On the rheology of partially molten synthetic granite

In an attempt to obviate some of the difficulties inherent in using natural rocks to study experimentally the rheology of partially-molten granitic rocks, we report some preliminary experiments using hot-pressed synthetic aggregates, with quartz as the solid phase and a quartz–albite glass to form the melt phase. Undrained constant displacement rate and stress relaxation experiments were performed at 1100 °C and 300 MPa total confining pressure using nominally 16 and 28 wt% anhydrous melt, and at 980 °C with 12 wt% of water-saturated melt. Strain rate varies non-linearly with stress, corresponding to a stress exponent of 3.6 for the anhydrous melt tests, and 1.9 with the wet melt. Strain rate is enhanced about 160-fold through doubling the melt fraction from 16 wt%. Intracrystalline plasticity appears to contribute insignificantly to the observed deformation, and the preservation of relatively equant grains implies a granular flow process. Microcracking occurs during isostatic hot-pressing, and also contributes to subsequent non-isostatic deformation at high stresses and strain rates. Models of flow of partially molten rock that assume diffusive transfer accommodation of granular flow predict linear viscosity but only at lower strain rates than accessed experimentally. It is suggested that the non-linear granular flow observed in these experiments involved the episodic unsticking of sintered contacts between grains by a rate-dependent cracking and/or dislocation accommodation process.     

Low frequency electrical impedance of partially molten gabbro: The effect of melt geometry on electrical properties

Electrical impedance of partially molten gabbro was measured as a function of the frequency of the applied electric field. The impedance of the partially molten gabbro was mainly controlled by the volume and geometry of partial melt, which were observed with an optical microscope study of the quenched sample. The experimental results are interpreted by a theory that formulates the electrical property of rocks containing partial melt, taking ionic diffusion in the melt into account. The theory evaluates the contributions from two types of melt-geometry distributed in the rock matrix, namely:

1. (a) nearly isolated melt pockets, and

2. (b) connected melt along grain boundaries.

According to the theory, connected melt mainly contributes to the increase of net conductivity, while melt pockets increase displacement current and cause the frequency dependence of impedance. The characteristic ionic diffusivity in melt pockets was determined from the peak frequency of the imaginary part of impedance. The magnitude of diffusion coefficient is estimated to be of the order of 10⁻⁷ to 10⁻⁶ cm²/sec. This suggests that sodium, iron, magnesium and/or calcium ions mainly carry the electric charge in partial melt. The observed frequency dependence of impedance is considered to reflect ionic polarization in melt pockets due to the migration of these alkali ions toward solid-melt interfaces.The present experimental and theoretical work suggests that electrical properties of the earth's upper mantle may involve an intrinsic frequency dependence associated with ionic diffusion in the frequency range of geomagnetic observation, if a partial melt zone exists. The analysis of the earth's conductivity structure may be inadequate if such intrinsic frequency dependence of mantle media is neglected.     

Characteristic features of ore gabbro formation in the third layer of the oceanic crust

Peculiarities of ore gabbro formation in slow-spreading mid-ocean ridges exemplified by the non-transform Sierra Leone fault and Marathon fault areas are considered. The formation of ore gabbros is most often connected with the rheologically weakened oceanic lithosphere mainly with active fault zones, in which basic magmatic melts intruded. Those faults provide ways for migration of differentiated melts at different depth levels. When intruding and moving along fault zones, melts interacting with the mantle and crust host rocks are often already hydrated. Such interaction occurs largely in conditions of subsolidus deformations resulting in enrichment of melts with volatile components.     

Thermal aspects of sea-floor spreading and the nature of the oceanic crust

By means of a plate-tectonics model we have computed radial temperature profiles for various spreading velocities for the region bounded by the bottom of oceanic layer two, the top of the lowvelocity zone, the centre of the ridge, and a vertical plane at 1000 km away from the ridge centre. The model we have used differs from previous models in that certain petrological aspects of basalt formation, partial melting, and latent heat effects have been taken into account. Oceanic heat flow was calculated from the ridge crest to 1000 km away. The thermal relationships in this region seem to rule out a gabbroic or amphibolitic third layer in the oceanic crust.     

