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Geochemical Journal, Vol. 45, pp. 157 to 167, 2011 INVITED REVIEW Petit-spot volcanism: A new type of volcanic zone discovered near a trench NAOTO HIRANO* Center for Northeast Asian Studies, Tohoku University, Kawauchi 41, Aoba-ku, Sendai 980-8576, Japan (Received March 23, 2010; Accepted December 8, 2010) One extremely young volcano (0.05–1 Ma) and other young volcanoes (1.8, 4.2, 6.0, and 8.5 Ma) composed of strongly alkaline magma were recently discovered on the abyssal plain of the Early Cretaceous (135 Ma) Pacific Plate. These volcanoes were dubbed “petit-spots”. The petit-spot volcanic province represents more than 8 Myr of activity over a large area (~600 km along the direction of plate motion), but with a relatively small volume of magma production, thus indicating a small supply of heat inconsistent with a hotspot. The low-flux petit-spot volcanoes may be related to the occurrence of a tensional field of lithosphere caused by plate flexure, with the ascending melt derived from a mantle source susceptible to partial melting. Rock samples from the young volcanoes are highly vesicular (up to 60%) despite high hydrostatic pressures at 6000 m water depth, indicating volatile-rich magmas. The depleted heavy rare earth elements and high radiometric isotopic ratios of noble gases indicate the magma was derived from upper mantle. Nevertheless, the low 143Nd/ 144Nd, high 87Sr/ 86Sr, low 206 Pb/ 204Pb, and low 207Pb/204 Pb ratios are similar to enriched or fertile compositions such as oceanic island basalts. These apparently conflicting data are explained by the extremely small degree of partial melting of recycled materials in the degassing mantle of the asthenosphere, probably with carbonate in the source. The petit-spot volcanoes, therefore, provide a unique window into the nature of the oceanic plate and underlying asthenosphere prior to subduction. Keywords: petit-spot, volcano, Pacific Plate, volatiles, basalt et al., 2001, 2006) has revealed a new type of volcanism, different from the three types mentioned above. Monogenetic petit-spot volcanoes located near the Japan Trench (Site A in Fig. 1) are less than 2 km in diameter and yield ages of 1.8, 4.2, 6.0, and 8.5 Ma, indicating the episodic eruption of magma over a period involving 600 km of plate motion, without any systematic spatial trend in age such that seen along oceanic island/seamount chains moving over a hotspot (Hirano et al., 2001, 2006, 2008). Moreover, these volcanoes represent 8 million years of activity over a large eruption area but with low volumes of magma production, indicating that this geological setting is distinct from that of hotspot volcanoes on the present-day Pacific Plate. Hirano et al. (2006) suggested that petit-spot magma originates within the asthenosphere, without the development of a hotspot, based on geological/geochronological grounds and geochemical data including noble gas isotope data and the abundance of heavy rare earth elements (HREEs). Hofmann and Hart (2007) noted that this type of small-volume, within-plate volcanism is not completely unknown, because previous studies have described alkaline volcanism that is unrelated to mantle plumes or hotspots (e.g., off-ridge seamounts: Batiza and Vanko, 1984; Cameroon Line: Fitton and Dunlop, 1985). How- INTRODUCTION Chemical and isotopic variability in the mantle is quantified by analyzing lavas from multiple sources collected at oceanic island volcanoes and submarine volcanoes. However, volcanoes occur only locally within the ocean, with three types recognized based on their magmatic system and tectonic setting: (1) those at divergent plate boundaries (mid-ocean ridges, MOR), (2) those at convergent plate boundaries (e.g., island arcs), and (3) those within plates (e.g., hotspots). MOR lavas are the only type to provide a window into the geochemistry of the shallow mantle (i.e., asthenosphere); the geochemistry of island arc and hotspot lavas reflects the input of components from the subducted slab and mantle plume, respectively. However, MOR lavas and their source mantle might show a compositional bias toward the asthenosphere, because the distribution of MOR is restricted to below newly producted lithosphere on Earth’s surface. The recent discovery of petit-spot volcanoes (Hirano *E-mail address: [email protected] Copyright © 2011 by The Geochemical Society of Japan. 