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Journal of Petrology | Volume 44 | Number 12 | Pages 2313-2339 | 2003
© Oxford University Press 2003; all rights reserved
A Petrologic Perspective of K
lauea Volcano's Summit Magma Reservoir
1 DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF HAWAI'I, HONOLULU, HI 96822, USA
2 DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF MASSACHUSETTS, AMHERST, MA 01003, USA
* Corresponding author. Telephone: 808-956-6641.E-mail: mogarcia{at}hawaii.edu
RECEIVED OCTOBER 23, 2002; ACCEPTED JUNE 19, 2003
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONTwo hundred years of magmatic history are documented by the lavas and tephra sampled from K
lauea's historical summit eruptions. This paper presents detailed petrographic and geochemical data for a comprehensive suite of samples erupted within or near Klauea's summit caldera since the 17th century. Our results elucidate the range of magmatic processes that operate within the volcano's summit magma reservoir and document two compositional trends that span nearly the entire known range for the volcano. Prior to the 1924 summit crater collapse, a trend of increasing incompatible element and CaO and decreasing SiO2 abundances (at a constant MgO) prevailed. Thereafter, the trend reversed direction and has persisted for the rest of the 20th century, including during the current Pu`u `
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eruption. The rapid and systematic nature of these temporal geochemical variations indicates that the summit reservoir is a single, relatively small body rather than a plexus of dikes and sills. Olivine fractionation is the dominant petrologic process within this reservoir. Petrographic observations and olivine and whole-rock geochemical data suggest that the summit reservoir has a crown of aphyric, more evolved, low-density magma. Differentiation within this crown involving clinopyroxene and plagioclase is more extensive than previously recognized in Klauea summit lavas. The effects of crystal fractionation are superimposed upon an evolving hybrid magma composition produced by mixing new, mantle-derived magmas with more fractionated reservoir magma. Frequent eruptions of these hybrid reservoir magmas document the rapid variation in parental magma composition. These compositional variations correlate with magma supply rate; both are thought to be influenced by the degree of melting of small-scale source heterogeneities within the Hawaiian plume. However, Klauea's source compositions and partial-melting processes have varied only within a narrow range over the past 350 kyr.
KEY WORDS: basalt; geochemistry; magma reservoir; Klauea Volcano; Hawai'i
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INTRODUCTION |
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The magmatic processes within active volcanoes are best delineated by monitoring the chemical and physical evolution of their lavas in space and time. The active Hawaiian volcanoes, Mauna Loa and K
lauea, are ideal natural laboratories for detailed case studies of basaltic volcanism because of their relatively easy access, frequent eruptions, continuous monitoring by the US Geological Survey's (USGS) Hawaiian Volcano Observatory, and prior study (e.g. Brigham, 1909
; Macdonald et al., 1983; Klein et al., 1987; Tilling & Dvorak, 1993; Barnard, 1995). From a petrological perspective, these volcanoes are particularly instructive because their lavas display rapid geochemical variations that are thought to result from short-term changes in the source composition and melting conditions within the Hawaiian plume (e.g. Wright, 1971; Wright & Fiske, 1971; Hofmann et al., 1984; Casadevall & Dzurisin, 1987; Tilling et al., 1987; Rhodes et al., 1989; Kurz et al., 1995; Rhodes & Hart, 1995; Pietruszka & Garcia, 1999a; Garcia et al., 2000; DePaolo et al., 2001; Pietruszka et al., 2001). These mantle-derived geochemical signatures are overprinted by crystal fractionation, magma mixing, and assimilation (e.g. Wright, 1971; Wright & Fiske, 1971; Wilkinson & Hensel, 1988; Rhodes, 1995; Garcia et al., 1998). To unravel these complexities and examine such issues as magma residence time (Pietruszka & Garcia, 1999b; Cooper et al., 2001), it is necessary to systematically examine a volcano's lava composition and mineralogy.
The 
200-year record of historical Klauea eruptions (Fig. 1) presents an excellent opportunity for a detailed petrologic investigation because the volcano's basic magmatic processes are well known (e.g. Tilling & Dvorak, 1993). Petrologic and geochemical studies have shown that the dominant compositional variation of Klauea lavas, particularly for those erupted at the summit of the volcano, is a consequence of olivine accumulation and fractionation (Powers, 1955; Wright, 1971; Wright & Fiske, 1971). In contrast, rift-zone magmas may undergo more extensive fractionation involving pyroxene and plagioclase during storage and cooling prior to eruption (Wright & Fiske, 1971), although the most primitive and many of its picritic lavas were erupted along Klauea's east rift zone (e.g. Garcia et al., 1989; Clague et al., 1991, 1995). More subtle compositional variations, a mild increase in K2O, TiO2 and CaO contents (at a constant MgO value), were noted for prehistoric and older historical lavas compared with younger historical lavas (Wright, 1971). These variations cannot be attributed to olivine control, and thus are thought to result from the influx of compositionally distinct magma batches into the summit magma reservoir (Wright, 1971; Wright & Fiske, 1971). Variations of incompatible trace element and Pb, Sr, and Nd isotopic ratios were also detected in Klauea's historical summit lavas, which were attributed to variations in the degree of partial melting of small-scale compositional heterogeneities within the Hawaiian mantle plume (Pietruszka & Garcia, 1999a; Pietruszka et al., 2001).
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In this study, we report petrologic data [X-ray fluorescence (XRF) whole-rock major and trace element chemistry, electron microprobe analyses of olivine and glass, and modal abundances of minerals] from a comprehensive suite of historical K
lauea summit lavas and tephra. These data are used to investigate the temporal and spatial variations in the magmatic processes within the summit magma reservoir (e.g. magma mixing, crystal fractionation and crustal assimilation) from the 17th century to the most recent summit eruption in September 1982 for this classic shield volcano. Our goals are to refine our previous model for the structure and size of Klauea's summit magma reservoir (Pietruszka & Garcia, 1999b), to determine the nature of the temporal and spatial variations in magmatic processes at the volcano's summit, and to link the variations in trace element and isotope geochemistry of historical Klauea lavas (Pietruszka & Garcia, 1999a; Pietruszka et al., 2001) with the petrologic evolution of the volcano. |
SUMMARY OF HISTORICAL SUMMIT ERUPTIONS |
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K
lauea's historical eruptive activity includes prolonged periods of quiescent effusion, interrupted by major collapses of the caldera floor (
100 m), and rare violent explosions (Table 1). The most dramatic and deadly historical eruptive event occurred in 1790 when violent explosions killed 80 Hawaiians. This eruption was thought to have involved four major explosive phases from vents within the caldera (McPhie et al., 1990). Recent fieldwork and new 14C dating indicate that some of these presumed 1790 phases are actually prehistoric (dating back to AD 1500; D. Swanson, personal communication, 2001). The next known event, between 1820 and 1823, involved high lava fountaining within the caldera, producing up to 2·8 m thick reticulite deposits (Sharp et al., 1987). The first written record of Klauea eruptive activity, for August 1823, describes vigorous eruptive activity from many vents in an 240 m deep caldera (Brigham, 1909). Summit lava-lake activity continued until January 1832 (Fig. 1), when an eruption between the caldera and Klauea Iki crater (Fig. 2) was followed by draining of the summit lava lake and a >100 m collapse of the caldera floor (Brigham, 1909). Lava-lake activity resumed and by 1840, the caldera was half full when it collapsed 100 m accompanying a major east rift zone eruption. Subsequent summit eruptive activity was centered at Halema‘uma‘u Crater, where a languid lava lake persisted until 1868 (Fig. 1), when Hawaii’s largest historical earthquake, estimated at M 7·9 (Wyss & Koyanagi, 1992), shook the Klauea region. Almost simultaneously, two-thirds of the caldera floor collapsed some 90–180 m and a lava flow erupted into Klauea Iki crater from the nearby caldera rim. Vigorous activity resumed in Halema‘uma‘u and continued until 1894 (Fig. 1), when the lake drained. The next 13 years saw only episodic eruptions (Brigham, 1909). Nearly continuous lava-lake activity resumed at Halema‘uma‘u in 1907 with major overflows and fissure eruptions from 1918 to 1921, which covered large portions of the caldera floor (Fig. 2).
