Eclogite Suite Research Articles

Dehydration‐driven deformation of eclogite: Interplay between fluid discharge and rheology

AbstractAqueous fluids released during dehydration of a subducting slab have a large effect on the rheology of the subduction interface. While high‐pressure experiments and natural‐case studies link deformation with critical dehydration reactions during eclogitization, the exact interplay between these processes remains ambiguous. To investigate fluid–rock interaction and associated deformation at high‐pressure, we studied a suite of eclogites from the Tsäkkok Lens of the Scandinavian Caledonides that record prograde metamorphism within an Early Palaeozoic cold subduction zone. Our results show that in‐situ dehydration during the blueschist to eclogite facies transition produces fluid fluxes leading to rheological weakening and densification, consequently promoting ductile‐brittle deformation. Petrographic evidence, supported by thermodynamic modelling and thermobarometry, attest to a prograde passage from lawsonite‐blueschist to peak eclogite facies of ~2.5 GPa and ~620°C. Phengite‐bearing eclogites imply interaction with an externally‐derived fluid, whereas rare phengite‐free, kyanite‐eclogites only record internally‐derived fluid production. Models predict that prograde breakdown of chlorite, lawsonite and amphibole between 500 and 610°C lead to progressive dehydration and release of up to 4.6 wt.% of aqueous fluid. Microstructural data reveal elongated shapes of highly strained omphacite porphyroblasts, displaying minor yet gradual changes in misorientation towards the grain boundaries. Occasionally, these intragranular structures form subgrain cells that have similar sizes to those of neoblasts in the rock matrix. These observations point to the potential onset of dynamic recrystallization processes via dislocation creep. Moreover, the omphacite neoblasts and rutile show non‐random crystallographic preferred orientations (CPOs), which are characterized by the subparallel alignment of point‐like maxima in rutile [001] and [100] axes to those of [001] and (010) of omphacite neoblasts, respectively. Additionally, the [001] axes of these minerals are also subparallel to the weak stretching mineral lineation, and the (100) of rutile and the (010) of omphacite neoblasts are distributed in the plane of the foliation. This suggests that the development of their CPOs was coeval and structurally controlled. Garnet microfractures normal to the foliation are dilated and sealed predominantly by omphacite. The lack of obliquity between CPO and foliation plane, as well as the systematic orientation of garnet microfracture orientations, are consistent with coaxial deformation at peak‐pressure conditions. Unlike other studies, we show that neither an external fluid source nor channelized fluid flow is needed to facilitate a ductile‐brittle deformation of eclogite in a subduction setting.

The system NaAlSi2O6‒CaMgSi2O6−CO2 at 3–6.5 GPa: implications for CO2 stability in the eclogitic suite at depths of 100–200 km

The system NaAlSi2O6‒CaMgSi2O6−CO2 at 3–6.5 GPa: implications for CO2 stability in the eclogitic suite at depths of 100–200 km

Paleo-Tethys subduction induced slab-drag opening the Neo-Tethys: Evidence from an Iranian segment of Gondwana

The Iranian plateau has a more straightforward relationship between the Paleo-Tethys and Neo-Tethys, than other regions in the Tethyan realm and is an ideal place to test the genetic connection of how the Paleo-Tethys affected the Neo-Tethys. Moreover, there is no consensus on how Paleo-Tethyan evolution influenced the Neo-Tethyan Wilson cycle. We studied the petrology , geochemistry , and metamorphic age of an eclogite suite at Rasht in northern Iran from the Paleo-Tethyan suture. The eclogitic protolith was a typical mid-ocean-ridge basalt and achieved its peak metamorphic condition of 2.7 GPa and 590 °C at 353 ± 8.9 Ma. Reconstructed pressure-temperature-time path of the eclogite-facies rocks suggests that the eclogitic protolith entered the trench at 356 ± 8.9 Ma. Mid-oceanic ridge rocks formed at ~380 Ma at the Rasht region and other places along the Alborz Mountain strike. A ~ 25 million years (Myr) period from the birth of oceanic crust to subduction at Rasht region implies that the mid-ocean-ridge was close to the trench. We propose that the Paleo-Tethyan mid-ocean-ridge was trench-parallelly subducted beneath the Eurasian boundary in northern Iran. This ridge-subduction propagated from east to west between 380 and 355 Ma. 47–27 Myr after ridge consumption, subduction of the remaining Paleo-Tethyan oceanic crust exerted a continuous extensional stress of ~0.1 GPa on the passive margin of Gondwana since the late Carboniferous. The drag-force eventually detached the Central Iranian Block from Gondwana, and opened the Neo-Tethyan ocean during the middle-late Permian .

