The Size and Shape of Kilauea Volcano S Summit Magma Storage

ELSEVIER Earth and Planetary Science Letters 167 (1999) 311±320

The size and shape of Kilauea Volcano's summit magma storage reservoir: a geochemical probe

Aaron J. Pietruszka Ł, Michael O. Garcia Hawaii Center for Volcanology, Department of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822, USA Received 28 September 1998; revised version received 15 February 1999; accepted 16 February 1999

Abstract

One of the most important components of the magmatic plumbing system of Kilauea Volcano is the shallow (2±4 km deep) magma storage reservoir that underlies the volcano's summit region. Nevertheless, the geometry (shape and size) of Kilauea's summit reservoir is controversial. Two fundamentally different models for the reservoir's shape have been proposed based on geophysical observations: a plexus of dikes and sills versus a single, `spherical' magma body. Furthermore, the size of the reservoir is poorly constrained with estimates ranging widely from 0.08 to 40 km3. In this study, we use the temporal variations of Pb, Sr, and Nd isotope and incompatible trace element (e.g., La=Yb and Nb=Y) ratios of Kilauea's historical summit lavas (1790±1982) to probe the geometry of the volcano's summit reservoir. These lavas preserve a nearly continuous, 200-year record of the changes in the composition of the parental magma supplied to the volcano. The systematic temporal variations in lava chemistry at Kilauea since the early 19th century suggest that the shape of the volcano's summit reservoir is relatively simple. Residence time analysis of these rapid geochemical ¯uctuations indicates that the volume of magma in Kilauea's summit reservoir is only ¾2±3 km3, which is smaller than most geophysical estimates (2±40 km3). This discrepancy can be explained if the volume calculated from lava chemistry represents the hotter, molten core of the reservoir in which magma mixing occurs, whereas the volumes estimated from geophysical data also include portions of the reservoir's outer crystal-mush zone and a hot, ductile region that surrounds the reservoir. Although our volume estimate is small, the amount of magma stored within Kilauea's summit reservoir since the early 19th century is an order of magnitude larger than the magma body supplying Piton de la Fournaise Volcano, another frequently active ocean-island volcano.  1999 Elsevier Science B.V. All rights reserved.

Keywords: Kilauea; volcanoes; Hawaii; lavas; geochemistry; magma chambers; residence time

1. Introduction ated melt is transported by a vertical conduit from a depth of ½60 km to a shallow (2±4 km deep) magma The basic magmatic plumbing system for an ide- reservoir beneath the volcano's summit region. This alized Hawaiian volcano was ®rst outlined in 1960 by reservoir stores magma prior to eruption at the vol- Eaton and Murata [1]. In their model, mantle-gener- cano's summit or lateral intrusion into the rift zones of the volcano (for further storage and=or eruption). This Ł Corresponding author. Present address: Carnegie Institution of model has been re®ned over the last four decades by Washington, Department of Terrestrial Magnetism, 5241 Broad continuous monitoring of the deformation and seis- Branch Road, N.W., Washington, DC 20015, USA. micity of Kilauea Volcano, one of the world's most

