Mechanisms of melt extraction during lower crustal partial melting

Progressive vapour‐absent partial melting of a closed rock system increases melt pressure due to an expansion in the volume of the mineral plus melt assemblage. For a locally closed system, we quantify the melt pressure increase per increment of partial melting of a metapelite using phase equilibria modelling and combine it with Mohr–Coulomb theory to examine the interplay between melt pressure and fracture behaviour. It is shown that very small increments of vapour‐absent partial melting (<1%) increase melt pore pressure by tens of MPa leading to inevitable brittle failure of locally closed systems. Fracturing will affect these systems, even if initially limited to the scale of a few grains, and a connected microfracture network will enhance permeability as partial melting progresses. This will lead to a conditionally open system, potentially limiting accumulation of melt in the source. Repeated and cyclic fracture as temperature progressively increases will drive migration of the melt into sites of low fluid pressure at all scales. Crystal‐plastic creep processes create deformation‐induced dilatancy gradients that dominate over buoyancy forces at all scales in the melt source. Brittle and ductile deformation therefore cooperate in the extraction of melt. Enhanced porosity and permeability in ductile shear zones result in lower fluid pressure, providing a potentially important driving force for melt migration and drainage ‘up’ shear zones and along larger scale fluid pressure gradients in the crust.     

Wetting behavior of partial melts during crustal anatexis: the distribution of hydrous silicic melts in polycrystalline aggregates of quartz

The equilibrium distribution of hydrous silicic melts in polycrystalline aggregates of quartz was characterized in a series of partial melting and melt distribution experiments in the systems quartz-albite-orthoclase-H₂O and quartz-anorthite-H₂O, at 650 to 1000 MPa and 800 to 900° C. Near-equilibrium textures in these experiments are characterized by very low quartz-quartz-melt wetting angles, and by a substantial number of thin melt films along grain boundaries. Wetting angles in the H₂O-saturated experiments are as follows: 18° at 800° C-1000 MPa, and 12° at 900° C-1000 MPa in the granitic system; 18° at 850° C-650 MPa, 15° at 900° C-650 MPa, and 15° at 900° C-1000 MPa in the quartzanorthite system. In the granitic system at 900° C-1000 MPa, a decrease of H₂O content in melt from 17 wt% (at saturation) to 6 wt%, results in a slight increase of wetting angle from 12° to 16°. These low wetting angles — and the observation that many grain boundaries are wetted by melt films-indicate that the ratio of quartz-quartz to quartz-melt interfacial energies (_ss/_s1) is high: 2. Secondary electron imaging of fracture surfaces of melt-poor samples provided a three-dimensional insight into the geometry of melt; at low melt fraction, melt forms an interconnected network of channels along grain edges, as predicted for isotropic systems with wetting angles below 60°. This high-permeability geometry suggests that the segregation of granitic melts is not as sluggish as previously anticipated; simple compaction calculations for a permeability range of 10^-12 to 10^-9 m² indicate that segregation may operate at low to moderate melt fractions (below 30 vol. %), within relatively short time-scales, i.e., 10⁵ to 10⁶ years. Quartzmelt textures show significant deviations from the equilibrium geometries predicted for isotropic partially molten systems. The most consistent deviation is the pervasive development of crystallographically-controlled, planar faces of quartz; these faces provide definitive evidence for non-isotropic quartz-melt surface energy. For most silicates other than quartz, the grain-scale distribution of partial melts deviates even more significantly from equilibrium distributions in isotropic systems; accordingly, in order to describe adequately melt distributions in most natural source regions, the equilibrium model should be modified to account for anisotropy of solid-liquid interfacial energy.Contribution CNRS-INSU-DBT n^o 651     

The generation of quartz-normative melts and corundum-bearing restites by crustal anatexis: petrogenetic modelling based on an example from the Lewisian of North-West Scotland

