Experimental Petrology Research Articles

Orogenic Lherzolites and Mantle Processes: Editorial

They contain vital information about melt transfer proThe papers in this Special Volume of the Journal of Petrology cesses related to the origin of mid-ocean ridge basalts. were presented at the Third International Lithosphere The Othris ophiolite, Greece, contains such spinel or Conference held in Pavia, Italy, in September 1999. The plagioclase lherzolites and Dijkstra et al. propose that meeting was attended by about 100 scientists who actively these shallow peridotites formed at a slow-spreading research mantle rocks and the processes of melt formation ridge. More significantly, they conclude that these rocks and extraction. The protoliths are garnet-, spinelor are the result of melt impregnation by mid-ocean ridge plagioclase-facies peridotites, rocks brought to the surface basalt (MORB) melts and that their ‘fertile’ compositions during the eruption of alkali to potassic volcanoes, diaare not primary. Similarly, an influx of melts of MORB tremes and maars. Mantle rocks are also exposed at the composition is proposed for the Horoman massif, Hoksurface of the Earth as a result of tectonic processes (i.e. kaido, by Saal et al. on the basis of Re–Os isotopic orogenic or alpine massifs) or mantle rocks are dredged data. Their data indicate a common petrogenesis for the or drilled on the ocean floor (i.e. abyssal peridotites). layered peridotite–pyroxenite sequence. In contrast, the Whereas the two previous lithosphere meetings in MontNikanbetsu plagioclase lherzolite from Hokkaido, Japan, pellier (1990) and Granada (1995) concentrated on orois believed by Takahashi to have experienced widespread genic massifs and xenoliths, the Pavia meeting dealt partial melting because of the evidence for incipient almost exclusively with the mineralogy, geochemistry and melting, melt migration and crystallization. experimental petrology of mantle rocks and their partial melt products.

Condensed matter chemistry under ‘extreme’ high pressure–high temperature conditions

Improved techniques for high-pressure experiments have led to new studies of the structure and physical properties of materials compressed to extremely high densities. Now we must fully enable the field of condensed matter chemistry under extreme high-pressure conditions. This will require development of strategies for the analysis and control of the chemical composition during reactions between solid, liquid and fluid phases. Such approaches already exist within the fields of experimental geochemistry and petrology, and they can be readily adapted to the wider area of chemistry. The first consideration is the manipulation and determination of stable and metastable pressure–temperature phase diagrams, illustrated here for the one-component system Si. Next, relationships between P, T and the chemical composition, X, expressed in terms of the chemical potential (μ) or the activity–composition relations, can be used to constrain and determine components within the system. This is illustrated by examples drawn from our recent work on high-pressure syntheses of boron suboxides (B6O1 − δ) and (Si, Ge)3N4 nitride spinels.

Experimental petrology of basement lavas from Ocean Drilling Program Leg 192: implications for differentiation processes in Ontong Java Plateau magmas

Abstract Melting relations of the basement lavas drilled from the Ontong Java Plateau during ODP Leg 192 were experimentally determined at 1150–1250°C and 0.1–190 MPa under the oxygen fugacity along the fayalite-magnetite-quartz (FMQ) and cobalt-cobalt oxide (CCO) buffers. The basement lavas were classified into two types according to phenocryst assemblage and whole-rock composition: one type is low in MgO (<8 wt%) and olivine + plagioclase + augite-phyric (Kwaimbaita type); and the other is rich in MgO (>8 wt%) and olivine-phyric (Kroenke type). One sample was chosen from each type as a starting material of the melting experiments. The experimental results demonstrate that the variations in phenocryst assemblage and whole-rock composition in the basement lavas can be modelled adequately by fractional crystallization processes in a shallow magma chamber (<6 km in depth). The experimentally determined mineral-melt equilibria, in combination with detailed petrographical investigation, revealed that the vast majority of phenocrysts are in equilibrium with their host magma composition, but some are not. The latter include unusually An-rich parts of plagioclase phenocrysts in the Kwaimbaita-type lavas. These An-rich parts probably crystallized in a mushy boundary layer along the wall of the magma chamber where the melt was relatively rich in H 2 O. Some olivine phenocrysts in the Kroenke-type lavas show reverse zoning, with core compositions that can be in equilibrium with the Kwaimbaita-type magmas. The cores of these olivine phenocrysts were most probably assimilated from a solidified pile of the Kwaimbaita-type lavas when the Kroenke-type magmas ascended through it.

