Showing papers on "Partial melting published in 2002"

Growth of early continental crust controlled by melting of amphibolite in subduction zones

Abstract: It is thought that the first continental crust formed by melting of either eclogite or amphibolite, either at subduction zones or on the underside of thick oceanic crust. However, the observed compositions of early crustal rocks and experimental studies have been unable to distinguish between these possibilities. Here we show a clear contrast in trace-element ratios of melts derived from amphibolites and those from eclogites. Partial melting of low-magnesium amphibolite can explain the low niobium/tantalum and high zirconium/samarium ratios in melts, as required for the early continental crust, whereas the melting of eclogite cannot. This indicates that the earliest continental crust formed by melting of amphibolites in subduction-zone environments and not by the melting of eclogite or magnesium-rich amphibolites in the lower part of thick oceanic crust. Moreover, the low niobium/tantalum ratio seen in subduction-zone igneous rocks of all ages is evidence that the melting of rutile-eclogite has never been a volumetrically important process.

Origin of mesozoic adakitic intrusive rocks in the Ningzhen area of east China: Partial melting of delaminated lower continental crust?

Abstract: To the best of our knowledge, modern adakites have not been documented in a nonarc environment. We report geochemical and isotopic data for Early Cretaceous Anjishan adakitic intrusive rocks that are in a continental setting unrelated to subduction. The Anjishan adakitic intrusive rocks, which are exposed in the Ningzhen area of east China, have high Sr/Y and La/Yb ratios coupled with low Yb and Y as well as relatively high MgO contents and Mg numbers (Mg#; 0.4-0.6), similar to products from slab melting. However, low ∈ N d ( t ) values (-6.8 to-9.7) and high ( 8 7 Sr/ 8 6 Sr) i (0.7053-0.7066) are inconsistent with an origin by slab melting. The tectonics and geochemistry lead us to conclude that adakitic magmas were most likely derived from partial melting of mafic material at the base of the continental crust. High Sr/Y and La/Yb ratios of the adakitic intrusive rocks suggest that garnet was stable as a residual phase during partial melting, implying that the crustal thickness exceeded 40 km in the Early Cretaceous. The present thickness of the crust in the Ningzhen area is only 30 km, and therefore the crust appears to have been thinned by at least ∼10 km since the Early Cretaceous. The relatively high MgO contents and Mg# of the Anjishan intrusive rocks suggest that adakitic magmas interacted with mantle rocks, possibly coinciding with lower-crustal delamination, which would also account for the observed thinning.

The pMELTS: A revision of MELTS for improved calculation of phase relations and major element partitioning related to partial melting of the mantle to 3 GPa

Abstract: [1] We describe a newly calibrated model for the thermodynamic properties of magmatic silicate liquid. The new model, pMELTS, is based on MELTS [Ghiorso and Sack, 1995] but has a number of improvements aimed at increasing the accuracy of calculations of partial melting of spinel peridotite. The pMELTS algorithm uses models of the thermodynamic properties of minerals and the phase equilibrium algorithms of MELTS, but the model for silicate liquid differs from MELTS in the following ways: (1) The new algorithm is calibrated from an expanded set of mineral-liquid equilibrium constraints from 2439 experiments, 54% more than MELTS. (2) The new calibration includes mineral components not considered during calibration of MELTS and results in 11,394 individual mineral-liquid calibration constraints (110% more than MELTS). Of these, 4924 statements of equilibrium are from experiments conducted at elevated pressure (200% more than MELTS). (3) The pMELTS model employs an improved liquid equation of state based on a third-order Birch-Murnaghan equation, calibrated from high-pressure sink-float and shockwave experiments to 10 GPa. (4) The new model employs a revised set of end-member liquid components. The revised components were chosen to better span liquid composition-space. Thermodynamic properties of these components are optimized as part of the mineral-liquid calibration. Comparison of pMELTS to partial melting relations of spinel peridotite from experiments near 1 GPa indicates significant improvements relative to MELTS, but important outstanding problems remain. The pMELTS model accurately predicts oxide concentrations, including SiO2, for liquids from partial melting of MM3 peridotite at 1 GPa from near the solidus up to � 25% melting. Compared to experiments, the greatest discrepancy is for MgO, for which the calculations are between 1 and 4% high. Temperatures required to achieve a given melt fraction match those of the experiments near the solidus but are � 60� C high over much of the spinel lherzolite melting interval at this pressure. Much of this discrepancy can probably be attributed to overstabilization of clinopyroxene in pMELTS under these conditions. Comparison of pMELTS calculations to the crystallization and partial melting experiments of Falloon et al. [1999] shows excellent agreement but also suffers from exaggerated calculated stability of clinopyroxene. Finally, comparison of pMELTS

