Orogenic Massifs Research Articles

Reworking of old continental lithosphere: Unradiogenic Os and decoupled Hf[sbnd]Nd isotopes in sub-arc mantle pyroxenites

Mantle lithologies in orogenic massifs and xenoliths commonly display strikingly different Hf- and Nd-isotope compositions compared to oceanic basalts. While the presence of pyroxenites has long been suggested in the source region of mantle-derived magmas, very few studies have reported their combined HfNd isotope compositions. We here report the first LuHf data along with ReOs data and S concentrations on the Cabo Ortegal Complex, where the pyroxenite-rich Herbeira massif has been interpreted as remnants of a delaminated arc root. The pyroxenites, chromitites and their host harzburgites show a wide range of whole-rock 187Re/188Os and 187Os/188Os (0.16–1.44), indicating that Re was strongly mobilized, partly during hydrous retrograde metamorphism but mostly during supergene alteration that preferentially affected low-Mg#, low Cu/S pyroxenites. Samples that escaped this disturbance yield an isochron age of 838 ± 42 Ma, interpreted as the formation of Cabo Ortegal pyroxenites. Corresponding values of initial 187Os/188Os (0.111–0.117) are relatively unradiogenic, suggesting limited contributions of slab-derived Os to primitive arc melts such as those parental to these pyroxenites. This interpretation is consistent with radiogenic Os in arc lavas being mostly related to crustal assimilation. Paleoproterozoic to Archean Os model ages confirm that Cabo Ortegal pyroxenites record incipient volcanic arc magmatism on the continental margin of the Western African Craton, as notably documented by zircon UPb ages of 2.1 and 2.7 Ga. LuHf data collected on clinopyroxene and amphibole separates and whole-rock samples are characterized by uncorrelated 176Lu/177Hf and 176Hf/177Hf (0.2822–0.2855), decoupled from Nd-isotope compositions. This decoupling is ascribed to diffusional disequilibrium during melt-peridotite interaction, in good agreement with the results of percolation-diffusion models simulating the interaction of an arc melt with an ancient melt-depleted residue. These models notably show that HfNd isotopic decoupling such as recorded by Cabo Ortegal pyroxenites and peridotites (ΔƐHf(i) up to +97) is enhanced during melt-peridotite interaction by slow diffusional re-equilibration and can be relatively insensitive to chromatographic fractionation. Finally, we discuss the hypothesis that arc-continent interaction may provide preferential conditions for such isotopic decoupling and propose that its ubiquitous recognition in peridotites reflects the recycling of sub-arc mantle domains derived from ancient, reworked SCLM.

Thermochronological Evidence of Early Orogenesis, Eastern Pyrenees, France

AbstractIn collisional orogens, distinguishing the thermal signature of early orogenesis from the preceding rift or from subsequent thermal events is a major challenge. We present an integrated geological and low‐temperature thermochronology study of the Paleozoic Agly‐Salvezines crustal block in the retrowedge of the eastern Pyrenees (France). The northern Pyrenees preserves one of the best geological records of a rift‐to‐collision transition. The Agly‐Salvezines block represents the inverted distal European margin of an Aptian–Cenomanian rift system. Seventeen samples were collected throughout the external orogenic massif and analyzed for low‐temperature thermochronology: zircon (U‐Th)/He dating documents the cooling history of the massif during the initiation and early phase of Pyrenean convergence, while apatite (U‐Th)/He dating completes the record of plate collision. Using inverse and forward modeling of new low‐temperature thermochronology data, we show that the Pyrenean retrowedge records two clear phases of orogenic cooling, Late Campanian–Maastrichtian and Ypresian–Bartonian, which we relate to early inversion of the distal rifted margin and main collision, respectively. An earlier, late Aptian–Turonian cooling history is detected, possibly related to rifting and/or postrift. No cooling is evidenced during the Paleocene during which tectonic quiescence is recorded in the adjacent Aquitaine retroforeland basin. Using our low‐temperature thermochronology data and geological constraints, we propose a crustal‐scale sequentially restored model for the tectonic and thermal transition from extension to peak orogenesis in the eastern Pyrenees, which suggests that both thrusting and underplating processes contributed to early inversion of the Aptian–Cenomanian rift system.