0012-821X/99/$ ± see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0012-821X(99)00036-9 312 A.J. Pietruszka, M.O. Garcia / Earth and Planetary Science Letters 167 (1999) 311±320 active basaltic volcanoes (see reviews of Decker [2] 2. The shape of Kilauea's summit magma storage and Tilling and Dvorak [3]). Kilauea's plumbing sys- reservoir tem has now been delineated in 3-D from the mantle conduit to the rift zones of the volcano [4,5]. One of Kilauea's summit magma storage reservoir is the most important components of this system is the thought to be located 1±2 km southeast of the pit summit magma storage reservoir, which is thought crater Halemaumau (Fig. 1) at a depth of 2±4 km to act as a control center for the volcano's magmatic based on measurements of the deformation associ- processes [2,3]. Nevertheless, the geometry (size and ated with the volcano's frequent activity [6,7]. These shape) of Kilauea's summit reservoir remains contro- geodetic studies also show that the apparent locus versial. Models for the reservoir's shape range from of deformation (or `pressure center') during periods a complex network of interconnected dike- and sill- of summit in¯ation migrates over a large area (up to shaped magma bodies [6] to a single, `spherical' 16 km2). This observation has led to contradictory magma body [7], and estimates of the reservoir's size conclusions about the shape of the volcano's summit vary widely (0.08 to 40 km3 [2,4,8±10]). A solution reservoir [3]. The classic interpretation (Fig. 2c) is to this `geometry problem' is required to better un- that the migration of the center of in¯ation results derstand Hawaiian shield volcanoes, which serve as from the sequential pressurization of a network of models for other ocean-island volcanoes. interconnected dikes and sills beneath the volcano's Geochemistry can help to decipher the geometry summit region (Fiske and Kinoshita [6]). However, of Kilauea's summit reservoir. The number, shape, and size of magma bodies beneath other frequently active volcanoes such as Vesuvius, Mt. Cameroon, Hekla, Piton de la Fournaise, and Mt. Etna have been inferred from the geochemical variations of their eruptive products [11±15]. Like these other volcanoes, Kilauea's historical summit and rift zone lavas (1790 to the present) show signi®cant temporal geochemical ¯uctuations [9,16±19]. Unfortunately, most of Kilauea's rift zone lavas cannot be used to probe the summit reservoir because they may be stored in the rift zones of the volcano for many years and=or mixed with unrelated magma prior to eruption [20], or may even bypass the summit reservoir [18,21]. In contrast, most of Kilauea's historical summit lavas are thought to have erupted directly from the summit reservoir [16]. The relatively uniform and unique major element chemistry of individual summit lavas (except for the effects of olivine fractionation or accumulation) has been used to suggest that these eruptions may tap chemically distinct and physically discrete magma batches that remain isolated within a compartmental storage reservoir [16,20]. However, this model has not been tested using high-resolu- Fig. 1. Simpli®ed geologic map of Kilauea's summit region tion geochemical tracers such as isotopes or trace (modi®ed from the unpublished map of J. Lockwood and T. elements. The purpose of this study is to use the Neal), showing the distribution of historically erupted lavas temporal variations of Pb, Sr, and Nd isotope and (white areas). The boundary of Kilauea Caldera is shown by the heavy solid line. Pit craters, such as Halemaumau, are marked incompatible trace element ratios in Kilauea's his- by the heavy hatchured lines. The `T's indicate the orientation torical summit lavas (1790±1982) to determine the of the cross-sections in Fig. 2. See [19] for the locations of the geometry of the volcano's summit reservoir. samples used in this study. A.J. Pietruszka, M.O. Garcia / Earth and Planetary Science Letters 167 (1999) 311±320 313

