Spinel Garnet Peridotite Research Articles

Two-Stage Model of Devonian Basic Magmatism in the Vilyui Paleorift (Siberian Platform)

Abstract —We report study results of basic intrusive bodies in the middle Paleozoic Vilyui paleorift (eastern Siberian Platform). Geochemical data for basic sills penetrated by boreholes in the rift’s dike swarms are presented, as well as our data on the time of formation of sills and dikes. We also studied mineral-hosted melt inclusions from a dolerite dike of the Vilyui–Markha dike swarm on the northwestern flank of the Vilyui paleorift. Data on the compositions of homogenous glasses of mineral-hosted melt inclusions yielded the P–T parameters of mantle sources of basic melts responsible for the formation of the Vilyui paleorift gabbro-dolerites. Two depth levels of basic melt generation have been established: 95–65 km at 1480–1400 °C and 55–45 km at 1360–1320 °C. Crystallization of the melts occurred at a shallow depth of 12–4 km with a decrease in temperature from 1185 to 1125 ºС. This occurrence of basic melt sources at two different depths, as well as the heterogeneity of the chemical composition of melt inclusions and their host minerals explains the presence of two pulses of Devonian basic magmatism. Our new numerical thermomechanical model of magma rise during melting of the lithospheric mantle above a mantle plume supports the existence of two chambers at the spinel–garnet peridotite boundary and under the base of the crust, as well as the two-stage nature of Devonian magmatism.

The Pressure–Temperature Path and the Origin of Phlogopite in Spinel–Garnet Peridotites from the Blanský Les Massif of the Moldanubian Zone, Czech Republic

A new lithotype of peridotite, phlogopite- and apatite-bearing spinel–garnet peridotite, associated with leucocratic granulite, has been recognized at the Plešovice quarry in the Gföhl Unit within the Moldanubian Zone of the Bohemian Massif, Czech Republic. There are three equilibrium stages in the Plešovice peridotite. The existence of Stage I, the precursor spinel ± garnet peridotite stage, is supported by the presence of an aluminous (Al2O3 ∼ 3·0 wt %) orthopyroxene megacryst in the matrix. The minimum temperature of Stage I was estimated to be 1020 ± 15°C. Stage II is defined by the cores of relatively large (<3 mm long) grains of olivine, low-Al orthopyroxene (Al2O3 ∼ 1·3–1·7 wt %), clinopyroxene, and chromian spinel [Cr/(Cr + Al) = 0·50–0·57], along with relatively small (<1 mm long) Ba-rich phlogopite (BaO = 1·0–4·0 wt %), Sr-rich apatite (SrO ∼1·7 wt %) and rare potassic (K2O ∼0·9–1·2 wt %) amphibole. Garnet generally occurs as large spheroidal grains (up to 20 mm in diameter). It contains inclusions of olivine, orthopyroxene, chromian spinel, and phlogopite, all of which have similar compositions to their matrix counterparts. Therefore, garnet appears to be in equilibrium with the matrix phases at Stage II. Application of appropriate geothermobarometers to the assemblage at Stage II yielded temperatures of 850–1030°C and pressures of 2·3–3·5 GPa. Stage III is defined by aluminous orthopyroxene (Al2O3 ∼ 2·1–4·0 wt %), aluminous clinopyroxene and aluminous spinel along with pargasitic amphibole and Ba-rich phlogopite in kelyphite; temperature conditions at this stage were estimated to be 730–770 (± 27)°C at 0·8–1·5 GPa. Multiphase solid inclusions, mainly composed of phlogopite, dolomite, apatite and calcite with minor amounts of chlorite and magnesiohornblende, are present only within large grains of chromian spinel, which are surrounded by kelyphites. The idiomorphic outline of the multiphase solid inclusions suggests that frozen remnants of carbonatite melts or supercritical fluids were trapped in the spinel. The mineral assemblage in the multiphase solid inclusions suggests relatively low-P and low-T conditions (T < 750°C; P < 1·6 GPa) for its crystallization. Furthermore, the timing of the crystallization of the multiphase solid inclusions appears to predate Stage II, as most multiphase solid inclusions are completely surrounded by the host chromian spinel. These data suggest that the Plešovice peridotite experienced cooling after Stage I and was then transformed to spinel–garnet peridotite by subsequent subduction processes.

