Because the maximum temperature reached in the slip zone is significant information for understanding slip behav- iors during an earthquake, the maturity of carbonaceous material (CM) is widely used as a proxy for detecting frictional heat recorded by fault rocks. The degree of maturation of CM is controlled not only by maximum tempera- ture but also by the heating rate. Nevertheless, maximum slip zone temperature has been estimated previously by −1 comparing the maturity of CM in natural fault rocks with that of synthetic products heated at rates of about 1 °C s , even though this rate is much lower than the actual heating rate during an earthquake. In this study, we investigated the kinetic effect of the heating rate on the CM maturation process by performing organochemical analyses of CM −1 −1 heated at slow (1 °C s ) and fast (100 °C s ) rates. The results clearly showed that a higher heating rate can inhibit the maturation reactions of CM; for example, extinction of aliphatic hydrocarbon chains occurred at 600 °C at a heat- −1 −1 ing rate of 1 °C s and at 900 °C at a heating rate of 100 °C s . However, shear-enhanced mechanochemical effects can also promote CM maturation reactions and may offset the effect of a high heating rate. We should thus consider simultaneously the effects of both heating rate and mechanochemistry on CM maturation to establish CM as a more rigorous proxy for frictional heat recorded by fault rocks and for estimating slip behaviors during earthquake. Keywords: Carbonaceous material, IR spectrometry, Raman spectrometry, Pyrolysis, Frictional heat, Heating rate Several temperature proxies have been proposed as Introduction indicators of frictional heat recorded by fault rocks Frictional heat generated in fault zones constitutes the (Rowe and Griffith 2015). These include pseudotachylyte largest part of the total seismic energy budget during formation (e.g., Cowan 1999; Di Toro et al. 2005), min- an earthquake (e.g., Chester et al. 2005), and it triggers eral transformations (e.g., Hirono et al. 2007; Mishima several kinds of fault-weakening mechanisms, including et al. 2009; Kameda et al. 2011; Evans et al. 2014), ther- thermal pressurization (Sibson 1973) and melt lubrica- mal decomposition of carbonate minerals (e.g., Han et al. tion (Hirose and Shimamoto 2005), which can strongly 2007; Oohashi et al. 2014), dehydration and dehydroxyla- affect earthquake energetics and fault slip behaviors. tion of clay minerals (e.g., Hirono et al. 2008; Schleicher Because the progression of such mechanisms is closely et al. 2015), and anomalies in fluid-mobile trace ele - dependent on the amount of heat produced by slip, to ment concentrations and strontium isotope ratios (e.g., understand fault slip behaviors during earthquakes it is Ishikawa et al. 2008; Honda et al. 2011). In particular, crucial to estimate the maximum temperature recorded the thermal maturity of carbonaceous material (CM) by the fault rocks. has received considerable attention as a new tempera- ture proxy (e.g., Savage et al. 2014; Hirono et al. 2015; *Correspondence: firstname.lastname@example.org Kaneki et al. 2016; Rabinowitz et al. 2017) because the Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan © The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat iveco mmons .org/licen ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Kaneki and Hirono Earth, Planets and Space (2018) 70:92 Page 2 of 10 organochemical characteristics of CM, including its ele- temperatures and fault slip behaviors may include uncer- mental compositions and molecular structures, change tainties due to the kinetic effect of heating rate. irreversibly with increasing temperature (e.g., Beyssac In this study, we quantitatively investigated the kinetic et al. 2002). The frictional heat recorded by CM in both effect of heating rate on the thermal maturation of CM by natural and experimental fault rocks has been inves- using infrared (IR) and Raman spectrometry and pyroly- tigated by spectroscopic analyses (e.g., Furuichi et al. sis–gas chromatography–mass spectrometry (py–GC/ 2015; Hirono et al. 2015; Kaneki et al. 2016, 2018; Ito MS) in conjunction with slow- and fast-heating experi- et al. 2017; Kouketsu et al. 2017; Kuo et al. 2017) and by ments. On the basis of our results, we show that the effect determining elemental compositions (Kaneki et al. 2016), of heating rate on thermal maturation of CM has implica- biomarker indexes (Polissar et al. 2011; Savage et al. 2014; tions for the use of CM maturity as a proxy for frictional Sheppard et al. 2015; Rabinowitz et al. 2017), and vitrin- heat. We consider information about this effect to be cru - ite reflectance (e.g., O’Hara 2004; Sakaguchi et al. 2011; cial for establishing a more rigorous fault geothermome- Kitamura et al. 2012; Maekawa et al. 2014; Hamada et al. ter and for estimating slip behaviors of past earthquakes. 