Omphacitic Pyroxene Research Articles

Eclogite and Garnet Pyroxenite Xenoliths from Kimberlites Emplaced Along the Southern Margin of the Kaapvaal Craton, Southern Africa: Mantle or Lower Crustal Fragments?

Abstract Eclogite xenoliths, together with garnet pyroxenites and some mafic garnet granulites, found in kimberlites located along the southern margin of the Kaapvaal craton in southern Africa have been analysed by electron microprobe and mass spectrometry techniques to determine their geochemical characteristics. The majority of eclogites are bimineralic with garnet and omphacitic clinopyroxene in subequal proportions, with rutile as the main accessory phase; a few contain kyanite. Based on K2O in clinopyroxene and Na2O in garnet, the eclogites can be classified as Group II eclogites, and the majority are high-Ca in character. Garnet pyroxenites comprise garnet clinopyroxenites and garnet websterites. Major and trace element concentrations and isotope ratios of reconstituted bulk rock compositions of the eclogites and garnet pyroxenites allow constraints to be placed on depth of origin and likely protolith history. Calculated Fe–Mg exchange equilibration temperatures for the eclogites range from 815 to 1000 °C, at pressures of 1·7 ± 0·4 GPa as determined by REE partitioning, indicating that they were sampled from depths of 50–55 km; i.e. within the lower crust of the Namaqua–Natal Belt. The garnet pyroxenites show slightly lower temperatures (686–835 °C) at similar pressures of equilibration. Initial 143Nd/144Nd and 87Sr/86Sr ratios (calculated to time of kimberlite emplacement) of both lithologies overlap the field for lower crustal samples from the Namaqua–Natal Belt. Further evidence for a crustal origin is found in the similar REE patterns shown by many of the associated garnet granulite xenoliths. Garnet pyroxenites are interpreted to have a similar origin as the associated eclogites but with the mafic protolith having insufficient Na (i.e. low modal plagioclase) to allow for development of omphacitic pyroxene. Metamorphism of the mafic protoliths to these eclogites and garnet pyroxenites is inferred to have occurred during crustal shortening and thickening associated with the collision of the Namaqua–Natal Belt with the Kaapvaal craton at 1–1·2 Ga.

Mineral and fluid inclusions in diamonds from the Urals placers, Russia: Evidence for solid molecular N2 and hydrocarbons in fluid inclusions

The compositions of mineral inclusions from a representative collection (more than 140 samples) of diamonds from the placer deposits in the Ural Mountains were studied to examine their compositional diversity. The overwhelming majority of rounded octahedral and dodecahedral stones typical of placers contain eclogitic (E-type) mineral inclusions (up to 80%) represented by garnets with Mg# 40–75 and Ca# 10–56, including the unique high calcic “grospydite” composition, omphacitic pyroxenes containing up to 65% of jadeite, as well as kyanite, coesite, sulfides, and rutile. Peridotitic (P-type) inclusions are represented by olivine, subcalcic Cr-pyrope, chrome diopside, enstatite and magnesiochromite that are typical for diamonds worldwide. Comparing the chemical composition of olivine, pyrope and magnesiochromite in diamonds of the Urals, north-east of the Siberian platform placers and Arkhangelsk province kimberlites show striking similarity. There are significant differences only in the variations of carbon isotopic composition of the diamonds from the placers of the Urals and north-east of the Siberian platform. One typical rounded dodecahedral diamond was found to contain abundant primary oriented submicrometer-sized (<3.0 µm) octahedral fluid inclusions identified by transmission electron microscopy, which caused the milky color of the entire diamond crystal. The electron energy-loss spectrum of a singular inclusion has a peak at ∼405 eV, indicating that nitrogen is present. The Raman spectra with peaks at 2346–2350 cm−1 confirmed that nitrogen exists in the solid state at room temperature. This means that fossilized pressure inside fluid inclusions may be over 6.0 GPa at room temperature, so the diamond may be considered sublithospheric in origin. However, identification of unique fluid inclusions in one typical placer diamond allows one to expand the pressure limit to at least more than 8.0 GPa. The volatile components of four diamonds from the Urals placers were analyzed by gas chromatography–mass spectrometry (GC–MS). They are represented (rel. %) by hydrocarbons and their derivatives (14.8–78.4), nitrogen and nitrogenated compounds (6.2–81.7), water (2.5–5.5), carbon dioxide (2.8–12.1), and sulfonated compounds (0.01–0.96). It is shown that high-molecular-weight hydrocarbons and their derivatives, including chlorinated, nitrogenated and sulfonated compounds, appear to be stable under upper mantle P-T conditions. A conclusion is drawn that Urals placer diamonds are of kimberlitic origin and are comparable in their high E-type/P-type inclusion ratios to those from the northeastern Siberian platform and in part to diamonds of the Arkhangelsk kimberlite province.