157 Kuril Trench Pacific Plate ~10 cm/yr. Site A outer rise Japan Site B Japan Trench Site C ts n un a o b Jo eam S J Se apa am ne Izuou se nts Ogasawara Trench –10000 –8000 –6000 Shtsky Rise –4000 Depth (m) –2000 0 Fig. 1. Bathymetric map of the NW Pacific Plate off NE Japan, based on data from Amante and Eakins (2009). The three petitspot volcanic fields are known as Sites A, B, and C. Black stars show the sites of petit-spot volcanoes from which samples have been retrieved. The circle indicates an area containing more than 80 potential petit-spots at Site C. The thick arrow represents the present-day constant plate motion (approximately 10 cm/yr; Gripp and Gordon, 2002). The thin arrow shows the possible eruption range of 0–8.5 Ma volcanoes. ever, the formation of such young volcanoes upon old, cold oceanic crust is uncommon, and none of these volcanoes described in the previous studies formed at sites located far from spreading centers, hotspots, or, more generally, areas of thermal upwelling (Hirano and Koppers, 2007). The petit-spot magmas, therefore, could represent the first discovery of melting product transported directly to the surface from the asthenosphere below an old plate prior to subduction. Early in the theory of plate tectonics, conditions at the boundary between the lithosphere and underlying asthenosphere were thought to be suitable for melting, as 158 N. Hirano indicated by observations of a seismic low-velocity zone, a layer with high electrical conductivity, and the experimentally determined peridotite solidus in the presence of H2O and CO2 (e.g., Wyllie, 1988). However, the possibility of melting in the asthenosphere has remained a topic of controversy since Karato (1990) and Karato and Jung (1998) proposed that the presence of small amounts of water in olivine could explain the above geophysical observations and may control the rheology of the mantle; consequently, they argued that the presence of a partial melt is unnecessary. On the other hand, laboratory measurements of Vs (shear-wave velocity) and Qs (shear-wave Site A Site B understandings of this newly discovered magmatism at the bottom of the ocean. Thus, the final section of this article will discuss possible future direction of the research associated with the petit-spot volcanoes. outer-rise Japan Trench Fig. 2. Model of the formation of petit-spot volcanoes, modified after Hirano et al. (2006, 2008). attenuation) in mantle material have recently been performed to investigate the existence of partial melt in the asthenosphere as a function of temperature and pressure (e.g., Faul and Jackson, 2005). Moreover, Mierdel et al. (2007) concluded that melting must occur in the asthenosphere because the minimum capacity of water in orthopyroxene coincides with the depth of the lowvelocity zone in the asthenosphere. The results of experimental studies have provided support for several incompatible processes proposed to explain the high electrical conductivity of the asthenosphere (Karato, 1990; Wang et al., 2006; Yoshino et al., 2006, 2009). Therefore, the presence of melt in the asthenosphere remains a topic of debate. The newly discovered petit-spot volcanoes may provide evidence relevant to the question of whether melting occurs in the asthenosphere, as the magma that feeds such volcanoes may escape to the surface from the asthenosphere along fractures caused by bending of the lithosphere (Fig. 2). If this interpretation is correct, then it is noteworthy that such volcanoes are ubiquitous on flexed and fractured ocean floor, and that acoustic reflective data indicate the possible presence of petit-spot volcanoes at another site off the southern Japan Trench (i.e., possibly more than 80 volcanoes are located close to Site C in Fig. 1), as well as on the oceanward slope of the Tonga Trench (Hirano et al., 2008). Although it is clear that the surface morphology and distribution of petit-spot volcanoes on the NW Pacific Plate are influenced by cracks in the lithospheric that reach the surface, it remains uncertain whether petit-spot volcanoes form wherever oceanic plate is flexed and fractured. An important objective of this contribution is to review current state of our understanding on the origin and mechanisim of formation of the newly recognized type of magmatism based mainly on our multidisciplinary studies of petit-spots and the subducting Pacific Plate. It will be shown that there are a lot of missing pieces for full THE DISCOVERY OF PETIT-S POT