diluted the water component of the vapour phase by using oxalic acid, which gives a fluid composition of equal amounts of H2O and CO2 (Holloway & Wood, 1988
). The results of three experiments are given in Table 2 and plotted in Fig. 3. Reducing the activity of water in the fluid phase results in the stabilization of plagioclase to higher pressures (>150 MPa) than in the water-saturated experimental runs, but the composition of the plagioclase remained about the same (Fig. 3), and not very different from the phenocryst composition (An40; Table 10).
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Comparison with the natural assemblage
A comparison of the experimentally determined phase equilibria with the petrological and textural features of the scoria can be used to constrain the conditions of storage of the magma. Hornblende forms euhedral phenocrysts and microphenocrysts in the scoria sample with oxidation rims absent or minute, whereas in the lava flow, hornblende is replaced by opacite almost completely. Experimental hornblendes exhibit some compositional variation as a function of pressure and temperature (Table 7) and those generated at PH2O of 100–130 MPa and temperatures between 1000 and 1020°C correspond most closely to those found in the phenocryst assemblage. One of the small differences between the natural and experimental results is K2O, which in the hornblende of the scoria is 0·26%, but in both the experiments and the ejected blocks (MAS 2003a), it is higher (
1·1%) (Table 7). Hornblende compositions are more Fe rich in the lower fO2 experiments, suggesting that the natural assemblage crystallized under the more oxidizing conditions similar to NNO + 2.
The average composition of olivine at temperatures above the onset of hornblende crystallization is Fo87 (Fig. 1), which corresponds within error (±1% Fo) to the composition of the most magnesian olivine cores in the Cerro la Pilita scoria (Fo88–85; Luhr & Carmichael, 1985). CO2 may have been only a minor component of the vapour phase, as the phenocrystic olivines compare well with those of water-saturated liquids (Fig. 1), rather than with those where CO2 is equal to H2O in the vapour phase (Table 2). Cr2O3-rich spinel (40%) occurs as inclusions in the cores of the olivine phenocrysts (Table 8) but such Cr-rich compositions have not been reproduced within the P–T field of the experiments, although spinel with less Cr2O3 (26–16%) is found at the highest P and T (Table 8). Otherwise all the experimental spinel approaches the titanomagnetite of the groundmass in composition.
Little systematic compositional variation is found among the experimental augites, which have the general composition Wo45·5En44·5Fs10 (Table 9). The augite phenocrysts of the scoria are zoned from Wo47En45·5Fs7·5 cores to Wo46·5En43·1Fs10·4 rims (Table 9, Luhr & Carmichael, 1985), whereas those from the biotite-bearing ejected blocks are more Mg rich (Wo45·4En46·8Fs7·8). The experimental augites most closely resemble the phenocryst rim compositions, suggesting that the magma may have initially crystallized at higher pressures than those explored experimentally.
Although biotite is absent in the scoria, it is present in the ejected blocks (MAS 2003a), and its composition is generally similar to those of the experimental biotites (Table 11), except that Na2O is higher, and K2O lower in the experimental phases; the Mg number of biotite is higher than in the coexisting experimental olivine, a characteristic feature of lavas with olivine and biotite phenocrysts (Carmichael et al., 1996). In our experiments, plagioclase feldspar that is compositionally similar to the high-Sr plagioclase microphenocrysts (An40, Table 10) is not stable in the presence of hornblende until PH2O is <60 MPa (3 wt % H2O) and the temperature is <1000°C (Fig. 1). We have interpreted the small size of the plagioclase microphenocrysts in the scoria, in contrast to the much larger and equant plagioclase phenocrysts in the ejected blocks (MAS 2003a), as resulting from decompressional crystallization on ascent, rather than as equilibration in the plagioclase stability field.
The absence of biotite in the natural phenocryst assemblage of Jor 46 constrains the phenocryst equilibration of the magma to PH2O <150 MPa, and to water concentrations of <4·5 wt % (Fig. 1). Both hornblende and biotite reach a temperature maximum of 1040°C at pressures of 200 MPa, but our experiments cannot separate their stability limits above 170 MPa. Consequently, there is a limited range of P–T conditions (<1000°C, <70 MPa) at which plagioclase and hornblende, without biotite, are capable of coexisting in this water-saturated trachybasalt liquid at NNO + 2. The presence of both biotite and plagioclase in a limited number of the eruptive products from Cerro la Pilita suggests that some of the magma passed through P–T conditions on or close to these phase boundaries during ascent. It is also worth noting that hornblende crystallization would be suppressed to significantly lower temperatures at lower fO2 values, reducing the P–T interval where hornblende could crystallize in the absence of plagioclase.
Crystallinity and liquid compositions
For each of the experimental runs that has an analysed glass composition (Table 5), the mode of that run has been calculated as described above, and the total amount of crystals (the crystallinity) ± 10% is given in Table 5. In Fig. 4, contours of crystallinity have been drawn using the values of Table 4, but the contours are not well constrained. However, their general form is correct, and show that with falling temperature and pressure, there is an increase in crystallinity; this same feature is found in the crystallinity curves of an andesite composition (Blatter & Carmichael, 2001).
