Petrology and geochemistry of peridotites and associated vein rocks of Zabargad Island,Red Sea,Egypt

Summary Zabargad (St. John's) Island in the Red Sea contains three ultramafic bodies, one of which bas produced the famous gem olivine (peridot). The ultramafic rock types consist of two major groups—the peridotites and the vein rocks within them. The peridotites are divided into three groups: primitive, depleted and metasomatized. The primitive peridotites are the most abundant and are represented by mainly pristine spinellherzolites which have chemical compositions representative of the subcontinental upper mantle. The depleted peridotites are mainly harzburgites and nome dunites and both are similar to worldwide occurrences. The most depleted peridotites also appear to have the greatest metasomatic additions of incompatible elements, as has been noted at other localities. Metasomatic additions were clearly accompanied by tectonic shearing. Metasomatism included infiltration of incompatible elements and the formation of porphyroblasts of clinopyroxene, amphibole, Al-spinel and plagioclase; il took place under a variety of p-T conditions and with fluids of differing compositions.The vein rocks are mainly monomineralic and comprise olivinites, orthopyroxenites, clinopyroxenites, websterites, hornblendites and plagioclasites. These rocks are believed to have formed from fluids similar to that which metasomatized the host rock, rather than by some kind of igneous process. The fluids were derived from peridotite reservoirs (fertile and depleted) and apparently were in equilibrium with these reservoirs. Highly abundant fluid inclusions document the hypersaline and CO₂-dominated character of these fluids. Monomineralic vein rocks are closely associated with metasomatic and tectonic processes, and there is a complete transition between metasomatic impregnation and formation of vein rocks. These processes may have also been active in other peridotite bodies of the world, as was earlier recognized and documented in the Seiad Ultramafic Complex, California. Metasomatism is evident along clinopyroxenite and hornblendite veins, whereas orthopyroxenites, olivinites and plagioclasites do not show any interaction with the wall rocks. Olivinites are probably the latest (lowest p-T) vein rock type, and the latest olivine which formed within their open cavities became the gem peridot.Zabargad ultramafic rocks preserve relic phases indicating an initial depth of origin greater than 85 km. Clinopyroxenites preserve the memories of the highest p-T conditions and they may be the first vein rock type formed in the peridotites. The p-T path of uplift coincides with the oceanic geotherm at great depth but deviates systematically from it with falling pressure in a series of tectonic stages accompanied by metasomatism and recrystallization. The p-T and petrologic history indicates rapid uplift, a feature which is supported by extensive contact metamorphism of the associated metasediments.

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Petrologie und Geochemie der Peridotite und der mit diesen vergesellschafteten Ganggesteine der Insel Zabargad, Rotes Meer, Ägypten