Abiotic Methane in Continental Serpentinization Sites: An Overview

Until 2008 methane in land-based serpentinized peridotites (MSP) was considered to be an unusual and rare occurrence of abiotic gas. Today, reports of MSP are increasing for numerous localities worldwide in low temperature settings in ophiolites, orogenic massifs or intrusions. CH4 emanates from focused seeps, hyperalkaline water springs or through diffuse seepage from the ground, typically along faults. MSP has a combination of stable C and H isotope composition that is different from that of biotic methane; it is likely produced by Fischer-Tropsch Type reactions between CO2 (or other C compounds) and H2 (from serpentinization) at low temperatures (typically <100°C) and its carbon is fossil, 14C free, which does not derive from the more recent C dissolved in the hyperalkaline waters. MPS is more common than previously assumed; it may have played a key role in the origin of life, may fuel microbial life in igneous rocks on Earth and other planets, and can be an additional source of gas in atypical petroleum systems hosting ultramafic rocks.

Refertilization Processes in the Subcontinental Lithospheric Mantle: the Record of the Beni Bousera Orogenic Peridotite (Rif Belt, Northern Morocco)

Correlations between major and minor transition elements in tectonically emplaced orogenic peridotites have been ascribed to variable degrees of melt extraction and melt–rock reaction processes, leading to depletion or refertilization. To elucidate how such processes are recorded in the subcontinental lithospheric mantle, we processed a large geochemical dataset for peridotites from the four tectono-metamorphic domains of the Beni Bousera orogenic massif (Rif Belt, northern Morocco). Our study reveals that variations in bulk-rock major and minor elements, Mg-number and modal mineralogy of lherzolites, as well as their clinopyroxene trace element compositions, are inconsistent with simple partial melting and mainly resulted from different reactions between melts and depleted peridotites. Up to 30% melting at < 3 GPa and cryptic metasomatism can account for the geochemical variations of most harzburgites. In Grt–Sp mylonites, melting and melt–rock reactions are masked by tectonic mixing with garnet pyroxenites and subsolidus re-equilibration. In the rest of the massif, lherzolites were mostly produced by refertilization of a refractory protolith (Mg-number ∼ 91, Ol ∼ 70%, Cpx/Opx = 0·4) via two distinct near-solidus, melt–rock reactions: (1) clinopyroxene and orthopyroxene precipitation and olivine consumption at melt/rock ratios <0·75 and variable mass ratio between crystallized minerals and infiltrated melt (R), which are recorded fairly homogeneously throughout the massif; (2) dissolution of orthopyroxene and precipitation of clinopyroxene and olivine at melt/rock ratios ≤1 and R = 0·2–0·3, which affected mainly the Ariegite–Seiland and Seiland domains. The distribution of secondary lherzolites in the massif suggests that the first refertilization reaction occurred prior to the differentiation of the Beni Bousera mantle section into petro-structural zones, whereas the second reaction was associated with the development of the tectono-metamorphic domains. Our data support a secondary, refertilization-related origin for most lherzolites in orogenic peridotite massifs.

Protolith of ultramafic rocks in the Kluane Schist, Yukon, and implications for arc collisions in the northern Cordillera

Mafic and ultramafic rocks crop out as decimetre- to centimetre-sized bodies of talc–antigorite–olivine (±orthopyroxene) and chlorite–amphibole schists interleaved in the pelitic Kluane Schist of southwestern Yukon. The metamorphic assemblages in ultramafic rocks exposed at Doghead Point overprint two generations of cleavage and are consistent with metamorphism reaching &gt;550 °C (talc + olivine) and &gt;750 °C (olivine + enstatite) in the contact aureole of the Eocene Ruby Range batholith. The bulk rock major and trace element patterns in the ultramafic schists (&gt;40 wt.% MgO, Mg/(Mg + Fe) &gt; 0.90) are unlike residual mantle from partial melting (i.e., ophiolite, orogenic massif, abyssal ocean floor) but are similar to peridotite or pyroxenite cumulates from arc magmas. Identical trace element concentrations and patterns are observed in several late Triassic basalts, pyroxenites, and websterites occurring to the southeast in Stikinia (present coordinates). A highly discordant U–Pb zircon date for one antigorite–talc–olivine schist sample (200–210 Ma) is within the range of U–Pb zircon ages for the late Triassic Lewes River – Stuhini arc in northwestern Stikinia (200–208 Ma, 216–220 Ma). When combined with other published age information, the ultramafic rocks in the Kluane Schist are interpreted as fragments of deeper arc-related mafic and ultramafic intrusive rocks introduced to the Kluane forearc basin between 95 and 82 Ma by exhumation along shear zones in northwestern Stikinia, most likely the Wann River or Llewellyn Faults. The Kluane Schist represents a west-facing forearc basin bordered to the east by arc-parallel strike-slip fault(s) that served to exhume and imbricate large knockers into the accretionary prism.

