A new source of water in seismogenic subduction zones

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 38,, doi: /2011gl048883, 2011 A new source of water in seismogenic subduction zones Jun Kameda, 1 Asuka Yamaguchi, 1 Saneatsu Saito, 2 Hiroshi Sakuma, 3 Katsuyuki Kawamura, 3 and Gaku Kimura 1 Received 28 July 2011; revised 13 October 2011; accepted 18 October 2011; published 18 November [1] Seismogenic plate boundary faults at accretionary margins (e.g., the Nankai margin, southwest Japan) may occur where the uppermost part of subducting oceanic crust, composed of basaltic rocks, is in contact with the overriding plate of a lithified accretionary prism. The plate boundary faults in ancient accretionary complexes typically record high velocity slip under fluid rich conditions. Although previous studies have emphasized the mechanical significance of fluids in terms of dynamic slip weakening, the source of fluid in seismogenic subduction zones remains poorly constrained. In this work, we focus on the hydrous smectite in the uppermost oceanic crust, an alteration product of intact basalt before arrival at the trench axis. A comparison between (1) new mineralogical data on basalt drillcore recovered by Integrated Ocean Drilling Program (IODP) Expedition 322 at site C0012, a reference site for subduction input to the Nankai Trough, and (2) mineralogical data on basalt within ancient oceanic crust embedded in a fossil accretionary complex of the Shimanto Belt, southwest Japan, suggests that progressive smectite chlorite conversion would liberate bound fluids at a rate of 0.34 to s 1 along the plate interface. This rate of fluid production appears to be more than an order of magnitude greater than that from other possible sources, including from overlying sediments via smectite illite conversion and the expulsion of pore fluids, and may facilitate seismic slip along plate boundary faults. Citation: Kameda, J., A. Yamaguchi, S. Saito, H. Sakuma, K. Kawamura, and G. Kimura (2011), A new source of water in seismogenic subduction zones, Geophys. Res. Lett., 38,, doi: /2011gl Introduction [2] Fluids circulating within subduction zones affect many aspects of dynamic processes in such environments, including seismicity, volcanism, and geochemical cycling [National Science Foundation, 2003]. Thus, the distribution, source, and flux of such fluids must be quantified to understand these phenomena. Large earthquakes in subduction zones appear to occur mainly within a specific temperature range ( C) [Hyndman et al., 1997; Hyndman, 2007]. The results of many geophysical and geological surveys suggest that seismogenic subduction faults are saturated with fluids [e.g., Bilek and Lay, 1999; Park et al., 2002; Ujiie 1 Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Tokyo, Japan. 2 Institute for Research on Earth Evolution, Japan Agency for Marine Earth Science and Technology, Yokohama, Japan. 3 Department of Earth and Planetary Sciences, Graduate School of Science, Tokyo Institute of Technology, Tokyo, Japan. Copyright 2011 by the American Geophysical Union /11/2011GL et al., 2007, 2008; Vannucchi et al., 2008; Brodsky et al., 2009; Ujiie et al., 2010]. Nevertheless, the source of the fluids under these temperatures remains debated [Moore and Saffer, 2001; Saffer et al., 2008]. Sediment entering subduction zones carries large volumes of interstitial water and bound water trapped in hydrous minerals, but most of this water is thought to escape during rapid tectonic loading and diagenesis, prior to entering the seismogenic zone [Moore and Saffer, 2001; Saffer et al., 2008]. [3] To identify the source of fluids within the seismogenic zone, we investigate the uppermost parts of oceanic crust. Before entering the subduction zone, such crust is covered by hemipelagic and terrigenous trench deposits. Accretionary processes at the subduction zone result in the scraping of most of these sediments. At depth within the seismogenic zone, the plate boundary fault is considered to lie near or within the oceanic crust [Ruff, 1989; Kimura and Ludden, 1995; Park et al., 2002]. Accordingly, to identify the primary fluid source along a seismogenic plate interface, it is necessary to examine the dehydration behavior of the uppermost oceanic