Zusammenfassung Auf der Insel Zabargad (St.John's Island) im Roten Meer befinden sich drei Peridotit-Körper von denen einer seit Jahrtausenden den berühmten Peridot (Edelolivin) geliefert hat. Die ultramafischen Gesteine von Zabargad gliedern sich in zwei Hauptgruppen: die Peridotite und die mit diesen vergesellschafteten Ganggesteine. Die Peridotite können in drei Gruppen gegliedert werden: die primitiven, die verarmten und die metasomatisch veränderten Peridotite. Am meisten verbreitet auf Zabargad sind die primitiven Peridotite. Diese sind meist Spinell-Lherzolithe mit einer chemischen Zusammensetzung, welche dem subkontinentalen Oberen Erdmantel entspricht. Die verarmten Peridotite werden hauptsächlich von Harzburgiten und einigen wenigen Duniten repräsentiert. Beide sind jenen aus anderen Vorkommen der Welt sehr ähnlich. Die am stärksten verarmten Peridotite scheinen auch die stärksten metasomatischen Veränderungen erfahren zu haben—ein Trend, der auch schon an anderen ultramafischen Komplexen erkannt wurde. Metasomatische Anreicherungen inkompatibler Spurenelemente sind häufig direkt mit tektonischer Verformung und Kataklase gekoppelt. Die Metasomatose ist als Infiltration inkompatibler Elemente erkennbar und führte auch zur Bildung von Porphyroblasten von Klinopyroxen, Amphibol, Al-Spinell und Plagioklas. Diese Bildungen fanden unter verschiedenen p-T-Bedingungen statt und erfolgten durch Fluide mit unterschiedlichen Zusammensetzungen.Die (meist ultramafischen) Ganggesteine sind häufig monomineralisch und umfassen Olivinite, Orthopyroxenite, Klinopyroxenite, Websterite, Hornblendite und Plagioklasite. Wir glauben, daß diese Gesteine von Fluiden gebildet wurden, welche ähnlich jenen waren, die die Metasomatosen der Peridotite verursachten. Diese Genese wird von uns der magmatischen vorgezogen. Die Fluide stammten aus peridotitischen Reservoiren (fertilen und verarmten) und waren mit diesen offenbar im Gleichgewicht. Die Ganggesteine sind sehr reich an fluid inclusions, welche allerdings keine Flüssigkeit enthalten, sondern nur Festkörper (Salze) und CO₂ (± N₂), also einen trockenen, hypersalinen Charakter haben. Auch die monomineralischen Ganggesteine sind eng mit tektonischen Prozessen verknüpft und somit auch mit metasomatischen Prozessen. Es existieren vollkommene Übergänge von metasomatischen Imprägnationen bis zu echten Ganggesteinen. Solche Prozesse waren offensichtlich auch weltweit in anderen ultramafschen Komplexen aktiv und wurden schon im Seiad Ultramafc Complex in Kalifornien erkannt und beschrieben. Metasomatismus begleitet überlicherweise die Klinopyroxenit- und Hornblendit-Gänge. Orthopyroxenite, Olivinite und auch Plagioklasite zeigen jedoch keine Wechselwirkung mit den Wirtgesteinen. Olivinite sind wahrscheinlich die zuletzt gebildeten Ganggesteine. Der zuletzt sich bildende Olivin wurde der schönste und zum gesuchten Peridot.Alle ultramafschen Gesteine von Zabargad enthalten Minerale aus verschiedenen Bildungsepochen. Einige Relikte erinnern an eine Herkunft aus einer Tiefe von mehr als 85 km. Klinopyroxenite konservierten die höchsten p-T-Bedingungen. Sie waren daher wahrscheinlich die ersten (noch erhaltenen) Ganggesteine, welche sich im peridotitischen Erdmantel unterhalb des heutigen Roten Meeres bildeten. Der p-T-Pfad der Zabargad Ultramafitite deckt sich in großer Tiefe mit der ozeanischen Geotherme. Mit abnehmender Tiefe entfernt sich dieser Pfad allerdings zunehmend von der Geotherme und läßt eine Reihe von tektonischen Aktivitäten verbunden mit Metasomatose und Rekristallisation erkennen. Die p-T-Geschichte der Zabargad Ultramaftite deuten auf einen raschen Aufstieg aus dem Erdmantel hin. Diese Daten werden durch die weitverbreitete und intensive Kontaktmetamorphose der mit den Peridotiten assoziierten Metasedimenten unterstützt.

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Dedicated to Prof. Josef Zemann on the occasion of his 70th birthday

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With 12 Figures     

An early Miocene age for a high-temperature event in gneisses from Zabargad Island (Red Sea,Egypt): mantle diapirism?

U–Pb zircon data from a felsic gneiss located at the contact zone with the central peridotite body of Zabargad Island (Red Sea, Egypt) provide an age of 23.2 ± 5.9 Myr consistent with the ²³⁸U–²⁰⁶Pb age of the youngest concordant grain (22.4 ± 1.3 Myr). Concordant grains indicate new zircon growth and/or resetting whereas slightly discordant analyses suggest participation of an older zircon component whose age cannot be defined precisely. SEM back-scattered imaging further reveals the occurrence of zoned domains almost completely erased by complex internal structures attributed to extensive recrystallization under metamorphic conditions. The 22.4 Myr Miocene age is thus interpreted as dating a high-temperature metamorphic event. The proximity between the studied sample and the peridotite supports previous conclusions which regard parts of the peridotites from Zabargad Island as an asthenopheric mantle diapir which intruded the thinned Pan-African continental crust during the early stages of the Red Sea opening.     

Ar/Ar dating of amphiboles from Zabargad Island (Red Sea) is precluded by interaction with fluids

Stepwise heating data on four amphiboles separated from various Zabargad rocks (a dolerite dike, a peridotite, a gneiss and a pyroxenite) invariably show saddle-shaped age spectra. The age information in the spectrum is generally masked by 1–2 nl/g excess ⁴⁰Ar. The spectrum least affected by excess Ar constrains the emplacement age of the dolerite dike (Z-2137) to be 20 Ma. The greatest concentration of excess Ar is found in the peridotite, implying that the ultramafic rocks were metasomatized by a very ⁴⁰Ar-rich fluid. A complex history of subsolidus reactions with hydrothermal fluids is suggested by the irregularities in the ³⁷Ar/³⁹Ar spectra.     