Cenozoic behind-arc volcanism in the Bolivian Andes, South America: implications for mantle melt generation and lithospheric structure

We develop a model of arc and behind-arc volcanism, as well as constraining the evolution of the lithospheric mantle structure beneath the Bolivian Andes during the last 25 Ma, using new geochronological and geochemical data on behind-arc mafic volcanic rocks, together with a regional isotopic study of geothermal helium emissions, and a comparison with seismic tomographic images of the same region. Helium isotopes measured in natural gas emissions in geothermal and mineral-water springs suggest recent mantle melting in a wide zone extending at c . 20°S from a few tens of kilometres west of the volcanic arc to over 300 km behind the arc, where the subducted slab is at depths increasing from c . 100 to c . 250 km. During the Neogene, there has been behind-arc mafic–felsic magmatism right across the high Andes between 17° and 22°S, in the Altiplano and western margin of the Eastern Cordillera. Modelling of Plio-Pleistocene basalts and basaltic andesites (&lt;5.2 Ma), using REE, suggests that the main melt source region is in spinel and spinel–garnet peridotite, at depths &lt;100 km. In detail, the best-fit REE inversions suggest two spatially distinct melt zones: (1) a c . 15% depleted mid-ocean ridge basalt (MORB) source at depths between c . 75 km and c . 110 km for mafic volcanic rocks that crop out &lt;75 km behind the arc; (2) a slightly enriched MORB source at depths between c . 65 km and c . 90 km for those that crop out &gt;100 km behind the arc. REE inversions for Oligo-Miocene shoshonites and basalts ( c . 25–21 Ma) suggest that they originated at significantly shallower depths ( c . 45 km to c . 100 km) from a slightly enriched MORB source. Eruption of shoshonitic lavas also occurred in the Altiplano between 13 and 11 Ma, and widespread behind-arc intermediate–felsic volcanism has been more or less continuous since the early Miocene. All this suggests that since c . 25 Ma there has been a thin (&lt;100 km) lithosphere beneath the high part of the Bolivian Andes. Long-lived mantle melting is best explained by wet adiabatic decompression in the upwelling part of the corner flow in the mantle wedge, controlled by the lithospheric thickness variations, with depleted melt advected into the melt zone beneath the arc. Widespread arc and behind-arc magmatism may have been triggered at c . 25 Ma by the opening up of a mantle wedge and inflow of hot asthenosphere, as a consequence of the steepening of the subducted slab and detachment of hydrated and weakened mantle lithosphere and possibly mafic lower crust.

Evidence from Antarctic mantle peridotite xenoliths for changes in mineralogy, geochemistry and geothermal gradients beneath a developing rift