Secular changes in tonalite-trondhjemite-granodiorite composition as markers of the progressive cooling of Earth

Abstract: Archean tonalite-trondhjemite-granodiorite associations (TTG) are classically thought to generate through partial melting of hydrous metabasalts. However, the chemical composition of the least differentiated TTG parental magmas evolved from 4.0 to 2.5 Ga. During this interval, the Mg# as well as the Ni and Cr contents increased, which is interpreted as reflecting increased interactions between felsic melts generated by metabasalt melting and mantle peridotite. Similarly, (CaO + Na2O) and Sr also increased over time, thus reflecting an increase in the abundance of plagioclase in the melt residue. The presence or absence of residual plagioclase is interpreted in terms of melting depth. The demonstrated interaction between TTG parental magmas and the mantle rules out their genesis by fusion of previously underplated metabasalt and favors the melting of subducted slab material. At 4.0 Ga, Earth's geothermal gradient was sufficiently high to allow slab melting at shallow depths where plagioclase was stable. Consequently, due to the small thickness of the overlying mantle wedge, felsic magmas interacted little with the mantle. At 2.5 Ga, however, owing to lower geothermal gradients, the melting depth was greater and plagioclase became no longer stable in the thick mantle wedge overlying the subducted slab. As a result, felsic magmas reacted strongly with the mantle peridotite. The changes of TTG composition during Archean time can be thus interpreted as reflecting the progressive cooling of Earth.

Adakites: some variations on a theme

Abstract: Adakites were proposed over a decade ago to be products of the melting of young subducted oceanic crust. In fact, several new localities have been discovered since the original work documented approximately ten localities in modern arcs (e. g., southwestern Japan, Trans Mexican Volcanic Belt, etc.). But work over the past ten years has also shown that adakites can be generated by other processes during subduction (e. g., along the edge of tears in the subducting slab, remnant slabs left in the upper mantle, etc.). In addition, adakites appear to be associated with a suite of rocks including high-Mg andesites resulting from either adakite interaction with the mantle (Adak-type) or melting of the mantle during adakite interaction (Piip-type), niobium enriched arc basalts (NEAB) that are believed to be derived from the partial melting of a mantle metasomatized extensively by adakites, and possibly boninites (several researchers have found an adakite component in boninites). A new rock suite, the adakite metasomatic volcanic series, has been proposed to account for the various associations. In addition, a large number of NEAB have been found to contain ultramafic mantle xenoliths with clear evidence of reaction between ultradepleted mantle and adakites.