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The experimental glass compositions (Table 5) are ne-normative at higher temperatures, becoming successively hy-normative with some of the most evolved liquids becoming q-normative at lower temperatures. This is a consequence of the precipitation of hornblende, which is very silica poor (Table 8) in contrast to the bulk composition of the lava (Table 5). These siliceous liquids are equivalent to high-K dacites according to the classifications of LeMaitre et al. (1989)
and Gill (1981), respectively. Because the highest pressures of our water-saturated experiments were only 200 MPa, the compositions of these liquids will not be representative of liquids generated by partial melting of a crystalline trachybasalt (saturated or undersaturated with water) at the base of the crust (>1 GPa), nor are the melt fractions (>40%) small enough to represent the compositional trend of small to moderate degrees of melting of underplated trachybasalt. Nevertheless, it is evident that q-normative liquids can be generated by partial fusion of an ne-normative basalt. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESANALYTICAL TECHNIQUESRESULTSDISCUSSIONREFERENCES |
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The Cerro la Pilita trachybasalt: intensive variables and the ascent path from the mantle
Comparison of the natural and experimental assemblage has provided several very important constraints on the depth of crystallization of the Cerro la Pilita magma. The absence of biotite in the Cerro la Pilita basaltic scoria provides a constraint on the depth of crystallization of the scoria phenocryst assemblage (Fig. 1). The phenocryst assemblage of hornblende, augite, apatite and olivine equilibrated between 1040°C and 970°C, with water contents between 4·5 and 2·5 wt % (Fig. 1). A combination of zoning in the augite phenocrysts, and Cr-rich spinel inclusions in the olivine, indicates that the trachybasalt magma was initially hotter than the phenocryst equilibration region of Fig. 1. The presence of biotite and plagioclase in the ejected blocks suggests that some portions of the magma equilibrated at greater depths and/or experienced partial degassing at some point prior to eruption.
In Fig. 5, the temperature of the trachybasalt magma has been extrapolated from the phenocryst equilibration stage to the base of the crust (1·0 GPa), using a calculated adiabat (TV
/Cp) that varies from 2·7°/108 Pa for an anhydrous magma to 4·7°/108 Pa for one containing 6% water. It can be seen (Fig. 5) that olivine may have precipitated in the ascending magma at about 600 MPa (18 km) followed by augite at lower pressures before the magma entered the hornblende stability field. An ascending magma appears far more likely to enter the biotite + hornblende field than it is to intersect the hornblende-only field, although only ejected blocks with biotite + hornblende have been found, whereas both scoria and a 2 km long lava flow lack biotite. A hydrous magma missing the stability fields of biotite + hornblende or hornblende would have phenocrysts of olivine, Cr-spinel and augite, and could suppress plagioclase, which is the characteristic phenocryst assemblage of the absarokites in western Mexico (Carmichael et al., 1996).
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The role of hornblende crystallization in prohibiting magma eruption
The analysis of sample crystallinities also affords the opportunity to assess physical changes in the magma state during ascent and eruption. From Marsh's (1981)
Aleutian modal data, magmas with more than 50–55% crystals are not found as lavas, presumably because their viscosity is too high to allow upward flow in the conduit. This limit reflects a change in the rheological regime, whereby magma behaves more like a brittle solid rather than a ductile mass, which occurs as a result of high crystal concentration within the magma. The experiments of LeJeune & Richet (1995) on a simple silicate liquid identified an abrupt change of rheology at crystal concentration of 40 vol. %, where clustered crystals start to strongly oppose shear deformation. In most silicate magmas the limit of crystal concentration beyond which magmas have an ‘infinite’ viscosity is assumed to be 60 vol. %, the value for a monodisperse suspension (Pinkerton & Stevenson, 1992). Thus for crystal concentrations of between 40 and 60 vol. %, eruption of a magma body becomes highly unlikely; the imposition of a pressure gradient across the body is more likely to result in the migration and percolation of any remaining melt via brittle cracks rather than ductile migration of the entire body (Bagdassarov & Dorfman, 1998).
It would therefore seem that an eruptible limit of 50% crystallinity would preclude trachybasalt magmas that had entered the plagioclase field (see Fig. 4) from erupting, and that unless the presence of CO2 shifts the plagioclase field to higher pressures (Fig. 3), and hence to lower crystallinities, plagioclase-bearing lavas with phenocrysts of hornblende, augite, apatite and olivine will not occur. However, there is one anomalous value of crystallinity in Fig. 4 (23 wt %) that is inconsistent with the pattern of crystallinities, and it is close to the value of the natural assemblage (20 vol. %); perhaps there is indeed an anomalous region of low crystallinity, but the evidence to support this is weak.