(a) (b) (c) Time Time Time ∆t ∆t Lava Output

∆R Input ∆R Geochemical Tracer Magma

NWHalemaumau SE

2 As ei sm i 3 c

o n

4 e

6 km

Fig. 2. Hypothetical cross-sections through Kilauea's summit region contrasting models for the shape and size of the volcano's shallow magma storage reservoir (lower panels) and the possible geochemical effects of each reservoir geometry (upper panels, modi®ed from AlbareÁde [14]). (a) Geometrically simple, small-volume magma reservoir. The dashed line marks the approximate limit of the volcano's aseismic region, which is the maximum possible volume of Kilauea's magma reservoir (¾40 km3 [4]). (b) Geometrically simple, large-volume magma reservoir. (c) Geometrically complex reservoir (modi®ed from Fiske and Kinoshita [6]). The composition of the input magma delivered to the volcano (represented by any geochemical tracer of parental magma composition such as incompatible trace element or isotope ratios) is assumed to vary systematically over time (shown here as a sinusoidal, solid line). These systematic parental magma changes will be progressively buffered (e.g., a ∆R decrease) and delayed (e.g., a ∆t increase) as the volume of the reservoir increases (models a to b; [14]), and=or destroyed by transport through a complex reservoir (model c). Thus, temporal variations in the output lava chemistry (gray circles connected by dashed lines) may be used to probe the geometry of the volcano's magma reservoir (see text for more details). dike intrusion during periods of summit in¯ation are thought to result from changes in the composi- may bias the loci of deformation towards the direc- tion of the parental magma delivered to the volcano tion of the dikes, leading to an apparent migration [19,22±24]. These temporal variations in lava chem- of the pressure center [7]. The removal of this effect istry may be profoundly affected by the shape of causes the locations of in¯ation centers to converge the volcano's magma reservoir (e.g., simple body within a 1 km2 area, which suggests that Kilauea's versus complex network). Lavas from eruptions that summit reservoir may instead be a single, `spherical' frequently tap a geometrically simple, well-mixed body (Yang et al. [7]; Fig. 2a or b). In this section, magma reservoir (e.g., a single, `spherical' magma we use lava chemistry to independently evaluate body) should preserve any progressive temporal these two radically different models for the shape of changes in the composition of the parental magma Kilauea's summit reservoir. delivered to the volcano (Fig. 2a or b). The geo- Hawaiian shield volcanoes display short-term, chemical variations of these lavas over time will only and possibly cyclic, geochemical ¯uctuations which be `buffered' and `delayed' relative to the composi- 314 A.J. Pietruszka, M.O. Garcia / Earth and Planetary Science Letters 167 (1999) 311±320 tional ¯uctuations of the parental magma due to the summit reservoir during the 1924 eruption [19,28], ®nite residence time of magma in the reservoir [14]. and are not considered in our modeling. During the In a geometrically complex magma reservoir (e.g., a second half of the 20th century, the highly over mod- plexus of dikes and sills), the geochemical signature erately incompatible trace element and Pb isotope of a particular lava will depend on a number of fac- ratios decreased rapidly, whereas the Sr and Nd iso- tors, including (1) the portion of the reservoir tapped tope ratios remained essentially constant. Although by the eruption, (2) the length of time the magma there have been no eruptions at Kilauea's summit spends in that area of the reservoir prior to eruption, since September 1982 [26], the compositional trends and (3) the degree of interaction between magmas in de®ned by the late 20th century summit lavas have different pockets of the reservoir. Changes in any of continued through the volcano's ongoing (1983 to these parameters between different eruptions will de- the present) Puu Oo rift zone eruption. stroy any systematic temporal variations in parental The variations of Pb, Sr, and Nd isotope and highly magma composition (Fig. 2c). Thus, a geochemical over moderately incompatible trace element (e.g., time-series analysis of a suite of lava samples may La=Yb and Nb=Y) ratios in Kilauea's historical sum- be used to infer (at least qualitatively) the shape of mit lavas over the last 200 years (Fig. 3) are thought the volcano's magma reservoir. to result from short-term changes in the composition The historical summit lavas of Kilauea preserve of the parental magma supplied to the volcano [19]. a nearly continuous, 200-year record of the changes Thus, these geochemical tracers can be used to probe in the volcano's lava chemistry (Fig. 3). Following the shape of Kilauea's summit reservoir. The remark- Kilauea's phreatomagmatic 1790 eruption [25], the ably systematic nature of the compositional trends highly over moderately incompatible trace element over time requires that the lavas erupted either from a ratios of the lavas increased until the early 20th cen- single magma body or a compartmental storage reser- tury (e.g., La=Yb or Nb=Y). The only lava that sig- voir in which the different magma pockets are well ni®cantly departs from the overall trends in trace el- connected. If these lavas are assumed to erupt from ement ratios erupted near Kilauea Iki Crater follow- an extremely complex reservoir system (such as the ing the great earthquake of 1868 [26]. The relatively one proposed by Fiske and Kinoshita [6]; Fig. 2c), low La=Yb and Nb=Y ratios of this lava (compared the systematic temporal changes in lava chemistry to contemporaneous intracaldera lavas) may have re- would require very ef®cient magma mixing between sulted from the intrusion of early 19th century magma different storage areas on the short time scale of Ki- from the summit reservoir to the area below Kilauea lauea's average repose period (less than a few years Iki Crater and storage for several decades prior to [8]). Otherwise, different eruptions might tap phys- its eruption in 1868. This interpretation is consistent ically and compositionally unrelated magma bodies with the moderately evolved composition of the 1868 (and produce scatter on plots of the temporal varia- lava (6.7 wt% MgO; M. Garcia, unpubl. data). The tions in lava chemistry). Instead, our results suggest temporal increase of the highly over moderately in- that the shape of Kilauea's summit reservoir must be compatible trace element ratios of the lavas also cor- relatively simple in order to transmit the progressive relates with a systematic change in their Pb, Sr, and changes in the volcano's parental magma composition Nd isotope ratios (87Sr=86Sr varies inversely), except observed during the 19th and early 20th centuries, and for the lavas erupted during the 1790 eruption. the late 20th century (Fig. 2a or b). This interpreta- In 1924, the collapse of the long-standing Hale- tion is consistent with the ground deformation study maumau lava lake resulted in 3 weeks of phreatic of Yang et al. [7]. explosions at Kilauea's summit [27]. This event cor- relates with a sudden shift in the ratios of highly over moderately incompatible trace elements and Pb, 3. The size of Kilauea's summit magma storage Sr, and Nd isotopes in Kilauea's summit lavas that reservoir persisted for up to 40 years (Fig. 3). These geochem- ically anomalous lavas may have been contaminated The size of Kilauea's summit magma storage by the assimilation of country rock into the volcano's reservoir is poorly constrained (0.08 to 40 km3 A.J. Pietruszka, M.O. Garcia / Earth and Planetary Science Letters 167 (1999) 311±320 315