Mobility of major and trace elements in the eclogite-fluid system and element fluxes upon slab dehydration

The equilibrium between aqueous fluids and allanite-bearing eclogite has been investigated to constrain the effect of temperature (T) and fluid composition on the stability of allanite and on the mobility of major and trace elements during the dehydration of eclogites. The experiments were performed at 590–800°C and 2.4–2.6GPa, and fluids were sampled as synthetic fluid inclusions in quartz using an improved entrapment technique. The concentrations and bulk partition coefficients were determined for a range of major (Mg, Ca, Na, Fe, Al, Ti) and 16 trace elements as a function of T and fluid composition. The results reveal a significant effect of T on element partitioning between the fluids and the solid mineral assemblage. The partition coefficients increase by more than an order of magnitude for most of the major and trace elements, and several orders of magnitude for light rare-earth elements (LREE) from 590 to 800°C. The addition of various ligand species into the fluid at 700°C results in distinctive trends on element partitioning. The concentrations and corresponding partition coefficients of most of the elements are enhanced upon addition of NaF to the fluid. In contrast, NaCl displays a nearly opposite effect by suppressing the solubilities of major elements and consequently affecting the mobility of trace elements that form stable complexes with alkali-(alumino)-silicate clusters in the fluid, e.g. high field strength elements (HFSE). The results further suggest that fluids in equilibrium with orthopyroxene and/or diopsidic clinopyroxene are peralkaline (ASI ∼0.1–0.7), whereas fluids in equilibrium with omphacitic pyroxene are more peraluminous (ASI ∼1.15). Therefore, natural aqueous fluids in equilibrium with eclogite at about 90km depth will be slightly peraluminous in composition. Another important finding of this study is the relatively high capacity of aqueous fluids to mobilize LREE, which may be even higher than that of hydrous melts.

In situ oxygen-isotope, major-, and trace-element constraints on the metasomatic modification and crustal origin of a diamondiferous eclogite from Roberts Victor, Kaapvaal Craton

A subducted oceanic crustal origin for most eclogite xenoliths in kimberlites has long been a cornerstone of tectonic models for craton development. However, eclogite xenoliths often have protracted and complex histories involving multiple metasomatic events that could overprint some of the key geochemical indicators typically taken as evidence of a subducted origin (e.g., garnet δ18O-values and mineral 87Sr/86Sr compositions). To assess the potential for disturbance of oxygen isotopic compositions in mantle eclogites via diamond-forming and other possible metasomatic fluids, we have conducted a multi-technique in situ study of a diamondiferous eclogite xenolith from the Roberts Victor kimberlite, S. Africa. Using SIMS we provide the first texturally-controlled in situ measurements of δ18O-values in eclogitic garnet in close proximity to diamond.Garnet and clinopyroxene modal proportions are heterogeneous in the xenolith and garnet compositions vary from Mg#=75.8–79.2; grossular proportions=8.05–10.14mol.%, and omphacitic pyroxene has Jd13–24 and Mg#=86.6–90.0. Rare earth element patterns of minerals across the xenolith, including grains close to diamond, are typical LREE-depleted garnets and markedly LREE-enriched pyroxenes. These silicate minerals also record detectable intra- and inter-grain LREE abundance variations. Clinopyroxenes of the studied xenoliths show HFSE and Sr abundance variations that are decoupled from LREE contents and major-element variations.Mineralogical constraints and bulk-rock reconstructions indicate that the studied sample likely experienced selective incompatible element enrichment during small-volume (<<0.03wt.%) infiltration of metasomatic fluid(s) potentially linked to ancient diamond evolution. Intra-grain major-element, LREE and HFSE variations in clinopyroxene resulted from late-stage metasomatism. Oxygen isotope compositions in garnet are decoupled from all major- and trace-element variations, with garnet δ18O-values being uniform across the xenolith in a wide variety of textural settings. Garnet δ18O-values of +6.5±0.2 ‰ are higher than the mean (+5.19±0.26 ‰) of the mantle garnet range (+4.8–5.5 ‰).Modelling of the buffering effect of mantle peridotite on CO2-rich and H2O-rich metasomatic fluids at temperatures within the diamond stability field indicates that the likelihood of a metasomatic fluid with exotic oxygen isotopic composition arriving at a mantle eclogite body with its isotopic composition unmodified, after percolative flow through dominantly peridotitic mantle at great depth, is very low. As we find no evidence of metasomatically induced garnet oxygen isotope variations in the studied diamondiferous eclogite xenolith we conclude that the most likely origin for the elevated garnet δ18O-values is via inheritance from a crustal protolith altered at relatively low temperatures. These results have broader relevance and support the hypothesis of a low-pressure protolith for mantle eclogite xenoliths, demonstrating the robust nature of garnet oxygen isotope compositions – even in diamond-bearing eclogites.

