Very High-Pressure Orogenic Garnet Peridotites

Very high-pressure orogenic garnet peridotites

J. G. Liou*, R. Y. Zhang, and W. G. Ernst Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305

Edited by Russell J. Hemley, Carnegie Institution of Washington, Washington, DC, and approved January 4, 2007 (received for review August 23, 2006)

Mantle-derived garnet peridotites are a minor component in many very high-pressure metamorphic terranes that formed during continental subduction and collision. Some of these mantle rocks contain trace amounts of zircon and micrometer-sized inclusions. The constituent minerals exhibit pre- and postsubduction microstructures, including polymorphic transformation and mineral exsolution. Experimental, mineralogical, petrochemical, and geochronological characterizations using novel techniques with high spatial, temporal, and energy resolutions are resulting in unexpected discoveries of new phases, providing better constraints on deep mantle processes.

ata on the composition of the These petrochemical findings lead to the index minerals coesite and/or dia- subcontinental lithospheric new challenges posed by critical tectonic mond at a minimum P Ͼ 2.7 GPa at mantle are essential for erect- questions: How were deep-seated (Ͼ200 T Ͼ 600°C (Fig. 1); such metamorphism ing realistic large-scale models km) mantle rocks transported to shallow is now well recognized in the geologic D depths? How were such peridotites in- of the Earth’s geochemical and tectonic community (25, 26). The discovery of evolution (1). Our knowledge of mantle corporated into subduction-zone oro- tracts of upper continental crust meta- composition and petrochemical pro- gens? How can we distinguish the morphosed under VHP conditions has cesses has been derived mainly from petrochemical/geochronological pro- revolutionized our understanding of col- studies of xenoliths and xenocrysts in cesses taking place in a mantle wedge lisional orogenic belts. The subduction kimberlites, mantle-derived volcanic setting from those affecting deeply sub- of sialic materials to mantle depths plays rocks, and experimental very high- ducted ultramafic rocks of the continen- a crucial role in crust–mantle interac- pressure (VHP) phase equilibria, and tal lithosphere? tions at convergent plate junctions. One from the interpretation of seismic tomo- In the spirit of synergy of 21st century of the most significant orogenic pro- graphic images. Recent studies of oro- science and technology, this article pre- cesses is the formation and subsequent genic peridotites provide additional sents an overview of VHP metamorphism exhumation of VHP rocks subducted to insights regarding upper mantle pro- of garnet peridotites and poses new chal- depths of 150 km or more. Several new cesses at convergent lithospheric plate lenges for petrochemical and experimental VHP terranes (Fig. 2) have recently boundaries. It was found that many oro- studies of mantle-derived orogenic perido- been identified on the basis of partially genic peridotites were derived from a tites. Specifically, we describe differences preserved trace index minerals (e.g., depleted, metasomatized mantle or in petrochemical features for mantle- coesite with or without diamond) in crustal cumulate, and later were wedge and subduction-zone processes strong containers such as zircon and/or subjected to subduction-zone VHP through examination of micrometer-sized garnet. metamorphism (e.g., refs. 2–6). Some minerals, exsolution textures, and polymorphic transformations. A recent study peridotites preserve a record of ultra- In Situ VHP Metamorphism. The volumetri- of garnet nodules in the Western Gneiss deep origin revealed by mineral ex- cally predominant rocks of VHP ter- Region of the Norwegian Caledonides (6) solution and the persistence of VHP ranes are felsic gneisses and schists, indicates that the interpretation of conti- many of which lack obvious evidence polymorphs (6–14), and several perido- nental subduction depths Ͼ200 km for of mantle-depth metamorphism. Recent tites contain dense hydrous magnesian some VHP terranes may be incorrect, in- observations (27) indicate that not all silicates (DHMS) that are stable only at asmuch as the deep-mantle origin of the garnet peridotites and eclogites are mantle depths (15, 16). It was also peridotites occurred before emplacement fault-bounded, as was previously found that some garnet peridotites, and in the subduction zone. In the following thought; some such VHP rocks preserve their host continental crust, underwent discussion, except for a few specific exam- evidence that their contacts with coeval subduction-zone VHP metamor- ples, we focus mainly on our own pub- gneissic rocks have retained structural phism under pressure–temperature lished and unpublished research in the coherence throughout subduction, meta- (P–T) conditions characterized by low Dabie–Sulu terrane of east-central China. Յ morphism, and exhumation. Mineralogi- thermal gradients ( 5°C/km), based on cal indicators of VHP metamorphism sensitive high-resolution ion microprobe VHP Metamorphism have been found in a variety of wall (SHRIMP) U–Pb ages of zircon sepa- Physical Conditions of Metamorphism. Since rock lithologies, including gneisses, rates from both rock types (e.g., refs. the initial discoveries of coesite in su- 17–20). Furthermore, VHP experiments pracrustal rocks (23, 24), VHP meta- have revealed that numerous hydrous morphism has become synonymous with Author contributions: J.G.L. and R.Y.Z. designed research; phases and nominally anhydrous miner- that portion of eclogite-facies conditions J.G.L. and R.Y.Z. performed research; W.G.E. analyzed data; and J.G.L., R.Y.Z., and W.G.E. wrote the paper. als containing substantial amounts of within the P–T stability field of coesite. The authors declare no conﬂict of interest. H2O are stable under such conditions. Understanding VHP tectonics is viewed Therefore, cold subduction zones are as a significant undertaking of consider- This article is a PNAS Direct Submission. Abbreviations: DHMS, dense hydrous magnesian silicates; the principal sites of H2O recycling back able importance, as underscored by the P–T, pressure–temperature; REE, rare earth element; into the mantle (for reviews, see refs. 21 abundance of recent task groups, work- SHRIMP, sensitive high-resolution ion microprobe; TEM, and 22). Such findings have advanced shops, conference sessions, and books transmission electron microscopy; VHP, very high pressure. our knowledge of the thermal structure devoted to the subject. VHP metamor- *To whom correspondence should be addressed. E-mail: of subduction zones and of the recycling phism refers to the transformation of [email protected]. of volatiles into the mantle. crustal rocks to assemblages containing © 2007 by The National Academy of Sciences of the USA