Explosive Eruptions Explosive Eruptions 1790 1924 1790 1924

18.65 2σ σ 2 8 18.60 Pb 7 La/Yb 204 18.55 18.50 Pb/ Kilauea 6 ? Iki 206 18.45 5 18.40 Puu Oo

2σ 0.70 0.70365 σ 2 0.65 Sr 0.70360 0.60 Nb/Y 86 0.55

Sr/ 0.70355 0.50 87 ? 0.45 0.70350 0.40 Rate (km

6.7 ? 0.10 Eruption 0.08 6.5 0.06 σ 6.3 2 0.04 3 /yr) 0.02

Nd 6.1 Eruptive ε History 5.9 ? 5.7

5.5 1800 1850 1900 1950 1800 1850 1900 1950 Eruption Date Eruption Date Fig. 3. Temporal geochemical variations of Kilauea's historical summit lavas. The gray sample symbols are grouped according to eruption date. The geochemical trends de®ned by these lavas have continued through the ongoing Puu Oo rift zone eruption (gray rectangles, excluding the early lavas that experienced magma mixing within the rift zone [18]). The vertical shaded lines mark the dates of Kilauea's explosive eruptions in 1790 and 1924. The 1868 lava may have been stored beneath Kilauea Iki Crater for several decades prior to eruption (dashed arrow). The open symbols are literature data selected from references summarized in [19]. The 2¦ error bars are given in the upper left corner of each plot. The lower right panel shows Kilauea's eruption rate and the eruptive activity of volcano's summit region (black bars) since August 1823 ([26] and references therein). Eruptions during the late 20th century have been concentrated along the volcano's east rift zone with the sustained Mauna Ulu (1969±1974) and Puu Oo (1983 to the present) eruptions (gray bars). The temporal variation in Kilauea's eruption rate is calculated using the data and approach of [38,39] as described in [19].