Abstract Rare layers of an aluminous, muscovite-rich rock from the Lewisian Complex at Stoer, North-West Scotland, display evidence which suggests that the rock has undergone local partial melting to form quartz-bearing veins and a corundum-bearing restite. The assemblages observed in these rocks match those predicted by modelling in the system KAlO₂-NaAlO₂-Al₂O₃-SiO₂-H₂O (KNASH) where certain bulk compositions melt peritectically to give corundum-bearing restites and quartz-normative melts. Study of the model system shows that the observed parageneses could have formed from a range of bulk compositions with a variety of possible values of a H₂O which could have been internally or externally buffered. The KNASH petrogenetic grid, together with another in the system CaO-Na₂O-FeO-Al₂O₃-SiO₂-H₂O (CNFASH), allows the P–T path of the rocks to be delineated and an estimate to be made of the conditions at the peak of metamorphism as > 11 kbar and 900-925°C. This estimate is in agreement with P–T estimates using thermobarometric methods on adjacent lithologies: The activity of H₂O in the system throughout metamorphism is calculated to have been >0.3.     

Geochronology and geochemistry of leucosomes in the North Dabie Terrane, East China: implication for post-UHPM crustal melting during exhumation

Migmatites are widespread in the North Dabie ultrahigh-pressure metamorphic terrane (NDT) of Dabie orogen, East China. Idiomorphic and poikilitic amphibole grains in both leucosome and melanosome contain inclusions of plagioclase, quartz and biotite, suggesting formation of leucosome by fluid-present melting of biotite + plagioclase + quartz-bearing protoliths at P = 5–7 kbar, T = 700–800 °C. Precise SIMS zircon U–Pb dating indicates that migmatization of Dabie orogen initiated at ~140 Ma and lasted for ~10 Ma, coeval with the formation of low-Mg^# adakitic intrusions in Dabie orogen. Based on mineralogical, petrographic and geochemical data, leucosomes in NDT can be subdivided into three groups. (1) High La/Yb_(N)–Medium Sr/Y group (Group I), whose high Dy/Yb_(N) but medium Sr/Y ratios are caused by amphibole and plagioclase residual during partial melting of dioritic to granodioritic gneisses. (2) Low La/Yb_(N)–Low Sr/Y group (Group II), whose flat HREE patterns are produced by entrainment of peritectic amphiboles into melts derived from partial melting of dioritic gneiss. (3) High La/Yb_(N)–High Sr/Y and Eu^# group (Group III), whose extremely high Sr and Eu but low other REE concentrations are caused by accumulation of plagioclase and quartz. Although Group I and III fall in the adakitic fields on La/Yb_(N)–Yb_(N) and Sr/Y–Y diagrams, they are chemically distinct from contemporary high-pressure adakitic intrusions in Dabie orogen in a series of geochemical indexes, for example, lower Dy/Yb_(N) and/or Sr/Y ratios at given La/Yb_(N) ratio, lower Sr/CaO ratios, lower Rb concentration but higher K/Rb ratios. Therefore, leucosomes are produced by anatexis of the exhumed ultrahigh-pressure metamorphic rocks at middle crustal level, instead of partial melting of thickened lower crust with garnet-rich and plagioclase-poor residual. The coeval occurrence of migmatites and high-pressure adakitic intrusions in Dabie orogen indicates large-scale partial melting of middle to thickened lower crustal column in the early Cretaceous. The required heat source may be the mantle heat conducting through the lithospheric mantle whose lower parts have been convectively removed.     

Anhydrous partial melting of an iron-rich mantle I: subsolidus phase assemblages and partial melting phase relations at 10 to 30 kbar

Anhydrous partial melting experiments, at 10 to 30 kbar from solidus to near liquidus temperature, have been performed on an iron-rich martian mantle composition, DW. The DW subsolidus assemblage from 5 kbar to at least 24 kbar is a spinel lherzolite. At 25 kbar garnet is stable at the solidus along with spinel. The clinopyroxene stable on the DW solidus at and above 10 kbar is a pigeonitic clinopyroxene. Pigeonitic clinopyroxene is the first phase to melt out of the spinel lherzolite assemblage at less than 20°C above the solidus. Spinel melts out of the assemblage about 50°C above the solidus followed by a 150° to 200°C temperature interval where melts are in equilibrium with orthopyroxene and olivine. The temperature interval over which pigeonitic clinopyroxene melts out of an iron-rich spinel lherzolite assemblage is smaller than the temperature interval over which augite melts out of an iron-poor spinel lherzolite assemblage. The dominant solidus assemblage in the source regions of the Tharsis plateau, and for a large percentage of the martian mantle, is a spinel lherzolite.     