2015). Several studies have succeeded in inferring slip behaviors on natural faults during past earthquakes from Materials and methods estimations of maximum temperatures recorded by CM Heating experiments (Savage et al. 2014; Hirono et al. 2015; Kaneki et al. 2016; To investigate the kinetic effect of heating rate on CM Mukoyoshi et al. 2018). maturation, we collected CM-bearing bulk-rock samples Maturation of CM is accompanied by the release of from non-deformed shale in the Cretaceous Nonokawa various volatile organic components (e.g., aliphatics, Formation, in the Shimanto accretionary complex, aromatics), resulting in the formation of a solid residue which crops out along the coast at Kure, Tosa Town, with extremely high carbon content (e.g., Spokas 2010). Japan (Mukoyoshi et al. 2006) (Fig. 1). After extracting This devolatilization process can be strongly affected not pure CM from the bulk samples by chemical treatment only by maximum temperature but also by other fac- (HCl–HF method; see Kaneki et al. 2016 for details), we tors such as a change in reactivity due to shear damage used a thermogravimetry–differential scanning calorim - (mechanochemical effect) and the kinetic effect of the eter apparatus (STA 449 C Jupiter balance, Netzsch) for heating rate (e.g., Alexander et al. 1986; Kitamura et al. our slow-heating experiments. About 30 mg of the CM 2012). Recently, Kaneki et al. (2018) experimentally dem- was placed in a covered Pt Rh crucible and heated 90 10 −1 onstrated that shear-enhanced mechanochemical effects under Ar gas flow at a rate of 50 °C min (approximately −1 can promote various organochemical reactions of CM at 1 °C s ) from an initial temperature of 50 °C to target relatively low temperature and suggested that maximum temperatures from 100 to 1000 °C at 100 °C intervals. temperatures estimated in the previous studies might be For the fast-heating experiments, we used a tube fur- overestimated. Although it is known that higher heating nace to heat each CM sample. About 10 mg of sample was rates generally lead to higher pyrolytic temperatures of enclosed in a quartz tube (outer diameter, 8 mm; thickness, CM (e.g., Alexander et al. 1986; Schenk et al. 1990; Huang 1 mm; length, 25 cm) under vacuum (≤ 10 Pa), and then, and Otten 1998; Burnham and Braun 1999; Lievens et al. the tube was inserted into the tube furnace apparatus, 2013), however, quantitative evaluation of the kinetic which had been preheated to the target temperature (100– effect of heating rate on the CM maturation process dur - 1000 °C at 100 °C intervals), for 10 s. We numerically simu- ing earthquakes remains unknown. Furthermore, several lated the heating rates during the experiments by adopting recent estimates of maximum temperatures are based on a CM particle diameter of 100 µm (determined from scan- heating experiments conducted only at slow heating rates ning electron microscope observations) and thermal dif- −1 −7 −7 2 −1 of about 1 °C s (Hirono et al. 2015; Kaneki et al. 2016; fusivities of 1.6 × 10 and 8.7 × 10 m s for CM and Mukoyoshi et al. 2018). For example, Kaneki et al. (2016) silica glass, respectively (Gustafsson et al. 1979; Turian inferred maximum temperatures and fault slip distances et al. 1991). Then, we simulated the time–temperature in the CM-bearing slip zone of an ancient plate-subduc- and time–heating rate relationships for a CM particle with tion fault developed in Kure Mélange, Shikoku, Japan these thermophysical properties during the fast-heating (Fig. 1), to be 500–600 °C and 2–9 m, respectively, from experiments (Additional file 1). The simulated temperature the results of heating experiments on host-rock CM con- profiles indicate that the heating rate increases as the tar - −1 ducted using a heating rate of 50 °C min (approximately get temperature increases, and the maximum heating rate −1 −1 1 °C s ). This rate is much lower than typical heat - that can be achieved is ≥ 100 °C s for all target tempera- −1 ing rates during earthquake slip (several tens to several tures except 100 °C (for which 50 °C s is the maximum hundreds of degrees per second); thus, these estimated rate) (Additional file 1). Hereafter, therefore, we refer to Kaneki and Hirono Earth, Planets and Space (2018) 70:92 Page 3 of 10 the slow- and fast-heating rates during the heating experi- Inc.) to obtain IR absorbance spectra of CM retrieved −1 ments as 1 and 100 °C s , respectively. We stopped the from the shale and from the products of heating at 1 or −1 heating as soon as the targeted temperature was achieved. 100 °C s . The samples were placed on a CaF plate and then hand-pressed to prevent saturation of the IR spec- Spectroscopic analyses tra. Before the measurements, plates and samples were To investigate the chemical structures of CM, we con- dried in an oven at 50 °C for several hours. To acquire ducted IR and Raman spectrometry on powdered CM one IR spectrum, 100 spectra were accumulated with a −1 samples. IR spectra of CM exhibit various peaks that wavenumber resolution of 4 cm , a wavenumber range −1 2 correspond to organic and inorganic chemical bonds of 4000–1000 cm , and an aperture size of 50 × 50 µm . (e.g., Stuart 2004). These include an O–H stretching Background intensities of the IR spectra were eliminated −1 band at around 3400 cm ; a sharp aromatic C–H band by measuring a blank CaF plate. −1 −1 at 3050 cm ; aliphatic hydrocarbon bands at 2960 cm Raman spectra of CM show significant peaks at 1355– −1 −1 (asymmetrical CH stretching), 2930 cm (asymmetri- 1380 and 1575–1620 cm , which are known as disor- −1 cal CH stretching), and 2860 cm (symmetrical CH dered (D) and graphite (G) bands, respectively (Tuinstra 2 2 −1 stretching); a C=O stretching band at 1680 cm ; an and Koenig 1970). Several spectral parameters have been −1 aromatic ring C=C stretching band at 1600 cm ; and used to evaluate the maturity of CM in metamorphic −1 weak aliphatic hydrocarbon bending bands at 1455 cm rocks (e.g., Beyssac et al. 2002; Aoya et al. 2010; Kouk- −1 (asymmetrical CH bending) and 1375 cm (sym- etsu et al. 2014; Nakamura et al. 2015) and fault rocks metrical CH bending). We followed the methods of (e.g., Hirono et al. 2015; Kaneki et al. 2016; Kouketsu Kaneki et al. (2016) to obtain our IR spectra. We used a et al. 2017; Kuo et al. 2017). We followed the methods Fourier transform IR spectrometer (FT/IR-4700, Jasco of Kaneki et al. (2016) to acquire our Raman spectra and Inc.) equipped with an IR microscope (IRT-5200, Jasco spectral parameters. We used a Raman microspectrom- eter (XploRA, Horiba Jobin–Yvon Inc.) equipped with a laser (532 nm) to obtain Raman spectra of CM powder derived from the shale and from the samples heated at 1 −1 130 °E 135 °E 140 °E and 100 °C s . Before the measurements, samples were dried in an oven at 50 °C for several hours. Although the plate boundary graphitic structure of CM has a crystallographic orienta- NAM tion (c-axis orientation), we did not control sample ori- EUR entation during our spectral measurements because the 35 °N orientation is unlikely to affect the Raman spectral fea - tures (Aoya et al. 2010). We used an exposure time of 10 s and a laser power of 0.09–0.11 mW to obtain spec- tra from the targeted surfaces to avoid thermal damage to the powder samples. Because Raman spectra obtained from the grain boundaries of CM particles might dif- PHS fer from those obtained from the body of CM particles 30 °N (Tuinstra and Koenig 1970), we adopted a laser spot size of 5 µm as sufficiently smaller than the average CM par - Fault ticle size (approximately 100 µm). We then used Peak- Fit 3.0 software (Systat Software Inc.) to fit the D and G bands to the acquired spectra after a linear baseline cor- −1 rection of 1000–1800 cm (Additional file 2). We deter- mined the intensities of both bands. To compare the spectral features among the acquired Raman spectra, we normalized the spectra so that the height of the strongest peak of each spectrum was the same among the spectra 1 m being compared. Ten spectra were obtained from each Fig. 