大陆俯冲带流体活动:超高压变质岩矿物水含量和稳定同位素制约

对超高压变质岩中含水矿物和名义上无水矿物的地球化学研究,极大地深化了我们对大陆碰撞带地壳俯冲和折返过程中流体体制的认识。就流体体制和化学地球动力学来说,有关研究在大别-苏鲁造山带进行的最为详细,因此已经成为研究大陆俯冲带变质的典型地区。本文以大别-苏鲁造山带为对象,从矿物水含量的角度,结合稳定同位素论述了大陆俯冲带流体活动。超高压变质岩中名义上无水矿物含有大量的水,以结构羟基和分子水形式存在。名义上无水矿物中结构羟基和分子水出溶与含水矿物分解共同构成了折返过程中退变质流体的主要来源。名义上无水矿物所释放的水以富集轻的氢氧同位素为特征,而含水矿物分解则提供了富集D的流体来源。折返过程中,名义上无水矿物降压脱水存在亏损D的分子水的优先丢失和不同形式水之间的相互转化。不同岩性的水含量差异导致了它们在折返过程中不同的流体活动行为。大陆板块俯冲和折返过程中,在不同矿物、不同岩性以及板片不同部位之间存在水的再分配;板片的一部分作为富水流体的源,而另一部分可能作为汇。     

Elemental mobility in subduction metamorphism: insight from metamorphic rocks of the Franciscan Complex and the Feather River ultramafic belt,California

The degree of element mobility in subduction metamorphism has generated much debate; some workers advocate considerable mobility during metamorphism, whereas others postulate minimal mobility. We assess this issue by examination of major and trace element concentrations and Pb-, Nd-isotopic data for 39 mafic metavolcanic rocks from the Franciscan subduction complex, related units of coastal California, and the Feather River ultramafic belt of the northern Sierra Nevada, California; these samples span a wide range of metamorphic grade. We conclude that these rocks, despite their metamorphism up to eclogite facies, preserve protolith major and trace elemental compositions and isotopic ratios, with the exception of some mobile large ion lithophile elements such as Ba, Pb, and to a smaller extent La, U, and Sr. Thus subduction metamorphism of these metabasalts occurred in a largely closed system. Lack of light rare earth element enrichment in the rocks demonstrates lack of chemical exchange with subducted metasediments. Relatively low SiO₂ content (<48 wt.%) of many of the metamorphic rocks and the lack of correspondence between silica depletion and metamorphic grade suggests that the silica depletion resulted from seafloor hydrothermal alteration before subduction. In spite of demonstrated mobility of Pb, and possible mobility of Nd, isotopic ratios of Pb and Nd were not modified during subduction metamorphism. In contrast to our results from metabasaltic rocks, our analysis of actinolite-rich rinds from high-grade Franciscan mélange blocks suggests some chemical exchange between metachert and the overlying mantle. The increasing enrichment in Ba and Pb with increasing metamorphic grade suggests that Ba- and Pb-rich fluids interacted more intensely with metabasalt at the higher grades of metamorphism. Comparison of these results with studies of the active Mariana forearc suggests that fluids interacting with the mantle wedge up-dip of the region of magma genesis are derived from subducting sediments overlying the down-going plate.     

Aqueous fluids and hydrous melts in high-pressure and ultra-high pressure rocks: Implications for element transfer in subduction zones

High-pressure (HP) and ultra-high pressure (UHP) terranes are excellent natural laboratories to study subduction-zone processes. In this paper we give a brief theoretical background and we review experimental data and observations in natural rocks that constrain the nature and composition of the fluid phase present in HP and UHP rocks. We argue that a fluid buffered by a solid residue is compositionally well defined and is either an aqueous fluid (total amount of dissolved solids < 30 wt.%) or a hydrous melt (H₂O < 35 wt.%). There is only a small temperature range of approximately 50–100 °C, where transitional solute-rich fluids exist. A review of available experimental data suggest that in felsic rocks the second critical endpoint is situated at 25–35 kbar and  700 °C and hence must be considered in the study of UHP rocks. Despite this, the nature of the fluid phase can be constrained by relating the peak metamorphic conditions of rocks to the position of the wet solidus even if the peak pressure exceeds the pressure where the wet solidus terminates at the second critical endpoint. Transitional solute-rich fluids are expected in UHP terrains (P > 30 kbar) with peak temperatures of about 700 ± 50 °C. At higher temperatures, hydrous granitic melts occur whereas at lower temperatures aqueous fluids coexists with eclogite-facies minerals. This argument is complemented by evidence on the nature of the fluid phase from high-pressure terrains. We show that in the diamond-bearing, high-temperature UHP rocks from the Kokchetav Massif there are not only hydrous felsic melts, but probably also carbonate and sulfide melts present.

