Polybaric crystallization differentiation of H2O-saturated island arc low-K tholeiite magmas: a case study of the Izu-Oshima volcano in the Izu arc
© Hamada et al.; licensee Springer. 2014
Received: 24 September 2013
Accepted: 24 March 2014
Published: 22 April 2014
Island arc low-K tholeiites are basaltic magmas erupting from frontal arc volcanoes of juvenile arcs associated with the subduction of old and cold plates. We investigated the origins of geochemical variation in volcanic rocks having multiple phase saturated liquid compositions from the Izu-Oshima volcano in the northern Izu arc. The geochemical variations in the liquids fall between two endmember trends, namely higher- and lower-Al/Si trends. Polybaric differentiation of H2O-saturated melts between a 4-km-deep magma chamber and degassed melts near the surface should be responsible for the observed variation in the liquids.
With a worldwide average of 3 to 5 wt.% H2O, island arc magmas are characterized by higher volatile concentrations than magmas erupting from other tectonic settings with <1 wt.% H2O (e.g., Stern 2002; Plank et al. 2013). At generally ≥90 mol%, H2O is the most abundant volatile component dissolved in island arc magmas (Shinohara 2008), with the occasional exception of CO2-rich magmas (e.g., Sisson and Bronto 1998). The dissolved H2O in melts contributes to the diverse geochemistry of island arc magmas, ranging from subalkaline (low-K, medium-K, and high-K series) to alkaline magma series (Kuno 1960; Kuno 1966; Miyashiro 1974; Tatsumi and Eggins 1995). In most cases, the alkalinity, or the K2O content, of volcanic rocks increases across the arc away from the trench. Subalkaline magma series are represented in oceanic island arc settings, whereas high-K and alkaline magma series are more common in active continental margin tectonic settings. The spatial correlations between the geochemistry of these magma series and geophysical perspectives on magma generation have been discussed since being first reported by Kuno (1966) and Sugimura (1967), respectively.
Among the island arc magmas, low-K series rocks, known as island arc low-K tholeiite magmas, are infrequently found. This rock series occurs only in frontal arc volcanoes associated with the subduction of old and cold plates and/or early stages of arc volcanism (e.g., Ishizuka et al. 2006). Examples of such arcs include the Izu-Bonin-Mariana, northeastern Japan, Kurile, and Tonga-Kermadec, South Sandwich, Lesser Antilles, and Bismarck arcs. The geochemical features of island arc low-K tholeiites include lower concentrations of TiO2, NiO, and Cr2O3 and higher concentrations of K2O, Rb, Ba, Cs, Pb, and Sr than those in mid-ocean ridge basalts (MORBs) (e.g., Jakeš and White 1972; Masuda and Aoki 1978; Perfit et al. 1980). In addition, island arc low-K tholeiites are more radiogenic than MORBs (Jakeš and Gill 1970).
Although most arc magmas exhibit calc-alkaline differentiation trends (e.g., Gill 1981), island arc low-K tholeiites are characterized by a tholeiitic differentiation trend marked by Fe enrichment in the early stages of differentiation. This tholeiitic differentiation trend has been reproduced in melting experiments with anhydrous basalts at 0.1 MPa and on low-H2O (≤2 wt.%) basalts at low pressure (≤200 MPa; e.g., Grove and Baker 1984; Hamada and Fujii 2008; Tatsumi and Suzuki 2009; Zimmer et al. 2010), which is consistent with the low H2O (≤2 wt.%) recorded in melt inclusions (e.g., Kazahaya et al. 1994; Saito et al. 2005).
The H2O concentration of island arc low-K tholeiites has been debated during the last decade. For example, island arc low-K tholeiite is characterized by Ca-rich plagioclase (~An90) as phenocrysts (e.g., Ishikawa 1951; Amma-Miyasaka and Nakagawa 2002). Crystallization of Ca-rich plagioclase requires hydrous basaltic melts with ≥3 wt.% dissolved H2O (Sisson and Grove 1993; Takagi et al. 2005; Feig et al. 2006; Kuritani et al. 2014). Other compositional attributes such as Ca/Na and Al/Si ratios are also critical in crystallizing Ca-rich plagioclase from basaltic melts (Hamada and Fujii 2007). While some researchers have debated whether island arc low-K tholeiite magma is dry or wet, other researchers have stated that both dry and wet primary magmas can be generated beneath frontal arc volcanoes (Tamura et al. 2005; England and Katz 2010).