Liquid immiscibility and phase relations in the system KAlSi3O8-CaMg(CO3)2 ± NaAlSi2O6 ± Na2CO3 at 6 GPa: Implications for diamond-forming melts

To evaluate the effect of Na on the carbonate-silicate liquid immiscibility in the diamond stability field, we performed experiments along some specific joins of the system KAlSi3O8-CaMg(CO3)2 ± NaAlSi2O6 ± Na2CO3 at 6 GPa. Melting in all studied joins begins at 1000–1050 °C. The melting in the Kfs + Dol system is controlled by the reaction 6 KAlSi3O8 (K-feldspar) + 6 CaMg(CO3)2 (dolomite) = 2 (Can,Mg1-n)3Al2Si3O12 (garnet) + Al2SiO5 (kyanite) + 11 SiO2 (coesite) + 3 K2(Ca1-n,Mgn)2(CO3)3 (carbonatitic melt) + 3 CO2 (fluid), where n ~ 0.3–0.4. A temperature increasing to 1300 °C yields an appearance of the silicic immiscible melt in addition to carbonatitic melt via the reaction K2CO3 (carbonatitic melt) + Al2SiO5 (kyanite) + 5 SiO2 (coesite) = 2 KAlSi3O8 (silicic melt) + CO2 (fluid or solute in melts). The silicic melt composition is close to KAlSi3O8 with dissolved CaMg(CO3)2 and molecular CO2. An addition of NaAlSi2O6 or Na2CO3 to the system results in partial decomposition of K-feldspar and formation of K-bearing carbonates, (K, Na)2Mg(CO3)2 and (K, Na)2Ca3(CO3)4. Their melting produces carbonatite melt with the approximate composition of 4(K, Na)2CO3·6Ca0.6Mg0.4CO3 and magnesite. Besides, the presence of NaAlSi2O6 in the studied system shifts the lower-temperature limit of immiscibility to 1500°С, while the presence of Na2CO3 eliminates the appearance of silicic melt by the following reaction: 2 KAlSi3O8 (in the silicic melt) + Na2CO3 = 2 NaAlSi2O6 (in clinopyroxene) + K2CO3 (in the carbonatitic melt) + SiO2 (coesite). Thus, an increase of the Na2O content in the system Na2O-K2O-CaO-MgO-Al2O3-SiO2-CO2 consumes Al2O3 and SiO2 from silicic melt to form clinopyroxene. We found that grossular-pyrope and diopside-jadeite solid solutions can coexist with CO2 fluid at 900–1500 °C and 6 GPa. Thus, CO2 fluid is stable in the eclogitic suite in the diamond stability field under temperature conditions of the continental lithosphere and subducting slabs. Variations in the Na2O content observed in carbonatitic melts trapped by natural in diamonds exceed those derived by the pelite melting. The present experiments show that an addition of NaAlSi2O6 to the Kfs + Dol system does not cause an increase of the Na2O content in the carbonatitic melt, whereas the addition of Na2CO3 at Na2O/Al2O3 > 1 yields the formation of the melts with the Na2O contents covering the entire range of natural compositions. Thus, only the presence of additional salt components can explain the elevated Na2O content in the melts trapped in lithospheric diamonds. In addition to carbonates, sodium can be hosted by chlorides, sulfates, etc.

Evidence for a dominantly reducing Archaean ambient mantle from two redox proxies, and low oxygen fugacity of deeply subducted oceanic crust

Oxygen fugacity (ƒO2) is an intensive variable implicated in a range of processes that have shaped the Earth system, but there is controversy on the timing and rate of oxidation of the uppermost convecting mantle to its present ƒO2 around the fayalite-magnetite-quartz oxygen buffer. Here, we report Fe3+/ΣFe and ƒO2 for ancient eclogite xenoliths with oceanic crustal protoliths that sampled the coeval ambient convecting mantle. Using new and published data, we demonstrate that in these eclogites, two redox proxies, V/Sc and Fe3+/ΣFe, behave sympathetically, despite different responses of their protoliths to differentiation and post-formation degassing, seawater alteration, devolatilisation and partial melting, testifying to an unexpected robustness of Fe3+/ΣFe. Therefore, these processes, while causing significant scatter, did not completely obliterate the underlying convecting mantle signal. Considering only unmetasomatised samples with non-cumulate and little-differentiated protoliths, V/Sc and Fe3+/ΣFe in two Archaean eclogite suites are significantly lower than those of modern mid-ocean ridge basalts (MORB), while a third suite has ratios similar to modern MORB, indicating redox heterogeneity. Another major finding is the predominantly low though variable estimated ƒO2 of eclogite at mantle depths, which does not permit stabilisation of CO2-dominated fluids or pure carbonatite melts. Conversely, low-ƒO2 eclogite may have caused efficient reduction of CO2 in fluids and melts generated in other portions of ancient subducting slabs, consistent with eclogitic diamond formation ages, the disproportionate frequency of eclogitic diamonds relative to the subordinate abundance of eclogite in the mantle lithosphere and the general absence of carbonate in mantle eclogite. This indicates carbon recycling at least to depths of diamond stability and may have represented a significant pathway for carbon ingassing through time.