Hydrous quartzo-feldspathic melts are mainly produced in high temperature UHP rocks and their composition is relatively well constrained from experiments and natural rocks. In contrast, constraining the composition of aqueous fluids is more problematic. The combined evidence from experiments and natural rocks indicates that aqueous fluids liberated at the blueschist to eclogite facies transition are dilute. They contain only moderate amounts of LILE, Sr and Pb and do not transport significant amounts of key trace elements such as LREE, U and Th. This indicates that there is a decoupling of water and trace element release in subducted oceanic crust and that aqueous fluids are unable to enrich the mantle wedge significantly. Instead we propose that fluid-present melting in the sediments on top of the slab is required to transfer significant amounts of trace elements from the slab to the mantle wedge. For such a process to be efficient, top slab temperature must be at least 700–750 °C at sub-arc depth. Slab melting is likely to be triggered by fluids that derive from dehydration of mafic and ultramafic rocks in colder (deeper) portions of the slab.     

新疆加尔塔斯克斯套斑岩锆石U-Pb-Hf同位素特征及其对洋脊俯冲的制约

新疆加尔塔斯克斯套斑岩铜矿床位于西准噶尔斑岩成矿带西段, 目前研究程度很低。对该矿床花岗闪长斑岩体进行LA-ICP-MS锆石U-Pb年龄、岩石地球化学及Hf同位素组成测试分析。结果显示, 该岩体的锆石U-Pb年龄为299.2 ± 5.3 Ma, 形成于晚石炭世末—早二叠世初; 岩石具有富SiO2、Al2O3、Na, 贫K, Mg#值较高, 以及高Sr、低Y和Yb含量的特点, 显示埃达克岩特征; 富集大离子亲石元素Cs、Ba、U和轻稀土元素, 亏损高场强元素Ti、Nb、Ta和重稀土元素, 无明显Eu异常。斑岩具有高的176 Hf/ 177 Hf值, 正高εHf (t)值(+10.1~+15.6, 平均+12.8), 以及年轻的两阶段模式年龄(TDM2=307.4~687.1 Ma), 显示亏损地幔源区属性, 鲜有壳源混染。研究认为, 该斑岩体可能形成于洋脊俯冲, 并伴有明显的Cu、Au矿化, 指示该斑岩铜矿床有良好的找矿潜力。     

Late Paleozoic-Mesozoic subduction and accretion of the Paleo-Pacific Plate:Insights from ophiolitic rocks in the Wandashan accretionary complex,NE China

The Wandashan accretionary complex(AC),consisting of the Raohe and Yuejinshan complexes,is located on the continental margin of Northeast Asia and represents an excellent source of information about Paleo-Pacific subduction and accretion.However,the protolith nature and tectonic evolution of the Wandashan AC are under debate.This contribution reports new geochronological,geochemical,and Sr-Nd-Pb-Hf isotopic data for ophiolitic rocks from the Wandashan AC.The 169-166 Ma plagioclasites and homogeneous gabbros from the Raohe complex are OIBs while 228-214 Ma homogeneous gabbros are continental VABs.Cumulate gabbros from the Yuejinshan complex formed at 280-278 Ma and～220 Ma and have similar characteristics with E-MORB and N-MORB,respectively.They are BABBs and their primary magma was derived from a source region between EMI and EMII that was affected by con-tinental crustal contamination as well as subduction-zone metasomatism.Combined with previous stud-ies,we suggest that the onset of subduction of the Paleo-Pacific Plate was in the Early Permian.Subsequently,a back-arc basin,whose present suture is on the eastern margin of the Jiamusi Massif,formed and widened during 280-232 Ma,after which the basin closed and BABBs were emplaced to form the Yuejinshan complex during 210-180 Ma.The formation of VABs of the Raohe complex is coincident with the closure of the back-arc basin,and together with the 169-166 Ma OIBs,they constitute a major part of the Raohe complex.The accretionary process was completed during 133-131 Ma.Taken together,the ophiolitic rocks indicating multistage magmatism in the Paleo-Wandashan region recorded the formation-closure process of back-arc basin and the accretionary process of the Wandashan AC,during the westward subduction of the Paleo-Pacific plate.The back-arc basin identified in our study sheds new lights on geodynamic evolution model of subduction and accretion of the Paleo-Pacific Plate on the continental margin of NE Asia.     

