Nature of heterogeneity of the upper mantle beneath the northern Philippine Sea as inferred from attenuation and velocity tomography

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1 Physics of the Earth and Planetary Interiors 140 (2003) Nature of heterogeneity of the upper mantle beneath the northern Philippine Sea as inferred from attenuation and velocity tomography Azusa Shito, Takuo Shibutan Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University, Gokasyo, Uji City, Kyoto , Japan Received 13 January 2003; accepted 15 September 2003 Abstract We investigated the physical properties in the upper mantle beneath the Philippine Sea using a theoretical relation derived by Karato [Mapping water content in the upper mantle. Subduction factory, AGU Monograph, in press]. From the attenuation model of Shito and Shibutani [Phys. Earth Planet. Interact., in press] and the velocity model of Widiyantoro et al. [Earth Planet. Sci. Lett. 173 (1999) 91], observed attenuation and velocity anomalies were evaluated to explain the temperature, water content, and chemical heterogeneities in the target area. The results indicate that the observed anomalies in the shallower regions ( km) may be due to chemical composition effects (e.g., concentration of iron), in addition to the temperature and water content anomalies. In contrast, for the deep upper mantle ( km), the observations can be explained by only the effects of high water content (10 50 times higher than the average mantle). These inferred properties of the mantle are consistent with the tectonic history of the Philippine Sea region, which has had a long history of subduction and active magmatism Elsevier B.V. All rights reserved. Keywords: Attenuation; Velocity; Upper mantle; Northern Philippine Sea; Chemical heterogeneity; Water 1. Introduction This paper presents the results of study of the mantle under the northern Philippine Sea, using both seismic attenuation and velocity information. In particular, we try to infer the physical properties of the mantle and go beyond the simple qualitative interpretation that Corresponding author. Present address: Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Hawen, CT 06520, USA. Tel.: (Japan)/ (USA); fax: (Japan)/ (USA). addresses: azusas@rcep.dpri.kyoto-u.ac.jp, azusa.shita@yale.edu (A. Shito). high velocity regions reflect low temperatures and low velocity regions reflect high temperatures. To explain lateral variations in seismic wave velocities, the importance of chemical composition, including partial melting and water content has been suggested in recent studies (Karato and Jung, 1998; Karato and Karki, 2001). The contributions of such effects on the seismic heterogeneities need to be evaluated to understand mantle dynamics. In order to quantitatively investigate the composition of the mantle, the spatial distributions of velocity and attenuation provide valuable information. The sensitivity of attenuation to temperature, water content, chemical composition and partial melting differs /$ see front matter 2003 Elsevier B.V. All rights reserved. doi: /j.pepi

2 332 A. Shito, T. Shibutan / Physics of the Earth and Planetary Interiors 140 (2003) from that of velocity, so comparing the attenuation and velocity distributions can help distinguish the cause of the seismic anomalies (Romanowicz, 2000). Distribution of the seismic attenuation is not as common as velocity distributions, because of the more complicated effects that need to be considered when analyzing amplitudes of seismic waves, compared to analyzing the travel times. However, with the improved quality of recent seismic data, more attenuation results are becoming available. In this study, we develop a theoretical relation between velocity and attenuation, and use the expression to infer physical properties of the mantle under the northern Philippine Sea, in terms of the temperature, water content, chemical composition, and partial melting. This is done by using the three-dimensional attenuation model of Shito and Shibutani (in press) along with the three-dimensional velocity model of Widiyantoro et al. (1999) and comparing the results to the theoretical models. The results provide some new constraints on the properties of the mantle. The study area is the northern Philippine Sea region, including the Izu-Bonin trench and the Shikoku Basin (Fig. 1). It is suggested that the Pacific slab has been subducting beneath the Philippine Sea plate since at least 48 Ma (Seno et al., 1993). Seismic velocity tomography studies (Fukao et al., 1992; Widiyantoro et al., 1999) reveal a large amount of subducted Pacific slab is in the transition zone. In the Shikoku Basin, back arc spreading started Ma (Okino et al., 1999). After the cessation of spreading, post-spreading volcanism (15 7 Ma) formed the Kinan Seamount Chain at the spreading center of the Shikoku Basin (Ishii et al., 2000; Sato et al., 2002). 2. Three-dimensional models and reference models For comparisons of the velocity and attenuation distributions, we used the three-dimensional attenuation model of Shito and Shibutani (in press) and the three-dimensional velocity model of Widiyantoro et al. (1999). We express the attenuation anomaly as the normalized difference in Q 1 between the tomographic model and a one-dimensional global reference model, PREM (Dziewonski and Anderson, 1981), Q 1 (Q 1 Q 1 0 )/Q 1 0. The velocity anomaly is the normalized difference in V between the tomographic model and a one-dimensional global reference model ak135 (Kennett et al., 1995), V (V V 0 )/V 0. Fig. 1. Map showing the target area and the major tectonic settings of the northern Philippine Sea region. The grid shows the modeled region in Fig. 2.

