minerals Article Post-Serpentinization Formation of Theophrastite-Zaratite by Heazlewoodite Desulfurization: An Implication for Shallow Behavior of Sulfur in a Subduction Complex Shoji Arai 1,* , Satoko Ishimaru 2, Makoto Miura 3, Norikatsu Akizawa 4 and Tomoyuki Mizukami 5 1 Kanazawa University, Kanazawa 920-1192, Japan 2 Department of Earth Sciences, Kumamoto University, Kumamoto 860-8555, Japan; [email protected] 3 Identification Department, GIA Tokyo Godo Kaisha, Yamaguchi Building 7, 4-19-9 Taito, Taito-ku, Tokyo 110-0016, Japan; [email protected] 4 Department of Ocean Floor Geoscience, Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan; [email protected] 5 Department of Earth Sciences, Kanazawa University, Kanazawa 920-1192, Japan; peridot@staff.kanazawa-u.ac.jp * Correspondence: ultrasa@staff.kanazawa-u.ac.jp Received: 23 August 2020; Accepted: 11 September 2020; Published: 13 September 2020 Abstract: Rare nickel hydroxide-hydroxyl carbonate, theophrastite (Ni(OH)2)-zaratite (Ni (CO )(OH) 4H O) aggregates were found from a partially serpentinized dunite from 3 3 4· 2 Fujiwara, the Sanbagawa metamorphic belt of high-pressure intermediate type, Japan. The dunite was regionally metamorphosed within the Sanbagawa subduction complex of Cretaceous age. The theophrastite-zaratite aggregate from Fujiwara most typically occurs in association with nickel sulfides, which form a composite grain with awaruite and magnetite within an antigorite-rich part of the rock. The theophraste-zaratite formed possibly together with millerite (NiS) from heazlewoodite (Ni3S2). This represents a partial desulfurization of heazlewoodite, which contains or interlocks with laths of antigorite, suggesting their cogenesis. The desulfurization occurred at an early stage of, or during, exhumation of the subduction complex toward the surface, where sulfur was oxidized and removed as sulfate ions. Serpentinization of olivine has not been associated with the formation of theophrastite-zaratite, and an oxidized condition has been kept at this post-serpentinization stage. The sulfate ions liberated in part precipitated anhydrite where calcium was available in the surrounding rocks. This shows one of the shallow migration pathways of sulfur in the subduction zone, especially to the forearc area. Keywords: theophrastite; zaratite; serpentinization; heazlewoodite; millerite; desulfurization; subduction; exhumation; Sanbagawa metamorphic belt; Japan 1. Introduction The behavior of sulfur in a subduction zone is very important and has been controversial [1–3]. Sulfur is transported to deeper parts via the subduction of sediments and altered crustal rocks such as meta-basalts [4]. It is also taken from sea water into peridotite on sea-floor serpentinization [5–7], and serpentinized peridotites gradually release sulfur upon progressive metamorphism in the subducted slab [8]. This is consistent with the relative sulfur enrichment in arc magmas [9]. However, Minerals 2020, 10, 0806; doi:10.3390/min10090806 www.mdpi.com/journal/minerals Minerals 2020, 10, x FOR PEER REVIEW 2 of 15 Minerals 2020, 10, 0806 2 of 16 the manners of delivery of sulfur from the slab to the overlying crust-mantle have not been thoroughly delineated so far. We would like to show one of the ways of sulfur recycling through theheazlewoodite, manners of delivery a nickel of sulfide, sulfur from in subducted the slab to serp the overlyingentinite, possibly crust-mantle to the have supra-subduction not been thoroughly zone. delineatedNickel so far.is compatible We would together like to show with onemagnesium of the ways in a of magmatic sulfur recycling system throughand concentrated heazlewoodite, in early a nickelformed sulfide, mafic minerals, in subducted especially serpentinite, in olivine possibly [10] in to addition the supra-subduction to some sulfides. zone. On the low temperature hydrationNickel is of compatible Mg-rich olivines, together nickel with then magnesium released in from a magmatic olivine is system reduced and to concentrated be crystallized in earlymainly formedas Ni-Fe mafic alloys minerals, (awaruites) especially inand/or olivine sulfides [10] in addition such as to someheazlewoodite sulfides. On [11–14]. the low temperatureHydrogen is hydrationsimultaneously of Mg-rich produced, olivines, coupled nickel then with released magnetite from pr olivineecipitation is reduced in the serpentinization to be crystallized process mainly [15]. as Ni-Fe alloysTheophrastite (awaruites) is anda rare/or sulfidesnickel suchhydroxide, as heazlewoodite first described [11–14 from]. Hydrogen highly isaltered simultaneously chromitite produced,(magnetite-chromite coupled with ore) magnetite from northern precipitation Greece in [16,17] the