A crustal origin for eclogites and a mantle origin for garnet peridotites: Strontium isotopic evidence from clinopyroxenes

Strontium isotopic data suggest that the classic eclogite-facies rocks of western south Norway described by Eskola (1921) formed from several parental materials in a variety of environments. Mineral separates from essentially basic, bi-minerallic (clinopyroxene and garnet) eclogites that occur as lens-shaped masses within high grade gneisses (country rock eclogites) have Sr⁸⁷/Sr⁸⁶ values that range from 0.704 for fine-grained varieties to 0.716 for coarse-grained, orthopyroxene-bearing varieties. These high, varied ratios contrast with the very low, restricted ratios (0.701 to 0.704) of similar minerals from ultrabasic, garnet-clinopyroxene-orthopyroxene-olivine assemblages (garnet peridotites) that occur as lenses within large peridotite bodies. The eclogite-facies metamorphism that generated the garnet peridotites may have occurred in the mantle. However, the metamorphism that generated at least the more radiogenic country-rock eclogites must have occurred in the crust. The high Sr⁸⁷/Sr⁸⁶ ratios of these eclogites could be generated either by forming them from crustal parental rocks or by contaminating mantle-derived parental rocks with radiogenic strontium from the country rocks. If this contamination occurred after intrusion and before eclogite-facies metamorphism, a rather contrived history must be postulated that involves intrusion, contamination accompanied by hydration, subsequent dehydration, and finally eclogite-facies metamorphism. These processes could have occurred within the long, complicated history of the enclosing country rocks. Alternatively, if the contamination occurred during eclogite-facies metamorphism, the presence of some hydrous fluid appears to be required to transport the radiogenic strontium from the enclosing country rocks. The eclogites with the highest Sr⁸⁷/Sr⁸⁶ ratios are also the most coarse-grained and it is possible that the presence of some intergranular fluid enabled these eclogites to recrystallize to a much larger grain size than would have been possible in a totally anhydrous environment. The garnet peridotites and fine-grained country rock eclogites may have formed from mantle material in the crust but escaped contamination by radiogenic strontium as a result of their position in a dry environment in the crust.Lamont-Doherty Geological Observatory Contribution No. 2443     

Clinopyroxenes from metasomatized spinel-peridotite mantle xenoliths from Nemby (Paraguay): crystal chemistry and petrological implications

Summary The Nemby mantle xenoliths are represented by spinel peridotites variably affected by “metasomatic processes” in Proterozoic times (1.2–0.5 Ga). Clinopyroxene compositional variations mainly reflect variably degrees of basalt extraction and mantle “metasomatic processes”. Clinopyroxene crystal chemistry allows to estimate the equilibration pressure by V(Cell) and V(M1) relationship. The investigated clinopyroxenes yielded a pressure range of 12–18 kb. Therefore, the Nemby xenoliths are representative of about 20 km thickness of subcontinental lithospheric mantle of the South America platform at Tertiary times (ca. 46 Ma).

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Zusammenfassung Klinopyroxene aus metasomatischen Spinell-Peridotit-Mantel Xenolithen aus Nemby (Paraguay): Kristallchemie und petrologische Bedeutung Die Mantelxenolithe aus Nemby sind Spinellperidodite, die in verschiedenem Ausma? im Proterozoikum (1.2 bis 0.5 Ga) durch “metasomatische Prozesse” beeinflusst wurden. Variationen in der Zusammensetzung der Klinopyroxene lassen variable Ausma?e von Basaltextraktion und Mantelmetasomatose erkennen. Die Kristallchemie der Pyroxene erm?glicht es, den Gleichgewichtsdruck durch V(Cell) und V(M1) Beziehungen zu bestimmen. Die untersuchten Klinopyroxene zeigten Drucke im Bereich von 12 bis 18 kb. Die Nemby Xenolithe sind deshalb repr?sentativ für einen ungef?hr 20 km dicken subkontinentalen lithosph?rischen Mantel der südamerikanischen Plattform im Terti?r (ca. 46 Ma).