1 Example of a CM-bearing fault. Location (upper) and photo- sample (one spectrum per CM particle), and the mean graph (lower) of an ancient plate-subduction fault developed in the values and standard deviations of the intensity ratios Kure Mélange, Shikoku, Japan. The sampling point of CM-bearing of the D and G bands (I /I ) were calculated. All of the non-deformed shale is shown by the black circle on the photograph D G calculated I /I ratios with their standard deviations are of the outcrop. EUR Eurasia plate, NAM North American plate, PHS D G Philippine Sea plate summarized in Additional file 3. Kaneki and Hirono Earth, Planets and Space (2018) 70:92 Page 4 of 10 Aromatic C=C Aromatic C=C Aromatic CH Aromatic CH a b Aliphatic CH Aliphatic CH Aliphatic CH Aliphatic CH O–HO–H –1 –1 1 °C s heating exp. 100 °C s heating exp. Samples Samples Starting Starting 100 °C 100 °C 200 °C 200 °C 300 °C 300 °C 400 °C 400 °C 500 °C 500 °C 600 °C 600 °C 700 °C 700 °C 800 °C 800 °C 900 °C 900 °C 1000 °C 1000 °C 4000 3000 2000 1000 4000 3000 2000 1000 –1 –1 Wavenumber (cm ) Wavenumber (cm ) −1 −1 Fig. 2 Representative IR spectra of the analyzed CM. IR spectra of CM from shale and from products heated at a rate of a 1 °C s or b 100 °C s . exp. experiment py–GC/MS C–H bond became weak at 600 °C and disappeared at To analyze the composition of gases released from the 700 °C, and the absorbance peaks of the aromatic C=C starting samples and from the samples that had been bond became weak at 700 °C and disappeared at 800 °C −1 −1 heated at 1 and 100 °C s , we followed the methods (Fig. 2a). In the spectra of samples heated at 100 °C s , of Kaneki et al. (2016). We used a py–GC/MS system the absorbance peaks of the aliphatic C–H bonds and the consisting of a model EGA/PY-3030D pyrolyzer (Fron- aromatic C–H bond became weak at 800 °C and disap- tier Lab) and a model GCMS-QP2010 SE GC/MS (Shi- peared at 900 and 1000 °C, respectively, and the absorb- madzu) with an UltraALLOY-5 column. About 1 mg of ance peaks of the aromatic C=C bond became weak at sample was pyrolyzed at 1000 °C for 1 min under vacuum 900 °C and disappeared at 1000 °C (Fig. 2b). (≤ 2 Pa), and chromatographs and chemical composi- The Raman spectra of all of the analyzed samples tions of the released gas were then analyzed. First, the showed distinct D and G band peaks, and the intensity intensities of the chromatographs were normalized by of the D band relative to that of the G band increased as the weight of the analyzed samples, and then, the inten- the target temperature increased (Fig. 3). The I /I ratios D G −1 sity ratios of toluene to benzene (I /I ) were of the samples heated at 1 °C s increased markedly at toluene benzene −1 determined. All of the calculated I /I ratios are ≥ 600 °C, whereas those of samples heated at 100 °C s toluene benzene summarized in Additional file 3. started to increase at ≥ 900 °C (Fig. 4). Results py–GC/MS Spectroscopic characteristics Chromatographs for the CM sample from shale and for The IR spectra of CM from the shale (starting material) the products of the heating experiments included clear showed sharp absorbance peaks for the aliphatic C–H peaks of various aromatic compounds, and the samples −1 (2960, 2930, 2860, 1455, and 1375 cm ), aromatic C–H were especially rich in benzene and toluene (Fig. 5). The −1 −1 (3050 cm ), and C=C bonds (1600 cm ) (Fig. 2). In chromatographs for the products of the heating experi- −1 the spectra of samples heated at 1 °C s , the absorb- ments showed a systematic decrease in peak intensi- ance peaks of the aliphatic C–H bonds and the aromatic ties as the target temperature increased. Intensities of Absorbance 1.0 Absorbance 1.0 Kaneki and Hirono Earth, Planets and Space (2018) 70:92 Page 5 of 10 ab −1 Fig. 3 Representative Raman spectra of the analyzed CM. Raman spectra of CM from shale and from products heated at a rate of a 1 °C s or b −1 100 °C s . exp. experiment and approached zero at 1000 °C. The I /I toluene benzene 1.4 −1 ratios of the CM sample heated at 1 °C s started to –1 1 °C s heating exp. decrease at 500 °C, whereas the ratios of samples heated –1 100 °C s heating exp. 1.2 −1 at 100 °C s showed a sudden decrease above 800 °C. −1 The I /I ratios in samples heated at 1 °C s toluene benzene −1 1.0 to ≥ 700 °C and in those heated at 100 °C s to 1000 °C could not be determined because of the extinction of the toluene and benzene peaks on those chromatographs. 