俯冲带Li同位素地球化学: 回顾与展望

板块俯冲伴随着多种复杂的地质过程,例如流体释放、流体岩石相互作用、可能的部分熔融、元素迁移及同位素分馏等。在俯冲带壳/幔循环过程中,Li同位素(6Li和7Li)高达16%的相对质量差异使其成为研究俯冲带各种地质过程的良好示踪剂。总结了近年来有关俯冲带Li同位素的研究进展,并按照相对时间序列系统地介绍了Li同位素在俯冲各个阶段的地球化学行为,包括俯冲初始物质的Li同位素组成特征,俯冲过程中Li随流体释放及与上覆地幔楔相互作用中Li同位素的行为,富Li流体与残余板片Li的去向,最终俯冲相关产物的Li同位素组成特征以及对全球Li循环的地学意义的综合介绍。随着Li同位素测试精度的提高和各种地球化学储库Li同位素数据库的不断完善,Li同位素体系可以为俯冲过程研究提供更多信息,成为一种可靠的示踪剂。     

Geochemistry of eclogites of the Tso Morari complex,Ladakh, NW Himalayas:Insights into trace element behavior during subduction and exhumation

Whole rock major and trace element compositions of seven eclogites from the Tso Morari ultra-high pressure(UHP) complex, Ladakh were determined with the aim of constraining the protolith origins of the subducted crust. The eclogites have major element compositions corresponding to sub-alkaline basalts. Trace element characteristics of the samples show enrichment in LILE's over HFSEs(Rb, Th, K except Ba) with LREE enrichments((La/Lu)n = 1.28-5.96). Absence of Eu anomaly on the Primitive Mantle normalized diagram suggests the absence of plagioclase fractionation. Positive correlation between Mg# with Ni and Cr suggests olivine fractionation of mantle melts. Narrow range of(La/Yb)n(2.1-9.4) and Ce/Yb(6.2-16.2) along with Ti/Y(435-735) ratios calculated for the Tso Morari samples is consistent with generation of melts by partial melting of a garnet free mantle source within the spinel peridotite field. Ternary diagrams(viz. Ti-Zr-Y and Nb-Zr-Y) using immobile and incompatible elements show that the samples range from depleted to enriched and span from within plate basalts(WPB)to enriched MORB(E-MORB) indicating that the eclogite protoliths originated from basaltic magmas.Primitive Mantle normalized multi element plots showing significant Th and LREE enrichment marked by negative Nb anomalies are characteristic of continental flood basalts. Positive Pb, negative Nb, high Th/Ta, a narrow range of Nb/La and the observed wide variation for Ti/Y indicate that the Tso Morari samples have undergone some level of crustal contamination. Observed geochemical characteristics of the Tso Morari samples indicate tholeiitic compositions originated from enriched MORB(E-MORB) type magmas which underwent a limited magmatic evolution through the process of fractional crystallization and probably more by crustal contamination. Observed geochemical similarities(viz. Zr, Nb, La/Yb, La/Gd,La/Nb, Th/Ta ratios and REE) between Tso Morari eclogites and the Group I Panjal Traps make the trap basalt the most likely protoliths for the Tso Morari eclogites.     

Role of subduction in the genesis of leucite-bearing rocks: Discussion

Conclusions The examples given above indicate a strong possibility of a genetic connection between high K volcanic rocks and subduction related processes, and suggest that the statements quoted from Cundari (1979) in the first paragraph of this paper cannot be substantiated. The origin of the high K rocks has been, and remains, one of the most puzzling problems in petrology. Petrologists are not yet in a position to exclude any aspects of their genesis as Cundari's statements imply.     