Garnet and spinel peridotite xenoliths associated with the Phanerozoic Lambert–Amery Rift in eastern Antarctica contain evidence for several stages in the development of the mantle beneath the rift. Despite the fact that equilibria were only partly attained, a combination of petrography, whole-rock geochemistry, mineral chemistry and thermobarometry can be used to decipher four stages prior to entrainment of the xenoliths in the host magma during the initial stages of the breakup of Antarctica, India and Madagascar. The first chronological stage is represented by harzburgitic protoliths represented by rare occurrences of low-Ca olivines and orthopyroxenes in spinel lherzolites: these yield the lowest temperatures of 830–850 °C, and are also characterized by distinct trace element contents; lower Ti, Cr, V and Zn in olivine and orthopyroxene, and additionally lower Cu, Ni, Ga and Li in orthopyroxene. Some garnets are subcalcic, indicating that the spinel–garnet lherzolites also formed from harzburgitic protoliths. The second stage is the formation of garnet due to a pressure increase probably related to collision at 1.1 Ga. The third stage is marked by the growth of clinopyroxene, demonstrably in cpx-poor spinel lherzolites but probably in all xenolith groups: equilibrium of clinopyroxene with olivine and orthopyroxene was not attained in all samples, so that the non-judicious use of thermobarometers can produce bewildering results. The fourth stage is an enrichment episode that affected all spinel–garnet peridotites and about half of the spinel peridotites. During this stage, reaction rims were produced on the clinopyroxenes that formed during stage 3, the modal content of olivine and Mg/(Mg + Fe) in the rocks was reduced, CaO, Al 2O 3 and trace elements were enriched, and garnets were almost completely transformed to kelyphites. A later stage is documented by interstitial glasses and films around spinels related to infiltration of melt from the host magma. These post-date, and are more enriched in alkalies than, partially melted rims on clinopyroxenes, demonstrating that all the three earlier episodes were pre-entrainment events. Pressures indicated by the spinel + garnet lherzolites are restricted to 20–24 kbar at 1040–1180 °C. Early harzburgitic assemblages are interpreted to represent an earlier, cooler geotherm, whereas the kelyphite assemblages indicate temperatures 180–200 °C hotter than the main xenolith geotherm. This event also caused recrystallization of the clinopyroxene rims and is attributed to heating during rifting, but not due to the host magma itself. The preservation of evidence for three progressively hotter geotherms can be related to the upward movement of isotherms during the development of the sub-rift mantle.

A redox profile of the Slave mantle and oxygen fugacity control in the cratonic mantle

The authors report a redox profile based on Mossbauer data of spinel and garnet to a depth of 210 km from mantle xenoliths of the northern (N) and southeastern (SE) Slave craton (northern Canada). The profile transects three depth facies of peridotites that form segments of different bulk composition, represented by spinel peridotite, spinel–garnet peridotite, low-temperature garnet peridotite, high-temperature garnet peridotite, and pyroxenite. The shallow, more depleted N Slave spinel peridotite records lower oxygen fugacities compared to the deeper, less depleted N Slave spinel–garnet peridotite, consistent with their different spinel Fe3+ concentrations. Garnet peridotites show a general reduction in Δlog fO2 (FMQ)s with depth, where values for garnet peridotites are lower than those for spinel–garnet peridotites. There is a strong correlation between depletion and oxygen fugacity in the spinel peridotite facies, but little correlation in the garnet peridotite facies. The strong decrease in Δlog fO2 (FMQ) with depth that arises from the smaller partial molar volume of Fe3+ in garnet, and the observation of distinct slopes of Δlog fO2 (FMQ) with depth for spinel peridotite compared to spinel–garnet peridotite strongly suggest that oxygen fugacity in the cratonic peridotitic mantle is intrinsically controlled by iron equilibria involving garnet and spinel.

Plume-Lithosphere Interactions in the Generation of the Basalts of the Kenya Rift, East Africa

Major and trace element and Sr–Nd–Pb isotopic data for mafic volcanic rocks are used to assess the number of mantle plumes contributing to the Tertiary–Holocene magmatism of the Kenya Rift Valley, current estimates of which vary from none to three. Rocks ranging in composition from nephelinite to hypersthene-normative basalt have been sampled from three lithospheric zones: the Tanzanian craton, the craton margin reworked during the late Proterozoic, and the Mozambique mobile belt. The magmas are interpreted as the products of variable degrees of partial melting within the spinel–garnet peridotite transition zone. Trace element and isotopic compositions from all three zones are broadly similar to those of oceanic island basalts, but there is considerable compositional variation, which is related to a strong overprint from the lithosphere on plume-derived melts. Sr and Nd isotopic ratios provide the only clear distinction between magmatic rocks from the three lithospheric domains. Within each setting, mafic magmatism has tended to become less silica undersaturated with time, and at any one locality magmatism has migrated towards the centre of the rift. Magmas may have formed as a result of the infiltration of plume-derived melts into the base of the lithosphere. The extent of interaction of inferred plume melts with the lithosphere has not varied systematically in time or space. The plume component appears to be similar to the source of oceanic island basalts.