0.8 Discussion and conclusions 0.6 The py–GC/MS chromatographs revealed that the domi - Starting nant components of the analyzed CM samples were 0.4 benzene and toluene, and the amounts of these com- 0 200 400 600 800 1000 pounds decreased as the target temperature increased Temperature (°C) (Fig. 5). The presence of toluene or phenol, which have Fig. 4 Temperature dependence of the I /I ratios of the analyzed D G aliphatic C–H or O–H bonds in their molecular struc- CM. Bars show standard deviations. exp. experiment tures, clearly indicates that the CM from shale was not derived from C–H–O-rich fluid at high temperature (≥ 500 °C) because such depositional CM is almost fully the benzene and toluene peaks of CM sample heated at graphitized (e.g., Luque et al. 2009). In samples heated −1 −1 1 °C s decreased markedly at 500 °C and approached at 1 and 100 °C s , the chromatographic peak of tolu- zero at 700 °C (benzene) and 600 °C (toluene). In con- ene disappeared (Fig. 5), and extinction of the aliphatic −1 trast, for the CM sample heated at 100 °C s , chroma- C–H absorbance peaks was observed on the IR spectra tograph peak intensities started to decrease at 800 °C (Fig. 2), at 600 and 900 °C, respectively. This result clearly I / I D G Kaneki and Hirono Earth, Planets and Space (2018) 70:92 Page 6 of 10 Fig. 5 Gas chromatographs of the analyzed CM. The two chromatographs at the top of each column show the composition of gas released from −1 the starting CM. The lower 20 chromatographs show the compositions of gas released from the products heated at 1 °C s (left column) and −1 100 °C s (right column). exp. experiment indicates that toluene dominantly controlled the amount ≥ 900 °C, respectively (Fig. 4). This result is well consist - of aliphatic C–H chain in the samples. Furthermore, ent with the findings of several prior studies that showed the almost simultaneous disappearance of the chroma- I /I ratios increased when intact CM was exposed to D G tographic peak for benzene and the absorbance peak of high temperatures of several hundreds of degrees Cel- the aromatic C=C bond at 700–800 and 1000 °C in the sius by heating or friction experiments (e.g., Furuichi −1 samples heated at 1 and 100 °C s , respectively (Figs. 2, et al. 2015; Hirono et al. 2015; Kaneki et al. 2016, 2018; 5), suggests that benzene was the main contributor to Ito et al. 2017). In these temperature ranges, no sig- the aromatic C=C absorbance peaks on the IR spectra. nificant change in the molecular compounds in gases The I /I ratios of the Raman spectra for the products released from the heating products was observed except D G −1 heated at 1 and 100 °C s began to increase at ≥ 600 and for a small decrease in the chromatographic peak of Kaneki and Hirono Earth, Planets and Space (2018) 70:92 Page 7 of 10 were completed within 20 min, we can ignore the effect 1.2 of water saturation. Starting The I /I ratio of Raman spectra has long been believed D G 1.0 to decrease with increasing CM maturity (e.g., Beyssac et al. 2002; Kuo et al. 2017), whereas we observed a com- 0.8 pletely opposite trend in our results (Fig. 4). However, 0.6 recent friction and heating experiments conducted with pure CM or CM-bearing samples have demonstrated 0.4 increases in the I /I ratios of CM with increasing tem- D G perature (e.g., Furuichi et al. 2015; Hirono et al. 2015; 0.2 –1 1 °C s heating exp. Kaneki et al. 2016, 2018; Ito et al. 2017). Furthermore, –1 100 °C s heating exp. Mukoyoshi et al. (2018) reported an increase in the I /I D G 0 200 400 600 800 1000 ratios of CM in a natural pseudotachylyte-bearing slip zone relative to the ratios in host-rock samples. These Temperature (°C) contradictory results might be explained by heterogeneity Fig. 6 Temperature dependence of the I /I ratios of the toluene benzene −1 analyzed CM. The ratios of the products heated to ≥ 700 °C at 1 °C s of the initial condition of the CMs among these studies. −1 and to 1000 °C at 100 °C s are not available owing to extinction of For example, Kuo et al. (2017) obtained lower I /I ratios D G the toluene and benzene peaks. exp. experiment for anthracite samples in a high-velocity friction experi- ment, whereas Furuichi et