[2,4,8±10]). This discrepancy probably results, in estimated size of a magma reservoir depends upon part, from the de®nition of `magma reservoir'. A which portion of the system is being measured. The typical magma body is thought to consist of two smallest previous estimate for size of Kilauea's sum- distinct rheological zones: a hotter, low-viscosity mit reservoir is ¾0.08 km3, which is the product core of essentially molten liquid surrounded by a of the volcano's average eruption rate and repose cooler, high-viscosity crystal `mush' [29]. Thus, the period [8]. This `active volume' is similar to the size 316 A.J. Pietruszka, M.O. Garcia / Earth and Planetary Science Letters 167 (1999) 311±320 of Kilauea's largest short-duration eruptions [8,26] the parental magma [14]. In contrast, the geochem- and probably represents the minimum amount of ical variations of lavas that erupt from a relatively eruptible magma in the reservoir. Geophysical es- large-volume magma body will be strongly damp- timates for the size of Kilauea's summit reservoir ened and shifted in time compared to the parental range widely from ¾2±13 km3 based on geodetic magma changes (Fig. 2b). data [9,10] to a maximum of ¾40 km3 based on the For our calculations, we use the model of Al- approximate volume of the volcano's aseismic zone bareÁde [14] except that we de®ne the residence time (Fig. 2a; [4]). In addition to the mostly liquid part of of magma in the reservoir relative to the magma the magma reservoir, these volume estimates based supply rate (rather than the lava eruption rate) to on geophysical data may also include portions of make the ®nal expression independent of short-term the reservoir's crystal mush zone and a hot, ductile changes in the reservoir volume (i.e., in¯ation or region that surrounds the reservoir [4,9]. In this sec- de¯ation [30]). Thus, the magma residence time (−m) tion, we use the temporal geochemical ¯uctuations in terms of the reservoir volume (V ) and supply rate in in of Kilauea's historical summit lavas to independently .Q / is simply −m D V=Q . Physically, this resi- estimate the volume of magma in the volcano's sum- dence time represents the average duration of magma mit reservoir. storage in the reservoir prior to eruption. The ®nal equation for the residence time of magma in the 3.1. The magma residence time model volcano's summit reservoir is: Cin Rin Rres Rres .Þ Þ / The volumes of the magma bodies beneath several − D 2 1 2 m res = = active volcanoes such as Vesuvius (two, 0.1±1.0 km3 C2 dR dt dR dt bodies), Mt. Cameroon (0.002±0.01 km3), Piton de The superscripts `in' and `res' refer to the input la Fournaise (0.1±0.3 km3), and Mt. Etna (a shallow, magma and magma in the reservoir (at the start of 0.3±1.6 km3 body connected to a deep, 30 km3 body) mixing), respectively. The variable R represents the have been calculated from the geochemical varia- ratio of two incompatible trace elements or isotopes tions of their lavas [11±14]. The volume of magma (DC1=C2), where Cn is the concentration of element in Kilauea's summit reservoir may also be estimated or isotope n. The rate of change of this ratio in using the technique of residence time analysis de- the lavas is denoted by dR=dt. Finally, Þn D s C veloped by AlbareÁde [14]. The basic idea of this r C xDn ,whereDn is the bulk partition coef®cient model is that the temporal geochemical ¯uctuations of an element or isotope n,ands, r,andx are observed in a time-series of lavas (such as Kilauea's the mass fractions of the input magma removed by historical summit lavas) result from the progressive summit eruptions, rift zone withdrawals, and crystal mixing of compositionally distinct input and resi- fractionation, respectively. Because we model ratios dent magmas within the volcano's magma reservoir. of trace elements (e.g., Nb=Y) that are highly to Thus, a volume calculated using this model re¯ects moderately incompatible in the minerals observed only the hotter, molten core of the reservoir (i.e., the in Kilauea's historical summit lavas (mostly olivine low-viscosity portion that undergoes magma mix- with minor clinopyroxene š plagioclase [16]), we ¾ ing [29]). For a given magma supply rate, the rate assume that xDn − 1 and, thus, Þ1 Þ2 D 0. This of variation in lava chemistry is inversely propor- leads to a simpli®cation of the expression for magma tional to the magma residence time and the volume residence time to: of magma in the reservoir (see equations below). Cin Rin Rres In general, lavas from eruptions that frequently tap − D¾ 2 m res = a relatively small-volume magma body should pre- C2 dR dt serve any systematic temporal changes in the compo- The volume of magma in the reservoir may be sition of the parental magma delivered to the volcano estimated from the de®nition of the magma residence in (Fig. 2a). The geochemical variations of these lavas time .V D −m ð Q /, using the residence time over time will only be slightly buffered and de- calculated from the geochemical ¯uctuations and the layed relative to the compositional ¯uctuations of volcano's magma supply rate [14]. A.J. Pietruszka, M.O. Garcia / Earth and Planetary Science Letters 167 (1999) 311±320 317