crust, in addition to underthrusted or accreted sediments, yet this topic has not been considered in previous studies in contrast to the well documented dehydration process at deeper setting where metamorphic reactions proceed (more than 300 C) [e.g., Peacock and Wang, 1999; Kuwatani et al., 2011]. The aim of the present study is to clarify the effects of diagenesis and low grade metamorphism on hydrous clay minerals in the uppermost part of the subducting oceanic crust, in the context of their dehydration potential within the seismogenic zone. 2. Alteration of the Uppermost Oceanic Crust Prior to Reaching the Trench [4] To constrain the reference physical and chemical states of the uppermost oceanic crust prior to entering the subduction zone, IODP Expedition 322 recovered a basaltic core ( 40 m thick) at site C0012 off the Kii Peninsula, central Japan (Figure 1a) [Saito et al., 2009]. Although the C0012 site locates at the crest of the bathymetric high (Kashinosaki Knoll), this topographic feature is supposed to be tectonic in origin and not due to the intra plate volcanic activity [Ike et al., 2008]. Petrological/geochemical signatures of the core samples also suggest that the basaltic basement at the drilling site is representative of the tholeiitic basalt in the Shikoku Basin [Saito et al., 2010]. In this study, we analyzed eight samples from this core (collected at depth intervals of 4 m from the upper surface of the basaltic basement) (Figure 1b). The samples are composed of plagioclase and clinopyroxene phenocrysts in a clay matrix (Figure 2a). In places, there occur clay mineral pseudomorphs after olivine (Figure 2b). Given that clay minerals are the main hydrous phase in the samples, they were identified and analyzed as 1of5

2 Figure 1. (a) Locality map of site C0012 of IODP Exp. 322, and analyzed on land basalt in the Mugi mélange. (b) Depths of the analyzed core samples. follows. First, we measured the X ray diffraction (XRD) patterns of air dried bulk and random mounts of the original samples (Figure 3a). All of the XRD patterns show similar profiles, dominated by reflections corresponding to smectite, plagioclase, and clinopyroxene. The strong, sharp smectite (001) peak has a d value of 15.5 Å, indicating a two layer hydrated form of the mineral. Figure 3b shows XRD patterns for oriented mounts of the samples in Mg saturated and ethylene glycolated states. The D2 value of 16 between the (002) and (005) basal reflections suggests that smectite occurs as an almost pure and discrete phase, without interstratification with chlorite (i.e., smectite content >90%) [Reynolds and Reynolds, 1997]. [5] The chemical composition of smectite was determined by energy dispersive X ray spectrometer (EDX) in trans- mission electron microscopy (TEM). The average structural formula is (Ca0.24K0.074Na0.02)(Al0.18Fe1.18Mg1.61Ti0.02) (Si3.22Al0.78)O10(OH)2 on the basis of 22 negative charges (Table S1 in the auxiliary material).1 The smectite is trioctahedral and has high contents of Fe and Mg, indicating saponite. Based on this structural formula, the saponite density in a dry state and in a two layer hydration state is estimated to be 2.92 and 2.24 (g/cm3), respectively. The saponite has a theoretical cation exchange capacity (CEC) of 133 (meq/100g), compared with an average value for bulk samples of 27 (meq/100g) (Table S2 in the auxiliary material). From these CEC values, the proportion of saponite in the 1 Auxiliary materials are available in the HTML. doi: / 2011GL Figure 2. (a) Photomicrograph showing the typical occurrence of matrix saponite surrounding plagioclase and clinopyroxene phenocrysts. CPX = clinopyroxene. PL = plagioclase. (b) Photomicrograph showing smectite pseudomorph after olivine. OP = olivine pseudomorph. PL = plagioclase. 2 of 5