Intergranular diamonds derived from partial melting of crustal rocks at ultrahigh-pressure metamorphic conditions

By reporting for the first time intergranular diamond in quartz–feldspar (Qtz–Kfs) aggregates, the processes of metamorphic diamond formation have to be reconsidered. Based on their Kfs/Qtz ratio, the texture of these aggregates are proposed to result from ‘granitic’ melt with a calculated composition that corresponds well with that of experimental data for the pelitic system. Taking into account experiments on CO₂ solubility in silicate melt under ultrahigh‐pressure conditions, a granitic melt is further suggested to act as a crystallization medium as well as a transport medium for producing metamorphic diamond.     

榴辉岩部分熔融过程中的同位素分馏

本文对榴辉岩部分熔融过程中不同同位素体系是否存在分馏这一当前研究热点进行了综述。榴辉岩作为研究洋陆俯冲、超高压变质以及壳幔相互作用的主要岩石类型,其部分熔融与地壳增生、板片折返过程以及俯冲隧道中元素的迁移分配等具有紧密的联系。作为典型的高压-超高压变质岩石,榴辉岩可通过俯冲带将壳源信息携带至地幔深部,影响地幔的化学组成,并可在大洋玄武岩中得以表现。近些年,随着仪器分析技术的发展,实验研究和理论计算均表明榴辉岩部分熔融过程中稳定同位素可以产生显著的分馏。作为常见的放射性成因子体同位素和传统稳定同位素Sr-Nd-Hf-O被广泛应用于源区示踪、岩浆混合以及结晶分异等过程。但目前有研究指出,在非平衡熔融过程中,熔体和源区的Sr-Nd-Hf-O同位素可发生解耦,导致二者的同位素组成不均一。另外,通过研究榴辉岩及其熔融产物的金属稳定同位素特征,发现榴辉岩部分熔融过程中,由于石榴石效应,会造成Ca、Mg、Fe、Li等金属稳定同位素的分馏。因此,当利用稳定同位素示踪榴辉岩熔体的源区时,需要考虑其分馏的影响。

     

Electrical conductivity of hydrous basaltic melts: implications for partial melting in the upper mantle

The Earth’s uppermost asthenosphere is generally associated with low seismic wave velocity and high electrical conductivity. The electrical conductivity anomalies observed from magnetotelluric studies have been attributed to the hydration of mantle minerals, traces of carbonatite melt, or silicate melts. We report the electrical conductivity of both H₂O-bearing (0–6 wt% H₂O) and CO₂-bearing (0.5 wt% CO₂) basaltic melts at 2 GPa and 1,473–1,923 K measured using impedance spectroscopy in a piston-cylinder apparatus. CO₂ hardly affects conductivity at such a concentration level. The effect of water on the conductivity of basaltic melt is markedly larger than inferred from previous measurements on silicate melts of different composition. The conductivity of basaltic melts with more than 6 wt% of water approaches the values for carbonatites. Our data are reproduced within a factor of 1.1 by the equation log σ = 2.172 − (860.82 − 204.46 w ^0.5)/(T − 1146.8), where σ is the electrical conductivity in S/m, T is the temperature in K, and w is the H₂O content in wt%. We show that in a mantle with 125 ppm water and for a bulk water partition coefficient of 0.006 between minerals and melt, 2 vol% of melt will account for the observed electrical conductivity in the seismic low-velocity zone. However, for plausible higher water contents, stronger water partitioning into the melt or melt segregation in tube-like structures, even less than 1 vol% of hydrous melt, may be sufficient to produce the observed conductivity. We also show that ~1 vol% of hydrous melts are likely to be stable in the low-velocity zone, if the uncertainties in mantle water contents, in water partition coefficients, and in the effect of water on the melting point of peridotite are properly considered.     