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6.0 G 211± 4 Ma enrichments and a marked negative Eu Peridotite G anomaly (31, 32). Consequently, identifi- Eclogite Sulu (A) cation of mineral inclusions and charac- 5.0 zone 160 terization of REE patterns of zoned Dabie- zircons have been used in conjunction

4.0 Forbidden Diamond with ion microprobe U–Pb dating to 231± 4 Ma Graphite o 120 elucidate the P–T time paths for some 5 C/km G Dry EC e > 680 Ma VHP terranes (e.g., refs. 33 and 34). 3.0 Lw-EC G Coesit rtz (A) Qua 231±4 Ma New isotopic ages support the hypoth- Depth km 80 esis that Dabie–Sulu eclogites, garnet Pressure [GPa] Amp-EC Protolith age: > 680 Ma 2.0 Ep-EC S peridotites, and the surrounding wall Qtz-bearing inherited core rocks were subjected to coeval VHP BS S UHP metamorphic age: 231±4 Ma HGR metamorphism at 220–240 Ma. Meta- (A) 40 1.0 > 680 Ma Coe-bearing mantle Retrograde metamorphism: 211±4 Ma morphic overgrowths on zircons from EA AM GR GS Qtz-bearing rim eclogites and country rock gneisses and 210 ± 4 Ma (B) 0 Exhumation rate: > 5 km/Ma schists yield virtually identical U–Pb 200 400 600 800 1000 Triassic ages (e.g., refs. 19, 35, and 36), Temperature [ oC] demonstrating that all units were metamorphosed at the same time. Zircon Fig. 1. P–T conditions of VHP mafic–ultramafic rocks. (Left) (A), P–T fields of VHP metamorphism, ‘‘forbidden-zone’’ (17), and stability of coesite and diamond; (B), P–T time paths for Dabie–Sulu eclogite separates from Dabie–Sulu VHP rocks and garnet peridotites. (Right) Zoned zircon domains with SHRIMP U–Pb ages for Sulu paragneiss. retain low-P mineral-bearing inherited cores, VHP mineral-bearing (e.g., coesite) mantles, and rims that contain quartzites, and marbles (27–30). De- to the P–T time path of a subduction low-P minerals such as quartz and pla- tailed studies of mineral compositions complex, inasmuch as this mineral is gioclase (37, 38). Ion microprobe U–Pb in Dabie felsic gneisses and schists show extremely stable and resistant over analyses of these zoned zircons have that they were metamorphosed together a wide range of conditions. During identified three discrete age groups, with intercalated coesite-bearing eclogite growth stages, individual zircon zonal shown schematically in Fig. 1: (i) the and garnet peridotite bodies under simi- domains may include and preserve in- latest Proterozoic protolith ages (Ͼ680 lar P–T conditions. clusions of minerals in equilibrium with Ma) in the inherited cores, (ii) a culmi- Evidence of mantle-depth meta- the matrix phase assemblage. For in- nating VHP metamorphic event in the morphism is typically preserved as rare stance, zircons that crystallized at man- coesite-bearing mantles at 220–240 Ma, mineral inclusions and relict phase as- tle depths in equilibrium with garnet and (iii) a late amphibolite-facies retro- semblages within host rocks that later display characteristic heavy rare-earth gressive overprint in rims at 210 Ϯ 10 reequilibrated under crustal conditions. element (REE) depletions and lack an Ma. The presence of anomalously low Among the various types of evidence, Eu anomaly, whereas those grown at ␦18O in VHP minerals, not only in coes- zircons from VHP rocks provide the crustal depths in equilibrium with pla- ite-bearing eclogites but also in the wall most useful information with regard gioclase have pronounced heavy REE rocks, suggests that both Dabie–Sulu