Phenocrysts versus xenocrysts in the youngest Toba Tuff: Implications for the petrogenesis of 2800 km3 of magma

Research Article| April 01, 2002 Phenocrysts versus xenocrysts in the youngest Toba Tuff: Implications for the petrogenesis of 2800 km3 of magma James E. Gardner; James E. Gardner 1Geophysical Institute and Department of Geology and Geophysics, University of Alaska, Fairbanks, Alaska 99775-7320, USA Search for other works by this author on: GSW Google Scholar Paul W. Layer; Paul W. Layer 1Geophysical Institute and Department of Geology and Geophysics, University of Alaska, Fairbanks, Alaska 99775-7320, USA Search for other works by this author on: GSW Google Scholar Malcolm J. Rutherford Malcolm J. Rutherford 2Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA Search for other works by this author on: GSW Google Scholar Author and Article Information James E. Gardner 1Geophysical Institute and Department of Geology and Geophysics, University of Alaska, Fairbanks, Alaska 99775-7320, USA Paul W. Layer 1Geophysical Institute and Department of Geology and Geophysics, University of Alaska, Fairbanks, Alaska 99775-7320, USA Malcolm J. Rutherford 2Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA Publisher: Geological Society of America Received: 05 Jul 2001 Revision Received: 10 Dec 2001 Accepted: 21 Dec 2001 First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2002) 30 (4): 347–350. https://doi.org/10.1130/0091-7613(2002)030<0347:PVXITY>2.0.CO;2 Article history Received: 05 Jul 2001 Revision Received: 10 Dec 2001 Accepted: 21 Dec 2001 First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation James E. Gardner, Paul W. Layer, Malcolm J. Rutherford; Phenocrysts versus xenocrysts in the youngest Toba Tuff: Implications for the petrogenesis of 2800 km3 of magma. Geology 2002;; 30 (4): 347–350. doi: https://doi.org/10.1130/0091-7613(2002)030<0347:PVXITY>2.0.CO;2 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 The petrogenesis of the 2800 km3 of magma erupted as the youngest Toba Tuff has been investigated using experimental petrology and 40Ar/39Ar dating of biotite, sanidine, hornblende, and plagioclase from the tuff. We find that hornblende does not crystallize experimentally from the magma at temperatures and pressures indicated by the natural mineral assemblage. Hornblende is also not in isotopic equilibrium with biotite and sanidine, both of which grew experimentally. Hornblende thus appears xenocrystic, despite being a major phase in the tuff. Some plagioclase is also xenocrystic, on the basis of Ar isotopes, but others are probably phenocrystic, because plagioclase grew experimentally. Crystal clots of hornblende + plagioclase observed in the tuff suggest that the xenocrysts came from a common source, which was at least 1.5 Ma (the oldest hornblende 40Ar/39Ar age). Our results suggest that the Toba Tuff magma resided at nearly water-saturated pressures of 100–150 MPa and that xenocrysts were entrained as recently as 10 yr before the eruption. The ubiquitous presence of hornblende in the tuff indicates that entrainment occurred throughout the 2800 km3 of magma. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The role of an H2O-rich fluid component in the generation of primitive basaltic andesites and andesites from the Mt. Shasta region, N California

This paper presents analyses of the trace element abundances and isotopic compositions in primitive lavas from the Mt. Shasta region, N California. These data are combined with estimates of pre-eruptive H2O contents and constraints from experimental petrology to develop a model of subduction zone magmatism. These lavas share geochemical characteristics of high-Mg andesites from the Setouchi volcanic belt in SW Japan and Adak-type high-Mg andesites of the western Aleutian arc. Estimates of the pre-eruptive water contents of the Shasta region lavas range from 8 wt % H2O. The pre-eruptive H2O content and an inferred melt of a harzburgitic residue are used to carry out a mass balance for the relative contributions from a mantle-derived melt and slab-derived fluid-rich component. We assume that elements are contributed either from melting of mantle peridotite or from a subduction-related fluid-rich component. Estimated fluid-rich component compositions are characterized by strong light rare earth element (LREE) enrichments ([La/Gd]N=3 to 7) and variable heavy rare earth element (HREE) depletions ([Dy/Yb]N=1 to 3). Sr and Ba abundances vary by approximately a factor of 2.5 in the fluid compositions calculated for the Mt. Shasta region lavas and large ion lithophile element (LILE) abundances are similar to those calculated by Stolper and Newman (1994) and Eiler et al. (2000). The major elements in the fluid-rich component are H2O (~55–68 wt%), Na2O (~25–33 wt%) and K2O (~5–13 wt%). This composition may be that of a supercritical fluid or a low-degree melt of the slab that has reacted with the overlying mantle wedge. Although the slab beneath Mt. Shasta is inferred to be hot (~600 – 650 °C), the calculated fluid-rich components do not resemble a pure slab melt. The calculated isotopic composition of the fluid-rich component is bimodal. One component has 87Sr/86Sr=0.7028 and eNd=+8, and is most similar to a MORB source. The second component has more radiogenic 87Sr/86Sr=0.7038 and eNd=+1 and is most similar to a sediment. These fluid-rich components probably represent a mixture of fluids and melts from the slab (serpentinized mantle, altered basalt, and sediment).

