Microdiamond in a low-grade metapelite from a Cretaceous subduction complex, western Kyushu, Japan

Microdiamonds in metamorphic rocks are a signature of ultrahigh-pressure (UHP) metamorphism that occurs mostly at continental collision zones. Most UHP minerals, except coesite and microdiamond, have been partially or completely retrogressed during exhumation; therefore, the discovery of coesite and microdiamond is crucial to identify UHP metamorphism and to understand the tectonic history of metamorphic rocks. Microdiamonds typically occur as inclusions in minerals such as garnet. Here we report the discovery of microdiamond aggregates in the matrix of a metapelite from the Nishisonogi unit, Nagasaki Metamorphic Complex, western Kyushu, Japan. The Nishisonogi unit represents a Cretaceous subduction complex which has been considered as an epidote–blueschist subfacies metamorphic unit, and the metapelite is a member of a serpentinite mélange in the Nishisonogi unit. The temperature condition for the Nishisonogi unit is 450 °C, based on the Raman micro-spectroscopy of graphite. The coexistence of microdiamond and Mg-carbonates suggests the precipitation of microdiamond from C–O–H fluid under pressures higher than 2.8 GPa. This is the first report of metamorphic microdiamond from Japan, which reveals the hidden UHP history of the Nishisonogi unit. The tectonic evolution of Kyushu in the Japanese Archipelago should be reconsidered based on this finding.


Geological background
The Nagasaki Metamorphic Complex (NMC) consists of high pressure (HP)-low temperature (LP) metamorphic rocks and is exposed in the Nishisonogi Peninsula (Nishisonogi unit), Nomo Peninsula (Nomo unit) and Amakusa-shimoshima Island (Amakusa-Takahama unit) in western Kyushu, Japan 19 (Fig. 1). Microdiamonds are present in several rocks within a serpentinite mélange at Yukinoura from the Nishisonogi unit (ca. 95-90 Ma for glaucophane and phengite using the 40 Ar/ 39 Ar method 20 , and ca. 85-60 Ma by the K-Ar method 21 ). The Nishisonogi unit is considered to represent an epidote-blueschist subfacies metamorphic unit that consists mainly of coherent schists (mostly pelitic and psammitic schists with minor amounts of basic schists) with minor serpentinites 19 . The serpentinite commonly occurs as a massive lenticular body, typically less than 500 m in length, which are enclosed in the coherent schists. Some serpentinite bodies are accompanied by various kinds of metabasites and minor pelitic and psammitic schists, representing the nature of serpentinite mélange 19 . The serpentinite mélange contains tectonic blocks of various lithologies and sizes embedded in a thin actinolite schist matrix or in schistose serpentinite (antigorite schist) 19 . The quartz-bearing jadeitites occur as tectonic blocks in one mélange (the Mie mélange) and exhibit a pressure and temperature of 1.5 GPa and 450 °C 19 . The Yukinoura mélange occurs at the westernmost part of the Nishisonogi unit along the Yobukono-Seto Fault (Fig. 1c) that bounds the Nishisonogi unit and the Oseto granodiorite (90 Ma) 22 . The granodiorite is overlain by the Palaeogene sediments, and there are no indications that it has thermally perturbed the Nishisonogi unit 22 .