Fig. 2. Distribution and peak metamorphic ages of recognized VHP terranes worldwide (modiﬁed after ref. 27).

Liou et al. PNAS ͉ May 29, 2007 ͉ vol. 104 ͉ no. 22 ͉ 9117 Downloaded by guest on September 29, 2021 Table 1. Dual origin of Dabie–Sulu garnet peridotites in Triassic VHP terrane Characteristic Type A (mantle-derived) Type B (crust-hosted)

Rock type Garnet peridotite Ϯ minor eclogite lenses Metagabbroic eclogites Ϯ peridotite cumulate layers Origin Mantle wedge or subducted subcontinent lithosphere Continental crust of the downgoing plate Emplacement Faulted into subduction zone Maﬁc–ultramaﬁc intrusion into crust prior to subduction Age of protolith Archean to Proterozoic Late Proterozoic to Paleozoic (Ϸ700–450 Ma) Age of zircons Ϸ220–240 Ma Ϸ700–220 Ma Extent of crustal metasomatism Minor Substantial

mafic–ultramafic and felsic rocks re- Morphologies of zoned zircon grains, mite in olivine from the Alpe Arami mained in mutual contact throughout and SHRIMP U–Pb isotopic analyses garnet lherzolite in the Central Alps (7). subduction and that the entire terrane obtained from their cores and rims, pro- Dobrzhinetskaya et al. (7) hypothesized underwent Triassic VHP closed-system vide important constraints regarding that the exsolved FeTiO3 lamellae were metamorphism (e.g., refs. 39 and 40). whether or not the mantle-derived gar- originally a high-P perovskite polymorph net peridotites experienced subduction- of ilmenite and that the precursor phase Mantle-Derived and Crust-Hosted Garnet zone metamorphism. Most zircons from formed at 10–15 GPa (300–450 km). Peridotites. Garnet-bearing ultramafic Chinese garnet peridotites are of the From the abundance, morphology, crys- rocks are widespread as a minor but rounded isometric form without inher- tallography, and topotaxy of these ox- significant component of the Dabie– ited cores, implying a metamorphic ori- ides, they argued that the inferred very Sulu VHP terrane (e.g., ref. 41). Al- gin. SHRIMP U–Pb dating of zircons high solubility of highly charged cations though most surface exposures are from both the peridotites and the en- (Ti and Cr) reflects previously unrecog- heavily serpentinized, fresh samples closed eclogite lenses in the Sulu region nized mantle conditions in recovered from quarries and drill-hole cores of the has yielded VHP metamorphic ages of rock samples. Despite considerable con- Chinese Continental Scientific Drilling 220–240 Ma (18–20, 44); the data are troversy, subsequent experiments under Project consist of garnet lherzolite, harz- consistent with VHP ages of 230 Ϯ 10 these conditions have confirmed that Ͼ1 burgite with or without minor wehrlite, Ma for the country rocks. However, re- vol % of TiO2 can be accommodated in and dunite. Dabie–Sulu garnet perido- connaissance study of Hf isotopic and olivine (46, 47). The additional observa- tites are classified as mantle-derived U–Pb upper-intercept ages of zircons tion of exsolved Ca-poor pyroxene dis- (type A) or crust-hosted (type B) on the yield Early Proterozoic, even Archean, playing antiphase domains in diopside basis of structural, geochemical, and iso- model ages for certain other Sulu garnet (8, 48, 49) also supports the idea of an Ͼ topic characteristics (Table 1 and Fig. peridotites (44), suggesting