Holloway & Burnham (1972), in a study on the melting relations of a Kilauea tholeiite, first recognized the large increase in the amount of liquid as the hornblende breakdown temperature was exceeded. Looked at from the perspective of magma cooling, there will be a large increase in crystallinity at the onset of hornblende precipitation, and Holloway & Burnham illustrated this with two isobaric sections. In Fig. 6, we have plotted the crystallinity of the trachybasalt at 200 MPa (Fig. 1; Table 5) which clearly shows this marked increase in crystallinity, and also the decrease in olivine and augite accompanying the precipitation of hornblende; this is a reaction relationship that has also been recognized by Helz (1973), Grove et al. (1997) and Moore & Carmichael (1998).
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In Fig. 7, the increase of crystallinity accompanying the isobaric crystallization of hornblende in hydrous basaltic, basaltic andesite and andesitic liquids is plotted for a group of experiments taken from the literature. As a larger proportion of the components of basalt can be taken up in hornblende (regardless of the exact composition of the basalt and hornblende), in comparison with andesite, the increase of crystallinity is larger in basalts than in andesites (Fig. 7). It is postulated that the paucity of subduction-related trachybasalts, or basaltic magmas, at the Earth's surface may not be a consequence of their absence within the suprasubduction-zone lithosphere, but rather a result of the onset of high crystallinities with the precipitation of hornblende near the base of the crust.
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, alkali basalt, Holloway & Burnham (1972)On leaving the upper mantle, an ascending hydrous magma will intersect a cooler crust, but rather than crystallize by decompression as it does on nearing the surface, it will be trapped and crystallize isobarically. Under these conditions the crystallinity curves depicted in Fig. 7 will influence the magma's capacity to flow, and once hornblende crystallizes, the magma's ascent is retarded by its high crystallinity. Successive hydrous basaltic magmas will become stock-piled at the base of the crust. This effect is more evident in basaltic magmas than in andesitic magmas because a greater proportion of the basaltic bulk composition can crystallize as hornblende than it can for andesite. Thus in more evolved melts the crystallization of hornblende increases the viscosity less, and the significant barrier to magma ascent and eruption is reduced.
Basaltic stock-piling
If hornblende crystallization were to effectively freeze and stall wet basaltic melts within or at the base of the crust then geological evidence for this process should be found within some subduction-related magmas. This should include: (1) hornblende-rich gabbros in exhumed subduction zones; (2) the presence of hornblende-rich mafic xenoliths or inclusions within more evolved volcanic suites or plutons; (3) petrological and geochemical evidence for the role of residual hornblende after subsequent remobilization, reheating and differentiation of these accumulated melts; (4) a paucity of hornblende-bearing basalts erupted as scoria and lava. The relative scarcity of erupted hornblende-bearing basalts has been described earlier, and Arculus & Wills (1980) contrasted this with the relative abundance of hornblende-bearing mafic xenoliths in island-arc settings. In the case of Cerro la Pilita we suggest that eruption occurred as a consequence of comparatively small degrees of undercooling within the hornblende stability field (as evidenced by the continued presence of augite).
Hornblende-bearing gabbros are a relatively common feature of exhumed subduction zones and in examples of the mafic lower crust [e.g. Onion Sill Gabbro, California, Sisson et al. (1996); Kamchatka, Russia, Kepezhinskas et al. (1993); the Coastal Batholith, Peru, Mason (1985); Adamello Massif, Italy, Ulmer et al. (1983), Blundy & Sparks (1992); lower-crustal examples from Rudnick & Fountain (1995)]. Modal proportions of hornblende can be as much as 50 vol. % of the rock and hornblende commonly forms as a reaction product around earlier formed clinopyroxene phenocrysts in an analogue to the reaction observed in our equilibrium experiments.
Additionally, the accumulation and subsequent remelting of hornblende-rich basaltic magmas within the lower crust has also been invoked to explain geochemical variations in some calc-alkaline volcanic suites and plutons [e.g. Lassen, California, Borg & Clynne (1998); North Cascades, Tepper et al. (1993); Aurora Volcanic Field, Lange & Carmichael (1996); central Aleutians, Alaska, Romick et al. (1992)].
Thus basaltic stock-piling does indeed occur in the lower crust and this mechanism in turn supports the concept that rapid hornblende crystallization at any point during the ascent of a hydrous basaltic melt would reduce the magma's eruptibility. Subsequent remelting and remobilization can generate a wide variety of melt compositions and these processes should thus be taken into account in models of subduction-zone magmatism.
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ACKNOWLEDGEMENTS |
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We would like to thank James Brophy and Richard Arculus for careful and useful reviews. The first author would like to thank Dawnika Blatter for assistance and advice in the laboratory and beyond, and John Donovan is thanked for his advice on the use of the electron microprobe. In another context Jim Brophy offered some elegant ideas on representing crystallinity, and Ethan Baxter provided some essential probe analyses at the last minute. To all the second author is indebted. The support of NSF EAR 97-06129 was essential to the research.
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FOOTNOTES |
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* Corresponding author. Present address: School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK. Fax: +44 (0)1603 507719. E-mail: j.barclay{at}uea.ac.uk
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