Multi-stage serpentinization of ultramafic rocks in the Manlay Ophiolite, southern Mongolia Mongolian Geoscientist

Serpentinization of ultramafic rocks in ophiolites is key to understanding the global cycle of elements and changes in the physical properties of lithospheric mantle. Mongolia, a central part of the Central Asian Orogenic Belt (CAOB), contains numerous ophiolite complexes, but the metamorphism of ultramafic rocks in these ophiolites has been little studied. Here we present the results of our study of the serpentinization of an ultramafic body in the Manlay Ophiolite, southern Mongolia. The ultramafic rocks were completely serpentinized, and no relics of olivine or orthopyroxene were found. The composition of Cr-spinels [Mg# = Mg/(Mg + Fe 2+ ) = 0.54 and Cr# = Cr/(Cr + Al) = 0.56] and the bulk rock chemistry (Mg/Si = 1.21-1.24 and Al/Si < 0.018) of the serpentinites indicate their origin from a fore-arc setting. Lizardite occurs in the cores and rims of mesh texture (Mg# = 0.97) and chrysotile is found in various occurrences, including in bastite (Mg# = 0.95), mesh cores (Mg# = 0.92), mesh rims (Mg# = 0.96), and later-stage large veins (Mg# = 0.94). The presence of lizardite and chrysotile and the absence of antigorite suggests low-temperature serpentinization (<300 °C). The lack of brucite in the serpentinites implies infiltration of the ultramafic rocks of the Manlay Ophiolite by Si-rich fluids. Based on microtextures and mineral chemistry, the serpentinization of the ultramafic rocks in the Manlay Ophiolite took place in three stages: (1) replacement of olivine by lizardite, (2) chrysotile formation (bastite) after orthopyroxene and as a replacement of relics of olivine, and (3) the development of veins of chrysotile that cut across all previous textures. The complex texture of the serpentinites in the Manlay Ophiolite indicates multiple stages of fluid infiltration into the ultramafic parts of these ophiolites in southern Mongolia and the CAOB.


INTRODUCTION
Serpentinization is the formation of serpentine group minerals as a result of the hydrothermal alteration of Mg-rich minerals such as olivine, orthopyroxene, and clinopyroxene in ultramafic rocks (Pinti, 2011). Serpentine is an important group of hydrous minerals that contain up to 13% H 2 O (e.g., Hattori and Guillot, 2007), and the three important polymorphs (lizardite, chrysotile, and antigorite) may form variously in response to a variety of temperatures and geological settings (Evans et al., 2013).