that some extremely deep origin ( 300 km) for 3). Type B igneous intrusions occur as bodies had long resident times in the the Alpe Arami garnet peridotite. minor ultramafic cumulates associated mantle wedge before involvement in Precursor majoritic garnet was postu- with dominant metagabbroic layers of the Triassic subduction. lated based on the identification of py- various compositions, whereas type A roxene lamellae exsolved from garnet in peridotites represent depleted, metaso- Microstructures of VHP Minerals a Norwegian orogenic garnet peridotite matized mantle fragments, some of Exsolution of Mineral Lamellae. Studies of (9, 10). Multistage processes for forma- which contain minor eclogite and garnet microminerals and exsolution structures tion of the majoritic garnet nodules subsequently were described for the hy- clinopyroxenite pods. Members of both have revealed numerous preserved, Ϸ types were subjected to Triassic subduc- deep-seated features formed at much pothesized 180 km depth of origin of tion-zone VHP metamorphism; some higher P than values estimated using the Otroy peridotite (10). Experimental were metamorphosed at mantle depths conventional (e.g., Grt–Opx) geobarom- simulation of the exhumation path of T under P–T conditions (Ͻ5°C/km) involv- eters (27, 45). The best example may be mantle material shows that high- (1,400°C) decompression of lherzolite ing pressures up to Ϸ6.7 GPa and T Ͻ the electrifying report of micrometer- from 14 to 12 GPa results in exsolution 950°C (15, 41–43) (Fig. 1). sized FeTiO rods and plates of chro- 3 of interstitial blebs of diopside and Mg2SiO4 (wadsleyite) lamellae from a parental majoritic garnet (47, 50). Later, N S research involving critical analyses of REE concentrations in minerals and Sino-Korean craton Yangtze craton other geochemical characteristics, as well as recalculation of the volume of Mantle wedge s Continental lithosphere exsolved pyroxene inclusions in garnet, s -100km G led researchers to reinterpret the origin G Footwall mantle s Spinel peridotites Mantle of Otroy garnet peridotite (6). Evi- wedge G Garnet peridotites dently, Archean (Ϸ3.5 Ga) deep-seated

Footwall mantle mantle peridotites containing majoritic

Ancient relict mantle garnets underwent exsolution during -200km upwelling from a depth of 350 km or Continental crust Mafic rocks Crustal more and were subjected to extensive Oceanic crust Ultramafic rocks partial melting; the residue formed a fluid + volatile garnet-bearing cratonic root. These Fig. 3. A schematic model for Triassic subduction of the Yangtze beneath the Sino-Korean cratons, lithospheric mantle fragments of Pro- showing the tectonic setting for mantle-derived (type A) and crustal-hosted (type B) garnet peridotites terozoic age (Ϸ1.8–1.4 Ga) were then (for details, see ref. 2). incorporated into subducting sialic crust

9118 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0607300104 Liou et al. Downloaded by guest on September 29, 2021 Fig. 4. Microstructures of VHP minerals. (A) Exsolution lamellae of garnet plus ilmenite in relict clinopyroxene from Sulu peridotite (for details, see ref. 14). (B and C) Exsolution lamellae of rutile plus amphibole in peridotite garnet from northern Qaidam VHP terrane (for details, see ref. 62).

