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Plate Tectonic Gemstones Plate tectonic gemstones Robert J. Stern1*, Tatsuki Tsujimori2*, George Harlow3*, and Lee A. Groat4* 1Geosciences Department, University of Texas at Dallas, Richardson, Texas 75083-0688, USA 2Pheasant Memorial Laboratory, Institute for Study of the Earth’s Interior, Okayama University, 827 Yamada, Misasa, Tottori 682-0193, Japan 3Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York 10024-5192, USA 4Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada ABSTRACT JADEITE: THE SUBDUCTION GEMSTONE The gemstones jadeite and ruby generally form as a result of Jade is a term ascribed to two different materials with similar proper- the plate tectonic processes subduction and collision. Jade made ties, toughness, and beauty that evolved in usage and signifi cance from of jadeite (jadeitite) forms when supercritical fl uids released from toolstones for axes, choppers, and hammers to one of the most highly subducting oceanic crust condense in the overlying mantle wedge, revered gemstones in the world. As a tool, jade was employed during the 20–120 km deep in the Earth. Jadeitite deposits thus mark the loca- Paleolithic (before 35,000 BCE) but was raised to high symbolic stature tion of exhumed fossil subduction zones. Ruby, the red gem vari- as a gemstone in proto-Chinese Hongshan and Liangzhu cultures by 3500 ety of corundum, forms during amphibolite- and granulite-facies BCE, in the Jomón culture of Japan by 3000 BCE, and in Central America by metamorphism or melting of mixed Al-rich and Si-poor protoliths, the Olmec of the Early Formative period by at least 1500 BCE, and later in 10–40 km deep in the crust. Suitable conditions generally exist where Mayan civilization. Both forms of jade, termed yü (玉) in China, are nearly passive-margin carbonates and shales are involved in continental monomineralic rocks: Jadeite jade (or jadeitite) consists predominantly of 硬玉 collision. Most ruby deposits formed during Ediacaran-Cambrian the pyroxene jadeite (NaAlSi2O8) and is hard jade (ying yü— ), while (ca. 550 Ma) collisions that produced the East African–Antarc- nephrite jade is tremolite-actinolite [Ca2(Mg,Fe)5Si8O22(OH)2] and is soft tic orogen and the supercontinent Gondwana, or during Cenozoic jade (ruan yü—軟玉). The term jade was derived from the Spanish piedra collisions in south Asia. Ruby is thus a robust indicator of conti- de yjada (loin stone) for talismans worn by the Aztec to ease abdominal nental collision. As a result of these diagnostic properties, we pro- pain, but was mistranslated to the word jade (Harlow et al., 2007). In New pose the term “plate tectonic gemstones” (PTGs) for jadeitite and Zealand nephrite is sometimes called greenstone, a favorite material of ruby. The PTGs are a new type of petrotectonic indicator that are the Maoris (their pounamu). Jadeitite is found in association with other mostly found in Neoproterozoic and younger rocks. The PTGs as high-pressure/low-temperature (HP-LT) metamorphic lithologies that are petrotectonic indicators that form deep in the Earth have the added diagnostic of fossil subduction zones. This assemblage typically includes advantage that their record is unlikely to be obliterated by erosion, subducted oceanic crust transformed to blueschist (glaucophane metaba- although the possibility of destruction via retrogression needs to be salt) and eclogite (Fig. 1), and mantle wedge material (serpentinized peri- further assessed. Recognition of the PTGs links modern concepts of dotite), typically as mélange matrix (Harlow et al., 2007). Such an assem- plate tectonics to economic gemstone deposits and ancient concepts blage probably represents an exhumed subduction channel (Vannucchi et of beauty, and may aid in exploration for new deposits. al., 2012), in which buoyancy-driven return fl ow above the plate interface has brought subducted and mantle-wedge materials back to the surface. INTRODUCTION Jadeitites form in this environment at a wide range of depths, typically Any mineral or stone beautiful enough to be sought, mined, and sold 20–60 km but occasionally as deep as 100 km (Fig. 2). for its beauty alone is a gemstone (Groat, 2012). The subclass of rocks Of the two jade rocks, jadeitite is the actual subduction indicator. and minerals that comprises gemstones—whether precious or semi-pre- Jadeite, by virtue of its density (3.4 g cm–2), is a high-pressure indica- cious—has mostly been established since antiquity (a few new gemstones tor, and thinking on its signifi cance predates plate tectonic theory (e.g., have been recognized more recently, for example tanzanite). Humans have Yoder, 1950a, 1950b; Miyashiro and Banno, 1958), but a rock essentially sought and prized gemstones since thousands of years before the science formed of jadeite is not simply interpreted as a metamorphic rock. With of geology was established. Because gemstones are rare by defi nition, the the realization that jadeitite is a precipitate or metasomatic replacement geological conditions