Garnet from Cr- and Ca-saturated mantle: implications for diamond exploration

An exotic trend of positively correlated Cr and Ca occurs in garnet from xenoliths in the Jericho kimberlite, northern Canada. The trend shows significantly less enrichment in Cr with increasing Ca than the common lherzolitic trend characteristic of kimberlite concentrate. The trend is found only in spinel–garnet peridotite xenoliths and is attributed to equilibration of garnet with spinel. The compositional variation of garnet within the trend is calculated to require buffering by equilibria involving Cr-spinel and clinopyroxene. Representation of garnet chemistry in terms of additive and exchange components shows that the chromite–clinopyroxene–garnet equilibrium (CCGE) is controlled by the exchange Mg 2Al 3↔Ca 2Cr 3. The equilibration of garnet with chromite and clinopyroxene reduces the extent of cation exchange in octahedral sites and eliminates Fe from active substitution in garnet relative to the common lherzolitic trend. A trend similar to the CCGE trend was described for spinel–garnet xenoliths from South Africa and Colorado Plateau and was produced experimentally for natural lherzolite compositions at high P–T conditions. The fact that the trend is extremely rare is explained by its low visibility and/or the lack of Ca-saturation in most spinel–garnet cratonic peridotite. The CCGE trend is more pronounced in garnet from peridotite xenoliths than in garnet from kimberlite concentrate where it is less visible due to sampling biases and dilution. The paucity of Ca-saturation results from harzburgitic composition of peridotitic mantle at depths of 120–180 km (spinel–garnet facies) under most Archean cratons. The Jericho and Colorado Plateau mantle segments are unique in having lherzolitic composition of the spinel–garnet mantle, where there is sufficient clinopyroxene to ensure the development of the CCGE trend. A CCGE trend in garnet composition reflects the presence of relatively fertile rocks at relatively shallow depths. In the Ca-saturated mantle, the CCGE trend can be used to constrain the depth interval of the mantle sampled by kimberlite. Coupled with the data on kimberlite spinel chemistry and on the local geotherm, the presence of the trend can reveal sampling of non-diamondiferous or diamondiferous mantle horizons.

Prograde eclogite in the Gföhl Nappe, Czech Republic: new evidence on Variscan high‐pressure metamorphism

A layer of relict, high‐temperature, prograde eclogite has been discovered within felsic granulite of the Gföhl Nappe, which is the uppermost tectonic unit in the Moldanubian Zone of the Bohemian Massif, the easternmost of the European Variscan massifs. Pressure‐temperature conditions for eclogite (≥890 °C, 18.0 kbar) and felsic granulite (c. 1000 °C, 16 kbar) place early metamorphism of the polymetamorphic Gföhl crustal rocks within the eclogite facies, and preservation of prograde compositional zoning in small garnet grains in high‐temperature eclogite requires very rapid heating, as well as cooling. Mantle‐derived garnet and spinel–garnet peridotites are associated with the high temperature‐high pressure crustal rocks in the Gföhl Nappe, and this distinctive lithological suite appears to be unique among European Phanerozoic orogenic belts, implying that tectonic processes during the late stages in evolution of the Variscan belt were different from those in the Caledonian and Alpine belts. The unusually high temperatures and pressures in Gföhl crustal rocks, mineralogical evidence for rapid heating and cooling, juxtaposition of lithospheric and asthenospheric mantle with crustal rocks, and widespread production of late‐stage granites indicate that culmination of the Variscan Orogeny may have been driven by lithospheric delamination and asthenospheric upwelling.

Oxygen fugacity constraints on the southern African lithosphere

Oxygen fugacities are calculated for olivine—spinel ±orthopyroxene assemblages recovered from diamonds and the concentrate of the Dokolwayo kimberlite, Swaziland. In addition thermobarometric oxygen fugacities are obtained for chrome spinel-garnet peridotites and diamonds from several other southern African kimberlites. The southern African lithosphere appears to be laterally homogeneous with respect to oxygen fugacity. Vertically the oxygen fugacity of the lithospheric upper mantle decreases with an increase in pressure. Locally, oxygen fugacities calculated for Dokolwayo mineral assemblages are indicative of an upper mantle characterised by diverse redox conditions within the range FMQ-IW. Reduced oxygen fugacities, calculated for the majority of the Dokolwayo samples, suggest that CH4 may be the dominant carbon volatile species in the lower lithosphere. These reduced conditions also suggest that the Dokolwayo kimberlite is unlikely to be a product of redox melting, but may be the product of a thermal anomaly. Calculated equilibrium temperatures for olivine-spinel pairs from Dokolwayo diamonds and concentrate indicate that the upper mantle in the vicinity of Dokolwayo was characterised by cool subsolidus conditions.