Mercury in some arc crustal rocks and mantle peridotites and relevance to the moderately volatile element budget of the Earth

We measured Hg concentrations in 37 igneous rocks from an arc crustal section and in 30 mantle peridotites from ophiolite, orogenic massif and xenolith settings. Mercury is heterogeneously distributed in the igneous rocks and shows a ‘nugget effect’, suggesting it is concentrated in a trace phase, likely sulfide. The abundance of Hg in the crustal samples varies from 0.9 to 8ppb and correlates with S and Cu but no other element indicative of differentiation. The average of our data produces 2.9±2.6 Hg for the bulk crust, a factor of 10 lower than previous estimates. The mantle peridotites contained 0.2–5ppb Hg and a correlation of Hg with Al, Cu, S or loss on ignition (LOI) depending on sample type. Secondary uptake of Hg due to low-temperature alteration or mantle metasomatism is evident in the ophiolite and orogenic massif samples, respectively. The primitive upper mantle (PUM) contains 0.4–0.6ppb Hg based on the depletion/enrichment trends in the fresh xenolith samples that demonstrably retained primary Cu/S during emplacement. During mantle melting to produce the crust, Hg behaves as a mildly incompatible element (DHg residue/melt~0.1), not unlike Cu. For a chondritic abundance of 310ppb Hg, our estimate for Hg in the mantle requires this element has a similar depletion to Se, Te or S in the bulk silicate Earth.

Subcontinental rift initiation and ocean-continent transitional setting of the Dinarides and Vardar zone: Evidence from the Krivaja–Konjuh Massif, Bosnia and Herzegovina

The Dinaride and Vardar zone ophiolite belts extend from the south-eastern margins of the Alps to the Albanian and Greek ophiolites. Detailed sampling of the Krivaja–Konjuh massif, one of the largest massifs in the Dinaride belt, reveals fertile compositions and an extensive record of deformation at spinel peridotite facies conditions. High Na2O clinopyroxene and spinel–orthopyroxene symplectites after garnet indicate a relatively high pressure, subcontinental origin of the southern and western part of Krivaja, similar to orogenic massifs such as Lherz, Ronda and the Eastern Central Alpine peridotites. Clinopyroxene and spinel compositions from Konjuh show similarities with fertile abyssal peridotite. In the central parts of the massif the spinel lherzolites contain locally abundant patches of plagioclase, indicating impregnation by melt. The migrating melt was orthopyroxene undersaturated, locally converting the peridotites to massive olivine-rich troctolites. Massive gabbros and more evolved gabbro veins cross-cutting peridotites indicate continued melt production at depth. Overall we infer that the massif represents the onset of rifting and early stages of formation of a new ocean basin. In the south of Krivaja very localized chromitite occurrences indicate that much more depleted melts with supra-subduction affinity traversed the massif that have no genetic relationship with the peridotites. This indicates that volcanics with supra-subduction affinity at the margins of the Krivaja–Konjuh massif record separate processes during closure of the ocean basin. Comparison with published compositional data from other Balkan massifs shows that the range of compositions within the Krivaja–Konjuh massif is similar to the compositional range of the western massifs of the Dinarides. The compositions of the Balkan massifs show a west to east gradient, ranging from subcontinental on the western side of the Dinarides to depleted mid-ocean ridge/arc compositions in the Vardar zone in the east. This is consistent with the hypothesis that both ophiolite belts originate in a single ocean, rather than from two separate basins. A distinct decrease in fertility occurs in the south of the Dinarides towards the Albanian ophiolites with supra-subduction affinity.

Highly unradiogenic lead isotope ratios from the Horoman peridotite in Japan

Peridotites from the Horoman massif have the least radiogenic lead isotope ratios reported from any mantle material, and unlike any inferred from the compositions of mid-ocean ridge basalts. These data hint at the existence of ancient mantle domains that are not sampled by mid-ocean ridge basalts. Basalts at mid-ocean ridges are generated by partial melting of the Earth’s upper mantle. As a result of this process, the upper mantle has become depleted over time in elements that are preferentially removed by melting1,2,3. Although mid-ocean-ridge basalts have traditionally been thought to reflect the chemical composition of such depleted mantle2,3,4,5,6,7, recent work has revealed the existence of domains in the upper mantle that are apparently not sampled by the basalts8. Here we present the lead (Pb), neodymium (Nd) and hafnium (Hf) isotope compositions of peridotites from the Horoman orogenic massif in Japan, which is considered to represent the residues of melting of the upper mantle. These peridotites exhibit the lowest Pb isotope ratios reported from any known mantle material, along with high Nd and Hf isotope ratios. These data suggest that chemical depletion of the peridotites occurred around a billion years ago, and that they represent ancient mantle domains that have escaped convective stirring and homogenization. We suggest that such domains—if abundant in the mantle—may constitute a hitherto unrecognized reservoir with highly unradiogenic lead.