The concentration of dissolved H2O in pre-eruptive basaltic melts, particularly that of primitive or primary melts, provides information on the pressure and temperature (P-T) conditions of their generation, differentiation pathways, and potential explosivity during eruption. However, a consensus with regard to the H2O concentration of island arc low-K tholeiitic melts remains elusive. In this paper, we investigated the conditions of crystallization differentiation, particularly the dissolved H2O concentration in melts by considering previously reported chemical compositions for volcanic rocks from the Izu-Oshima volcano, a frontal arc volcano in the northern Izu arc, along with the results of hydrous melting experiments on relevant magmas to evaluate the dissolved H2O concentration in island arc low-K tholeiitic melts.
Geological overview of the Izu-Oshima volcano
The Izu-Oshima volcano is an active, frontal arc volcano located approximately 110 km SSW of Tokyo (34°44′ N, 139°24′ E) at the vent of the central scoria cone and has erupted low-K island arc tholeiite magmas throughout its history. The Izu-Oshima volcano includes three distinct stratigraphic units: the Senzu Group (>40,000 YBP), the pre-caldera Older Oshima Group (40,000 to 1,500 YBP), and the co- and post-caldera Younger Oshima Group (<1,500 YBP; Nakamura 1964; Isshiki 1984; Kawanabe 1991). During the past 1,500 years, the Younger Oshima Group has experienced 12 major eruptions with a total volume of erupted magma of >0.6 km3 dense rock equivalent (DRE; Nakamura 1964). Each eruption begins with scoria and ash falls, which are followed by lava flows. The most voluminous eruption occurred in 1777 with the emergence of the central cone of 1 km wide and 200 m high. The latest eruptions occurred in 1986 to 1987 and consisted of summit eruptions and fissure eruptions from two vents. The volume of eruptive products from the summit was 1.8 × 10−2 km3 DRE and that from the fissure vents was 3.4 × 10−2 km3 DRE (Endo et al. 1988). Continued inflation of the volcanic edifice since the 1986 to 1987 eruptions (Onizawa et al. 2013) and detection of volcanic CO2 gas in soil (Watanabe 2013) suggest that the magma accumulation rate beneath the Izu-Oshima volcano has recently accelerated as has the potential for eruptions in the near future.
Geochemistry of erupted rocks from the Izu-Oshima volcano
Aphyric lava flows with <5 vol.% phenocrysts and porphyritic lava flows with up to 20 vol.% plagioclase phenocrysts are scattered in drilling core samples from the Izu-Oshima volcano (Fujii et al. 1996; Okayama 2005). The apparent volume ratio of aphyric versus porphyritic lavas is approximately 1:1 in the drilling core samples of the Older Oshima Group (Fujii et al. 1996). During the post-caldera stage (the Younger Oshima Group), porphyritic lavas with accumulated plagioclase phenocrysts erupted at the early stage of each summit eruption and were followed by eruptions of aphyric lavas (Nakano and Yamamoto 1991). These observations suggest that plagioclase accumulated by floating in the basaltic melt during the dormant stage, which resulted in the formation of a porphyritic magma layer in the upper part of the magma chamber and its emission at the onset of each eruption (Aramaki and Fujii 1988).
Hydrous melting experiments on island arc low-K tholeiite magmas
Comparison of the geochemical variations in the liquids from the Izu-Oshima volcano (Figure 3) with those in the experimental results (Figure 5) suggests that the higher- and lower-Al/Si trends can be reproduced under more and less hydrous conditions, respectively. Figure 6 also demonstrates that the higher-Al/Si trend was derived at a higher pressure less than 200 MPa than the lower-Al/Si trend at a lower pressure.
Olivine, a minor but common mineral occurring in volcanic rocks from the Izu-Oshima volcano, was not crystallized from either MA43 or MA44 melts at 250 MPa or at 0.1 MPa (Hamada 2002). This result proves that the olivine was not actually in equilibrium with the MA43 and MA44 melts, which should be situated on the augite-pigeonite-plagioclase cotectic in the basalt tetrahedron rather than in the primary field of olivine. The olivine may have been crystallized from a less differentiated melts (MgO ≥ 6 wt.%; Ikehata et al. 2010) and incorporated into slightly differentiated magmas such as MA43 and MA44 (MgO approximately 5 wt.%).
Results and discussion
Implications for the origin of the lower-K subgroup liquids
Three types of magma have been identified in island arc low-K tholeiites from the Izu-Oshima volcano: (i) a lower-K subgroup, (ii) higher-Al/Si trend of a higher-K subgroup, and (iii) lower-Al/Si of a higher-K subgroup. The lower-K and higher-K subgroups exhibit distinct differences in their trends at a given MgO content (Figure 2), which indicates that the primary magmas of these two subgroups are different.