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Received February 1, 1999; revised version accepted November 26, 1999     

Four- and five-phase peridotites from a continental rift system: evidence for upper mantle uplift and cooling at the Ross Sea margin (Antarctica)

Upper mantle plagioclase+spinel- and spinel-peridotite xenoliths occur in basanitic and tephritic lavas of the 2.7 my to Recent Mt. Melbourne Volcanic Field (Antarctica). This field belongs to the Cenozoic McMurdo volcanic group which is located between the deep western trough of the Ross Sea rift system and the uplifted rift shoulder of the Transantarctic Mountains. Our samples cover the transition zone between rift and shoulder. We examined texture and composition of plagioclase+spinel and normal spinel peridotites and determined temperatures and pressures of formation using the internally consistent Ca-ol/cpx and 2px-thermobarometer of Köhler and Brey (1990) and Brey and Köhler (1990). Distinct calcium distribution patterns in olivines correspond to three different petrographic textures: type ELZ have equigranular textures, and low calcium concentrations of 60 ppm in the olivine cores which are strongly zoned to 200 ppm in their rims. Type PLH are protogranular to porphyroclastic and have low and homogeneous calcium contents in the range of 120 to 200 ppm. Type EHH peridotites are equigranular and have olivines with high and homogeneous calcium values of 467–485 ppm. The application of the 2 px-thermometer give rim temperatures of 800 to 860 °C for Type ELZ, 900 to 1080 °C for type PLH and 1030 to 1050 °C for type EHH. Pressures of 13 to 17 kb calculated with the Ca-ol/epx-barometer for EHH peridotites are consistent with the Ross Rift geotherm. For the other two types, this barometer yields unreasonable high pressures exceeding 30 kb for both, plagioclase-bearing and normal spinel-peridotites. This indicates disequilibrium and continued calcium-loss from the olivines during cooling below the closure temperature for the 2 px-thermometer. Inversion of the Ca-in-olivine-barometer into a thermometer and application to core compositions of ELZ olivines (60 ppm) suggests that cooling occurred to temperatures of ca. 580 °C. Based on petrographical and geothermobarometric results and diffusion arguments, a four stage model is developed for the evolution of the upper mantle beneath the margin of the Ross Rift: (1) adiabatic uplift into the plagioclase/spinel-peridotile field; (2) subsequent cooling below the blocking temperature (800 °C) of the 2 px-thermometer to about 600 °C as indicated by low Ca in olivine cores; followed by (3) reheating to 760 °C as suggested by zonation to high calcium concentrations of up to 200 ppm in olivine rims. Calcium concentrations of up to 800 ppm were measured in one ELZ-olivine in the outermost rim (10 m) reflecting (4) a last heating event during transport and ascent in the basanitic host magma. Timing for the latter two stages has been roughly calculated from Ca-diffusivities in olivine. A minimum duration of 1200 to a few million years is indicated for stage 3 and 90 h to 22–23 days for stage 4, respectively. This timing of events correlates to increased mantle temperatures for the duration of magmatic activity of the Mt. Melbourne Volcanic Field and the short-term transport in host magmas. Our results also indicate that anomalous shallow mantle exists at the transition from the Ross Rift into the uplifted Transantarctic Mountains.     

Geochemistry of arc-related mantle peridotites and gabbros from the Chaldoran ophiolite,NW Iran

ABSTRACT

The Neo-Tethys-related Chaldoran ophiolite peridotites in NW Iran are remnants of mantle lithosphere, exhumed tectonically during the Late Cretaceous. Harzburgite is the predominant peridotite type, associated with oceanic lower crust cumulate gabbros occasionally. The ophiolite rocks are unconformably overlain by Late Cretaceous-Paleocene sediments. New whole-rock geochemistry of the variably serpentinized harzburgites shows a depleted nature, exemplified by low Al₂O₃, CaO, TiO₂, V and Y and high Ni, Cr and Mg and also low rare earth element (REE) contents. The harzburgites present LREE enrichment. Positive correlations between some LREEs and high field strength elements (HFSE) suggest enrichment of LREEs by melt re-fertilization processes. Cr-spinels have Cr number of [Cr# = Cr/(Cr + Al) = 0.53–0.67], showing medium to high degree of partial melting (F = ~17-20%). Both whole-rock and mineral chemistry data show a supra-subduction zone setting and progressive depletion along with increase in spinel Cr# (MOR to fore arc). The cumulate gabbros have high MgO and SiO₂, low TiO₂ and Ti/V < 10 and also low chondrite normalized Dy (<8.5). The gabbro samples show enriched LREEs and LILEs and depleted HREEs and HFSEs with respect to MORBs.