Does continental crust transform during eclogite facies metamorphism?

AbstractEclogites within exhumed continental collision zones indicate regional burial to depths of at least 60 km, and often more than 100 km in the coesite‐stable, ultra‐high pressure (UHP) eclogite facies. Garnet, omphacitic pyroxene, high‐Si mica, kyanite ± coesite should grow at the expense of low‐P minerals in most felsic compositions, if equilibrium obtained at these conditions. The quartzofeldspathic rocks that comprise the bulk of eclogite facies terranes, however, contain mainly amphibolite facies, plagioclase‐bearing assemblages. To what extent these lower‐P minerals persisted metastably during (U)HP metamorphism, or whether they grew afterwards, reflects closely upon crustal parameters such as density, strength and seismic character. The Nordfjord area in western Norway offers a detailed view into a large crustal section that was subducted into the eclogite facies. The degree of transformation in typical pelite, paragneiss, granitic and granodioritic gneiss was assessed by modelling the equilibrium assemblage, comparing it with existing parageneses in these rocks and using U/Th–Pb zircon geochronology from laser ablation ICPMS to establish the history of mineral growth. U–Pb dates define a period of zircon recrystallization and new growth accompanying burial and metamorphism lasting from 430 to 400 Ma. Eclogite facies mafic rock (~2 vol.% of crust) is the most transformed composition and records the ambient peak conditions. Rare garnet‐bearing pelitic rocks (&lt;10 vol.% of crust) preserve a mostly prograde mineral evolution to near‐peak conditions; REE concentrations in zircon indicate that garnet was present after 425 Ma and feldspar broke down after 410 Ma. Felsic gneiss – by far the most abundant rock type – is dominated by quartz + biotite + feldspar, but minor zoisite/epidote, phengitic white mica, garnet and rutile point to a prograde HP overprint. Relict textures indicate that much of the microstructural framework of plagioclase, K‐feldspar, and perhaps biotite, persisted through at least 25 Ma of burial, and ultimately UHP metamorphism. The signature reaction of the eclogite facies in felsic rocks – jadeite/omphacite growth from plagioclase – cannot be deduced from the presence of pyroxene or its breakdown products. We conclude that prograde dehydration in orthogneiss leads to fluid absent conditions, impeding equilibration beyond ~high‐P amphibolite facies.

P-T estimates and timing of the sapphirine-bearing metamorphic overprint in kyanite eclogites from Central Rhodope, northern Greece

Sapphirine-bearing symplectites that replace kyanite in eclogites from the Greek Rhodope Massif have previously been attributed to a high-pressure granulite-facies metamorphic event that overprinted the eclogitic peak metamorphic assemblage. The eclogitic mineralogy consisted of garnet, omphacitic pyroxene, rutile and kyanite and is largely replaced by low-pressure minerals. Omphacite was initially replaced by symplectites of diopside and plagioclase that were subsequently replaced by symplectites of amphibole and plagioclase. Garnet reacted during decompression to form a corona of plagioclase, amphibole and magnetite. Rutile was partly transformed to ilmenite and kyanite decomposed to produce a high-variance mineral assemblage of symplectitic spinel, sapphirine, plagioclase and corundum. The presence of quartz and corundum in the kyanite eclogites is evidence for the absence of bulk equilibrium and obviates a conventional analysis of phase equilibria based on the bulk-rock composition. To circumvent this difficulty we systematically explored the pressure-temperature-composition (P-T-X) space of a thermodynamic model for the symplectites in order to establish the pressure-temperature (P-T) conditions at which the symplectites were formed after kyanite. This analysis combined with conventional thermometry indicates that the symplectites were formed at amphibolite-facies conditions. The resulting upper-pressure limit (∼0.7 GPa) of the sapphirine-producing metamorphic overprint is roughly half the former estimate for the lower pressure limit of the symplectite forming metamorphic event. Temperature was constrained (T ∼ 720°C) using garnet-amphibole mineral thermometry. The P-T conditions inferred here are consistent with thermobarometry from other lithologies in the Rhodope Massif, which show no evidence of granulite-facies metamorphism. Regional geological arguments and ion-probe (SHRIMP) zircon dating place the post-eclogite-facies metamorphic evolution in Eocene times.

