Partial Melting Experiments Research Articles

Partial melting of garnet lherzolite with water and carbon dioxide at 3 GPa using a new melt extraction technique: implications for intraplate magmatism

The origin and source rocks of alkali-rich and SiO2-undersatured magmas in the Earth’s upper mantle are still under debate. The garnet signature in rare earth element patterns of such magmas suggests a garnet-bearing source rock, which could be garnet lherzolite or garnet pyroxenite. Partial melting experiments were performed at 2.8 GPa and 1345–1445 °C in a piston-cylinder using mixtures of natural lherzolite with either 0.4 wt% H2O and 0.4 wt% CO2 or 0.7 wt% H2O and 0.7 wt% CO2. Different designs of AuPd capsules were used for melt extraction. The most successful design included a pentagonally shaped disc placed in the top part of the capsule for sufficient melt extraction. The degrees of partial melting range from 0.2 to 0.04 and decrease with decreasing temperature and volatile content. All samples contain olivine and orthopyroxene. The amounts of garnet and clinopyroxene decrease with increasing degree of partial melting until both minerals disappear from the residue. Depending on the capsule design, the melts quenched to a mixture of quench crystals and residual glass or to glass, allowing measurement of the volatile concentrations by Raman spectroscopy. The compositions of the partial melts range from basalts through picrobasalts to foidites. Compared to literature data for melting of dry lherzolites, the presence of H2O and CO2 reduces the SiO2 concentration and increases the MgO concentration of partial melts, but it has no observable effect on the enrichment of Na2O in the partial melts. The partial melts have compositions similar to natural melilitites from intraplate settings, which shows that SiO2-undersaturated intraplate magmas can be generated by melting of garnet lherzolite in the Earth’s upper mantle in the presence of H2O and CO2.

Anatexis at the roof of an oceanic magma chamber at IODP Site 1256 (equatorial Pacific): an experimental study

Replenished axial melt lenses at fast-spreading mid-oceanic ridges may move upward and intrude into the overlying hydrothermally altered sheeted dikes, resulting in high-grade contact metamorphism with the potential to trigger anatexis in the roof rocks. Assumed products of this process are anatectic melts of felsic composition and granoblastic, two-pyroxene hornfels, representing the residue after partial melting. Integrated Ocean Drilling Program Expeditions 309, 312, and 335 at Site 1256 (eastern equatorial Pacific) sampled such a fossilized oceanic magma chamber. In this study, we simulated magma chamber roof rock anatectic processes by performing partial melting experiments using six different protoliths from the Site 1256 sheeted dike complex, spanning a lithological range from poorly to strongly altered basalts to partially or fully recrystallized granoblastic hornfels. Results show that extensively altered starting material lacking primary magmatic minerals cannot reproduce the chemistry of natural felsic rocks recovered in ridge environments, especially elements sensitive to hydrothermal alteration (e.g., K, Cl). Natural geochemical trends are reproduced through partial melting of moderately altered basalts from the lower sheeted dikes. Two-pyroxene hornfels, the assumed residue, were reproduced only at low melting degrees (<20 vol%). The overall amphibole absence in the experiments confirms the natural observation that amphibole is not produced during peak metamorphism. Comparing experimental products with the natural equivalents reveals that water activity (aH2O) was significantly reduced during anatectic processes, mainly based on lower melt aluminum oxide and lower plagioclase anorthite content at lower aH2O. High silica melt at the expected temperature (1000–1050 °C; peak thermal overprint of two-pyroxene hornfels) could only be reproduced in the experimental series performed at aH2O = 0.1.

The Fate of Sulfur During Fluid-Present Melting of Subducting Basaltic Crust at Variable Oxygen Fugacity