3 passed through the 150 C paleo isotherm and would have been incorporated in the accretionary prism at this temperature via duplex underplating [Matsumura et al., 2003; Ikesawa et al., 2005]. A representative basaltic slab ( 20 m thick), composed of hyaloclastite, massive basalt, pillow basalt, and pillow breccia, is exposed along one of the subduction thrusts. The mineralogy of the basalt is dominated by mixed layer saponite chlorite (S C) ( 30 wt%) followed by albite, calcite, quartz, and minor opaque minerals such as magnetite, hematite, and titanite [Kameda et al., 2011]. S C occurs as a matrix clay mineral surrounding euhedral albite crystals. XRD analyses of the clay phase reveal that 30% 35% of the layers in the S C are expandable saponite. This persistence of saponite layers in S C at 150 C is an important factor when considering the source of fluid in the seismogenic zone. Figure 3. (a) Bulk XRD patterns for random mounts of powdered specimens in an air dried state. The smectite (001) reflection has a d value of 15.5 Å. (b) Bulk XRD patterns for oriented mounts in Mg saturated and ethyleneglycolated states. bulk sample is estimated to be 20 wt%. Given the typical density of basalt (3.0 g/cm 3 ), this value ( 20 wt%) is equivalent to 27 vol%. Such an abundance of saponite may have resulted from low temperature interaction between intact basalt and seawater [Alt, 2004] during the long journey from the ridge to the trench (for more than 20 Ma based on the depositional age of the sediment just above the basaltic basement) [Saito et al., 2009]. The relatively homogeneous mineralogy throughout the analyzed core samples suggests that seawater circulation infiltrated to a depth of at least 40 m (greater than the drilling depth). 3. Diagenesis of the Oceanic Crust Within the Subduction Zone [6] The mineral assemblage of ocean plate basalt is modified by prograde diagenesis during subduction. An advanced state of such a reaction sequence is recorded by a basalt within an on land, ancient accretionary complex, the Shimanto belt (Mugi mélange) of southwest Japan (Figure 1b). The basalt 4. Discussion [7] Kinetic simulations performed by Saffer et al. [2008], focusing on the smectite illite transition in underthrust sediments along the Muroto and Ashizuri transects across the Nankai Trough, indicate that the proportion of smectite in mixed layer smectite illite (S I) is less than 20% at the décollement where it intersects the 150 C isotherm. Although the authors estimated a larger proportion of detrital smectite (45 wt%) in the sediments compared with the proportion of S C in the present samples, a higher proportion of smectite appears to survive in mixed layer S C packets than in S I. Consequently, the amount of bound water stored in the uppermost oceanic crust is similar to that in sediments across the plate interface. If the interlayer water is stably bounded in S C, further subduction would release the fluids during progressive chloritization. However, in contrast to the S I transition, the higher content of structural water in chlorite than in the starting saponite may result in the self consumption of released water, which is likely to be dependent on the reaction mechanism. According to Bettison Varga and Mackinnon [1997], there are two potential reaction pathways for the S C transition, one of which produces a volume increase, and the other a volume decrease. The former reaction requires high influxes of Mg and Fe; however, this is unlikely to occur because the possible sources of such elements (olivine) are largely absent (replaced by S C) in the analyzed on land basalts; hence, the latter process is more likely. In this case, the following reaction may occur: ðca 0:24 K 0:074 Na 0:02 ÞðAl 0:18 Fe 1:18 Mg 1:61 Ti 0:02 ÞðSi 3:22 Al 0:78 ÞO 10 ðohþ 2 4:5H 2 O þ 0:88H þ! R 2þ ; R 3þ ð Si1:44 Al 0:56 ÞO 5 ðohþ 4 þ dissolved cations Ca 2þ ; K þ ; Na þ ; Mg 2þ ; Fe 2þ þ 1:78SiO 2 þ 3:94H 2 O ð1þ where R in chlorite structural formula is cation occupying octahedral site. To construct the equation (1), we employ the averaged composition of saponite determined by TEM EDS (Table S1 in the auxiliary material), Si content of chlorite in a metabasic rock (chlorite zone) of the Sanbagawa belt [Okamoto and Toriumi, 2005], and consider that saponite contains 4.5 moles of water per structural formula in a twolayer hydrated state, equivalent to those in aluminous smectite (montmorillonite) [Ransom and Helgeson, 1994]. Equation (1) indicates that 12% of the released water is selfconsumed by the chloritization reaction. 3of5