Mongolian Geoscientist
Serpentinization commonly proceeds at slow to ultraslow spreading ridges, in mantle wedges (Hattori and Guillot, 2007;Deschamps et al., 2013), and along strike-slip faults Uno and Kirby, 2019). In terms of the stability of serpentine minerals, lizardite is stable at temperatures below 300 °C, the transition from lizardite to antigorite starts at around 300 °C, and chrysotile is considered metastable at any temperature (e.g., Moody, 1976;Deschamps et al., 2013;Evans et al., 2013). The precipitation of magnetite, brucite, and talc that often accompanies serpentinization provides information on the temperatures, water -rock ratios, and chemistry of interacting fluids during the serpentinization process (e.g., Bach et al., 2004;Frost and Beard 2007;Klein et al., 2014;Schwarzenbach et al., 2016;Oyanagi et al., 2020). Serpentinization plays a significant role in the global circulation of water and the geochemical cycles of subduction zones. Serpentine minerals incorporate water and fluid-mobile elements into their structures; therefore, they play an effective role in the transport of elements from slabs to mantle wedges and from mantle wedges to arc magmas (Hattori and Guillot, 2007). The buoyant property of serpentinite enables it to facilitate the exhumation of ultrahigh-and highpressure metamorphic rocks to the surface, and its rheology and dehydration cause deep seismic activity in subduction zones (e.g., Schwartz et al., 2001;Dobson et al., 2002;Hattori and Guillot, 2007). The serpentinization of ultramafic rocks is accompanied by the production of hydrogen, which can contribute to sustainable development by generating clean energy and sustaining ecosystems on the deep seafloor (e.g., Martin et al., 2008;Ohara et al., 2012;Wang et al., 2019). Furthermore, partially serpentinized ultramafic rocks and serpentinites are highly reactive to carbonation processes and have a high capacity for sequestering anthropogenic carbon dioxide emissions (e.g., Power et al., 2013;Wang et al., 2019). The ultramafic rocks of ophiolites preserve crucial information on the processes of serpentinization and carbonation. The Central Asian Orogenic Belt (CAOB), also known as the Altaids (Şengőr et al., 1993), is one of the best-preserved and largest orogenic belts. The belt is surrounded by the Siberian, North China, and Tarim cratons, and it extends from the Urals in the west through Kazakhstan, NW China, Mongolia, and NE China to the Okhotsk Sea in East Russia (Jahn et al., 2004). The CAOB records the opening and closure of the Paleo-Asian Ocean from the Neoproterozoic to the Paleozoic (Blight, 2010), but its tectonic evolution remains controversial. Şengőr et al. (1993) suggested that it formed by the crustal growth of subduction-accretionary complexes along a single magmatic arc. In contrast, Badarch et al. (2002) and Windley et al. (2007) suggested that it is an accretionary-type orogenic belt that formed by the amalgamation of fragments of island arcs, ophiolite complexes, seamounts, accretionary wedges, and microcontinents. The CAOB is considered an important area for studying juvenile crustal growth. Furthermore, the belt contains numerous Neoproterozoic to Triassic ophiolite complexes, which can provide vital information on (1) how the orogenic belt originated in the place of a former ocean (Furnes and Safonova, 2019) and (2) how hydration (serpentinization) and/or carbonation processes took place during the evolution of the belt (Dandar et al., 2019;Dandar et al., 2021). However, the metamorphism and hydrothermal alteration of ophiolites in the CAOB have not been studied sufficiently, except for the Khantaishir Ophiolite (Dandar et al., 2019;Dandar et al., 2021). In this paper, we present our results from a study of the completely serpentinized ultramafic rocks of the Manlay Ophiolite, which is one of the best -preserved and most significant ophiolites in southern Mongolia and the CAOB. We first present details of the microstructures and mineral chemistry of the ultramafic rocks. We then discuss plausible conditions for the serpentinization, the multiple stages of serpentinization, and how our results contribute to an understanding of the alteration of ultramafic rocks in the ophiolites of southern Mongolia and the CAOB.