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In 1924, the lava lake in Halema‘uma‘u drained, its floor and walls collapsed, and numerous violent phreatic explosions occurred (Jaggar, 1947
). Subsequent summit eruptions were sporadic, with seven within Halema‘uma‘u lasting a few days to 1 month between 1924 and 1934 (Finch, 1940; Jaggar, 1947). Halema ‘uma‘u remained quiet for the next 17 years; subsequent eruptions in 1952, 1954, 1961, 1967–1968, 1971, 1974, 1975, and 1982 were mostly short-lived from fissures in and near the crater (Fig. 1; Macdonald et al., 1983). Short summit eruptions also occurred outside the Halema‘uma‘u area in 1959, 1971, 1974, and 1982 (Fig. 1). The 1959 eruption in Klauea Iki Crater was unusual for its high lava fountains (up to 600 m) and its picritic lavas (Richter et al., 1970). There have been no summit eruptions since 1982.
Major eruptions have also occurred along the subaerial portion of Klauea's east and SW rift zones. The east rift zone had major fissure eruptions in 1790, 1840 and 1955 (Holcomb, 1987). Since 1955, it has been the site of frequent eruptions and intrusions including the current 20 years long, Pu`u `` eruption (e.g. Klein et al., 1987; Wolfe et al., 1987; Garcia et al., 1996b, 2000). Eruptions on the SW rift have been less frequent, with only two major (in 1823 and 1919) and three minor eruptions in 1868, 1971, 1974 (Macdonald et al. 1983). The well-documented record of historical activity at Klauea provides an excellent opportunity to gain insights into processes within its summit magma reservoir.
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SAMPLING |
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Halema‘uma‘u Crater and adjacent areas were the primary targets for our sampling of historical K
lauea summit eruptions. We collected from all exposed surface flows and tephra deposits (see Fig. 2 for sample locations and Fig. 1 for age distribution). Additional samples were obtained from the collections of the Smithsonian Institution, US Geological Survey, and the University of Hawaii for some eruptions whose products are now covered by younger deposits. Samples are numbered here according to their date of eruption, unless they were numbered in a previous study. We collected samples from deposits mapped as from the 1790 eruption (McPhie et al., 1990), which are now 14C dated as middle to late 17th century (D. Swanson, personal communication, 2001). They include juvenile tephra from layer 6 of McPhie et al. (1990), and lava and spatter from vents along a circumferential fracture 1·4 km SW of the caldera. Two large clasts of reticulite (10 cm long) were taken from the 1820 s ‘golden pumice’ deposit (e.g. Sharp et al., 1987). Two samples of the 1832 lava flow were collected between the caldera and the Klauea Iki pit crater. This crater was the site of two other historical eruptions in 1868 and 1959, which were also sampled. The oldest sample obtained from the caldera floor was erupted in 1866. A lava erupted in 1877 on the eastern caldera rim was also examined. Our samples of eruptions between 1885 and 1921 are from overflows or fissures extending from Halema‘uma‘u crater onto the caldera floor. Multiple samples were taken from the larger flows (1885, 1894, 1919 and 1921) to determine intra-flow compositional variations. Pele's hair from eruptions in 1912 and 1923 collected by T. Jaggar was obtained for this study. Deposits from these eruptions are now buried. None of the products from the brief and small eruptions during the 30 year period following the 1924 summit explosions are now exposed. However, we obtained samples from the 1929, 1931, 1934 and 1952 eruptions from institutional collections. At least one sample was obtained from each post-1952 summit eruption.
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PETROGRAPHY |
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The basic mineralogy of K
lauea basalts has been discussed in many papers dating back to Dana (1889); however, few of these studies provide modal data, especially for rocks erupted before 1955. New modes were determined for 52 Klauea historical lavas to: (1) establish what minerals were present in the magma reservoir; (2) assess how these minerals might affect whole-rock compositions; (3) evaluate temporal and spatial variations in rock type. All of the historical summit lavas appear fresh in thin section. Many contain brown glass and are strongly vesicular (>20 vol. %); others have a fine-grained, dark gray matrix consisting of plagioclase, clinopyroxene and opaque minerals (mostly Ti-magnetite), with or without olivine. Although olivine is commonly thought to be absent in the groundmass of tholeiitic lavas (e.g. McBirney, 1993), it is ubiquitous in the more MgO-rich (>7 wt %) Klauea summit lavas.
Historical Klauea summit lavas are overwhelmingly weakly olivine-phyric (85% have 1–3 vol. % phenocrysts; Table 2). More porphyritic lavas were erupted in 1820 from Halema‘uma‘u when eruption rates were high, and in 1959, August 1971 and December 1974 from vents on the margin of the caldera (Table 2). Phenocrysts (>0·5 mm across) in these lavas are almost always small (<2 mm across) except in lavas from the December 1974 (Kolysko-Rose, 1999) and 1959 eruptions (Helz, 1987). The small and sparse nature of phenocrysts indicates that whole-rock analyses of most Klauea historical summit lavas are essentially representative of magmatic compositions.
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Olivine is the most common mineral in historical K
lauea summit lavas. It usually occurs as equant euhedral crystals but may also be present as skeletal or strongly elongate crystals. Deformed and resorbed olivines (xenocrysts) are rare or absent in most of these lavas (Table 2). Inclusions of glass and euhedral chromite are common in these olivine crystals. Thus, olivine and chromite were probably the only minerals growing in the magma prior to eruption, except for many of the 1885–1921 lavas, which contain other minerals.
Clinopyroxene and plagioclase phenocrysts are absent or rare (
0·4 vol. %) in historical Klauea summit lavas. When present, they are small (1 mm in width), subhedral to euhedral without signs of resorption or deformation. Both these minerals are more common as microphenocrysts (usually 0·1–0·3 mm across), which may have grown during and just after eruption of their host lava. Lavas erupted before 1877 and after 1924 contain rare or no clinopyroxene and plagioclase microphenocrysts, except those from 1968 (Table 2). Open-textured, small (<1 cm across) glomerocrystic clots and small (<1 cm across) gabbroic and pyroxenitic xenoliths occur in some summit lavas but are common only in those erupted during periods of long-lived lava lake activity, especially during the period from 1885 to 1894 (Table 2).
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ANALYTICAL METHODS |
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Olivine and glass compositions were determined using the University of Hawaii, five-spectrometer, Cameca SX-50 electron microprobe using SAMx automation. For olivine analyses, the operating conditions were a minimum spot size (
1 µm), 15 kV and 20 nA beam current. For glasses, a broader beam (15 µm) and lower current (10 nA) was used to minimize degradation of the sample. Na was analyzed first in the glasses for only 40 s to minimize its loss. Peak counting times for other elements were 65–75 s for glass and 60 s for olivine. The raw data were corrected using ZAF-PAP procedures (e.g. Reed, 1993). The reported analyses are an average of at least three spots in the core of each olivine; rims also were analyzed to check for compositional zoning. The glass data are an average of 5–20 spot analyses.