Ages and sources of mantle eclogites: ID-TIMS and in situ MC-ICPMS Pb-Sr isotope systematics of clinopyroxene

Strontium and Pb isotopic compositions of clinopyroxene (cpx) in selected samples from three well-characterised eclogite suites with oceanic crustal protoliths (Lace/Kaapvaal craton, Orapa/Zimbabwe craton and Koidu/West African craton) were acquired by high-precision isotope dilution thermal ionisation mass spectrometry (ID-TIMS) and in situ multicollector-laser ablation-inductively-coupled plasma mass spectrometry (MC-LA-ICPMS). The aims of this study are twofold: (1) assess their utility to obtain formation or resetting age constraints and identify elemental signatures that enhance the chances of successful age dating, and (2) to confirm the veracity and utility of results obtained by novel MC-LA-ICPMS techniques. Strontium-Pb isotope systematics of eclogitic cpx measured in this study are decoupled and may reflect addition of unsupported radiogenic Sr during seawater alteration or interaction with oceanic sediments in subduction mélanges, and/or disturbance due to mantle metasomatism, to which the more incompatible Pb is more susceptible. Despite a complex history, subsets of samples yield meaningful model dates. Clinopyroxene fractions from Lace with high Pb contents (3–6 ppm), unradiogenic Pb isotopic compositions (206Pb/204Pb = 13.57–13.52) and low 238U/204Pb (1.0–1.5) give single-stage model Pb dates of 2.90–2.84 Ga. In contrast, samples from Orapa plot to the right of the Geochron and do not yield meaningful Pb model ages. However, these data do define secondary isochrons that can be modelled to yield minimum age constraints on major events affecting the cratonic lithosphere. Within the uncertainties, the resultant 2.18 ± 0.45 Ga age obtained for Koidu eclogites reflect disturbance of the Pb isotope system due to subduction beneath the craton linked to the Eburnean orogeny, while they retained their unradiogenic 87Sr/86Sr (0.7016). Similarly, the age for samples from Orapa (2.20 ± 0.54 Ga) is interpreted as an overprint age related to Palaeoproterozoic accretion at the western craton margin.Gabbroic eclogites (Eu/Eu* > 1) with plagioclase-rich protoliths having low time-integrated Rb/Sr and U/Pb retain the least radiogenic Sr and, in part, Pb. High model μ (9.0 to 9.1) for several eclogites from Lace with elevated LREE, Th and Pb abundances reflects ca. 3.0 Ga addition of a sedimentary component, possibly derived from reworking of a high-μ basaltic protocrust, as observed on other cratons. We suggest that sample targeting can be usefully guided by fast-throughput in situ LA-ICPMS techniques, which largely yield results identical to ID-TIMS, albeit at lower precision, and which can further help identify kimberlite contamination in the mineral separates used for solution work.

Episodic diamond growth beneath the Kaapvaal Craton at Jwaneng Mine, Botswana

Important implications for the interior workings of the Earth can be drawn by studying diamonds and their inclusions. To better understand the timing and number of diamond forming events beneath the NW margin of the Kaapvaal Craton, a comprehensive reassessment of Jwaneng’s diamond populations has been undertaken. We report new inclusion abundance data from the visual examination of ~130,000 diamonds that validate the predominance of an eclogitic diamond suite (up to 88%) with on average 5% inclusion-bearing diamonds (with inclusions >10 μm in size). From this population, polished plates from 79 diamonds of eclogitic and peridotitic paragenesis have been studied with cathodoluminescence (CL) imaging and infrared spectroscopy (FTIR) traverses. The majority (80%) record major changes in N concentration and aggregation states, as well as sharp boundaries in the CL images of individual plates that are interpreted to demarcate discrete diamond growth events. In addition, bulk FTIR data have been acquired for 373 unpolished diamonds. Silicate inclusions sampled from distinct growth zones define 2 compositional groups of omphacites and pyrope-almandines associated with different N contents in their diamond hosts. These findings reinforce previous observations that at Jwaneng at least seven individual diamond forming events can be identified – 3 peridotitic and 4 eclogitic. The results demonstrate that detailed examination of diamond plates by CL imaging and FTIR traverses is necessary to unveil the complex history recorded in diamonds.

