Academic literature on the topic 'Upper mantle melting'

Deeply subducted carbonates likely cause low-degree melting of the upper mantle and thus play an important role in the deep carbon cycle. However, direct seismic detection of carbonate-induced partial melts in the Earth’s interior is hindered by our poor knowledge on the elastic properties of carbonate melts. Here we report the first experimentally determined sound velocity and density data on dolomite melt up to 5.9 GPa and 2046 K by in-situ ultrasonic and sink-float techniques, respectively, as well as first-principles molecular dynamics simulations of dolomite melt up to 16 GPa and 3000 K. Using our new elasticity data, the calculated VP/VSratio of the deep upper mantle (∼180–330 km) with a small amount of carbonate-rich melt provides a natural explanation for the elevated VP/VSratio of the upper mantle from global seismic observations, supporting the pervasive presence of a low-degree carbonate-rich partial melt (∼0.05%) that is consistent with the volatile-induced or redox-regulated initial melting in the upper mantle as argued by petrologic studies. This carbonate-rich partial melt region implies a global average carbon (C) concentration of 80–140 ppm. by weight in the deep upper mantle source region, consistent with the mantle carbon content determined from geochemical studies.

Abstract Ophiolite pulses, which are periods of enhanced ophiolite generation and emplacement, are thought to have a relevance to highly active superplumes (superplume model). However, the Cambrian-Ordovician pulse has two critical geological features that cannot be explained by such a superplume model: predominance of subduction-related ophiolites and scarcity of plume-related magma activities. We addressed this issue by estimating the mechanism and condition of magma generation, including mantle potential temperature (MPT), from a ~500 Ma subduction-related ophiolite, the Hayachine-Miyamori ophiolite. We developed a novel method to overcome difficulties in global MPT estimation from an arc environment by using porphyritic ultramafic dikes showing flow differentiation, which have records of the chemical composition of the primitive magma, including its water content, because of their high pressure (~0.6 GPa) intrusion and rapid solidification. The solidus conditions for the primary magmas are estimated to be ~1450 °C, ~5.3 GPa. Geochemical data of the dikes show passive upwelling of a depleted mantle source in the garnet stability field without a strong influence of slab-derived fluids. These results, combined with the extensive fluxed melting of the mantle wedge prior to the dike formation, indicate sudden changes of the melting environment, its mechanism, and the mantle source from extensive fluxed melting of the mantle wedge to decompressional melting of the sub-slab mantle, which has been most plausibly triggered by a slab breakoff. The estimated MPT of the sub-slab mantle is ~1350 °C, which is very close to that of the current upper mantle and may reflect the global value of the upper mantle at ~500 Ma if small-scale convection maintained the shallow sub-slab mantle at a steady thermal state. We, therefore, conclude that the Cambrian-Ordovician ophiolite pulse is not attributable to the high temperature of the upper mantle. Frequent occurrence of slab breakoff, which is suggested by our geochemical compilation of Cambrian-Ordovician ophiolites, and subduction termination, which is probably related to the assembly of the Gondwana supercontinent, may be responsible for the ophiolite pulse.