Deciphering high-pressure metamorphism in collisional context using microprobe mapping methods: Application to the Stak eclogitic massif (northwest Himalaya)

Research Article| February 01, 2013 Deciphering high-pressure metamorphism in collisional context using microprobe mapping methods: Application to the Stak eclogitic massif (northwest Himalaya) Pierre Lanari; Pierre Lanari 1ISTerre, University of Grenoble 1, CNRS, 1381 rue de la Piscine, 38041 Grenoble, France Search for other works by this author on: GSW Google Scholar Nicolas Riel; Nicolas Riel 1ISTerre, University of Grenoble 1, CNRS, 1381 rue de la Piscine, 38041 Grenoble, France Search for other works by this author on: GSW Google Scholar Stéphane Guillot; Stéphane Guillot 1ISTerre, University of Grenoble 1, CNRS, 1381 rue de la Piscine, 38041 Grenoble, France Search for other works by this author on: GSW Google Scholar Olivier Vidal; Olivier Vidal 1ISTerre, University of Grenoble 1, CNRS, 1381 rue de la Piscine, 38041 Grenoble, France Search for other works by this author on: GSW Google Scholar Stéphane Schwartz; Stéphane Schwartz 1ISTerre, University of Grenoble 1, CNRS, 1381 rue de la Piscine, 38041 Grenoble, France Search for other works by this author on: GSW Google Scholar Arnaud Pêcher; Arnaud Pêcher 1ISTerre, University of Grenoble 1, CNRS, 1381 rue de la Piscine, 38041 Grenoble, France Search for other works by this author on: GSW Google Scholar Keiko H. Hattori Keiko H. Hattori 2Department of Earth Sciences, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information Pierre Lanari 1ISTerre, University of Grenoble 1, CNRS, 1381 rue de la Piscine, 38041 Grenoble, France Nicolas Riel 1ISTerre, University of Grenoble 1, CNRS, 1381 rue de la Piscine, 38041 Grenoble, France Stéphane Guillot 1ISTerre, University of Grenoble 1, CNRS, 1381 rue de la Piscine, 38041 Grenoble, France Olivier Vidal 1ISTerre, University of Grenoble 1, CNRS, 1381 rue de la Piscine, 38041 Grenoble, France Stéphane Schwartz 1ISTerre, University of Grenoble 1, CNRS, 1381 rue de la Piscine, 38041 Grenoble, France Arnaud Pêcher 1ISTerre, University of Grenoble 1, CNRS, 1381 rue de la Piscine, 38041 Grenoble, France Keiko H. Hattori 2Department of Earth Sciences, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada Publisher: Geological Society of America Received: 13 Apr 2012 Revision Received: 04 Jul 2012 Accepted: 22 Jul 2012 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2013 Geological Society of America Geology (2013) 41 (2): 111–114. https://doi.org/10.1130/G33523.1 Article history Received: 13 Apr 2012 Revision Received: 04 Jul 2012 Accepted: 22 Jul 2012 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Pierre Lanari, Nicolas Riel, Stéphane Guillot, Olivier Vidal, Stéphane Schwartz, Arnaud Pêcher, Keiko H. Hattori; Deciphering high-pressure metamorphism in collisional context using microprobe mapping methods: Application to the Stak eclogitic massif (northwest Himalaya). Geology 2013;; 41 (2): 111–114. doi: https://doi.org/10.1130/G33523.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Stak massif, northern Pakistan, is a newly recognized occurrence of eclogite formed by the subduction of the northern margin of the Indian continent in the northwest Himalaya. Although this unit was extensively retrogressed during the Himalayan collision, records of the high-pressure (HP) event as well as a continuous pressure-temperature (P-T) path were assessed from a single thin section using a new multiequilibrium method. This method uses microprobe X-ray compositional maps of garnet and omphacitic pyroxene followed by calculations of ∼200,000 P-T estimates using appropriate thermobarometers. The Stak eclogite underwent prograde metamorphism, increasing from 650 °C and 2.4 GPa to the peak conditions of 750 °C and 2.5 GPa, then retrogressed to 700–650 °C and 1.6–0.9 GPa under amphibolite-facies conditions. The estimated peak metamorphic conditions and P-T path are similar to those of the Kaghan and Tso Morari high- to ultrahigh-pressure (HP-UHP) massifs. We propose that these three massifs define a large HP to UHP province in the northwest Himalaya, comparable to the Dabie-Sulu province in China and the Western Gneiss Region in Norway. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

P-TEstimates and Timing of the Sapphirine-Bearing Metamorphic Overprint in Kyanite Eclogites from Central Rhodope, Northern Greece