Formation of diamondiferous kyanite–eclogite in a subduction mélange

Diamond- and kyanite-bearing eclogites from the Lace kimberlite on the Kaapvaal craton have common picritic to gabbroic oceanic protoliths with bimineralic eclogites, lying on arrays of Eu∗ and ∑REE that are consistent with accumulation and fractionation of plagioclase and olivine. However, they also show significant compositional differences, such as more grossular-rich garnet and aluminous clinopyroxene (cpx), which require the operation of additional processes. Their nature is elucidated using mineral major- and trace-element compositions, as well as Sr isotope ratios determined by in situ techniques.Highly variable major-element compositions across the co-genetic eclogite suites exert a strong effect on the trace-element distribution between garnet and cpx, whereby Sc, Ge, Sr, Y, Cd, REE, Th and U partition more strongly into garnet with increasing grossular-content. Thus, significant differences between the trace-element compositions of garnet can ensue from crystal-chemical effects alone, making their use as petrogenetic indicators potentially ambiguous. After correcting for these compositional effects, garnet in kyanite-/diamond eclogites, and in eclogites devoid of accessory minerals but with similar signatures, shows depletion (or dilution) in Sc, Ge, Y, In, Zr, Hf and the HREE, and enrichment in the LREE and Th compared to garnet in bimineralic eclogites. This is interpreted as the signature of a pelite-derived melt, which was transferred by addition of aluminous cpx that later exsolved kyanite and garnet, as observed in other aluminous eclogite suites. Continental input can explain initial (at 2.9Ga) 87Sr/86Sr⩾0.714 measured in cpx in eleven samples with low 87Rb/86Sr (<0.01). The association of diamond with kyanite suggests that diamond formation is also linked to this event, possibly due to diamond formation by oxidation of reduced carbon, such as methane, and attendant reduction of Fe3+ in garnet.This model of sediment melt–oceanic crust interaction reconciles evidence for both low- and high-pressure igneous processes in some aluminous eclogites. We suggest that a subduction mélange is a favourable setting for the transfer of a sediment-derived signature into oceanic crust, leading to formation of diamondiferous kyanite–eclogites from bimineralic eclogites. Diapirism, fluxed by the presence of partial melt, may have facilitated dispersal of the eclogites in the lithosphere column, consistent with their widely varying equilibration pressures ranging from ∼5 to 8GPa.

Kyanite/corundum eclogites from the Kaapvaal Craton: subducted troctolites and layered gabbros from the Mid- to Early Archean

An oceanic crustal origin is the commonly accepted paradigm for mantle-derived eclogites. However, the significance of the aluminous members of the eclogite suite, containing kyanite and corundum, has long been underrated and their role neglected in genetic models of cratonic evolution. Here, we present a geochemical and petrological study of a suite of kyanite- and corundum-bearing eclogites from the Bellsbank kimberlite, S. Africa, which originate from depths between 150 and 200 km. Although clearly of high-pressure provenance, these rocks had a low-pressure cumulative origin with plagioclase and olivine as major cumulate phases. This is shown by the very pronounced positive Eu anomalies, low REE abundances, and δ 18O values lower than the Earth’s mantle. Many chemical features are identical to modern-day troctolitic cumulates including a light REE depletion akin to MORB, but there are also distinguishing features in that the eclogites are richer in Na, Fe, and Ni. Two of the eclogites have a minimum age of ~3.2 Ga, defined by the extremely unradiogenic 87Sr/86Sr (0.7007) in clinopyroxene. Phase equilibria indicate that the parent melts were formed by partial melting below an Archean volcanic center that generated (alkali-)picritic to high-alumina tholeiitic melts from a mantle whose oxygen fugacity was lower than today. Fractional crystallization produced troctolites with immiscible sulfide melt droplets within the mafic crust. Instability of the mafic crust led to deep subduction and re-equilibration at 4–6 GPa. Phase relationships plus the presence of a sample with appreciable modal corundum but no Eu anomaly suggest that kyanite- and corundum-bearing eclogites may also originate as plagioclase-free, higher pressure cumulates of highly aluminous clinopyroxene, spinel, and olivine. This is consistent with the crystallizing phase assemblage from an olivine tholeiitic to picritic magma deeper in the Archean oceanic crust or uppermost mantle. We postulate that the magmatic and subduction processes driving modern plate tectonics already existed in the Meso- to Early Archean.

