Academic literature on the topic 'Eclogite melting'

The phase relationships have been experimentally studied at eclogitization of basalts and the melting of H2O‑containing eclogite in the basalt-H2O system at P = 3,7-4,0 GPa, T = 1000-1300 C. It is established that the phase relationships depend on temperature. The formation of a supercritical fluid-melt occurs at T = 1000 C, P = 3,7 GPa, conversion eclogite-granatite occurs at T = 1000-1100 C, P = 3,9 GPa, partial melting of eclogite with the formation of Na-alkali silicate melt and clinopyroxenite restite at 1150 C and 1300 C. The supercritical fluid-melt has a high reactivity, resulting in the formation of megacrists of garnet, its enrichment with Ti, the replacement of garnet with clinopyroxene, the formation of ilmenite, K‑containing amphibole, the conversion of eclogite into garnetite as a result of mass crystallization of garnet.

Abstract. Metamorphic textures and a pressure–temperature (P–T) path of zoisite eclogite are presented to better understand the metamorphic evolution of the North-East Greenland eclogite province and this particular type of eclogite. The eclogite contained the mineral assemblage garnet, omphacite, kyanite, phengite, quartz and rutile at peak pressure. Partial melting occurred via breakdown of hydrous phases, paragonite, phengite and zoisite, based on (1) polymineralic inclusions of albite and K-feldspar with cusps into host garnet, (2) small euhedral garnet with straight boundaries against plagioclase, (3) cusps of plagioclase into surrounding phases (such as garnet), and (4) graphic intergrowth of plagioclase and amphibole next to anhedral zoisite grains. Isochemical phase equilibrium modeling of a melt-reintegrated composition, along with XNa-in-omphacite and Si-in-phengite isopleths, yields a peak pressure of 2.4±0.1 GPa at 830±30 ∘C. A peak temperature of 900±50 ∘C at 1.9±0.2 GPa is determined using the rim composition of small euhedral garnet, as predicted by modeling a crystallized melt pocket. Zoisite growth at the expense of kyanite suggests that the P–T path crossed the fields of zoisite growth at ∼1.9 GPa, 800–900 ∘C on the modeled phase diagram of the bulk rock. A point on the exhumation path at ∼1.3 GPa and 750 ∘C is derived from hornblende-plagioclase thermometry and Al-in-hornblende barometry. The study demonstrates that paragonite, phengite and zoisite could contribute to partial melting of eclogite at near-peak P and during exhumation.

Mineralogy of diamondiferous eclogite xenolites showing metasomatosis evidence from the Udachnaya kimberlite pipe is discussed. The paper also reviews features of diamonds they contain, compositions of primary garnets and omphacites as well as alteration of structural and species compositions of original garnets and clinopyroxenes during metasomatosis. Based on pyrope structure update, two-phase garnet composition is suggested, which is mostly represented by complex pyrope associated with Ca-pyrope. In all samples, primary omphacite is replaced by another clinopyroxene variety depleted in Na2O, which is typical of partial melting products. Geothermometry results suggested that the eclogites formed within a temperature range of 1,000–1,2000 °C. Based on diamond morphology, data on total N content in diamonds and its aggregation, multiple stages of diamond formation in eclogites and the most probable growth of later diamond generations impacted by metasomatizing mantle fluids containing carbon are postulated. It is suggested that certain diamond formation stages probably had a time gap of several hundred million years.