Sapphirine bearing symplectites that replace kyanite in eclogites from the Greek Rhodope Massif have previously been attributed to a high pressure granulite facies metamorphic event that overprinted the eclogitic peak metamorphic assemblage. The eclogitic mineralogy consisted of garnet, omphacitic pyroxene, rutile and kyanite and is largely replaced by low pressure minerals. Omphacite was initially replaced by sym plectites of diopside and plagioclase that were subsequently replaced by symplectites of amphibole and pla gioclase. Garnet reacted during decompression to form a corona of plagioclase, amphibole and magnetite. Rutile was partly transformed to ilmenite and kyanite decomposed to produce a high variance mineral assemblage of symplectitic spinel, sapphirine, plagioclase and corundum. The presence of quartz and corun dum in the kyanite eclogites is evidence for the absence of bulk equilibrium and obviates a conventional anal ysis of phase equilibria based on the bulk rock composition. To circumvent this difficulty we systematically explored the pressure temperature composition (P–T–X) space of a thermodynamic model for the symplec tites in order to establish the pressure temperature (P T) conditions at which the symplectites were formed after kyanite. This analysis combined with conventional thermometry indicates that the symplectites were formed at amphibolite facies conditions. The resulting upper pressure limit (~0.7 GPa) of the sapphirine producing metamorphic overprint is roughly half the former estimate for the lower pressure limit of the sym plectite forming metamorphic event. Temperature was constrained (T ~ 720°C) using garnet amphibole mineral thermometry. The P T conditions inferred here are consistent with thermobarometry from other lithologies in the Rhodope Massif, which show no evidence of granulite facies metamorphism. Regional geo logical arguments and ion probe (SHRIMP) zircon dating place the post eclogite facies metamorphic evo lution in Eocene times. DOI: 10.1134/S0869591113050032

A unique omphacite, amphibole, and graphite-bearing clast in Queen Alexandra Range (QUE) 99177: A metamorphosed xenolith in a pristine CR3 chondrite

A metamorphosed, unshocked, chondritic clast, containing approximately equal volumes of matrix and chondrules, has been identified in the primitive CR3 chondrite QUE 99177. The most striking features of this clast are the presence of ferroan omphacitic pyroxenes, amphiboles (pargasite and pargasitic hornblende) and coarse-grained (up to 70μm) graphite laths. The clast also contains relatively equilibrated FeO-rich olivine, low-Ca pyroxene, and sulfides (pentlandite and troilite). The clast’s matrix contains coarse, interlocking crystals. The chondrules are extensively recrystallized, contain no mesostasis or metal, and present poorly-defined boundaries. Based on its compositional and crystallographic features, omphacite appears to have formed at high pressures (>6GPa). Amphibole formation probably postdated graphite formation and occurred by reaction of olivine, clinopyroxene, and omphacite in the presence of an oxidizing, Na-rich fluid at high temperatures (>700°C). The absence of metal suggests that this fluid was also rich in S.

Experimental and crystal chemical study of the basalt–eclogite transition in Mars and implications for martian magmatism

This paper represents the start of a campaign to identify possible source regions for pyroxene-phyric “enriched” and “depleted” martian basalts using high pressure studies on martian meteorite compositions that represent liquids or near liquids. Our first experiments focus upon mineralogical and crystal chemical aspects of the basalt–eclogite transformation on Mars. It is anticipated that like Earth, eclogites are not the dominant upper mantle assemblage. However, like Earth they may be important hosts for P, Cl, F, OH, Ti, REE, Sr, Y, high-field-strength elements, Hf, and Zr in the upper mantle. This initial experimental study evaluates the major and trace element crystal chemistry of potential martian eclogite assemblages using a martian melt composition (QUE 94201). This composition is appropriate for this study because it is evolved, so it is pyroxene-phyric, contains abundant phosphate, which is important for storage of REE, and is very well studied. In the high pressure experiments, garnet and omphacitic pyroxene are the dominant phases. The garnet has a compositional range from Al56.9Gr+An24.6Py18.5 to Al41.5Gr+An28.1Py30.4 to Al40.3Gr+An22.6Py37.1 (where Al = almandine, Gr+An = grossular+andradite, and Py = Pyrope) and the pyroxene has jadeite and acmite components of up to 26.0 and 9.9%, respectively. The garnet is enriched in REE over omphacite; this is especially true for the HREE which likely remain sequestered in residual garnet during melting. This observation suggests that garnet may play a key role in producing the relatively flat REE patterns of enriched martian basalts. The prime substitution couple that incorporates REE into garnet is: REE3+X Site+Mg2+Y Site↔R2+X Site+Al3+Y Site, where R2+=Ca, Mn2+, Fe2+, and Mg. This is a very effective couple that accounts for both charge balance and ionic size restrictions in individual sites. The prime substitution couple that incorporates REE into omphacite is: REE3+M2 Site+Na1+M2 Site for 2 Ca2+M2 Site. This is a less effective couple than that for garnet because Na is pulled in two directions in omphacite; it must balance the Al in the M1 site in the jadeite component and it also has to provide a charge deficiency for each REE3+ cation in the M2 site. The potential for the presence of eclogite in the martian mantle, has implications for bulk trace element partitioning differences between eclogite and peridotite assemblages that are closely linked to garnet and pyroxene compositions. Further, the occurrence of eclogites may have implications for the capability of the martian mantle to produce magmas, and the major element composition of those magmas.