Subduction initiation (SI) model in a fore-arc/proto-fore-arc environment is suggested for the upper mantle evolution of the Chaldoran ophiolite. The rocks have experienced depletion in a second melting process at the later stages of SI and compositions were probably modified by extraction of island arc tholeiitic (IAT) and possibly boninitic (BON) melts. The chemostratigraphic progression for ‘subduction initiation rule (SIR)’ is likely traceable in Chaldoran mafic-ultramafic sequence, which corresponds to the most Neo-Tethyan ophiolites and is similar to MOR to supra-subduction zone (SSZ) evolution of most Iranian ‘Inner’ and ‘Outer Zagros’ ophiolitic peridotites.     

内蒙古柯单山蛇绿岩地幔橄榄岩铂族元素(PGEs)的分布特征及分异机制探讨

采用Carius管结合MC-ICPMS法分析了内蒙古柯单山蛇绿岩地幔橄榄岩中Ir、Ru、 Pt 和Pd 的含量,与典型的地幔橄榄岩进行对比研究,发现柯单山地幔橄榄岩中Ir和Ru明显亏损,Pt和Pd强烈富集,具有极高的Pd/Ir值,PGEs地幔标准化配分模式具有较陡的正斜率,明显不同于通常观测到的代表部分熔融残留相中铂族元素配分模式(负斜率或平坦型)。柯单山地幔橄榄岩的Ir和Ru与MgO呈正相关关系,表明Ir和Ru的亏损可能与部分熔融过程中硫化物的消耗程度有关,而与PGEs在硫化物/硅酸盐间的能斯特分配系数没有直接关系; Pt、Pd的富集表明本区的地幔橄榄岩不仅仅是经历过部分熔融的残余,而与来自深海的橄榄岩和大陆岩石圈地幔(SCLM)中的方辉橄榄岩相似,因此推测,本区地幔橄榄岩在部分熔融后又经历了富Pd的熔/流体交代,而熔/流体的来源可能是在岩浆分异演化过程中"熔离"出来的硫化物。     

西藏西南部拉昂错地幔橄榄岩的地球化学特征及其构造意义

拉昂错蛇绿岩位于西藏西南部雅鲁藏布缝合带(YZSZ)的西段,由地幔橄榄岩和侵入其中的基性岩墙组成.拉昂错地幔橄榄岩普遍发育碎斑结构及熔体注入和交代结构,尖晶石的Cr#值具有较广泛的变化(0.32～0.70),大多数样品富集LREE并伴随HFSE的明显增加,少数亏损LREE,前者部分熔融程度为15%～23%,后者为10%左右,这表明它们并不是地幔单阶段部分熔融的残余物,而是MORB型亏损橄榄岩在俯冲过程中再度部分熔融后熔体与残余地幔相互作用的产物,由于熔体不同程度的混合与交代,形成了各种再饱满程度不同的橄榄岩.对拉昂错地幔橄榄岩岩石学和地球化学特征的研究,为探讨YZSZ蛇绿岩带所代表的特提斯洋盆的形成和演化提供了新的证据.     

Petrological and geochemical characteristics of Plio-Pleistocene Volcanics from Ponza Island (Tyrrhenian Sea, Italy)

Summary Volcanic rocks on Ponza Island (Tyrrhenian Sea, central Italy) consist of Pliocene submarine rhyolites and Pleistocene subaerial trachyte and comendite lavas. Chemical variations and the homogeneous Sr and Nd isotopic signatures within the analyzed Pliocene rocks are ascribed to crystal fractionation. The absolute isotopic values, however, indicate the important role of a crustal component in the origin of these magmas. The very high-silica rocks were probably derived from a superimposed mechanism which may have been connected to the ascent of hydrothermal magmatic fluids. Compositional and ⁸⁷Sr/⁸⁶Sr variations at constant ¹⁴³Nd/¹⁴⁴Nd values in the Pleistocene rocks are likely due to fractionation of the observed phenocryst assemblage, possibly coupled with minor crustal interaction. These processes, however, cannot account for the extreme enrichment of many incompatible trace elements in the comendites. Some evidence suggests the influence of a halogen- and/or CO₂-rich volatile phase. Received February 17, 2000; revised version accepted November 29, 2000     

Interaction between protokimberlite melts and mantle lithosphere: Evidence from mantle xenoliths from the Dalnyaya kimberlite pipe,Yakutia (Russia)