during the Caledonian continental colli- biologic C-isotope signatures for micro- VHP experiments demonstrating the sion at Ϸ400 Ma. The conclusion of diamonds (54), allow the conclusion that solubility of K in clinopyroxene (60, 61). presubduction exsolution was based on subducted crustal felsic and carbonate Similar exsolution lamellae of rutile detailed Re-Os and Nd-Sm age data and protoliths reached mantle depths Ͼ180 plus sodic amphibole plus apatite in gar- on REE concentrations and partitioning km. Transmission electron microscopy net from the Qaidam garnet peridotite between exsolved pyroxenes and the (TEM) identification of nanometric of western China (Fig. 4 B and C) have host garnet. inclusions of aragonite (CaCO3) and been suggested (62) to represent decom- Numerous examples of garnet and magnesite (MgCO3) in microdiamonds, pression products from depths Ͼ200 km clinopyroxene exsolution from Dabie– together with the experimental stability of supersilicic majorite crystals typified Sulu peridotites and eclogites have been of these carbonates, further suggests by high concentrations of Na2O (0.3 hypothesized to have been derived from that the diamond-bearing rocks of the wt %) and hydroxyl (up to 1,000 ppm). a majoritic garnet precursor formed at Kokchetav massif were subducted to a Thus, in addition to DHMS and nomi- considerable mantle depths. Coarse- depth of Ϸ190–280 km (55). Appar- nally anhydrous silicates, majoritic gar- grained clinopyroxenes from Rizhao ently, exsolution occurred during de- net and supersilicic clinopyroxene could garnet clinopyroxenites contain up to compression/exhumation of VHP rocks. be important reservoirs of H2O at man- 25 vol % exsolved garnet and 4 vol % In addition, nanometer-thick (Ͻ2 nm) tle depths. ilmenite (Fig. 4A). Petrologic and exper- lamellae of ␣-PbO2-type TiO2 occur be- imental studies suggest that the precur- tween multiple twinned rutile crystals in Polymorphic Transformations. Intergrowths sor of such intergrowths was majoritic both diamond-bearing felsic rocks from of ortho- and clinoenstatite lamellae are garnet in which Ca2ϩTi4ϩ 3 2Al3ϩ, the Erzgebirge (56) and coesite-bearing common within Chinese orogenic garnet Mg2ϩSi4ϩ 3 2Al3ϩ, and NaϩTi4 3 Dabie eclogites (57). The occurrence of peridotites. Clinoenstatite lamellae in Ca2ϩAl3ϩ in octahedral sites (14, 42, these lamellae implies subduction of orthoenstatite may have formed either 43). Exsolved needles of pyroxene, continental materials to a depth Ͼ200 by inversion from orthoenstatite or by rutile, and apatite along garnet (111) km. These observations, together a displacive transformation from VHP planes from the Yangkou eclogite layer with inferred supersilicic titanite in clinoenstatite during decompression (8, within peridotite (11, 45, 51) are based Kokchetav marble, allow the conclu- 12). Experiments indicate that orthoen- on optical and SEM observations. sion that some continental supercrustal statite transforms to VHP clinoenstatite Similar exsolution lamellae, suggesting rocks have been subducted to depths at P Ͼ 8 GPa, 900°C, corresponding to a that a majoritic garnet precursor formed of at least 300 km before being re- mantle depth of Ϸ300 km (e.g., refs. 63 at depths Ͼ200 km, have also been turned to the surface. These depths of and 64) (Fig. 5). The growth of high-P reported in the Erzgebirge massif of metamorphism demonstrate that coun- clinoenstatite in mantle-derived perido- Germany, another VHP terrane (13). try rocks, although perhaps not as tite, and the inferred majoritic garnet Despite numerous reports of majoritic deeply buried as some garnet perido- precursor, may have formed at great garnets, however, formation of VHP tites, have ascended from astonishing depth in the mantle wedge long before solid-solution phases (e.g., majorite) subduction depths. insertion into the downgoing continental may have taken place either in the deep lithospheric plate, then recrystallized upper mantle, and then later been se- Exsolution of Hydrous Phases. K-bearing during subduction-zone metamorphism. questered in the mantle wedge such as pargasite [KCa2(Mg,Fe)4AlSi6Al2O22 in Norway, or in a subduction zone such (OH,F)2] lamellae in clinopyroxene in- Synergy of VHP Metamorphic Studies as in the Kokchetav of northern Kazak- clusions within garnet megacrysts, and with Mineral Physics stan (see below). This problem remains phlogopite lamellae in lherzolitic diop- The above review concludes that some to be satisfactorily investigated. side from Sulu, show topotactic inter- sections of continental crust have Supersilicic titanite was suggested as a growths and are confined to the cores of reached subduction depths approaching precursor phase of coesite lamellae in ti- the host clinopyroxene (14, 58). These or exceeding 200 km, involving passive- tanite from a Kokchetav impure marble K- and OH-bearing exsolved phases sug- margin lithologies, including carbonate, that formed at P Ͼ 6 GPa (52). Other gest that the primary clinopyroxene may felsic, pelitic, and minor mafic–ultra- exsolution lamellae of quartz or K- have incorporated a considerable mafic protoliths. Some peridotites now feldspar with or without phengite in amount of K2O and H2O under VHP hosted in continental orogens may have diopside from diamond-bearing marble conditions, as documented in clinopy- been formed even deeper. However, the and gneiss, and quartz exsolution in roxene inclusions in Kokchetav zircons recognition of mineral exsolution in eclogitic omphacite (53), together with (59). This conclusion is consistent with mantle-derived garnet peridotites result-