Differentiation Processes of Deccan Trap Basalts: Contribution from Geochemistry and Experimental Petrology

EARTHQUAKE RESEARCH INSTITUTE, THE UNIVERSITY OF TOKYO, TOKYO 113-0032, JAPAN INSTITUTE FOR GEOTHERMAL SCIENCES, KYOTO UNIVERSITY, NOGUCHIBARU, BEPPU 874-0903, JAPAN PLOT NO. 89, FRIENDS HOUSING SOCIETY LAYOUT-4, DEENDAYALNAGAR, NAGPUR 400 022, INDIA FACULTY OF GEO-ENVIRONMENTAL SCIENCES, RISSHO UNIVERSITY, KUMAGAYA 360-0194, JAPAN DEPARTMENT OF EARTH SCIENCES, NIHON UNIVERSITY, TOKYO 156-0045, JAPAN

Ig nobel marriage vows

Two geologists at Arizona State University (ASU), AGU member Will Stefanov and Lisa Danielson, celebrated their “Ig Nobel” wedding on October 4. The couple married during a 60‐second ceremony in front of a crowd of 1200, which included many other scientists and four actual Nobel prize winners at this year's offbeat Ig Nobel Prizes ceremony in Sanders Theater on the campus of Harvard University in Cambridge, Massachusetts.The wedding was the culmination of the Ig Nobel ceremony, an annual fete sponsored by the science humor magazine, Annals of Improbable Research (AIR), which honors oddball scientific achievements. Danielson, a doctoral candidate at ASU whose interests include experimental petrology and geochemistry was one of about 20 people who responded to a notice to wed during the Ig Nobel.

Eclogites and their geodynamic interpretation: a history

Haüy coined the term eclogite, meaning “chosen rock”, in 1822, but de Saussure had already observed rocks of this type in the Alps four decades earlier. Throughout the 19th century, the origin of eclogite remained an enigma, in spite of great progress in our knowledge of this rock. The first chemical analyses, carried out around 1870, showed that its bulk composition was the same as gabbro. Therefore, eclogite was thought to be either an igneous rock of gabbroic composition or a metamorphosed gabbro. This second hypothesis became preferred when progressive transitions were observed between gabbros and eclogites. In 1903, simply by comparing the molar volumes of gabbroic and eclogite parageneses, Becke inferred that eclogite was the high-pressure equivalent of gabbro. In 1920, eclogite was involved in the conception of the metamorphic facies by Eskola. However, a few researchers denied the existence of an eclogite facies, and claimed that high stress instead of high lithostatic pressure could generate eclogites. In the 1960s, consideration of the water pressure parameter also favoured the belief that eclogite was simply the anhydrous equivalent of amphibolite. Finally, eclogite was definitely considered as a high-pressure metamorphic rock following the development of experimental petrology and the application of thermodynamics. In recent years, the discovery of ultrahigh-pressure coesite-bearing rocks in the crust has drastically changed geologists' ideas concerning the limits of eclogite-facies crustal metamorphism. Eclogites have been involved in several geodynamic theories. Around 1900, kimberlite studies favoured the idea that eclogite might be abundant in the interior of the Earth. In 1912, Fermor predicted the existence of a dense eclogite-bearing zone in the mantle. This “eclogite layer” hypothesis was still envisaged as late as 1970. The alternative “peridotite” hypothesis became preferred when experimental investigations demonstrated that the gabbro-to-eclogite transition could not coincide with a sharp Mohorovičić discontinuity. Before plate tectonics, high-pressure belts were interpreted as remnants of ophiolite-bearing “geosynclines”, metamorphosed by loading during thrust faulting. After the acceptance of plate tectonics, around 1970, the same high-pressure Alpine-type belts came to be considered as former oceanic crust, transformed into eclogite within subduction zones, and subsequently incorporated into mountain belts. Surprisingly, formation of eclogite in “subsidence” zones (i.e. subduction zones) had already been envisaged as early as 1931 by Holmes, the inventor of a convection-current theory. In the 1980s, many authors tried to apply the model of Alpine-type high-pressure belts to eclogites enclosed within the gneisses of ancient orogens, but the question remains obscure nowadays. These eclogites have been involved in the “in situ versus foreign” controversy and in the unresolved enigma of ultrahigh-pressure metamorphism. The latter came under scrutiny in 1984 after the discovery of coesite and diamond in some eclogite-facies rocks. It has been a matter of considerable interest during the last two decades. Currently, the debate is focused on the geodynamic mechanisms responsible for the exhumation of these rocks, a question that will probably remain unresolved for part of the coming century.