GEOLOGICAL SETTINGS
In terms of its geology, Mongolia can be subdivided into northern and southern domains that are separated by the Main Mongolian Lineament (Fig. 1a). According to Badarch et al. (2002), the northern domain consists mainly of Precambrian and lower Paleozoic metamorphic rocks, Neoproterozoic ophiolite complexes, late Paleozoic island-arc, volcanic, and volcaniclastic rocks, Devonian to Carboniferous sediments, Permian volcanic-plutonic belts and marine and non-marine sediments, and granitoids of various ages. The southern domain consists mainly of island arc terranes, back-arc and fore-arc basins, and cratonic blocks. It contains an abundance of lower to middle Paleozoic arc-related volcanic and volcaniclastic rocks with fragments of ophiolite and serpentinite melange. In the western part of the domain, Lower Devonian to middle Carboniferous carbonate, siliciclastic, and volcanic rocks are distributed (Lamb and Badarch, 1997). In the eastern part, Silurian and Devonian fossil-rich limestones, terrigenous, and volcaniclastic rocks, Carboniferous to Permian volcanic rocks, Permian limestones, and turbidites occur. In addition, Mesozoic granite plutons are exposed and overlapped by Upper Jurassic to Cretaceous non-marine volcanic and sedimentary rocks (Badarch et al., 2002). South Mongolia has become a crucial area for mineral exploration, and there has been much research into subduction-related porphyry copper deposits such as the Late Devonian Oyu Tolgoi and Tsagaan Suvarga and the Carboniferous Shuteen and Kharmagtai deposits (Lamb and Cox, 1998;Perello et al., 2001) as well as research into the relationships of various mineral deposits and granitoids (Blight, 2010). Nevertheless, there is a general lack of geochemical and geochronological data on many of the dismembered ophiolites that are distributed in south Mongolia. Our study was focused on the Manlay ophiolite complex of Cambrian age, which has suprasubduction signatures, and which is thought to be the basement of late Paleozoic arc formations and correlate with the Lake Zone in western Mongolia. The ophiolite is located on the border of the Gurvansayhan and Mandalovoo islandarc terranes in southern Mongolia (Fig. 1a;Zhu et al., 2014a). The Gurvansayhan Terrane is interpreted to be the result of island arc accretion and rifting (Batkhishig et al., 2010), and consists of dismembered ophiolite and melange, Ordovician to Silurian greenschistfacies metamorphosed sandstones, argillites, cherts, and volcaniclastic rocks, upper Silurian to Lower Devonian radiolarian cherts, tholeiitic pillow basalts, andesites, and tuffs, and Middle Devonian to early Carboniferous volcaniclastic rocks, cherts, and minor olistostromes with coral limestone clasts. The terrane was intruded by Permian granitoids, and overlapped by Carboniferous, Permian, Jurassic, and Cretaceous volcanic and sedimentary rocks (Badarch et al., 2002). The central and northern parts of the terrane contain serpentinite bodies and highly deformed metamorphosed siliciclastic sequences (Lamb and Badarch, 1997). In comparison, the Mandalovoo Terrane is a long narrow belt that consists of Ordovician and Silurian sandstones, argillites, and limestones; Lower Devonian conglomerates, sandstones, limestones, and felsic tuffs; Upper Devonian pillow basalts, andesites, volcaniclastic sandstones, and cherts; lower Carboniferous marine sedimentary rocks; and Devonian plutonic rocks (Badarch et al., 2002). The geochemical and geochronological characteristics of crustal rocks in the Manlay ophiolite complex show that the ophiolite is a basement of Devonian and early Carboniferous arc rocks that could be linked to the Lake Zone in western Mongolia (Zhu et al., 2014a). The Manlay ophiolitic complex consists mainly of ultramafic rocks, basalts, and cherts (Fig. 1b). The western part is characterized by basalts with minor chert, ultramafic rocks, plagiogranites, gabbros, and pyroxenites, the eastern part is dominated by carbonated ultramafic rocks, intrusions of dolerite, and thin chromitite veins, the southeastern part exposes pillow lavas, and the northwestern part contains andesites and tuffaceous sandstones interlayered with siliceous shale. The plagiogranites and gabbros yield U-Pb ages of 482-509 Ma. Whole-rock geochemical data indicate that the origin of the Manlay Ophiolite was a supra-subduction zone, because the basalt and diabase samples are enriched in large-ion lithophile elements (LILEs; Rb, Sr, Pb and K) and light rare-earth elements (LREEs; La, Ce, Pr, Nd, and Sm) with moderate to strong negative Nb, Ta, and Ti and weak Eu anomalies. The ultramafic rocks have very high loss on ignition (LOI) values that correspond to the extensive serpentinization and carbonation (Zhu et al., 2014a).