The NMC has been correlated with the Sanbagawa Metamorphic Belt (SMB) 20,23 in terms of similarity in the age and nature of the metamorphism 20,24 . The SMB is a high pressure (HP)-low temperature (LT) metamorphic belt of Cretaceous in age, occurring along the Median Tectonic Line in the Outer Zone of Southwest Japan 25 (Fig. 1a). It trends east to west, and terminates at the eastern extremity of Kyushu ('SMB' in Fig. 1a). The NMC occurs at the western extremity of Kyushu, trending north to south (Fig. 1a). However, detailed comparison of the NMC and the SMB sheds a light on the essential differences between the two. The Nishisonogi unit is characterized by the occurrence of serpentinite mélanges involving jadeitites 19 , whereas no such serpentinite mélanges have been reported from the SMB. Eclogites are common in the SMB [26][27][28] , but not in the Nishisonogi unit 19 . The prograde P-T path of the pelitic schists in the SMB shows a linear increment in pressure and temperature from 0.6 GPa and 300 ℃ to 1 GPa and 600 ℃ based on geobarometers 29 , and the application of the Gibbs method to garnet zoning gives a steeper P/T path from 0.6 GPa and 470 ℃ to 0.9 GPa and 520 ℃ 30 . The pelitic schists in the Nishisonogi unit show almost constant temperature of 450 ℃ 31 , which is apparently lower than that in the high grade part of the SMB. The prograde P-T path of eclogites in the SMB goes up to 2 GPa and 600 ℃ 28 or to 3 GPa and 800 ℃ 27 . No such high-grade rocks have been known from the Nishisonogi unit 19 . In the theory of a correlation between the NMC and the SMB, the main difficulty is the difference in stretching lineation trends between western Kyushu and Southwest Japan 20 . The trend of the stretching lineation is compatible to the general distribution trend of these metamorphic belts: N-S in the NMC and E-W in the SMB 25 (Fig. 1a). Possible tectonics which caused this discrepancy ('bending' of the belt at western Kyushu) may be a clockwise rotation of Southwest Japan related to the opening of the Sea of Japan, during which western Kyushu remained fixed with respect to Eurasia 20 . However, all geological units in the Outer Zone of Southwest Japan such as the Chichibu Belt and the Shimanto Belt are bending southward at the southwestern part of Kyushu 32 (Fig. 1a), extending towards the Ryukyu Arc. Therefore, it is very difficult to assume that only the SMB is bending northward. We conclude that the NMC is a geological unit independent of the SMB.

petrography
We have found four types of microdiamond occurrences. Type 1: inclusions in chromite from a chromitite layer in serpentinite, Type 2: inclusions in pseudotachylyte in a magnesite-quartz rock (carbonated serpentinite), Type 3: inclusions in pyrite porphyroblasts in a metapelite, and Type 4: aggregates in the matrix of a metapelite. We focus on the microdiamonds in the metapelite, Types 3 and 4, especially on the last type of microdiamond, and we will describe the former two types elsewhere. Here we describe some important lithologies related to the microdiamond. the microdiamond-bearing metapelite. The microdiamond-bearing metapelite is a block, several tens of meters long, in the serpentine mélange. It has a distinct feature in that dolomite layers develop parallel to the schistosity (S1) and are folded (F2) asymmetrically together with the schistosity (Fig. 2a). Later dolomite veins cut these structures obliquely. The metapelite at Yukinoura has a mineral assemblage of graphite + chlorite + phengite + albite + quartz + pyrite + dolomite + magnesite + pseudomorph after titanite (rutile and/ or anatase + quartz ± calcite). Representative mineral analyses are presented in SI Table S1. Neither lawsonite nor epidote occurs in the metapelite, although epidote (or clinozoisite) is common in the coherent schists in the Nishisonogi unit. Breakdown texture of titanite (Fig. 2b,c) into rutile (and/or anatase) and quartz with or without calcite is commonly observed in all rock types involving metabasite, which will be described later, in the Yukinoura mélange. The breakdown reaction will be driven either by increase in X CO2 in the fluid (CO 2 -rich fluid infiltration) or by pressure increment. The pressure increment is more likely, because ubiquitous occurrence of pseudomorph after titanite in the Yukinoura mélange precludes the possibility of a local phenomenon such as fluid infiltration. Si content in phengite ranges from 3.33 to 3.43 in atoms per formula unit (apfu), and Mg content does from 0.48 to 0.53 apfu (SI Fig. S1). The empirical phengite barometer 33 gives 1.95 to 1.99 GPa for T = 450 