The Composition and Evolution of Lithospheric Mantle: a Re-evaluation and its Tectonic Implications

The composition of the subcontinental lithospheric mantle (SCLM) is broadly related to the tectonothermal age of the overlying crust, suggesting a secular change in SCLM-forming processes. Most estimated compositions of Archean SCLM, based on well-studied suites of xenoliths and xenocrysts, are depleted garnet lherzolites with high orthopyroxene/olivine. However, these compositions make it difficult to account for the high shear-wave velocities measured in the cores of large cratons, and predict deeper geoid anomalies and higher elevations than are observed in most cratons. Global and regional seismic tomography indicates that most cratonic xenolith suites represent material from the lower-velocity margins of lithospheric blocks. This implies that previous compositional estimates are strongly biased toward metasomatized material. We suggest that most Archean SCLM originally consisted of highly depleted dunites/harzburgites, similar to the Archean orogenic massifs of western Norway. Incorporation of such rocks in the cold upper parts of the cratonic SCLM satisfies the seismic and gravity data, suggesting that large volumes of these rocks are preserved in the cores of cratons, but are poorly sampled by volcanic rocks. The roots of most Proterozoic shields probably consist of refertilized Archean SCLM; the juvenile SCLM beneath Proterozoic and Phanerozoic mobile belts reflects only moderate depletion of Primitive Mantle compositions. Rather than a gradual evolution in SCLM-forming processes, we suggest a sharp dichotomy between Archean and younger tectonic regimes. The differences in buoyancy and viscosity between these two types of SCLM have played a major role in the construction, preservation and recycling of continental crust. If originally Archean SCLM is more widespread than currently recognized, models of crustal growth rates and recycling may need to be revised.

Permian exhumation of the Buffalo Pitts orogenic peridotite massif, northern Cordillera, Yukon

We report the results of a geochemical and U–Pb zircon geochronological study aimed at constraining the timing and tectonic setting of the exhumation of an orogenic peridotitic mantle massif in central Yukon within northern Canadian Cordillera. The Buffalo Pitts orogenic massif is inferred to have been exhumed into continental metasedimentary rocks within the pericratonic Yukon–Tanana terrane. Structurally admixed with the peridotite were boudins of metaleucogabbro and metatroctolite. A metamorphic aureole, defined by migmatite with abundant leucosome, characterizes the metasedi mentary wall rocks to the massif. Whole-rock chemical analyses indicate significant light rare-earth element enrichment of the leucogabbro and the metatroctolite, characteristics commonly ascribed to within-plate or rift settings. Crystallization of the leucogabbro occurred at 261.5 ± 2.3 Ma. The metatroctolite yields a similar crystallization age. These ages are coeval with metamorphism of the wall rocks to the orogenic massif, as indicated by leucosome crystallization at 262.3 ± 0.43 Ma. These geochemical and geochronological data are consistent with the orogenic massif having been exhumed within a continental rift at about 262 Ma, giving rise to metamorphism of the upper crustal rocks into which the massif was exhumed, and coeval with rift-related magmatism. Regional considerations suggest that rifting occurred within the back arc of a northeast-facing magmatic arc, represented by the Klondike schist. Coeval eclogite and blueschist along the northeast margin of the Yukon–Tanana terrane may mark the paleo-trench, along which a southwest-dipping slab is assumed to have subducted beneath Yukon–Tanana terrane.

Sedimentary record for exhumation of ultrahigh pressure (UHP) rocks in the northern Cordillera, British Columbia, Canada

We use field relationships, paleoflow indicators, petrography, and major and trace element mineral chemistry to examine the protolith and provenance of detrital mantle-derived ultrahigh pressure (UHP—2.8 GPa) minerals in immature clastic sediments of an Early Jurassic basin (Laberge Group) in northwestern British Columbia, Canada. Our results show that fresh mantle detritus in the Laberge Group was derived from mantle lithosphere that equilibrated at >2.8 GPa and temperatures of 850–1100 °C, exhumed in orogenic massifs and quickly deposited over a restricted time interval. Two models are proposed for the exhumation and denudation of the UHP rocks in either (1) an arc-continent collision between the Stikinia and Yukon Tanana terranes, or (2) an exposed forearc and accretionary melange in a convergent margin in the Cache Creek terrane. A collision between the Stikinia and Yukon Tanana terranes has not previously been documented, but this scenario is the most consistent with widespread evidence for rapid exposure of deep-seated rocks at the required time period for deposition of UHP detritus ca. 185 Ma.