Samples and analytical techniques
The analyzed serpentinized ultramafic rocks from the Manlay Ophiolite (Fig. 1b [N44°0 8'44'';E107°54'56''] and Table 1) are massive and show dark gray to brownish colors (Fig. 1c). In outcrop, the serpentinites are cut by white to brownish carbonate veins (Fig. 1c). The mineral assemblages and microtextures were observed in thin sections with an Olympus BX51 microscope equipped with an Olympus DP21 camera at Tohoku University, Japan. Raman spectroscopy was performed with a Horiba XploRa Raman spectrometer and Olympus BX51 microscope at Tohoku University to distinguish the minerals of the serpentine group (lizardite, chrysotile, and antigorite). The calibrations were performed using synthetic silicon and the 520 cm −1 band. The estimated spectral resolution was 1.0 cm −1 . We used a grating of 1800 g/mm, a x100 objective, a 1 μm laser spot size, and a green laser with a wavelength of 532 nm, and the integration time was 5 s. The serpentine minerals were identified using information from Auzende et al. (2004) and Schwartz et al. (2013), and spectral regions were from 200 to 1200 cm −1 for lower wavelengths and from 3600 to 3800 cm −1 for higher wavelengths (Fig. 2). The Raman spectra of each mineral have been double-checked against the RRUFF Raman database. The chemical compositions of the minerals were determined using an electron probe microanalyzer (EPMA; JEOL JXA-8200) at Tohoku University. The natural and synthetic standards used for calibration were wollastonite for Ca and Si, rutile for Ti, eskolaite for Cr, hematite for Fe, manganosite for Mn, periclase for Mg, albite for Na, and K-feldspar for K. The accelerating voltage and beam currents were set at 15 kV and 12 nA for the quantitative analyses. The focused beam diameter was 1-2 µm. The counting times for the peaks and backgrounds were 10 and 5 s, respectively. Elemental mappings were performed in order to understand the distribution and correlations of elements. The accelerating voltage and beam currents were set at 15 kV and 120 nA for mapping, and the dwell time was 30 s per pixel. The concentrations of major elements (SiO 2 , TiO 2 , Al 2 O 3 , Fe 3 O 2 , MnO, MgO, CaO, Na 2 O, K 2 O, and P 2 O 5 ) were determined using wavelength dispersive X-ray fluorescence spectrometry (WDXRF; Rigaku ZSX Primus IV) with relative mean square error (RMSE) ranging from 0.77% for SiO 2 to 0.003% for MnO and correlation coefficient (R) of the linear regression lines of >0.99, whereas the minor and some trace elements (V, Cr, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Sn, Cs, Ba, La, Ce, Pr, Nd, Pb, and Th) were measured using energy dispersive X-ray fluorescence spectrometry (EDXRF; PANalytical Epsilon 5; Table 2) with RMSE of < 28 ppm and R of > 0.96, except for Co with R = 0.89 and Pr with R = 0.84. In terms of sample preparation, ~100 g of each rock sample was crushed and milled to get a homogeneous powder. In order to eliminate moisture, 6 grams of powder from each sample were dried for 2 h at 105 °C. The LOI values were determined after each sample had been heated at 950 °C in an electric furnace for 2 hours. The fusion glass bead method was used for the determination of major elements. Each glass bead was made from a mix of 5.4 g of lithium borate flux (LiBO 2 = 49.5%, Li 2 B 4 = 49.5%, and LiBr = 0.5%) and 0.6 g of powder sample that was then fused at 1150 °C. The glass beads were exposed to Xrays using WDXRF (Rigaku ZSX Primus IV) and a Rh anode tube with an acceleration voltage of 75 kW and a beam current of 8 mA.
Polyvinyl chloride (PVC) rings filled with homogenous powder from each sample were subjected to pressures of 150 kNPa and 200 kNPa for 5 min, respectively, to make the pressed powder pellets used for estimating the concentrations of minor and trace elements. To measure V, Cr, Co, Ni, Cu, and Zn, the pellets  were exposed to X-rays using EDXRF, a Sc/W anode tube, a 75 kW acceleration voltage, and an 8 mA beam current. A 100 kW acceleration voltage and 6 mA beam were used to measure Rb, Sr, Y, Zr, Nb, Sn, Cs, Ba, La, Ce, Pr, Nd, Pb, and Th. Standard samples provided by the Geological Survey of Japan (GSJ; JP-1) were also measured during the XRF analyses. The WDXRF and EDXRF instruments was calibrated using 26 geological and environmental standard reference materials obtained from GSJ (Yamasaki et al., 2011).