Geochronology and geochemistry of the Late Triassic Longtan pluton in South China: termination of the crustal melting and Indosinian orogenesis

The Indosinian orogeny is recorded by Triassic angular unconformities in Vietnam and South China and by widely occurring granitoids in the Yunkai-Nanling and the Xuefengshan belts of South China. The Longtan pluton in the northwestern part of the Xuefengshan belt is a typical high-K, calc-alkaline, I-type granitoid, which can shed light on the relationship between the Indosinian tectonic and magmatic activity in the region. Three precise zircon U–Pb ages yielded a mean of 218 ± 0.8 Ma, which is taken as the age of crystallization. The pluton consists of both granodiorite (64.59–68.01 % SiO₂ and 3.25–4.22 % K₂O) and granite (70.49–71.80 % SiO₂ and 4.07–4.70 % K₂O). The granodiorites are characterized by relatively high Mg# (54–57), low contents of Na₂O (3.2–4.3 wt%), low abundances of incompatible elements (LILE, Nb and P), high initial ⁸⁷Sr/⁸⁶Sr (0.7175–0.7184) and negative ε_Nd(t) (?9.98 to ?9.72). REE patterns show moderate fractionation ((La/Yb)_cn = 8.07–18.80) with negative Eu anomalies (Eu/Eu* = 0.62–0.86). Compared with the granodiorite, the granite has a wider range of Mg# (49–59), lower contents of Na₂O (2.8–4.2 wt%), higher initial ⁸⁷Sr/⁸⁶Sr (0.7232–0.7243) and more negative ε_Nd(t) (?12.07 to ?11.24) values. REE patterns are relatively flat ((La/Yb)_cn = 14.73–29.37) with smaller negative Eu anomalies (Eu/Eu* = 0.48–0.63). The granodiorite has lower K₂O/Na₂O and Al₂O₃/(MgO + FeO_Tot) values than the granite. Based on major and trace element geochemistry and Sr–Nd isotopes, we interpret the Longtan granodioritic magma to have been derived by partial melting of interlayered Proterozoic metabasaltic to metatonalitic source rocks, whereas the granite was probably derived from a mixture of Proterozoic metagraywackes and metaigneous rocks. Field, petrographic and geochemical evidence indicate that partial melting and fractional crystallization were the dominant mechanism in the evolution of the pluton. The Longtan granodiorites and granites are petrologically and geochemically similar to typical Indosinian varieties and are considered to have been produced in a similar manner. The Indosinian granitoids in the region show a magmatic peak age of ~238 Ma from the Yunkai-Nanling belt in the southeast and a magmatic peak age of ~218 Ma of the Xuefengshan belt to the northwest. These early and late magmatic episodes of the Indosinian granitoids also display slight variations of regular compositions, ε_Nd(t) values and T _DM ages. Thus, we propose a syncollisional extension model that Indosinian granitoids were generated by decompressional partial melting of crustal materials triggered by two extensions during collision of the Indochina and South China blocks. The Longtan pluton in the northwesternmost part of the orogenic belt marks the termination of the Indosinian magmatism and orogenesis.     

The Stannern trend eucrites: Contamination of main group eucritic magmas by crustal partial melts

We report on the petrology of a new eucrite belonging to the Stannern trend and discuss the origin of this trend. The eucrite Northwest Africa 4523 (NWA 4523) is an equilibrated eucrite consisting of dark clasts embedded in a fine-grained crystallized matrix. Two types of clasts have been observed: medium-grained ophitic/subophitic clasts, and very fine-grained clasts. Despite textural differences, the clasts display the same mineralogy, in particular the same kind of pyroxenes with pigeonitic cores containing sparse exsolution lamellae, and augitic rims, zoned plagioclases and the occurrence of K-feldspar. The major and trace element abundances of a large medium-grained clast are very similar to Stannern or Bouvante.The Stannern trend eucrites are characterized by high incompatible trace element abundances. Their trace element patterns normalized to a representative Main Group eucrite, exhibit significant Eu, Sr and Be negative anomalies. In this paper, we show that contamination of Main Group eucritic magmas by melts derived by partial melting of the asteroid’s crust can successfully explain both the high incompatible trace elements concentrations and the distinctive Eu, Sr, Be anomalies shown by the Stannern trend eucrites. This model is in agreement with the view that Stannern and some Main Group-Nuevo Laredo trend eucrites have been contemporaneously erupted, and with the probable assumption that Stannern trend eucrites formed rather late in the history of the 4-Vesta’s crust.     