Ferric iron in orogenic lherzolite massifs and controls of oxygen fugacity in the upper mantle

The bulk Fe 2O 3 contents and inter-mineral distributions of Fe 3+ were investigated in a suite of samples from several orogenic lherzolite massifs, including Beni Bousera (Morocco), Ronda (Spain) and Lherz (France). Ferric iron contents were determined for each phase by Mössbauer spectroscopy and these results were combined with microprobe data and modal abundances to determine the bulk Fe 2O 3 contents. The notion that Fe 3+ is moderately incompatible during partial melting in the mantle is supported by the observed decrease in bulk Fe 2O 3 content with increasing MgO, as well as by the generally lower Fe 3+ content of clinopyroxene in samples with low modal abundances of this phase. The partitioning of Fe 3+ between orthopyroxene and clinopyroxene is consistent with literature data for spinel peridotite xenoliths. The partitioning of Fe 3+ between clinopyroxene and spinel is composition dependent, changing with the Cr/Al of spinel. Thus partitioning is expected to be different in the spinel peridotite and plagioclase peridotite facies of the upper mantle. The ƒO 2 of orogenic massifs varies over several log units relative to the FMQ buffer. The values recorded by spinel-based and clinopyroxene-based oxygen barometry are generally comparable, indicating redox equilibrium between these two phases even at the relatively low temperature conditions existing in parts of the lithospheric mantle. Calculated bulk Fe 2O 3 contents range from 0.03 to 0.27 wt.%. The combination of modal abundance and major element composition means that orthopyroxene is a major contributor to the bulk Fe 2O 3 budget of peridotites, although clinopyroxene and spinel are much richer in Fe 3+ on a per formula unit basis. Residual Cr-rich spinel is the dominant source of Fe 3+ in plagioclase peridotites. In terms of the geochemical behaviour of Fe 3+, it can be concluded that orogenic peridotites exhibit essentially the same behaviour as spinel peridotite xenoliths. In terms of the controlling factors of ƒO 2 in the upper mantle, our data set records a certain degree of decoupling of ƒO 2 from whole rock Fe 2O 3 content, even if a correlation between these two parameters is generally apparent. This decoupling is because, whereas both whole rock Fe 2O 3 content and ƒO 2 are influenced by partial melting and melt extraction, additional processes such as metasomatism and phase changes can effectively reset ƒO 2 without always causing a concomitant change in bulk Fe 2O 3 content. Modelling the oxidation of spinel reveals that the ƒO 2 can be readily reset under initially reducing conditions, but the incorporation of progressively more and more Fe 3+ in spinel is required to further raise ƒO 2. A quasi-limit of ΔlogƒO 2 = FMQ + 1 is expected. These results are consistent with the general redox behaviour observed for spinel peridotites. Our data imply that Fe 3+–Fe 2+ equilibria have an important, if not dominant influence on ƒO 2 in the spinel peridotite facies of the upper mantle.

Upper Devonian biostratigraphy of the Iberian Pyrite Belt, southwest Spain - Part One: Miospores

The Iberian Pyrite Belt (IPB), which forms part of the Variscan orogenic massif, is renowned for the magnitude and extent of its massive sulfide mineralization. The stratigraphic record of the IPB consists of Upper Palaeozoic sedimentary and igneous rocks. In ascending order, these comprise the thick Phyllite-Quartzite Group attributed to the Middle and Upper Devonian and characterized by shales and quartzites with conglomeratic and carbonate intercalations towards the top; the appreciably thinner Volcano-Sedimentary Complex, a heterogeneous uppermost Devonian-Mississippian unit embodying diverse volcanic, subvolcanic, and sedimentary rocks that host the massive sulfide deposits; and the shaly and sandy, turbiditic Culm Group (Carboniferous). This entire succession was folded and faulted during the Asturian phase of the Variscan Orogeny that gave rise to a thin-skinned type structure. The present study constitutes a detailed blostratigraphic investigation of palynologically productive samples representative of the Phyllite-Quartzite Group and the basal (anoxic) portion of the Volcano-Sedimentary Complex. These were collected from surface and mine exposures variously located in the Spanish part of the IPB; out of 282 samples processed, 117 proved to be productive palynologically. The aim of this project is to provide comprehensive palynostratigraphic data applicable to precise dating and correlation of the IPB's stratigraphic succession (i.e., of the two sampled lithostratigraphic units), which has hitherto been investigated biostratigraphically on a relatively localized basis. The results are incorporated in two successive parts. The first of these, i. e., the present paper, focuses on the systematic analysis of the terrestrial (miospore) component of the palynological assemblages. The second part, devoted to the marine, organic-walled microphytoplankton (acritarchs and prasinophytes), will evaluate the stratigraphic significance of the IPB palynofloras and their application to elucidating the geological history of the region. In the systematic-descriptive section, which occupies the bulk of this paper, 55 species of trilete miospores are described and are allocated among 34 genera, two of which (Cristicavatispora and Epigruspora) are newly instituted herein. The majority of the species are either positively identifiable or closely affiliable with previously named species. The nine newly established species are as follows: Camptozonotriletes confertus, Indotriradites diversispinosus, Cristicavatispora dispersa (type species), Epigruspora regularis (type species), Ancyrospora? implicata, Endosporites tuberosus, Rugospora explicata, Spelaeotriletes plicatus, and Teichertospora iberica.