Thesis (MSc)--Stellenbosch University, 2015.
ENGLISH ABSTRACT: Entrainment and assimilation of xenolithic material during kimberlite ascent is considered to be important in shaping the chemistry of the magma and fuelling magma ascent by driving CO2 exsolution. Previous, but as yet unpublished experimental work from Stellenbosch University has demonstrated that orthopyroxene has a key role in this. Orthopyroxene is a very rare xenocrystic constituent of kimberlite but makes up a considerable fraction of the entrained xenolithic material. The initial study used a natural kimberlite composition (ADF1) doped with a peridotite mineral suite (by weight); 88 % ADF1 5% olivine, 5% orthopyroxene and 2% garnet-spinel intergrowth as a starting composition. The subsequent high PT experiments (1100 to 1300°C and 2.0 to 3.5GPa) established that equilibrium orthopyroxene is stable at 1100°C above 2.5GPa, at 1200°C above 2.5GPa and at 1300°C between 2.0 and 3.5GPa. At lower pressures orthopyroxene is completely digested by the experimental melt by the reaction; Mg2Si2O6 (opx) = Mg2SiO4 (ol) + SiO2 (in liquid). In contrast, clinopyroxene is a common phase in kimberlite and often occurs as more than one generation of crystals. Xenocrystic clinopyroxene is dominated by diopside compositions. However, rare omphacite is sometimes also inherited from an eclogite source. The Omphacite, like orthopyroxene, displays textural evidence of severe disequilibrium and may also contribute to the evolution of kimberlitic melt. Thus, a second study produced experiments on the ADF1 kimberlite material at upper mantle PT conditions (1100 to 1300°C and 2.0 to 4.0GPa) as well as an omphacite doped starting material (ADF1+O). These experiments examine the behaviour of pyroxene in kimberlite magma including the influence this may have on magma buoyancy. Within this PT range omphacitic clinopyroxene breaks-down via complex multipart reactions. At 1100°C and 2.0GPa reaction textures around remnant omphacite suggest that omphacite melts incongruently in a complex reaction similar to: Omp + Melt = Ap + Cr-diop + SiO2-enriched Melt. At 1300°C omphacite melts completely and is perceived to produce peritectic Cr-diopside, calcium-rich olivine, carbonate in the melt as well as enrich the melt in SiO2. The melts produced by both the ADF1+O and ADF1 compositions at 1300°C and 4.0GPa are reduced in SiO2 content and have increased TiO2, Cr2O3, Al2O3, MnO, CaO, K2O and P2O5 compared to their respective starting compositions. However, significantly higher proportions of Ca, Na and Fe observed within the ADF1+O melt is a direct consequence of omphacite melting. The ADF1+O starting composition produced equilibrium orthopyroxene above 1100°C and 4.0GPa as well as at 1300°C above 2.0GPa. At lower pressure the orthopyroxene melts incongruently to form peritectic olivine and more silica-rich melt compositions. This digestion favours CO2 exsolution. The effect of orthopyroxene melting can be seen in the melt compositions produced by the peridotite doped starting material (ADF1+P) of the initial study. At 1300°C and 2.0GPa, ADF1+P produced a siliceous melt (37.0 wt.% SiO2) enriched in Al and alkalis compared to the starting ADF1+P composition. This behaviour is directly attributed to xenocrystic orthopyroxene melting at high temperature. In contrast, at the same PT the original kimberlite (ADF1) composition produces a melt with 28.9 wt.% SiO2 and high Ca and Mg contents. Overall, with an increase in pressure the melts become enriched in alkalis and Al2O3 as a direct result of xenocrystic pyroxene melting. In addition, increased pressure allows for a greater solubility of CO2 within the melt. This results in a lower SiO2 melt content and the increased stabilization of equilibrium silica-rich mineral phases (i.e. olivine and equilibrium orthopyroxene). Within the peridotite doped static system (unpublished) the mineral separates with an average crystal size of 115μm ±10μm were all effectively digested in less than 48hours. Similarly, the omphacite doped experiments consumed the 150μm (±10μm) xenocrysts in under 24 hours. Thus, it is suggested that xenocrystic pyroxene is unstable in these experimental kimberlitic melt compositions and is likely to be efficiently assimilated in less than 24 hours. These experimental melts most likely resemble those of natural systems under upper mantle PT conditions. Therefore, pyroxene melting increases the silica content of the melt which in turn drives CO2 exsolution and ascent.
AFRIKAANSE OPSOMMING: Meevoering en assimilasie van xenolitiese materiaal gedurenende kimberliet bestyging is beskou as belangrik in verband met die vorming van die chemie van die magma, en bevorder magma bestyging deur die aandrywing van CO2 ontmenging. Vorige, maar ongepubliseerde eksperimentele werk vanaf Stellenbosch Universiteit het gedemonstreer dat ortopirokseen ‘n sleutelrol hierin het, omrede ortopirokseen ‘n baie skaars xenokristiese bestanddeel van kimberliet is maar ‘n aansienlike fraksie van die meevoerde xenolitiese materiaal moet opmaak. Hierdie studie het ‘n natuurlike primere kimberliet komposisie (ADFI) gedoop met ‘n peridotiet mineraal reeks (per gewig); 88 % ADF1 5% olivien, 5% ortopirokseen en 2% granaat-spinel ingroeiing as begin komposisie gebruik. Die daaropvolgende hoë DT eksperimente (1100 tot 1300°C en 2.0 tot 3.5GPa) het vasgestel dat ewewigsortopirokseen stabiel is teen 1100°C bo 2.5GPa, 1200°C bo 2.5GPa en teen 1300°C vanaf 2.0 tot 3.5Gpa. Teen laer druk word ortopirokseen geheel verteer deur die eksperimentele smelting volgens die reaksie Mg2Si2O6 (opx) = Mg2SiO4 (ol) + SiO2 (in vloeistof). In kontras hiermee is clinopirokseen algemeen in kiemberliet en kom dikwels voor as meer as een generasie se kristalle. Diopsiet komposisies domineer xenokristiese klinopirokseen. Seldsame omfasiet is tog somtyds ook geërf vanaf ‘n eklogiet bron. Die omfasiet, soos ortopirokseen, vertoon teksturuele bewys van ernstige disekwilibrium en mag ook bydra tot die evolusie van kimberlitiese smelt. Dus was daar addisionele eksperimente uitgevoer op die ADF1 kimberliet material teen hoër mantel DT kondisies (1100 tot 1300°C en 2.0 tot 4.0GPa), asook ‘n begin materiaal gedoop met omfasiet (ADF1+O). Hierdie eksperimente ondersoek die gedrag van pirokseen in kiemberliet magma, asook die invloed wat dit sal hê op die dryfvermoë van die magma. Binne hierdie DT reeks breek omfasitiese klinopirokseen af via komplekse multideel reaksie prosesse. Teen 1100°C en 2.0Gpa stel reaksie teksture rondom die oorblywende omfasiet voor dat omfasiet ongelykvormig smelt deur ‘n komplekse reaksie soortgelyk aan: Omp + Smelt = Ap + Cr-diop + SiO2-verrykde Smelt. Teen 1300°C smelt omfasiet volkome en is waargeneem om peritektiese Cr-diopsiet, kalsiumryke olivien en kalsiet te produseer, sowel as dat dit die smelt verryk in SiO2. Die smeltings geprodiseer deur die ADF1+O en ADF1 massa komposisies teen 1300°C en 4.0GPa is verlaag in SiO2 inhoud en bevat verhoogde TiO2, Cr2O3, Al2O3, MnO, CaO, K2O en P2O5 in vergelyking met die onderskeie begin komposisies. Aansienlike hoër proporsies van Ca, Na en Fe is egter waargeneem in die ADF1+O smelt en is ‘n direkte gevolg van die smelting van omfasiet. Die ADF1+O begin samestelling het ewewigsortopirokseen bo 1100°C en 4.0Gpa geproduseer en massa teen 1300°C en 2.0 tot 4.0GPa. Teen laer druk smelt hierdie pirokseen inkongruent om peritektiese olivien en meer silika-ryke smelt samestellings te vorm, en ontmeng CO2. Die effek van ortopirokseen smelting kan aanskou word in die smelt samestellings wat produseer is deur die begin materiaal wat gedoop is in peridotiet (ADF1+P), in die oorspronklike studie. Teen 1300°C en 2.0GPa het ADF1+P ‘n silikahoudende smelt (37.0 wt.% SiO2) produseer wat verryk is in Al en alkalies in vergelyking met die ADF1+P massa samestelling. Hierdie gedrag is direk toegeskryf aan die xenokristiese ortopirokseen wat smelt teen hoë temperatuur. In kontras hiermee, teen dieselfde DT kondisies produseer die oorspronklike kiemberliet (ADF1) massa ‘n smelt met 28.86 gewigspersentasie SiO2 en hoë Ca en Mg inhoud. In die algeheel word die smeltings verryk in alkalies en Al2O3 teen verhoogde druk as ‘n derekte gevolg van xenokristiese pirokseen smelting. Verder laat verhoogde druk toe vir hoër oplosbaarheid van CO2 in die smelt, wat lei tot laer SiO2 inhoud en ‘n toename in stabilisering van ewewigs silika-ryke mineraal fases (dws. olivien en ewewigsortopirokseen). In die peridotiet gedoopde statiese sisteem (ongepubliseerd), was die mineraal skeiding met ‘n gemiddelde kristal grootte van 115μm ±10μm almal effektief verteer in minder as 48 ure. Soortgelyk hieraan het die omfasiet gedoopde eksperimente die 150μm (±10μm) sade onder 24 ure verteer. Dus stel dit voor dat xenokristiese pirokseen in naatuurlike sisteme onstabiel is in kiemberlietiese smelt samestellings en sal waarskynlik geassimileer wees in miner as 24 ure en ‘n meer silica-ryke kiemberlietiese smelt samestelling produseer terwyl dit CO2, ontmenging en bestyging aandryf.