Peritectic mechanisms, controlling fractional ultrabasic-basic evolution of the upper mantle magmatism and genesis of the peridotitepyroxeniteeclogite rock series, are substantiated in theory and experiment. Melting phase relations of a differentiated mantle material are studied with polythhermal section method in the multicomponent olivineclinopyroxene/omphacitecorundumcoesite system with boundary compositions duplicated these of peridotitic and eclogitic minerals. The peritectic reaction of orthopyroxene and melt with formation of clinopyroxene (the opthopyroxene clinopyroxenization reaction) has been determined at a liquidus surface of the ultrabasic olivineorthopyroxeneclinopyroxenegarnet system. As a result of the reaction the temperature-regressive univariant curve olivine + clinopyroxene + garnet + melt is formed. A further evolution of magmatism has experimentally studied at 6 GPa in the ultrabasic-basic olivinediopsidejadeitegarnet system with changeable compositions of the diopsidejadeite solid solutions (controlling the clinopyroxene omphacite mineralogy). Peritectic reaction of olivine and melt with formation of garnet was established on the liquidus surface of the ternary olivinediopsidejadeite system as the mechanism of olivine garnetization and going to the univariant curve omphacitegarnetmelt with formation of bimineral eclogites. Structure of the liquidus surface for the olivinediopsidejadeitegarnet system is inferred, and its role as a physic-chemical bridge between ultrabasic olivinebearing peridotitepyroxenitic and basic silica-saturated eclogitic compositions of the garnetperidotite facies matter. The new experimental physic-chemical results reveal the genetic links between ultrabasic and basic rocks as well as mechanisms of the uninterrupted fractional magmatic evolution and petrogenesis from the olivinebearing peridotitepyroxenitic to silica-saturated eclogite-grospyditicrocks. This provides an explanation for the uninterrupted composition trends for rock-forming components in clinopyroxenes and garnets of the differentiated rocks of the garnetperidotite facieis.

Thesis (PhD)--Stellenbosch University, 2013.
ENGLISH ABSTRACT: The sodic and leucocratic Tonalite, Trondhjemite and Granodiorite (TTG) granitoid series of rocks characterise Paleo- to Meso- Archaean felsic continental crust, yet are uncommon in the post-Archaean rock record. Consequently, petrogenetic studies on these rocks provide valuable insight into the creation and evolution of Earth’s early continental crust. The highpressure (HP)-type of Archaean TTG magmas are particularly important in this regard as their geochemistry requires that they are formed by high-pressure melting of a garnet-rich eclogitic source. This has been interpreted as evidence for the formation of these magmas by anatexis of the upper portions of slabs within Archaean subduction zones. In general, TTG magmas have been assumed to arise through fluid-absent partial melting of metamafic source rocks. Therefore, very little experimental data on fluid-present eclogite melting to produce Archaean TTG exist, despite the fact that water drives magmatism in modern arcs. Consequently, this study experimentally investigates the role of fluid-present partial melting of eclogite-facies metabasaltic rock in the production of Paleo- to Meso-Archaean HP-type TTG melts. Experiments are conducted between 1.6 GPa and 3.0 GPa and 700 ºC and 900 ºC using natural and synthetic eclogite, and gel starting materials of low-K2O basaltic composition. Partial melting of the natural and synthetic eclogite occurred between 850 ºC and 870 ºC at pressures above 1.8 GPa, and the melting reaction is characterised by the breakdown of sodic clinopyroxene, quartz and water: Qtz + Cpx1 + H2O ± Grt1 = Melt + Cpx2 ± Grt2. The experimental melts have the compositions of sodic peraluminous trondhjemites and have compositions that are similar to the major, trace and rare earth element composition of HPtype Archaean TTG. This study suggests that fluid-present eclogite melting is a viable petrogenetic model for this component of Paleo- to Meso-Archaean TTG crust. The nature of the wet low-K2O eclogite-facies metamafic rock solidus has been experimentally defined and inflects towards higher temperatures at the position of the plagioclase-out reaction. Therefore, the results indicate that a crystalline starting material is necessary to define this solidus to avoid metastable melting beyond temperatures of the Pl + H2O + Qtz solidus at pressures above plagioclase stability. Furthermore, this study uses numerical and metamorphic models to demonstrate that for reasonable Archaean mantle wedge temperatures within a potential Archaean subduction zone, the bulk of the water produced by metamorphic reactions within the slabs is captured by an anatectic zone near the slab surface. Therefore, this geodynamic model may account for HP-type Archaean TTG production and additionally provides constraints for likely Archaean subduction. The shape of the relevant fluid-present solidus is similar to the shape of the pressure-temperature paths followed by upper levels of the proposed Archaean subducting slab, which makes water-fluxed slab anatexis is very dependant on the temperature in the mantle wedge. I propose that cooling of the upper mantle by only a small amount during the late Archaean ended fluid-present melting of the slab. This allowed slab water to migrate into the wedge and produce intermediate composition magmatism which has since been associated with subduction zones.