Significance of Accessory Chrome Spinel in Identifying Serpentinite Paragenesis

Chrome spinels are common accessory minerals in a wide series of ultrabasic rocks occurring in various geological settings and formed over a wide P-T range. These rocks include kimberlites, lamproites, and peridotites of orogenic massifs, part of the ophiolitic association. In many cases, these rocks are heavily serpentinized such that alterations cause difficulties in identifying the presence and, in particular, compositions of olivines and pyroxenes. Chrome spinels, however, may be present in heavy concentrates as the sole surviving primary mineral. This study shows that the compositional features of accessory chromites from xenoliths of granular peridotites from the Udachnaya kimberlite pipe, Yakutia, and chrome spinel from kimberlite and lamproite heavy concentrates, display a wide range in Mg# (50-80) and Cr# (20-90). Chromites occurring as inclusions in diamonds are notable exceptions, showing relatively narrow Cr# (mostly from 75 to 90) but a wide range in Mg# (50-80). Accessory chrome spinels, similar to diamonds in kimberlites and lamproites, and zircons in UHP metamorphic rocks, may be considered as good containers of primary minerals, including olivine, pyroxenes, and amphiboles. This is confirmed for a variety of chrome spinels studied from kimberlites, lamproites, and detrital spinels from unknown sources within the Siberian platform. The examination of chrome spinels from heavily serpentinized peridotites and serpentinites for the presence of mineral inclusions may widen the possibilities of identifying their paragenesis and P-T estimations.

Mildly incompatible elements in peridotites and the origins of mantle lithosphere

The abundances of the mildly incompatible elements Al, Cr, V, Sc and Yb in more than 1700 mantle peridotite bulk rock analyses are interpreted in the light of a fractional melting model based on experimentally measured partition coefficients ( D) and melting reaction stoichiometries. All peridotites examined, irrespective of sample type (abyssal peridotites, orogenic massifs, ophiolites, on/off craton xenoliths), tectonic environment (divergent/convergent/passive margin, intraplate) or the pressure ( P) they last equilibrated at in the mantle (plagioclase-, spinel- , or garnet facies), originated as residues at less than 3 GPa, mainly within the spinel-facies. Mantle rocks currently in the garnet facies likely were originally spinel-facies lithosphere underthrust or subducted to greater depths in convergent margins. This view is inescapable even within the widest range of D values employed in the calculations, and is furthermore strengthened when metasomatic effects on the abundances of the mildly incompatible elements in residues are considered. A pressure of origin of below ∼3 GPa for most mantle lithosphere creates difficulties for any model ascribing a significant volume of deep, cratonic mantle roots to plume sub-cretion or any other vertical tectonic mechanism.

Nature and origin of eclogite xenoliths from kimberlites

Eclogites from the Earth's mantle found in kimberlites provide important information on craton formation and ancient geodynamic processes because such eclogites are mostly Archean in age. They have equilibrated over a range of temperatures and pressures throughout the subcratonic mantle and some are diamond-bearing. Most mantle eclogites are bimineralic (omphacite and garnet) rarely with accessory rutiles. Contrary to their overall mineralogical simplicity, their broadly basaltic-picritic bulk compositions cover a large range and overlap with (but are not identical to) much younger lower grade eclogites from orogenic massifs. The majority of mantle eclogites have trace element geochemical features that require an origin from plagioclase-bearing protoliths and oxygen isotopic characteristics consistent with seawater alteration of oceanic crust. Therefore, most suites of eclogite xenoliths from kimberlites can be satisfactorily explained as samples of subducted oceanic crust. In contrast, eclogite xenoliths from Kuruman, South Africa and Koidu, Sierra Leone stem from protoliths that were picritic cumulates from intermediate pressures (1–2 Ga) and were subsequently transposed to higher pressures within the subcratonic mantle, consistent with craton growth via island arc collisions. None of the eclogite suites can be satisfactorily explained by an origin as high pressure cumulates from primary melts from garnet peridotite.