3 A. Shito, T. Shibutan / Physics of the Earth and Planetary Interiors 140 (2003) Fig. 2. Vertical cross-sections of attenuation and velocity anomaly ( Q 1 P and V P ) models used in this study along with recent seismicity (circles). The cross-sections are oriented in east west directions at latitudes of 35, 34, 33 and 32 N from top to bottom (Shito and Shibutani, in press; Widiyantoro et al., 1999).

4 334 A. Shito, T. Shibutan / Physics of the Earth and Planetary Interiors 140 (2003) The attenuation and velocity models are shown in Fig. 2. The averaged standard error of the attenuation model is less than 10%. The standard errors of the velocity model are unknown, however, the average error of the regional travel time tomography is estimated to be 10% (Bijwaard et al., 1998). For the temperature, a one-dimensional geotherm (Turcotte and Schubert, 1986) is used as a reference model. All of these reference models are shown in Fig. 3. Depth, km Depth, km (c) (a) (b) Depth, km Q V, km/s T, K Fig. 3. Reference models: (a) one-dimensional attenuation model PREM (Dziewonski and Anderson, 1981) (Q P and Q S are shown by solid and dashed lines, respectively); (b) one-dimensional velocity model ak135 (Kennett et al., 1995) (V P and V S are shown by solid and dashed lines, respectively); (c) one-dimensional geotherm model (Turcotte and Schubert, 1986). 3. Interpretation of attenuation and velocity anomalies In order to investigate the physical properties that may cause the observed seismic heterogeneities in the mantle, first we examine the effects of temperature and water content, then we look at the effects of chemical composition. It is very difficult to separate these effects in the observed data, however, we can carry out some end member calculations and compare them to our data to make some inferences about the physical structures under the northern Philippine Sea Effect of water and temperature on attenuation and velocity Recent experimental studies under high pressure and high temperature have established that a significant amount of water can be dissolved in nominally anhydrous minerals, such as olivine (Kohlstedt et al., 1996). We consider only the effect of the water in anhydrous minerals because hydrous minerals are not thought to be stable deep in the high temperature regions of the mantle. Water incorporated in anhydrous minerals as hydrogen-related point defects enhance the anelasticity. Only recently has the exact functional relationship between water content and anelasticity been investigated (Mei and Kohlstedt, 2001a,b; Karato and Jung, 2003). The following is a modified version of Eq. (3) from Karato (in press): ( H Q 1 (f,t,c OH ) (AC roh )) α exp, (1) RT where A is a constant depending on the mineral, r the water content (C OH ) exponent, H the activation enthalpy, R the gas constant, T the temperature, and α = for the frequencies. The water content C OH is defined as the concentration of OH, with the units of H/Si in parts per million (ppm). Actually, Eq. (1) based on the results of creep experiment. The application of these results to seismic attenuation needs the assumption that the mechanism of seismic attenuation is related to that of creep through the frequency dependence α. We should notice that we have no evidence which strongly supports or denies the assumption. The lateral variation of Q 1 as related to the lateral variation in temperature and water content can be