VOLCANOES The discovery of a sample of young alkali basalt (ca. 6 Ma; Hirano et al., 2001), collected during dive 10K#56 by ROV KAIKO (of the Japan Agency for Marine-Earth Science and Technology, JAMSTEC) in the Japan Trench, was unexpected because nobody had anticipated such volcanism on the Cretaceous Pacific Plate. The dive revealed continuous outcrops and collapsed angular blocks of basalt for 50 m along a steep cliff on the oceanward slope of the trench (Kawamura, 2007). To explore these young volcanoes, extensive surveys were carried out from 2003 to 2005 using the research ships R/V Kairei and Yokosuka, and the submersible Shinkai6500 of JAMSTEC. High-resolution acoustic multibeam surveys of the ocean floor are required to detect these volcanoes because they are only 1–2 km in diameter and only several hundred meters in height (Hirano et al., 2006). It has been shown that the acoustic reflectivity of a petit-spot is more than three times as high as that of the surrounding abyssal plain on the old plate (Hirano et al., 2008). In the present case, areas of high acoustic reflectivity are probably hard ocean floor, covered with only a thin layer of soft pelagic sediment, much thinner than the surrounding pelagic sediment layer on the old Pacific Plate. Following sections will explore the geological and lithological overviews about the petit-spot volcanoes and rock samples recovered from them. Geology and lithology The volume of each petit-spot knoll is less than 1 km3 (Hirano et al., 2008), and the many knolls at Site A are aligned, defining three lines of WNW to ESE. The alignment of monogenetic volcanoes has generally been explained in terms of the maximum horizontal compression (Koyama and Umino, 1991) or upward bending of the plate (Natland, 1980; Ozawa et al., 2005). Rock samples were recovered by dredges and submersible dives from several “spotty” areas at Sites A and B that show high acoustic reflectivity (Figs. 3A and B) (Hirano et al., 2006). Most of the rocks are highly vesicular, and specimens with quench features are associated with lava lobes and breccias within the pelagic sediments. Volcanic cones and ridges at Site A can be distinguished from the surrounding seafloor based on bathymetric and acoustic reflectivity data, where the slope is characterized by trench-parallel normal faults (horsts and graben) resulting from extensional bending of the subducting Pacific Plate, and where the cliffs generally Petit-spot volcanism 159 indicate 100–500 m of vertical displacement upon the faults (Ogawa et al., 1997). An important difference between the area with trench-parallel normal faults and other parts of the subducting plate is the presence of hummocky structures within the former area (Site A in Fig. 3A) (Hirano et al., 2001). The volcanic features consist of moderately or highly vesicular lavas (~10–60 vol% vesicles). Elsewhere, lavas recovered from along the fault outcrops below the ridges are generally less vesicular (0– 20 vol%). Erosion of sediments by the strong bottom currents in the trench (Owens and Warren, 2001), together with localized normal faulting, has exposed the bases of the volcanoes, which consist of non-vesicular sheet-flow lavas. The sequences and lithologies at Site B represent only the exposures at the top of volcanic cones, and typically represent just one eruptive phase. These rocks include highly vesicular pillow lava, water-chilled bombs, hyaloclastite, peperite, and contact-metamorphosed mud. Petrology and geochemistry Notable and obvious feature of the specimens recovered from Sites A and B are significant freshness of the samples (even the interior surfaces of vesicles and olivine crystals are unaltered). This feature is in clear contrast with lavas obtained from Cretaceous seamounts (Koppers et al., 2000; Awaji et al., 2004). This is because most of the Cretaceous volcanic rocks have undergone a long period of submarine alteration. Large olivine crystals within the analyzed samples have been interpreted as xenocrysts from the mantle, from as deep as 14 km (Hirano et al., 2004). In fact, the petit-spot lavas contain several kinds of xenoliths and xenocrysts 2–100 mm in diameter, representing material brought up from the oceanic crust and the upper mantle, including dolerite, olivine gabbro, and peridotite (Hirano et al., 2010; Yamamoto et al., 2009). XRF analyses of major elements in whole-rock petitspot lavas, together with electron microprobe analyses of