Thermobarometry, diffusion modelling and cooling rates of crustal garnet peridotites: Two examples from the Moldanubian zone of the Bohemian Massif

Different origins for crustal garnet peridotites are illustrated by two Variscan bodies in adjacent, tectonically juxtaposed terranes in the Moldanubicum of the Bohemian Massif (eastern Moravia). The Nové Dvory garnet peridotite contains lenses of eclogite and is allochthonous and allofacial with respect to surrounding Gföhl gneiss. The Mohelno spinel-garnet peridotite is devoid of eclogite and has concordant contacts with surrounding felsic granulite. The bulk of the Mohelno body is spinel peridotite, and garnet occurs only along its margins, where it has formed at the expense of spinel. Thermobarometry of mineral cores from garnet-bearing assemblages yields values for Nové Dvory from 1195°C, 54,6 kbar to 970°C, 41.8 kbar and for Mohelno from 1265°C, 27.9 kbar to 1070°C, 23.4 kbar. The compositional zoning profiles in garnets at both localities are matched closely by diffusion modelling in which bilinear cooling rates are utilized. At Nové Dvory initial cooling rates of 50–200°C/Ma are followed by slower rates of 10–50°C/Ma. In contrast, at Mohelno an extremely rapid initial cooling rate of 1°C/year was followed by one of 500°C/Ma. An idealized tectonic model is proposed for the emplacement of the Mohelno peridotite to account for its cooling history. The predicted garnet zoning profiles that result from conductive cooling agree closely with the observed zoning profiles. Both peridotites were derived from the upper mantle. The Nové Dvory body was emplaced from a deep fault zone into the crust as garnet peridotite, and the Mohelno body was emplaced as a hot slab of spinel peridotite, in which garnet formed only along contacts as the body cooled into the garnet stability field.

Multiple spinel–garnet peridotite transitions in upper mantle: evidence from a harzburgite xenolith

THE transition between spinel- and garnet-peridotites, caused by the reaction olivine + garnet ⇌ orthopyroxene + spinel, has been investigated by petrographic study of xenoliths1,2 and by reconnaisance experiments on phase equilibria. We report here an unusual spinel-garnet harzburgite xenolith from the Liqhobong kimberlite (north-west pipe), Lesotho, that shows evidence of a complex history.

Statistical Classification of Garnets from Kimberlite and Associated Xenoliths: Addendum