al. (2015) reported an increase in the I /I ratios of brown coal in a similar friction exper- D G benzene (Fig. 5). This result suggests that the changes in iment. If it is assumed that the direction of change in the the I /I ratios are possibly attributable to pyrolysis of D G I /I ratios of Raman spectra with increasing temperature D G residual benzene with relatively strong bonds within the depends on the initial maturity of the starting CM (e.g., graphitic sheets accompanied by the formation of a dis- Kouketsu et al. 2014, 2017; Schito et al. 2017), the increas- ordered structure as a result of pyrolytic rearrangement. ing I /I ratios of the Raman spectra of CM with increas- D G The abrupt decreases in the I /I ratios at ≥ 500 toluene benzene ing temperature in our study might be attributable to the −1 and ≥ 800 °C in the products heated at 1 and 100 °C s , relatively low maturity of the starting CM (bituminous respectively, might be due to the difference in the pyro - coal). Thus, to characterize fully the temperature–matu - lytic temperatures of benzene and toluene (Fig. 6). rity relationship, further friction and heating experiments On the basis of these results, we inferred that the domi- and organochemical analyses should be performed using nant maturation process controlling the changes in the CM samples with various initial maturities. organochemical characteristics of the starting CM during We obtained experimental evidence for the first time that the heating experiments was the thermal decomposition a kinetic effect of heating rate is involved in various organo - of benzene and toluene, which resulted in the extinction chemical reactions of CM. The results of our organochemi - of absorbance peaks in the IR spectra at higher tempera- cal analyses clearly indicate that a higher heating rate can tures, and the subsequent rearrangement of residual aro- inhibit various CM maturation reactions, including the matic nuclei, which in turn increased the I /I ratios of the D G thermal decomposition of several aromatic compounds and Raman spectra. Although Kaneki et al. (2016) attributed structural rearrangement, thus causing extinction of some changes in the characteristics of IR and Raman spectra of IR spectral absorbance peaks, increases in the I /I ratios D G heated CM to the thermal decomposition of toluene and of Raman spectra, and decreases in the I /I ratios toluene benzene the growth of aromatic rings, our series of organochemi- on py–GC/MS chromatographs (Fig. 7). These results sug - cal analyses, including py–GC/MS analyses, revealed that gest that the maximum temperatures reported previously thermal decomposition not only of toluene but also of ben- (Hirono et al. 2015; Kaneki et al. 2016; Mukoyoshi et al. zene, along with subsequent structural rearrangement that 2018) might be too low. On the other hand, Kaneki et al. increased the I /I ratios, may have played a significant D G (2018) demonstrated that shear-induced mechanochemi- role in maturation process of our CM at high temperature. cal effects can increase a reactivity of various organochemi - Although our CM heating experiments were conducted cal reactions, thus lowering the temperatures necessary for under dry conditions, the in situ environment of natural the occurrence of CM maturation reactions by approxi- fault rocks is usually water-saturated. Water saturation mately 100 °C under a normal stress of 3 MPa and a slip may affect the CM maturation process by providing an distance of 10 m (Fig. 7). Although this study focused only exogenous source of hydrogen (e.g., Lewan 1997). How- on the kinetic effect of heating rate, to understand the CM ever, this hydrothermal effect is reported to appear only maturation process during earthquake slip and to estab- after a long reaction time of ≥ 70 h at temperatures of lish a more rigorous fault geothermometer based on CM ≥ 330 °C (Lewan 1997). Because our heating experiments I / I toluene benzene Kaneki and Hirono Earth, Planets and Space (2018) 70:92 Page 8 of 10 Frictional heat (°C) 0100 200300 400500 600 700 800900 1000 IR aliphatic C–H peak IR aromatic C–H peak IR aromatic C=C peak Raman I /I ratio increase D G I /I ratio decrease toluene benzene –1 Mechanochemical effect 1 °C s heating exp. –1 Heating rate effect 100 °C s heating exp. Fig. 7 Summary of