Le magmatisme post-collisionnel du Nord-Ouest de Borneo, produit de la fusion d'un fragment de croute oceanique ancre dans le manteau superieur

Abstract Magmatic activity linked to syn- or post-collisional zones leads to the emplacement of remarkably heterogeneous rocks: calc-alkaline, high-K calc-alkaline or shoshonitic series variably contaminated by continental crust; anatectic granites and ignimbrites derived from the latter; and finally alkali potassic to ultrapotassic basalts [Harris et al., 1990; Pearce et al., 1984, 1990; Arnaud et al., 1992; Benito et al., 1999]. The main sources of these magmas are either the upper mantle (sub-oceanic or subcontinental) frequently metasomatized by hydrous fluid originating from the subducted slab; or the continental crust, which can act as a contaminant [Benito et al., 1999; Miller et al., 1999] or melt directly [Harris et al., 1990; Zingg et al., 1990]. The purpose of the present paper is to document the role of a third source: the subducted oceanic crust, as evidenced by the occurrence of Miocene adakites in Sarawak (NW Borneo). The studied rocks have been sampled from western Sarawak (fig. 1), and their location is shown on the geological map [Tan, 1982] of figure 2. They mostly occur as stocks, dykes and sills which crosscut the Paleozoic to Miocene sedimentary units. Two kinds of intrusions can be distinguished. High-K calc-alkaline to medium-K calc-alkaline diorites and microdiorites occur in the northern part of the studied area, in Salak Island and Santubong Peninsula. Microtonalites and dacites occur near Kuching and in the southern part of Sarawak (Kuap and Bau areas). Whole-rock K-Ar data (table I) demonstrate that these two associations are of different ages: high-K calc-alkaline diorites were emplaced during the Lower Miocene (22.3 to 23.7 Ma), whereas the microtonalites and dacites are younger by ca. 8 Ma or more (Middle to Upper Miocene, 14.6 to 6.4 Ma). Major and trace element data (table II) show that the Lower Miocene diorites display all the usual characteristics of subduction-related magmas. The Middle to Upper Miocene microtonalites and dacites share some of these characteristics, but in addition they display typical adakitic features: SiO 2 -rich (65.5-70%) and sodic (Na 2 O/K 2 O&amp;gt;2) character (table II and figure 3); lack or rare occurrence of pyroxenes, usually replaced by early-crystallized (near-liquidus) amphiboles (table III); very low Y and HREE contents, consistent with the presence of residual garnet in their source, and leading to characteristically high La/Yb and Sr/Y ratios (fig. 4, 5). Their titanomagnetite-hemoilmenite associations reflect equilibrium features [Bacon and Hirschman, 1988] indicating moderate temperatures (&amp;lt;900 degrees C) and highly oxidizing (NNO+1) crystallization conditions [Ghiorso and Sack, 1991]. The Lower Miocene Sarawak diorites are typically subduction-related from a geochemical point of view. They likely derive from the evolution of island-arc basaltic magmas, which themselves originated from the partial melting of upper mantle peridotites previously metasomatized by hydrous fluids expelled from the subducting oceanic slab [Tatsumi et al., 1986; Tatsumi, 1989]. The origin of the Middle-Upper Miocene adakitic microtonalites and dacites is different. According to previous studies, they likely derive from the partial melting of metabasalts (garnet amphibolites or eclogites) from subducted oceanic crust [Defant and Drummond, 1990; Defant et al., 1991, 1992; Drummond et al., 1996; Maury et al., 1996; Martin, 1993, 1999]. Their position in the hybrid tonalite+peridotite system [Caroll and Wyllie, 1989] shows that they crystallized within the garnet stability field and likely interacted with the upper mantle during their ascent (fig. 7). This feature is not consistent with their genesis through melting of metabasalts accreted at the base of the Borneo continental crust. In addition, the less evolved Sarawak adakites display mineralogical and geochemical features remarkably similar to those of the 1991 Mt Pinatubo dacite, the experimental petrology of which has been extensively studied at low [2 kbar; Scaillet and Evans, 1999; Rutherford and Devine, 1996] to medium pressures [4 to 20 kbar; Prouteau et al., 1999]. Such dacitic magmas are not in equilibrium with garnet at pressures lower than or equal to 20 kbar, which rules out their derivation from metabasalts tectonically or magmatically accreted to the base of the North Borneo continental crust. We propose, instead, that they originated from the partial melting of basalts from a fragment of oceanic lithosphere within the upper mantle. Like the adakites of Central Mindanao, Philippines [Sajona et al., 1994, 1997 and 2000; Maury et al., 1996] and those from Aird Hills, Papua-New Guinea [Smith et al., 1979; Defant and Drummond, 1990] the Sarawak adakites represent potential markers of the occurrence at depth of oceanic crust slivers, which could be much more common in collision zones than previously thought.