Jadeitite from the Monviso meta-ophiolite, western Alps: occurrence and genesis

A block is described, which is exposed in the antigorite serpentinite of Vallone Bule, belonging to the Basal Serpentinite Unit of the Monviso massif (Piemonte Zone of calcschists with meta-ophiolites). The block consists of a quartz-jadeite rock core and a jadeitite rim, very similar to the rocks used by prehistoric men to make stone axeheads. In spite of their different bulk-rock compositions, both core and rim show the same trace and rare earth elements patterns, suggesting the same protolith. The quartz-jadeite rock exhibits a major, trace and rare earth elements composition consistent with that of oceanic plagiogranite, most likely a dyke cutting across upper mantle peridotites, later hydrated to serpentinites. Conversely, the jadeitite, which consists mainly of zoned jadeite crystals progressively enriched in the diopside component from core to rim, is significantly depleted in Si but enriched in Mg and Ca with respect to the quartz-jadeite rock. The trace and rare earth elements similarities and the ubiquitous presence of small zircons suggest that the jadeitite and the quartz-jadeite rock both derive from a plagiogranite; however, jadeitite would have undergone a metasomatic process involving a significant desilication and Mg- and Ca-enrichment, connected to the host peridotite serpentinization. The process, responsible for the transformation of the plagiogranite into a jadeitite, should have occurred during prograde Alpine high-pressure (eclogite-facies) metamorphism, since the first Na-pyroxene formed is jadeite, corroded and partly replaced during the metasomatic process by a progressively more omphacitic pyroxene. Because similar rocks – mostly jadeitites, but even their plagiogranite protoliths – are reported from other localities from the Western and Maritime Alps, it is likely that the raw materials of most jadeitites used to make stone axeheads, which are spread all over the Western Europe, have a similar origin and derive from the western Alps as long suggested.

Evidências de metamorfismo de alta pressão na faixa de dobramentos Pajeú – Paraíba, Província Borborema, nordeste do Brasil: petrografia e química mineral de rochas metamáficas

Metamafic rocks occur as concordant lenses in a sequence of migmatites - orthogneisses of probably paleoproterozoic age. Petrographic and mineral chemistry investigations on metamafic rocks from Itatuba - Itabaiana, the Pajeú-Paraíba Belt of the Borborema Province, are reported. The presence of Mg- Ca-rich garnet, amphibole with Fe-edenite composition as coronas on the garnet grains and tschermakitic amphiboles in the symplectitic intergrowths with plagioclase around clinopyroxene suggest that the studied rocks were metamorphosed under high pressure conditions and that they were reequilibrated during a retrograde metamorphic path successively under temperature and pressure conditions of granulitic, amphibolitic and greenschist facies. In spite of the lack of omphacitic pyroxene in the studied rocks, the texture of the rocks and field relationships suggest the occurrence of a metamorphic event under eclogitic conditions. The studied rocks may mark a continental Paleoproterozoic suture zone in the Transversal Zone Domain of the Borborema Province.

Evidências de metamorfismo de alta pressão na faixa de dobramentos Pajeú – Paraíba, Província Borborema, nordeste do Brasil: petrografia e química mineral de rochas metamáficas

Metamafic rocks occur as concordant lenses in a sequence of migmatites – orthogneisses of probably paleoproterozoic age. Petrographic and mineral chemistry investigations on metamafic rocks from Itatuba – Itabaiana, the Pajeu-Paraiba Belt of the Borborema Province, are reported. The presence of Mg- Ca-rich garnet, amphibole with Fe-edenite composition as coronas on the garnet grains and tschermakitic amphiboles in the symplectitic intergrowths with plagioclase around clinopyroxene suggest that the studied rocks were metamorphosed under high pressure conditions and that they were reequilibrated during a retrograde metamorphic path successively under temperature and pressure conditions of granulitic, amphibolitic and greenschist facies. In spite of the lack of omphacitic pyroxene in the studied rocks, the texture of the rocks and field relationships suggest the occurrence of a metamorphic event under eclogitic conditions. The studied rocks may mark a continental Paleoproterozoic suture zone in the Transversal Zone Domain of the Borborema Province.

An order-disorder model for omphacitic pyroxenes in the system jadeite-diopside-hedenbergite-acmite, with applications to eclogitic rocks

A new thermodynamic model for sodic pyroxenes involving jadeite-diopside-hedenbergite-acmite is presented. This model allows for ordering of Mg, Al, Fe2+, and Fe3+ on the M1 sites, with coupled Na and Ca ordering on the M2 sites. It is calibrated on the basis of experiments in three chemical subsystems together with available information on ordering in different pyroxenes and on the limited calorimetric data. Central to the determination of the parameters of the model is the use of relationships among the end-member Gibbs energies and the interaction energies in the various possible non-independent sets of end-members. An important aspect of this model, which uses the symmetric formalism, is that Fe-Mg (FM) and Al-Fe3+ (AF’) mixing is not assumed to be ideal. The model accounts successfully for the experiments at both 600 °C and at higher temperatures involving ordered and disordered pyroxenes coexisting with albite and quartz in NCMAS, NCFAS, NF’AS systems as well as the available calorimetry. It is also able to predict the positions and slopes of tielines in coexisting jadeite + omphacite and omphacite + augite found in nature at lower temperatures. Although the model requires a large number of energy parameters, some of these are not critical to the behavior of the model, while for others the constraints from experiment, calorimetry, ordering state and solvi lead to very limited allowable combinations. The model places some restrictive constraints on the shape of the phase relations in the jadeite–augite–acmite system. Petrological applications of the model are illustrated via a phase diagram example for a MORB bulk composition eclogite.