Critical phenomena in the rheology of partially melted rocks

Theoretical considerations and exploratory experiments indicate that most of the extreme change of large-strain “viscosities” between the solidus and the liquidus of rocks is concentrated about a non-zero rheological critical melt percentage (RCMP). Usually the RCMP is within the range of 20 ± 10 vol%. Below it, both bulk shear and segregative-compactive filter pressing are drastically slowed down, requiring strains in the solid fraction and squeezing of thin melt films. Many geological consequences of these critical phenomena may be far reaching. In particular, most of the upper-mantle low-velocity zone (LVZ), if partially melted, is likely to be stabilized somewhat below the RCMP, with runaway excesses of heat, melt and shear strain creating local excursions over the RCMP. Flow may be thus concentrated in narrow high-melt channels, within a relatively unsheared LVZ.     

Structure and composition of the oceanic lithosphere created at different spreading rates

The specific features of the oceanic lithosphere (the petrography, the mineral composition, and the petrochemistry of igneous rocks and restites) that indicate its formation at different spreading rates, from the extremely slow to fast, are considered. This evidence may be used for solution of the inverse problem of estimating, at least qualitatively, the rate of paleospreading from the structure and composition of rocks pertaining to the ophiolitic association. The use of petrochemical data as the criteria of paleospreading rate is limited. The anomalous composition and structure of the oceanic crust may be due to factors unrelated to the spreading rate. The well-studied cases of ophiolites interpreted as fragments of the ancient oceanic lithosphere formed under conditions of fast, slow, and extremely slow spreading rates are discussed. It is concluded tentatively that the fast spreading is typical of the ophiolites obducted on passive margins (the Periarabian, Uralian, and Appalachian-Caledonian belts) as fragments of ensimatic suprasubduction basins formed at the final stages of the evolution of paleooceans (Tethys and Iapetus). Ophiolites as products of slow spreading are commonly localized in accretionary (subduction-related) orogens at the present-day and older active continental margins.     

花岗质超糜棱岩的微观构造与变形机制

花岗质岩石的变形方式和过程决定大陆地壳的流变学特性。本文聚焦藏南拆离系超糜棱岩化的花岗质岩石，借助传统显微构造分析方法和扫描电镜、阴极发光、矿相自动分析系统和电子背散射衍射等新技术手段，开展微观组分、结构、组构定量化观测和分析。超糜棱岩主要造岩矿物为钾长石、斜长石、石英、黑云母等，显微构造呈现为单矿物相域与多相矿物混合域交织结构。相平衡模拟与斜长石钙含量等值线变形温度估算结果为390～410℃。单相域的矿物集合体条带主要分为钾长石条带与石英条带，其中钾长石条带内变形颗粒呈现典型的核 幔构造。组构分析表明钾长石颗粒具有强烈的晶格优选定向，残斑与动态重结晶的钾长石颗粒具有相似的晶格优选方位(CPOs)特征。施密特因子法分析揭示钾长石残斑变形过程中主要活动的滑移系为(100)\[010\]、(010)\[001\]和(001)\[100\]，基质钾长石颗粒形成机制主要为位错蠕变驱动的亚颗粒旋转重结晶。在混合相域，矿物颗粒发生强烈细粒化而只含有少量残斑，基质颗粒主要为斜长石，斜长石颗粒间广泛分布微米级黑云母颗粒。斜长石无组构或弱组构，主导变形机制为颗粒边界滑动。在单相域条带与混合相域基质内，石英颗粒均发生强烈细粒化，颗粒表面发育溶蚀结构以及细小的新晶晶核，石英＜c＞轴晶格优选定向及形态学长轴优选定向皆平行于线理X方向，变形机制为溶解 沉淀蠕变。这显示在由单相域向混合相域的演化过程中，流体作用至关重要，流体与单相域钾长石进行交代使其分解为细粒的斜长石与石英，并导致花岗质岩石变形机制由位错蠕变向非位错蠕变转换，并诱发岩石的流变弱化。     

Microstructural constraints on the timing of Proterozoic deformation in Central New Mexico