Platinum group element geochemistry of komatiites from the Alexo and Pyke Hill areas, Ontario, Canada

Thirty-three whole-rock drill core samples and thirteen olivine, chromite, and sulfide separates from three differentiated komatiite lava flows at Alexo and Pyke Hill, Canada, were analyzed for PGEs using the Carius tube digestion ID-ICP-MS technique. The emplaced lavas are Al-undepleted komatiites with ∼27% MgO derived by ∼50% partial melting of LILE-depleted Archean mantle. Major and minor element variations during and after emplacement were controlled by 30 to 50% fractionation of olivine Fo93–94. The emplaced lavas are characterized by (Pd/Ir)N = 4.0 to 4.6, (Os/Ir)N = 1.07, and Os abundances of ∼2.3 ppb. Variations in PGE abundances within individual flows indicate that Os and Ir were compatible (bulk DOs,Ir = 2.4–7.1) and that Pt and Pd were incompatible (bulk DPt,Pd < 0.2) during lava differentiation, whereas bulk DRu was close to unity. Analyses of cumulus olivine separates indicate that PGEs were incompatible in olivine (DPGEsOl-Liq = 0.04–0.7). The bulk fractionation trends cannot be accounted for by fractionation of olivine alone, and require an unidentified Os-Ir-rich phase. The composition of the mantle source (Os = 3.9 ppb, Ir = 3.6 ppb, Ru = 5.4 ppb, Pt and Pd = 5.7 ppb) was constrained empirically for Ru, Pt, and Pd; the Os/Ir ratio was taken to be identical to that in the emplaced melt, and the Ru/Ir ratio was taken to be chondritic, so that the absolute IPGE abundances of the source were determined by Ru. This is the first estimate of the PGE composition of a mantle source derived from analyses of erupted lavas. The suprachondritic Pd/Ir and Os/Ir of the inferred Abitibi komatiite mantle source are similar to those in off-craton spinel lherzolites, orogenic massif lherzolites, and enstatite chondrites, and are considered to be an intrinsic mantle feature. Bulk partition coefficients for use in komatiite melting models derived from the source and emplaced melt compositions are: DOs,Ir = 2.3, DRu = 1.0, DPt,Pd = 0.07. Ruthenium abundances are good indicators of absolute IPGE abundances in the mantle sources of komatiite melts with 26 to 29% MgO, as Ru fractionates very little during both high degrees of partial melting and lava differentiation.

Orogenic Lherzolites and Mantle Processes: Editorial

They contain vital information about melt transfer proThe papers in this Special Volume of the Journal of Petrology cesses related to the origin of mid-ocean ridge basalts. were presented at the Third International Lithosphere The Othris ophiolite, Greece, contains such spinel or Conference held in Pavia, Italy, in September 1999. The plagioclase lherzolites and Dijkstra et al. propose that meeting was attended by about 100 scientists who actively these shallow peridotites formed at a slow-spreading research mantle rocks and the processes of melt formation ridge. More significantly, they conclude that these rocks and extraction. The protoliths are garnet-, spinelor are the result of melt impregnation by mid-ocean ridge plagioclase-facies peridotites, rocks brought to the surface basalt (MORB) melts and that their ‘fertile’ compositions during the eruption of alkali to potassic volcanoes, diaare not primary. Similarly, an influx of melts of MORB tremes and maars. Mantle rocks are also exposed at the composition is proposed for the Horoman massif, Hoksurface of the Earth as a result of tectonic processes (i.e. kaido, by Saal et al. on the basis of Re–Os isotopic orogenic or alpine massifs) or mantle rocks are dredged data. Their data indicate a common petrogenesis for the or drilled on the ocean floor (i.e. abyssal peridotites). layered peridotite–pyroxenite sequence. In contrast, the Whereas the two previous lithosphere meetings in MontNikanbetsu plagioclase lherzolite from Hokkaido, Japan, pellier (1990) and Granada (1995) concentrated on orois believed by Takahashi to have experienced widespread genic massifs and xenoliths, the Pavia meeting dealt partial melting because of the evidence for incipient almost exclusively with the mineralogy, geochemistry and melting, melt migration and crystallization. experimental petrology of mantle rocks and their partial melt products.

