Hydrothermal systems in peridotites at slow-spreading ridges. Modeling phase transformations and material balance: Role of gabbroids

This work was aimed on the estimation of role of gabbro in the ore mobilization during hydrothermal transformation of oceanic (gabbro-peridotite) crust at slow-spreading mid-ocean ridges. Kinetic-thermodynamic modeling was used to reconstruct the geochemical and mineralogical trends of evolution of gabbroids during their hydrothermal interaction with marine fluid. The results of our simulation offered a new insight into some problems of material balance and ore formation during hydrothermal process in the slow-spreading mid-ocean ridges. It was shown that the root zones of all known MAR hydrothermal fields related to serpentinites are made up of ultrabasic rocks and located within peridotite protolith near hot and uncooled gabbroic bodies. It was also demonstrated that the observed mineral and geochemical diversity of metagabbros of slow-spreading mid-ocean ridges was provided by the interaction of hydrothermal fluid percolating through the Hess-type oceanic crust with gabbro bodies. It was established that almost cooled gabbroid bodies, being involved in hydrothermal circulation in the shallow root zones, may play an important role in the redistribution of the material within the Hess-type oceanic crust.     

Structural Petrology of Plagioclase Peridotites in the West Othris Mountains (Greece): Melt Impregnation in Mantle Lithosphere

We present the results of a structural and petrological studyof mantle rocks from the strongly dismembered Othris Ophiolite.Part of the mantle section was impregnated with melt, crystallizingplagioclase and clinopyroxene as cumulate phases and refertilizingpreviously depleted peridotites. Melt impregnation occurredlate in the deformation history of the host peridotites. Thedeformation took place at stresses of 13–26 MPa and attemperatures around 1000–1200°C, at the base of thethermal lithosphere. The melt therefore impregnated relativelycold mantle rocks, implying that the thermal lithosphere reachedinto the mantle during magmatic activity. We conclude that theOthris Ophiolite represents a spreading environment with a relativelythick lithosphere, such as that near an axial discontinuityor transform fault of a slow-spreading ridge. The proposed magmaticand deformation history of the peridotites is in agreement withepisodic magmatism at slow-spreading ridges. We thus concludethat the heterogeneous character of the mantle section of theOthris Ophiolite results from melt impregnation processes. Wesuggest that the presence of lherzolitic ophiolite types amongharzburgitic ophiolite types in the Hellenic–Dinaric chainreflects variable degrees of melt impregnation and refertilizationrather than partial melting and melt extraction. KEY WORDS: lithospheric mantle deformation; melt impregnation; microstructures; Othris Ophiolite; plagioclase peridotites     

Petrogenetic conditions at 18°–20° N MAR: Interaction between hydrothermal and magmatic systems

The paper presents petrological and geochemical data on mantle peridotite, basalt, and metamorphic rocks sampled in Cruise 36 of the R/V Professor Logachev at the MAR axial zone between 17° and 20° N. These data are interesting not only as providing new information on the inner structure of the oceanic crust in the still-poorly known axial MAR segment but also in the context of the fundamental problem of interaction between magmatic and hydrothermal systems in slow-spreading mid-oceanic ridges. The MAR axial zone between 17° and 20° N was determined to host both weakly and strongly depleted residual peridodites, which suggests that the degree of mantle source melting significantly varied along the ridge axis in this segment. The MAR crest zone comprises slabs of serpentinized peridotite brought to the seafloor surface at various time. The most strongly depleted mantle peridotites likely uplifted later than the mildly and weakly depleted rocks in the same areas. A mantle reservoir beneath the MAR axial zone at 20° N is not isotopically related to the mantle source of the parental MORB melts, and high-Mg metabasites exposed at 17°56- N were derived from a crustal source that was modified near the root zone of a high-temperature hydrothermal system. The studied area seems to display traces of an extinct hydrothermal field and likely an ore occurrence related to it.     