quenched glass, show that the rocks can be classified as potassium-rich shoshonite, trachybasalt, basanite, and sodium-rich basanite (SiO 2 = 45–54 wt%, total alkalis = 5–9 wt%). MgO contents range from 4 to 14 wt%, with the Mg# (100 × Mg/[Mg + Fe]) in the range 47–70 (Fig. 4). The rare earth element (REE) contents of quenched glasses (measured using ICP-MS with the ArF Excimer laser ablation system) and bulk compositions (measured using ICP-MS) show a depletion in HREEs, as represented by high [La/Yb] CN values (La/Yb normalized by chondrite) of 19–40 (Fig. 5). AGE DETERMINATIONS Determining the age of petit-spot eruptions is important in gaining an understanding of the tectonic setting of 160 N. Hirano the eruption site and temporal variations in the magmatic process. During submersible dives over petit-spots at Site B, water-chilled bombs, covered with only a thin layer of pelagic sediment, were observed scattered on the seafloor around the volcanic cones. This finding suggests a recent eruption (Hirano et al., 2006), as also indicated by the thickness of palagonite in quenched glass rims (1–4 mm; Hirano et al., 2006), which is estimated to have formed via alteration in the period from 0.05 to 1 Ma (0.003– 0.02 µm/yr: Moore et al., 1985). At Site A, non-quenched, crystalline rocks from the interior of the volcano were exposed by strong bottom currents and were easily sampled in areas that contained good outcrops of basement lava along the horst and graben fault structures in the trench (Hirano et al., 2006). In contrast, the three petit-spot volcanoes at Site B are only exposed along the top, quenched parts of the volcano, showing eruptive products such as hyaloclastite, peperite, volcanic bombs, volcaniclastic rocks, and pillow lavas, which are all composed mainly of glasses and the alteration product palagonite (Hirano et al., 2006; Fujiwara et al., 2007), with intersertal texture. Such materials are unsuitable for Ar–Ar age dating because in amorphous material, when 39 Ar is created from 39K by a fast neutron in the nuclear reactor during irradiation of the sample, the 39Ar may move from one phase to another in a process called “recoil”, creating a disturbed age spectrum (McDougall and Harrison, 1988). Accordingly, Ar–Ar methods for dating petit-spot volcanics were only successful for the holocrystalline rocks at Site A. This approach was also supported by Iwata and Kaneoka (2000), who concluded that fresh, holocrystalline rocks should be analyzed for precise Ar–Ar dating. Base on above notion, four holocrystalline rock samples collected from petit-spot volcanics at Site A have been subjected to age determination by the Ar–Ar method. As shown in Fig. 6, the Ar–Ar method yield ages ranging from 1.8 to 8.5 Ma. The data for sample 6K#880-R3B (Fig. 6) indicate an age of 1.76 ± 0.58 Ma, based on the inverse isochron. This age is clearly younger than the plateau age, reflecting excess Ar of 340 ± 21 for the initial 40 Ar/ 36Ar ratio for the inverse isochron, which is higher than atmospheric values (approximately 296). The three other samples show atmospheric values in the initial 40Ar/ 36 Ar ratio for the inverse isochron, along with successFig. 4. Major element compositions of petit-spot lavas, modified after Valentine and Hirano (2010). The ages in the SiO2 vs. Na2O + K 2O diagram are based on Ar–Ar data from representative samples of each volcano (color-coded). The different trends obtained for each volcano in the SiO2 vs. Mg# diagram show the variable degrees of fractionation of each petit-spot magma (i.e., fractionation occurred independently at each volcano). A) JapanTrench axis –7500 –7000 –6500 –6000 –5500 depth (m) 465.387 –5000 0 5 1563.920 5419.260 10 km B) 140 –6000 –5800 –5600 –5400 –5200 –5000 –4800 –4600 depth (m) 0 10 300 350 400 450 500 600 700 800 1000 8700 20 km Fig. 3. Maps showing the bathymetry (left-hand panels) and acoustic reflectivity (right-hand panels) obtained from on-board multibeam surveys at Sites A and B (see Fig. 1). Yellow lines in upper panels A) of Site A and arrows in lower panels B) of Site B indicate sites sampled by dredges and dives during 2003–2005 (Hirano et al., 2006). Site A Site B KR03-07 D1, 10K#056 6K#878 KR03-07 D2 KR04-08 D07 & 08 KR04-08 D02 6K#880 R1,R2,R5 6K#880 R3 REE normalized to concentration in chondrite 1000 Na2O + K2O (wt%) 9 8 4.2 Ma 7 0~1 Ma 1.8 Ma 6 8.5 Ma 6.0 Ma 5 4 70 10 1 La Ce Pr Nd 65 Mg # 100 60 Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Fig. 5. Chondrite-normalized rare earth element (REE) patterns. The data are from Hirano et al. (2006). REE abundances are normalized to C1 chondrite values (Sun and McDonough, 1989). Color legends are the same as in Fig. 4. 