Vanadium in peridotites, mantle redox and tectonic environments: Archean to present

New measurements of partition of vanadium (V) between spinel, garnet and a pigeonite-like high pressure ( P) pyroxene and magnesian liquids on the mantle solidus as a function of oxygen fugacity (fO 2) are presented. The spinel-liquid experiments show an effect of Cr/Al in spinel on partitioning, and further suggest that V exists as V 3+, V 4+ and V 5+ in melts at terrestrial fO 2. Vanadium is mildly incompatible in both ‘pigeonitic’ pyroxene and garnet between 4.5 and 6.5 GPa on the mantle solidus. Analysis of the fO 2-sensitive partitioning for V between mantle minerals and melts are combined with compositional data from peridotite melting experiments to model the covariation of V and Al in peridotite residues produced by non-modal, fractional melting under different fO 2 from 1.0 to 7.0 GPa. The partial melting models at 1.0 to 3.0 GPa fit the covariation of V and Al in abyssal peridotites quite well at fO 2s similar to those of mid-ocean ridge basalt. Many orogenic massifs and spinel lherzolite xenoliths represent mantle that formed at fO 2 higher than that produced at mid-ocean ridges in a range of tectonic environments. A large proportion of spinel-facies Archean cratonic lithosphere formed at fO 2s significantly higher than those of abyssal peridotites possibly linking its formation to a convergent margin (arc) tectonic setting. The case for garnet-facies cratonic mantle is equivocal; it may have formed at significantly higher pressures (7.0 GPa), or within the spinel-facies at lower pressures but at significantly higher fO 2 than is observed for abyssal peridotites. The imbrication of both oceanic garnet-facies mantle with spinel-facies arc mantle may explain the datasets for some Archean cratons. Overall, the data for Archean mantle melts and residues make clear that models cannot look to reduced, mantle-derived volcanic gases containing H 2 and CO to engender early life synthesis, or to promote hydrogen escape and gradual oxygenation of the Archean earth system.

The age of continental roots

Determination of the age of the mantle part of continental roots is essential to our understanding of the evolution and stability of continents. Dating the rocks that comprise the mantle root beneath the continents has proven difficult because of their high equilibration temperatures and open-system geochemical behaviour. Much progress has been made in the last 20 years that allows us to see how continental roots have evolved in different areas. The first indication of the antiquity of continental roots beneath cratons came from the enriched Nd and Sr isotopic signatures shown by both peridotite xenoliths and inclusions in diamonds, requiring isolation of cratonic roots from the convecting mantle for billions of years. The enriched Nd and Sr isotopic signatures result from mantle metasomatic events post-dating the depletion events that led to the formation and isolation of the peridotite from convecting mantle. These signatures document a history of melt– and fluid–rock interaction within the lithospheric mantle. In some suites of cratonic rocks, such as eclogites, Nd and Pb isotopes have been able to trace probable formation ages. The Re–Os isotope system is well suited to dating lithospheric peridotites because of the compatible nature of Os and its relative immunity to post-crystallisation disturbance compared with highly incompatible element isotope systems. Os isotopic compositions of lithospheric peridotites are overwhelmingly unradiogenic and indicate long-term evolution in low Re/Os environments, probably as melt residues. Peridotite xenoliths from kimberlites can show some disturbed Re/Os systematics but analyses of representative suites show that beneath cratons the oldest Re depletion model ages are Archean and broadly similar to major crust-forming events. Some locations, such as Premier in southern Africa, and Lashaine in Tanzania, indicate more recent addition of lithospheric material to the craton, in the Proterozoic, or later. Of the cratons studies so far (Kaapvaal, Siberia, Wyoming and Tanzania), all indicate Archean formation of their lithospheric mantle roots. Few localities studied show any clear variation of age with depth of derivation, indicating that >150 km of lithosphere may have formed relatively rapidly. In circum-cratonic areas where the crustal basement is Proterozoic in age kimberlite-derived xenoliths give Proterozoic model ages, matching the age of the overlying crust. This behaviour shows how the crust and mantle parts of continental lithospheric roots have remained coupled since formation in these areas, for billions of years, despite continental drift. Orogenic massifs show more systematic behaviour of Re–Os isotopes, where correlations between Os isotopic composition and S or Re content yield initial Os isotopic ratios that define Re depletion model ages for the massifs. Ongoing Sr–Nd–Pb–Hf–Os isotopic studies of massif peridotites and new kimberlite- and basalt-borne xenolith suites from new areas, will soon enable a global understanding of the age of continental roots and their subsequent evolution.