Hydrothermal systems hosted in peridotites at slow-spreading ridges. Modeling phase transformations and material balance: Upwelling limb of the hydrothermal cell

The research was centered on the estimation of geochemical and mineralogical effects related to the transport of hydrothermal fluid to the seafloor surface in the upwelling limb of a hydrothermal system hosted in peridotites at slow-spreading mid-oceanic ridges. The three variants of the location of the root zone of the circulation cell considered in this research were as follows: (1) shallow-depth, with T = 107°C, P = 1.14 kbar; (2) moderate low depths, with T = 151°C, P = 1.4 kbar; and (3) deep, with T = 500°C, P = 4 kbar. The modeling results demonstrate that ore material is accumulated in the discharge zones of serpentinite-related hydrothermal systems only at a high temperature of the fluid in the discharge zone of the upwelling limb of the circulation cell. The root zones at hydrothermal fields that meet this condition should be situated at a significant depth in the crustal section. It was also established that a significant volume of ore material involved in hydrothermal material exchange between the peridotites and fluid is redeposited in the downwelling limb of the hydrothermal system and gives rise to disseminated ore mineralization, which is typical of many serpentinized abyssal peridotites. The activity of moderately low-temperature and low-temperature hydrothermal systems in peridotites does not concentrate ore material in the discharge zone, and no hydrothermal edifices can grow at such systems.     

Thermodynamic model of ore-forming processes in a submarine island-arc hydrothermal system

A thermodynamic model suggested for ore-forming processes in a hydrothermal system (HS) in an island arc is based on the technique suggested earlier in [1] for simulating ore-forming hydrothermal systems in mid-oceanic ridges. This technique make use of the principle of flow-through multistep reactor and encompasses (a) the region where hydrothermal solutions are generated when seawater interacts with rocks (descending convection branch); (b) the region where material is transported with the solution at decreasing pressure (feeder channel); and (c) the region where the ore material is deposited (orebody). Hydrothermal systems in island arcs exhibit the following distinctive features taken into account in the model: (1) the composition of the host crustal rocks (rocks of mafic-acid composition instead of basalt and serpentinite) and (2) possible significant involvement of magmatic gases in the feeding of the hydrothermal system. The naturally occurring prototype of the simulated system is the hydrothermal system in the caldera of a submarine volcano in an island arc. The model is simulated in a number of variants in which the hydrothermal fluid is exogenic (heated seawater convecting through hot volcanic rocks), magmatic, or mixed (magmatic plus exogenic) is involved. The simulations were carried out using the HCh version 4.3 [2] program package for the multisystem H-O-K-Na-Ca-Mg-Fe-Al-Si-C-S-Cl-Cu-Zn-Pb-As-Sb-Ag-Au at temperatures of 25?C370°C and pressures of 10?C500 bar. The multisystem included 88 possible solid phases and aqueous solution with 95 species. The thermodynamic properties of compounds were calculated using the UNITHERM databank. The model is underlain by the principle of multiwave flow-through multistep reactor (MFTMR) with a starting rock/water (R/W) ratio of 1: 1. As progressively more solution portions passed through the rocks, the participation of fresh rock in the interaction accordingly diminished because the rock material was gradually exhausted in the system. The magmatic fluid had a composition selected based on data on fumaroles at Kudryavyi volcano [3] with a correction for the degassing pressure. The evolution of ore deposition was simulated in compliance with the scheme described in [4], which was implemented using the technology of ??openness from above?? [3]. The model was simulated with various compositions of the host rocks (basalts, andesites, dacites, and rhyolites) and the origin of the fluid (magmatic fluid alone, seawater alone, and variable proportions of both). Our simulation results indicate that the metallogeny (relative enrichment in Pb, As, Sb, or Ag) of island-arc ore deposits is controlled by the abundances of metals in the host rocks predominant in the hydrothermal system. The mineralogy and geochemistry of ores generated in arc hydrothermal systems are predetermined by the effective transport of metalloids (S, As, and Sb) that have a high migration capacity in these systems. Magmatic gases introduced in the hydrothermal systems play dualistic roles in the ore-forming processes. If the hydrothermal fluid in a hydrothermal system is dominated by magmatic components, deposits of native sulfur are formed, and the precipitation of base metal is thereby suppressed because of the high acidity of the generated hydrothermal solutions. The involvement of magmatic gases in an amount of a few percent in a hydrothermal system enhances the overall oregenerating potential of the system in terms of sulfide ores.     