To constrain the effect of redox state on sulfur transport from subducting crust to mantle wedge during fluid-present melting and the stability of sulfur-bearing phases in the downgoing ocean crust, here we report high-pressure phase equilibria experiments on a H2O-saturated mid-ocean ridge basalt with 1 wt % S at variable oxygen fugacity (⁠⁠). Double-capsule experiments were conducted at 2·0 and 3·0 GPa and 950–1050°C, using Co–CoO, Ni–NiO, NixPd1–x–NiO, and Fe2O3–Fe3O4 external buffers. Sulfur content at sulfide saturation (SCSS) or sulfur content at sulfate saturation (SCAS) of experimental hydrous partial melts was measured by electron microprobe. All experiments were fluid-saturated and produced either pyrrhotite- or anhydrite-saturated assemblages of silicate glass, clinopyroxene, garnet, and rutile or titanomagnetite, ± amphibole ± quartz ± orthopyroxene. The silicate partial melt composition evolves from rhyolitic at 950°C to trachydacitic and trachyandesitic at 1050°C with increasing ⁠. At pyrrhotite saturation, melt S contents range from ∼30 ppm S at < FMQ – 1 to ∼500 ppm S at FMQ < ≤ FMQ + 1·1, whereas at anhydrite saturation (⁠ ≥ FMQ + 2·5) melt S concentrations range from ∼700 ppm S to 0·3 wt % S. Mass-balance calculations suggest that the aqueous fluid phase at equilibrium may contain as much as ∼15 wt % S at 1050°C at pyrrhotite saturation (≤ FMQ + 1·1), in agreement with previous estimates, and up to 8 wt % S at anhydrite saturation. Our data also show that decreases markedly with increasing at pyrrhotite saturation, from several thousand at < FMQ – 1 to ∼ 200–400 at FMQ < ≤ FMQ + 1·1, owing to the increase of melt S content. At anhydrite saturation, is very low (<100) but increases with decreasing temperature, in an opposite way to previous observations at pyrrhotite saturation. As a consequence, at T ≤ 900°C, might be in the range 200 ± 100, irrespective of ⁠. The present study confirms that slab partial melts saturated with pyrrhotite are unable to efficiently transport S from slab to mantle wedge, and suggests that slab partial melts in equilibrium with anhydrite also have very limited power to enrich the mantle wedge in S. Importantly, slab-derived aqueous fluids appear to be efficient vectors for the transport of sulfur from slab to mantle wedge at all ⁠. Therefore, S transfer from ocean crust to wedge mantle is not dependent and could take place over a range of conditions, and oxidized slab conditions are not necessarily required to enrich the mantle wedge in S. Finally, depending on the initial amount of sulfur in the slab, the proportion of residual anhydrite and pyrrhotite in the dehydrated slab below the region of formation of arc magmas is likely to be significant and may efficiently be recycled into the deep mantle.

The geochemical signature of a felsic injection complex in the continental crust: Opinaca Subprovince, Quebec

The Archean Opinaca Subprovince of the Superior Province in northern Québec is an injection complex consisting of lower granulite facies metasedimentary terrane that was pervasively injected with a large volume of leucogranitic melt which forms innumerable thin dykes and veins.The typical microstructure of most leucogranites can be divided into two groups, 1) one has an open framework constructed of K-feldspar crystals in which the interstices are filled with albite-rich plagioclase and quartz and, 2) a microstructure defined by a framework of albite-rich plagioclase crystals in which the interstices are filled by quartz and, in some cases, small pink garnets. However, pegmatitic and aplitic textures also occur.The leucogranites that form the injection complex are all SiO2-rich (above 70 wt.%) and have low (FeOt+MgO+TiO2) contents compared to granitic melts obtained from partial melting experiments. The mafic minerals were fractionated from the magma before its emplacement into the injection complex as leucogranite. The leucogranites form a coherent trend on major element diagrams that is best explained by the removal of a cumulate dominated by K-feldspar from a granitic “initial melt” composition. The cumulate rocks are enriched in K2O, whereas the residual melts are richer in SiO2 and Na2O.Based on these results, leucogranites with a K-feldspar framework are interpreted as cumulate rocks. This indicates that the early-crystallising calcic plagioclase had already been fractionated from the magma before its injection into the complex. The leucogranites have rare earth element (REE) patterns typical of feldspar accumulation. The residual melts extracted after the K-feldspar framework developed are those that have the framework of albite-rich plagioclase. These have REE patterns typical of highly evolved melts from which accessory minerals have also been fractionated.The highly evolved nature of the leucogranites that were injected into granulite facies host rocks in the Opinaca Subprovince has several important implications for crustal evolution. 1) The large volume of evolved melts emplaced at depth released a significant amount of H2O into the adjacent crust, thus injection complexes should be considered as a potentially important source of H2O in the middle crust that is capable of rehydrating the granulite terranes into which they are injected. 2) A corollary of the evolved nature of the leucogranite in the injection complex is that there should be evidence of accumulation of the early fractionated material still deeper in the crust. 3) These findings are natural examples that corroborate predictions from numerical modelling which suggest that the whole process from melting to crystallisation of leucogranite can be confined to deep levels of the crust, below, or close to, the solidus. This has important impact on our general view of the transfer of leucogranites in the continental crust which is the main mechanism of crustal differentiation.

Microstructure of Cr&amp;ndash;V&amp;ndash;Mo Steel Processed by Recrystallization and Partial Melting and Its Effect on Mechanical Properties

Recrystallization and partial melting (RAP) experiments were conducted on Cr–V–Mo steel using a multistage hot compression test machine. The dendritic microstructure of the cast material was refined owing to the recrystallization and austenization that occurred during RAP. The refinement of the microstructure is affected by the preparation of the initial cast billet. A higher cooling rate during casting results in a finer RAP-processed globular microstructure. Owing to the core segregation during casting and the phase segregation during RAP, alloying elements are distributed inhomogeneously in RAP-processed specimens. Optimal post heat treatments were designed to increase the homogeneity of the microstructure of RAP-processed specimens. Quenching from 1323 K followed by tempering at 833 K improve the consistency of the Vickers hardness, toughness, and high-temperature wear resistance of RAP-processed specimens.