AFRIKAANSE OPSOMMING: Die reeks natruimhoudende en leukokraties Tonaliet, Trondhjemiet en Granodioriet (TTG) felsiese stollingsgesteentes is kenmerkend in die Paleo- tot Meso-Argeïkum felsiese kontinentale kors, maar is ongewoon in die post-Argeïese rots rekord. Gevolglik, petrogenetiese studies op hierdie rotse verskaf waardevolle insig in die skepping en evolusie van die aarde se vroeë kontinentale kors. Die hoë-druk (HD)-tipe van die Argeïkum TTG magmas is veral belangrik in hierdie verband as hulle geochemie vereis dat hulle gevorm word deur hoë druk smelting van 'n granaat-ryk eklogitiese bron. Dit word interpreteer as bewys vir die vorming van hierdie magmas deur smelting van die boonste gedeeltes van die blaaie in Argeïese subduksie sones. TTG magmas in die algemeen, is veronderstel om op te staan deur middel van water-afwesig gedeeltelike smelting van metamafiese bron rotse. Daarom bestaan baie min eksperimentele data op water-teenwoordig eklogiet smelting om Argeïkum TTG te produseer, ten spyte van die feit dat water magmatisme dryf in moderne boë. Gevolglik is hierdie studie ‘n eksperimentele ondersoek in die rol van water-teenwoordig gedeeltelike smelting van eklogiet-fasies metamafiese rots in die produksie van Paleo- tot Meso-Argeïkum HD-tipe TTG smelte. Eksperimente word uitgevoer tussen 1.6 GPa en 3.0 GPa en 700 ºC en 900 ºC met behulp van natuurlike en sintetiese eklogiet, en gel begin materiaal van lae-K2O basaltiese samestelling. Gedeeltelike smelting van die natuurlike en sintetiese eklogiet het plaasgevind tussen 850 ºC en 870 ºC te druk bo 1.8 GPa, en die smeltings reaksie is gekenmerk deur die afbreek van natruimhoudende klinopirokseen, kwarts en water: Qtz + Cpx1 + H2O ± Grt1 = Smelt + Cpx2 ± Grt2. Die eksperimentele smelte het die komposisies van natruimhoudende trondhjemites en is soortgelyk aan die hoof-, spoor- en seldsame aard element samestelling van HD-tipe Argeïkum TTG. Hierdie studie dui daarop dat water-teenwoordig eklogiet smelting 'n lewensvatbare petrogenetiese model is vir hierdie komponent van Paleo- tot Meso-Argeïkum TTG kors. Die aard van die nat lae-K2O eklogietfasies metamafiese rock solidus is eksperimenteel gedefinieër en beweeg na hoër temperature by die posisie van die plagioklaas-out reaksie. Daarom dui die resultate daarop dat 'n kristallyne materiaal nodig is om hierdie solidus te definieër en metastabiele smelting buite temperature van die Pl + H2O + Qtz solidus druk bo plagioklaas stabiliteit te vermy. Verder maak hierdie studie gebruik van numeriese en metamorfiese modelle om aan te dui dat die grootste deel van die water geproduseer deur metamorfiese reaksies binne die blaaie bestaan vir redelike Argeïkum mantel wig temperature binne 'n potensiële Argeïkum subduksie sone, en word opgevang deur 'n smelting sone naby die blad oppervlak. Daarom kan hierdie geodinamies model rekenskap gee vir HD-tipe Argeïkum TTG produksie en dit bied ook die beperkinge vir waarskynlik Argeïese subduksie. Die vorm van die betrokke waterteenwoordig solidus is soortgelyk aan die vorm van die druk-temperatuur paaie gevolg deur die boonste vlakke van die voorgestelde Argeïkum subderende blad, wat water-vloeiing blad smeltingbaie afhanklik maak van die temperatuur in die mantel wig. Ons stel voor dat afkoeling van die boonste mantel met slegs 'n klein hoeveelheid gedurende die laat Argeïese, die water-vloeiing smelting van die blad beëindig. Dit het toegelaat dat die blad water in die wig migreer en intermediêre samestelling magmatisme produseer wat sedert geassosieer word met subduksie sones.