4 Figure 4. (a) Estimated rate of fluid production during S C conversion along the Ashizuri and Muroto transects, plotted on a diagram proposed by Saffer et al. [2008], also showing other sources of dehydration fluid (i.e., I S transition and sediment compaction). The rate of fluid production from I S transition was estimated under the heat flux of 120 mw m 2 for the Ashizuri transect and 180 mw m 2 for the Muroto transect [Saffer et al., 2008]. (b) Suppression of I S derived fluids resulting from the underplating of underthrust sediments at horizontal distances of 10 and 30 km arcward of the trench [Saffer et al., 2008], represented by two dotted lines branched from the solid line. The solid line marked as no underplating corresponds to the normal dehydration rate curve along the Muroto transect, equivalent to that drawn in Figure 4a. Also shown is the estimated rate of fluid production by S C conversion. [8] Based on the inferred reaction of saponite to chlorite conversion in S C (equation (1)), we next estimate the rate of fluid production associated with this dehydration reaction within the seismogenic zone. Assuming a negligible increase in porosity, the rate of fluid production G(s 1 ) is expressed as follows [Bethke, 1986; Bekins et al., 1994]: H C ð1 Þ ð2þ where H is the volume content of bound water per saponite layer, C is the volume fraction of S C in the bulk sample, and F is porosity. Here we simply assume that H = 0.4 (corresponding to a hydrated two layer form [Saffer et al., 2008]), 5% volume expansion of water by expulsion [Saffer et al., 2008], and C = We adopt 0.1% of porosity, that was reported as an average value of the altered basalt incorporated in the Okitsu mélange of the Shimanto accretionary complex [Kato et al., 2004]. [9] The first term on the right hand side in equation (2) is the rate of conversion from saponite to chlorite in S C; however, there is no appropriate kinetic equation with which to describe this term. Consequently, a somewhat simple situation is considered where the saponite content in S C shows a linear decrease with the increasing temperature that accompanies downward movement along the plate interface within the seismogenic zone. The initial saponite content in S C is 35% at the up dip limit of the seismogenic zone ( 150 C) [Kameda et al., 2011]. Because hydrated saponite layers in S C (corrensite and randomly interstratified S C) are likely to disappear above 260 C [Hillier, 1993], this temperature condition is defined as an end point of the dehydration reaction. Accordingly, if the geometry of the accretionary wedge is known (i.e., taper angle, geothermal gradient, and rate of relative plate convergence), the reaction rate can be obtained as a function of these variables. [10] Here we discuss fluid production along the Muroto and Ashizuri transects across the Nankai Trough. For this calculation, we use the parameters adopted by Saffer et al. [2008]: taper angles of 4 5 and 8 10, and geothermal gradients of 72 and 48 C/km for the Muroto and Ashizuri transects, respectively, that are derived from reported heat flow values (180 and 120 mw/m 2 ) and a normal heat conductivity of the accretionary wedge (2.5 W/m/K) [Hyndman et al., 1995]. A relative convergence rate of 68 km/myr (= plate convergence rate+ prism outbuilding rate) is used for both transects. Equation (1) yields fluid production rates of 0.34 to (s 1 ) and 0.46 to (s 1 ) for the Muroto and Ashizuri transects, respectively. In Figure 4a, this rate is plotted on a diagram given by Saffer et al. [2008], showing fluid production from underthrust sediments due to S I conversion and from a compaction induced source. Because we assume the reaction occurs at a steady rate, the rate of fluid production is invariable in a landward direction. A comparison of these estimates suggests that around the upper limit of the seismogenic zone, fluid production from basaltic rocks is comparable to that from S I conversion and sediment compaction, whereas at deeper levels, the former is much greater than the latter. [11] Saffer et al. [2008] further argued that if sediment is underplated to a certain depth and is incorporated into the overriding accretionary prism, fluid production is strongly suppressed from this point due to a reduction in the subduction velocity of the sediment. Figure 4b shows this scenario when underplating occurs at horizontal distances of 10 and 30 km from the trench