The Dalnyaya kimberlite pipe(Yakutia,Russia) contains mantle peridotite xenoliths(mostly Iherzolites and harzburgites) that show both sheared porphyroclastic(deformed) and coarse granular textures,together with ilmenite and clinopyroxene megacrysts.Deformed peridotites contain high-temperature Fe-rich clinopyroxenes,sometimes associated with picroilmenites,which are products of interaction of the lithospheric mantle with protokimberlite related melts.The orthopyroxene-derived geotherm for the lithospheric mantle beneath Dalnyaya is stepped similar to that beneath the Udachnaya pipe.Coarse granular xenoliths fall on a geotherm of 35 mWm-2 whereas deformed varieties yield a 45 mWm-2)geotherm in the 2-7.5 GPa pressure interval.The chemistry of the constituent minerals including garnet,olivine and clinopyroxene shows trends of increasing Fe~#(=Fe/(Fe+Mg))with decreasing pressure.This may suggest that the interaction with fractionating protokimberlite melts occurred at different levels.Two major mantle lithologies are distinguished by the trace element patterns of their constituent minerals,determined by LA-ICP-MS.Orthopyroxenes,some clinopyroxenes and rare garnets are depleted in Ba,Sr,HFSE and MREE and represent relic lithospheric mantle.Re-fertilized garnet and clinopyroxene are more enriched.The distribution of trace elements between garnet and clinopyroxene shows that the garnets dissolved primary orthopyroxene and clinopyroxene.Later high temperature clinopyroxenes related to the protokimberlite melts partially dissolved these garnets.Olivines show decreases in Ni and increases in Al,Ca and Ti from Mg-rich varieties to the more Fe-rich,deformed and refertilized ones.Minerals showing higher Fe~#(0.11-0.15) are found within intergrowths of low-Cr ilmenite-clinopyroxene-garnet related to the crystallization of protokimberlite melts in feeder channels.In P-f(O_2) diagrams,garnets and Cr-rich clinopyroxenes indicate reduced conditions at the base of the lithosphere at-5 log units below a FMQ buffer.However,Cr-poor clinopyroxenes,together with ilmenite and some Fe-Ca-rich garnets,demonstrate a more oxidized trend in the lower part of lithosphere at-2 to 0 log units relative to FMQ.Clinopyroxenes from xenoliths in most cases show conditions transitional between those determined for garnets and megacrystalline Cr-poor suite.The relatively low diamond grade of Dalnyaya kimberlites is explained by a high degree of interaction with the oxidized protokimberlite melts,which is greater at the base of the lithosphere.     

Early cretaceous ophiolites of the Yarlung Zangbo Suture Zone: insights from dolerites and peridotites from the Baer upper mantle suite,SW Tibet (China)

The Baer ophiolitic massif is located in the northern sub-belt of the western segment of the Yarlung Zangbo Suture Zone (YZSZ) and mainly consists of a lherzolite-dominant mantle suite, dolerite intrusions and limited crustal outcrops. The dolerites show sub-ophitic texture and light rare earth element-depleted chondrite-normalized rare earth element patterns similar to normal-mid-ocean ridge basalts (N-MORB); though, they display enrichments in fluid-mobile elements (Rb, Ba, and Sr) and marked depletions in Th and Nb. The U–Pb ages of several magmatic zircon grains recovered from two dolerite samples indicate that the intrusion of the dikes into the Baer lherzolitic mantle occurred at 125.6–126.3 Ma, consistent with the widespread mafic magmatism between 120 and 130 Ma in the Yarlung Zangbo ophiolites. The dolerites have slightly more radiogenic ⁸⁷Sr/⁸⁶Sr ratios (0.7043–0.7054) in comparison to N-MORB, whereas they show ¹⁴³Nd/¹⁴⁴Nd values (0.513067–0.513114) similar to N-MORB and high zircon Hf-isotope compositions. They have a limited range of Nd-isotope (ε_Nd(t) values: +8.2 to +9.1) and juvenile Hf-isotope compositions (ε_Hf(t) values: +8.4 to +14.2 and +10.0 to +15.1) indicating derivation from mantle melts. The moderate spread in the ε_Hf (t) values of zircons indicates derivation of the dolerites parental magma from a weakly contaminated spinel-bearing mantle source. This is also corroborated by the geochemical signatures of the Baer dolerites (enrichment in LILE and depletion in HFSE) suggesting minor slab input to the mantle source of the dike-filling melt. We suggest that the genesis of the dolerite dike-forming melt happened at a stage of subduction initiation in a sub-oceanic mantle domain mildly affected by fluids emanating from the downgoing slab. Our data combined with literature data allow us to presume that the intrusion of the dolerites into the Baer mantle corresponds to an early phase of subduction initiation beneath a developing forearc basin.     