Petrology of the mafic rocks of the Xigaze ophiolite,Tibet

The Xigaze ophiolite (Yarlung-Zangbo suture zone, Southern Tibet, China) shows an unusual crustal sequence characterized by a lack of large masses of cumulate gabbros, by dolerites intrusive throughout the whole ophiolite sequence, and by the injection of dolerites in already serpentinized peridotites. The abyssal tholeiitic nature of all the mafic rocks indicates that they have been generated at an oceanic ridge. All the geological arguments and petrological and textural data on the mafic rocks point to very low heat production and large heat losses through widespread intensive sea-water circulation, for the spreading centre in which they have been formed, in good agreement with a slow-spreading ridge origin.     

Hydrothermal serpentinization of peridotite within the oceanic crust: Experimental investigations of mineralogy and major element chemistry

Mantle derived ultramafic rocks form a significant portion of lithosphere created at slow-spreading mid-ocean ridges. These rocks are ubiquitously serpentinized, due at least in part to interaction with seawater, at temperatures below approximately 500°C. To evaluate reaction pathways, primary mineral reaction rates, major element exchange between rock and solution, and alteration mineral formation, interaction of equigranular peridotites with seawater and seawater derived solutions has been investigated experimentally at 200°C and 300°C, 500 bars.Seawater chemistry changed greatly during the experiments. Initially, the concentrations of Mg, Ca, and SO₄ decreased, as did pH. During Iherzolite experiments, however, the trend of dissolved Ca concentrations reversed with time, first decreasing, then increasing. pH also increased during the latter part of the experiments. Mg, Ca, SiO₂, Fe, Cl and ΣCO₂ decreased as pH increased Fe^II oxidation is shown to be affected by solution pH, being greatly enhanced under alkaline conditions. Resulting solution composition and reaction pathway are dependent on initial solution composition, particularly initial concentrations of Mg in solution. Consistent with changes in solution chemistry, the peridotites were significantly altered. Substantial amounts of olivine, relatively minor amounts of diopside and all the enstatite dissolved. Alteration products included serpentine + anhydrite ± magnesium hydroxide sulfate hydrate ± magnetite ± brucite ± tremolite-actinolite or truscottite.From the changes in solution chemistry and examination of the alteration products, the reaction rates (moles per unit time) of olivine to enstatite to diopside during 300°C Iherzolite-seawater experiments are estimated to be approximately 1.0/1.0/0.1. These rates correspond to constant surface area rates of 1.5:5:1 (moles per unit time per unit surface area), which are consistent with experimental data on the dissolution kinetics of these minerals and emphasize the importance of initial rock texture on reaction rates.     

Analyses d’ouvrages

New structural data and P–T estimates of syn-deformational assemblages within the Beni Bousera peridotites and their crustal envelope are used to explain their Alpine exhumation. The Beni Bousera peridotites occur as thin sheets within high grade crustal units of the lower Sebtides (inner Rif, Morocco) and are composed of weakly deformed spinel lherzolite in the core of the massif and garnet-spinel mylonite at the rim. The main foliation trajectories in both the peridotites and overlying crustal units show systematic rotation towards their mutual contact, indicating a kilometer-scale top to the NW shearing with a dextral component along this crust/mantle contact. Widespread top to the NW shear criteria within the crustal units overlying the peridotite support this feature. Available ages constrain the development of the main foliation in both the peridotites and crustal rocks between 25 and 20 Ma. New P–T data from the peridotites show that deformation occurs during decompression from ≈ 22 kbar, 1050°C to ≈ 9-15 kbar, 800°C. As a consequence, exhumation of the Beni Bousera peridotites takes place during the Oligo-Miocene lithosphere thinning in the footwall of a lithospheric extensional shear zone. The exceptional preservation of garnet within the mylonitic peridotites results from rapid cooling of the border of the massif due to the juxtaposition with colder crustal rocks along this shear zone. Uplifting of the hot mantle rocks simultaneously induces high temperature metamorphism in the overlying crustal units. These new findings allow us to reconstruct the deformation history of the Beni Bousera region and the Alboran domain in the framework of the western Mediterranean geodynamics during the last 40 Myrs.     