Subduction-related origin of eclogite xenoliths from the Wajrakarur kimberlite field, Eastern Dharwar craton, Southern India: Constraints from petrology and geochemistry

Major and trace elements as well as the first oxygen isotopes are reported on eclogite xenoliths from the Mesoproterozoic KL2 and P3 kimberlite pipes of the Wajrakarur kimberlite field, Eastern Dharwar craton, Southern India. Garnets in kyanite-bearing samples are rich in grossular, whereas they are predominantly pyrope-almandines in the bimineralic (kyanite-free) samples. The kyanite eclogite from the P3 pipe is more Mg-rich than those from KL2 pipe. Equilibration temperatures indicate derivation from 4.5 to 5.3GPa and 1060 to 1220°C for the KL2 samples and 3.6GPa, 918°C for the P3 sample.Garnet rare earth element patterns show two characteristic types, one with relatively low and flat heavy rare earth element patterns: Wajrakarur Group 1 and a second with lower light to heavy rare earth element ratios: Wajrakarur Group 2. Most samples in Wajrakarur Group 1 show pronounced positive Eu anomalies in garnet and positive Eu and Sr anomalies in the reconstructed whole rock trace element patterns; these are among the strongest anomalies in eclogite xenoliths worldwide. In contrast, Wajrakarur Group 2 samples show only subtle positive Eu anomalies.Oxygen isotopic ratios of garnets range between +5.3‰ and +7.8‰ δ18O. This range extends significantly beyond the range for unchanged mantle. Similar to many other eclogite suites worldwide, the Wajrakarur Group 1 and Group 2 eclogite suites shows evidence for an origin as crustal gabbroic material, likely once part of the oceanic crust, which was subducted and imbricated under the Eastern Dharwar craton. Their surface origin therefore lends support to geodynamic models that favor amalgamation of the Dharwar craton by subduction.

Apatite, SiO2, rutile and orthopyroxene precipitates in minerals of eclogite xenoliths from Yakutian kimberlites, Russia

Eclogite mantle xenoliths from the central part of Siberian craton (Udachnaya and Zarnitsa kimberlite pipes) as well as from the northeastern edge of the craton (Obnazhennaya kimberlite) were studied in detail. Garnet and clinopyroxene show evident exsolution textures. Garnet comprises rutile, ilmenite, apatite, and quartz/coesite oriented inclusions. Clinopyroxene contains rutile (±ilmenite) and apatite precipitates. Granular inclusions of quartz in kyanite and garnet usually retain features of their high-pressure origin. According to thermobarometric calculations, the studied eclogitic suite was equilibrated within lithospheric mantle at 3.2–4.9GPa and 813–1080°C. The precursor composition of garnets from Udachnaya and Zarnitsa eclogites suggests their stability at depths 210–260km. Apatite precipitation in clinopyroxenes of Udachnaya and Zarnitsa allows us to declare that original pyroxenes could have been indicative of their high P–T stability. Raman spectroscopic study of quartz and coesite precipitates in garnet porphyroblasts confirms our hypothesis on the origin of the exsolution textures during pressure–temperature decrease. With respect to mineralogical data, we suppose the rocks to be subjected to stepwise decompression and cooling within mantle reservoir.

Water contents of Roberts Victor xenolithic eclogites: primary and metasomatic controls

A suite of eclogites from the Roberts Victor kimberlite has been extensively characterized in terms of petrology and geochemical compositions (Greau et al. in Geochim Cosmochim Acta 75(22):6927–6954, 2011; Huang et al. in Lithos 142–143:161–181, 2012a). In the present study, the water contents of eclogitic garnet and omphacite were analyzed by Fourier transform infrared spectrometry. Garnet does not contain measureable OH in any sample. The water content of omphacite in the studied eclogites ranges from 211 to 1,496 ppm. Mantle metasomatism has modified the water content of some of the eclogites, while others retain water contents characteristic of their original environment. The OH contents of the metasomatized eclogites may be mainly controlled by the H2O fugacity and mineral compositions. The OH contents of the non-metasomatized samples are interpreted to be more sensitive to their mantle equilibration temperature, pressure, and the local fugacities of H2O and O2. The calculated water content of the metasomatic medium is similar to that of carbonatitic–kimberlitic melts/fluids. Eclogites contain more water than peridotites recorded in the literature (341 ± 161 vs 122 ± 54 ppm) and represent an important water reservoir in the lithospheric mantle wherever they occur. This is an important parameter to be considered in the interpretation of mantle processes and geophysical data such as seismic wave speeds and electrical conductivity, and in geodynamic modeling.

Diamondiferous subcontinental lithospheric mantle of the northeastern Siberian Craton: Evidence from mineral inclusions in alluvial diamonds