Phosphate oxygen isotopes: Insights into sedimentary phosphorus cycling from the Benguela upwelling system

Marine sediments are the main sink in the oceanic phosphorus (P) cycle. The activity of benthic microorganisms is decisive for regeneration, reflux, or burial of inorganic phosphate (P_i), which has a strong impact on marine productivity. Recent formation of phosphorites on the continental shelf and a succession of different sedimentary environments make the Benguela upwelling system a prime region for studying the role of microbes in P biogeochemistry. The oxygen isotope signature of pore water phosphate (δ¹⁸O_P) carries characteristic information of microbial P cycling: Intracellular turnover of phosphorylated biomolecules results in isotopic equilibrium with ambient water, while enzymatic regeneration of P_i from organic matter produces distinct offsets from equilibrium. The balance of these two processes is the major control for δ¹⁸O_P.Our study assesses the importance of microbial P cycling relative to regeneration of P_i from organic matter from a transect across the Namibian continental shelf and slope by combining pore water chemistry (sulfate, sulfide, ferrous iron, P_i), steady-state turnover rate modeling, and oxygen isotope geochemistry of P_i.We found δ¹⁸O_P values in a range from 12.8‰ to 26.6‰, both in equilibrium as well as pronounced disequilibrium with water. Our data show a trend towards regeneration signatures (disequilibrium) under low mineralization activity and low P_i concentrations, and microbial turnover signatures (equilibrium) under high mineralization activity and high P_i concentrations. These findings are opposite to observations from water column studies where regeneration signatures were found to coincide with high mineralization activity and high P_i concentrations. It appears that preferential P_i regeneration in marine sediments does not necessarily coincide with a disequilibrium δ¹⁸O_P signature. We propose that microbial P_i uptake strategies, which are controlled by P_i availability, are decisive for the alteration of the isotope signature. This hypothesis is supported by the observation of efficient microbial P_i turnover (equilibrium signatures) in the phosphogenic sediments of the Benguela upwelling system.     

Geodynamics of the divergent double subduction along the Bangong-Nujiang tethyan suture zone: Insights from late mesozoic intermediate-mafic rocks in central Tibet

Bangong-Nujiang Suture Zone (BNSZ) in central Tibet plays an important role in evaluating the formation and uplift mechanism of Tibetan Plateau. However, its Mesozoic tectonic evolution is ambiguous and intensely debated. In this study, Early Cretacesous adakites and sodium-rich arc rocks are identified in Western Qiangtang (WQ) and Northern Lhasa (NL) sub-terranes. Forty-four adakite samples from both WQ and NL have akin geochemical features, and are derived from partial melting of subducted oceanic crust with amphibole residual. Nineteen sodium-rich samples originated from a mixed source region between crustal sediment and enriched lithospheric mantle. These two parallel arc belts separated by the Bangong-Nujiang Suture Zone (BNSZ) represent the divergent double subduction of the Bangong-Nujiang Tethyan Ocean (BNTO). Combined with the previous studies, our new data suggest three significant magmatic flare-ups at ∼240–140 Ma, 135–105 Ma and 92–60 Ma in the WQ and BNSZ, and two at 135–105 Ma and 92–60 Ma in the NL. These asymmetrical magmatic activities indicate that the southern subduction may have commenced at about 135 Ma and experienced slab breakoff at the latest Early Cretaceous, and the northern subduction could trace back to L-Triassic (228 Ma) and experienced episodic low-angle subduction, slab rollback (190-140 Ma) and oceanic ridge subduction (135-100 Ma). The 100–92 Ma magmatic gap, 92–60 Ma magmatic flare-up and L-Cretaceous angular unconformities indicate that the double-sided subduction of the BNTO resulted in soft collision with oceanic lithosphere detachment.     