Subduction Fluxes of Water, Carbon Dioxide, Chlorine, and Potassium

Abstract: [1] The alteration of upper oceanic crust entails growth of hydrous minerals and loss of macroporosity, with associated large-scale fluxes of H2O, CO2, Cl−, and K2O between seawater and crust. This age-dependent alteration can be quantified by combining a conceptual alteration model with observed age-dependent changes in crustal geophysical properties at DSDP/ODP sites, permitting estimation of crustal concentrations of H2O, CO2, Cl−, and K2O, given crustal age. Surprisingly, low-temperature alteration causes no net change in total water; pore water loss is nearly identical to bound water gain. Net change in total crustal K2O is also smaller than expected; the obvious low-temperature enrichment is partly offset by earlier high-temperature depletion, and most crustal K2O is primary rather than secondary. I calculate crustal concentrations of H2O, CO2, Cl−, and K2O for 41 modern subduction zones, thereby determining their modern mass fluxes both for individual subduction zones and globally. This data set is complemented by published flux determinations for subducting sediments at 26 of these subduction zones. Global mass fluxes among oceans, oceanic crust, continental crust, and mantle are calculated for H2O, Cl−, and K2O. Except for the present major imbalance between sedimentation and sediment subduction, most fluxes appear to be at or near steady state. I estimate that half to two thirds of subducted crustal water is later refluxed at the prism toe; most of the remaining water escapes at subarc depths, triggering partial melting. The flux of subducted volatiles, however, does not appear to correlate with either rate of arc magma generation or magnitude of interplate earthquakes.

Unexpectedly high-PGE chromitite from the deeper mantle section of the northern Oman ophiolite and its tectonic implications

Abstract: Unusually high, platinum-group element (PGE) enrichments are reported for the first time in a podiform chromitite of the northern Oman ophiolite. The chromitite contains ≤1.5 ppm of total PGE, being highly enriched in the IPGE subgroup (Ir, Os and Ru) and strongly depleted in the PPGE subgroup (Rh, Pt and Pd). Its platinum-group minerals (PGMs) are classified into three types arranged in order of abundance: (1) sulphides (Os-rich laurite, laurite–erlishmanite solid solution and an unnamed Ir sulphide), (2) alloys (Os–Ir alloy and Ir–Rh alloy), and (3) sulpharsenides (irarsite and hollingworthite). The high PGE concentrations are observed only in a discordant chromitite deep in the mantle section, which has high-Cr# (>0.7) spinel with an olivine matrix. All the other types of chromitite (in the Moho transition zone (MTZ) and concordant pods in the deeper mantle section) are poor in PGEs and tend to have spinels with lower Cr# (up to 0.6). This diversity of chromitite types suggests two stages of magmatic activity were responsible for the chromitite genesis, in response to a switch of tectonic setting. The first is residual from lower degree, partial melting of peridotite, which produced low-Cr#, PGE-poor chromitites at the Moho transition zone and, to a lesser extent, within the mantle, possibly beneath a fast-spreading mid-ocean ridge. The second chromitite-forming event involves higher degree partial melting, which produced high-Cr#, PGE-rich discordant chromitite in the upper mantle, possibly in a supra-subduction zone setting.

Boninitic volcanism in the Oman ophiolite: Implications for thermal condition during transition from spreading ridge to arc

Abstract: The discovery of boninite, a typical high-MgO andesite, in the Oman ophiolite is reported. The boninites in the Oman ophiolite occur as lavas and dikes of the Alley volcanic sequence that overlie or crosscut the spreading-ridge-derived lavas (Geotimes volcanic sequence) and sheeted dikes. The phenocryst mineral assemblage and the major and trace element compositions observed for these boninites resemble those of the Izu-Mariana forearc boninites, indicating that the Alley boninites represent primitive melt generated by partial melting of hydrous peridotite. The occurrence of boninite provides strong thermal and chemical constraints on the formation of the Oman ophiolite that require hot, hydrous shallow mantle (>1250 °C at <30 km depth) to have underlain the proto-Oman ophiolite at the time of boninite generation. The initiation of subduction of the young, hot oceanic lithosphere (and obduction of the future Oman ophiolite) near the spreading ridge and the resultant melting of the highly depleted, shallow-mantle wedge metasomatized by slab-derived fluid represent the most favorable mechanism for the genesis of the Alley boninites.