Diamonds from the Buffalo Head Hills, Alberta: Formation in a non-conventional setting

Kimberlite pipes K11, K91 and K252 in the Buffalo Head Hills, northern Alberta show an unusually large abundance (20%) of Type II (no detectable nitrogen) diamonds. Type I diamonds range in nitrogen content from 6 ppm to 3300 ppm and in aggregation states from low (IaA) to complete (IaB). The Type IaB diamonds extend to the lowest nitrogen concentrations yet observed at such high aggregation states, implying that mantle residence occurred at temperatures well above normal lithospheric conditions. Syngenetic mineral inclusions indicate lherzolitic, harzburgitic, wehrlitic and eclogitic sources. Pyropic garnet and forsteritic olivine characterize the peridotitic paragenesis from these pipes. One lherzolitic garnet inclusion has a moderately majoritic composition indicating a formation depth of ∼ 400 km. A wehrlitic paragenesis is documented by a Ca-rich, high-chromium garnet and very CaO-rich (0.11–0.14 wt.%) olivine. Omphacitic pyroxene and almandine-rich garnet are characteristic of the eclogitic paragenesis. A bimodal δ 13C distribution with peaks at − 5‰ and − 17‰ is observed for diamonds from all three kimberlite pipes. A large proportion (∼ 40%) of isotopically light diamonds ( δ 13C < −10‰) indicates a predominantly eclogitic paragenesis. The Buffalo Head Terrane is of Lower Proterozoic metamorphic age (2.3–2.0 Ga) and hence an unconventional setting for diamond exploration. Buffalo Hills diamonds formed during multiple events in an atypical mantle setting. The presence of majorite and abundance of Type II and Type IaB diamonds suggests formation under sublithospheric conditions, possibly in a subducting slab and resulting megalith. Type IaA to IaAB diamonds indicate formation and storage under lower temperature in normal lithospheric conditions.

Combined in situ X-ray diffraction and Raman spectroscopy on majoritic garnet inclusions in diamonds

Mineral inclusions in Sao Luiz diamonds have been characterized using angle dispersive X-ray diffraction and Raman spectroscopy. We identified two different garnet phases coexisting with an omphacitic pyroxene. They represent disintegration products of a former homogeneous majorite-rich garnet phase. The two garnets have significantly different cell parameters but are tightly intergrown with their unit cells parallel to each other. The garnets are oriented relative to the diamond host with [100] garnet parallel to [110] diamond. Combining the measured cell parameters of the garnet inclusions with chemical analyses of similar inclusions from the same source allows the extraction of a residual pressure between 1 and 3 GPa, depending on the exact chemical composition assumed. Depth-resolved Raman spectra at the diamond–garnet interface indicate a residual pressure of about 1 GPa. Such a low residual pressure is unexpected at first glance for a garnet assemblage from the transition zone. The inclusion pressure is lowered due to pyroxene crystals, which surround the garnet inclusions and act as cushions reducing the residual inclusion pressure.

Petrology of the diamond‐grade eclogite in the Kokchetav Massif, northern Kazakhstan

Abstract The Kokchetav Massif of northern Kazakhstan is unique because of the abundant occurrence of microdiamond inclusions in garnet, zircon and clinopyroxene of metasediments. In order to determine precise pressure–temperature (P–T) conditions, we have systematically investigated mineral inclusions and the compositions of major silicates in Ti–clinohumite–garnet peridotite and diamond‐grade eclogite from Kumdy–Kol. It was found that garnet peridotites from Kumdy–Kol contain assemblages of garnet, olivine, Ti–clinohumite and ilmenite. The garnet contains inclusions that are indicative of both ultrahigh pressure (UHP) and retrograde conditions. Inclusions of hydrous phases such as chlorite, amphibole and zoisite were formed at the post‐UHP stage. The study also found that eclogite from Kumdy–Kol contains albite–augite symplectites after omphacitic pyroxene. The core of pyroxene (sodic augite) contains high K2O (up to 1wt%; average 0.24wt%). Phengite is included in the core. Applying the K2O‐in‐augite geobarometry, which is based on recent experiments, and the garnet–clinopyroxene (Grt–Cpx) geothermometer for peak metamorphism, the eclogites yield P–T estimates of &gt; 6 GPa and &gt; 1000 °C, and the diamond‐grade eclogites yield lower temperature estimates at 900–1000 °C and 5 GPa.