Early Paleozoic magmatism and metallogeny related to Proto-Tethys subduction: Insights from volcanic rocks in the northeastern Altyn Mountains,NW China

Remnants of the Proto-Tethys are mainly preserved in the region between south of the North China-Tarim Block and north of Qiangtang-Sibumasu/Baoshan Blocks. Magmatic-metallogenic events related to the Proto-Tethyan subductions were rarely reported, and the subduction history and polarity of the Proto-Tethyan are still under debate. Here, we presented new data of zircon UPb ages, whole-rock Sr–Nd–Pb isotopes, major and trace elements and zircon Hf isotopes for the volcanic rocks in the northeastern Altyn Mountains. Information of over 14 volcanic-hosted deposits/prospects in the region has been compiled. These volcanic ore hosts consist mainly of basaltic andesite, andesite, dacite and rhyolite rocks. The andesite and rhyolite rocks are newly zircon UPb dated to be Late Cambrian-Early Ordovician (andesite: 490.5 ± 5.2 Ma; rhyolite: 492.6 ± 2.9 Ma and 491.6 ± 5.6 Ma), representing the timing of volcanism and VMS (Volcanogenic Massive Sulfide) mineralization. All the volcanic rocks belong to the high-K calc-alkaline and shoshonite series: the andesite rocks from the Kaladawan area in north of the region display arc geochemical affinities and contain (⁸⁷Sr/⁸⁶Sr)_i (0.7082–0.7083) and ε_Nd(t) (−9.7 to −7.6), indicating that they were likely formed by partial melting of the mantle wedge with subducted sediment inputs. The rhyolite rocks from the Kaladaban area in south of the region are characterized by high SiO₂ (64.46–78.55 wt%), low alkali (Na₂O + K₂O, 3.46–7.17 wt%), and contain (⁸⁷Sr/⁸⁶Sr)_i (0.7063–0.7095), ε_Nd(t) (−6.6 to −1.5), and zircon ε_Hf(t) (−5.5 to 5.4), indicating that they were likely derived from partial melting of the lower crust with depleted mantle inputs. Rock assemblage and geochemistry suggest that volcanic rocks in the northeastern Altyn Mountains may have formed in a continental arc setting. Their spatial distributions with respect to the ophiolites in the region suggest that the subduction was likely south-dipping. This subduction-related arc magmatism may have formed the many important VMS and porphyry–skarn deposits in the region.     

Variations of slab dip and overriding plate tectonics during subduction: Insights from analogue modelling

We present small-scale laboratory models of oceanic subduction in which plates motion is imposed by lateral boundary conditions. The oceanic plate moves trenchward at constant speed and subducts below a fixed overriding plate. In this configuration, the long-term process of subduction is not steady-state. Slab interaction with the upper mantle-lower mantle boundary results in periods of slab flattening during which the dip of the slab diminishes, followed by periods of slab steepening. The overriding plate tectonic regime is influenced by the dynamics of subduction, slab anchoring favouring trench perpendicular shortening. When the slab is anchored, slab flattening further favours shortening, while slab steepening favours extension or smaller shortening rates. Non-steady-state long-term subduction may explain part of the variability of slab geometries evidenced by statistical analyses of present-day subduction zones. Experiments suggest that, despite boundary conditions applied on the converging plates do not change, tectonics pulses within the overriding plate may be caused by this non-steady-state behaviour.     

Oxygen and nitrogen isotopes as tracers of fluid activities in serpentinites and metasediments during subduction

Summary N and O isotope systematics of a suite of high-pressure (HP) and ultrahigh-pressure (UHP) metasediments of the Schistes Lustrés nappe and metaperidotites of the Erro Tobbio Massif from the Alpine-Appennine system are compared with their unmetamorphosed or hydrothermally-altered equivalent from the same localities and from the South West Indian Ridge (SWIR). The HP and UHP rocks studied represent a sequence of pelagic sediments and altered ultramafic rocks subducted to different depths of down to 90 km along a cold geothermal gradient (8 °C/km). Unmetamorphosed and HP metasediments show the same range in δ¹⁵N values irrespective of their metamorphic grade and bulk nitrogen concentrations. Together with several other geochemical features (K, Rb and Cs contents, δD), this indicates that δ¹⁵N values were unaffected by metamorphism and N was not released during subduction. N isotope analysis of serpentinites coupled with δ¹⁸O systematics suggests the involvement of a mafic (crustal) component during partial deserpentinization of the subducted oceanic mantle at the depth locus of island arc magmatism. This does not imply large-scale fluxes as the metagabbros are spatially associated with the analyzed serpentinites. It rather indicates preservation of presubduction chemical and isotopic heterogeneities on a local scale as documented for the metasediments.     