Les propriétés de transport des roches mantelliques sont des paramètres importants pour interpréter aussi bien qualitativement que quantitativement les informations géophysiques, telles que la vitesse des ondes sismiques, les flux de chaleurs et les profils magnétotelluriques terrestres. L’origine des anomalies géophysiques du manteau supérieur, comme la zone de faible vitesse (LVZ ; 70-150km de profondeur) et le niveau de faible vitesse (LVL ; 350-410 km de profondeur), est peu renseignée et demande des contraintes expérimentales. Au cours de cette thèse, nous avons étudié les propriétés électriques, sismiques et thermiques des péridotites solides et partiellement fondues via le développement de techniques géophysiques in situ. Nos expériences à hautes pressions et températures, en presse multi-enclumes, nous ont permis d’établir les effets de la fusion sur ces différentes propriétés physiques aux conditions mantelliques. Nous avons, pour la première fois, réalisé des mesures combinées de conductivité électrique et de vitesses des ondes sismiques en une seule et même expérience. Grâce à cette technique, nous avons réconcilié les mesures du taux de fusion impliquées dans la LVZ estimées par les deux signaux géophysiques avec 0.3-0.8%vol de fusion partielle. L’équilibre textural entre les phases liquides et solides s’est révélé être fondamental pour la comparaison des mesures en laboratoire. Nous avons ensuite procédé à la première reproduction de fusion par déshydratation durant l’ascension des péridotites hydratées depuis la zone de transition mantellique vers le manteau supérieur (entre 12 et 14 GPa). Au cours de la fusion partielle, les signaux sismiques et électriques mesurés sont comparables aux observations géophysiques confirmant l’hypothèse de fusion au niveau de la LVL. Les taux de liquides impliqués à la base du manteau supérieur seraient alors modestes (< 2 %vol). La composition des magmas produits précise le rôle de filtre chimique de ce niveau situé entre les manteaux supérieur et profond. La densité estimée du magma confirme sa flottabilité neutre, favorisant la stabilité de ce niveau au cours des temps géologiques. Les analyses des éléments volatils et les modélisations des transferts d’hydrogène prouvent que ce niveau est un réservoir potentiel d’eau profond et favorise l’hypothèse d’une hydratation par la base du manteau supérieur. Enfin, des méthodes de mesure de diffusivité thermique (Angström, pulse) ont été adaptées à la presse multi-enclumes du LMV. Des procédures de traitement et des modélisations des transferts thermiques ont été développées Les premières mesures de diffusivité thermique de verres et liquides réalistes aux conditions mantelliques ont pu ainsi être réalisées. De plus, la caractérisation d’échantillons aux structures variées a pu être effectuée à l’aide de la méthode Angström (périclase, olivine, péridotite partiellement fondue)
The transport properties of mantle rocks are key parameters to qualitatively and quantitatively interpret direct and indirect geophysical information such as seismic velocities, heat fluxes and electromagnetic profiles across Earth’s and planetary interiors. The origins of upper mantle geophysical anomalies such as the Low Velocity Zone (70-150 km deep) and the Low Velocity Layer (350-410 km deep) are poorly known and require experimental constraints. In this PhD thesis, we have explored the electrical, seismic and thermal properties of realistic solid and partially molten peridotites via the development of geophysical in situ techniques. Performed at high pressures and temperatures in multi-anvil apparatus, our experiments allowed the characterization of the effect of melting on these different physical properties at mantle conditions. We performed the first experimental combination of electrical conductivity and sound wave velocity in a single multi-anvil experiment. Thanks to this technique, we reconciled electrical and seismic estimations of the melt fraction implied in the LVZ with 0.3-0.8 Vol.% of partial melting. The textural equilibration between melt and solid phases was found to be crucial for the comparison of laboratory estimations. We then realized the first reproduction of the dehydration melting process during the ascend of hydrous peridotites from the mantle transition zone to the upper mantle, between 12 and 14 GPa. Measurements during partial melting gave acoustic and electrical signals comparable to geophysical observations favoring partial melting explanation of the LVL anomaly. The implied melt fractions at upper mantle base were quantified to be moderate (<2 Vol.%). The chemical composition of produced melts confirmed the role of chemical filter of this melt layer located between upper and deep mantle. The calculated density confirmed the neutral buoyancy of the melt layer, making it a stable feature over geological times. Volatiles analyses and hydrogen transfer modeling confirmed this layer as a potential deep water reservoir and favored a bottom-up hydration of Earth’s upper mantle. Thermal diffusivity characterization techniques (Angström and pulse heating methods) were adapted to the LMV multi-anvil apparatus. Improved treatment procedures were elaborated for thermal transfer characterization under HP and HT conditions. The first thermal diffusivity characterization of glasses and melts at realistic mantle conditions were performed. In addition, thermal diffusivities of various samples (periclase, olivine, peridotite) were investigated with different structures (solid, solid+melt etc.) using Angström method