Headless placer diamond deposits occur in Cenozoic alluvium within the Paleoproterozoic Khapchan fold belt of the Olenek province in the northeastern part of the Siberian Platform, yet the known kimberlite pipes from this region are low-grade or non-diamondiferous. To characterise the subcontinental diamondiferous lithospheric mantle of this region, the mineral inclusions in 265 diamonds were exposed and analysed. Inclusions of the eclogite suite are predominant (>72%): garnet, omphacitic clinopyroxene, coesite, K-feldspar, rutile and corundum. The garnet and Cpx inclusions are within the range of eclogitic inclusions worldwide. Diamonds of the peridotitic contain olivine, Сr-pyrope garnet, ortopyroxene and chromite. The olivines have Fo contents between 89.7 and 93.8mol. % (average – 92.4). Majoritic garnets of both peridotitic and eclogitic parageneses were identified in four diamonds. Most eclogitic diamonds display positive Eu anomalies. High-Ca garnets are LREE-depleted, show strong positive Eu (up to 4.25) and Sr anomalies and have HREE contents lower than those of the lowand intermediate-Ca garnets. The presence of majorite inclusions indicates that some of the diamonds may have sublithospheric origins.Although diamonds with peridotitic-suite garnets constitute a minor percentage of the volume, the composition of these diamonds indicates a depleted composition for the peridotitic mantle. Diamonds of harzburgitic paragenesis compose 57% of the peridotitic suite, which is close to the world average. For an assumed pressure of 5GPa, eclogitic garnet and clinopyroxene gave temperatures in the range of 1028–1290ºC. The composition of mineral inclusions of the peridotitic suite and the equilibrium temperatures of the eclogitic suite suggest that the lithospheric mantle of the Olenek province had a composition and thermal regime that was similar to those of the Archean cratonic mantle at the time the diamonds were brought to the surface.

Multi-stage origin of Roberts Victor eclogites: Progressive metasomatism and its isotopic effects

Xenolithic eclogites are high-pressure, high-temperature garnet–clinopyroxene rocks brought from the mantle to the surface by kimberlites and other primitive magmas. Their origins have been controversial for decades: do they represent metamorphosed subducted oceanic crust or magmatic rocks originally crystallized in the deep earth? The answer has important implications for the definition of major Earth processes.The extensive eclogite suite from the Roberts Victor kimberlite (South Africa) has previously been divided into two types (Type I and II); this study proposes five subgroups (IA, IB, IK; IIA, IIB) based on mineral assemblages and compositions. All of these eclogites were derived from depths of 170–200km, near the base of the contemporary subcontinental lithospheric mantle.The fresh, microstructurally equilibrated Type IIA eclogites are the protoliths of Type I, which were heavily metasomatized by carbonatitic–kimberlitic melts/fluids to form Types IAIBIK. All the Type I rocks show extensive textural disequilibrium, and have abundant fluid inclusions, secondary phases, and higher δ18O than Type II (IA: 5.0–8.0‰; IB: 4.1–6.8‰; IK:2.2–6.8‰; IIA=3.5‰; IIB:2.2–3.9‰). Type I are richer in LREE and LILE than Type IIA; they also are isotopically more homogeneous, with higher 87Sr/86Sr and less radiogenic Nd and Hf isotopes. Type I eclogites give gnt–cpx isochron ages (SmNd (103±10Ma); LuHf (132±16Ma)) similar to the kimberlite eruption age (128±15Ma). Type IIA and IIB eclogites have unradiogenic Sr and radiogenic Nd and Hf isotopic compositions, and give a range of Proterozoic two-mineral “ages” (SmNd: 738–1143Ma; LuHf: 1148–1544Ma), reflecting some preservation of their original isotopic compositions. Type IIB are broadly similar to Type IIA, but have lower MgO; their mutual relationships are not clear.Neither Type I nor Type II eclogites are similar to modern or Archean oceanic crust when all the chemical evidence is compared. In any case, the data from Type I eclogites cannot be used to support a “subduction model” of origin because their major- and trace-element chemistry, and radiogenic- and stable-isotope compositions, reflect the extensive metasomatism, rather than any primary characteristics.Type II eclogites are the key samples for the study of eclogite origins. Their bulk compositions and microstructures (exsolution of gnt and rutile) suggest that most or all the garnet in Type II samples might have formed by exsolution from high-Al clinopyroxenes, crystallized from mafic–ultramafic magmas in the deep mantle. Therefore, the xenolithic eclogites from Roberts Victor Mine probably were originally magmatic rocks crystallized at depth, rather than subducted oceanic crust; as a suite, they record a major metasomatic event in the deep lithospheric mantle, close to the time of kimberlite eruption.

High-Pressure Tourmaline Formation and Fluid Activity in Fe–Ti-rich Eclogites from the Kreuzeck Mountains, Eastern Alps, Austria

A tourmaline-bearing eclogite from the Kreuzeck Mountains, Eastern Austria, is described in this study as part of an eclogite suite situated SE of theTauern Window.These rocks formed during Eoalpine high-P metamorphism of the Austroalpine basement. They have high and variable Fe, Ti, P, Zr and Sr contents and unlike other Eoalpine eclogites their Ti^V^Zr^P^Nb^Y systematics indicates a within-plate rather than mid-ocean ridge basalt type environment for the formation of their protoliths. Major and trace element variations are consistent with progressive differentiation of a mafic magma with Fe^Ti oxide and apatite as important igneous phases and with metasomatic addition of K and Sr. The eclogites