Fluid flow in subduction zones: evidence from Nd- and Sr-isotope variations in metabasalts of the Franciscan complex,California

Greenstone, blueschist and eclogite metabasaltic blocks from the Franciscan complex of California preserve extensive petrographic and chemical evidence for interaction with hydrous fluids at high-P, low-T metamorphic conditions. The Nd and Sr isotope variations within and among the blocks constrain the origin of the basaltic protoliths, the nature of the fluid metasomatism that occurred within the upper levels (15–45 km) of the paleosubduction zonc, and the character and provenance of the rock that generated the hydrous fluids within the paleosubduction zone. Samples with little or no petrographic evidence of retrograde alteration and unaltered garnet separates have _Nd. With increasing degrees of retrograde alteration, Nd isotope compositions are consistently lower, ranging down to _Nd(160)=5. Actinolitic alteration rinds which are present on some blocks have the least radiogenic compositions with _Nd=1.6 to 6.1. While Nd isotope compositions of unaltered blockes are in the range expected for basalt derived from normal depleted mantle, the Sr isotope compositions are more radiogenic ranging from _Sr(160)=–5 to +11. Compositions of unaltered eclogite and blue-schist blocks are consistent with a protolith origin in normal oceanic crust derived from depleted mantle. The Sr isotopy systematics indicate that the protoliths were modified by seawater alteration in an ocean-floor hydrothermal system. Isotopic compositions of samples from parts of blocks that have a retrograde metamorphic overprint show a strong correlation between less radiogenic Nd compositions and the extent of retrograde metamorphism. Maximum Nd isotope ratios of the metasomatizing fluid are provided by analyses of actinolitic rinds, and range from _Nd(160)=1.6 to 6.1. A possible source for fluids of this composition is subducted sediment that was derived from a continental craton. Because rind formation occurred while the basaltic blocks were within an ultramafic matrix, the fluids must have migrated from sediments in the accretionary wedge into an overlying wedge of mantle material imbricated with blocks of oceanic crust. This suggests possibly km-scale movement of fluids that carry an amount of the rare-earth elements sufficient to significantly modify the trace-element budget of subducted basalt.     

宁芜地区中生代富钾和富钠火山岩的源区特征:岩石学和地球化学证据

宁芜中生代火山盆地广泛产出早白垩世中基性火山岩,钾质岩石主要为龙王山组英安岩、粗安岩,钠质火山岩主要为大王山组玄武粗安岩、粗安岩、英安岩以及钠质安山玢岩类。钠质辉石安山玢岩的锆石U-Pb年龄为128.9±1.9Ma,为早白垩世产物。稀土元素球粒陨石标准化配分曲线均表现为一致的平缓右倾型式,Eu负异常不明显,富集大离子亲石元素,亏损高场强元素,具Nb、Ta的负异常。钾质龙王山组火山岩总体Cs、Rb、Ba、Th、U等LILE含量较钠质大王山组火山岩和钠质安山玢岩高,并具有较高的Si、K,较低的Ca、Mg,高Rb/Sr,高Isr(0.70521~0.70857);钠质火山岩具有较低的Si、富Na、高Sr、低I_(sr)(0.70544~0.70768),Mg~#相对较高。钠质辉石闪长玢岩斑晶中有斜长石的矿物结构和成分不平衡现象,锆石Hf同位素特征为ε_(Hf)(t)=-6.2~+0.6,变化较大,指示岩浆混合过程。地幔源区可能是由中元古代俯冲板块释放出流体发生交代作用后形成的富集地幔,钾质火山岩地幔源区交代矿物可能以金云母为主,崆岭群TTG片麻岩为其地壳端元。而钠质火山岩地幔源区交代矿物可能以角闪石和单斜辉石为主,中元古上溪群变质基性岩或新生下地壳基性岩为其地壳端元。     