L'etude de la distribution des elements du groupe du platine (pge: ir, ru, rh, pt et pd) et de l'or dans les peridotites des massifs ultrabasiques orogeniques (ronda, beni bousera) permet de mettre en evidence le comportement differentiel de cu, pt et pd par rapport a ni, ir et ru au cours de la fusion partielle. Ce comportement resulte d'une extraction de cu, pt et pd par les liquides silicates sous forme d'une phase sulfuree. A l'inverse, ir et ru restent pieges dans les residus de fusion probablement sous forme de grains refractaires interstitiels. Le rh qui montre un comportement refractaire a ronda contrairement a beni bousera ne peut pas etre attribue a son incorporation dans le spinelle mais suggere plutot des fs#2 plus faibles dans les peridotites de ronda que dans celles des beni bousera. Par ailleurs, la distribution des pge dans les xenolites de peridotite a spinelle des basaltes alcalins (france, sud de l'espagne) est caracterisee par des spectres des teneurs normalisees au manteau relativement plats et ne semble pas etre influencee par le processus de fusion. Elle refleterait une refertilisation ou une retention d'une fraction sulfuree riche en pt et en pd par un manteau plus ou moins deprime, a la suite de multiples extractions basaltiques. Les spectres des teneurs en pge et en au de pente pd/ir positive et variable, couples aux compositions en elements majeurs et en traces, illustrent l'heterogeneite geochimique des pyroxenites des massifs de ronda et des beni bousera. Ces pyroxenites peuvent etre regroupees dans deux lignees distinctes. Une premiere (pyroxenites a grenat s. S. Et pyroxenites a spinelle s. L. ) est caracterisee par un rapport al#2o#3/feotot <2. 7. Les basses teneurs en ir, ru et rh peuvent etre reliees a une precipitation precoce de ces elements en reponse a une augmentation de fo#2 tandis que les teneurs en pt, pd et au sont gouvernees par les sulfures. Certaines similitudes entre les pyroxenites a grenat s. S et les pyroxenites a spinelle s. L. Suggerent un lien genetique entre ces deux facies. La seconde lignee, correspondant a des clinopyroxenites a grenat corindon tres alumineuses (al#2o#3/feotot > 2. 7), montre des niveaux de teneurs tres faibles, ou au contraire, tres riches en pge. La variation des teneurs observee ne concerne que les ppge et l'au et peut etre attribuee a une segregation d'une faible fraction de sulfures.