The fluid-absent partial melting of a zoisite-bearing quartz eclogite from 1.0 to 3.2 GPa : Implications for melting in thickened continental crust and for subduction-zone processes

Abstract: Fluid-absent melting experiments on a zoisiteand phengite-bearing INTRODUCTION eclogite (omphacite, garnet, quartz, kyanite, zoisite, phengite and Zoisite or clinozoisite is present in many high-pressure rutile) were performed to constrain the melting relations of these eclogites, and many occurrences are known from the hydrous phases in natural assemblages, as well as the melt and Scandinavian Caledonides (e.g. Holsnoy: Austrheim & mineral compositions produced by their breakdown. From 1·0 to Mork, 1988; Jamtveit et al., 1990; Western gneiss region: 3·2 GPa the solidus slopes positively from 1·5 GPa at 850°C to Griffin et al., 1985; Seve Nappe: Kullerud et al., 1990). 2·7 GPa at 1025°C, but bends back at higher pressures to 975°C Indeed, most eclogites worldwide contain minor amounts at 3·2 GPa. The melt fraction is always low and the melt of zoisite or other hydrous phases, according to the compositions always felsic and become increasingly so with increasing compilations of Smith (1988) and Carswell (1990). During pressure. The normative Ab–An–Or compositions of the initial prograde eclogitization, zoisite forms by breakdown of melts vary from tonalites at 1·0 GPa to tonalite–trondhjemites at the anorthite component of plagioclase in the presence 1·5 GPa, adamellites at 2·1 and 2·7 GPa, and to true granites of a hydrous fluid phase. The origin of these hydrous at 3·2 GPa. At pressures 2·5 GPa zoisite and phengite break fluids is controversial. One possibility is that they are down more or less simultaneously. At 3·2 GPa and 1000°C zoisite introduced from below during continental collision, for is unreacted whereas phengite is absent, so that the first formed melt example, if wet continental sedimentary rocks are deeply at these conditions is granitic. Our experiments show that if subducted. During subduction of oceanic crust zoisite sufficiently high temperatures (of the order of 1000°C) are attained, forms by prograde metamorphism of hydrothermally zoisiteand phengite-bearing eclogites can produce small fractions altered oceanic crust, which in its upper levels contains of silicic melts of a wide range of compositions. These melts are low-temperature hydrous Ca-rich phases (e.g. Poli & rich in water and, probably, in Sr and other incompatible elements, Schmidt, 1997). At the gabbro to dyke transition zone so that they can act as metasomatic agents in the mantle wedge. the temperature and pressure are high enough to allow

Source contamination and mantle heterogeneity in the genesis of Italian potassic and ultrapotassic volcanic rocks: Sr–Nd–Pb isotope data from Roman Province and Southern Tuscany