Seeking the primary compositions of mantle xenoliths: Isotopic and elemental consequences of sequential leaching treatments on an eclogite suite

Three leaching methods have been tested on garnet and clinopyroxene separates from Roberts Victor xenolithic eclogites, in an attempt to remove secondary materials and uncover the primary compositions of the samples. A “typical” leaching treatment commonly used to clean mineral separates proved insufficient. However, the secondary material can be completely removed by treating the separates in 80°C 6N HCl for approximately 14h, followed by 6N HNO3 and then washing in MQ water ultrasonically for 20min three times. Further leaching produced no change in the isotopic composition (Sr, Nd, Hf) of the residual minerals. The leaching process produces largest relative changes in Sr-isotope compositions, and less in Nd- and Hf-isotope compositions. After this leaching process, measurement of the primary isotopic compositions of Sr, Nd and Hf in the eclogitic garnets can be achieved. The HREE patterns of the leachates are similar to those of the host minerals, but the leachates are enriched in LREE and LILE and show pronounced positive Eu anomalies. The leachable material is a mixture of the primary host minerals and secondary minerals (±fluid inclusions) deposited by a metasomatic melt/fluid. The leachates have very radiogenic Sr-isotope compositions (87Sr/86Sr=0.706–0.712 at the time of kimberlite eruption time (128Ma)) and this range suggests that the melt/fluid was heterogeneous in terms of Sr-isotope composition, and that it may be related to diamond-forming fluids. The results for these samples suggest that some Eu anomalies, commonly used to shallow crustal processes, such as plagioclase fractionation, in eclogite protoliths were actually introduced by a secondary melt/fluid percolating in the lithospheric mantle.

U–Pb geochronology of two discrete Ordovician high-pressure metamorphic events in the Seve Nappe Complex, Scandinavian Caledonides

U–Pb geochronology for eclogites in two different areas of the Seve Nappe Complex (SNC) in Sweden confirms previous indications of discrete Ordovician high-pressure events affecting various parts of the complex. In Norrbotten, just north of the Arctic circle, eclogites from the Tsakkok and Vaimok Lenses yield identical metamorphic zircon ages of 482 ± 1 Ma. Titanite in a metagabbro from the Vaimok Lens retains an older age of 607 ± 2 Ma, which may date a protolith coeval with mafic dikes in the overlying Sarek Lens; high-U zircon cores in one of the eclogites also indicate a similar age. Farther south, in Jamtland, the Tjeliken eclogite yields a significantly younger metamorphic age of 446 ± 1 Ma. Although they support the age discrepancy between the Norrbotten and Jamtland eclogites, the U–Pb ages of both eclogite suites are ca. 20 m.y. younger than previously reported Sm–Nd ages. The latter may either represent early prograde growth or be spuriously too old due to isotopic disequilibrium. The SNC has traditionally been taken to represent the outermost margin of Baltica, linking the Early Caledonian eclogite–forming events to subduction of Baltica below an offshore arc. Alternatively, the coincidence of these eclogite-forming events with orogenic phases recorded on the Laurentian margin may point to an origin from other regions of the Iapetus Ocean.

Nitrogen recycling in subducted oceanic lithosphere: The record in high- and ultrahigh-pressure metabasaltic rocks

This paper provides the first measurements of the nitrogen (N) concentrations and isotopic compositions of high- and ultrahigh-pressure mafic eclogites, aimed at characterizing the subduction input flux of N in deeply subducting altered oceanic crust (AOC). The samples that were studied are from the Raspas Complex (Ecuador), Lago di Cignana (Italy), the Zambezi Belt (Zambia) and Cabo Ortegal (Spain), together representing subduction to 50–90 km depths. The eclogites contain 2–20 ppm N with δ 15N air values ranging from −1 to +8‰. These values overlap those of altered oceanic crust, but are distinct from values for fresh MORB (for the latter, ∼1.1 ppm N and δ 15N air ∼ −4‰). Based on N data in combination with other trace element data, the eclogite suites can be subdivided into those that are indistinguishable from their likely protolith, AOC, with or without superimposed effects of devolatilization (Lago di Cignana, Cabo Ortegal), and those that have experienced metasomatic additions during subduction-zone metamorphism (Zambezi Belt, Raspas). For the former group, the lack of a detectable loss of N in the eclogites, compared to various altered MORB compositions, suggests the retention of N in deeply subducted oceanic crust. The metasomatic effects affecting the latter group can be best explained by mixing with a (meta)sedimentary component, resulting in correlated enrichments of N and other trace elements (in particular, Ba and Pb) thought to be mobilized during HP/UHP metamorphism. Serpentinized and high-pressure metamorphosed peridotites, associated with the eclogites at Raspas and Cabo Ortegal, contain 3–15 ppm N with δ 15N air values ranging from +3 to +6‰, significantly higher than the generally accepted values for the MORB mantle (δ 15N air ∼ −5‰). Based on their relatively high N contents and their homogeneous and positive δ 15N values, admixing of sedimentary N is also indicated for the serpentinized peridotites. One possible pathway for the addition of sediment-derived N into eclogites and peridotites involves mixing with fluids along the slab–mantle wedge interface. Alternatively, sedimentary N could be incorporated into peridotites during serpentinization at bending-related faults at the outer rise and, during later deserpentinization, released into fluids that then infiltrate overlying rocks. Deep retention of N in subducting oceanic crust should be considered in any attempt to balance subduction inputs with outputs in the form of arc volcanic gases. If materials such as these eclogites and serpentinized peridotites are eventually subducted to beyond sub-arc depths into the deeper mantle, containing some fraction of their forearc–subarc N inventory (documented here), they could deliver isotopically heavy N into the mantle to potentially be sampled by plume-related magmas.