Exhumation of high-pressure and ultrahigh-pressure eclogites in large orogens and associated petrological change during the process remain enigmatic problems. This dissertation examines eclogites from high-pressure (HP) and ultrahigh-pressure (UHP) terranes in the North-East Greenland Eclogite Province, aiming to decipher their metamorphic pressure-temperature (P-T) paths, evaluate spatial variation of P-T paths, and understand petrological changes during the exhumation. Kyanite-bearing UHP eclogites from North-East Greenland contain a peak mineral assemblage of phengite, garnet, omphacite, kyanite, coesite, rutile and probably epidote-group minerals. Thermodynamic modeling with an XRF-derived bulk composition yielded a peak P-T condition of 3.4 GPa and 920 °C. Petrographic textures, such as graphic intergrowth of amphibole and plagioclase, cusps of plagioclase into garnet and quartz, and neoblasts of garnet indicate that the eclogites were partially melted through dehydration melting of phengite and epidote-group minerals. Since thermodynamic modeling could not yield a satisfactory solidus curve, experimental phase relations were considered in interpreting the melting process, and show a near isothermal decompression path across the epidote mineral melting curve. Additional thermodynamic modeling of a symplectite after omphacite, consisting of amphibole, plagioclase and clinopyroxene, yields a P-T condition of ~ 1.2 GPa and 800 °C. Thermodynamic modeling of a melt pocket yields a further P-T constraint of 1.4 GPa and 740 °C. The HP zoisite eclogites from the Storstrømmen shear zone in the Sanddal area preserve partial melting textures both in garnet and in the matrix. The textures include multiphase solid inclusions of albite and K-feldspar in garnet, graphic intergrowth of amphibole and plagioclase, cuspate textures, and leucosome. Thermodynamic modeling combined with mineral composition and modes yielded an exhumation P-T path from subsolidus conditions at ~1.95 GPa and ~670 °C, to ~1.85 GPa and 715 °C at suprasolidus, to ~1.45 GPa and 640 °C. Paragonite, phengite, and amphibole were the major dehydration melted phases along the exhumation path. The HP kyanite eclogite from the Danmarkshavn area contains disequilibrium textures developed during retrograde stages. Petrographic observation documents two groups of textures: a strongly zoned plagioclase (anorthite to andesine) enclosing a poorly developed symplectite of sapphirine + spinel + plagioclase after kyanite, and a less zoned plagioclase (labradorite to andesine) enclosing a fully developed symplectite after kyanite. Thermodynamic modeling of the bulk rock returns a peak P-T condition of 1.9 GPa and 840 °C. Thermodynamic modeling of a symplectite domain yields poor P-T constraints of 0.8 – 1.3 GPa and 700 – 900 °C. Modeling also indicates the plagioclase development would be richer in Ca during decompression while progressive replacement of kyanite induced the plagioclase rim to be less Ca-rich. This study reveals that HP and UHP eclogites may experience partial melting on their exhumation path. Dehydration melting of hydrous minerals (e.g. phengite and zoisite) is the most plausible way in partially melt the eclogites, because of limited amounts of free fluid. The partial melting does not trigger exhumation of the eclogites, but may facilitate the exhumation process. The near-isothermal exhumation path for the UHP terranes suggests that it was initially exhumed through vertical extrusion. Lateral extrusion by the Storstrømmen and Germania Land shear zones is suggested to have further exhumed the HP and UHP rocks, which is analogous to the lateral escape tectonics in the Tibetan Plateau