The nature and extent of deformation associated with 1.4 Ga tectonism in the south-western USA are poorly understood. Two models have been proposed. Both agree that Proterozoic crustal accretion occurred at 1.65 Ga and that the rocks remained at mid-crustal conditions ( c . 12 km depth) until 1.4 Ga. However, one model suggests that 1.4 Ga deformation was regionally extensive, the other that it was localized around 1.4 Ga plutons. Following 1.4 Ga tectonism, the crust cooled below 300 °C. Detailed studies of quartz mylonite microfabrics in samples both adjacent to and removed from 1.4 Ga plutons in the Manzano Mountains, central New Mexico, are used to discriminate between these models of mid-Proterozoic thermotectonic history. In this area, as in much of northern New Mexico, the metamorphic conditions prior to emplacement of 1.4 Ga plutons were 500 °C and 4 kbar. The quartz mylonite microfabrics include ribbon grains, recrystallized grains with serrated boundaries, and strong c-axis crystallographic preferred orientations, which indicate no post-deformational modification. All of these microfabrics are consistent with deformation at upper greenschist/lower amphibolite facies conditions, and could have formed during either 1.65 or 1.4 Ga tectonism. Microfabrics formed during 1.65 Ga tectonism, however, should have been substantially modified by annealing recrystallization during residency in the middle crust and/or thermal/mechanical effects associated with 1.4 Ga tectonism. The observed microstructures are consistent with regional deformation associated with metamorphism at 1.4 Ga. The effects of deformation at 1.4 Ga in New Mexico are therefore more widespread than previously thought.     

Carbonate metasomatism of the oceanic mantle beneath Fernando de Noronha Island, Brazil

Evidence is presented for a carbonate-immiscibility phenomena in the upper mantle based on data from a set of strongly metasomatized xenoliths in a basanitic lava flow from Fernando de Noronha Island (southwest Atlantic). A petrological and geochemical study of lherzolitic and harzburgitic xenoliths reveals that the oceanic mantle of this region has been affected by very strong carbonate metasomatism. The metasomatism led to wehrlitization of the primary mantle mineral assemblage (ol, opx, sp). The wehrlitization was the result of interaction between a possibly ephemeral sodic dolomitic melt or fluid with the mantle peridotite according to the following reactions, which include sodic components: 4MgSiO₃+CaMg(CO₃)₂=2Mg₂SiO₄+CaMgSi₂O₆+2CO2 3CaMg(CO₃)₂+CaMgSi₂O₆=4CaCO₃+2Mg₂SiO₄+2CO₂ The olivine has abundant micro-inclusions consisting of Na-Al-Si-rich glass, Fe, Ni and Cu-monosulfide, Ca-rich carbonate and dense CO₂. The interrelationships between the glass, sulfide and carbonate inclusions permit speculation that silicate, sulfide, and Ca-rich carbonatite melts were in equilibrium with each other and originated from partial melting of metasomatized and wehrlitized peridotite underneath Fernando de Noronha Island. These results support a two-stage model of Ca-rich carbonatite formation: first stage - metasomatic wehrlitization and carbonatization of mantle rocks; second stage - partial melting of the carbonate-bearing wehrlitic rock resulting in the formation of immiscible silicate, sodic carbonate and sulfide liquids and the ultimately generation of calciocarbonatites.     

Chemical and isotopic constraints on the generation and transport of magma beneath the East Pacific Rise