Abstract: The Tyrrhenian border of the Italian peninsula has been the site of intense magmatism from Pliocene to recent times. Although calc-alkaline, potassic and ultrapotassic volcanism overlaps in space and time, a decrease of alkaline character in time and space (southward) is observed. Alkaline ultrapotassic and potassic volcanic rocks are characterised by variable enrichment in K and incompatible elements, coupled with consistently high LILE/HFSE values, similar to those of calc-alkaline volcanic rocks from the nearby Aeolian arc. On the basis of mineralogy and major and trace element chemistry two different arrays can be recognised among primitive rocks; a silica saturated trend, which resulted in formation of leucite-free mafic rocks, and a silica undersaturated trend, charactrerised by leucite-bearing rocks. Initial 87Sr/86Sr and 143Nd/144Nd values of Italian ultrapotassic and potassic mafic rocks range from 0.70506 to 0.71672 and from 0.51173 to 0.51273, respectively. 206Pb/204Pb values range between 18.50 and 19.15, 207Pb/204Pb values range between 15.63 and 15.70, and 208Pb/204Pb values range between 38.35 and 39.20. The general eSr vs. eNd array, along with crustal lead isotopic values, clearly indicates that a continental crustal component has played an important role in the genesis of these magmas. The main question is where this continental crustal component has been acquired by the magmas. Volcanological and petrologic data indicate continental crustal contamination to be a leading process along with fractional crystallisation and magma mixing. Considering, however, only the samples thought to represent primary magmas, which have been in equilibrium with their mantle source, a clearer picture emerges. A large variation of eSr vs. eNd is still observed, with eSr from −2 to +180 and eNd from + 2 to −12. A bifurcation of this array is observed in the samples that plot in the lower right quadrant, with mafic leucite-bearing Roman Province rocks buffered at eSr = + 100 whereas the mafic leucite-free potassic and ultrapotassic rocks point to strongly radiogenic Sr compositions. We may argue that mafic leucite-bearing Roman Province rocks point to eSr and eNd values similar to those of Miocene carbonate sediments whereas mafic leucite-free potassic and ultrapotassic rocks point to a silicate upper crust end-member. Lead isotopes plot well inside the field of island arcs, overlapping the values of pelagic sediments as well, but bifurcation between the samples north and south of Rome is observed. The main characteristic for the mantle source of Italian potassic and ultrapotassic magmas is the clear upper crustal signature acquired prior to partial melting through metasomatic agents released by the subducted slab. In addition, one lithospheric mantle source in the north and an asthenospheric mantle source, pointing to an HIMU reservoir, in the south were recognised. The chemical and isotopic differences observed between the northern and southern sectors of the magmatic region were possibly due to the presence of a carbonate-rich component in the crustal enriching agent in the south. One crustal component might have been generated by melting of silicate metasedimentary rocks or sediments from an ancient subducted slab. The second one might reflect the activity of mostly CO2-rich fluid released more recently by the incipient subduction of carbonate sedimentary rocks.

Partial melting of sulfide ore deposits during medium- and high-grade metamorphism

Abstract: Minor elements, such as Ag, As, Au and Sb, have commonly been remobilized and concentrated into discrete pockets in massive sulfide deposits that have undergone metamorphism at or above the middle amphibolite facies. On the basis of our observations at the Broken Hill orebody in Australia and experimental results in the literature, we contend that some remobilization could be the result of partial melting. Theoretically, a polymetallic melt may form at temperatures as low as 300°C, where orpiment and realgar melt. However, for many ore deposits, the first melting reaction would be at 500°C, where arsenopyrite and pyrite react to form pyrrhotite and an As–S melt. The melt forming between 500° and 600°C, depending on pressure, will be enriched in Ag, As, Au, Bi, Hg, Sb, Se, Sn, Tl, and Te, which we term low-melting point chalcophile metals. Progressive melting to higher T ( ca . 600°–700°C) will enrich the polymetallic melt progressively in Cu and Pb. The highest-T melt (in the upper amphibolite and granulite facies) may also contain substantial Fe, Mn, Zn, as well as Si, H2O, and F. In our model, we suggest that the presence of polymetallic melts in a metamorphosed massive sulfide orebody is recorded by: (1) localized concentrations of Au and Ag, particularly in the presence of low-melting-point metals, (2) multiphase sulfide inclusions in high-T gangue minerals, (3) low interfacial angles between sulfides or sulfosalts suspected of crystallizing from the melt and those that are likely to have been restitic, (4) sulfide and sulfosalt fillings of fractures, and (5) Ca- and Mn-rich selvages around massive sulfide deposits. Using these criteria, we identify 26 ore deposits worldwide that may have melted. We categorize them into three chemical types: Pb- and Zn-rich deposits, either of SEDEX or MVT origin, Pb-poor Cu–Fe–Zn deposits, and disseminated Au deposits in high-grade terranes.