Presidential Address: Presented 2000 August 28, Chicago, Illinois, USA The eucrite/Vesta story

Abstract— Many lines of evidence indicate that meteorites are derived from the asteroid belt but, in general, identifying any meteorite class with a particular asteroid has been problematical. One exception is asteroid 4 Vesta, where a strong case can be made that it is the ultimate source of the howardite‐eucrite‐diogenite (HED) family of basaltic achondrites. Visible and near‐infrared reflectance spectra first suggested a connection between Vesta and the basaltic achondrites. Experimental petrology demonstrated that the eucrites (the relatively unaltered and unmixed basaltic achondrites) were the product of approximately a 10% melt. Studies of siderophile element partitioning suggested that this melt was the residue of an asteroidal‐scale magma ocean. Mass balance considerations point to a parent body that had its surface excavated, but remains intact. Modern telescopic spectroscopy has identified kilometer‐scale “Vestoids” between Vesta and the 3:1 orbit‐orbit resonance with Jupiter. Dynamical simulations of impact into Vesta demonstrate the plausibility of ejecting relatively unshocked material at velocities consistent with these astronomical observations. Hubble Space Telescope images show a 460 km diameter impact basin at the south pole of Vesta. It seems that nature has provided multiple free sample return missions to a unique asteroid. Major challenges are to establish the geologic context of the HED meteorites on the surface of Vesta and to connect the remaining meteorites to specific asteroids.

Silicate-melt inclusions in magmatic rocks: applications to petrology

Silicate-melt inclusions in igneous rocks provide important information on the composition and evolution of magmatic systems. Such inclusions represent accidentally trapped silicate melt (±immiscible H 2O and/or CO 2 fluids) that allow one to follow the evolution of magmas through snapshots, corresponding to specific evolution steps. This information is available on condition that they remained isolated from the enclosing magma after their entrapment. The following steps of investigation are discussed: (a) detailed petrographic studies to characterise silicate-melt inclusion primary characters and posttrapping evolution, including melt crystallisation; (b) high temperature studies to rehomogenise the inclusion content and select chemically representative inclusions: chemical compositions should be compared to relevant phase diagrams. Silicate-melt inclusion studies allow us to concentrate on specific topics; inclusion studies in early crystallising phases allow the characterisation of primary magmas, while in more differentiated rocks, they unravel the subsequent chemical evolution. The distribution of volatile species (i.e., H 2O, CO 2, S, Cl) in inclusion glass can provide information on the degassing processes and on recycling of subducted material. In intrusive rocks, silicate melt inclusions may preserve direct evidence of magmatic stage evolution (e.g., immiscibility phenomena). Melt inclusions in mantle xenoliths indicate that high-silica melts can coexist with mantle peridotites and give information on the presence of carbonate melt within the upper mantle. Thus, combining silicate-melt inclusion data with conventional petrological and geochemical information and experimental petrology can increase our ability to model magmatic processes.