5 A. Shito, T. Shibutan / Physics of the Earth and Planetary Interiors 140 (2003) obtained by differentiating Eq. (1): ( ) αr ln Q 1 ln Q 1 COH 0 = ln αh ( 1 C OH0 R T 1 ), T 0 Q 1 = αh ( (T T 0 ) + RTT ( ) αr ) 0 RTT 0 αh ln COH αh = R(T 0 + δt)t 0 C OH0 where T 0 is the reference temperature defined as the one-dimensional geotherm. C OH0 is the reference water content defined as the one-dimensional global mean. The value for the global mean water content, is not known, but in this discussion, we always use the ratio C OH /C OH0. δt = T T 0 is the difference in temperature from the one-dimensional reference model. Eq. (2) describes the fact that high water content has a similar effect to high temperature on the anelasticity. Therefore, a region of high water content would look like high temperature region in attenuation observations. To quantify this point, Karato (in press) introduced a quantity called rheologically effective temperature T eff. Here, we define δt eff which is the temperature anomaly including the effect of water content as follows: δt eff δt + R(T ( ) 0 + δt)t αr 0 COH αh ln. (3) C OH0 Taking the rheologically effective temperature derivative of Eq. (1), we obtain a relationship between attenuation anomaly and rheologically effective temperature as follows: δt eff = RT2 eff αh Q 1. (4) Using Eq. (4), values of δt eff can be directly estimated from the observed attenuation anomalies. The distribution of the estimated δt eff for the mantle under the northern Philippine Sea, as derived from the three-dimensional attenuation model of Shito and Shibutani (in press), is shown in Fig. 4. The maximum value of δt eff is about +425 K at 250 km depth. The relationship between attenuation and velocity anomalies due to temperature and water content can be described as (Karato, in press): ( δt + R(T 0 + δt)t 0 αh ( ) αr ) (2) COH ln, C OH0 V(f, T, C OH ) ( ) (ln V (T)) = δt F(α)Q 1 Q 1, (5) T where F(α) = 2 1 cot ( 2 1 απ). (6) Fig. 4. Distribution of the difference of effective temperature δt eff estimated from observed attenuation anomalies.

6 336 A. Shito, T. Shibutan / Physics of the Earth and Planetary Interiors 140 (2003) Table 1 Anelastic parameters Sample Temperature ( C) Pressure (MPa) Frequency (Hz) α H (kj mol 1 ) Fo90 olivine a ± ± 30 Dunite b ± 30 Forsterite c ± ± 50 a Fo90 olivine, synthetic, polycrystal, 50 m (Tan et al., 1997). b Dunite, natural rock,13 m (Jackson et al., 1992). c Forsterite, synthetic, single crystal (Gueguen et al., 1989). In Eq. (5), the first term on the right side represents the anharmonic effect. The water has little effect on this term (Karato, 1995), therefore, this term is affected by only pure temperature change. The second term on the right side represents the anelastic effect, which implicitly includes both temperature and water effects through the physical dispersion effect. The pure thermal anomaly, δt, in the anharmonic term is difficult to estimate, therefore, we consider two end member cases, δt = δt eff, and δt = 0. The first case, δt = δt eff, means that the observed attenuation and velocity anomaly is explained by the temperature effect only. Therefore, δt = δt eff can be estimated from the observed attenuation anomaly. The second case, δt = 0, means the origin of the observed attenuation and velocity anomalies are due to water content only. In the second case, we can omit the anharmonic term. Then, we can calculate the predicted relationship between the attenuation and velocity anomalies through Eqs. (4) and (5). In the calculation, we need some anelastic and elastic parameters. Some of the available measurements of those parameters are shown in Tables 1 and 2. The differences among the results are small, so we chose the measurements of Fo90 olivine, because it had the most complete set of mea- surements. Then, we used H = 420 kj mol 1 (Tan et al., 1997), V P / T = km s 1 K 1 and V S / T = km s 1 K 1 (Anderson et al., 1992), and α = 0.3 (Tan et al., 1997) which is consistent with the results of Shito and Shibutani (in press). Fig. 5 shows the observed and calculated relationships between seismic attenuation and velocity anomalies for various depths of P-wave. In the shallower portions ( km) the observed velocity anomalies are much larger than any of the predicted values. At those depths, the observed attenuation and velocity anomalies cannot be explained by either temperature only or water only effects, or the combination of the two effects. Therefore, we have to consider other explanations. At depths of about 250 km, the observed anomalies plot between the two lines, suggesting that the observed δt eff can be explained by the combination of temperature and water effects. Although, the real temperature anomaly δt and water content cannot be inferred, we can estimate the expected combination of δt and C OH /C OH0 using Eq. (3) and r = 1.2 (Karato and Jung, 2003). Fig. 6 shows the relationship between δt and C OH /C OH0 for various δt eff at a depth of 250 km. At this depth, the lowest Q P value of 111 Table 2 Partial derivatives corresponding to temperature change Sample Temperature (K) ( V P / T) p ( 10 4 km s 1 K 1 ) Fo90 olivine a Fo92 olivine a Fo100 olivine b a Anderson et al. (1992). b Isaak (1992). ( V S / T) p ( 10 4 km s 1 K 1 )