Dolomite-bearing orogenic garnet peridotites witness fluid-mediated carbon recycling in a mantle wedge (Ulten Zone,Eastern Alps,Italy)

We document the presence of dolomite ± apatite in orogenic peridotites from the Ulten Zone (UZ, Italian Alps), the remnants of a Variscan mantle wedge tectonically coupled with eclogitized continental crust. These dolomite peridotites are associated with dominant carbonate-free amphibole peridotites, which formed in response to infiltration of aqueous subduction fluids lost by the associated crustal rocks during high-pressure (HP) metamorphism and retrogression. Dolomite-free and dolomite-bearing peridotites share the same metamorphic evolution, from garnet- (HP) to spinel-facies (low-pressure, LP) conditions. Dolomite and the texturally coexisting phases display equilibrium redistribution of rare earth elements and of incompatible trace elements during HP and LP metamorphism; clinopyroxene and amphiboles from carbonate-free and carbonate-bearing peridotites have quite similar compositions. These features indicate that the UZ mantle rocks equilibrated with the same metasomatic agents: aqueous CO₂-bearing fluids enriched in incompatible elements released by the crust. The P–T crystallization conditions of the dolomite peridotites (outside the field of carbonatite melt + amphibole peridotite coexistence), a lack of textures indicating quench of carbonic melts, a lack of increase in modal clinopyroxene by reaction with such melts and the observed amphibole increase at the expense of clinopyroxene, all suggest that dolomite formation was assisted by aqueous CO₂-bearing fluids. A comparison of the trace element compositions of carbonates and amphiboles from the UZ peridotites and from peridotites metasomatized by carbonatite and/or carbon-bearing silicate melts does not help to unambiguously discriminate between the different agents (fluids or melts). The few observed differences (lower trace element contents in the fluid-related dolomite) may ultimately depend on the solute content of the metasomatic agent (CO₂-bearing fluid versus carbonatite melt). This study provides strong evidence that C–O–H subduction fluids can produce ‘carbonatite-like’ assemblages in mantle rocks, thus being effective C carriers from the slab to the mantle wedge at relatively low P–T. If transported beyond the carbonate and amphibole solidus by further subduction, dolomite-bearing garnet + amphibole peridotites like the ones from Ulten can become sources of carbonatite and/or C-bearing silicate melts in the mantle wedge. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. In memory of Lauro Morten 1941–2006.     

Platinum-group element signature of the primitive mantle rejuvenated by melt-rock reactions: evidence from Sumail peridotites (Oman Ophiolite)

Platinum-group element (PGE) abundances in mantle rocks are generally considered to result from a late meteorite addition to the early Earth, post-dating the core separation event. As such, PGEs are key tracers for the Earth accretion history. For decades, the PGEs systematics of undepleted mantle peridotites has been used to constrain the composition of meteorite impactors involved in the late veneer material. Despite multiple evidence of considerable modifications by partial melting, harzburgites from the Sumail ophiolite (Oman) display a mean PGE composition very akin to recently refined estimates for the Primitive Upper Mantle (PUM) of the Earth. These rocks document a resetting of the PUM signature by percolating basaltic melts, which precipitated Pd-enriched Cu–Ni sulphides within a strongly Pd-depleted residual harzburgitic protolith. Such a resetting casts doubt on both the reliability of any PUM estimates and relevance of the PUM concept itself, at least for PGEs.     

Crystal chemistry of clinopyroxenes from Oligo-Miocene volcanics of Montresta (Sardinia, Italy)

Summary Augite and diopside crystals from the Oligo-Miocene high-Mg and high-Al basalts, basaltic andesites and andesites of Montresta (Sardinia, Italy) have been analysed by means of X-ray single crystal diffraction and electron microprobe. It has been found that crystallization or equilibration processes took place within the crust at pressures below 5 kbar. In particular, samples from high-Al basalts, basaltic andesites and andesites crystallised at pressures below one kbar while those from high-Mg basalts crystallised in a range from about 1.5 to 4.5 kbar. The intracrystalline temperatures range between about 550 °C for high-Mg basalts and 750 °C for andesites. The M2′ site is occupied in each of the studied crystals, which implies slow cooling rates.     