Assimilation and partial melting of continental crust: evidence from the mineralogy and geochemistry of autoliths and xenoliths

This paper presents a model for the partial melting of quartz diorite and greywacke in the upper crust based on the mineralogy and geochemistry of enclaves within the Loch Doon granitic intrusion of southern Scotland. The melting of quartz diorite was modelled using autoliths, which represent fragments of cogenetic igneous rocks that became incorporated in the fractionating magma. Compared to their quartz diorite parents, the autoliths are enriched to varying degrees in some elements (notably Rb, Nb, Ta, Sm, Y, Yb) and depleted in others (Sr, and Ba); Eu and P are also depleted in the more assimitated autoliths. The compositions of melts that could be derived from assimilation of the autoliths have also been calculated: their REE patterns reveal a light REE enrichment, low concentrations of heavy REEs (1–3 x chondrite) and a positive Eu anomaly. The calculated degrees of melting vary from 35% in the least assimilated to 84% in the most assimilated autolith (assuming a bulk distribution coefficient of 10 for the most compatible element). Results from modelling of xenolith compositions (derived from metasediments) are also reported, but because of uncertainties in the composition of the parental sediment, these data are subject to larger errors. They do, however, indicate that resultant partial melts are distinctly different from those derived by partial melting of autoliths. In particular, the REE pattern of a greywacke-derived melt shows a slight enrichment in light REEs, greater concentrations of heavy REEs (10 x chondrite) and a small negative Eu anomaly. The calculated degrees of melting of the xenoliths fall in the range of 66–88% (assuming a bulk distribution coefficient of 10 for the most compatible element). The results have direct implications for assimilation and melting of the upper crust. By taking into account how the nature of residual phases is likely to change with depth, it can be demonstrated that some Archaean tonalite gneisses could represent liquids derived by partial melting of igneous material.     

Behavior of apatite in granitic melts derived from partial melting of muscovite in metasedimentary sources

Fluid-absent and fluid-fluxed melting of muscovite in metasedimentary sources are two types of crustal anatexis to produce the Himalaya Cenozoic leucogranites. Apatite grains separated from melts derived from the two types of parting melting have different geochemical compositions. The leucogranites derived from fluid-fluxed melting have relict apatite grains and magmatic crystallized apatite grains, by contrast, there are only crystallized apatite grains in the leucogranites derived from fluid-absent melting. Moreover, apatite grains crystallized from fluid-fluxed melting of muscovite contain higher Sr, but lower Th and LREE than those from fluid-absent melting of muscovite, which could be controlled by the distribution of partitioning coefficient (D_Ap/Melt) between apatite and leucogranite. D_Ap/Melt in granites derived from fluid-absent melting is higher than those from fluid-fluxed melting. So, not only SiO₂ and A/CNK, but also types of crustal anatexis are sensitive to trace element partition coefficients for apatite. In addition, due to being not susceptible to alteration, apatite has a high potential to yield information about petrogenetic processes that are invisible at the whole-rock scale and thus is a useful tool as a petrogenetic indicator.©2021 China Geology Editorial Office.     

Wet or dry? The difficulty of identifying the presence of water during crustal melting

Partial melting of continental crust and evolution of granitic magmas are inseparably linked to the availability of H₂O. In the absence of a free aqueous fluid, melting takes place at relatively high temperatures by dehydration of hydrous minerals, whereas in its presence, melting temperatures are lowered, and melting need not involve hydrous minerals. With the exception of anatexis in water‐saturated environments where anhydrous peritectic minerals are absent, there is no reliable indicator that clearly identifies the presence of a free aqueous fluid during anatexis. Production of Ab‐rich magmas or changes in LILE ratios, such as an increase in Sr and decrease in Rb indicating increased involvement of plagioclase, are rough guidelines to the presence of aqueous fluids. Nevertheless, all indicators have caveats and cannot be unequivocally applied, allowing for the persistence of a bias in the literature towards dehydration melting. Investigation of mineral equilibria modelling of three metasedimentary protoliths of the Kangaroo Island migmatites in South Australia, shows that the main indicator for the presence of small volumes of excess water under upper amphibolite to lower granulite facies conditions (660–750°C) is the melt volume produced. Melt composition, modal content or chemical composition of peritectic minerals such as cordierite, sillimanite or garnet are relatively insensitive to the presence of free water. However, the mobility of melt during open system behaviour makes it difficult to determine the melt volume produced. We therefore argue that the presence of small volumes of excess water might be much more common than so far inferred, with large impact on the buffering of crustal temperatures and fertility, and therefore rheology of the continental crust.     