Lithium isotopes in the system Qz-Ms-fluid:An experimental study

The fractionation of lithium isotopes among quartz, muscovite, and a chloride-bearing aqueous fluid has been investigated experimentally at 400°-500°C and 50-100 MPa. Experiments were performed for 15-60 days in cold seal vessels with natural mineral specimens. Lithium was introduced primarily through the fluid, which also contained KCl and HCl. In most runs, the fluid was prepared with the L-SVEC standard (δ⁷Li = 0) and was 1 M in total chloride with K/Li/H = 100/10/1. In two experiments, a ⁶Li spike was employed. The experiments demonstrate that quartz and muscovite are susceptible to pronounced, rapid shifts in Li isotopic composition by diffusion through interaction with a Li-bearing fluid, particularly at 500°C. At 500°C, fractionation factors were determined to be Δ_{quartz-aqueous} ≅ +8 to + 12‰ and Δ_{muscovite-aqueous} ≅ +18 to + 20‰. An intermineral fractionation factor is given by Δ_{muscovite-quartz} ≈ +9‰. At 400°C, the results suggest Δ_{quartz-aqueous} ≈ +4 to + 6‰. The study provides evidence of systematic fractionation in lithium isotopes at the temperatures of some magmatic processes, such as those associated with porphyry-type ore systems and pegmatites.     

Interaction of deformation and metamorphism during subduction and exhumation of hydrated oceanic mantle: Insights from the Western Alps

Petrological investigations supported by multi‐scale structural analysis of eclogitized serpentinite in the Zermatt–Saas Zone of the Western Alps allows for the determination of mineral assemblages related to successive fabrics, upon which the P–T–d–t path of these hydrated mantle rocks can be inferred. Serpentinites of the upper Valtournanche, with lenses and dykes of metagabbro and meta‐rodingite, display an Alpine polyphase metamorphic evolution from eclogite to epidote‐amphibolite facies conditions associated with three successive foliations having different parageneses in these rocks. Serpentinite mainly consists of serpentine with minor magnetite; however, where S1 and S2 foliations are pervasive, metamorphic olivine, together with Ti‐clinohumite and clinopyroxene, are also found. The mineral assemblage associated with D1 includes serpentine1, clinopyroxene1, opaque minerals, titanite ± olivine1, Ti‐clinohumite1 and ilmenite; the D2 assemblage is the same (±chlorite) but minerals have different compositions. The assemblage associated with D3 comprises serpentine3, opaque minerals, ±chlorite3, ilmenite and amphibole3. Ti‐clinohumite is associated with veins that are older than D2 and pre‐date D3. Veins that post‐date D3 are characterized by amphibole + chlorite or by serpentine. P–T conditions for S2 parageneses evaluated using two pseudosections for different bulk compositions suggest that these rocks experienced pressures >2.5 ± 0.3 GPa at temperatures slightly higher than 600 °C. The late epidote–amphibolite facies re‐equilibration associated with D3 and D4 developed during late syn‐exhumation deformation related to folding and testifies to a small temperature decrease. These results, which were integrated in the regional framework, suggest that different portions of the Zermatt–Saas Zone registered different P–T peak conditions and underwent different exhumation paths. In addition, the inferred P–T–d–t path suggests that the Valtournanche serpentinites re‐equilibrated close to the UHP conditions registered by the Cignana meta‐cherts. These results imply that tectonic slices exhumed after UHP metamorphism might be wider than previously reported or that small‐size UHP units, tectonically sampled during the Alpine convergence, are more abundant than those that have been detected to date.     

Some aspects of the role of intergranular fluids in the compositional evolution of metamorphic rocks

Minerals that react with each other during the progressive evolution of metamorphic terranes are not always in physical contact. As such, an “intergranular fluid” could play a major role in element transfer and chemical evolution. However, the nature of this fluid and its specific role remains somewhat elusive. Recent experiments in our laboratory shed some light on the behavior of such a fluid. Here we present a simple mathematical model which accounts for diffusion within crystals and fluid, solubility in the fluid and mass balance between the various reservoirs. The model elucidates the nature of element exchange between two minerals via the mediation of an intergranular fluid. It is shown that a coupling of thermodynamics and kinetics controls the evolution of the system and the concentration of an element in the intergranular fluid is a key parameter of interest. The results have important implications for standard tools of metamorphic petrology such as geothermometers and barometers, geospeedometry and the closure of isotopic systems. For example, homogeneity of mineral grains may be a poor criterion for equilibrium and the rim compositions of minerals showing diffusion zoning may be out of equilibrium with distant exchange partners, even in the presence of a fluid in which transport is fast.