Peridotites from the Mariana Trough: first look at the mantle beneath an active back-arc basin

Two dives of the DSV Shinkai 6500 in the Mariana Trough back-arc basin in the western Pacific sampled back-arc basin mantle exposures. Reports of peridotite exposures in back-arc basin setting are very limited and the lack of samples has hindered our understanding of this important aspect of lithospheric evolution. The Mariana Trough is a slow-spreading ridge, and ultramafic exposures with associated gabbro dykes or sills are located within a segment boundary. Petrological data suggest that the Mariana Trough peridotites are moderately depleted residues after partial melting of the upper mantle. Although some peridotite samples are affected by small-scale metasomatism, there is no evidence of pervasive post-melting metasomatism or melt-mantle interaction. Spinel compositions plot in the field for abyssal peridotites. Clinopyroxenes show depletions in Ti, Zr, and REE that are intermediate between those documented for peridotites from the Vulcan and Bouvet fracture zones (the American-Antarctic and Southwest Indian ridges, respectively). The open-system melting model indicates that the Mariana Trough peridotite compositions roughly correspond to theoretical residual compositions after ~7% near-fractional melting of a depleted MORB-type upper mantle with only little melt or fluid/mantle interactions. The low degree of melting is consistent with a low magma budget, resulting in ultramafic exposure. We infer that the mantle flow beneath the Mariana Trough Central Graben is episodic, resulting in varying magma supply rate at spreading segments.     

Structures et cinématique liées á la mise en place des péridotites de Ronda (Cordilléres Bétiques,Espagne)

The interaction of the Mid-Atlantic Ridge with the North Atlantic Mantle Plume has produced a magmatic plateau centred about Iceland. The crust of this plateau is 30 km thick on average. This abnormal thickness implies that, unlike other slow-spreading ridges, addition of magmatic material to the crust is not balanced by crustal stretching. The thermal effect of the plume also reduces the strength of the lithosphere. Both mechanisms affect the rifting process in Iceland. A structural review, including new field observations, demonstrates that the structure of the Iceland plateau differs from that of other slow-spreading oceanic ridges. Lithospheric spreading is currently accommodated in a 200 km wide deformation strip, by the development of a system of half-grabens controlled by growth faults. Similar extinct structures, with various polarities, are preserved in the lava pile of the Iceland plateau. These structures are identified as lithospheric rollover anticlines that developed in hanging walls of listric faults. We introduce a new tectonic model of accretion, whereby the development of the magmatic plateau involved activation, growth and decay of a system of growth fault/rollover systems underlain by shallow magma chambers. Deactivation of a given extensional system, after a lifetime of a few My, was at the expense of the activation of a new, laterally offset, one. Correspondingly, such systems formed successively at different places within a 200 km wide diffuse plate boundary. Unlike previous ones, this new model explains the lack of an axial valley in Iceland, the dip pattern of the lava pile, the complex geographical distribution of ages of extinct volcanic systems and the outcrops of extinct magma chambers.     