Experimental constraints on major and trace element partitioning during partial melting of eclogite

Isobaric partial melting experiments were performed on an Fe-free synthetic composition to simulate partial melting of subducted oceanic crust. Nominally anhydrous experiments at 3.0 GPa yielded melts in equilibrium with garnet (13 to 16 mol.% grossular) and aluminous clinopyroxene (14 to 16 wt.% Al₂O₃). Melt compositions show decreasing Si and alkalis and increasing Ca, Mg, and Ti contents with increasing temperatures. Experiments at 1200 and 1300°C were rutile saturated, whereas experiments at 1400°C contained no residual rutile. We argue that during the initial stages of subduction, accessory rutile is likely to be stable in subsolidus eclogites of average midocean ridge basalt composition and that only large degrees of partial melting will eradicate rutile from an eclogitic source. At 3 GPa, any eclogites with a bulk TiO₂ content of ≥1.5 wt.% rutile will produce rutile-saturated partial melts, except at very high degrees of melting. At higher pressures, all bulk Ti may dissolve in clinopyroxene and garnet, leaving no accessory rutile.Trace element partition coefficients for 24 trace elements between clinopyroxene, garnet, and melt were determined by secondary-ion mass spectrometry analysis of experimental run products at 1400°C and 3 GPa. Partition coefficients for the rare earth elements agree well with previous studies and have been evaluated using the lattice strain model. Partitioning data for high-field strength elements indicate complementary D_Zr/D_Hf for clinopyroxene and garnet. Partial melting of an eclogitic component of different modal compositions may therefore explain both subchondritic and superchondritic Zr/Hf ratios. Superchondritic Zr/Hf has recently been observed in some ocean island basalts (OIB), and this may be taken as further evidence for components of recycled oceanic crust in OIB. The data also indicate slight Nb/Ta fractionation during partial melting of bimineralic eclogite, which is not, however, sufficient to explain some recently observed Nb/Ta fractionation in island arc rocks. Accessory rutile, however, can explain such fractionation.     

The petrogenesis of Tertiary cone-sheets in Ardnamurchan, NW Scotland: petrological and geochemical constraints on crustal contamination and partial melting

Abundant cone sheets form one of the last magmatic stages in the Tertiary central complex on the Scottish peninsula of Ardnamurchan and can be grouped into a younger inner and an older outer suite relative to a gabbro intrusion. Most of the cone-sheets consist of tholeiitic to transitional basalt with MgO contents between 7.5% and 4%, although more evolved rocks also occur (to 0.5% MgO). The mafic samples are slightly enriched in the light rare earth elements (Chondrite-normalized La/Sm ∼1.1), the enrichment increases in the more evolved rocks. The compositional variation of the basaltic rocks is mainly due to crystal fractionation of olivine and clinopyroxene at depths of ∼10 km but trace elements show simultaneous assimilation of Archean Lewisian granulite crust. The andesitic to rhyolitic lavas formed by fractional crystallization from the contaminated basaltic magma coupled with assimilation of Proterozoic Moine metasediments at uppermost crustal levels. The occurrence of composite cone-sheets with basaltic and rhyolitic parts and mixtures between these magmas implies that the melts ascended successively but within a short period of time. The parental magmas of the Ardnamurchan cone-sheets must have formed at relatively shallow depths in the mantle and are comparable to the youngest tholeiitic lavas from the neighbouring island of Mull. Received: 5 June 1997 / Accepted: 12 November 1997     