axis. In this case, the fluid production rate from the subducting oceanic crust is more than an order of magnitude greater than that from S I. This situation is achieved if the deeper part (i.e., next to be underplated) of the basaltic crust, which is exposed once the top of the basement has been peeled off, is also altered to produce a similar quantity of hydrous saponite. This scenario is applicable in the case of the analyzed basaltic slab in the Shimanto belt, because the thickness of this slab ( 20 m) is less than the extent of the alteration zone observed at the C0012 site. [12] Our mineralogical analyses exhibit that S C in the uppermost part of an oceanic plate is potentially a significant 4of5

5 carrier of fluids into seismogenic subduction zones. However, whether the fluids released during S C conversion are trapped in the plate boundary and contribute to seismic faulting depends on the competition between the rates of dehydration and diffusion. Quantitative argument on the behavior of such expelled fluids is now in progress, but low permeability of the onland altered basalt (as low as m 2 [Kato et al., 2004]) probably prevents diffusion of dehydrated fluids, so that fluid release associated with S C conversion result in facilitating seismic slip along subduction faults, as recorded in ancient subduction zones. [13] Acknowledgments. This research used samples and data provided by IODP. We thank Kei Matsuura (The University of Tokyo) for his assistance with the ICP measurement of the samples. We also thank editor M. Wysession and two anonymous reviewers for their critical comments and suggestions. This work was supported by Grants in Aid from JSPS for Research Activity Start up ( ) and for Scientific Research on Innovative Areas from MEXT ( ). [14] The Editor thanks the two anonymous reviewers for their assistance in evaluating this paper. References Alt, J. C. (2004), Alteration of the upper oceanic crust: Mineralogy, chemistry, and processes, in Hydrogeology of the Oceanic Lithosphere, edited by E. E. Davis and H. Elderfield, pp , Cambridge Univ. Press, New York. Bekins, B. A., A. M. McCaffrey, and S. J. Dreiss (1994), Influence of kinetics on the smectite to illite transformation in the Barbados accretionary prism, J. Geophys. Res., 99, 18,147 18,158, doi: /94jb Bethke, C. M. (1986), Inverse hydrologic analysis of the distribution and origin of Gulf Coast type geopressured zone, J. Geophys. Res., 91, , doi: /jb091ib06p Bettison Varga, L., and I. D. R. Mackinnon (1997), The role of randomly mixed layered chlorite/smectite in the transformation of smectite to chlorite, Clays Clay Miner., 45, , doi: /ccmn Bilek, S. L., and T. Lay (1999), Rigidity variations with depth along interpolate megathrust faults in subduction zones, Nature, 400, , doi: / Brodsky, E. E., C. D. Rowe, F. Meneghini, and J. C. Moore (2009), A geological fingerprint of low viscosity fault fluids mobilized during an earthquake, J. Geophys. Res., 114, B01303, doi: /2008jb Hillier, S. (1993), Origin, diagenesis, and mineralogy of chlorite minerals in Devonian lacustrine mudrocks, Orcadian basin, Scotland, Clays Clay Miner., 41, , doi: /ccmn Hyndman, R. D. (2007), The seismogenic zone of subduction thrust faults: What we know and don t know, in The Seismogenic Zone of Subduction Thrust Faults, edited by T. Dixon and J. C. Moore, pp , Colombia Univ. Press, New York. Hyndman, R. D., K. Wang, and M. Yamano (1995), Thermal constraints on the seismogenic portion of the southwestern Japan subduction thrust, J. Geophys. Res., 100, 15,373 15,392, doi: /95jb Hyndman, R. D., M. Yamano, and D. A. Oleskevich (1997), The seismogenic zone of subduction thrust faults, Isl. Arc, 6, , doi: / j tb00175.x. Ike, T., G. F. Moore, S. Kuramoto, J. O. Park, Y. Kaneda, and A. Taira (2008), Tectonics and sedimentation around Kashinosaki Knoll: A subduction basement high in the eastern Nankai Trough, Isl. Arc, 17, , doi: /j x. Ikesawa, E., G. Kimura, K. Ikehara Ohmori, Y. Kitamura, A. Yamaguchi, K. Ujiie, and Y. Hashimoto (2005), Tectonic incorporation of the upper part of oceanic crust to overriding plate of a convergent margin: An example from the Cretaceous early Tertiary Mugi mélange, the Shimanto belt, Japan, Tectonophysics, 401, , doi: / j.tecto