The trend observed for the lower-K subgroup with a nearly constant K2O content and a decreasing MgO content or increasing SiO2 content is enigmatic. The constant K2O content may be explained by fractionation of amphibole (Davidson et al. 2013) because K is a compatible element of this mineral (Tiepolo et al. 2007). Amphibole can be crystallized from basaltic melts and basaltic andesite melts as a near-liquidus phase under high H2O conditions (≥5 wt.%; Adam et al. 2007; Almeev et al. 2013) and from andesites under lower H2O conditions (approximately 4 wt.%; Eggler and Burnham 1973), although no direct evidence exists to prove that crystallization of amphibole occurred beneath the Izu-Oshima volcano. Geochemical trends similar to those of the lower-K subgroup have been termed ‘low SiO2 group’ at the Iwate volcano in the northeastern Japan arc (Nakagawa 1993), which suggests that this cryptic trend is actually a ubiquitous feature of island arc low-K tholeiite magmas.
Two endmember trends resulting from polybaric crystallization
Two endmember trends, the higher- and lower-Al/Si trends, were identified in the geochemical variation of liquids (Figure 3); all of the plotted liquid compositions can be explained either by the mixing of these two endmember trends or by differentiation at intermediate conditions between those responsible for the endmembers. The higher-Al/Si trend is characterized by suppressed enrichment of SiO2 and FeO* and delayed decreases in Al2O3 and the Al2O3/SiO2 ratio with decreasing MgO compared with those of the lower-Al/Si trend. Results of the hydrous melting experiments on IAT60, described in Section ‘Hydrous melting experiments on island arc low-K tholeiite magmas’, show that the two endmember trends were derived from crystallization differentiation under different H2O conditions and at different depths. The higher-Al/Si trend can be explained by crystallization differentiation under more hydrous conditions and at higher pressures less than 200 MPa. The lower-Al/Si trend can be explained by less anhydrous conditions and at lower pressure.
Ca-rich plagioclase (An≥90) can be crystallized from moderately hydrous melts of the higher-Al/Si trend with ≥3 wt.% H2O but cannot be crystallized from melts of the lower-Al/Si trend at any H2O concentration. In contrast, Ca-poor plagioclase rims (~An75) cannot be crystallized from melts of the higher-Al/Si trend and were likely crystallized from the melts of the lower-Al/Si trend under H2O-poor conditions (Figures 3 and 7). These experimental constraints, reported by Hamada and Fujii (2007), demonstrate that higher Al/Si ratio and H2O content in melts are critical for crystallizing Ca-rich plagioclase.
The H2O concentration can vary between liquids of higher- and lower-Al/Si trends, characterized by higher H2O and higher pressure and by lower H2O and lower pressure, respectively. Beneath the Izu-Oshima volcano, 4-km-deep magma chamber and 8-10-km-deep magma chamber were detected as seismic scatters (Mikada et al. 1997). This geophysical constraint is consistent with the estimated pressure at which multiple phase saturated liquids differentiate, which is less than 200 MPa or shallower than the 8-10-km-deep magma chamber, as determined by using the pseudo-ternary diagram as a geobarometer (Figure 6). Hamada et al. (Hamada et al. 2011; Hamada et al. 2013) demonstrated that the melts beneath the Izu-Oshima volcano dissolved >5 wt.% H2O at the 8-10-km-deep magma chamber, which resulted in the interpretation that the H2O concentration in the melts at depths shallower than the 8-10-km-deep magma chamber were controlled by the solubility of H2O as a function of pressure. Using MA43 as an example and assuming 1,100°C, if pre-eruptive liquids dissolve approximately 3 wt.% H2O, they become saturated with H2O at pressures of 60 MPa (Papale et al. 2006). With upper crustal densities in the range of 2,200 kg m−3 for volcanic edifices to 2,600 kg m−3 for basement rocks and constraints of the seismic velocity structure in crust beneath the Izu-Oshima volcano (Onizawa et al. 2002), these pressures would equate to the onset of H2O saturation at depths of approximately 3 km. Geochemical arguments developed herein suggest that the erupted liquids are final products of H2O-saturated crystallization differentiation between the geophysically imaged 4-km-deep magma chamber and the surface.
Although differentiation processes in the 8-10-km-deep magma chamber are not clearly constrained by using the composition of liquids including aphyric lavas and groundmasses of porphyritic lavas, they may have yielded the non-primitive liquids (6 ≤ MgO ≤ 8 wt.%) detected in the olivine-hosted melt inclusions (Ikehata et al. 2010) and crystallized Ca-rich plagioclase (Hamada et al. 20112013).