serpentinization, as well as from process altered [15 ].chromitites in Unst, ShetlandTheophrastite (Scotland) is [18]. a rare In nickeladdition hydroxide, to theophrast firstite, described hydroxyl fromnickel highly carbonates altered such chromitite as zaratite (magnetite-chromite[Ni3CO3(OH)4·4H2O], ore) otwayite from northern [Ni2CO Greece3(OH)2·H [162O],,17 ],nullaginite as well as [Ni from2CO altered3(OH)2] chromititesand hellyerite in Unst,[NiCO Shetland3·6H2O] (Scotland)and Ni-rich [ carbonates18]. In addition such as togaspéite theophrastite, [(Ni, Fe, Mg)CO hydroxyl3] are nickel precipitated carbonates during such low- astemperature zaratite [Ni alterationCO (OH) or4H weatheriO], otwayiteng of serpentinites [Ni CO (OH) andH relatedO], nullaginite nickel-rich [Ni oresCO [19–24].(OH) ]In and low- 3 3 4· 2 2 3 2· 2 2 3 2 hellyeritetemperature [NiCO alteration6H O] andproce Ni-richsses of carbonatesserpentinites, such nickel as gasp (II)é isite also [(Ni, incorporated Fe, Mg)CO in] are hydrous precipitated silicates 3· 2 3 during(clay low-temperatureminerals) suchalteration as pecoraite or weathering [Ni3SiO of5(OH) serpentinites4] and nickel-rich and related sepiolite nickel-rich around ores [19 –nickel24]. Insulfides/oxides low-temperature [21–25]. alteration processes of serpentinites, nickel (II) is also incorporated in hydrous silicatesAll (clay of minerals)the previous such works, as pecoraite however, [Ni3 SiOhave5(OH) given4] andus nickel-richrather weak sepiolite genetic around constraints nickel on sulfidestheophrastite/oxides [and21–25 hydroxyl]. nickel carbonates, as well as weak petrological implications. Aggregates of Alltheophrastite, of the previous Ni(OH) works,2, and however, a nickel have hydroxyl given us carbonate rather weak (zaratite genetic constraints= Ni3(CO3)(OH) on theophrastite4·4H2O) were andfound hydroxyl in a nickel partly carbonates, serpentinized as well (antigoritized) as weak petrological dunite implications. from Fujiwara Aggregates (Ehime of Prefecture), theophrastite, the Ni(OH)Sanbagawa, and ametamorphic nickel hydroxyl belt, carbonate southwest (zaratite Japan= (FNiigure(CO 1).)(OH) They4H showO) were a very found clear in amode partly of 2 3 3 4· 2 serpentinizedoccurrence, (antigoritized)that is, a close dunite association from Fujiwara with ni (Ehimeckel sulfides Prefecture), (heazlewood the Sanbagawaite and metamorphicmillerite). The belt,Fujiwara southwest theophrastite-zaratite Japan (Figure1). They aggregate show a veryseems clear quit modee different of occurrence, in the mode that is,of aoccurrence close association from the withother nickel reported sulfides occurrences, (heazlewoodite which and were millerite). mostly The alon Fujiwarag shear theophrastite-zaratiteplanes in altered chromitites aggregate and seems nickel quiteores. di ffInerent this in article, the mode we ofdescribe occurrence the fromnickel-rich the other minerals, reported especially occurrences, theophrastite-zaratite which were mostly along(nickel shearhydroxide-hydroxyl planes in altered chromititescarbonate), and in a nickel partly ores. antigo In thisritized article, dunite, we describe and discuss the nickel-rich their origin minerals, with an especiallyimplication theophrastite-zaratite of their occurrence (nickel in a subduction hydroxide-hydroxyl complex. carbonate),More general in apetrological partly antigoritized characteristics dunite, of andthe discuss Fujiwara their meta-dunites origin with anare implication available from of their Arai occurrence et al. [26]. in a subduction complex. More general petrological characteristics of the Fujiwara meta-dunites are available from Arai et al. [26]. FigureFigure 1. Location1. Location of the of Fujiwarathe Fujiwara dunite dunite in the in Sanbagawa the Sanbagawa metamorphic metamorphic belt, Japan. belt, Modified Japan. Modifiedfrom [27, 28from]. (a)[27,28]. The Sanbagawa (a) The metamorphicSanbagawa metamorphic belt in Shikoku belt Island, in Shikoku southwest Island, Japan. southwest (b) Part of Japan. the Sanbagawa (b) Part beltof the andSanbagawa the location belt of theand Fujiwara the locati bodyon of ( cthe). Fujiwara body (c). Minerals 2020, 10, 0806 3 of 16 Minerals 2020, 10, x FOR PEER REVIEW 3 of 15 2. Geological Background and Petrography The dunite sample (T4-8) was obtained from the Fujiwara body, one of the abundant peridotite masses [[29,30]29,30] inin thethe SanbagawaSanbagawa metamorphic metamorphic belt belt in in Shikoku Shikoku Island, Island, southwest southwest Japan Japan [31 [31,32],32] (Figure (Figure1). They1). They are are associated associated with with crystalline crystalline schists schists of high-pressureof high-pressure intermediate intermediate type type [29 [29],],