Microtextures of the serpentinites
Serpentinites in the Manlay Ophiolite are made up dominantly of chrysotile and lizardite along with minor magnetite, calcite, and Cr-spinel ( Table 1). The primary olivines and orthopyroxenes of the original ultramafic rocks are completely gone from all the serpentinite samples. The low wavenumber Raman spectra peaks (124-126, 225-226, 379-380, and 1099-1100 cm −1 ) and the high wavenumber peaks (3680-3681 and 3701 cm −1 ) indicate lizardite (Fig. 2a). The peaks at low wavenumbers (126-127, 225-230, 381-385, 685-690, and 1100 cm −1 ) are similar to those of lizardite, but a single high wavenumber peak (3692-3694 cm −1 ) indicates chrysotile ( Fig. 2a and b). Chrysotile occurs commonly as a bastite texture (orthopyroxene pseudomorphs) and in large veins, and small amounts also occur in mesh texture cores and rims. In contrast, lizardite occurs only as mesh texture (Fig. 3a-e). Mesh cores and large veins show three high wavenumber peaks (3686-3687, 3692, and 3699 -3700 cm −1 ) indicating a mixture of chrysotile and lizardite at the microscopic level (less than the size of the beam spot of the micro-Raman spectroscope; Fig. 2b). We refer to this mixture of lizardite and chrysotile in mesh cores and large veins as chrysotile because (1) the Al and Fe contents of the chrysotile mesh cores are similar to the Al and Fe contents of the chrysotile in bastite textures and mesh rims, and (2) the chrysotile large veins are crack-filling veins in which lizardite particles could be included (Fig. 3). If chrysotile occurs in the mesh rims and bastite textures, it is colorless, but the large veins have a yellowish color (Fig.   3a-c). Moreover, lizardite is colorless, but if it occurs in the mesh cores, it takes on a brownish color (Fig. 3b). The grain size of the lizardite varies from 50 to 200 μm. The chrysotile grains in bastites are ~10 μm in size and size of the bastite varies from 125 to 2000 μm. Finegrained (mostly <10 μm, rarely up to 100 μm) calcite occurs as veins and patchy textures that crosscut the serpentine minerals and are associated with fractures (Fig. 3b). In contrast, fine-grained (10-100 μm) magnetite forms veins within the bastite that follow the former cleavages of the orthopyroxene (Fig. 3b-e), and it is also found inside mesh textures ( Fig. 3e and d) and locally within calcite. Cr-spinel occurs as anhedral to subhedral crystals 100-2000 μm in size (Fig. 3a, d, and e), and it is usually rimmed by magnetite.

Bulk rock major element compositions
The bulk rock compositions of three serpentinite samples from the Manlay Ophiolite are shown in Table 2. The SiO 2 , MgO, and FeO contents of the serpentinites show ranges of 44.5-45.2, 42.0-43.5, and 7.84-9.12 wt.%, respectively. The Al 2 O 3 and CaO contents exhibit ranges of 0.59-0.99 and 0.67-1.28 wt.%, respectively. The LOI values vary from 14.5 to 15.1 wt.% (Table 2). The ratios of Mg/Si in the serpentinites vary from 1.219-1.245 and the Al/Si ratios are lower than 0.018 (Table 2).

Plausible conditions of the serpentinization
Primary minerals of the original ultramafic rocks, such as olivine and orthopyroxene, have not been observed in our serpentinite samples. The mesh serpentines are low in Al and Cr, whereas the bastite serpentines are high in Al and Cr ( Fig. 4a and b). The elemental mapping and thin section observations show that the bastite serpentines, which replaced orthopyroxene, occupy ~12% of the serpentinite samples, which suggests that the ultramafic protolith was a harzburgite (Fig 3). The values of Mg# (0.51-0.59) and Cr# (0.50-0.59) in the Cr-spinels of the serpentinites are similar, which indicates a fore-arc peridotite (Fig 4d). In order to understand the serpentinization process, the various serpentine mineral species were identified. Lizardite and chrysotile are dominant in our samples and antigorite is absent, which suggests the serpentinization took place at temperatures below 300 °C  ; Fig 2), since many studies have emphasized that the transformation of lizardite to antigorite occurs at around 300-400 °C in a subduction zone setting (e.g., Deschamps et al., 2013. The Si + Al and Mg + Fe compositions of the serpentine minerals indicate that brucite is absent from our analyzed samples (Fig 4c). The (MgO-FeO)/SiO 2 ratios of the serpentinites are slightly lower than those of the original harzburgite (based on Ol = 87 vol.%, Opx = 12 vol.%, spinel = 1 vol.%), which suggests that a Si-rich fluid infiltrated and reacted with the harzburgites (Fig 5) Hattori and Guillot, 2007). Based on the identification of serpentine mineral species, mineral chemistry, and bulk rock compositions, we propose, therefore, that the ultramafic rocks of the Manlay Ophiolite interacted with silicarich fluids to become serpentinized at shallow levels of the mantle wedge (at temperatures lower than 300 °C).