o C 31,34 . The pressure is apparently lower than that for the graphite-diamond transition, probably because of the retrograde re-equilibration during exhumation. Garnet (Prp 3 Alm 60 Sps 20 Grs 17 in the core and Prp 2 Alm 68 Sps 6 Grs 24 in the rim) rarely occurs in the metapelite, and contains quartz as inclusions. Radial cracks have developed around the inclusion, and in one example cracks are filled with quartz emerging from the quartz inclusion, which are dammed by chlorite derived from the matrix (Fig. 2d). This texture strongly suggests the transformation from coesite to quartz during exhumation of the Yukinoura mélange. Pyrite occurs as strongly deformed porphyloblasts, containing inclusions of quartz, graphite and microdiamond (Fig. 2e). Microdiamond is confirmed by Raman microspectroscopy (SI Fig. S2). Electron probe microanalysis-soft X-ray emission spectroscopy (EPMA-SXES) analysis also showed a broad band derived from the sp 3 -bonded carbon of diamond (SI Fig. S3) in the spectra collected from inclusions in pyrite. These lines of evidence strongly suggest that the present mineral assemblage of the metapelite represents a retrograde one except microdiamond.

characterization of microdiamond
The microdiamond aggregates in Type 4 are always associated with Mg-carbonates (mostly dolomite and occasionally magnesite) and show irregular shapes of 10-50 μm in size that consist of numerous diamond grains embedded in phengite ( Fig. 4a-d). The phengite is very fine-grained (~ 500 nm in size), and is confirmed with scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS) and by the electron diffraction method. Each diamond crystal is subhedral to euhedral and 0.3-0.6 μm in diameter (Fig. 5a,b). The selected area electron diffraction (SAED) pattern is obtained from a grain (Fig. 5c)  Raman microspectroscopy measurements of the aggregates shows a strong band at 1,332 cm −1 , with a full width at half maximum (FWHM) of ~ 5 cm −1 , which corresponds to the T 2g mode for diamond (SI Fig. S5a). Most aggregates do not show any graphite bands, with two exceptions; one shows a weak and broad D1 band (1,350 cm −1 ) due to disordered graphite associated with a strong band of diamond at 1,332.5 cm −1 (SI Fig. S5b), and the other shows a diamond band at 1,316 cm −1 with a broad G band of graphite at around 1,570 cm −1 (SI Fig. S5c). These lines of evidence clearly indicate the presence of natural microdiamonds.

Discussion
temperature condition estimated by Raman microspectroscopy of graphite. The peak metamorphic temperature of the metapelite was estimated to be 450 °C according to Raman microspectroscopy of graphite 31,34 . Graphite shows two kinds of occurrence. The first type of graphite occurs along the cleavages of chlorite and phengite and also occurs as inclusions in albite and in quartz (Fig. 2b). The second type of graphite www.nature.com/scientificreports/ occurs as tiny grains in pseudomorphs after titanite (Fig. 2c). Only graphite inclusions in albite and in quartz (graphite grains beneath the surface of a transparent mineral) are measured by Raman microspectroscopy to estimate the formation temperature of graphite 31,34 , because graphite exposed on the surface of thin section (e.g., the case of the second type) is damaged by polishing to give lower temperatures 36 . The first type of graphite may have originated from carbonaceous matter in the original sediments, because it occurs along the cleavages of chlorite and phengite and also as inclusions in albite and in quartz. The second type of graphite may have precipitated from a C-O-H fluid during the breakdown of titanite into rutile + quartz + calcite. The first type of graphite may be a product of prograde graphitization, which is an irreversible reaction 37 . Therefore, the temperature estimated for the first type of graphite by Raman microspectroscopy can be regarded as a peak metamorphic temperature. Laboratory deformation experiments showed that the mechanical modification of graphite structure owing to deformation makes the estimation of metamorphic temperature by Raman microspectroscopy ambiguous 38 . However, graphite inclusions in albite and quartz can escape such effects of strong deformation; they will give a meaningful metamorphic temperature. The graphite could easily persist to diamond stability field without overcoming the barrier of activation energy to convert to diamond. If the formation temperature for the present microdiamond is the same as that of graphite (450 °C), the pressure condition will be higher than 2.8 GPa based on the graphite-diamond equilibrium (SI Fig. S6).