Kameda, J., K. Ujiie, A. Yamaguchi, and G. Kimura (2011), Smectite to chlorite conversion by frictional heating along a subduction zone thrust, Earth Planet. Sci. Lett., 305, , doi: /j.epsl Kato, A., A. Sakaguchi, S. Yoshida, H. Yamaguchi, and Y. Kaneda (2004), Permeability structure around an ancient exhumed subduction zone fault, Geophys. Res. Lett., 31, L06602, doi: /2003gl Kimura, G., and J. Ludden (1995), Peeling oceanic crust in subduction zones, Geology, 23, , doi: / (1995)023<0217: POCISZ>2.3.CO;2. Kuwatani, T., A. Okamoto, and M. Toriumi (2011), Thermodynamic forward modeling of progressive dehydration reactions during subduction of oceanic crust under greenschist facies conditions, Earth Planet. Sci. Lett., 307, 9 18, doi: /j.epsl Matsumura,M.,Y.Hashimoto,G.Kimura,K.Ohmori Ikehara, M. Enjyoji, and E. Ikesawa (2003), Depth of oceanic crust underplating in a subduction zone: Inferences from fluid inclusion analyses of crack seal veins, Geology, 31, , doi: /g Moore, J. C., and D. M. Saffer (2001), Updip limit of the seismogenic zone beneath the accretionary prism of southwest Japan: An effect of diagenetic to low grade metamorphic processes and increasing effective stress, Geology, 29, , doi: / (2001)029<0183: ULOTSZ>2.0.CO;2. National Science Foundation (2003), NSF MARGINS Science Plans 2004, 170 pp., Lamont Doherty Earth Obs., Columbia Univ., New York. Okamoto, A., and M. Toriumi (2005), Progress of actinolite forming reactions in mafic schists during retrograde metamorphism: An example from the Sanbagawa metamorphic belt in central Shikoku, Japan, J. Metamorph. Geol., 23, , doi: /j x. Park, J. O., T. Tsuru, S. Kodaira, P. R. Cummins, and Y. Kaneda (2002), Splay fault branching along the Nankai subduction zone, Science, 297, , doi: /science Peacock, S. M., and K. Wang (1999), Seismic consequences of warm versus cool subduction metamorphism: Examples from southwest and northeast Japan, Science, 286, , doi: /science Ransom, B., and H. C. Helgeson (1994), A chemical and thermodynamic model of aluminous dioctahedral 2:1 layer clay minerals in diagenetic processes: Regular solution of representation of interlayer dehydration in smectite, Am. J. Sci., 294, , doi: /ajs Reynolds, D. M., and R. C. Reynolds (1997), X Ray Diffraction and the Identification and Analysis of Clay Minerals, Oxford Univ. Press, Oxford, U. K. Ruff, L. J. (1989), Do trench sediments affect great earthquake occurrence in subduction zones?, Pure Appl. Geophys., 129, , doi: / BF Saffer, D. M., M. B. Underwood, and A. W. McKiernan (2008), Evaluation of factors controlling smectite transformation and fluid production in subduction zones: Application to the Nankai Trough, Isl. Arc, 17, , doi: /j x. Saito, S., M. B. Underwood, Y. Kubo, and the Expedition 322 Scientists (2009), NanTroSEIZE stage 2: Subduction inputs, Proc. Integr. Ocean Drill. Program, 322, doi: /iodp.proc Saito, S., et al. (2010), Reconstruction of volcanic history from volcanic and volcaniclastic rocks in subducting Shikoku Basin: Results from IODP Expedition 322, Abstract T13A 2146 presented at 2010 Fall Meeting, AGU, San Francisco, Calif., Dec. Ujiie, K., A. Yamaguchi, G. Kimura, and S. Toh (2007), Fluidization of granular material in a subduction thrust at seismogenic depths, Earth Planet. Sci. Lett., 259, , doi: /j.epsl Ujiie, K., A. Yamaguchi, and S. Taguchi (2008), Stretching of fluid inclusions in calcite as an indicator of frictional heating on faults, Geology, 29, Ujiie, K., M. Kameyama, and A. Yamaguchi (2010), Geological records of thermal pressurization and earthquake instability of subduction thrusts, Tectonophysics, 485, , doi: /j.tecto Vannucchi, P., F. Remitti, and G. Bettelli (2008), Geological record of fluid flow and seismogenesis along an erosive subducting plate boundary, Nature, 451, , doi: /nature J. Kameda, G. Kimura, and A. Yamaguchi, Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Hongo, Bunkyo ku, Tokyo , Japan. (kameda@eps.s.u tokyo.ac.jp) K. Kawamura and H. Sakuma, Department of Earth and Planetary Sciences, Graduate School of Science, Tokyo Institute of Technology, Ookayama, Meguro ku, Tokyo , Japan. S. Saito, Institute for Research on Earth Evolution, Japan Agency for Marine Earth Science and Technology, Showa machi, Kanazawa ku, Yokohama , Japan. 5of5