55 50 45 45 46 47 48 49 50 51 52 53 54 SiO2 (wt%) Fig. 4. Petit-spot volcanism 161 10K#056 R002 Site A; Japan Trench (fault wall) 0.004 0.004 no isochron 0.003 0.003 36Ar/40Ar 36Ar/40Ar 0.002 0.002 Isochron Age: 5.69 ± 0.43 Ma N = 8 (all fractions) (40Ar/36Ar)t = 0: 304.9 ± 9.4 MSWD: 1.1 0.001 0 0 30 0.439Ar/40Ar 0.6 0.2 0.001 0 0 30 0.8 20 Age (Ma) 5.95 ± 0.31 Ma Plateau Age: Released 39Ar : 94% (n = 4) 10 0 0.004 0.439Ar/40Ar 0.6 0.8 50 % 39Ar cumulative 100 6K#880-R3B Site A; Japan Trench (fault wall) 0 0.004 0.003 0.003 36Ar/40Ar 0.002 0 1.76 ±1000 0.58 Ma C) N = 7 (650 (40Ar/36Ar)t = 0: 340 ± 21 0 MSWD: 0.15 0 0.4 30 0.839Ar/40Ar 1.2 20 0 0 30 20 Age (Ma) 0 50 % 39Ar cumulative 0.2 0.439Ar/40Ar 0.6 0.8 8.53 ± 0.18 Ma Age (Ma) Plateau Age: 2.54 ± 0.76 Ma Released 39Ar : 83% (n = 8) 10 100 Isochron Age: 8.3 ± 1.2 Ma N = 5 (860 1200 C) (40Ar/36Ar)t = 0: 302 ± 24 MSWD: 10.7 0.001 1.6 50 % 39Ar cumulative KR04-08 D02-009 Site A; Japan Trench (fault wall) 0.002 Isochron Age: 0.001 4.23 ± 0.19 Ma Plateau Age: Released 39Ar : 89% (n = 3) 10 36Ar/40Ar 0 0.2 20 Age (Ma) 0 KR03-07 D2-211 Site A; Japan Trench (knoll) Plateau Age: Released 39Ar : 91% (n = 6) 10 100 0 0 50 % 39Ar cumulative 100 Fig. 6. Results of Ar–Ar dating of petit-spot samples. All the samples, namely 10K#056 R002 (Hirano et al., 2001), KR03-07 D2211, KR04-08 D02-009 (Hirano et al., 2006), and 6K#880-R3B (Hirano et al., 2008), are from Site A. Analytical methods were adopted from Saito et al. (1991) and Ebisawa et al. (2004). fully obtained plateau ages (Fig. 6). Radiometric dating and geochronological analysis of several monogenetic petit-spot volcanoes at Sites A and B reveal that the volcanoes erupted independently from each other, from different sources, possibly as a result of squeezing out of magma during the period 0–8.5 Ma (Hirano et al., 2001, 2006, 2008). As mentioned above, these dates are geologically important in recording petitspot activity during a period involving a total plate motion of 600 km, which is inconsistent with the occurrence 162 N. Hirano of a single hotspot on the Pacific Plate. If the asthenosphere contains a zone of partial melt, then the process of petit-spot formation may potentially occur anywhere on the ocean floor, triggered by tectonic events that enable the melt to ascend from the mantle. Thus, as will be further discussed in the next section, it is anticipated that the occurrence of petit-spot volcanoes might be controlled by the condition of the lithospheric part of the ocean directly above the zone of possible melt production. hinge of outer rise trench oceanward slope subdu upper lithosphere's stress field in concave-warping petit-spots ction pelagic sediments oceanic crust lithospheric mantle asthenospheric mantle not to scale lower lithosphere's stress field in concave-warping Fig. 7. Model of ascending dikes in concavely warped lithosphere (modified after Valentine and Hirano, 2010; Hirano et al., 2006, 2008). White arrows and white dotted lines show the tensional field at the base of lithosphere, under the petitspot. The magma accumulates and ascends due to tension in the lower lithosphere and subjacent asthenosphere that reflects concave warping of the plate; otherwise, the dike ascends through the upper lithosphere, entraining xenoliths, and experiences a 90° rotation in σ 3 when passing through the middle lithosphere (black ellipses). Consequently, the petit-spot volcanoes are aligned perpendicular to the hinge line of the flexure in the outer rise. ROLE OF LITHOSPHERIC FLEXURE Old, cold lithosphere behaves elastically, and it may be deformed or flexed as a result of ocean-island or seamount loading, or flexed as the plate enters a subduction zone (McAdoo and Martin, 1984; Watts and Zhong, 2000). The NW Pacific Plate, close to the Japan Trench, rises 300–800 m higher than predicted in models of plate subsidence (Parsons and Sclater, 1977; Stein and Stein, 1992; Carlson and Johnson, 1994). This area is called the outer rise, representing a convex flexing that occurs during the initiation of plate subduction and that yields a positive gravity anomaly (Sandwell and Smith, 1997). The petit-spot magmas are likely to rise to the surface along fractures