By definition, island arc low-K tholeiites are characterized by a tholeiitic differentiation trend. The origin of the tholeiitic versus calc-alkaline differentiation trend is essentially controlled by the H2O concentration in melts. The tholeiitic differentiation trend can be reproduced under low H2O (≤2 wt.%) conditions (e.g., Grove and Baker 1984; Hamada and Fujii 2008; Tatsumi and Suzuki 2009; Zimmer et al. 2010). Consistent with such conditions, the multiple phase saturation point for the MA44 was approximately 1,150°C and approximately 2 wt.% H2O (Figure 7c), where Ca-poor plagioclase (An75), augite, and pigeonite co-crystallize as liquidus phases; this temperature is also consistent with the estimated temperature of the basaltic magmas that erupted in 1986 based on pyroxene geothermometry (Fujii et al. 1988). The tholeiitic differentiation trend observed for the Izu-Oshima volcano may have been controlled by the lower-Al/Si trend, which was reproduced under low H2O conditions (≤2 wt.% H2O) in the melting experiments. Such low H2O conditions can be explained by degassing of magma at a low pressure (≤40 MPa assuming 1,100°C; Papale et al. 2006). We inferred that smaller amounts of melts with ≥3 wt.% H2O of the higher-Al/Si trend, including Ca-rich plagioclase, ascended primarily from the 4-km-deep magma chamber and also from the 8-10-km-deep magma chamber (Hamada et al. 2011) before injecting into shallower, low-H2O magmas of the lower-Al/Si trend. The geochemical variations in the liquids, shown in Figure 3, can be interpreted either as the mixing of liquids of these two endmember trends or by differentiation at intermediate depths between those responsible for the endmember trends throughout the eruptive history of the Izu-Oshima volcano.
The origins of geochemical variations in the island arc low-K tholeiites from the Izu-Oshima volcano were investigated using the liquid compositions obtained from aphyric rocks and groundmasses of porphyritic rocks in addition to the results of hydrous melting experiments. Three types of liquids were distinguished using geochemical data from volcanic rocks: (i) a lower-K subgroup, (ii) higher-Al/Si trend of a higher-K subgroup, and (iii) lower-Al/Si trend of a higher-K subgroup. Fractionation of amphibole may have been responsible for the lower-K subgroup, although its origin remains unknown. For liquids of the higher-K subgroup, higher- and lower-Al/Si trends were identified as endmember trends. Geochemical variations in the higher-K subgroup liquids can be explained either by mixing of these two endmember trends or by differentiation at intermediate depths between those of the endmember trends. By applying the results of melting experiments on hydrous basalts, the higher- and lower-Al/Si trends were reproduced by upper crustal crystallization differentiation of H2O-saturated magmas in approximately 4-km-deep magma chamber (moderately hydrous melts with approximately 3 wt.% H2O) and near the surface (nearly degassed melts), respectively. Such polybaric crystallization of H2O-saturated magmas should be a ubiquitous feature of island arc low-K tholeiites. Ca-rich plagioclase (An≥90), commonly found in island arc low-K tholeiites, can be crystallized from moderately hydrous melts of the higher-Al/Si trend but not from melts of the lower-Al/Si trend at any H2O concentration.
MH is a scientist at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). He earned his Ph.D. in igneous petrology at the University of Tokyo in 2006. He studies the role of water in differentiation and eruption of arc basaltic magmas by conducting hydrous melting experiments and analyses of melt inclusions. His current research focuses on trace amounts of hydrogen accommodated in plagioclase as a tracer of H2O in arc basaltic magmas. YO studied the evolution of magma of the Izu-Oshima volcano throughout its eruptive history as a master’s course graduate student at the University of Tokyo during 2003 to 2005. She provided the geochemical data for volcanic rocks from the Izu-Oshima volcano used in this paper. She is currently a science communicator at the National Museum of Emerging Science and Innovation in Tokyo. TK and AY are staff scientists in igneous petrology and volcanology at the Earthquake Research Institute, University of Tokyo and analyzed the drilled volcanic rocks from Izu-Oshima volcano. TF is an igneous petrologist and volcanologist who is currently a professor emeritus of the University of Tokyo. He also provided the geochemical data for volcanic rocks from the Izu-Oshima volcano used in this paper. He has provided valuable discussion and advice to MH for more than 15 years on the geochemical evolution of magmas beneath the Izu-Oshima volcano.
We thank professors Eiichi Takahashi, Jun-Ichi Kimura, and Jon Blundy for their discussion and Professor Yasuo Ogawa and Dr. Tatsuhiko Kawamoto for their editorial handling. This manuscript has been significantly improved by critical reviews and encouragement from two anonymous reviewers. This study was partially supported by KAKENHI (Grant-in-Aid for Young Scientists (B) No. 24740355 to MH) from the Japan Society for the Promotion of Science.
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