Genesis of microdiamond. Such aggregates of microdiamonds described above are rare in ultrahigh-pressure metamorphic (UHPM) terranes. Compared to polycrystalline microdiamonds reported from Kokchetav 5 and also from Erzgebirge 11 , the Yukinoura microdiamonds occur as aggregates of many single crystalline microdiamonds. The microdiamonds have the following distinct features. Firstly, the size of each microdiamond grain (0.2-0.6 μm) is much smaller than that (10-80 μm) of microdiamonds from other UHPM terranes 3 . Secondly, Yukinoura microdiamond has almost euhedral cuboid or octahedral forms, whereas those reported to date from www.nature.com/scientificreports/ UHPM terranes show a wide range of morphologies from skeletal, rose-like, and hopper to perfect euhedral shapes 3 . The morphology of diamond is dependent on the ratio of the driving force for nucleation and growth to the reaction kinetics 41 , and the latter is dependent on several factors, such as the temperature, the fluid composition, and the presence of melt. High temperature and the presence of melt facilitate the formation of euhedral diamond 41 ; however, that is not the case in Yukinoura. The coexistence of Mg-carbonates strongly suggests the presence of a C-O-H fluid. The C-O-H fluid may play a crucial role for the formation of highly crystalline microdiamond, even at low temperature, as shown in the case of Lago di Cignana (Western Alps, Italy), in which microdiamonds occur as inclusions in garnet formed at approximately 600 °C 16 . In the case of the Maksyutov Metamorphic Complex, South Ural Mountains 42 , the poor crystallinity is compatible with relatively low temperature, solid state growth in the absence of both melt and a C-O-H-N fluid 42 . The contrast between the two cases of low-temperature microdiamond clearly shows the potential importance of fluid in determining the crystallinity and morphology of microdiamond. The mechanism for the formation of microdiamond in metapelites is an interesting issue to be discussed. The possibility of detrital origin of diamond can be precluded, because microdiamonds will be decomposed into graphite during the prograde metamorphism. Conversely, microdiamonds can be preserved during the retrograde metamorphism because the activation energy may not be available for the transformation to graphite. Therefore, our discussion will be based on an assumption of the metamorphic origin of the microdiamonds. First, we discuss the meaning of the formation temperature of graphite estimated by Raman microspectroscopy and the relation between microdiamond and graphite. The first type of graphite may have originated from carbonaceous matter in the original sediments, because it occurs along the cleavages of chlorite and phengite and also as inclusions in albite and in quartz. The second type of graphite may have precipitated from a C-O-H fluid during the breakdown of titanite into rutile + quartz + calcite. The first type of graphite will be a product of prograde graphitization, which is an irreversible reaction 37 . Therefore, the temperature estimated for the first type of graphite by Raman microspectroscopy can be regarded as a peak metamorphic temperature. It is experimentally confirmed that hexagonal diamond (lonsdaleite) stably formed from well crystallized graphite by solid state transformation only at temperatures above 1,000 o C 43 . Therefore, graphite could easily persist to diamond stability field without converting to diamond in low-grade metamorphic rocks. If the formation temperature for the present microdiamond is the same as that of graphite (450 °C), the pressure condition will be higher than 2.8 GPa based on the graphite-diamond equilibrium. Such conditions correspond to the lawsonite eclogite facies 4 (see SI Fig. S3). The first question is the possibility of the metastable growth of microdiamond in the stability field of graphite. www.nature.com/scientificreports/ Simakov 44,45 showed that nanosized diamond can form from a C-O-H fluid under low pressures in the graphite stability field. Manuella 46 