L’obiettivo di questo studio è quello di caratterizzare il trasferimento di massa tra crosta e mantello. A questo scopo sono stati considerati due terreni metamorfici di alta pressione (HP) dove peridotiti a granato affiorano all’interno di rocce crostali di alto grado, i.e. l’area del Monte Duria (falda Adula-Cima Lunga, Alpi centrali, N Italia) e la zona d’ Ultimo (falda del Tonale, Alpi orientali, N Italia). Nell’area del Monte Duria, peridotiti a granato affiorano in contatto diretto con eclogiti migmatitiche (Borgo). Sia le peridotiti che le eclogiti registrano condizioni di picco in HP a 2.8 GPa e 750 ° C e un riequilibratura statica a 1.0 GPa e 850 ° C. Le peridotiti mostrano abbondanti anfibolo, dolomite, flogopite e ortopirosseno (su olivina), suggerendo che le peridotiti registrano metasomatismo ad opera di agenti crostali arrichiti in SiO2, K2O, CO2 e H2O. Le peridotiti mostrano anche un frazionamento in LREE (La/Nd = 2.4) legato alla presenza di anfibolo e clinopirosseno. Questi minerali sono equilibrio con il granato, indicando che il metasomatismo è avvenuto in HP. Nelle eclogiti, microstrutture di fusione come aggregati microcristallini a Kfs+Pl+Qz+Cpx e Cpx+Kfs sono allineate lungo la foliazione a Zo+Omp+Grt, indicando che le eclogiti hanno subito un evento di fusione parziale in HP. Il contatto tra le peridotiti e le eclogiti di Borgo è marcato dalla presenza di un livello di tremolitite. Boudins di tremolititi si ritrovano anche trasposti lungo la foliazione a granato della peridotite, indicando che il boudinage delle tremolititi è avvento in alta pressione. Le tremolititi mostrano aggregati a Phl+Tc+Chl+Tr interpretati come psudomorfi su granato. Tali pseudomorfi si sviluppano in condizioni statiche post-datando la formazione dei boudins, suggerendo che le tremolititi derivano da precursori a granato. Le tremolititi mostrano Mg# > 0.90 e Al2O3 = 2.75 wt.% tipici di composizioni ultramafiche ma allo stesso tempo presentano arricchimenti in SiO2, CaO, e LREE, indicando che esse rappresentano il prodotto dell’interazione in alta pressione tra le peridotiti e i fusi derivati dalle eclogiti. Per testare questa ipotesi abbiamo sviluppato un modello termodinamico a P = 3 GPa e T = 750 °C. I nostri risultati indicano che l’interazione fuso-peridotite produce una paragenesi a Opx+Cpx+Grt, suggerendo che le tremolititi rappresentano il prodotto di retrocessione di una westerite a granato. Nella zona d’Ultimo, numerose lenti di peridotite affiorano all’interno di rocce crostali di alto grado. Le peridotiti mostrano una transizione da lherzoliti a spinello protogranulari a peridotiti milonitiche a granato e anfibolo. Le pirosseniti trasposte lungo la foliazione della peridotite mostrano un’evoluzione simile, da pirosseniti a spinello a pirosseniti a granato. Questa evoluzione riflette il passaggio indotto dal corner flow del mantello da condizioni in facies a spinello a a granato. Come consguenza, il granato forma corone intorno allo spinello ed essoluzioni all’interno dei porfiroclasti di pirosseno, e cristallizza lungo la foliazione delle pirosseniti e delle peridotiti Evidenze tessiturali e dati cristallografici indicano che la transizione spinello-granato avviene in un contesto deformativo. I porfiroclasti di pirosseno mostrano evidente CPO, alte frequenze delle misorientazioni a basso angolo, e distribuzione non-random degli assi di misorientazione per misorientazioni a basso angolo, indicando che i pirosseni si deformano per dislocation creep. Il dislocation creep è contemporaneo a processi di ricristallizzazione dinamica e alla transizione spinello-granato. Ciò induce una riduzione della grana e una transizione permanente da disclocation creep nei porfiroclasti a grain-size sensitive creep nei grani ricristallizzati che risulta in un forte indebolimento delle pirosseniti e delle peridotiti quando queste vengono tettonicamente accoppiate alle rocce crostali.