oriented perpendicular to the direction of the maximum horizontal compression, controlled by the stress field in the down-warping Pacific Plate at the eastern edge of the outer rise. Of note, the WNW-ESE alignment of volcanic cones at Site A is essentially perpendicular to the hinge line of the bending plate at the outer rise (Hirano et al., 2006). Valentine and Hirano (2010) suggested that the tensional field in the mantle results in the accumulation and ascent of melt, producing low eruption-flux volcanoes within intra-plate volcanic provinces, because regional tectonic deformation controls the way in which partial melts accumulate. Regarding the NW Pacific petit-spots, flexuring of the plate creates tension in the lower lithosphere and subjacent asthenosphere as a result of concave warping of the plate (Hirano et al., 2006, 2008), whereas regional tectonics results in the formation of dikes that ascend under the eruption sites and experience a 90° rotation of σ 3 in the middle lithosphere (Fig. 7). The base of the lithosphere, therefore, is extended so that σ3 is perpendicular to the flexure axis, while in the upper lithosphere, the least compressive horizontal stress is parallel to the flexure axis. This stress rotation may cause many of the ascending dikes to stall in the lithosphere (Valentine and Hirano, 2010) (Fig. 7), one result being the observed variable degree of fractionation in the volcanic products (Fig. 4). Dikes that propagate upward from these temporary reservoirs are oriented perpendicular to the least horizontal compressive stress in the upper lithosphere, perpendicular to the flexure axis (Hirano et al., 2006; Valentine and Hirano, 2010). Petit-spot volcanoes may be ubiquitous on the ocean floor, with magma being squeezed upward by tectonic forces associated with plate flexure if the source mantle is susceptible to partial melting. For example, potential petit-spot volcanoes have been identified on the oceanward slope of the Tonga Trench, where horst and graben structures are developed in association with subduction of the southern Pacific Plate (Hirano et al., 2008). Ranero et al. (2005) produced bathymetric maps near the Chile Trench, showing tiny knoll-like petit-spots on the subducting plate (the authors anticipate hotspot-related volcanoes from bathymetry). The above features may represent good examples of fracture-related petit-spot volcanism. Pending more detailed rock sampling on the oceanward slopes of trenches and further age dating of lavas, it is suggested that petit-spot volcanic activity may prove to be a ubiquitous phenomenon on the flexed parts of all subducting tectonic plates. Fissures that form within outer rises appear to provide the mechanism for asthenospheric melts to escape to the surface, forming petit-spot volcanoes. A MANTLE SOURCE FOR PETIT-SPOT MAGMA AND AN ISOTOPIC P ARADOX As noted earier, the depleted HREEs in petit-spot lavas indicates that the magma was derived from garnetbearing mantle, suggesting a source deeper than 90 km (Arth, 1976), possibly corresponding to the low-velocity zone (asthenosphere) below the base of the lithosphere, as detected by seismography. Previous studies have interpreted the presence of interstitial magma in the asthenosphere (Lambert and Wyllie, 1968; Mierdel et al., 2007); therefore, the asthenosphere is a potential source of petit-spot magmas (Hirano et al., 2001, 2006). Petit-spot volcanism 163 Isotopic ratios in petit-spot lavas provide further information on the source region in the mantle. Hirano et al. (2006) examined the isotopes of noble gases because they reveal whether the magma originated from the deep mantle or shallow mantle. MORB and oceanic island basalt (OIB) are generally regarded to be derived from depleted mantle and primordial mantle, respectively (e.g., Kaneoka, 2000; Ozima and Podosek, 2002). Secondary nuclides are expected to differentiate most clearly the nature of the mantle source. Thus, 21Ne/ 22Ne and 40Ar/ 36 Ar ratios in MORBs are comparatively higher than those in OIBs, due to the addition of radiogenic 21Ne and 40Ar from the radioactive decay in a source depleted in nonradiogenic isotopes ( 22 Ne and 36 Ar). The isotope geochemistry of petit-spot lavas examined in the present study was determined after extracting the gases from quenched glass by applying the vacuum crushing method to glass rind in samples collected from Site B (Hirano et al., 2006). This approach was taken because the