Previous articleNext article No AccessStatistical Classification of Garnets from Kimberlite and Associated Xenoliths: AddendumJ. B. Dawson and W. E. StephensJ. B. Dawson Search for more articles by this author and W. E. Stephens Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 84, Number 4Jul., 1976 Article DOIhttps://doi.org/10.1086/628216 Views: 4Total views on this site Citations: 18Citations are reported from Crossref Copyright 1976 University of ChicagoPDF download Crossref reports the following articles citing this article:Agnes T. Fung, Stephen E. Haggerty Petrography and mineral compositions of eclogites from the Koidu Kimberlite Complex, Sierra Leone, Journal of Geophysical Research: Solid Earth 100, no.B10B10 (Sep 2012): 20451–20473.https://doi.org/10.1029/95JB01573Doris V. Hills, Stephen E. Haggerty Petrochemistry of eclogites from the Koidu Kimberlite Complex, Sierra Leone, Contributions to Mineralogy and Petrology 103, no.44 (Jan 1989): 397–422.https://doi.org/10.1007/BF01041749J.B. Dawson, J.V. Smith Relationships between eclogites and certain megacrysts from the Jagersfontein kimberlite, South Africa, Lithos 19, no.3-43-4 (Oct 1986): 325–330.https://doi.org/10.1016/0024-4937(86)90031-9M. Ito Kimberlites and their ultramafic xenoliths from western Kenya, TMPM Tschermaks Mineralogische und Petrographische Mitteilungen 35, no.33 (Jan 1986): 193–216.https://doi.org/10.1007/BF01082085R. L. Hervig, J. V. Smith, J. B. Dawson Lherzolite xenoliths in kimberlites and basalts: petrogenetic and crystallochemical significance of some minor and trace elements in olivine, pyroxenes, garnet and spinel, Transactions of the Royal Society of Edinburgh: Earth Sciences 77, no.33 (Nov 2011): 181–201.https://doi.org/10.1017/S026359330001083XColin H. Donaldson Kinetics of pyrope megacryst reactions in ascending basaltic magma – relevance to high-pressure magmatic crystallization at Elie Ness, East Fife, Geological Magazine 121, no.66 (May 2009): 615–620.https://doi.org/10.1017/S0016756800030752 References, (Jan 1984): 435–466.https://doi.org/10.1016/B978-0-444-42273-6.50037-7 References, (Jan 1984): 361–393.https://doi.org/10.1016/B978-0-444-42274-3.50036-6 Suzanne Mahlburg Kay , William T. Snedden , Brayton P. Foster , and Robert W. Kay Upper Mantle and Crustal Fragments in the Ithaca Kimberlites, The Journal of Geology 91, no.33 (Sep 2015): 277–290.https://doi.org/10.1086/628771SUZANNE Y. WASS, J. D. HOLLIS Crustal growth in south-eastern Australia?evidence from lower crustal eclogitic and granulitic xenoliths, Journal of Metamorphic Geology 1, no.11 (Mar 1983): 25–45.https://doi.org/10.1111/j.1525-1314.1983.tb00263.xD.B. Clarke, G.G. Pe-Piper Multiply exsolved clinopyroxene megacrysts from the Frank Smith Mine, Cape Province, South Africa, Lithos 16, no.11 (Jan 1983): 75–84.https://doi.org/10.1016/0024-4937(83)90036-1R.L. Hervig, J.V. Smith, I.M. Steele, J.B. Dawson Fertile and barren AlCr-spinel harzburgites from the upper mantle: Ion and electron probe analyses of trace elements in olivine and orthopyroxene: Relation to lherzolites, Earth and Planetary Science Letters 50, no.11 (Oct 1980): 41–58.https://doi.org/10.1016/0012-821X(80)90118-1N. P. Lock, J. B. Dawson Garnet-olivine reaction in the upper mantle: evidence from peridotite xenoliths in the Letseng-la-Terae kimberlites, Lesotho, Transactions of the Royal Society of Edinburgh: Earth Sciences 71, no.11 (Nov 2011): 47–53.https://doi.org/10.1017/S0263593300013481J. B. DAWSON, J. V. SMITH, J. S. DELANEY Multiple spinel–garnet peridotite transitions in upper mantle: evidence from a harzburgite xenolith, Nature 273, no.56655665 (Jun 1978): 741–743.https://doi.org/10.1038/273741a0F.C. Bishop, J.V. Smith, J.B. Dawson Na, K, P and Ti in garnet, pyroxene and olivine from peridotite and eclogite xenoliths from African kimberlites, Lithos 11, no.22 (Apr 1978): 155–173.https://doi.org/10.1016/0024-4937(78)90006-3J. B. Dawson, J. S. Delaney, J. V. SmitH Aspects of the mineralogy of aln�itic breccia, Malaita, Solomon Islands; comparison with continental kimberlites, Contributions to Mineralogy and Petrology 67, no.22 (Jan 1978): 189–193.https://doi.org/10.1007/BF01046575 W. E. Stephens , and J. B. Dawson Statistical Comparison between Pyroxenes from Kimberlites and Their Associated Xenoliths, The Journal of Geology 85, no.44 (Sep 2015): 433–449.https://doi.org/10.1086/628317