1 3.2. Magma residence time calculation for Kilauea −m D .27=25/ ð [.0:66 0:44/=0:0022 yr ] D 108 years, and from 1961 to 1982 the residence time, 1 The residence time of magma in Kilauea's sum- −m D .27=29/ ð [.0:43 0:64/=0:0060 yr ] D 33 mit reservoir is estimated using the Nb=Y ratios of years. These calculations indicate that the residence lavas from the two periods (early 19th to early 20th time of magma in Kilauea's summit reservoir de- centuries and late 20th century) that display sys- creased by a factor of ¾3 from the early 19th±early tematic temporal geochemical trends (Fig. 3). We 20th centuries to the late 20th century. This large use Nb=Y because this ratio shows the largest range change in residence time may be due to a decrease in and the most systematic variation, although similar the reservoir volume, an increase in the magma sup- results are obtained with other highly over moder- ply rate, and=or an incorrect model assumption. In ately incompatible trace element (e.g., La=Yb) or Pb the following discussion, we evaluate each of these isotope ratios. Eight historical samples are excluded possibilities. from the analysis for the following reasons: the six Although the Nb=Y ratios and Y concentrations lavas that erupted from 1924 to 1954 may have been of the resident magmas are probably correct (be- contaminated by crustal assimilation during the 1924 cause lavas with these compositions were erupted), collapse of Halemaumau lava lake [19,28], the 1868 the actual values for the input magmas are unknown. lava may have been stored beneath Kilauea Iki Crater The relative Y abundances of the input magmas are for several decades prior to eruption (Fig. 3), and the probably not much different from the assumed values magma supplying the 1959 Kilauea Iki eruption may because the concentrations of moderately incompat- have bypassed the summit reservoir [31,32]. ible trace elements such as Y and Yb in Kilauea From 1820 to 1921 the Nb=Y ratios of Kilauea's lavas are essentially constant at a given MgO value historical summit lavas increased from 0.44 to 0.66, [17,18]. Therefore, an incorrect assumption for the Y whereas from 1961 to 1982 the Nb=Y ratios de- abundances cannot explain the apparent temporal decreased from 0.64 to 0.49 (Fig. 3). These temporal crease in the magma residence time. At most, the Y Nb=Y ratio variations correspond to rates of 0.0022 concentrations of the input magmas might be lower yr1 and 0.0060 yr1, respectively, based upon a due to a more ma®c parental magma. If a higher, 15 linear regression of the data. In order to calculate the wt% MgO parental magma is used instead (the most residence time, the composition of the reservoir and ma®c Hawaiian glass composition ever observed input magmas for each period must be estimated. For [33]), the residence times decrease proportionally the earlier period, we assume the geochemical trends from 108 to 92 and 33 to 29 years. In contrast, incor- result from mixing an input magma of `early 20th rect, assumed Nb=Y ratios for the input magmas may century' composition (1918±1921 lava average [19]) yield residence times that are too short (if the true with an initial resident `early 19th century' magma ratios are more extreme). We evaluate this possibil- composition (1820 lava average [19]). For the later ity by recalculating the maximum magma residence period, we assume that an initial resident magma of times for each period using the total observed range `mid-20th century' composition (1961 lava [19]) was of Nb=Y ratios for Kilauea lavas (0.36±0.81 [34,35]) mixed with an input magma of `late 20th century' and a 10 wt% MgO parental magma: −m D 182 years composition (1990 Puu Oo rift zone lava average from 1820 to 1921 and −m D 43 years from 1961 to [18]). The input magma compositions are normal- 1982. Thus, an incorrect choice for the Nb=Y ratio of ized to 10 wt% MgO by olivine addition assuming the input magma is unlikely to explain the apparent complete incompatibility of Nb and Y in olivine decrease in magma residence time from the early (see [19] for details). This MgO content probably 19th±early 20th centuries to the late 20th century. represents a minimum for Kilauea's parental magma Our best estimate for the residence time of magma because the most ma®c, weakly olivine-phyric Puu in Kilauea's summit reservoir during the late 20th Oo rift zone lavas and some glasses from the 1959 century (¾30±40 years) is signi®cantly shorter than Kilauea Iki eruption (which are both thought to have a previous ¾100±120 year estimate for this pe- bypassed the summit reservoir) have ¾10 wt% MgO riod (based on geodetic data [2,9]). As expected, [18,31]. Thus, from 1820 to 1921 the residence time, our magma residence time is much shorter than the 318 A.J. Pietruszka, M.O. Garcia / Earth and Planetary Science Letters 167 (1999) 311±320