Ultra‐high‐pressure (UHP) marble and eclogite in the Su‐Lu UHP terrane, eastern China

A large mass of dolomitic marble including many eclogite blocks occurs in orthogneisses of the Rongcheng area of the Su‐Lu province, eastern China. The marble consists mainly of dolomite, calcite (formerly aragonite), graphite, forsterite, diopside, talc, tremolite and phlogopite. Aggregates of talc and calcite occur at the boundary between dolomite and diopside. Tremolite is a reaction product between talc and calcite. Eclogite blocks are rimmed by dark green amphibolite. The primary mineral assemblage in the core of eclogite is Na‐bearing garnet (up to 0.2 wt% Na2O), omphacitic pyroxene, clintonite and rutile. Secondary minerals are pargasitic/edenitic amphibole, plagioclase, sodic diopside, chlorite, zoisite and titanite. The peak metamorphic conditions, based on stability of the dolomite+forsterite+aragonite (now calcite)+graphite assemblage, under conditions where tremolite is unstable, are estimated at T =610–660 °C and P=2.5–3.5 GPa (for XCO=0.001). A reaction between dolomite and diopside to form talc under tremolite‐unstable conditions indicates a temperature decrease under ultra‐high‐pressure conditions (P &gt;2.4 GPa, XCO&lt;0.0013). The formation of secondary tremolite is consistent with a nearly adiabatic pressure decrease post‐dating the ultra‐high‐pressure metamorphism. The temperature decrease under ultra‐high‐pressure conditions preceding decompression may reflect the underplating of a cold slab, and the rapid decompression probably corresponds to the upwelling stage promoted by the delamination of a downwelling lithospheric root. The P–T conditions of the amphibolitization stage are estimated at &lt;0.9 GPa and &lt;460 °C, and are similar to conditions recorded by the surrounding orthogneisses.

Calcium environment in omphacitic pyroxenes: XANES experimental data versus one-electron multiple scattering calculations

Calcium environment in omphacitic pyroxenes: XANES experimental data versus one-electron multiple scattering calculations

Silicate-carbonate liquid immiscibility in upper-mantle eclogites: Implications for natrosilicic and carbonatitic conjugate melts

Two eclogites from the Jagersfontein kimberlite, South Africa, contain silicate-carbonate immiscible liquids, a very rare feature in upper-mantle xenoliths. The two eclogites have unusual chemistries relative to other eclogites from Jagersfontein; JEC 90-2 contains 5.7 wt% Na 2O and JEC 91-11 contains 1.2 wt% BaO, and both eclogites belong to a group of four Jagersfontein eclogites which all contain natrocarbonatitic mineral assemblages (NMA) defined by carbonate, hydrous Na-Al silicate (natrolite), barite and pectolite. Immiscible liquid-bearing eclogites display veined metasomatism, whereas the two other eclogites (JEC 91-15 and 91-17) are pervasively metasomatized. Garnets in these four eclogites average Py 50Alm 38Gr 12; however, “mantled” garnets in JEC 90-2 have rims ranging from Py 61 to Py 65. Pyroxenes average Di 58Jd 42 in composition. Amphiboles in these eclogites range in composition from pargasite to kaersutite. Pectolite occurs in patches of calcite or as minute needle inclusions in Na-Al silicate ocelli. Calcite and natrolite occur in patches or as ocelli in Na-rich (~13 wt%) or K-rich (~6 wt%) devitrified glasses. Textural evidence for liquid immiscibility includes sharp ocelli boundaries, kidney-and teardrop-shaped ocelli, and ocelli trapped in the process of pinching off. Silicate and carbonate immiscible liquids in the two eclogites demonstrate good correlation with experimental and other natural silicate-carbonate conjugate liquids, and calculated bulk liquid compositions for these eclogite-hosted immiscible liquids fall within the two-liquid field as determined by Kjarsgaard and Hamilton (1989). The silicate liquid in JEC 91-11 is similar in composition to a melilitite or melanephelinite, whereas the silicate liquid from JEC 90-2 is comparable to a phonolite. Strongly zoned garnets imply a multi-stage history for these eclogites: equilibration in the upper mantle, followed by a higher temperature melting episode and reprecipitation in the uppermost mantle or lower crust. A model is presented for the generation of these immiscible liquids that involves partial melting of omphacitic pyroxene (±phlogopite), the release of Na and K; and injection of a CO 2-H 2O rich “fluid” or melt. The unmixed carbonate liquid is immiscible in the alkaline silicate fraction, and two melts are produced. Occurrence on a large scale could generate conjugate silicate (melilititic to phonolitic) and carbonate magmas (sövites) typical of natural carbonatite complexes.