橄榄岩-熔体的相互作用:岩石圈地幔组成转变的重要方式

橄榄岩-熔体/岩浆的相互作用常被用来解释蛇绿岩套橄榄岩、造山带橄榄岩、超镁铁质侵入杂岩体、地幔橄榄岩捕虏体中某些具有不平衡结构和矿物组成的岩石的形成过程。橄榄岩-熔体的反应主要有两种方式,即消耗橄榄石(和单斜辉石)生成斜方辉石或消耗斜方辉石生成橄榄石(和单斜辉石)。反应的结果不仅造成矿物百分含量的变化,而且造成矿物组成的变化;后者更重要但未引起足够的重视。华北东部中生代玄武质岩石中具有环带状结构的橄榄石和辉石捕虏晶,特别是具有环带状结构的地幔橄榄岩捕虏体的发现,暗示这种橄榄岩-熔体的相互作用在华北东南部中生代岩石圈地幔中很可能普遍存在,为岩石圈地幔组成转变和快速富集的重要方式。这是全球首例由橄榄岩-熔体相互反应造成的岩石圈地幔大规模的组成变化。反应熔体来源途径主要有地壳来源和软流圈地幔来源。来源不同的熔体与橄榄岩的反应造成的组成变化完全不同。     

Characteristic features of ore gabbro formation in the third layer of the oceanic crust

Peculiarities of ore gabbro formation in slow-spreading mid-ocean ridges exemplified by the non-transform Sierra Leone fault and Marathon fault areas are considered. The formation of ore gabbros is most often connected with the rheologically weakened oceanic lithosphere mainly with active fault zones, in which basic magmatic melts intruded. Those faults provide ways for migration of differentiated melts at different depth levels. When intruding and moving along fault zones, melts interacting with the mantle and crust host rocks are often already hydrated. Such interaction occurs largely in conditions of subsolidus deformations resulting in enrichment of melts with volatile components.     

Hot contacts of garnet peridotites in middle/upper crustal levels: new constraints on the nature of the late Variscan high-T/low-P event in the Moldanubian (Central Vosges/NE France)

P–T  paths based on parageneses in the immediate vicinity of former high-temperature contact zones between mantle peridotites and granulitic country rocks of the Central Vosges (NE France) were derived by applying several conventional thermometers and thermobarometric calculations with an internally consistent dataset. The results indicate that former garnet peridotites and garnet–spinel peridotites were welded together with crustal rocks at depths corresponding to 1–1.2 GPa. The temperature of the crustal rocks was about 650–700 °C at this stage, whereas values of 1100 °C (garnet peridotites) and 800–900 °C (garnet–spinel peridotites) were calculated for the ultramafic rocks. After emplacement of the mantle rocks, exhumation of the lower crust took place to a depth corresponding to 0.2–0.3 GPa. The temperatures of the incorporated peridotite slices were still high (900–1000 °C) at this stage. This is indicated by the presence of high- T  /low- P parageneses ( c . 800 °C, 0.2–0.3 GPa) in a small (1–10 m) contact aureole around a former garnet peridotite. Crustal rocks distant to the peridotites equilibrated in the same pressure range at lower temperature (650–700 °C). High cooling rates (10²–10³ °C Ma⁻¹) were calculated for a garnet–biotite rock inclusion in the peridotites and for the crustal rocks at the contact by applying garnet–biotite diffusion modelling. Minimum rates of 0.75–7.5 cm a⁻¹ are required for vertical ascent of rock units (30 km vertical distance) derived from the crust–mantle boundary, resulting in a late Variscan (340 Ma) high- T  /low- P event.     

Evolution of the petrological and seismic Moho-implications for the continental crust-mantle boundary