Whole-rock XRF analyses were made for major and trace (Nb, Rb, Zr, Sr, Y, V, Zn, Ni, Cr) elements at the University of Massachusetts. The methods and the analytical precision for the XRF analyses have been given by Rhodes (1996).
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OLIVINE CHEMISTRY |
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Compositions were determined for olivine phenocrysts and microphenocrysts from a wide range of K
lauea historical summit lava compositions (5·9–13·0 wt % MgO). Olivines were previously examined in the high-MgO 1959 eruption products and were found to have core compositions of forsterite (Fo) 83·5–88·7 (Helz, 1987). New olivine analyses were made of other Klauea summit lavas to ascertain whether their olivines were in equilibrium with their host magma. Although olivine can re-equilibrate rapidly at magmatic temperatures (e.g. Roeder & Emslie, 1970; Ulmer, 1989), analyses of olivine in lavas can provide evidence of magma mixing, crystal accumulation and other magmatic processes (e.g. Nakamura, 1995; Garcia, 1996).
The olivine core compositions in historical Klauea summit lavas range from Fo 78 to 89 (Table 3) and, in general, Fo values increase with the whole-rock Mg number (Fig. 3). Most olivines are normally zoned or unzoned, which is typical of Klauea (Garcia et al., 1992; Clague et al., 1995) and tholeiitic lavas from other Hawaiian volcanoes (e.g. Garcia, 1996). However, all of the summit lavas with Mg number 63·5 contain two populations of olivine; a normally zoned, high-Fo group (Fo 86–89) and reversely zoned, lower forsterite olivine (Fo 83–85%). The presence of reversely zoned olivine in these lavas is indicative of magma mixing. The reverse zoning occurs only at crystal rims (outer 10–20 µm), suggesting that the mixing occurred during or shortly before eruption. These lavas were erupted from fissures on the margin of the caldera, except for the 1820 tephra.
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errors are smaller than the symbol size. Representative analyses given in Most of the olivines were in Fe/Mg equilibrium with their host rock when erupted (Fig. 3), which is significant given the wide range in rock Mg number (51–68). Some of the lavas with Mg numbers 58–63 have euhedral olivines with Fo 88 ± 1, which is 1–4% Fo too high for their bulk composition. These undeformed, high-Fo olivines have moderate CaO contents (0·20–0·35 wt %) suggesting they were not formed in the mantle (e.g. Stormer, 1973
; Jurewicz & Watson, 1988). Instead, these olivines probably formed early in the crystallization history of their host magmas, and their presence reflects incomplete crystal fractionation (Maaloe et al., 1988). These high-Fo olivines indicate that Klauea summit lavas are probably derived from parental magmas with Mg numbers of at least 69 ± 1, which is consistent with the discovery of Klauea submarine glasses with Mg numbers of 69–70 (15 wt % MgO; Clague et al., 1991). Sample 1832-1 contains rare deformed olivines, which yielded the highest measured Fo content (89·2). This lava was erupted from the same area outside the caldera as the 1959 lavas (Fig. 2), which also contain abundant high-forsterite olivine xenocrysts (up to 88·7% Fo; Helz, 1987). The xenocrysts in these lavas may be derived from deformed cumulates from a prehistoric magma reservoir that is thought to underlie this area (Holcomb, 1987). |
WHOLE-ROCK GEOCHEMISTRY |
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Major elements
XRF major-element analyses were made of 82 rocks from K
lauea summit eruptions; 77 are from 32 separate historical eruptions and five are from mid- to late 17th century eruptions (Table 4). Major-element analyses for lavas from many of these eruptions have been reported previously (see Wright, 1971). We report here the first analyses for the 17th century extra-caldera flows, the 1820, 1929, 1934 and 1982 Halema‘uma‘u eruptions, and 1982 caldera margin lavas.
The Klauea historical summit lavas display a wide range in MgO content (6·0–17·8 wt %). This range is nearly identical to that reported for the Hilina basalts (5·8–18·0 wt %; Fig. 4), which are Klauea's oldest exposed lavas (>23 ka) and may span 75 kyr of its history (Easton, 1987; Chen et al., 1996). Klauea lavas with MgO concentrations <6·8 wt % MgO are considered to have differentiated beyond olivine-only control and none were thought to be present at the summit (Wright, 1971; Wright & Fiske, 1971). Eleven of our summit samples (mainly from 1885 and 1894 eruptions), however, have MgO concentrations below 6·8 wt % (Table 4). The lower CaO/Al2O3 (<0·81) for these rocks compared with more MgO-rich samples (Fig. 4) and the presence of rare phenocrysts and abundant microphenocrysts of clinopyroxene or plagioclase (Table 2) confirm that these lavas differentiated beyond olivine control. Nevertheless, we concur with previous studies (e.g. Wright, 1971) that olivine is the dominant mineral crystallizing in Klauea summit lavas and that major-element variations are best shown on MgO diagrams.
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K2O (Fig. 4), TiO2, and P2O5 are incompatible in historical K
lauea summit lavas, forming curved trends when plotted against MgO. These oxides display considerable variation at the same MgO value compared with their analytical error (Fig. 4). The CaO and Fe2O3 data show broad trends with a bend at 7·0 wt % MgO. In contrast, the Al2O3 data define one relatively tight trend to 6·5 wt % MgO, where Al2O3 decreases slightly possibly reflecting plagioclase fractionation. The occurrence of clinopyroxene fractionation in summit lavas is suggested by the distinctly lower CaO/Al2O3 for lavas with MgO 7·0 wt % (Fig. 4). However, there is considerable variation in CaO/Al2O3 even among olivine-only lavas with MgO >7·5 wt %, pointing to possible variations in parental magma composition (Fig. 4). Similar variations have been observed for other suites of Hawaiian tholeiitic lavas (e.g. Frey & Rhodes, 1993).
Klauea historical summit lavas vary systematically in composition with age (Fig. 5) when lavas with MgO >7·1 wt % (those that have not experienced significant fractionation beyond olivine control) are normalized to 10 wt % MgO by addition or subtraction of equilibrium composition olivine in small steps (0·5 mol %). Incompatible element abundances and CaO increase and SiO2 decreases from the 19th century to 1923 ending with the 1924 summit collapse and explosions. Lavas erupted from 1929 to 1961 show little change but subsequent summit lavas (1968–1982) and those from the current eruption at Pu`u `` have lower incompatible element and CaO contents and higher SiO2 (Fig. 5), indicating a reversal in compositional variation. It is important to note that this variation is not related to alteration (a common problem in older Hawaiian tholeiites; e.g. Lipman et al., 1990) because the summit lavas have K2O/P2O5 ratios of 1·6–2·0, which is characteristic of unaltered Klauea lavas (e.g. Wright, 1971; Garcia et al., 2000), and they appear unaltered in thin section.
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Many of the eruptions centered in the Halema‘uma‘u crater area (Fig. 1) had limited compositional variations (from 0·1 wt % MgO for the 1 day April 1982 eruption to 0·7 wt % MgO for the voluminous eruptions during 1919; Table 4). In contrast, there was a large compositional variation (3·1 wt % MgO) for the 1921 eruptions. Eruptions on the flanks of the caldera, even those of short duration, typically have large compositional variations (e.g. 2·7 wt % MgO for the <1 day August 1971 eruption). The origins of these two distinct compositional trends for individual eruptions are discussed below.