that produced them must have been exceptional. from hydrous fl uids released during dehydration of subducted oceanic Thus, there is a confl uence of economic, esthetic, and academic interest crust, the “jadeite problem” was resolved. High pressure in subduction in gemstones. In this contribution we build on this common interest by zones enhances dissolved solute concentrations in hydrous fl uids released exploring the plate tectonic signifi cance of two gemstones, both of which from subducted materials, enriched in Na, Al, and Si, such that the pri- are generally produced by plate tectonic processes: jadeitite and the gem mary saturated phase is jadeite (Manning 1998, 2004). These hydrous variety of corundum, ruby. These gemstones are products of plate conver- fl uids are buoyant and fl ow up to infi ltrate and react with the overlying gence, and refl ect end-member processes of subduction and collision and mantle wedge sole, which itself becomes pervasively altered (Kimura et thus different protoliths and thermal regimes. We summarize how jade— al., 2009), forming jadeitite veins. The occurrence of relict chromian spi- specifi cally the variety jadeite—is the characteristic beautiful product of nel in many jadeitites further indicates reaction between jadeitite and host normal oceanic lithosphere subduction and that rubies are the character- ultramafi c rocks (Tsujimori and Harlow, 2012). Jadeitites thus serve as a istic beautiful products of continental collision. We further explore what proxy for the related mass transfer within a subduction zone at relatively these “plate tectonic gemstones” (PTGs) can add to our understanding of shallow depths (<100 km). the fundamental processes that produced them: “collision-type (A-type)” Jadeitites form under P-T conditions that are somewhat hotter than and “Pacifi c-type (B-type)” plate tectonic regimes (Maruyama et al., expected for the subduction interface, even compared to hot subduction 1996; Liou et al., 2004). zones where young crust is subducted, for example beneath southwest- ern Japan (Fig. 2). This further suggests that jadeite forms in the warmer mantle wedge, above the subduction interface (Fig. 2). Jadeite deposits *E-mails: [email protected]; [email protected]; gharlow@ are found in the northern continents, especially North America and Eur- amnh.org; [email protected]. asia, where 15 deposits are documented (Table DR1 in the GSA Data GEOLOGY, July 2013; v. 41; no. 7; p. 1–4; Data Repository item 2013202 | doi:10.1130/G34204.1 | Published online XX Month 2013 GEOLOGY© 2013 Geological | July Society2013 | ofwww.gsapubs.org America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. 1 Jade (jadeitite) A Forearc basement Jade (jadeitite) Dia Volcanic front HP serpentinite Tsujimori and Harlow (2012) Accretionary prism Gr mélange Ruby and sapphire 120 Trench (metamorphic gem (GPa) corundum) Dry EC MagmaticMagmatic arcarc crustcrust P t Simonet et al. (2008) ic crus Fluid-fluxed OceanicOcean crust 3.0 Coe melting Qz Serpentinized forearc Exhuming HP rocks Lithospheric mantle NE Japan slab surface Ep-EC Hydrous eclogite Blueschist-facies 80 metamorphism Lw-EC Jd + Qz Dsp Ab Eclogite-facies Crn Dry eclogite Asthenosphere 30 km metamorphism 30 km 2.0 Amp-EC ite Ruby and sapphire (gem corundum) B ran ite Collisional metamorphic terranes Hinterland sediments Anatectic migmatite Passive margin Solidus Wet G sediments Solidus Oceanic crust HGR Wet Tonal Jd 40 SW Japan slab surface + Nph ental crust 1.0 ContinentalContin crust BS EA Ab l crust tinentaal crust Jd ayan ContinentCon AM GR Anl Himal SCLM Amphibolite- and granulite- GS ter facies metamorphism Dsp Grea Sequence Exhumed Crn Anl UHP domain inental Depth (km) (SCLM) Ab + Nph Subcont 30 km 0 lithospheric mantle 30 km 200 400 600 800 T (°C) Figure 1. Cross sections showing characteristic tectonic environ- Figure 2. Pressure-temperature (P-T) diagram showing P-T condi- ments where plate tectonic gemstones form. A: Pacifi c-type subduc- tions recorded by plate tectonic gemstones jadeite (rounded black tion zone (modifi ed after Gerya, 2011), where jadeitite forms. B: Con- symbols; Tsujimori and Harlow, 2012) and gem corundum (gray rect- tinental collision zone (after model R2 of Warren et al., 2008), where angles; Simonet et al., 2008). Thick, light gray lines show collision ruby forms. HP—high pressure; UHP—ultra-high pressure. zone P-T paths, from Jamieson et al. (2006). Thick dark lines show modeled P-T paths (model D80 of Syracuse et al., 2010) for slab surfaces, both hot (southwestern Japan/Nankai) and cool (north- Repository1; Fig. 3). Another jadeite occurrence is found in New Guinea, eastern Japan/Tohoku) subduction zones. Mineral abbreviations: Ab—albite; Anl—analcime; Coe—coesite; Crn—corundum; Dia—di- part of the Gondwana fragment Australia. These deposits are entirely Pha- amond; Dsp—diaspore; Gr—graphite; Jd—jadeite; Nph—nepheline; nerozoic (Fig. 4B), products of subduction leading to the formation of Qz—quartz.
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