organochemical reactions of CM as a function of temperature. Quantitative data on the mechanochemical effect on reaction temperature are from Kaneki et al. (2018), who reported that the mechanochemical effect can cause CM maturation reactions to occur at tempera- tures approximately 100 °C lower under a normal stress of 3 MPa and a slip distance of 10 m. exp. experiment Abbreviations maturity, these two, possibly opposite, effects should be CM: carbonaceous material; EUR: Eurasia plate; I /I : intensity ratio of the D D G considered simultaneously. and G bands of Raman spectra; IR: infrared; I /I : intensity ratio of the toluene benzene In this study, we focused on how the heating rate might benzene and toluene peaks on gas chromatographs; NAM: North American plate; PHS: Philippine Sea plate; py–GC/MS: pyrolysis–gas chromatography– affect the maturation of CM during earthquake slip, and mass spectrometry. demonstrated experimentally that a high heating rate can inhibit various organochemical reactions of CM. Our results Authors’ contributions Both authors designed the study. SK carried out all of the experiments, analy- suggest that the maximum slip zone temperatures estimated ses, and numerical simulations. Both authors contributed to the interpretation previously by slow-rate CM heating experiments (Hirono of the results, collaborated in writing the early drafts. Both authors read and et al. 2015; Kaneki et al. 2016; Mukoyoshi et al. 2018) might approved the final manuscript. be underestimated. Furthermore, mechanochemical effects Acknowledgements during earthquake slip can also strongly affect the matura - The authors thank Tadashi Kondo for help with our heating experiments. We tion of CM (Kaneki et al. 2018). Therefore, comprehensive are also grateful to Takuji Yamada for editing this paper and to Patrick Fulton and an anonymous reviewer for giving many constructive comments. consideration of the effects on CM maturation of both heating rate and mechanochemistry, as well as of the initial Competing interests maturity of the starting CM, is needed to establish a more The authors declare that they have no competing interests. rigorous proxy of frictional heat recorded in fault rocks and Availability of data and materials to infer fault slip behaviors during earthquakes. All data used in this study are available in the figures, additional files, and references. The data are also available from the corresponding author upon Additional files request. Consent for publication Additional file 1. Numerical simulation results for the heating experi- Not applicable. ments. Description of data: Simulated experimental relationships between a temperature and b heating rate and time for a 100-µm-diameter Ethics approval and consent to participate particle of CM. Not applicable. Additional file 2. Peak decomposition method for Raman spectra. Description of data: Representative Raman spectrum (800 °C, fast-rate Funding heating experiment) with peaks decomposed for calculation of the I /I SK was supported by a Grant-in-Aid for Japan Society for the Promotion of D G ratio. Our I /I ratios roughly correspond to I /I ratios calculated by Science (JSPS) Fellows (KAKENHI No. 17J01607), and TH was supported by a D G D1 D2 several prior studies (e.g., Furuichi et al. 2015; Ito et al. 2017). Grant-in-Aid for Scientific Research (B) (KAKENHI No. 15H03737) from JSPS and by Grants-in-Aid for Scientific Research on Innovative Areas (Crustal Dynamics, Additional file 3. Estimated Raman spectral parameters and gas KAKENHI No. 26109004) from the Ministry of Education, Culture, Sports, Sci- composition ratios. Description of data: Average I /I ratios with standard D G ence and Technology of Japan. deviations and I /I ratios, for the starting CM and the products toluene benzene of heating experiments. Kaneki and Hirono Earth, Planets and Space (2018) 70:92 Page 9 of 10 fault in the Shimanto accretionary complex. Geophys Res Lett 38:L06310. Publisher’s Note https ://doi.org/10.1029/2011G L0467 22 Springer Nature remains neutral with regard to jurisdictional claims in pub- Huang WL, Otten GA (1998) Oil generation kinetics determined by DAC-FS/IR lished maps and institutional affiliations. pyrolysis: technique development and preliminary results. 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