Trace elements
Historical Klauea summit lavas exhibit substantial trace element variations (Table 4). Enrichment factors (total compositional variation) for incompatible elements in these lavas range from 1·9 for Rb and Nb (and K), and 1·5 for Sr and Zr, to 1·3 for Y and V. These factors are somewhat less than observed for Hilina basalts with the same MgO range (3·4–1·9; Chen et al., 1996), although the largest Hilina variations (3·4 for Rb and 2·8 for K) are related to alteration. Variations for alteration-resistant elements are also larger for Hilina lavas (e.g. 2·5 for Nb and 1·8 for Y). However, ratios of these elements (Nb/Y) yield identical, although somewhat offset ranges (0·61–0·97 for Hilina vs 0·53–0·89 for historical summit lavas) for data collected in the same laboratory (Chen et al., 1996; Table 4). Thus, the extent of partial melting and source composition variations were comparable for these two rock groups, which is surprising because of the very different age ranges represented by these basalts (75 ka vs 300 years).
Rb, Nb, Zr and Sr data for the historical summit lavas define broad trends well beyond analytical error on K variation plots (Fig. 6). As with the major element data, the variations can be subdivided into age groups, with both older and younger lavas having lower concentrations of incompatible elements at a given K, relative to intermediate-age lavas (Fig. 6). The slight kink in the Sr vs K plot for 1885 lavas (Fig. 6) supports our interpretation, based on modal and Al2O3 evidence, for plagioclase fractionation in the more evolved lavas (<6·5 wt %; Table 2; Fig. 4). Some of the other lavas have relatively low Sr contents (1868, 1959, December 1974), which may be related to mixing with fractionated magma.
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Ratios of incompatible trace elements (e.g. Nb/Y) show systematic variations among the lavas, increasing in value from 1820 to 1959 and then decreasing during the rest of the 20th century including during the Pu`u `
` eruption (Fig. 7). Similar temporal variations were observed for inductively coupled plasma mass spectrometry (ICP-MS) determined trace elements and Pb isotopic ratios (Pietruszka & Garcia, 1999a), but the trend reversal occurs after the 1924 collapse of the summit crater (Fig. 7). Unfortunately, we have no XRF trace element data for the 1929–1934 samples because of their very small size. However, our XRF database is more extensive for post-1934 eruptions and these data reveal that the August 1971 and July 1974 high-MgO lavas erupted from caldera-rim vents (Fig. 2) have lower Nb/Y (<0·60) than coeval lavas from intracaldera vents (Fig. 7).
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Another interesting observation is that the composition of the 1959 K
lauea Iki eruption lava plots within the distinctive field for 1952 and 1954 Halema‘uma‘u eruptions (Figs 7 and 8). Previous studies had suggested that one component of the 1959 lava composition bypassed the summit reservoir (Helz, 1987) and the other component was of the same composition as 1929–1934 reservoir lava (Wright & Fiske, 1971). Our whole-rock major and trace element results indicate that the 1959 lavas are chemically identical to those for reservoir lavas from the 1950 s (Figs 7 and 8) except for their high olivine contents. In contrast, subsequently erupted lavas are compositionally distinct.
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Ni (Fig. 6) and Cr are compatible in the historical K
lauea summit lavas and display considerable variations when plotted against a highly incompatible element such as K. These variations can be subdivided into subparallel age trends as observed for the major elements (Figs 5 and 6). Lavas from the current Pu`u `` eruption also show a temporal Ni–K trend with a progressive decrease in K for the younger lavas (Garcia et al., 2000). Zn concentrations in summit lavas are nearly constant for all but two samples (115 ± 15 ppm; Table 4). |
GLASS GEOCHEMISTRY |
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K
lauea summit lavas usually have glassy crusts. Thirty-three glassy crusts from 19 separate eruptions were analyzed for major elements by electron microprobe (Table 5) to better understand the liquid lines of descent for Klauea lavas. Glasses from lavas with bulk MgO contents <8·0 wt % typically have only slightly lower MgO content than the whole rock. This similarity in composition reflects the combined effects of sparse olivine phenocrysts in most of these samples and rapid quenching. Among the samples with higher MgO, only the 1820 tephra and August 1971 lava from caldera-rim vents have glasses with high MgO contents (9·3 wt %). The other MgO-rich lavas have lower MgO glasses reflecting either accumulation of olivine (e.g. 1959) or extensive cooling prior to eruption. Estimates of glass-quenching temperatures range from 1130 to 1200°C based on the glass MgO geothermometer for Klauea compositions (Helz & Thornber, 1987). The relatively high temperature (>1150°C) for many of these glasses reflects sample collection at or near its vent (Fig. 2).
Crystallization of clinopyroxene and plagioclase in Klauea summit glasses occurs at 7·0–7·2 and 6·4–6·8 wt % MgO, respectively, based on inflections of CaO and Al2O3 trends on MgO-variation diagrams (Fig. 9) and the presence of these minerals as microphenocrysts (Table 2). This crystallization sequence is consistent with previous petrographic observations of Klauea lava lake glasses (e.g. Wright & Peck, 1978), experimental work on Klauea lavas (e.g. Helz & Thornber, 1987), and results from calculations using the MELTS program (Fig. 9) of Ghiorso & Sack (1995).
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K
lauea summit glasses show temporal groups on MgO variation plots (Fig. 9) with the older and younger groups having similar composition, like the whole-rock analyses (Fig. 4). These data indicate that Klauea historical magmas had at least two distinct liquid lines of descent. |
TEMPORAL AND SPATIAL VARIATIONS |
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Mineralogy
Substantial mineralogical variation has occurred in K
lauea lavas during the last 100 kyr (Table 6). The 23–100 ka Hilina basalts are predominantly olivine-phyric (Easton & Garcia, 1980). Historical summit lavas are mostly weakly olivine-phyric to aphyric lavas (Tables 2 and 6). This overall mineralogical variation to less porphyritic lavas is the opposite of those reported by Macdonald (1949) and Easton & Garcia (1980). If lavas from the rift zones are included in this comparison, the overall trend towards less olivine-phyric lava remains, because the voluminous Pu`u `` eruption (which is comparable in lava production with the summit from 1820 to 1982; 2 km3) has to date only produced weakly olivine-phyric lava (e.g. Garcia et al., 2000). However, this eruption is thought to be supplied only partly from the summit reservoir (Garcia et al., 1996b).
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The nature of K
lauea's recent mineralogical change is revealed in the walls of its caldera. All but the lowermost of the 20 flows overlying the 2·1 ka Uwekahuna Ash are moderately to strongly olivine-phyric (>5 vol. %) with high MgO contents (9–20 wt %; Casadevall & Dzurisin, 1987). In contrast, the subsequent 30 prehistoric flows are all aphyric or weakly olivine-phyric basalts (<2 vol. %) with moderate MgO contents (6·3–8·0 wt %; Casadevall & Dzurisin, 1987). With few exceptions and only from vents outside the caldera (1959 and December 1974; Table 2), this tendency to erupt weakly porphyritic to aphyric basalt has persisted during historical summit eruptions. However, historical eruptions of olivine-rich basalts are more common along Klauea's rift zones (e.g. 1790, 1840, 1921, 1960, 1968 and 1969–1974). Therefore, the dominance of weakly porphyritic lavas at Klauea's summit during the last 2000 years probably reflects processes in the summit magma reservoir.
A spatial mineralogical variation is also evident for Klauea's 20th century summit lavas. Lavas erupted in or from fissures extending from Halema‘uma‘u crater since the 1924 summit collapse are weakly olivine- phyric or aphyric (Table 2). In contrast, many of the lavas erupted during the last half of the 20th century from vents on the fringes of the caldera contain more olivine. These more olivine-rich lavas follow the overall temporal geochemical trend for summit lavas (Fig. 6), so they presumably must have been derived from the same magma reservoir.