Mineralogy of the mid-ocean-ridge basalt source from neodymium isotopic composition of abyssal peridotites

Abstract: Inferring the melting process at mid-ocean ridges, and the physical conditions under which melting takes place, usually relies on the assumption of compositional similarity between all mid-ocean-ridge basalt sources. Models of mantle melting therefore tend to be restricted to those that consider the presence of only one lithology in the mantle, peridotite. Evidence from xenoliths and peridotite massifs show that after peridotite, pyroxenite and eclogite are the most abundant rock types in the mantle. But at mid-ocean ridges, where most of the melting takes place, and in ophiolites, pyroxenite is rarely found. Here we present neodymium isotopic compositions of abyssal peridotites to investigate whether peridotite can indeed be the sole source for mid-ocean-ridge basalts. By comparing the isotopic compositions of basalts and peridotites at two segments of the southwest Indian ridge, we show that a component other than peridotite is required to explain the low end of the (143)Nd/(144)Nd variations of the basalts. This component is likely to have a lower melting temperature than peridotite, such as pyroxenite or eclogite, which could explain why it is not observed at mid-ocean ridges.

Peridotites from the Mariana Trough: first look at the mantle beneath an active back-arc basin

Abstract: Two dives of the DSV Shinkai 6500 in the Mariana Trough back-arc basin in the western Pacific sampled back-arc basin mantle exposures. Reports of peridotite exposures in back-arc basin setting are very limited and the lack of samples has hindered our understanding of this important aspect of lithospheric evolution. The Mariana Trough is a slow-spreading ridge, and ultramafic exposures with associated gabbro dykes or sills are located within a segment boundary. Petrological data suggest that the Mariana Trough peridotites are moderately depleted residues after partial melting of the upper mantle. Although some peridotite samples are affected by small-scale metasomatism, there is no evidence of pervasive post-melting metasomatism or melt-mantle interaction. Spinel compositions plot in the field for abyssal peridotites. Clinopyroxenes show depletions in Ti, Zr, and REE that are intermediate between those documented for peridotites from the Vulcan and Bouvet fracture zones (the American- Antarctic and Southwest Indian ridges, respectively). The open-system melting model indicates that the Ma- riana Trough peridotite compositions roughly corre- spond to theoretical residual compositions after 7% near-fractional melting of a depleted MORB-type upper mantle with only little melt or fluid/mantle interactions. The low degree of melting is consistent with alow magma budget, resulting in ultramafic exposure. We infer that the mantle flow beneath the Mariana Trough Central Graben is episodic, resulting in varying magma supply rate at spreading segments.

Constraints on the bulk composition and root foundering rates of continental arcs: A California arc perspective

Abstract: [1]Garnetpyroxenites are the most common deep lithospheric xenolith assemblages found in Miocene volcanic rocks that erupted through the central part of the Sierra Nevada batholith. Elemental concentrations and isotope ratios are used to argue that the Sierra Nevada granitoids and the pyroxenite xenoliths are the melts and the residues/cumulates, respectively, resulting from partial melting/fractional crystallization at depths exceeding 35-40 km. The estimated major element chemistry of the protolith resembles a basaltic andesite. Effectively, at more than about 40 km depth, batholith residua are eclogite facies rocks. Radiogenic and oxygen isotope ratios measured on pyroxenites document unambiguously the involvement of Precambrian lithosphere and at least 20-30% (mass) of crustal components. The mass of the residual assemblage was significant, one to two times the mass of the granitic batholith. Dense garnet pyroxenites are prone to foundering in the underlying mantle. An average removal rate of 25-40 km 3 /km Myr is estimated for this Cordilleran-type arc, although root loss could have taken place at least in part after the cessation of arc magmatism. This rate is matched by the average subcrustal magmatic addition of the arc (∼23-30 km 3 /km Myr), suggesting that the net crustal growth in this continental arc was close to zero. It is also suggested that in order to develop a convectively removable root, an arc must have a granitoid melt thickness of at least 20-25 km. Residues of thinner arcs should be mostly in the granulite facies; they are not gravitationally unstable with respect to the underlying mantle.