Density and sound speed measurements on model basalt (An–Di–Hd) liquids at one bar: New constraints on the partial molar volume and compressibility of the FeO component

Density and sound speed measurements were obtained over a wide range of temperature for three model basalt liquids in the An–Di–Hd (CaAl2Si2O8–CaMgSi2O6–CaFeSi2O6) system. High-temperature (1585–1838 K) double-bob density measurements were combined with low-temperature (943–930 K) measurements at the limiting fictive temperature for each sample to provide liquid volume data over a temperature interval of ∼900 K. In addition, relaxed sound speeds were obtained with a frequency sweep acoustic interferometer from 1665–1876 K. An ideal mixing model for molar volume, thermal expansivity, and isothermal compressibility recovers the new data from this study and leads to the following fitted values (±2σ) at 1723 K for V¯FeO (12.86±0.32 cm3/mol), ∂V¯FeO/∂T((3.69±1.16)×10−3 cm3/mol-K) and β¯T,FeO((4.72±0.46)×10−2 GPa−1). These volumetric properties for the FeO component are estimated to reflect Fe2+ in an average coordination of 5.7 (±0.2), based on the relationship between V¯FeO and Fe2+ coordination derived by a comparison to mineral molar volumes (Guo et al., 2013). Application of these volumetric data to a calculation of the pressure dependence of the Fe–Mg exchange reaction between orthopyroxene and basaltic liquid results in a small decrease in Fe–MgKD with pressure. In contrast, partial melting experiments of peridotite show a small increase in Fe–MgKD(opx-liq) with pressure (e.g., Walter, 1998). This difference in the pressure dependence is proposed to reflect the role of alkalis in reducing the average coordination number of Fe2+ toward five compared to the alkali-free model basalt compositions in this study, thus changing the volume and compressibility of the FeO liquid component. The results from this study may be most appropriately applied to lunar basalts, which are impoverished in alkalis.

The reaction mechanism of fluid-induced partial melting of gabbro in the oceanic crust

Hydrous partial melting of oceanic gabbro as a consequence of fluid/rock interaction at very high temperatures is regarded as an important process for the formation of felsic lithologies within the oceanic crust. In order to investigate the principal reaction mechanism of hydrous partial melting in detail, water-saturated partial melting experiments were conducted in an internally heated pressure vessel in the temperature range 860-1020 °C and 200 MPa pressure, fO 2 ~NNO, with mm-sized blocks of unmodified olivine gabbro (microrocks) used as a coarse-grained starting material. Although far from global equilibrium, the observed phase relations and compositional trends are as expected from a similar experimental survey designed for achieving global equilibrium, using a fine-grained powder of the same starting gabbro. The experimental results allow us to formulate a general fluid-induced reaction for hydrous partial melting of olivine gabbro as a function of temperature: at T ≤ 980°C; olivine + clinopyroxene + plagioclase(I) + H 2 O) amphibole + orthopyroxene + plagioclase(II) + hydrous plagiogranitic melt; at T ≥ 980°C; olivine(I) + clinopyroxene(I) + plagioclase(I) + H 2 O) olivine (II) + clinopyroxene(II) + plagioclase(II) + hydrous plagiogranitic melt: Textural features and compositional characteristics obtained in these partial-melting experiments on microrock are in good agreement with corresponding features of late-stage assemblages in some natural oceanic gabbros from slow-spreading ridges, implying that some of the natural late-stage parageneses can be better explained as residual phases left back after a hydrous partial-melting event rather than by crystallization of a late evolved percolating melt. One experiment performed at 940 °C produced pools of silicate melt containing droplets of a second melt strongly enriched in typical trace elements (P, Zr, Cl as proxies), which is interpreted as an effect of liquid immiscibility. The observation of unusually high concentrations of some trace elements within the first melts is in contradiction with trace-element concentrations derived from modelling using equilibrium partition coefficients. A model is implied that during the very first stage of hydrous partial melting, submicrometre sized phases like apatite, zircon and oxides crystallized on grain boundaries from a late-stage melt in the host gabbro may be forced into the very first melt which incorporated their trace-element inventory. A possible consequence is that the very first melts of fluid-induced partial melting of oceanic gabbro may be regarded as one possible source for the contamination of natural MORB or of corresponding felsic lithologies generated at mid-ocean ridges.

A petrologic, thermodynamic and experimental study of brachinites: Partial melt residues of an R chondrite-like precursor