The existence, creation and modification of compositional heterogeneities within the Earth's upper mantle and their influences on the petrogenesis of primitive magmas have been highly debated for decades. The possible origins of mantle heterogeneities within the Earth's peridotite-dominated mantle are numerous and complex. One major mechanism invokes an important role for recycling of oceanic crust at subduction zones, and the storage of these recycled mafic (and sedimentary) materials in the deep mantle, before it is thought to be incorporated in the adiabatically upwelling upper mantle sources of some primitive mantle-derived magmas. This data is then combined with the results of earlier experimental investigations to constrain the complex melting behaviours of (residual) eclogite bodies in upwelling mantle, the nature of the interactions of their partial melts with ambient peridotitic mantle, and the nature of re-melting of such metasomatised mantle assemblages, and a brief evaluation of the contributions of these processes to the genesis of some primitive mantle-derived magmas. This study highlights that mantle heterogeneites caused by upwelling of subducted oceanic crust and its residues 'breed' an infinite number of various heterogeneities, ranging from Si02-oversaturated to Si02-undersaturated. To assess the influence of recycled oceanic crust on the creation of mantle heterogeneities and on the petrogenesis of primitive magmas in homogeneous, buoyant, upwelling peridotite-dominated mantle, high pressure experimental investigations of the melting behaviour of subducted oceanic crust and its residues will be presented. I track a sequential process in which melts are redistributed from the (initially) low temperature melting of average oceanic crust (Spandler et al., 2008), and then from the residues (nominally anhydrous eclogitic compositions) of such subducted oceanic crust (Res2 and REC, this study). This study establishes the phase and melting relations and compositions of reasonable melting residues, which are themselves derived directly from subducted oceanic crust at high pressures and temperatures (GA2; Spandler et al., 2008). In particular, this study has systematically determined the solidus temperatures, phase relations, and partial melt compositions xv during upwelling of both, a residual quartz/coesite-bearing eclogite (Res2) and a residual quartz/coesite-free eclogite (REC) from 5 to 3 GPa and 1500-1200{u00B0}C.

Peridotite in the earth's upper mantle undergoes polybaric, fractional melting as it rises adiabatically beneath mid-ocean spreading ridges. As liquid is continually extracted, peridotite becomes increasingly depleted in incompatible components. The amounts and compositions of partial melts of depleted peridotite are important parameters in models of MORB petrogenesis, but have not been well-constrained previously. I present partial melting experiments on a depleted peridotite composition at 10 kbar and 1250–1390°C. My experiments make use of small aggregates of glassy carbon particles into which partial melt is extracted at high temperature. I have been able to analyze low degree partial melts (<10%) and quantify the effects of incompatible element depletion on the melting behavior of peridotite. Special tests of the approach to equilibrium in this study confirm the validity of the aggregate melt extraction technique, which has sparked much debate in the literature (see Chapters 2 and 3 for details).

Melts of depleted peridotite differ in important ways from melts of fertile peridotite, mostly due to lower alkali contents and chemical consequences thereof. At low melt fractions, melts of depleted peridotite have less SiO₂, more CaO, and higher CaO/Al₂O₃ than melts of fertile peridotite at the same melt fraction. According to these results and others in the literature, solidus temperature is a linear function of incompatible major element content. Melt fraction at cpx-out is proportional to normative cpx in source peridotite.

Liquid compositions from this study are in good agreement with calculations using the quantitative models of Kinzler and Grove (1992a), Langmuir et al. (1992), and Ghiorso and Sack (1995). Calculations of polybaric, fractional melting of primitive mantle using the models of Langmuir et al. (1992) and Asimow (1997) indicate that about half of all liquid contributed to MORB is formed by partial melting of depleted peridotite.