Interpretation of U-series disequilibria in midocean ridge basalts is highly dependent on the bulk partition coefficients for U and Th and therefore the mineralogy of the mantle source. Distinguishing between the effect of melting processes and variable source compositions on measured disequilibria (²³⁸U-²³⁰Th-²²⁶Ra and ²³⁵U-²³¹Pa) requires measurement of the radiogenic isotopes Hf, Nd, Sr, and Pb. Here, we report measurements of ²³⁸U-²³⁰Th-²²⁶Ra and ²³⁵U-²³¹Pa disequilibria; Hf, Nd, Sr, and Pb isotopic; and major and trace element compositions for a suite of 20 young midocean ridge basalts from the East Pacific Rise axis between 9°28′ and 9°52′N. All of the samples were collected within the axial summit trough using the submersible Alvin. The geological setting and observational data collected during sampling operations indicate that all the rocks are likely to have been erupted from 1991 to 1992 or within a few decades of that time. In these samples, ²³⁰Th excesses and ²²⁶Ra excesses are variable and inversely correlated. Because the eruption ages of the samples are much less than the half-life of ²²⁶Ra, this inverse correlation between ²³⁰Th and ²²⁶Ra excesses can be considered a primary feature of these lavas. For the lava suite analyzed in this study, ²²⁶Ra and ²³⁰Th excesses also vary with lava composition: ²²⁶Ra excesses are negatively correlated with Na₈ and La/Yb and positively correlated with Mg#. Conversely, ²³⁰Th excesses are positively correlated with Na₈ and La/Yb and negatively correlated with Mg#. Th/U, ²³⁰Th/²³²Th, and ²³⁰Th excesses are also variable and correlated to one another. ²³¹Pa excesses are large but relatively constant and independent of Mg#, La/Yb, Th/U, and Na₈. The isotope ratios ¹⁴³Nd/¹⁴⁴Nd, ¹⁷⁶Hf/¹⁷⁷Hf, ⁸⁷Sr/⁸⁶Sr, and ²⁰⁸Pb/²⁰⁶Pb are constant within analytical uncertainty, indicating that they were derived from a common source. The source is homogeneous with respect to parent/daughter ratios Lu/Hf, Sm/Nd, Rb/Sr, and Th/U; therefore, the measured variations of Th/U, ²³⁰Th, and ²²⁶Ra excesses and major and trace element compositions in these samples are best explained by polybaric melting of a homogeneous source, not by mixing of compositionally distinct sources.     

Geochemical constraints on the origin of the late Jurassic proto-Caribbean oceanic crust in Hispaniola

The nature of the oceanic crust produced through rifting and oceanic spreading between North and South America during the Late Jurassic is a key element for the Caribbean plate tectonic model reconstruction. Located in the Cordillera Central of Hispaniola, the Loma La Monja volcano-plutonic assemblage (LMA) is composed of gabbros, dolerites, basalts, and oceanic sediments, as well as metamorphic equivalents, which represent a dismembered fragment of this proto-Caribbean oceanic crust. Petrologic and geochemical data show that the LMA have a relatively broad diversity in composition, which represent the crystallization products of a typical low-pressure tholeiitic fractionation of mid-ocean ridge basalts (MORB)-type parental magmas, ranging from N- to E-MORB. Three geochemical groups have been distinguished in the volcanic sequence: LREE-flat to slightly LREE-enriched basalts of groups II and III occur interlayered in the lower stratigraphic levels; and LREE-depleted basalts of group I in the upper levels. Mantle melt modeling suggests that group III magmas are consistent by mixing within a mantle melt column of low-degree (<1%) melts of a deep garnet lherzolite source and high-degree (>15%) melts of a shallow spinel source, and groups II and I magmas are explained with moderate to high (14–18%) and very high (>20%) fractional melting degrees of a shallower spinel mantle source, respectively. Thus, upward in the volcanic sequence of the LMA, the magmas represent progressively more extensive melting of shallower sources, in a plume-influenced spreading ridge of the proto-Caribbean oceanic crust. Nb/Y versus Zr/Y systematics combined with recent plate tectonic model reconstructions reveal that Caribbean Colombian oceanic plateau fragments in Hispaniola formed through melting of heterogeneous mantle source regions related with distinct plumes during at least from Aptian–Albian (>96 Ma) to Late Campanian.     