Multi-stage serpentinization
The ultramafic rocks of the Cambrian Manlay Ophiolite may have been affected by several stages of serpentinization, and we identify three possible stages based on cross cutting relationships and mineral chemistry (Fig 6). First, compared with orthopyroxenes and clinopyroxenes, natural olivines tend to hydrate readily at low temperatures and pressures (e.g., Moody, 1976). The serpentinization of olivine produces mesh textures that develop during volume expansion (~64%; Holland and Powell, 1998) and hydration. In the Manlay Ophiolite, lizardite was found in the mesh textures, according to the results of Raman spectroscopic analysis (Fig 2). Consequently, we define Stage I of the ser pentinization pr ocess as the formation of lizardite owing to the serpentinization of olivine, as shown in Eq. 1.

Olivine Water Silica Lizardite
The infiltration of Si-rich fluids into an ultramafic rock at low temperatures (<300 °C) tends to inhibit brucite precipitation (e.g., Bach et al., 2004;Sonzogni et al., 2017), and the absence of brucite in our specimens indicates that the olivine of ultramafic rocks in the Manlay Ophiolite reacted with Si-rich fluids to generate lizardite without brucite (Fig 4a-c). The serpentinization of orthopyroxene produces bastite textures, which are pseudomorphs of orthopyroxene and easily distinguished by the preservation of the pyroxene cleavage positions (e.g., Deschamps et al., 2013), and we define Stage II of the ser pentinization pr ocess as the formation of chrysotile through serpentinization of orthopyroxene (see Eq. 2 below ; Fig 3b and

Olivine Water Silica Chrysotile
Coupled reactions such as Eqs 2 and 3, where silica is released during the serpentinization of orthopyroxene and the released silica then reacts with olivine to form serpentine, have been observed in hydrothermal experiments at 250 °C (Ogasawara et al., 2013). According to Evans et al. (2013), chrysotile is found mainly as a filling of fractures that crosscut the serpentinite, indicating it is a common process that occurs as the last stage of serpentinization. In the Manlay Ophiolite, mesh and bastite serpentines are cut by chrysotile veins (Fig 3a and e). Therefore, we define Stage III, the last stage of the ser pentinization process, as the development of large veins of chrysotile (Figs 3a and 6).
Although the multi-stage serpentinization of the Cambrian Manlay Ophiolite may be representative of the history of serpentinization in other Cambrian ophiolites (Gobi-Altai, Gurvan Saykhan-Zoolen, Biluutiin Ovoo, and Namdain Hundy) in south Mongolia, further studies of those other ophiolites are still needed if we are to reach a better understanding of the serpentinization and carbonation of ultramafic rocks during the opening of the Paleo-Asian Ocean in southern Mongolia. In addition, the Carboniferous (?) Altan-Uul and Permian Solonker ophiolites may provide key information on the processes of serpentinization and carbonation during the closure of the Paleo-Asian Ocean. These studies indicate clearly that the ophiolites of the CAOB record crucial information on mantle-fluid interactions during both the opening and closure of the Paleo-Asian Ocean.

CONCLUSIONS
The ultramafic rocks of the Manlay Ophiolite in southern Mongolia, originally composed of olivine and orthopyroxene, have been serpentinized pervasively. Studies of the mineralogy, microstructures, and mineral chemistry of these rocks have revealed many details of the processes of their serpentinization. The compositions of relict Cr-spinels indicate a fore-arc setting for the rocks of the Manlay Ophiolite. Lizardite and chrysotile are the dominant serpentine species, and they demonstrate that the serpentinization took place at low temperatures (<300 °C). The ultramafic rocks of the Manlay Ophiolite were serpentinized by Si-rich fluids, generating serpentines without brucite. Analyses of the microstructures and mineral chemistry of these rocks reveal three stages of serpentinization: (1) the alteration of most olivine to lizardite; (2) the alteration of orthopyroxene and remnant cores of olivine to chrysotile; and (3) the development of chrysotile veins that cut across previous serpentines. This multi-stage process of serpentinization provides new insights into the metamorphic history of ophiolite complexes in southern Mongolia and the CAOB, and similar work on the numerous ophiolite complexes in the CAOB should improve our understanding of mantlefluid interactions during the opening and closure of the Paleo-Asian Ocean.