The primitive achondrites provide a window into the initial melting of asteroids in the early solar system. The brachinites are olivine-dominated meteorites with a recrystallized texture that we and others interpret as evidence of partial melting and melt removal on the brachinite parent body. We present a petrologic, thermodynamic and experimental study of the brachinites to evaluate the conditions under which they formed and test our hypothesis that the precursor material to the brachinites was FeO-rich compared to the precursors of other primitive achondrites. Petrologic analysis of six brachinites (Brachina, Allan Hills (ALH) 84025, Hughes 026, Elephant Moraine (EET) 99402, Northwest Africa (NWA) 3151, and NWA 4969) and one brachinite-like achondrite (NWA 5400) shows that they are meteorites with recrystallized texture that are enriched in olivine (⩾80vol.%) and depleted in other minerals with respect to a chondritic mineralogy. Silicates in the brachinites are FeO-rich (Fa32–36). Brachinite-like achondrite Northwest Africa 5400 is similar in mineralogy and texture to the brachinites but with a slightly lower FeO-content (Fa30). Thermodynamic calculations yield equilibration temperatures above the Fe,Ni–FeS cotectic temperature (∼950°C) for all meteorites studied here and temperatures above the silicate eutectic (∼1050°C) for all but two. Brachina formed at an fO2 of ∼IW, and the other brachinites and NWA 5400 formed at ∼IW−1. All the meteorites show great evidence of formation by partial melting having approximately chondritic to depleted chondritic mineralogies, equilibrated mineral compositions, and recrystallized textures, and having reached temperatures above that required for melt generation. In an attempt to simulate the formation of the brachinite meteorites, we performed one-atmosphere, gas-mixing partial melting experiments of R4 chondrite LaPaz Ice Field 03639. Experiments at 1250°C and an oxygen fugacity of IW−1 produce residual phases that are within the mineralogy and mineral compositions of the brachinites. These experiments provide further evidence for the formation of brachinites as a result of partial melting of a chondritic precursor similar in mineralogy and mineral compositions to the R chondrites.

Water and the composition of Martian magmas

Shergottites, the most abundant martian meteorites, represent the best source of information about Mars’ mantle and its dissolved water. If the mantle was wet, magmatic degassing could have supplied substantial water to the martian surface early in its history. Researchers have attempted to reconstruct the volatile contents of shergottite parental magmas, with recent analyses confirming that the shergottites contained significant water. However, water is not a passive tracer; it directly affects magma chemistry and physical properties. Deciphering the history of water on Mars requires understanding how that water affected the chemistry of the shergottites and how they fit within Mars’ geologic history. Both topics present difficulties, as no shergottite-like rock has been found in stratigraphic context and there is debate over the timing of eruptions of shergottite-like magmas. Partial melting experiments on terrestrial basalts and new data from orbiters and rovers on Mars provide the information needed to overcome these difficulties and explain the role of water in shergottite magmas. Here we show that shergottite compositions and their martian geologic context can be explained by melting of an originally wet mantle that degassed over time. We also demonstrate that models for the evolution of the martian mantle that do not consider water fail to account for the shergottite compositions, surface distributions, and ages. Finally, we suggest that dehydration of the martian mantle has led to changes in magmatic chemistry over time, with shergottites representing melts of water-bearing mantle and rocks similar to nakhlites representing melts of other mantle sources.

Metamorphism and melting of picritic crust in the early Earth

Partial melting experiments with models of Archean oceanic crust (MAOC; with 11, 13 and 15wt.% MgO) are used to investigate the role of metamorphism and melting of primary picritic compositions in the formation of TTG-like melts and continental crust on the early Earth. The approach investigates the possibility that the average early crust composition was comparatively MgO-rich and evolved to lower magnesium content during the secular cooling of the Earth. High-pressure partial melting experiments indicate a transition of melt compositions from aluminous basaltic melts in MAOC 15 to predominantly tonalitic melts in MAOC 11 and higher melting temperatures with increasing magnesium in the bulk composition. Tonalitic melts were generated in MAOC 11 and 13 at pressures ≥12.5kbar along with the residual phases garnet+clinopyroxene+plagioclase±quartz (±orthopyroxene in the presence of quartz and at lower pressures) in the absence of amphibole. Basaltic melts were generated at pressures ≤15kbar predominantly in the presence of granulite facies residues such as amphibole±garnet±plagioclase+orthopyroxene that lack quartz in all MAOC compositions.The tonalitic melts generated in MAOC 11 and 13 indicate that thicker oceanic crust with more magnesium than that of a modern MORB is a viable source for the generation of early Archean high-Si, low-MgO melts, and therefore TTG-like continental crust in the Archean. The favoured settings for the generation of these melts at pressures up to 15kbar are the base of oceanic crust much thicker than today or melting of slabs in shallow subduction zones, both without interaction of the melts with the mantle during passage to the surface. Tonalitic melts may have formed in deeper subduction zones at 20kbar beyond plagioclase stability but it is unlikely that such melts could migrate to shallower levels without further mantle interaction. This process may have become more important during the progressive cooling of the Earth.

Partial melting of lower crust at 10–15 kbar: constraints on adakite and TTG formation