Microstructural constraints on the mechanisms of the transformation to reidite in naturally shocked zircon

Zircon (ZrSiO₄) is used to study impact structures because it responds to shock loading and unloading in unique, crystallographically controlled manners. One such phenomenon is the transformation of zircon to the high-pressure polymorph, reidite. This study quantifies the geometric and crystallographic orientation relationships between these two phases using naturally shocked zircon grains. Reidite has been characterized in 32 shocked zircon grains (shocked to stages II and III) using a combination of electron backscatter diffraction (EBSD) and focused ion beam cross-sectional imaging techniques. The zircon-bearing clasts were obtained from within suevite breccia from the Nördlingen 1973 borehole, close to the center of the 14.4 Ma Ries impact crater, in Bavaria, Germany. We have determined that multiple sets (up to 4) of reidite lamellae can form in a variety of non-rational habit planes within the parent zircon. However, EBSD mapping demonstrates that all occurrences of lamellar reidite have a consistent interphase misorientation relationship with the host zircon that is characterized by an approximate alignment of a {100}_zircon with a {112}_reidite and alignment of a {112}_zircon with a conjugate {112}_reidite. Given the tetragonal symmetry of zircon and reidite, we predict that there are eight possible variants of this interphase relationship for reidite transformation within a single zircon grain. Furthermore, laser Raman mapping of one reidite-bearing grain shows that moderate metamictization can inhibit reidite formation, thereby highlighting that the transformation is controlled by zircon crystallinity. In addition to lamellar reidite, submicrometer-scale granules of reidite were observed in one zircon. The majority of reidite granules have a topotaxial alignment that is similar to the lamellar reidite, with some additional orientation dispersion. We confirm that lamellar reidite likely forms via a deviatoric transformation mechanism in highly crystalline zircon, whereas granular reidite forms via a reconstructive transformation from low-crystallinity ZrSiO₄ within the reidite stability field. The results of this study further refine the formation mechanisms and conditions of reidite transformation in naturally shocked zircon.     

Experimental constraints on the evaporation of partially oxidized acid-sulfate waters at the martian surface

Isothermal evaporation experiments were carried out on an acidic (pH 2), partially oxidized (Fe²⁺/Fe_T ∼0.5) brine with a cation composition consistent with derivation from the chemical weathering of martian basalt. During evaporation, the brine composition evolved to a highly acidic (un-scaled pH −1.3) Mg-Fe-SO₄-Cl brine depleted in Ca, Al and K. Evaporite minerals identified throughout the course of the experiment include (in order of crystallization): gypsum, Mg-rich voltaite, (Mg_0.7, )SO₄·7H₂O and rhomboclase. The solid solution compositions of voltaite and (Mg_0.7, )SO₄·7H₂O, although uncommon in analogous environments on Earth, result from the distinct chemistry of evaporating martian surface fluids. Analysis of brine compositions with available thermodynamic models indicates that, although gypsum and rhomboclase precipitate at equilibrium saturation, kinetic controls on the precipitation of copiapite-group minerals affect the subsequent sulfate mineralogy and evolving chemistry of the entire system. In addition, geochemical simulations of the experimental evaporation process suggest that the appearance of voltaite and rhomboclase indicate a “metastable” evaporation pathway for martian fluids whereby bilinite and copiapite-group minerals did not form despite thermodynamic saturation. Comparison of the experimentally-produced assemblage to available observations of saline minerals at the martian surface represents a step toward systematically characterizing evaporite mineralogy as a function of Fe-oxidation in the initially dilute fluid. Deconvolving the complexity of Fe-sulfate formation in martian environments ultimately will help to exploit the sensitivity of these minerals to pH and redox conditions present at the ancient martian surface.     