Several models have been proposed to explain mineralogical variations for Hawaiian shield volcanoes. A stratified magma reservoir tapped at different levels was invoked by Macdonald (1942) to explain the eruption of olivine-rich magmas on the flanks of these volcanoes. An updated version of this model (Ryan, 1988) advocates that lower density (2·7 g/cm3), weakly olivine-phyric magmas are able to ascend into the summit reservoir, whereas the higher density (2·8– 2·9 g/cm3), olivine-rich magma is stored at depth and periodically supplied to the rift zones. This model is consistent with the observation that olivine-rich lavas are preferentially erupted along the submarine rift zones of Hawaiian shield volcanoes (Garcia et al., 1989, 1995).
Alternatively (or in addition), during periods of high-magma supply olivine may be flushed from the base of the summit magma reservoir to produce olivine-rich lavas (e.g. Rhodes, 1995). If so, this would imply a general decrease in magma supply to Klauea during the last few thousand years. However, there is no evidence for a long-term decrease in Klauea's magma supply or of a major change in magma composition during the last 350 kyr (Quane et al., 2000), as would be expected if magma supply rates (and extents of melting) had changed significantly. Furthermore, olivine-rich subaerial lavas were erupted along Klauea's rift zones throughout most of the historical period (1840–1974) despite wide variations in the magma-supply rate (0·01–0·18 km3/yr; Dvorak & Dzurisin, 1993; Cayol et al., 2000). Olivine-rich lavas are more common at the summit of Klauea during the periods of higher magma supply (1790–1820 and 1959–present), so fluctuations in magma-supply rate may play a role in influencing temporal variations in mineralogy. However, except those from the 1959 eruption (Helz, 1987), olivines in Klauea lavas are overwhelming undeformed, euhedral and were probably grew in their host magma before eruption (Fig. 3). Thus, periods of higher magma supply probably allow less time for crustal-level differentiation and favor eruption of olivine-rich magma.
Long-term geochemical trends
Historical Klauea summit lavas display two distinct geochemical variation trends with a change in direction following the 1924 summit collapse (Fig. 5). This reversed trend is continuing during the current Pu`u `` eruption (Figs 5 and 7). Systematic compositional variations are also observed for lavas from other Hawaiian shield volcanoes (e.g. Mauna Loa; Rhodes & Hart, 1995, and Mauna Kea; Albarède, 1996; Yang et al., 1996; Blichert-Toft et al., 2003) and from other frequently active ocean island volcanoes (e.g. Piton de la Fournaise; Albarède & Tamagnan, 1988). The geochemical variations at Klauea and other volcanoes must be related to small-scale source heterogeneities because they are accompanied by rapid Pb, Sr, and Nd isotope ratio variations (e.g. Fig. 7). Systematic variations in the proportions of source components in Hawaiian magmas appear to be fundamental in the magma genesis for these volcanoes. It is our interpretation that Klauea's two distinct compositional variation trends (Fig. 7) are predominantly controlled by fluctuations in the composition of parental magmas delivered from the mantle rather than the result of magma mixing in the summit reservoir over periods of decades to centuries (Pietruszka & Garcia, 1999a).
Klauea's current trend of decreasing incompatible element abundance typifies the ‘common’ melting trend of Reiners (2002), which was identified for relatively primitive basalts from eruptions and eruptive sequences in a variety of tectonic settings including the Pu`u `` eruption. However, Klauea's current summit trend does not show decrease in MgO (Garcia et al., 2000) as predicted for the common trend from mixing melts produced by different degrees of melting of two compositionally distinct sources with different solidi and melt productivities (peridotite and pyroxenite; Reiners, 2002). Another complication for the two-source common melting model is the need for three source components to explain the compositional and isotopic data for Klauea's historical lavas (Garcia et al., 2000). The nature of these components remains enigmatic but the lack of a systematic MgO variation (Fig. 4) and the relatively low Os isotopic and SiO2 contents of Klauea lavas (Hauri, 1996) argue that neither source component can be pyroxenite as proposed by Reiners (2002). Instead, the source components are probably all peridotites based on the inferred high MgO content of Klauea's parental magmas (e.g. >16 wt %; Clague et al., 1995). The 19th to early 20th century summit trend with increasing incompatible elements (Figs 5 and 7) is opposite to that predicted by the common melting trend.
Historical lavas (1843–1984) from neighboring Mauna Loa volcano display systematic variations in ratios of incompatible elements, although not in major elements (Rhodes & Hart, 1995). The chemical variations of lavas from these two volcanoes are out of phase with each other and they have distinct Pb, Sr and Nd isotopic ratios (e.g. Rhodes & Hart, 1995; Garcia et al., 2000). Thus, historically the lavas from these volcanoes are chemically distinct and are not being supplied by the same mantle conduit, as was suggested based on earthquake locations that appeared to merge at 60 km below these two volcanoes (Klein, 1982) and by the simultaneous inflation of both volcanoes in May 2002 (Miklius & Cervelli, 2003). However, the strong overlap of some Kilauea lavas erupted from 0·4 to 2 ka with Mauna Loa compositions led Rhodes et al. (1989) to propose that Mauna Loa magma moved laterally at shallow levels within the crust and mixed with Klauea magma during that period. There is no evidence of historical magmatic communication between the two volcanoes.
Short-term compositional trends
Significant compositional variation occurred within several brief Klauea historical summit eruptions. Lavas became less MgO-rich during the 14 h September 1982 eruption (8·7–6·8 wt %; Table 4), the 10 h August 1971 eruption (11·5–7·3 wt %; Duffield et al., 1982) and the 2·5 day July 1974 eruption (11·0–6·7 wt %; Lockwood et al., 1999), all of which occurred on the margins of the caldera. The lava MgO content decreased progressively with time during the September 1982 eruption. In contrast, MgO variation for the 1971 and 1974 eruptions was apparently spatially controlled. High-MgO lavas (10·0–11·5 wt %) were erupted on the caldera rim, whereas lower-MgO lavas (6·7–7·6 wt %) were erupted from fissures on the caldera floor (Duffield et al., 1982; Lockwood et al., 1999). It is unclear whether these two eruptions had a temporal variation in composition because no lavas were collected inside the caldera during the early part of each eruption when the MgO-rich lavas were erupted on the caldera rim. The early caldera floor lavas were buried by later flows. Overflows of Halema‘uma‘u lava lake during the early months of 1921 also record large MgO variation (7·5–10·6 wt %). The volumes of magma erupted during these four eruptions are relatively small (6 x 106 m3 for 1921; 10 x 106 m3 for August 1971; 6·5 x 106 m3 for July 1974; 3 x 106 m3 for September 1982; Duffield et al., 1982; Macdonald et al., 1983; Lockwood et al., 1999), compared with the nearly homogeneous (7·4–7·6 wt % MgO) and much larger (80·3 x 106 m3) 1967–1968 eruption in Halema‘uma‘u crater (Wright, 1971; Macdonald et al., 1983).
Olivines from Klauea historical lavas demonstrate the complexities in accounting for the short-term MgO variations. Olivines from the nearly aphyric to weakly porphyritic September 1982, and the lower-MgO 1971 and 1974 lavas, are normally zoned and forsterite contents are in equilibrium with their whole-rock compositions (Fig. 3). In contrast, the MgO-rich August 1971 and July 1974 lavas have lower Fo content crystals with narrow, reversely zoned rims and higher forsterite crystals with normal zoning, which is indicative of mixing of lower and higher MgO content magmas during these eruptions. The variably olivine-phyric 1921 lavas (1·0–4·1 vol. %; Table 1) have a wide range in olivine composition (Fo 81–88), although most are Fo 81–82 despite the wide range in whole-rock Mg number (Fig. 3). The wide compositional range for 1921 lavas cannot be explained solely by variable amounts of accumulated olivine because insufficient olivine is observed (3·1 vol. % total variation) compared with the amount required by mass balance calculations (8 vol. %).