The chemical and petrological composition of mafic rocks from the lower continental crust are discussed by comparing mafic granulites and meta-gabbroic rocks from the Ivrea Zone and the Northern Hessian Depression (NHD) xenolith suite. Both regions contain contrasting types of meta-mafic lithologies (i) former basaltic rocks with trace element patterns ranging from MORB-Iike to subduction-related or intra-plate tholeütes and (ü) Ca-and Al-enriched, plagiodase-dominated gabbroic rocks showing positive Eu-anomalies generated by complex deep crustal magmatic processes such as fractionation, accumulation of plagiodase and pyroxene, and crustal contamination. The absence of typical garnet-omphadte parageneses in these rocks indicates that the eclogite stability field was not reached during Palaeozoic orogenic processes. A compilation of experimentally determined P-wave velocities and densities for mafic granulites, gabbroic rocks, eclogites and peridotites is used to evaluate key physical properties of lower crustal mafic rocks during crystal thickening caused by continent-continent collision. In a step-by-step scenario it is demonstrated that the position of the seismic Moho (defined as a first-order velocity discontinuity) and the petrological Moho (defined as the boundary between non-peridotitic crustal rocks and olivine-dominated rocks) is not identical for the case that mafic rocks are transformed into edogites at the base of orogenically thickened crust. P-wave velocities of eclogites largely overlap with those of peridotites, although their densities are significantly higher than common upper mantle rocks. As a consequence, refraction seismic field studies may not detect edogites as crustal rocks. This means that the seismic Moho detected by refraction seismic field studies appears at the upper boundary between edogites and overlying crustal units. Since edogites generally have higher densities than peridotites, they might be recycled into the deeper lithosphere thereby transferring excess Eu into the upper mantle. This process could be a due for understanding the negative Euanomaly in the upper continental crust which is apparently not balanced quantitatively by the abundance of common mafic crustal rocks.     

Mantle-Crust Interaction in Petrogenesis of the Gabbro-Granite Association in the Preobrazhenka Intrusion,Eastern Kazakhstan

The paper reports results of petrological-geochemical, isotope, and geochronological studies of the Preobrazhenka gabbro–granitoid massif located in the Altai collisional system of Hercynides, Eastern Kazakhstan. The massif shows evidence for the interaction of compositionally contrasting magmas during its emplacement. Mineralogical–petrological and geochemical studies indicate that the gabbroid rocks of the massif were formed through differentiation of primary trachybasaltic magma and its interaction with crustal anatectic melts. Origin of the granitoid rocks is related to melting of crustal protoliths under the thermal effect of mafic melts. The mantle–crust interaction occurred in several stages and at different depths. A model proposed here to explain the intrusion formation suggests subsequent emplacement of basite magmas in lithosphere and their cooling, melting of crustal protolith, emplacement at the upper crustal levels and cooling of the granitoid and basite magmas. It was concluded that the formation of gabbro-granitoid intrusive massifs serves as an indicator of active mantle–crust interaction at the late evolutionary stages of accretionary–collisional belts, when strike-slip pull-apart deformations causes the high permeability of lithosphere.     

造山带橄榄岩记录的大陆俯冲带多期壳幔相互作用

俯冲带是地壳与地幔之间物质交换的主要场所.前人对大洋俯冲带壳幔相互作用进行了大量研究,但是对俯冲带壳幔相互作用的物理化学过程和机理仍缺乏明确认识.在大陆俯冲带出露有造山带橄榄岩,它们来自俯冲板片之上的地幔楔,是解决这个问题的理想样品.通过对大别-苏鲁和柴北缘造山带橄榄岩进行系统的岩石学和地球化学研究,发现地幔楔橄榄岩由于俯冲地壳的交代作用而含有新生锆石和残留锆石,它们能为地壳交代作用时间、交代介质来源、性质和组成提供制约.地幔楔橄榄岩在大陆碰撞过程的不同阶段受到了俯冲大陆地壳衍生的多期不同性质流体的交代作用.地幔楔橄榄岩还受到了陆壳俯冲之前古俯冲洋壳衍生流体的交代作用.深俯冲陆壳衍生熔体与橄榄岩反应形成的石榴辉石岩具有高的水含量,能提供高水含量的地幔源区.      