7 A. Shito, T. Shibutan / Physics of the Earth and Planetary Interiors 140 (2003) km 300 km 100 km 350 km 150 km 400 km 200 km 450 km 250 km 500 km Fig. 5. Observed and predicted relationships between seismic attenuation and velocity for various depths for P-waves. Solid and open circles indicate observations on the grid in the mantle and slab, respectively. Solid and dashed lines indicate predicted curves for the cases of δt = 0 (water effect only) and δt eff = δt (temperature effect only), respectively. Examples of the uncertainty range of the predicted curves are shown by thin lines at the depth of 50 km.

8 338 A. Shito, T. Shibutan / Physics of the Earth and Planetary Interiors 140 (2003) km 10 3 δteff=500 (Qp=100) 10 2 COH/COH0 δteff=400 (Qp=115) δteff=300 (Qp=135) 10 1 δteff=200 (Qp=160) δteff=100 (Qp=210) δt, K Fig. 6. Predicted relationship between δt and C OH /C OH0 for various δt eff at a depth of 250 km. from the model of Shito and Shibutani (in press) was obtained in the attenuation tomography, and the corresponding δt eff was estimated to be 425 K. For example, if we assume the real temperature anomaly δt = 200, the estimated water content is C OH /C OH0 = 10. In the deep upper mantle ( km), the data points are close to the predicted curve for the water effect only. We cannot rule out the possibility that there are also temperature effects, but these results suggest that the effects of water may dominant in this depth range Water content in the upper mantle From our results we obtained that there is almost no temperature anomaly (δt 0) in the deep upper mantle ( km), then C OH /C OH0 can be estimated from Eq. (3) using r = 1.2 (Karato and Karki, 2001). Fig. 7 shows the relationship between δt eff and C OH /C OH0, when δt = 0. In the depth range of km, the values of C OH /C OH are estimated from δt eff of K. Recent experimental studies under high pressure and high temperature have established that a significant amount of water can be dissolved in nominally anhydrous minerals such as olivine. Hirth and Kohlstedt (1996) estimated the water content in a typical oceanic mantle olivine is about 800 H/10 6 Si. Kohlstedt et al. (1996) investigated the solubility of hydroxyl in olivine, wadsleyite, and ringwoodite. Observed maximum hydroxyl solubility reaching a value of H/10 6 Si at 13 GPa and H/10 6 Si at 19.5 GPa. The result suggest that the estimate of C OH /C OH may be an acceptable value Partial melt and chemical composition In the shallower part of the model ( km), the observed seismic anomalies cannot be explained by temperature and/or water effects. Therefore, we have

9 A. Shito, T. Shibutan / Physics of the Earth and Planetary Interiors 140 (2003) δt= km 200 km 300 km 400 km 500 km 10 2 COH/COH δteff, K Fig. 7. Relationship between δt eff and C OH /C OH0 for various depths when δt = 0 is assumed. to provide other explanations of the heterogeneities. One possibility is the presence of partial melt. In a solid with partial melt, fluid flow between pores during compression leads to large bulk attenuation on the order or 5 50% of the shear attenuation (Durek and Ekstrom, 1995). Although, the estimated value of Q P /Q S = 2.15 in Shito and Shibutani (in press) is weekly constraint, it suggest that there is almost no bulk attenuation and that there is no significant amount of partial melt in the target area. Another more realistic candidate to explain the observed seismic heterogeneity may be due to anomalies in chemical composition. Partial derivatives corresponding to changes in chemical composition for olivine, d( ln V P )/dx and d( ln V S )/dx where X = Fe/(Fe+Mg), were calculated by using compiled data in Bass (1995), and listed in Table 3. These results suggest that concentrations of Fe (iron) largely reduce both P- and S-wave velocities. Moreover, a chemical composition anomaly can change elastic moduli directly without changing the anelasticity (Karato, 1989). This is consistent with our observations. If we assume that all of the observed velocity anomaly V = δ( ln V P ) = is due to the change in the ratio of Fe/(Fe + Mg), then the estimated δ(fe/(fe + Mg)) is The possible high content of Fe may be explained by an accumulation in the shallower mantle from frequent production of partial melt, where Fe tends to concentrate. Shallower regions, which have undergone frequent partial melting may have larger amounts of Fe in frozen melt. The value of Fe/(Fe + Mg) in typical mantle zenolith ranges from to 0.15 (Takahashi, 1997). The estimated value of δ(fe/(fe + Mg)) =+0.08 may be reasonable. Table 3 Partial derivatives corresponding to chemical composition change d( ln V P )/dx d( ln V S )/dx X = Fe/(Fe + Mg)