The data presented in this thesis provide information about amounts and compositions of partial melts formed from depleted peridotite, an important upper mantle constituent beneath mid-ocean ridges, and can be used to improve quantitative models of MORB primary magma formation and further our understanding of MORB petrogenesis.

(1) The presence of small amounts of water dissolved within nominally-anhydrous minerals in the earth has significant effects on the chemistry of melting in the Earth’s mantle. Upwelling rock containing water will melt at greater depths than the same rock would if it were volatile-free, and the chemistry of these hydrous melts is expected to be quite different from that of anhydrous melts. We have developed new experimental techniques and applied them to melting under pressures where hydrous melting is of the greatest natural importance. We have also controlled the content of carbon, another volatile element, to produce melts from a range of compositions not previously sampled experimentally.

The liquid composition shows a number of interesting properties. Compared to anhydrous melts from the same pressure, it shows decreased modal olivine and increased silica content. Compared to carbon-containing experiments, it suggests that carbon interacts with water when both volatiles are present, and may act to oppose the effects of water. The presence of a hydrous liquid also has an important effect on the coexisting solid chemistry. High-aluminum clinopyroxenes are commonly observed at this pressure in anhydrous systems. However, in all of our volatile-containing experiments, the clinopyroxenes show a substantial decrease in aluminum content and an increase in calcium content. Many elements, including water, enter into the clinopyroxene structure by coupled substitution with aluminum, and thus reduced clinopyroxene aluminum content during natural melting will decrease the partitioning of these elements during melting.

(2) Variations in the modal abundance of olivine are the main mineralogical differences amongst typical Hawaiian lavas. A large quantity of olivine must crystallize from the Hawaiian parental liquids prior to eruption to produce the erupted lavas. The chemistry and abundance of these olivines reflects the behavior of the magmatic system in a number of ways. We have used the chemistry of these olivines and lavas to estimate the parental liquid compositions in Hawaiian volcanoes, to infer the relationship between the olivines and the lavas that host them, and to probe the evolution of Hawaiian volcanoes over time.

Peridotites occuring in orogenic massifs provide important insights into geochemical processes of the Earth's upper mantle by providing direct evidence of mantle evolution throughout Earth history. It has been previously demostrated (e.g. Medaris et al., 1990, 2005) that the uppermost tectonic unit of the Bohemian Massif - the Gföhl Nappe hosts a variety of peridotites that originated from different sources, including subcontinental lithosphere, suboceanic asthenosphere, and possible ultramafic layered intrusive complex. The Czech peridotites of the Gföhl Nappe has been divided into three groups, defined by Medaris et al. (1999), according to their chemical compositions, identity and relations of the aluminous phases, ortopyroxen compositions and estimated P-T conditions. According to Medaris et al. (2005) "Type I" peridotites - represented by Mohelno and Biskoupky bodies - equilibrated in low P-T regime (recording the highest equilibration temperatures - up to 1335 ⁰C at 29 kbar - among the Gföhl peridotites) consist of spinel peridotite with garnet appearing only at its margins. Peridotites are enclosed in granulites that have been extensively recrystallized mostly at amphibolite-facies conditions. Many studies have been done on this locality and a wide range of mineralogy and P-T histories has...

‘Minerals and the interior of the Earth’ looks at the role of minerals in plate tectonics during the processes of crystallization and melting. The size and range of minerals formed are dependent on the temperature and pressure of the magma during its movement through the crust. The evolution of the continental crust also involves granite formation and processes of metamorphism. Our understanding of the interior of the Earth is based on indirect evidence, mainly the study of earthquake waves. The Earth consists of concentric shells: a solid inner core; liquid outer core; a solid mantle divided into a lower mantle, a transition zone, and an upper mantle; and then the outer rigid lithosphere.

Ampferer-type subduction is a term that refers to the foundering of hyper-extended continental or embryonic oceanic basins (i.e., ocean-continent transitions) at passive continental margins. The lithospheric mantle underlying these rift basins is mechanically weaker, less dense, and more fertile than the lithospheric mantle underlying bounded continents. Therefore, orogens resulting from the closure of a narrow, immature extensional system are essentially controlled by mechanical processes without significant thermal and lithologic changes. Self-consistent, spontaneous subduction initiation (SI) due to the density contrast between the lithosphere and the crust of ocean-continent transitions is unlikely to occur. Additional far-field external horizontal forces are generally required for the SI. When the lithosphere subducts, the upper crust or serpentinized mantle and sediments separate from the lower crust, which becomes accreted to the orogen, while the lower crust subducts into the asthenosphere. Subduction of the lower crust, which typically consists of dry lithologies, does not allow significant flux-melting within the mantle wedge, so arc magmatism does not occur. As a result of melting inhibition within the mantle wedge during Ampferer-type subduction zones, the mantle beneath the resulting orogenic belts is fertile and thus has a high potential for magma generation during a subsequent breakup (i.e., magma-rich collapse).