The pressure–temperature (P–T) conditions for producing adakite/tonalite–trondhjemite–granodiorite (TTG) magmas from lower crust compositions are still open to debate. We have carried out partial melting experiments of mafic lower crust in the piston-cylinder apparatus at 10–15 kbar and 800–1,050 °C to investigate the major and trace elements of melts and residual minerals and further constrain the P–T range appropriate for adakite/TTG formation. The experimental residues include the following: amphibolite (plagioclase + amphibole ± garnet) at 10–15 kbar and 800 °C, garnet granulite (plagioclase + amphibole + garnet + clinopyroxene + orthopyroxene) at 12.5 kbar and 900 °C, two-pyroxene granulite (plagioclase + clinopyroxene + orthopyroxene ± amphibole) at 10 kbar and 900 °C and 10–12.5 kbar and 1,000 °C, garnet pyroxenite (garnet + clinopyroxene ± amphibole) at 13.5–15 kbar and 900–1,000 °C, and pyroxenite (clinopyroxene + orthopyroxene) at 15 kbar and 1,050 °C. The partial melts change from granodiorite to tonalite with increasing melt proportions. Sr enrichment occurs in partial melts in equilibrium with <20 wt% plagioclase, whereas depletions of Ti, Sr, and heavy rare earth elements (HREE) occur relative to the starting material when the amounts of residual amphibole, plagioclase, and garnet are >20 wt%, respectively. Major elements and trace element patterns of partial melts produced by 10–40 wt% melting of lower crust composition at 10–12.5 kbar and 800–900 °C and 15 kbar and 800 °C closely resemble adakite/TTG rocks. TiO2 contents of the 1,000–1,050 °C melts are higher than that of pristine adakite/TTG. In comparison with natural adakite/TTG, partial melts produced at 10–12.5 kbar and 1,000 °C and 15 kbar and 1,050 °C have elevated HREE, whereas partial melts at 13.5–15 kbar and 900–1,000 °C in equilibrium with >20 wt% garnet have depressed Yb and elevated La/Yb and Gd/Yb. It is suggested that the most appropriate P–T conditions for producing adakite/TTG from mafic lower crust are 800–950 °C and 10–12.5 kbar (corresponding to a depth of 30–40 km), whereas a depth of >45–50 km is unfavorable. Consequently, an overthickened crust and eclogite residue are not necessarily required for producing adakite/TTG from lower crust. The lower crust delamination model, which has been embraced for intra-continental adakite/TTG formation, should be reappraised.

Geochemical constraints on a mixed pyroxenite–peridotite source for East Pacific Rise basalts

The extent to which the lithological heterogeneity of the mantle source controls mid-oceanic ridge basalt (MORB) compositions remains largely debated. The recycling of oceanic crust into the mantle is widely considered to be an important cause of mantle source heterogeneity. We provide data for major and trace elements and Sr–Nd–Pb isotopes of thirteen mid-ocean ridge basalt (MORB) samples from the East Pacific Rise (EPR) to investigate the mantle source heterogeneity introduced by recycled oceanic crust (ROC). Relationships of Sr–Nd–Pb isotopes and incompatible trace element ratios, such as Sm/Yb and Hf/Zr, and ratios of Zn/Fe, Co/Fe and (Pb/Pb⁎)N indicate a geochemically and lithologically heterogeneous mantle source. Mixing between components from depleted mantle (DMM) and enriched ROC-derived pyroxenite is required to explain these relationships. The relatively high Sm/Yb and the good correlation of 87Sr/86Sr vs. Sr/Nd are in concert with variable contributions from a pyroxenite component derived from recycled plagioclase-fractionated E-MORB. The narrow range of MgO contents (7.49wt.% to 8.58wt.%) and the correlations of major element contents with 87Sr/86Sr and Zn/Fe ratios indicate the control of a mixed pyroxenite–peridotite source on the major element compositions of these lavas. To further investigate the major element contribution from the lithologically heterogeneous mantle source (peridotite/pyroxenite), major element compositions were corrected for fractional crystallization both to a given MgO wt.% and a given liquidus temperature. After correction for fractional crystallization, the FeOT and Na2O increase and CaO decreases with increasing melt proportion from the enriched end-member. Based on the observations of this study and the results from partial melting experiments on pyroxenite and peridotite, a melting model, in which magma forms by mixing a deep and enriched melt (pyroxenite melt) with a shallow and depleted DMM melt (peridotite melt), provides a likely mechanism that can produce the major element systematics.