magmatic gas would have been trapped in the glass when the magma was quenched during eruption. The low 21Ne/22Ne values and high 40Ar/36Ar values obtained from the samples suggest that the petit-spot magma was derived from a MORBtype mantle source (Hirano et al., 2006). Isotopic compositions of Nd, Pb and Sr of samples from Sites A and B are investigated by Machida et al. (2009). It was reported that petit-spot samples from Sites A and B yielded relatively low 143Nd/ 144Nd (0.5125– 0.5127) and 206Pb/204Pb (16.87–17.93) ratios with relatively elevated 87Sr/86Sr (0.7042–0.7047) ratios. Note that such a feature is similar to those found in ocean island basalts derived from enriched or fertile compositions, as in Pitcairn, Tristan Da Cunha, and Walvis Ridge (e.g., an EM-1 mantle component: Zindler and Hart, 1986; the Dupal isotopic signature: Dupré and Allègre, 1983). The results reported by Machida et al. appear to be at odds with the isotopic compositions of noble gases, which indicate the magma was derived from depleted mantle. However, this apparent discrepancy is resolved by considering the extremely small degrees of partial melting involved and the higher mobility of noble gases in the mantle due to their more volatile behavior when compared with other solid elements (Machida et al., 2009). Compared with a depleted peridotite source, the solidus of an enriched component (e.g., recycled former oceanic crust) would be more than 100°C lower at pressures of 3 GPa in the asthenosphere (Takahashi et al., 1998; Hirschmann, 2000). Therefore, the effect of enriched materials may prevail, even though they are minor constituents in the depleted mantle, if the small degrees of partial melting occur in the asthenosphere. Other key geochemical component of a mantle source is carbon dioxide. Petit-spot lavas are highly vesicular (up to 60%) despite the high hydrostatic pressures en- 164 N. Hirano countered at 6000 m water depth (Hirano et al., 2006). This observation suggests that the vesicularity is caused by CO2, as the solubility of CO2 is very low in alkaline magmas (Dixon, 1997) compared with the high solubility of H2O (ca. 300 ppm versus 0.5–1.0 wt%, respectively). Because a few percent of melt might be present if small amounts of H 2 O or CO 2 are present in the asthenosphere (Wyllie, 1995), it is anticipated that petitspot magmas originate in the asthenosphere as incipient partial melts that form as a result of the presence of CO2. More recently, carbonatite melt has been proposed as a key material in explaining the electrical conductivity of oceanic asthenosphere (Gaillard et al., 2008; Yoshino et al., 2010). The preliminary observation of high CO2 contents in petit-spot lavas, based on their vesicularity, raises the possibility that CO2 affects the source components and their melting. However, it is necessary to carefully estimate the fluctuations in CO2 content that occur in ascending magma due to degassing (Gonnerman and Mukhopadhyay, 2007) in order to investigate the significance of CO2 in an asthenospheric source of petit-spot. Furthermore, the presence of entrained carbonate rocks (e.g., submarine limestone) might increase the CO2 content of the magma (Allard et al., 1991). As stated above, there is a lack of direct evidence for melting in the asthenosphere with which to complement experimental and geophysical observations (Karato, 1990; Karato and Jung, 1998; Faul and Jackson, 2005; Wang et al., 2006; Yoshino et al., 2006, 2009, 2010; Mierdel et al., 2007; Gaillard et al., 2008). In addition, geophysical data are ambiguous regarding the specific nature of the asthenosphere below the NW Pacific Plate (Shimamura et al., 1983; Shinohara et al., 2008). In this context, petitspots on the Pacific Plate provide a potential window into the occurrence of partial melting of the asthenosphere. FUTURE D IRECTIONS The discovery of petit-spot volcanoes upon old subducting plates requires the revision of some established ideas in geosciences. If we could determine that melt is present in the source mantle, petit-spots would be recognized as a ubiquitous phenomena on Earth; otherwise, the widespread occurrence of petit-spots could be indicated by the discovery of petit-spots at other sites (e.g., off the Tonga and Chile trenches). In such a case, the distribution and alignment of monogenetic petit-spot volcanoes from various areas may reveal that fractures in the lithosphere are important controls on petit-spot