10 340 A. Shito, T. Shibutan / Physics of the Earth and Planetary Interiors 140 (2003) In fact, the observed seismic anomalies in the shallower part of the model are likely produced by a combination of temperature, water, and chemical composition anomalies (and small amount of present day partial melt) effects. It is difficult to estimate quantitatively contributions of each effect. However, results indicate that the observed seismic anomalies in the shallower part cannot be explained only temperature and/or water effects. We suggest that chemical composition anomalies play an important role in producing the observed velocity and attenuation distributions. 4. Tectonic implications The results of this study investigated two important issues related to the seismic heterogeneities in the upper mantle. First, in addition to the temperature and water content anomaly, a chemical composition difference may exist in the shallower region ( km). Second, in the deep upper mantle ( km) the effect of high water content (10 50 times higher than the average mantle) is dominant. These facts can be explained by the tectonic history of the northern Philippine Sea region. It is suggested that the Pacific slab has been subducting beneath the Philippine Sea plate since at least 48 Ma (Seno et al., 1993). During the long history of subduction, a large amount of water may accumulate in the mantle through the dehydration from the subducted slab. This is consistent with our results of high water content in the deep upper mantle ( km). The addition of water lowers the solidus (Hirose, 1997), and partial melting becomes more likely to be produced. The frequent production of partial melt can explain the inferred chemical composition anomaly in the shallower portion ( km) and frequent back arc spreadings that have occurred in this region of northern Philippine Sea. 5. Conclusions Using a theoretical relation between the observed velocity and attenuation distributions, we interpreted the models of Shito and Shibutani (in press) and Widiyantoro et al. (1999) in terms of the temperature, water content, and chemical content in the upper mantle beneath the northern Philippine Sea. We found that the observed anomalies in the shallower regions ( km) may be due to chemical composition effects (concentration of iron), in addition to the temperature and water content anomalies. In contrast, for the deep upper mantle ( km), the observations can be explained by only the effects of high water content (10 50 times higher than the average mantle). We suggest that the high water content in the mid mantle accumulated during the long period of active subduction on the surrounding plate boundaries. Also, this high water content gives rise to the frequent episodes of back arc spreading that have occurred within the northern Philippine Sea. Acknowledgements The authors would like to thank Prof. Jim Mori for his critical comments, suggestions, and continuous encouragement. The comments from Prof. Shun-ichiro Karato were very helpful to improve the paper. Many discussions with Dr. Junichi Nakajima were always constructive. Dr. Yu Nishihara gave us helpful suggestions. High quality and resolution three-dimensional velocity structure used in this study were provided by Dr. Widiyantoro. This research was supported by a Research Fellowship for Young Scientists from the Japan Society for the Promotion of Science. References Anderson, O.L., Isaak, D., Oda, H., High-temperature elastic constant data on minerals relevant to geophysics. Rev. Geophys. 30, Bass, J.D., Elasticity of minerals, glasses, and melts. In: Ahrens, T.J. (Ed.), Mineral Physics and Crystallography. American Geophysics Union, pp Bijwaard, H., Spakman, W., Engdahl, E.R., Closing the gap between regional and global travel time tomography. J. Geophys. Res. 103, Durek, J.J., Ekstrom, G., Evidence of bulk attenuation in the asthenosphere from recordings of the Bolivia earthquake. Geophys. Res. Lett. 22, Dziewonski, A.M., Anderson, D.L., Preliminary reference Earth model. Phys. Earth Planet. Interact. 25, Fukao, Y., Obyahi, M., Inoue, H., Nenbai, M., Subducting slabs stagnant in the mantle transition zone. J. Geophys. Res. 97,

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