Water-present eclogite melting to produce Earth's early felsic crust

The geochemistry of well preserved Archaean Tonalite–Trondhjemite–Granodiorite (TTG) rocks, such as the trondhjemites of the Meso-Archaean Barberton granitoid–greenstone terrain in South Africa, provides insight into the origins of Earth's early felsic continental crust. This is particularly well demonstrated by the high pressure (HP)-type of Archaean TTG magmas, where the geochemistry requires that they are formed by high-pressure melting of a garnet-rich eclogitic source. This has consequently been interpreted as evidence for the formation of these magmas by anatexis of the upper portions of slabs within Archaean subduction zones. Most of the experimental data relevant to TTG genesis has been generated by studies of fluid-absent melting of metabasaltic sources. However, water drives arc magmatism within Phanerozoic subduction zones and thus, understanding the behaviour of water in Archaean subduction zones, may have considerable value for understanding the genesis of HP-type Paleo- to Meso-Archaean trondhjemites. Consequently, this study investigates the role of high-pressure water-present melting of an eclogite-facies starting material, in the production of Paleo- to Meso-Archaean HP-type TTG melts. Water-saturated partial melting experiments were conducted between 1.9 and 3.0GPa; and, 870°C and 900°C. The melting reaction is characterised by the breakdown of the jadeite molecule in the clinopyroxene, together with quartz and water, to form melt in conjunction with a less sodic clinopyroxene: Qtz+Cpx1+Grt1+H2O=Melt+Cpx2+Grt2 and produced melt compositions that, if allowance is made for the low Mg# and almost K2O-free character of the starting material, are an excellent major element match with HP-type TTG compositions. In two of the experimental run products, melt segregated efficiently from residual crystals, allowing for the measurement of a full range of trace elements, via Laser Ablation Inductively Coupled Plasma Mass Spectroscopy (LA-ICP-MS). The experimental glasses produced are Light Rare Earth Element (LREE) (24–44ppm La; 50–86ppm Ce) and Zr (101–228ppm) enriched; and Heavy Rare Earth Element (HREE) (0.4ppm Yb; 50.9 La/Ybn), Y (2.2–3.2ppm; 82–142 Sr/Y), Sm (2.1–4.3ppm; 40.9–50.8 Zr/Sm) and Nb (1.7–2.8ppm) depleted compared to the DSE4 starting material. Thus, the REE element compositions of the experimental glasses are also similar to HP-type TTG compositions, and in more particular they are strikingly similar to Meso-Archaean Barberton trondhjemites. Additionally, we propose that due to Cpx being a major reactant, Ni and Cr contents (4–29ppm Ni and 43ppm Cr) of the glasses which where analysed for these elements, are within the compositional range displayed by HP-type Archaean TTG. This study suggests that the geochemistry of HP-type Archaean trondhjemites may reflect high pressure water-present partial melting of an eclogite facies metabasaltic source. Consequently, we propose that water-present melting of an eclogitic source is a viable mechanism for the genesis of this component of Paleo- to Meso-Archaean TTG crust.

Hadean greenstones from the Nuvvuagittuq fold belt and the origin of the Earth's early continental crust

To investigate formation of the Earth9s earliest continental crust, partial-melting experiments were conducted (at 900–1100 °C and 0.5–3.0 GPa) on two greenstones from the 4.3 Ga Nuvvuagittuq complex of Quebec, Canada. For comparison, experiments were also conducted on a compositionally similar but modern arc volcanic (a Tongan boninite). At 1.5–3.0 GPa and 950–1100 °C, the experimentally produced melts are compositionally similar to the tonalite-trondhjemite-granodiorite (TTG) granitoids that compose most of Earth9s early continental crust, including a 3.66 Ga tonalite that encloses the Nuvvuagittuq Complex. Because the degree of melting needed to produce the TTG-like melts is comparatively high (>30%), the relative concentrations of most incompatible elements in the melts are similar to those in their greenstone parent rocks. These greenstones have compositional affinities with modern subduction zone magmas and do not resemble mid-oceanic ridge basalts. That arc-like mafic rocks could have been selectively involved in TTG formation (in spite of their volumetrically subordinate status in most greenstone terrains) must reflect tectonic circumstances that were specific to their generation. These must have enabled accumulations sufficiently deep to melt at the 1.5–3.0 GPa needed to generate TTG magmas from eclogitic sources. They are also likely to have been related to some form of crustal recycling whereby mafic crust and water were returned to the mantle and arc-like mafic magmas generated as a consequence. To what degree these circumstances replicated modern plate tectonics is difficult to say, but it seems likely that, as in the modern Earth, the Hadean crust was organized into different tectonic environments and that one of these gave rise to the first continental crust.

Microstructures of melt inclusions in anatectic metasedimentary rocks

AbstractThe occurrence of crystallized and glassy melt inclusions (MI) in high‐grade, partially melted metapelites and metagraywackes has opened up new possibilities to investigate anatectic processes. The present study focuses on three case studies: khondalites from the Kerala Khondalite Belt (India), the Ronda migmatites (Spain), and the Barun Gneiss (Nepal Himalaya). The results of a detailed microstructural investigation are reported, along with some new microchemical data on the bulk composition of MI. These inclusions were trapped within peritectic garnet and ilmenite during crystal growth and are therefore primary inclusions. They are generally isometric and very small in size, mostly ≤15 μm, and only rarely reaching 30 μm; they occur in clusters. In most cases inclusions are crystallized (‘nanogranites’) and contain a granitic phase assemblage with quartz, feldspar and one or two mica depending on the particular case study, commonly with accessory phases (mainly zircon, apatite, rutile). In many cases the polycrystalline aggregates that make up the nanogranites show igneous microstructures, e.g. granophyric intergrowths, micrographic quartz in K‐feldspar and cuneiform rods of quartz in plagioclase. Further evidence for the former presence of melt within the investigated inclusions consists of melt pseudomorphs, similar to those recognized at larger scale in the host migmatites. Moreover, partially crystallized inclusions are locally abundant and together with very small (≤8 μm) glassy inclusions may occur in the same clusters. Both crystallized and partially crystallized inclusions often display a diffuse nanoporosity, which may contain fluids, depending on the case study. After entrapment, inclusions underwent limited microstructural modifications, such as shape maturation, local necking down processes, and decrepitation (mainly in the Barun Gneiss), which did not influence their bulk composition. Re‐homogenized nanogranites and glassy inclusions show a leucogranitic and peraluminous composition, consistent with the results of partial melting experiments on metapelites and metagraywackes. Anatectic MI should therefore be considered as a new and important opportunity to understand the partial melting processes.