The high-Fo olivines in lavas from both the overflows of Halema‘uma‘u crater from 1918 to 1921 and caldera rim eruptions in September 1982, August 1971 and July 1974 require the presence of MgO-rich magmas within the summit reservoir. The progressive compositional change during the September 1982 eruption, with olivines in equilibrium with the host rock, suggests gradual compositional zonation within the reservoir. The decrease in MgO during this eruption may reflect the tapping the flanks of a vertically zoned body, as proposed by Macdonald (1942). Utilizing this model, the sharp compositional changes during the August 1971 and July 1974 eruptions are the result of tapping separate, compositionally distinct parts of the reservoir. The implications of these results for the nature of Klauea's summit magma reservoir are discussed below.
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PROCESSES WITHIN THE SUMMIT MAGMA RESERVOIR |
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The detailed sampling of the output of K
lauea's summit magma reservoir for the past few hundred years allows us to examine the nature, and determine the relative importance, of various petrologic processes that controlled lava composition. Below, we examine magma mixing, crystal fractionation, and assimilation processes in Klauea's summit magma reservoir as recorded in its historical lavas.
Magma mixing vs crystal fractionation
Magma mixing is an important process for many volcanoes including Klauea (e.g. Wright & Fiske, 1971). The progressive compositional variation of Klauea historical summit lavas (Fig. 7) is undoubtedly a consequence of mixing new magma, supplied nearly continuously from the mantle (Tilling & Dvorak, 1993), with magma resident in its summit reservoir. The presence of reversely zoned olivine in some mafic lavas (Fig. 3) reaffirms that mixing occurs in the summit magma reservoir. The coherence of the geochemical variation for Klauea summit lavas indicates that new magma is mixed relatively rapidly (<10 years) with magma in the reservoir. For example, lavas erupted during the same year (1982) in different parts of the summit area are compositionally similar (i.e. lie along the same liquid line of descent; Fig. 4) but those erupted in the same area 10 or more years apart are compositionally distinct (e.g. 1960 s vs 1970 s; Fig. 4). Thus, magma mixing is an important crustal process controlling the overall compositional variation of Klauea summit lavas. The kinked and curved major element trends (Fig. 4) are not indicative of magma mixing; rather, they suggest that crystal fractionation has played an important role in controlling the composition of the lavas erupted at Klauea.
MELTS modeling (Ghiorso & Sack, 1995) was undertaken to better understand crystal fractionation processes at Klauea and to calculate liquid lines of descent for comparison with the observed compositional trends. Two parental magma compositions that span the range of observed compositions (1820–1821 and 1921–1925; Table 4, Fig. 4) were used for modeling. The modeling conditions were fractional crystallization, oxygen fugacity of QFM – 0·2 (where QFM is the quartz–fayalite–magnetite buffer) (within the range observed for Klauea; Moore & Ault, 1965), pressure of 0·5 kbar (consistent with a shallow magma reservoir) and water contents of 0·2–0·35 wt % for the parental magmas (somewhat below expected water contents for these parental magmas, 0·48–61 wt %; Wallace & Anderson, 1998). Higher water values delayed the start of plagioclase crystallization beyond its observed point of crystallization (e.g. 0·5 wt % H2O delayed plagioclase crystallization to 6·1 wt % MgO vs observed 6·4–6·8 wt % MgO; Figs 4 and 9). This delay may be an artifact of the approximations in MELTS algorithms and not necessarily an indication of the water content of Klauea parental magmas.
The crystal fractionation modeling results demonstrate that the large compositional variations observed for several short-lived Klauea eruptions (e.g. August 1971, July 1974, and September 1982) and the overall variation for various time segments parallel the calculated liquid lines of descent (Fig. 4). Thus, the observed curved and kinked variation trends are probably related to crystal fractionation rather than to magma mixing, which would produce linear trends. However, the effects of crystal fractionation are superimposed on compositions produced by magma mixing. Olivine crystallization is the dominant control for much of the compositional variation, although many summit lavas also show the effects of clinopyroxene and plagioclase fractionation (Fig. 4). Fractionation of plagioclase and clinopyroxene is especially important during periods of low magma supply (e.g. late 19th to early 20th century; Fig. 10). For example, lavas erupted during 1885–1894 exhibit the greatest extent of fractionation (i.e. contain more plagioclase and clinopyroxene crystals and have lower MgO contents; Fig. 10). During periods of higher eruption rate (e.g. early 19th century), more primitive magmas are erupted as evidenced by the higher forsterite content of olivines and the virtual absence of plagioclase and clinopyroxene in these lavas (Fig. 10; Table 2). No other mineral is observed as a phenocryst or microphenocryst in Klauea historical summit lavas (Table 2) or inferred from compositional data (Fig. 4, Table 4) to have crystallized in its summit reservoir. In contrast, some Klauea rift zone lavas exhibit petrographic and chemical evidence of orthopyroxene and iron–titanium oxide crystallization (Wright & Fiske, 1971; Ho & Garcia, 1988).
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Accumulation of crystals is another potentially important crustal process that can modify lava composition (e.g. Bowen, 1928
). Samples from the 1959 and Dec. 1974 Klauea summit eruptions have abundant olivine phenocrysts (15 vol. %) with forsterite contents mostly too low to be in equilibrium with their bulk composition (Helz, 1987; Fig. 3). These picritic basalts probably accumulated significant amounts of olivine. In contrast, most Klauea historical summit lavas are weakly olivine-phyric (3 vol. %), although they may have MgO contents of 9–10 wt % (Table 2). Thus, among Klauea historical summit lavas only the bulk compositions of the picritic basalts have been significantly modified by the mineral accumulation. Therefore, the compositions of the other rocks are representative of pre-eruptive magmatic compositions.
Assimilation
Crustal contamination is another potentially important process affecting the composition of Klauea (e.g. Kyser et al., 1982; Hemond et al., 1994) and other oceanic island lavas (e.g. Eiler et al., 1996; Thirlwall et al., 1997). Oxygen isotope analyses demonstrate that crustal assimilation has affected some Klauea historical lavas, including those from the current Pu`u `` eruption (Garcia et al., 1998). Early lavas from this eruption have low matrix oxygen isotope values (
18O = 4·6–5·0
), which are out of equilibrium with oxygen in coexisting olivine (Garcia et al., 1988). Later lavas have higher matrix oxygen isotope values (5·0–5·25) and are in equilibrium with coexisting olivine. The values for these later lavas are identical to those for summit lavas erupted after the 1924 collapse of Halema‘uma‘u crater (Garcia et al., 1996a) but are below normal mantle values (5·5–6·0; Ito et al., 1987). In contrast, pre-1924 summit lavas have oxygen isotope values of 5·5 ± 0·1 (Kyser et al., 1982; Garcia et al., 1996a). The lower oxygen isotope values for post-1924 lavas are not accompanied by any major compositional change (Figs 5 and 6; Table 4), although there are minor differences in Pb, Sr, and Nd isotopes for post-1924 lavas (Pietruszka & Garcia, 1999a).