Liou et al. PNAS ͉ May 29, 2007 ͉ vol. 104 ͉ no. 22 ͉ 9119 Downloaded by guest on September 29, 2021 ple, kokchetavite, a new hexagonal polymorph of K-feldspar, was discovered as a metastable phase together with ␣-cristobalite plus phengite plus sili- 18 A ceous glass with or without phlogopite/ titanite/calcite/zircon as multiphase 9 HCLEN (C2 /c) cloudy inclusions in clinopyroxene and 8 garnet from a diamond-grade Kokchetav MgSiO3 7 XFe=0.1 garnet-pyroxene rock (73). This prob- ␮ 6 lematic phase (2- to 7- m-sized plates) LCLEN can be misidentified as K-feldspar by 5 (P2 1 /c) OREN conventional techniques and misinter- (Pbc)a 4 preted as an exsolved phase. 3 Focused ion beam techniques and Pressure (GPa) 2 TEM studies of microdiamonds from 1 the Erzgebirge (74) have revealed 0 400 800 1200 1600 numerous nanometric crystalline inclu- Temperature ( oC) sions, including phases of known stoi- chiometries such as SiO2 and Al2SiO5 Fig. 5. Phase transformations in enstatite. (Upper) and minerals with different combina- TEM image of enstatite with clinoenstatite lamellae tions of Si, K, P, Ti, Fe, and O . These from Sulu garnet peridotite. (Lower) P–T path for 2 Fig. 6. Schematic P–X (Cpx–Grt) diagram for iso- such transformation. (For details, see ref. 12.) phases need to be investigated by em- thermal decompression path of majoritic garnet to ploying synchrotron radiation. Such form garnet plus ilmenite lamellae in clinopyrox- approaches are only now beginning to ene host (Fig. 4A) (for details, see ref. 14). ing from decompression in a mantle- be applied to VHP rocks. For exam- wedge setting, or due to exhumation in ple, metamorphic diamonds from the a subduction zone, remains to be deter- Erzgebirge have been examined using minerals is highly dependent on pressure mined, except in cases for which age synchrotron infrared absorption, Ra- and temperature. Many hydroxyl-bearing constraints, such as those in the Western man scattering, and fluorescence spec- phases, such as OH-topaz and phase A, P T Gneiss Region, are conclusive. With the troscopy. The characteristic features of are only stable under mantle – condi- O O tions involving geotherms considerably recent breakthroughs in VHP technol- C C and C H bonds, molecular Ϫ 2Ϫ less than 5°C/km. Because such condi- ogy and new-generation synchrotron, H2O, OH and CO3 radicals, and N tions are essentially transient and inevi- neutron, and laser facilities for charac- impurities all support the concept of tably are followed by a period of terization of nano-sized materials, new diamond crystallization from a COH- thermal relaxation, these phases have in-depth research on VHP minerals and rich supercritical fluid (66, 75). little chance of surviving T increase and rocks is now within reach; such studies P decrease on return to the surface are just beginning (see refs. 65, 66, and Characterization of Mineral Exsolution and Phase Transformations. Exsolution inter- through erosional and tectonic pro- 83). Three general fields of mineral growths are common in minerals of cesses. However, one possibility for the physics research are highlighted below decompressed VHP rocks; however, preservation of these phases is as to illustrate some of the opportunities. the exsolution mechanisms are poorly minute inclusions in high mechanical- These fields all have relevance to a understood. Each lamellae-bearing host strength, impervious containers like fuller understanding of crust–mantle mineral preserves information con- garnet, zircon, or diamond (69). tectonics. cerning the composition and physical Interpretation of the occurrence of ma- conditions of formation of the homo- joritic garnets in