formation. The article of “perspectives” by McNutt (2006) on the paper by Hirano et al. (2006) (in the same issue of Science) noted that the model of petit-spot formation may also apply to hotspot volcanoes, because some workers consider that the magma at hotspots ascends along a crack in the plate from the asthenosphere to the surface, rather than being caused by plume activity (e.g., McNutt et al., 1997). The contrasting views regarding the ascent of hotspot magma constitute the “plume debate”. As mentioned above, the sizes and volumes of hotspot volcanoes are several orders of magnitude greater than those of petitspot volcanoes, with the latter being less than 1 km3 in volume (Hirano et al., 2008) and being underlain by small sills (Fujiwara et al., 2007). Therefore, in order to accept an alternative to the hotspot plume model, an additional explanation should be given how such a large volume of melt could be produced solely by melting of the asthenosphere. Otherwise, knowledge about formation of petit-spots would not directly be applicable to the plume debate (Hirano and Koppers, 2007; Hofmann and Hart, 2007). Although the formation of fissures upon an outer rise seems to be the logical mechanism for enabling asthenospheric melt to escape to the surface, Valentine and Hirano (2010) suggested that the presence of fracture-controlled pathways is not necessary because ascending dikes can propagate their own fractures. The distribution of petit-spot volcanoes may depend on the presence of melt in the source. Therefore, if the accumulation of partial melts due to tension (Valentine and Hirano, 2010) occurs in the mantle, the distribution of petit-spots may be sufficiently limited to prevent them forming linear arrays along tectonic fissures at the surface. On the other hand, if horizontal melt-rich layers are embedded in otherwise meltless mantle in the asthenosphere (Kawakatsu et al., 2009), petit-spot volcanoes could be formed ubiquitously wherever the plate flexes under the influence of plate tectonics. A multidisciplinary research project is currently being undertaken on the NW Pacific Plate, including oceanbottom seismic observations, measurements of electrical conductivity, and a broad survey using multi-channel seismic equipment. The detailed results of this project promise to increase our understanding of the asthenosphere and the structures in the lithosphere beneath petit-spot volcanoes. Further work on the concentrations of isotopes and volatiles in petit-spot lavas could provide important information on the chemical composition of the asthenosphere. Moreover, a petrological, rheological and geochemical study of xenoliths entraining petit-spot lavas, together with information from the 21st Century SuperMohole Project (Hirano et al., 2010), provides us with the opportunity to sample and understand the entire oceanic lithosphere. The fourth type of volcanic zone recognized on Earth, the petit-spot, provides a unique window into the nature of old plates, in areas where bathymetry was largely ignored prior to the discovery of petit-spots. Acknowledgments—I would like to thank H. Kagi and the Executive Editor Y. Sano for encouraging me to write this paper. The present work on petit-spot volcanoes was fully supported by the author’s former supervisors: K. Saito, Y. Ogawa, T. Ishii, E. Takahashi, I. Kaneoka, H. Staudigel, A. A. P. Koppers, H. Kagi, and T. Morishita. I gratefully acknowledge the assistance of I. Kaneoka, K. Nagao, K. Saito, Y. Takigami, K. Fukunaga, Y. Miura, T. Hanyu, N. Iwata, K. Sato, H. Tamura, H. Sumino, J. Yamamoto, and N. Ebisawa in performing the Ar–Ar and K–Ar analyses, as well as the Radioisotope Center of the University of Tokyo, the International Research Center for Nuclear Materials Science, and the Institute for Materials Research, Tohoku University. I also owe my thanks to T. Yoshida, S. Arai, T. Nakano, T. Hirata, J. Kimura, S. Takizawa, M. Kurosawa, H. Sato, S. Haraguchi, S. P. Ingle, A. Tamura, S. Takeuchi, S. Machida, S. Okumura, K. Sato, and S. Yoshimura for their help and discussions on geochemical matters. I also thank Captains S. Ishida, Y. Imai, O. Yukawa, and H. Tanaka, the crews of R/V Kairei, R/V Yokosuka, and the submersible Shinkai6500, K. Tamaki, G. Valentine, M. Nakanishi, T. Fujiwara, S. Umino, K. Okino, J. Smith Jr., T. Kanamatsu, N. Abe, M. Ichiki, B. Eakins, K. Baba, S. Haraguchi, T. Sasaki, B. 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