Granites constitute the main rock components of the Earth’s continental crust, which suggested to be formed in variable geodynamics environments. The different types of granitic rocks, their compositional characteristics, tectonic settings and magma sources are outlined. Mineralogical classification of granites includes four rock types: tonalites, granodiorites, granite (monzogranite and syenogranites) and alkali-feldspar granites. Alphabetical classification subdivided granites into: I-type, S-type, A-type and M-type granites. Moreover, formation of granitic magmas requires distinctive geodynamic settings such as: volcanic arc granite (Cordilleran); collision-related granites (leucogranites); intra-plate and ocean ridge granites. The Eastern Desert of Egypt (ED) forms the northern part of Nubian Shield. Both older and younger granites are widely exposed in the ED. Old granites (OG) comprise tonalites and granodiorites of syn- to late-orogenic granitoid assemblages. They are calcalkaline, I-type, metaluminous and display island arc tectonic setting. Younger granites (YG) on the other hand, include granites, alkali-feldspar granites and minor granodiorites. They are of I- and A-type granites and of post-orogenic to anorogenic tectonic settings. The majority of the YG are alkaline, A-type granite and of within-plate tectonic setting (WPG). The A-type granites are subdivided into: A2-type postorogenic granites and A1-type anorogenic granites. Granite magma genesis involves: (a) fractional crystallization of mafic mantle-derived magmas; (b) anatexis or assimilation of old, upper crustal rocks (c) re - melting of juvenile mafic mantle – derived rocks underplating the continental crust. Generally, older I-type granitoids were interpreted to result from melting of mafic crust and dated at approximately 760–650 Ma, whereas younger granites suggested to be formed as a result of partial melting of a juvenile Neoproterozoic mantle source. Moreover, they formed from anatectic melts of various crustal sources that emplaced between 600 and 475 Ma.

ABSTRACT Prolonged slow cooling (average 1–3 °C/m.y.) of Ottawan phase granulite-facies gneisses (peak temperature ≥850 °C ca. 1090–1080 Ma) through the argon closure temperatures (TC) of hornblende ca. 980–920 Ma and biotite ca. 890–820 Ma in the western Grenville Province and in an inlier in the central Appalachians is well established, but its tectonic setting has not been systematically investigated. Here, the case is made that this slow cooling occurred in the suprasolidus cores of large metamorphic core complexes that were exhumed during mid-Ottawan (ca. 1050 Ma) extensional orogenic collapse. The ductile midcrustal metamorphic cores of the large metamorphic core complexes are overlain across gently dipping extensional detachments by a brittle-ductile cover composed of upper orogenic crust, parts of which preserve evidence of relict pre-Ottawan fabrics and peak prograde Ottawan temperatures of &lt;500 °C (TC of Ar in hornblende), collectively implying thermal, structural, and rheological decoupling across the detachments. Slow average rates of cooling of the orogenic midcrust for &gt;150 m.y. imply an anomalously hot upper mantle and mask short periods of more rapid cooling indicated by analyses of retrograde diffusional mineral zoning patterns. It is suggested that these slow average rates of cooling, coupled with slow average rates of exhumation of ≤0.1 km/m.y. modeled for one data set, were a result of decompression melting of rising asthenosphere and emplacement of voluminous mafic intrusions within or at the base of the crust, which reduced the buoyancy of the residual thinned lithosphere. This process is compatible with either delamination of subcontinental lithospheric mantle or slab rollback. The high-strain extensional detachments of the large metamorphic core complexes are sites of amphibolite-facies retrogression, suggesting a feedback between ingress of hydrous fluid, which was likely derived from beneath the detachment during crystallization of migmatite, and strain. Extensional juxtaposition of the hot midcrust (T &gt;850 °C) and cooler cover (T &lt;500 °C) across the detachments led to conductive heating of the base of the cover, locally raising its temperature above 500 °C, as recorded by amphibolite-facies metamorphism and young cooling ages. The slow cooling and exhumation of Grenvillian large metamorphic core complexes contrast with much faster rates in smaller metamorphic core complexes in other settings (e.g., North American Cordillera). The slow rates of these processes in large metamorphic core complexes are attributed to the prolonged high temperature and low viscosity of their metamorphic cores due to proximity of the asthenosphere, and to the intrusion of voluminous asthenospheric mafic magmas that both advected heat and reduced lithospheric buoyancy.