The crustal tongue melting model and the origin of massive anorthosites

Recent detailed field studies in several anorthosite complexes have shown that anorthosites are frequently associated with weakness zones in the crust which may have favoured their emplacement at mid-crust levels. Recent experimental data have shown that the parent magma compositions of various anorthosite massifs lie on thermal highs in the relevant phase diagrams at 10–13 kbar, indicating that these magmas cannot be derived by fractionation of peridotitic mantle melts but by melting of gabbronoritic sources in the lower crust at 40–50 km depths. In the Sveconorwegian Province terne boundaries have been traced in deep seismic profiles to Moho offsets or to tongues of lower crustal material underthrust to depths higher than 40 km. In Southern Norway, we suggest that a lithospheric-scale weakness zone (the Feda transition zone?) has channelled the Rogaland anorthosites through linear delamination, asthenospheric uprise and melting of a mafic lower crustal tongue.     

The influence of submarine weathering of basalts on their partial melting during subduction

Compositional changes beyond that of simple hydration are present in most of the analyzed examples of submarine weathering of basalt. Recalculations of 278 analyses from the literature of altered basalts to simplified basalt, amphibolite and eclogite mineral assemblages result in significant increases of the quartz contents of the rock relative to fresh basalt in most examples. The increased quartz contents result in mineral assemblages that have enhanced potentials to generate andesitic partial melts from the basalt or amphibolite forms of most altered basalts and trondhjemitic partial melts from the eclogite form of most altered basalts.     

Francolite geochemistry—compositional controls during formation,diagenesis, metamorphism and weathering

Francolite in unaltered Tertiary phosphorites is highly substituted with about 1.2% Na, 0.25% Sr, 0.36% Mg and 2.7% SO₄ within the structure in substitution for Ca and PO₄. These substitutions reflect the composition of the solution from which francolite forms and are not influenced by the mechanism, rate or redox conditions of formation. The remarkably constant composition for a wide range of samples reflects the relative constant composition of sea water, during Tertiary times. Francolite in sulphate-reducing sediments forms in an uppermost zone where bioturbation maintains the concentrations of Na, Sr, Mg and SO₄ at sea water concentrations. The structura-CO₂ content of unaltered francolites vary from 1.4% to 6.3%, in response to variations in pH. The maximum level of carbonate substitution is limited by the disruption of the francolite structure at levels above 6.3% CO₂. For the samples considered here F/P₂O₅ ratios vary with X-ray CO₂ contents showing that PO₄^3? is replaced by (CO₃^2? + F?) and not by CO₃^2? alone. In Phosphoria Formation francolite the Na/P₂O₅ ratios and X-ray CO₂ contents show areal trends related to burial depth and thermal metamorphism. These two processes together with weathering and fresh water diageneses explain all observed variation in francolite composition.     

800MPa下泥质岩部分熔融实验研究：残留相及熔体相REE地球化学特征

利用JL-3600t压机实验研究了800MPa、不同温度条件下泥质岩部分熔融过程,利用EMPA和LA-ICPMS分别测定了熔体相和残留相中主要化学组成以及微量元素（包括REE）组成。实验结果表明,泥质岩低程度部分熔融（〈25％）形成的熔体中REE含量分布于308．8-3565μg／g较大范围内,显示较大的不均匀性,其REE球粒陨石标准化分布模式显示弱的M型REE“四分组效应”,而残留相矿物石榴子石中REE含量变化于167．5—1008μg／g范围,也显示有明显的不均匀性,其REE球粒陨石标准化分布模式显示明显的W型REE“四分组效应”,尤以第一段La-Nd最为显著;随着部分熔融程度的增加（〉30％）,其形成的熔体中REE集中在523．2—1130μg／g范围,残留相石榴子石中BEE集中在288．6—512．7μg／g范围,均显示相对均匀;熔体相和残留相石榴子石矿物的REE球粒陨石标准化分布模式不发育REE“四分组效应”。实验前后Cl质量平衡计算的结果表明该实验过程中并没有产生岩浆挥发分相。上述特征表明S型花岗岩中的REE“四分组效应”现象很可能与泥质岩低程度部分熔融具有成因联系。