The 1924 collapse of Halema‘uma‘u caused the crater to grow markedly in size (increasing its depth from 100 to 400 m and volume by 0·2 km3; Dvorak & Dzurisin, 1993). Stoping of older Klauea lavas into the summit magma reservoir probably accompanied this collapse (Pietruszka & Garcia, 1999a). The minor compositional and isotopic differences of lavas erupted just after the 1924 collapse can be explained by mixing 3·5% of a melt formed by 5% melting of the stoped blocks with 46% resident reservoir magma and 50·5% new, MgO-rich magma (Pietruszka & Garcia, 1999a). Thus, crustal assimilation may affect some Klauea summit magmas, especially following a major summit collapse. Our sample suite for the 19th century is too sparse to evaluate whether the summit collapses in 1832, 1840 and 1868 were accompanied by significant compositional variation. However, the impact of the contamination associated with the 1924 collapse on the overall geochemistry of summit lavas appears to be minor. Crustal contamination also had little effect on the composition of Pu`u `` lavas (Garcia et al., 1998).
In summary, magma mixing and crystal fractionation are both important magmatic processes controlling compositional variation of Klauea historical summit lavas. The effects of crystal fractionation process are superimposed on a changing resident magma composition caused by the mixing of compositionally distinct parental magmas. Catastrophic collapse of Halema‘uma‘u crater in 1924 may have caused assimilation of Klauea rocks but this had a minor effect on the composition of subsequent lavas. The 1924 collapse, however, heralded a much larger magmatic event; the start of a new compositional trend that has continued for over 75 years.
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GEOMETRY OF K LAUEA'S SUMMIT MAGMA RESERVOIR
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The shape and size of K
lauea's summit reservoir have fundamental implications for how the volcano's magmatic plumbing system operates. Estimates for these features have varied greatly. One model (e.g. Tilling & Dvorak, 1993) based on ground deformation data suggests that the summit reservoir is a plexus of interconnected dikes and sills located 2–6 km beneath the summit (Fiske & Kinoshita, 1969; Decker, 1987). Alternatively, modeling of the same and more recent ground deformation data indicates that the reservoir is a single spherical body (Yang et al., 1992). Estimates of the size of the summit reservoir have varied from 0·08 to 40 km3 (Klein, 1982; Decker, 1987; Klein et al., 1987). Unfortunately, seismic studies to date have been unable to resolve these discrepancies (e.g. Rowan & Clayton, 1993).
The coherence of the major and trace element whole-rock data (especially the intracaldera lavas; Fig. 6) and isotopic ratios with time for Klauea summit historical lavas requires that magma in the summit reservoir mixes efficiently (Pietruszka & Garcia, 1999a). Efficient mixing on a time scale of less than a decade in unlikely in a plexus of dikes and sills. Thus, the geochemical data suggest that Klauea's summit reservoir is a well-mixed single body (Fig. 11). Our schematic drawing of Klauea's summit reservoir utilizes the single spherical body model of Yang et al. (1992). This body underlies the southern margin of the caldera near Halema‘uma‘u (Klein et al., 1987), the site of frequent summit eruptions (Fig. 11), and is thought to be supplied at its base by new mantle-derived magma (Tilling & Dvovak, 1993).
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The size of the summit reservoir was estimated using ICP-MS trace element data (Nb/Y) for historical summit lavas and residence time analysis (Pietruszka & Garcia, 1999b
). For the period from 1820 to just before the collapse of Halema‘uma‘u in 1924, the reservoir volume was estimated to have been 2·3–4·6 km3; from 1961 to 1982, the size was 1·7–2·7 km3. The range in values reflects the choice of parental magma composition; the smaller values are based on observed historical compositions and the larger values reflect end-member Klauea compositions (Pietruszka & Garcia, 1999b).
Eruption rates and lava compositions changed significantly from 1820 to 1954 (Figs 5, 7, 8 and 10). Thus, we decided to re-examine the implications of these changes on the size of the reservoir for this period using a trace element ratio (Ba/Sm) that appears to be more sensitive to source compositional variations (Fig. 12) and the residence time analysis model of Albarède (1993). Additional details on this modeling, including a minor correction, have been given by Pietruszka & Garcia (1999b). The Ba/Sm variation of the lavas is linear from 1820 to 1894. If we assume that the parental magma being added to the reservoir had 10 wt % MgO and a Ba/Sm ratio comparable with that of the 1894 lava, the reservoir volume is 2·6 km3, consistent with the results of Pietruszka & Garcia (1999b). Varying the MgO content of the input magma from 6·6 to 15 wt % causes only a minor change in volume (±0·2 km3). However, if Klauea's highest Ba/Sm is used for the parent magma (from Hilina Basalts; Chen et al., 1996), the size of the reservoir nearly doubles (up to 5·1 km3 depending on the MgO value of the input magma). This is somewhat larger than the upper value reported by Pietruszka & Garcia (1999b). From 1894 to 1921, Ba/Sm decreased (Fig. 12) unlike Nb/Y (Fig. 7). Magma residence time calculations based on this shift to lower Ba/Sm and the lower lava eruption rates for this period (Fig. 10) suggest a dramatic decrease in reservoir size during this period to 0·3 km3 using an average 1920 s era parental magma composition or a decrease to 0·9 km3 using Klauea's lowest Ba/Sm (1790–1). Based on a shift to higher Ba/Sm values after 1924 (Fig. 12) and the extremely low eruption rates to 1954 (Fig. 10), the calculations suggest a further decrease in reservoir size to <0·1 km3 using the observed or end-member ratios. Thus, regardless of the input parent magma composition, there appears to have been a dramatic decrease in the size of Klauea's summit reservoir from 1820 to 1954.
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K
lauea had multiple major summit collapses during the period from 1820 to 1954 (Table 1). It is our interpretation that the changes in lava composition and the summit collapses are consequences of a decrease in magma supply rate to the summit magma reservoir (Fig. 10). This decrease in magma supply is thought to be a direct consequence of the gradual decrease in the melt fraction during the 19th to early 20th century at Kilauea (Pietruszka & Garcia, 1999a). Subsequently, there was a 17 year gap in Klauea eruptive activity, which may have been a period of magma reservoir inflation. The reservoir continued to inflate during a period of increased magma supply and is thought to have reached a size of 2 km3 by 1982, before it began to shrink 10% during the Pu`u `` eruption (Denlinger, 1997). Interestingly, the late 20th century is inferred to be a period in which the degree of partial melting increased by a factor of 2 (Pietruszka & Garcia, 1999a). Overall, these variations in the melt fraction are thought to result from differences in the fertility of the source region (Pietruszka et al., 2001).
The main mass of Klauea's summit reservoir is compositionally zoned. This interpretation is based on the extremely wide range in whole-rock MgO content for historically erupted Klauea lava (6·0–17·8 wt %; Table 4) and in the systematic compositional variation during some historical eruptions. A single, compositionally zoned reservoir model is supported by the presence of euhedral, undeformed high forsterite olivines (Fo 87–88) in more evolved lavas (e.g. 1832 and 1889 eruptions; Fig. 3), which indicates mixing between more primitive magma (from the lower part of the reservoir freshly fed from the mantle conduit) and more evolved magma in the upper part of the reservoir. This primitive component was tapped during three caldera rim eruptions in the 1970 s, which produced MgO-rich lavas with somewhat lower Nb/Y (<0·60) than coeval intracaldera lavas. The summit reservoir is also zoned in temperature based on glass compositions from the 1970 s eruptions, which indicate at least 40°C for individual eruptions and as much as 60°C for eruptions a few months apart (Table 5).