ABSTRACT The amalgamation of Laurentia’s Archean provinces ca. 1830 Ma was followed by ~700 m.y. of accretionary orogenesis along its active southeastern margin, marked by subduction of oceanic lithosphere, formation of arcs and back-arcs, and episodic accretion. This prolonged period of active-margin tectonic processes, spanning the late Paleoproterozoic and Mesoproterozoic eras, resulted in major accretionary crustal growth and was terminated by closure of the Unimos Ocean (new name). Ocean closure was associated with rapid motion of Laurentia toward the equator and resulted in continental collision that led to profound reworking of much of the accreted Proterozoic crust during the ca. 1090–980 Ma Grenvillian orogeny. The Grenvillian orogeny resulted in formation of a large, hot, long-duration orogen with a substantial orogenic plateau that underwent extensional orogenic collapse before rejuvenation and formation of the Grenville Front tectonic zone. The Grenvillian orogeny also caused the termination and inversion of the Midcontinent Rift, which, had it continued, would likely have split Laurentia into distinct continental blocks. Voluminous mafic magmatic activity in the Midcontinent Rift ca. 1108–1090 Ma was contemporaneous with magmatism in the Southwestern Laurentia large igneous province. We discuss a potential link between prolonged subduction of oceanic lithosphere beneath southeast Laurentia in the Mesoproterozoic and the initiation of this voluminous mafic magmatism. In this hypothesis, subducted water in dense, hydrous Mg-silicates transported to the bottom of the upper mantle led to hydration and increased buoyancy, resulting in upwelling, decompression melting, and intraplate magmatism. Coeval collisional orogenesis in several continents, including Amazonia and Kalahari, ties the Grenvillian orogeny to the amalgamation of multiple Proterozoic continents in the supercontinent Rodinia. These orogenic events collectively constituted a major turning point in both Laurentian and global tectonics. The ensuing paleogeographic configuration, and that which followed during Rodinia’s extended breakup, set the stage for Earth system evolution through the Neoproterozoic Era.

ABSTRACT The Providencia island group comprises an extinct Miocene stratovolcano located on a shallow submarine bank astride the Lower Nicaraguan Rise in the western Caribbean. We report here on the geology, geochemistry, petrology, and isotopic ages of the rocks within the Providencia island group, using newly collected as well as previously published results to unravel the complex history of Providencia. The volcano is made up of eight stratigraphic units, including three major units: (1) the Mafic unit, (2) the Breccia unit, (3) the Felsic unit, and five minor units: (4) the Trachyandesite unit, (5) the Conglomerate unit, (6) the Pumice unit, (7) the Intrusive unit, and (8) the Limestone unit. The Mafic unit is the oldest and forms the foundation of the island, consisting of both subaerial and subaqueous lava flows and pyroclastic deposits of alkali basalt and trachybasalt. Overlying the Mafic unit, there is a thin, minor unit of trachyandesite lava flows (Trachyandesite unit). The Breccia unit unconformably overlies the older rocks and consists of crudely stratified breccias (block flows/block-and-ash flows) of vitrophyric dacite, which represent subaerial near-vent facies formed by gravitational and/or explosive dome collapse. The breccias commonly contain clasts of alkali basalt, indicating the nature of the underlying substrate. The Felsic unit comprises the central part of the island, composed of rhyolite lava flows and domes, separated from the rocks of the Breccia unit by a flat-lying unconformity. Following a quiescent period, limited felsic pyroclastic activity produced minor valley-fill ignimbrites (Pumice unit). The rocks of Providencia can be geochemically and stratigraphically subdivided into an older alkaline suite of alkali basalts, trachybasalts, and trachyandesites, and a younger subalkaline suite composed dominantly of dacites and rhyolites. Isotopically, the alkali basalts together with the proposed tholeiitic parent magmas for the dacites and rhyolites indicate an origin by varying degrees of partial melting of a metasomatized ocean-island basalt–type mantle that had been modified by interaction with the Galapagos plume. The dacites are the only phenocryst-rich rocks on the island and have a very small compositional range. We infer that they formed by the mixing of basalt and rhyolite magmas in a lower oceanic crustal “hot zone.” The rhyolites of the Felsic unit, as well as the rhyolitic magmas contributing to dacite formation, are interpreted as being the products of partial melting of the thickened lower oceanic crust beneath Providencia. U-Pb dating of zircons in the Providencia volcanic rocks has yielded Oligocene and Miocene ages, corresponding to the ages of the volcanism. In addition, some zircon crystals in the same rocks have yielded both Proterozoic and Paleozoic ages ranging between 1661 and 454 Ma. The lack of any evidence of continental crust beneath Providencia suggests that these old zircons are xenocrysts from the upper mantle beneath the Lower Nicaraguan Rise. A comparison of the volcanic rocks from Providencia with similar rocks that comprise the Western Caribbean alkaline province indicates that while the Providencia alkaline suite is similar to other alkaline suites previously defined within this province, the Providencia subalkaline suite is unique, having no equivalent rocks within the Western Caribbean alkaline province.