喀喇昆仑地体甜水海地区102Ma辉长岩的发现及其对区域中生代构造演化的约束

喀喇昆仑地体是青藏高原的重要构造单元。近来,我们对其中的林济塘中生代盆地地层进行了调查,并在喀喇昆仑地体甜水海地区褶皱的中侏罗统龙山组灰岩中发现了辉长岩侵入体。辉长岩具有高Al2O3(15.07%~15.32%)与TiO2(1.31%~1.34%)、贫P2O5(0.19%~0.21%)和低碱(Na2O+K2O=3.49%~3.72%)的特征,明显富集轻稀土(La/Yb=6.17~7.39),富集Th、La、Nd与Tb,亏损Rb、Sr、P与Y,辉长岩Sr-Nd同位素表现出富集的特征(εNd(t)值分别为-5.8与-5.9)。LA-ICP-MS锆石U-Pb测年结果显示辉长岩岩浆锆石206Pb/238U年龄为102±1Ma(MSWD=0.92;n=14),指示岩体形成于早白垩世晚期。辉长岩中含有大量继承锆石,38颗继承锆石年龄分布在2450~232Ma之间,其中16颗为前寒武纪年龄锆石,意味着该地区可能存在前寒武纪基底。这些特征意味着辉长岩在形成的过程中经历壳源物质的强烈混染(Nb/U、Ta/U、Ce/Pb分别为26.6~29.1、1.69~1.76、9.72~11.3)。这种经历壳源物质的强烈混染岩浆的形成可能与软流圈上涌有关,即102Ma辉长岩形成时甜水海地区处于伸展背景。结合区域地质特征,指出102Ma时研究区进入新的伸展与海盆发育阶段,并建立了喀喇昆仑地体中生代三阶段构造演化模型,即侏罗纪-早白垩世早期的正常俯冲与弧盆演化、早白垩世的平板俯冲与造山阶段及晚白垩世的正常俯冲与弧盆演化。     

Melt migration in the mantle beneath spreading zones and formation of replacive dunites: a review

The state of the art of the problem of mantle melt transport through the upper mantle beneath spreading zones is reviewed. The geochemical data and physical properties of the mantle constraining the melt migration processes are considered. The review concentrates on the ways of mantle magma transport and mechanisms of the localization of diffuse intergranular porous melt percolation into the channel flow. The hypothesis of the reaction of migrating mantle magmas with wall peridotites and the formation of replacive dunites as a result of this process are discussed. We examine the publications on the field, theoretical, and geochemical studies of the reactive melt migration and its role in the dunite formation in the mantle sections of the ophiolites. Alternative models of the dunite origin in the mantle are also tested. The results of experimental studies of the melt/peridotite interaction are presented.     

Petrologic evolution of partially molten cumulate: the Atok section of the Bushveld Complex

The postcumulus evolution of a portion of the Bushveld Complex that includes the Merensky reef is inferred from the study of a continuous 56 m drill core. The core penetrated the basal orthopyroxenites of the Merensky and Bastard units, the massive anorthosites overlying the two pyroxenites and about 10 m of norite underlying the Merensky pyroxenite. Detailed profiles of major, minor and rare earth element (REE) contents of clinopyroxene and orthopyroxene were determined by electron and ion probe. Good correlations exist between textural and lithological variations and the REE contents of the pyroxenes. Specifically, enrichments in pyroxene REE abundances are observed in the basal pyroxenites of the Merensky and Bastard units relative to the underlying and overlying rocks. In the pyroxenites the Nd content of clinopyroxene is typically over 12 ppm and reaches nearly 40 ppm (≈90 × chondrite), and Nd/Yb ratio is in the range 8 to 25. These extreme enrichments in REE are not accompanied by large variations in major element contents. Computations of the compaction parameters relevant to the conditions of crystallization of the Bushveld Complex combined with a consideration of cooling history confirm the importance of compaction as a post-cumulus process. This analysis indicates that the geochemical variability is a result of redistribution of interstitial melt driven by compaction and cannot reflect variations in the initial porosity of the accumulating crystal pile. A model for the development of the Atok section is developed. The Merensky anorthosite is interpreted to have served as a barrier to the upward porous flow of late-stage, hydrous and incompatible-element enriched melt, which was thus trapped in the underlying Merensky pyroxenite. The flow was driven by compaction of the 350+ meter-thick section of predominantly norite beneath the anorthosite. The introduction and accumulation of this melt in the pyroxenite and subsequent cooling resulted in partial dissolution, recrystallization and REE enrichment of the rock forming minerals, and in the formation of the main lithologic features of the Merensky pyroxenite. Further upward percolation of the interstitial melt through the Merensky anorthosite was restricted to channels due to the relatively impermeable nature of the cemented anorthosite. This melt accumulated in and metasomatized the Bastard pyroxenite in the same manner as the Merensky pyroxenite, having again been trapped by the overlying Bastard anorthosite. Received: 10 July 1996 / Accepted: 27 February 1997