¾3000 year residence time for the entire magmatic in the residence time. It is not necessary to invoke a plumbing system of the volcano (based on geophysi- large (factor of 3) change in the volume of magma cal data [36]), and longer than the ¾10 year storage stored within the summit reservoir between the early time for the 1959 Kilauea Iki eruption lavas (based 19th±early 20th centuries and the late 20th century, on crystal size distribution [37]) which may have which is consistent with the relatively small varia- bypassed the summit reservoir [31,32]. No previous tions in the amount of magma storage since 1959 (a estimates for the residence time of magma in Ki- maximum of 0.4 km3 with little net change, based lauea's summit reservoir during the early 19th±early on geodetic data [38]). The limits on the volume of 20th centuries are available to compare with our magma in Kilauea's summit reservoir can be esti- result of ¾90±180 years. mated using the range of residence times for each period (as calculated above): 2.3±4.6 km3 from 1820 − 3 3.3. Magma volume estimate for Kilauea's summit to 1921 (from m D 92±182 years) and 1.7±2.6 km − reservoir from 1961 to 1982 (from m D 29±43 years). Thus, the magma volume during the early 19th to early Estimates of the magma supply rates for the early 20th centuries must have been less than ¾5km3 19th±early 20th centuries and the late 20th century (probably ¾2±3 km3 based on the shorter residence are required to calculate the volume of magma in Ki- times), whereas the magma volume during the late lauea's summit reservoir. The average magma supply 20th century was ¾2±3 km3 (a size that is well rate to Kilauea between 1959 and 1990 is estimated constrained from the full range of residence times). to be ¾0.06 km3=yr [38]. We use this value for the Our best estimate for the volume of magma in Ki- late 20th century. The best (and only) estimate of the lauea's summit reservoir since the early 19th century supply rate before 1959 comes from the average lava is ¾2±3 km3 (Fig. 2a), which is smaller than most eruption rate of the volcano, which was ½0.1 km3=yr estimates based on geophysical data (2±40 km3 [4, from 1823 to 1840 [39] and ¾0.009 km3=yr be- 9,10]). Since the residence time model assumes that tween 1840 and 1950 [38]. Over periods longer than the temporal geochemical trends result from mix- several tens of eruptions, Kilauea's eruption rate is ing compositionally distinct magmas within the vol- thought to be proportional to its magma supply rate cano's summit reservoir [14], the volume calculated [38]. Thus, the changes in Kilauea's eruption rate using this technique probably represents the hotter, since the early 19th century (Fig. 3) probably re¯ect molten core of the reservoir (i.e., the dominantly liq- variations in the volcano's supply rate [38,39], rather uid part of the system [29]). However, the short-term than a large difference in the amount of magma variations in lava chemistry at Kilauea's summit may storage. Assuming supply rates of 0.1 km3=yr from also re¯ect progressive, mantle-derived changes in 1823 to 1840 and 0.01 km3=yr from 1840 to 1924 the composition of the parental magma delivered to (Fig. 3), we calculate an average from 1823 to 1924 the volcano (e.g., ¯uctuations in the extent of partial of ¾0.025 km3=yr. This value (which we use for the melting and source composition within the Hawai- early 19th±late 20th centuries) is signi®cantly lower ian plume [19]), rather than simple magma mixing. than the supply rate during the late 20th century. A In this case, the residence time estimates, and thus, lower magma supply rate for this period is consistent the volume of magma in Kilauea's summit reservoir with the more evolved nature of Kilauea's summit for each time period, are strictly maximum values. lavas erupted during the late 19th century (as low as Therefore, a lack of magma mixing cannot explain 5.9 wt% MgO; M. Garcia, unpubl. data). the relatively small volume calculated from residence The volume of magma in Kilauea's summit reser- time analysis compared to the estimates based on voir, estimated from our preferred magma residence geophysical data. Instead, this difference can be ex- times and supply rates, is similar for both periods plained if the geophysical estimates also include por- (¾2±3 km3): 1820±1921 (0.025 km3=yr ð 108 years tions of the reservoir's outer crystal-mush zone and D 2.7 km3); and 1961±1982 (0.06 km3=yr ð 33 a hot, ductile region that surrounds the reservoir (the years D 2.0 km3). Thus, the inferred increase in the aseismic zone on Fig. 2a; [4,10]). Although our vol- magma supply rate accounts for most of the decrease ume estimate is small, the amount of magma stored A.J. Pietruszka, M.O. Garcia / Earth and Planetary Science Letters 167 (1999) 311±320 319 within Kilauea's summit reservoir since the early References 19th century (¾2±3 km3) is approximately an order of magnitude larger than the volume of the magma [1] J.P. Eaton, K.J. Murata, How volcanoes grow, Science 132 body supplying Piton de la Fournaise Volcano, an- (1960) 925±938. [2] R.W. Decker, Dynamics of Hawaiian volcanoes: an other frequently active ocean-island volcano [14]. overview, U.S. Geol. Surv. Prof. Pap. 1350 (1987) 997± 1018. [3] R.I. Tilling, J.J. Dvorak, Anatomy of a basaltic volcano, 4. Conclusions Nature 363 (1993) 125±133. [4] F.W. Klein, R.Y. Koyanagi, J.S. Nakata, W.R. Tanigawa, The seismicity of Kilauea's magma system, U.S. Geol. 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