theoretically examined the metastable growth of diamond in serpentinite-hosted hydrothermal systems based on the Laplace-Young Equation 47 , which gives the additional curvature-induced pressure associated with the formation of spherical and quasi-isotropic nanocrystals. The size-dependent internal pressure Pa is defined as Pa = 4γ/d, where γ stands for surface free energy and d does size (nm). The transition pressure P decreases by Pa: P (GPa) = 0.003 T (K) + 0.4171 -Pa. The size of metastable diamond is determined by the following equation: d = -14.8/(P − 0.003 T − 0.4171) 47 . Based on this theory, Manuella 46 showed that the size of diamond will be smaller than 6 nm at temperatures between 700 and 1200 K. The Yukinoura microdiamonds are much larger (0.2-0.6 μm) than this; therefore, they are unlikely to have formed metastably. If Yukinoura microdiamonds formed in the diamond stability field, then the pressure would be as high as at least 2.8 GPa, which corresponds to the lawsonite-eclogite facies as mentioned above. We have several lines of evidence supporting high pressure (HP) to UHP condition: (1) pseudomorph after coesite (Figs. 2d, 3b), (2) high Si content in phengite as described in Petrography, and (3) pseudomorph (rutile + quartz ± calcite) after titanite (Fig. 2b,c), and (4) the possibility of nearby lawsonite-garnet assemblages (Fig. 3). A possible mechanism for the stable formation of microdiamond could be direct precipitation from a C-O-H fluid that consists of neutral species such as CO 2 , CH 4 , H 2 and H 2 O, in response to a change of oxygen fugacity 16  implications on the tectonics of the Southwest Japan. This is the first report of the occurrence of microdiamonds from a palaeo-subduction complex in Japan, which urges reconsideration of geotectonic evolution of the Japanese Archipelago. Kyushu is divided into the Inner Zone (northern part) and Outer Zone (southern part) by the Usuki-Yatsushiro tectonic line ('UYTL' in Fig. 6), along which a narrow sedimentary basin of the Cretaceous to the Palaeogene develops 23 . It merges with the Arita graben, trending north to south, at the east of the NMC. The Arita graben is mostly covered by Quaternary volcanic rocks (basalt and dacite with minor rhyolite) with a Palaeogene basement 48 . The NMC is separated by the graben, both from the Inner Zone and from the Outer Zone 23 . To the west of the NMC, the Oseto granodiorite ('OG' in Fig. 6) of ca. 90 Ma occurs in fault contact with the NMC, and has not had a thermal effect on the NMC 22 . The fault is named the Yobukono-Seto fault 22 ('YF' in Fig. 6), which is a dextral strike slip fault. To the west of the Oseto granodiorite, a Miocene volcanic arc developed in a north to south orientation 23 . A volcanic arc of the same age also occurs along the southeast part of Kyushu 23 . The parallel alignment of the two Miocene volcanic arcs suggests that the tectonics of Kyushu is not solely governed by the subduction of the Philippine Sea Plate. The NMC may have formed along a narrow zone between the Chinese craton and Kyushu by subduction of the marginal part of the Chinese craton, and displaced along the strike-slip fault system involving the Tsushima-Goto fault ('TGF' in Fig. 6) and the Yobukono-Seto fault during the opening of the Sea of Japan 48 . Although the lack of geophysical data such as seismic tomography in this region prevents us to construct a detailed tectonic model involving the underground structure, we will briefly discuss a possible tectonic model which may explain the development of the UHP unit in the western part of Kyushu. Guillot et al. 49 proposed three types of subduction regime to explain different styles of exhumation: the accretionary-type, the serpentinite-type and the continental-type. The Nishisonogi unit may represent the coexistence of an accretionary wedge and a serpentinite subduction channel in a single subduction zone 49 , because the unit consists mainly of metasedimentary rocks with several thin (less than 350 m) and elongated bodies of serpentinite melanges 19 . The existence