orogenic peridotites re- Identification of Nano-Sized Minerals. A geneous precursor phase, as well as a quires better experimental data, and some host of peculiar minerals and mineral portion of the inferred P–T path dur- syntheses using natural peridotite samples compositions have been described in ing decompression. Compositional and have been accomplished (47, 51). For ex- the Western Gneiss Region (38 miner- structural characterization of lamellae– ample, as shown in Fig. 6, the depths for als in all) (67) and in the Dora Maira host mineral pairs will provide impor- the formation of majoritic garnet are Ͼ massif ( 7 minerals) (68) VHP rocks tant new constraints on the physical based on reconnaissance investigation by conventional methods. Novel tech- conditions of crystallization/recrystalli- of the pseudobinary system MgSiO3– niques developed for experimental zation. Such studies should guide sub- Mg3Al2Si3O12 (81). Although experiments studies with high spatial, temporal, and sequent experimentation to delineate involved testing the effects of Fe and Ca, energy resolution should be applied for the P–T conditions and mechanisms the effect of Ti has not been explored. microanalysis of solid and fluid inclu- for the formation of the primary VHP Solubilities of Ti, K, OH, and other trace sions in naturally occurring VHP min- minerals. elements in olivine, garnet, and pyroxene erals, including unexplored trace but in mafic–ultramafic systems need to be widespread opaque phases (e.g., Fe-Ni Experimental Phase Relations and Composi- examined, inasmuch as natural analogs sulfides) in garnet peridotites and co- tional Variations. Experimental investiga- exhibit numerous microstructures. These rundum-bearing garnetite (69). Ubiqui- tions of the KMASH, CMASH, and phases also contain minute inclusions as tous micrometer-sized mineral and KNCMASH systems have revealed the yet unidentified that may turn out to be fluid inclusions in tough, rigid VHP possibility of occurrences of hydrous VHP phases or DHMS previously synthe- mineral hosts, including zircon, dia- phases in VHP pelitic and peridotitic sized only in diamond-cell or multianvil mond, garnet, and pyroxenes (e.g., refs. rocks (76–78). Experimental investiga- experiments (e.g., see ref. 82). Discovery 65 and 70–72), require analytical elec- tion of hydroxyl solubility in clinopyrox- of natural representatives of these syn- tron microscopy to characterize their ene, orthopyroxene, and olivine (79, 80) thetic phases would be a major step for- structures and compositions. For exam- indicates that the OH content of these ward in understanding mantle processes,

9120 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0607300104 Liou et al. Downloaded by guest on September 29, 2021 including the role of hydrous phases as synchrotron x-ray diffraction, and high- We thank Dave Mao, Larissa Dobrzhi- storage sites for H2O. resolution TEM) used by experimentalists netskaya, Harry Green, and Nick Sobolev to identify synthetic analogs. Such studies for support and reviews, including review Summary Statement are needed to bridge the gap between of a draft version of this manuscript. This work was supported by Stanford Uni- The search for mantle minerals in oro- mantle petrology and mineral physics. A versity and by National Science Founda- genic VHP garnet peridotites should be review of recent progress involving such tion Continental Dynamic Program conducted with the same sophisticated an experimental approach is detailed by Grants EAR 00-03355 and 05-06901 (to techniques (microRaman spectroscopy, Dobrzhinetskaya (83). J.G.L.).

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