Continental exhumation triggered by partial melting at ultrahigh pressure

Research Article| December 01, 2011 Continental exhumation triggered by partial melting at ultrahigh pressure L. Labrousse; L. Labrousse 1UPMC U. Paris 6, ISTeP, F-75005 Paris, France2CNRS, UMR 7193, F-75005 Paris, France Search for other works by this author on: GSW Google Scholar G. Prouteau; G. Prouteau 3ISTO, UMR 6113, CNRS/U. Orléans, 1A rue de la Férollerie, F-45071 Orleans, France Search for other works by this author on: GSW Google Scholar A.-C. Ganzhorn A.-C. Ganzhorn 1UPMC U. Paris 6, ISTeP, F-75005 Paris, France2CNRS, UMR 7193, F-75005 Paris, France Search for other works by this author on: GSW Google Scholar Author and Article Information L. Labrousse 1UPMC U. Paris 6, ISTeP, F-75005 Paris, France2CNRS, UMR 7193, F-75005 Paris, France G. Prouteau 3ISTO, UMR 6113, CNRS/U. Orléans, 1A rue de la Férollerie, F-45071 Orleans, France A.-C. Ganzhorn 1UPMC U. Paris 6, ISTeP, F-75005 Paris, France2CNRS, UMR 7193, F-75005 Paris, France Publisher: Geological Society of America Received: 05 Apr 2011 Revision Received: 20 Jul 2011 Accepted: 03 Aug 2011 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2011 Geological Society of America Geology (2011) 39 (12): 1171–1174. https://doi.org/10.1130/G32316.1 Article history Received: 05 Apr 2011 Revision Received: 20 Jul 2011 Accepted: 03 Aug 2011 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 L. Labrousse, G. Prouteau, A.-C. Ganzhorn; Continental exhumation triggered by partial melting at ultrahigh pressure. Geology 2011;; 39 (12): 1171–1174. doi: https://doi.org/10.1130/G32316.1 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 Partial melting textures, observed in most continental crust buried in ultrahigh-pressure (UHP) conditions, have mostly been related to their retrograde evolution during exhumation in collisional orogens. Analysis of leucosomes from the Western Gneiss Region (WGR, Norway) UHP and HP domains in the Caledonides show a wide scatter of their chemistries, from early ones close to trondhjemites restricted to UHP domains, to granites in late occurrences or associated with HP domains. Nearly trondhjemitic compositions compare with hydrous melts produced in felsic systems at high pressure (>2 GPa) and moderate temperature (<900 °C). Partial melting experiments at higher temperatures or in dry conditions produce granitic glasses similar to late leucosomes from the WGR. Comparison of pressure-temperature paths for Caledonian eclogites with melting and dehydration reactions for the surrounding gneiss suggest that (1) the continental crust remained partially hydrated during its subduction to ultrahigh pressure, and (2) partial melting reactions producing the trondhjemitic melts started as soon as the WGR rocks reached their hydrated solidus, at the peak pressure recorded by the eclogites. The limited partial melting degree at the peak conditions induced weakening of the continental crust, decoupling from the lithospheric root, and initiation of exhumation. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Experimental investigation on low-degree dehydration partial melting of biotite gneiss and phengite-bearing eclogite at 2 GPa

The ultrahigh-pressure (UHP) eclogite and gneiss from the Dabie (大别)-Sulu (苏鲁) oro- gen experienced variable degrees of partial melting during exhumation. We report here dehydration partial melting experiments of biotite gneiss and phengite-bearing eclogite at 2 GPa and 800-950 ℃. Our results show that the partial melting of gneiss is associated with the breakdown of biotite into almandine-rich garnet starting at 900 ℃. About 10% granitic melt can be produced at 950 ℃. In con- trast, the partial melting of phengite-bearing eclogite exists at slightly lower temperatures (800-850 ℃). The melt fraction is in general more in biotite gneiss than in phengite-bearing eclogite under similar pressure and temperature conditions. Both melts are rich in silica and alkali, but poor in FeO, MgO and CaO. These results suggest that low-degree partial melting of gneiss and eclogite is often associated with dehydration of hydrous mineral, such as micas. The dehydration temperature and melt composi- tion can place important constraints on the partial melting phenomena (granitic leucosome and multi-phase mineral inclusions) recorded in UHP rocks.