Rocks mineralogy controls the metasomatic activities: an indication from water/rock ratio modeling

The final ratio equation of an isotopic element in a rock, derived from water/rock formula of McCulloch et al. Earth Planet Sci Lett 46:201-211, 1980, McCulloch et al. J Geophys Res 86:B4 2721-2735, 1981 is used to assess the behavior of diverse suites of rocks towards the alteration effect, and what implications can give about hydrothermal alteration in terms of isotopic compositions. Due to their higher Sr and lower Nd initial ratios than seawater, rocks of metamorphic and sedimentary signatures such as carbonates and Precambrian basement rocks show similar but inverse mixing curves compared with igneous rocks. Sr composition of rocks immediately alters by seawater, while Nd composition keeps unchanged until large volumes of water are added. Although, this can be attributed to the very low Nd concentration in seawater, it indicates that Nd-exchange may only take place under seawater, possibly hydrothermally by circulated seawater, and Nd-concentration of less altered crustal rocks are apparently primary. The isotopic composition and rock mineralogy seem to be the main factors controlling the volume of water required to cause isotopic alteration in rocks. Crustal rocks require higher water volumes due to their relatively low temperature minerals, whereas, mantle peridotites mainly consist of residual olivine minerals that are highly susceptible to alteration and lack of Sr and Nd compositions, and so need less amount of water for metasomatism. This property reduces the limited penetration effect as the mafic affinity increases at depth in the oceanic crust, and enables modified (probably acidified) circulated fluids to maintain ion exchanging and leaching throughout their passageway.     

A general model for the intrusion and evolution of 'mantle' garnet peridotites in high-pressure and ultra-high-pressure metamorphic terranes

Garnet‐bearing peridotite lenses are minor but significant components of most metamorphic terranes characterized by high‐temperature eclogite facies assemblages. Most peridotite intrudes when slabs of continental crust are subducted deeply (60–120 km) into the mantle, usually by following oceanic lithosphere down an established subduction zone. Peridotite is transferred from the resulting mantle wedge into the crustal footwall through brittle and/or ductile mechanisms. These ‘mantle’ peridotites vary petrographically, chemically, isotopically, chronologically and thermobarometrically from orogen to orogen, within orogens and even within individual terranes. The variations reflect: (1) derivation from different mantle sources (oceanic or continental lithosphere, asthenosphere); (2) perturbations while the mantle wedges were above subducting oceanic lithosphere; and (3) changes within the host crustal slabs during intrusion, subduction and exhumation. Peridotite caught within mantle wedges above oceanic subduction zones will tend to recrystallize and be contaminated by fluids derived from the subducting oceanic crust. These ‘subduction zone peridotites’ intrude during the subsequent subduction of continental crust. Low‐pressure protoliths introduced at shallow (serpentinite, plagioclase peridotite) and intermediate (spinel peridotite) mantle depths (20–50 km) may be carried to deeper levels within the host slab and undergo high‐pressure metamorphism along with the enclosing rocks. If subducted deeply enough, the peridotites will develop garnet‐bearing assemblages that are isofacial with, and give the same recrystallization ages as, the eclogite facies country rocks. Peridotites introduced at deeper levels (50–120 km) may already contain garnet when they intrude and will not necessarily be isofacial or isochronous with the enclosing crustal rocks. Some garnet peridotites recrystallize from spinel peridotite precursors at very high temperatures (c. 1200 °C) and may derive ultimately from the asthenosphere. Other peridotites are from old (>1 Ga), cold (c. 850 °C), subcontinental mantle (‘relict peridotites’) and seem to require the development of major intra‐cratonic faults to effect their intrusion.     

Gabbroic intrusions and magmatic metasomatism in harzburgites from the Garrett transform fault: implications for the nature of the mantle–crust transition at fast-spreading ridges