of a decoupling zone within the upper part of the subducting slab is essential for the exhumation of HP to UHP rocks, because only slices of the upper decoupled part will be exhumed in contrast to the downgoing major part of the slab 50 . The serpentinite mélange may have acted as a decoupling zone in the case of the Nishisonogi unit. According to the numerical simulation 51 , the evolution of the subduction zone consists of two distinct stages: (1) the early stage without return flow from depths exceeding about 20 km and (2) the mature stage with intense return flow from greater depths. The hydration of the mantle wedge will control the transition between these two stages. The hydration of the hanging wall mantle may cause the widening of the subduction channel, resulting in the onset of forced return flow 51 . The serpentinite mélange will form by this forced return flow and is subsequently exhumed to the surface. Among several factors necessary for exhumation of HP to UHP metamorphic rocks 49 , thrusting associated with normal faulting and return flow in the subduction channel may be the important factors in the case of the Nishisonogi unit. The western border fault ('WBF' in Fig. 6) of the Arita graben will be a normal fault, and the Yobukono-Seto fault will be a thrust with a dextral slip component. At the mature stage of the subduction zone, the exhumation of the Nishisonogi unit may have been triggered by forced return flow owing to the hydration of hanging wall, and serpentinite mélanges may have acted as a detachment zone which enhanced the exhumation. Simultaneous activities of the western border fault of the Arita graben and the Yobukono-Seto fault were possibly associated with the return flow. As a result, the serpentinite mélange could have exhumed from the depth of UHP condition. Thus, finding of microdiamonds from the Yukinoura mélange unravels the hidden UHP history of the Nishisonogi unit, the Nagasaki Metamorphic Complex, which has been considered as an epidote-blueschist subfacies metamorphic unit until this finding.

Methods
Polished petrological thin sections were prepared with Al 2 O 3 lapping sheets (Sankyo Rikagaku Co. Ltd.) on a grooved glass plate to avoid contamination of diamond from diamond paste, after grinding with SiC. The sections were coated with Pt for SEM-EDS analysis. The same sections were used for micro-Raman spectroscopy. Raman spectra were measured with a Raman spectrometer (Horiba Jovin Yvon LabRAM HR800) equipped with a diode-pumped solid state laser (Fandango 50, 514.5 nm) at Kumamoto University. A silicon standard was used for calibration before each measurement. The laser power was set at 50 mW on the sample surface, and the laser was focused on the sample using a microscope (Olympus BX41) with a 100 × objective lens (LMPlan FL: NA = 0.80). The operating conditions were a 1,000 μm pinhole, a 25 μm slit, an 1,800 line/mm grating, and an acquisition time of 30 s.
Mineral chemistry was determined with a SEM-EDS system (JEOL JSM-7001F field emission scanning microscope equipped with an Oxford Aztec energy-dispersive X-ray spectrometer) operating at an accelerating voltage of 15 kV with a beam current of 1.02 nA.
A focused ion beam (FIB) system (FEI, Scios) was used to prepare transmission electron microscopy (TEM) foils (typically, 15 × 10 × 0.1 μm) from the target areas marked by the micro-Raman and FE-SEM examinations. Prior to milling, the target area was coated with a Pt layer (~ 1 μm thick) using a gas injection system equipped on the FIB to protect the outermost surface from the ion beam. TEM observations were performed using a microscope (JEOL JEM-2100F) operated at 200 kV and equipped with CCD cameras (Gatan Orius 200D and UltraScan 1,000) and an EDS detector (JEOL JED-2300T). Bright-field imaging and selected area electron diffraction (SAED) were used for microtexture observation and phase identification. Scanning TEM (STEM) imaging and chemical analysis with EDS were conducted using a focused electron beam (with a spot size of 0.5 nm) and a camera length of 40 cm.