Compositional trends of Icelandic basalts: Implications for short-length scale lithological heterogeneity in mantle plumes

Lithological variations in the mantle source regions under mid-ocean ridges and ocean islands have been proposed to play a key role in controlling melt generation and basalt composition. Here we combine compositional observations from Icelandic basalts and modelling of melting of a bi-lithologic peridotite-pyroxenite mantle to demonstrate that, while short-lengthscale major element variation is present in the mantle under Iceland, source heterogeneity does not make an important contribution to excess melt production. By identifying the major element characteristics of endmember Icelandic melts, we find enriched melts to be characterised by low SiO2 and CaO, but high FeO. We quantitatively compare endmember compositions to experimental partial melts generated from a range of lithologies, pressures and melt fractions. This comparison indicates that a single source composition cannot account for all the major element variation; depleted Icelandic melts can be produced by depleted peridotite melting, but the major element composition of enriched melts is best matched by melting of mantle sources that have been refertilised by the addition of up to 40% mid-ocean ridge basalt. The enriched source beneath Iceland is more fusible than the source of depleted melts, and as such will be over-represented in accumulated melts compared with its abundance in the source. Modelling of peridotite-pyroxenite melting, combined with our observational constraints on the composition of the Icelandic mantle, indicates that crustal thickness variations in the North Atlantic must be primarily due to mantle temperature and flow field variations.

Encapsulation of Dye Molecules and Nanoparticles in Hollow Organogel Fibers of a Nonchiral Polyurethane Model Compound

We present the case of a nonchiral organogelator that forms hollow fibers and encapsulates silver nanoparticles (SNP) and a dye molecule. The biscarbamate molecule (a model compound for polyurethanes), which has two hydrogen-bonding motifs symmetrically attached to n-dodecyl side chains (C(12)), gels benzonitrile with hollow fibrillar morphology. The C(12) molecules form sheets that eventually wrap into hollow fibers to form the gel network. Herein, two-component gels were prepared with C(12) as one component and SNP, phthalocyanine (Pc), or perylene (Pe) as the other. Microscopic analysis and partial melting experiments confirmed the inclusion of the silver nanoparticles and phthalocyanine into the hollow fibers. On the other hand, Pe molecules tend to form crystals at the outer surface of the C(12) fibers, which results in a significant increase in the width of the gel fibers. This difference in the behavior of Pc and Pe molecules were accounted for by their crystal geometry and significantly different crystal growth rate compared with that of C(12) fiber formation in the gels. Pc crystallizes in a needle shape that facilitates occlusion in the gel fibers, whereas Pe forms large platelets. X-ray diffraction and spectroscopic analysis of the two-component gels along with their neat components confirmed that there was no change in the packing behavior of the Pc and Pe molecules in the gels. Therefore, these are examples of two-component physical gels in which the Pc crystals are occluded within the hollow fibers of C(12) by physical mixing of the components without the aid of any inter-molecular interactions between the different components. We have thus shown that lumen-loaded gel fibers with nanoparticles and dye molecules can be prepared by the two-component gel route, provided that the above growth rate, shape, and size conditions are satisfied.

Carbonate-fluxed Melting of MORB-like Pyroxenite at 2{middle dot}9 GPa and Genesis of HIMU Ocean Island Basalts

We present partial melting experiments on a carbonate-added, mid-ocean ridge basalt (MORB)-like pyroxenite composition (G2C; 5 wt % CO2). Experiments were conducted at 2·9 GPa and 1000^15008C and the resulting partial melt compositions were compared with those of alkalic ocean island basalts (OIBs). The solidus is estimated between 1000 and 10508C and the liquidus is between 1450 and 14758C. The subsolidus assemblage is cpx þ garnet þ rutile þ calcio-dolomitic solid solution, and the near-solidus melt is carbonatitic (5 5 wt % SiO2, 51· 3w t % TiO2,50· 5w t % Al2O3 ,3 1wt %5CaO525 wt %). At 1245^ 12758C, with the disappearance of rutile, a carbonated basaltic melt is found to coexist with carbonatitic melt, cpx, and garnet. The silicate melts are alkalic basalts with SiO2 of � 44^47 wt % on a volatile-free basis, and the melt becomes most silica-poor and CO2rich at the temperature of complete mixing of carbonate and silicate melt (i.e. at 1345^13758C). The onset of carbonated silicate melting in our study is � 60^708C cooler than the solidus of the carbonate-free MORB-pyroxenite at a similar pressure. G2C-derived carbonated silicate partial melts are similar to nephelinitic to basanitic ocean island basalts in general, and those derived from the HIMU mantle end-member in particular.The key similarities in major and minor element signatures include low SiO2 and highTiO2 ,F eO*, CaO, and Na2O.The main discrepancies between G2C partial melts and natural alkalic OIBs are higher Al2O3, lower CaO/Al2O3, and lower MgO of the former. We hypothesize that such discrepancies might be resolved if carbonated MORB-like pyroxenite produces partial melts at somewhat higher pressures and if a hybrid peridotite^carbonated pyroxenite source is considered. Geodynamic consideration of carbonated silicate melting of pyroxenite bodies beneath ocean islands suggests that volatile-enriched alkalic OIBs with the HIMU signature are probably generated from subducted, carbonated crust over a depth range, with the onset of melting as deep as 180^200 km, for a potential temperature of � 15008C. However, rather than a direct decompression of carbonated ocean crust, the key processes involved in the generation of a carbonated silicate melt in equilibrium with MORB-pyroxenite may involve melt^rock reaction and melt^melt mixing.