 Mafic and ultramafic rocks sampled in the Garrett transform fault at 13°28′S on the East Pacific Rise (EPR) provide insight on magmatic processes occurring under a fast-spreading ridge system. Serpentinized harzburgite from Garrett have modal, mineral and bulk chemical compositions consistent with being mantle residue of a high degree of partial melting. Along with other EPR localities (Terevaka transform fault and Hess Deep), these harzburgites are among the most residual and depleted in magmatophile elements of the entire mid-ocean ridge system. Geothermometric calculations using olivine-spinel pairs indicate a mean temperature of 759 ± 25 °C for Garrett residual harzburgite similar to the average of 755 °C for tectonite peridotites from slow-spreading ridges. Results of this study show that mid-ocean ridge peridotites are subject to both fractional melting and metasomatic processes. Evidence for mantle metasomatism is ubiquitous in harzburgite and is likely widespread in the entire Garrett peridotite massif. Magma-harzburgite interactions are very well preserved as pyroxenite lenses, plagioclase dunite pockets or dunitic wall rock to intrusive gabbros. Abundant gabbroic rocks are found as intrusive pockets and dikes in harzburgite and have been injected in the following sequence: olivine-gabbro, gabbro, gabbronorite, and ferrogabbro. The wide variety of magmas that crystallized into gabbros contrast sharply with present-day intratransform basalts, which have a highly primitive composition. Ferrogabbro dikes have been intruded at the ridge-transform intersection and as they represent the last event of a succession of gabbros intrusive into the peridotite, they likely constrain the origin of the entire peridotite massif to the same location. In peridotite massifs from Pacific transform faults (Garrett and Terevaka), primitive to fractionated basaltic magmas have flowed and crystallized variable amounts of dunite (±plagioclase) and minor pyroxenite, followed by a succession of cumulate gabbroic dikes which have extensively intruded and modified the host harzburgitic rocks. The lithosphere and style of magmatic activity within a fast-slipping transform fault (outcrops of ultramafic massif, discontinuous gabbro pockets intrusive in peridotite, magnesian and phyric basalts) are more analogous to slow-spreading Mid-Atlantic Ridge type than the East Pacific Rise. Received: 13 October 1997 / Accepted: 5 February 1999     

Chromian spinel as a petrogenetic indicator in abyssal and alpine-type peridotites and spatially associated lavas

The composition of chromian spinel in alpine-type peridotites has a large reciprocal range of Cr and Al, with increasing Cr# (Cr/(Cr+Al)) reflecting increasing degrees of partial melting in the mantle. Using spinel compositions, alpine-type peridotites can be divided into three groups. Type I peridotites and associated volcanic rocks contain spinels with Cr#<0.60; Type III peridotites and associated volcanics contain spinels with Cr#>0.60, and Type II peridotites and volcanics are a transitional group and contain spinels spanning the full range of spinel compositions in Type I and Type II peridotites. Spinels in abyssal peridotites lie entirely within the Type I spinel field, making ophiolites with Type I alpine-type peridotites the most likely candidates for sections of ocean lithosphere formed at a midocean ridge. The only modern analogs for Type III peridotites and associated volcanic rocks are found in arc-related volcanic and intrusive rocks, continental intrusive assemblages, and oceanic plateau basalts. We infer a sub-volcanic arc petrogenesis for most Type III alpine-type peridotites. Type II alpine-type peridotites apparently reflect composite origins, such as the formation of an island-arc on ocean crust, resulting in large variations in the degree and provenance of melting over relatively short distances. The essential difference between Type I and Type III peridotites appears to be the presence or absence of diopside in the residue at the end of melting.Based on an examination of co-existing rock and spinel compositions in lavas, it appears that spinel is a sensitive indicator of melt composition and pressure of crystallization. The close similarity of spinel composition fields in genetically related basalts, dunites and peridotites at localities in the oceans and in ophiolite complexes indicates that its composition reflects the degree of melting in the mantle source region. Accordingly, we infer from the restricted range of spinel compositions in abyssal basalts that the degree of mantle melting beneath mid-ocean ridges is generally limited to that found in Type I alpine-type peridotites. It is apparent, therefore, that the phase boundary OL-EN-DI-SP +meltOL-EN-SP+melt has limited the degree of melting of the mantle beneath mid-ocean ridges. This was clearly not the case for many alpine-type peridotites, implying very different melting conditions in the mantle, probably involving the presence of water.