Thallium isotopes as a potential tracer for the origin of cratonic eclogites

Cratonic eclogites are inferred to originate either from subducted ocean crust or mantle melts accreted onto the roots of continents. These models have different implications for the growth of continents, but it is currently difficult to determine the origin of individual eclogite suites. Upper ocean crust altered at low temperatures and marine sediments both display high thallium (Tl) concentrations and strongly fractionated Tl isotope signatures relative to the ambient upper mantle. In this study we carry out the first examination of the suitability of Tl isotopes as a tracer for an ocean-crust origin of cratonic eclogites. We have analysed the Tl isotope composition of clinopyroxene and garnet in six eclogites from the Kaalvallei and Bellsbank kimberlite pipes in South Africa. Minerals were pre-cleaned with an HCl leaching technique and the leachates display variably light Tl isotope ratios. These most likely reflect low-temperature hydrothermal alteration occurring after eruption of the kimberlite that carried the eclogites to the surface. The leached mineral pairs all display identical Tl isotope ratios, strongly suggesting that the source of the analysed Tl is identical for each mineral pair. It is, however, not possible to exclude the possibility that the analysed Tl originates from kimberlitic material that was not removed by the cleaning procedure. Only one of the six samples exhibits a Tl isotope composition different from ambient mantle. Assuming that the Tl isotope signatures indeed represent the eclogite minerals and not any form of contamination, the Tl isotope composition in this sample is consistent with containing a minor component (<3%) of ocean crust altered at low temperatures. Thallium isotopes may become one of the most sensitive indicators for the presence of low-T altered ocean crust because of the stark contrast in Tl concentration and isotopic composition between the mantle and altered ocean crust. In fact, no other chemical or isotopic tracer could have provided an indication that any of the samples studied here had a subduction origin. However, much work is still required before it becomes clear if Tl isotope measurements are a viable means to establish the origin of cratonic eclogites.

The origin of high-MgO diamond eclogites from the Jericho Kimberlite, Canada

A unique suite of diamond-rich eclogites from the Jericho kimberlite, Nunavut, Canada, has a uniform but unusual geochemical and isotopic composition that is unlike any other eclogite suite worldwide. Compared to other eclogite suites, garnets from Jericho diamond-bearing eclogites have high MgO (19.6–21.2 wt.%), Cr 2O 3 (0.28–0.60 wt.%), Sc (88–113 ppm), and Zr (32–36 ppm) contents and are characterized by consistent highly fractionated chondrite-normalized HREE patterns ([Lu/Gd] N = 4.0–6.6). Na-poor clinopyroxene has a uniform LREE-enriched pattern and radiogenic 87Sr/ 86Sr values between 0.7057 and 0.7061, higher than other eclogite groups at Jericho and the Jericho kimberlite itself. The distinct geochemical and isotopic compositions of the Jericho diamond eclogites are not directly compatible with either the subducted oceanic crust or high-pressure mantle melt models commonly invoked for the origin of mantle eclogites. We propose a multi-stage origin that involves multiple metasomatic events coupled with hybridization between basaltic eclogite and mantle peridotite. Emplacement of basaltic eclogites in the diamond stability field was followed by metasomatism by a carbon-bearing, LREE-enriched metasomatic fluid or melt as recorded by clinopyroxene and diamond inclusions in garnet. Subsequent or concurrent partial melting of eclogite produced melts that facilitated diffusional elemental exchange between residual eclogite and ambient peridotite. This process enabled the eclogites to attain their distinct high-Mg and Cr composition. Finally, carbonatite-like modal metasomatism grew phlogopite, carbonate and apatite and potentially also facilitated additional diamond growth, producing the extreme diamond enrichments found in these eclogites.