In the Monte Duria area (Adula-Cima Lunga unit, Central Alps, N Italy) garnet peridotites occur in direct contact with migmatised orthogneiss (Mt. Duria) and eclogites (Borgo). Both crustal and ultramafic rocks share a common high pressure (HP) peak at 2.8 GPa and 750 °C and post-peak static equilibration at 0.8-1.0 GPa and 850 °C. Garnet peridotites show abundant amphibole, dolomite, phlogopite and orthopyroxene after olivine, suggesting that they experienced metasomatism by crust-derived agents enriched in SiO2, K2O, CO2 and H2O. Peridotites also display LREE fractionation (La/Nd = 2.4) related to LREE-rich amphibole and clinopyroxene grown in equilibrium with garnet, indicating that metasomatism occurred at HP conditions. Kfs+Pl+Qz+Cpx interstitial pocket aggregates and Cpx+Kfs thin films around symplectites after omphacite parallel to the Zo+Omp+Grt foliation in the eclogites suggest that they underwent partial melting at HP.The contact between garnet peridotites and associated eclogites is marked by a tremolitite layer, which also occurs as layers within the peridotite lens, showing a boudinage parallel to the garnet layering of peridotites, flowing in the boudin necks. This clearly indicates that the tremolitite boudins formed when peridotites were in the garnet stability field. Tremolitites also show Phl+Tc+Chl+Tr pseudomorphs after garnet, both crystallised in a static regime postdating the boudins formation, suggesting that they derive from a garnet-bearing precursor. Tremolitites have Mg# > 0.90 and Al2O3 = 2.75 wt.% pointing to ultramafic compositions but also show enrichments in SiO2, CaO, and LREE suggesting that they formed after the reaction between the eclogite-derived melt and the garnet peridotite at HP. To test this hypothesis, we performed a thermodynamic modelling at fixed P = 3 GPa and T = 750 °C to model the chemical interaction between the garnet peridotite and the eclogite-derived melt. Our results show that this interaction produces a Opx+Cpx+Grt assemblage + Amp+Phl, depending on the water activity in the melt, suggesting that tremolitites likely derive from a previous garnet websterite with amphibole and phlogopite. In the Ulten Zone (Tonale nappe, Eastern Alps, N Italy), peridotite bodies occur within high-grade crustal rocks. Peridotites show a transition from coarse spinel-lherzolites to mylonitic garnet-amphibole peridotites. Pyroxenites veins and dikes, transposed along the peridotite foliation, show a similar evolution from coarse garnet-free websterites to fine-grained garnet + amphibole clinopyroxenites. This coupled evolution has been interpreted to reflect cooling and pressure increase of pyroxenites and host peridotites from spinel- (1200 °C, 1.3-1.6 Gpa) to garnet-facies conditions (850 °C and 2.8 Gpa) likely induced by mantle corner flow. As a consequence, garnet formed coronas around spinel and exsolved from porphyroclastic, high-T pyroxenes, and finally crystallised along the pyroxenite and peridotite foliations. Textural evidences and CPO data indicate that the transition from spinel- to garnet-facies conditions was assisted by intense shearing and deformation. Pyroxene porphyroclasts in garnet clinopyroxenites show well-developed CPOs, high frequencies of low-angle misorientations, and non-random distribution of the low-angle misorientation axes, indicating that pyroxene porphyroclasts primarily deform by dislocation creep. Dislocation creep is accompanied by reaction-induced dynamic recrystallisation during the spinel to garnet phase transition, which promotes a sudden reduction of the grain size and a shift from dislocation creep in the porphyroclast to grain-size sensitive creep (GSS) in the recrystallised grains. This results in a dramatic rheological weakening of pyroxenites at HP peak conditions when pyroxenites and host peridotites were coupled with crustal rocks.