A crown of lower density [2·56–2·58 g/cm3; densities calculated using the procedures of Niu & Batiza (1991)] magma overlies the summit reservoir beneath Halema‘uma‘u (Fig. 11). This interpretation is based on the restricted overall compositional range of lavas erupted in and around Halema‘uma‘u since its 1924 catastrophic collapse (total MgO range is 7·0–7·7 wt %; Table 4), the presence of olivines in equilibrium with their host rock composition (Fig. 3), and the higher density of the more MgO-rich lavas (2·60–2·78 g/cm3). Furthermore, the compositional variation for lavas from individual eruptions is remarkably small (e.g. 7·4–7·6 wt % MgO for the large, 80·3 x 106 m3, 1967–1968 eruption; Wright, 1971; Macdonald et al., 1983) compared with the much smaller volume eruptions on the caldera margins (e.g. 6·7–11·0 wt % for 6·5 x 106 m3 of lava during July 1974; Lockwood et al., 1999). The limited compositional variation of these lavas is probably related to two processes: (1) an increase in the enthalpy of crystallization, especially as clinopyroxene begins to crystallize at 7·2 wt % MgO, as pointed out by Ghiorso (1995) for basaltic magmas; (2) nearly continuous influx of new magma (Dzurisin et al., 1984). During periods of low magma supply and small, infrequent eruptions, the volume of the crown probably increases and the magma becomes more differentiated (as evidenced by the lavas from the 1885 and 1894 eruptions; Fig. 4). In contrast, during periods of high magma supply rates, more MgO-rich magmas are erupted from the Halema‘uma‘u crater area (e.g. 1820 eruption).
One enigmatic aspect of the Klauea's magmatic plumbing system is the connection between the summit reservoir and the rift zones. Shallow-level (1–3 km deep) earthquake locations (e.g. Klein et al., 1987) have been used to infer that dikes from the shallow portions of the summit reservoir fed the rift zones (Dzurisin et al., 1984). However, at least some intrusions into the more active east rift zone seem to bypass the summit reservoir, based on seismic and geochemical evidence (Ryan, 1988; Garcia et al., 1996b). It has also been reported that samples of compositionally distinct magma batches are erupted along the east rift zone before they appear at the summit (Wright & Fiske, 1971), although this is not true for the Pu`u `` eruption (Garcia et al., 1992). However, the summit reservoir has shrunk by 10% during this eruption (Denlinger, 1997), so there must be a connection between the reservoir and the east rift zone. More work is needed on Klauea's other rift zone eruptions to gain a better understanding of the interplay between its summit reservoir and rift zones now that we have a geochemical baseline for summit eruptions.
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COMPOSITION OF KLAUEA PARENTAL MAGMAS: IMPLICATIONS FOR MANTLE MELTING PROCESS AND SOURCE HETEROGENEITY
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The minimum MgO content of the parental magmas for historical K
lauea lavas can be inferred from the compositions of their most forsteritic euhedral, undeformed olivine phenocrysts (e.g. Wilkinson & Hensel, 1988). Over a long time span (1820–1974), Klauea has erupted lavas with high forsterite content olivine phenocrysts (88–89%; Fig. 3). Similar olivines have been found in Klauea historical rift zone lavas (e.g. Nicholls & Stout, 1988; Wilkinson & Hensel, 1988). Using the experimentally determined equilibrium relationship between olivine and whole-rock compositions (Roeder & Emslie, 1970; Ulmer, 1989), the MgO content of the parental magmas for Klauea summit lavas was calculated to be at least 13–14 wt % (assuming 90% of the total iron is Fe2+). This interpretation is consistent with the discovery of submarine Klauea glasses with up to 15 wt % MgO (Clague et al., 1991) and estimates for the minimum MgO content for Klauea rift zone parental magmas (e.g. Nicholls & Stout, 1988; Wilkinson & Hensel, 1988; Clague et al., 1995). It also demonstrates that Klauea's mantle conduit supplies relatively primitive magma to both the summit reservoir and its rift zones.
The rate of compositional change of Klauea parental magmas has varied during the last 200 years, with more rapid changes during the latter part of the 20th century (Figs 5 and 7). This rapid change is continuing during the current Pu`u `` eruption, which produced 50–75% of the summit historical compositional variation during its first 15 years, although only 20% of the summit Pb isotopic variation. Roughly equal amounts of lava were produced historically at the summit and from the current eruption (2 km3, adjusted to dense rock; Dvorak & Dzurisin, 1993; Garcia et al., 2000). Modeling of trace element and isotopic data indicates that the extent of partial melting for Klauea magmas has varied by a factor of 2 during historical times and that the proportions of its source components have varied markedly (Pietruszka & Garcia, 1999a). The change in source proportions correlates with large variations in magma supply (<0·01 to 0·18 km3/yr; Dvorak & Dzurisin, 1993; Cayol et al., 2000) with the lowest supply rates related to the depleted source during the early 20th century (Pietruszka & Garcia, 1999a). Although Klauea is likely to have experienced endogenous growth (e.g. Francis et al., 1993) during the last 200 years, the strong link between magma supply rate and source composition suggests that melting processes are the dominant factor controlling the compositional variation of historical summit lavas.
Despite the rapid historical variations, Klauea's overall compositional variation appears to be limited. Lavas from a 1·7 km deep drill core into the volcano's east rift zone show roughly the same compositional variation as historical Klauea lavas (Quane et al., 2000). These drill core lavas date back to 350 ka (Quane et al., 2000). Thus, although the extent of mantle melting is variable (Pietruszka & Garcia, 1999a), Klauea has shown no long-term trend in compositional variation.
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CONCLUSIONS |
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Historical K
lauea summit lavas record magmatic processes within the summit magma reservoir during the past 200 years. These lavas reveal well-defined systematic compositional variations that imply the summit magma reservoir is a single, well-mixed body. The restricted range in mineralogy and geochemistry of lavas erupted in and near Halema‘uma‘u Crater since 1924 suggests that a crown of relatively low-density (2·56–2·58 g/cm3), aphyric magma overlies more crystal-rich, denser (2·60–2·78 g/cm3) magma in the main mass of the reservoir. Magma in this crown undergoes more extensive crystal fractionation, especially during periods of lower magma supply, than previously envisioned. The compositions of glasses from some eruptions indicate temperature gradients of atleast 40–60°C within the summit reservoir. The effects of crystal fractionation are superimposed on the hybrid magmas produced by mixing of new mantle-derived magma of varying composition with preexisting magma residing within the summit reservoir. Underlying the crown is more olivine-rich magma that is occasionally tapped by eruptions on the caldera margin. Crustal contamination apparently occurred following the 1924 collapse of the summit, although the overall geochemical effects were minor. Variations in parental magma composition for Klauea summit lavas correlate with lava eruption rate and indicate significant changes in its source composition and extent of partial melting over the last 200 years. However, the ranges in source composition and extents of partial melting have varied little at Klauea for 350 kyr. |
ACKNOWLEDGEMENTS |
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Many people helped make this study possible. Our warmest aloha to Tom Casadevall for sharing unpublished modes for summit prehistoric lavas, Sorena Sorensen for lending critical samples from the Smithsonian collection, Tina Neal for sharing her formerly unpublished K
lauea summit map, J. Rayray, J. Parker and Kelly Kolysko for sample preparation, Nancy Baker for collecting some of the summit samples, Rachel Konishi, Joann Romano, Kate Bridges, Kristina Garcia and Joe Horrell for help in the field, Pete Dawson for able maintenance of the XRF laboratory at the University of Massachusetts, and Mike Vollinger for unwavering expert XRF analyses. Thanks go to Amy Gaffney, Robert Tilling and Pete Reiners for helpful comments on the manuscript for this paper. This study was supported by an NSF grant (EAR00-01123). This is SOEST contribution 6209. |
FOOTNOTES |
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Present address: Department of Geological Sciences, San Diego State University, San Diego, CA 92182, USA. 
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