Redox state of deep off-craton lithospheric mantle: new data from garnet and spinel peridotites from Vitim, southern Siberia

Oxygen fugacity (fO₂) affects melting, metasomatism, speciation of C–O–H fluids and carbon-rich phases in the upper mantle. fO₂ of deep off-craton mantle is poorly known because garnet-peridotite xenoliths are rare in alkali basalts. We examine the redox and thermal state of the lithospheric mantle between the Siberian and North China cratons using new Fe³⁺/ΣFe ratios in garnet and spinel obtained by M?ssbauer spectroscopy, major element data and P–T estimates for 22 peridotite xenoliths as well as published data for 15 xenoliths from Vitim, Russia. Shallow spinel-facies mantle is more oxidized than deep garnet peridotites (average, ?0.1 vs. ?2.5 ΔlogfO₂(FMQ)). For intermediate garnet–spinel peridotites, fO₂ estimates from spinel-based oxybarometers are 1.5–3.2 ΔlogfO₂(FMQ) lower than those from garnet-based oxybarometers. These rocks may be out of phase and chemical inter-mineral equilibrium because the spinel–garnet reaction and concomitant changes in mineral chemistry do not keep up with P–T changes (e.g., lithospheric heating by recent volcanism) due to slow diffusion of trivalent cations and because gar-, gar-spl and spl-facies rocks may coexist on centimeter–meter scale. The spinel-based fO₂ estimates may not be correct while garnet-based fO₂ values provide conditions before the heating. The T (780–1,100?°C) and fO₂ ranges of the Vitim xenoliths overlap those of coarse garnet and spinel cratonic peridotites. However, because of a higher geothermal gradient, the deepest Vitim garnet peridotites are more reduced (by 0.5–2.0 ΔlogfO₂(FMQ)) than cratonic garnet peridotites at similar depths, and the “water maximum” conditions (>80?% H₂O) in the off-craton mantle exist in a more shallow and narrow depth range (60–85?km) than in cratonic roots (100–170?km). The base of the off-craton lithospheric mantle (≥90?km) at 2.5?GPa and 1,150?°C has fO₂ of ?3.0 ?logfO₂(FMQ), with dominant CH₄ and H₂O and minor H₂ in the fluid. Melting near the base of off-craton mantle lithosphere may be induced by increasing water share in migrating fluids due to oxidation of methane.     

西藏班公湖存在MOR型和SSZ型蛇绿岩——来自两种不同地幔橄榄岩的证据

在班公湖怒江蛇绿岩带的西端日土出露两种不同的地幔橄榄岩:①角砾状方辉橄榄岩,由80%橄榄石(Fo=90.76~91.84,平均91.09)、15%斜方辉石(Mg#=90.97~91.41,平均91.16)、2%单斜辉石(Mg#=93.24~94.60,平均93.96)、3%棕色尖晶石(Cr#=0.20~0.25,平均0.23<0.60)和磁铁矿组成,以低MgO(41.41%~42.02%)、高Al2O3(1.63%~1.94%)、CaO(1.34%~1.60%)和Ti(133.04~134.52μg/g)为特征,亏损REE,ΣREE为球粒陨石的17%~22%,估算其为原始地幔岩经过10%~15%部分熔融的残留物;②块状方辉橄榄岩,由85%橄榄石、13%斜方辉石、2%红褐色尖晶石(Cr#=0.69~0.74,平均0.71>0.60)和磁铁矿组成,不含单斜辉石,相对于角砾状方辉橄榄岩,高MgO(42.96%~44.69%),低Al2O3(0.23%~0.61%)、CaO(0.08%~0.11%)和Ti(68.55~68.82μg/g),强烈亏损REE,ΣREE仅为球粒陨石的3%~5%,估算其为原始地幔橄榄岩经过30%~40%部分熔融的残留物。初步研究认为角砾状方辉橄榄岩是古大洋岩石圈在板块汇聚过程中构造侵位于缝合带中的残留物,是MOR型蛇绿岩的地幔橄榄岩;块状方辉橄榄岩是古大洋岩石圈在俯冲消减过程中再度发生熔融的残留物,是SSZ型蛇绿岩的地幔橄榄岩,这与本区发育MOR型蛇绿岩熔岩洋中脊拉斑玄武岩(P-MORB)和SSZ型蛇绿岩熔岩玻安岩(Boninite)是一致的。