Thermodynamic data from redox reactions at high temperatures. VI. Thermodynamic properties of CoO–MnO solid solutions from emf measurements

Activities of CoO in (Co,Mn)O solid solutions in contact with metallic Co have been determined on ten compositions ranging from 0.12 to 0.84 XCoO in order to calibrate the divariant equilibrium between (Co,Mn)O oxide solutions and Co metal as an oxygen fugacity sensor for application in experimental petrology. Experiments were conducted over the temperature range 900–1300 K at 1 bar, using an electrochemical technique with oxygen-specific calcia-stabilized zirconia (CSZ) electrolytes. Co + CoO or Fe + FeO was used as the reference electrode. Compositions of the (Co,Mn)O solid solutions were measured after each run by electron microprobe, and these were checked for internal consistency by measuring the lattice parameter by X-ray diffraction. Activity–composition relations were fitted to the Redlich–Kister formalism. (Co,Mn)O solid solutions exhibit slight positive deviations from ideality, which are symmetrical (corresponding to a regular solution mixing model) across the entire composition range with A0G = 3690(±47) Jmol−1. Excess entropies and enthalpies were also derived from the emf data and gave Sex=0.77(±0.08) JK−1 mol−1 and Hex=4558(±90) Jmol−1 respectively. The experimental data from this study have been used to formulate the (Co,Mn)O/Co oxygen fugacity sensor to give an expression:

D.A. Young N.L. Bowen and Crystallization-Differentiation: the Evolution of a Theory. Washington, D.C. Mineralogical Society of America Monograph No. 4, l998, xii + 296 pp. Price $16.00 ($12.00 for MSA members). ISBN: 0939950472

On the brink of the twenty-first century, petrology is a very different subject than it was in its heyday in the twentieth, and this book tells the story of N.L. Bowen, the great pioneer of experimental petrology, and of the extraordinary impact that he has had on petrological thought. From the start, Bowen sought to explain the features of igneous rocks in terms of liquid–solid equilibria in simple silicate systems, and how the residual liquids changed composition in response to the separation of crystals. This is crystallization-differentiation, which is more popularly known as fractional crystallization. To gain acceptance, this required that contemporary thought be rid of the concept of eutectic crystallization, which Bowen did with evident gusto. From today’s perspective fractional crystallization has been a phenomenally successful concept underlying almost all of igneous petrology, and this book details some of the great petrological controversies that emerged in the period 1913–1958, fostered by the research of Bowen. Two most bitter, sometimes acrimonious, disputes described in this book are iron-enrichment in residual liquids, observed in nature by Fenner (a colleague of Bowen at the Geophysical Laboratory), and the …

CASTRO, A., FERNÁNDEZ, C. &amp; VIGNERESSE, J. L. (eds) 1999. Understanding Granites: Integrating New and Classical Techniques. Geological Society Special Publication no. 168. vi+278 pp. London, Bath: Geological Society of London. Price £70.00, US $117.00; members' price £32.00, US $53.00; AAPG/SEPM/GSA price £42.00, US $70.00; hard covers. ISBN 1 86239 058 4.

‘Granite magmatism represents a major contribution to crustal growth and recycling and, consequently, is one of the most important mechanisms to have contributed to the geochemical differentiation of the Earth’s crust since the Archean times’, so say the editors of this book. Their aim was to present an integrated approach to granite studies using geophysics, geochemistry, experimental petrology, structural geology, scale modelling and field geology. The book ‘was inspired by the international workshop’ on modern and classical techniques in granite studies held in the University of Huelva, Spain in 1997. Seventeen chapters in the book were written by 30 authors, and many chapters are review papers discussing the development and achievements using particular techniques, such as geophysical imaging and crystal size distribution (CSD). Others describe problem solving using a variety of techniques, e.g. structural, gravitational and geochemical, to interpret pluton emplacement …

Experimental Aspects of UHP Metamorphism: Granitic Systems

The outstanding development of recent years in the experimental petrology of granite systems with water is the in situ determination of critical curves in pressure-temperature diagrams, along which the hydrous melts and silicate-bearing vapors coexisting at low pressures merge compositionally, and form a single-phase supercritical fluid stable at higher pressures. Their locations, extending from pressures of ∼15 kbar near 1000°C to ∼25 kbar near 700°C, suggest that previous determinations of excess-water granite melting at higher pressures were influenced by quench problems. Under the conditions of ultrahigh-pressure metamorphism, therefore, granites in the presence of water do not melt, but are more or less gradually dissolved in the supercritical fluid with rising temperature, i.e., without any sharp “melting temperature.” The anhydrous compositions of the supercritical fluids change from strongly incongruent, mainly potassium silicatic at low temperatures via ultrapotassic, leucititic compositions to congruent granitic near the temperature of complete dissolution. The presence of hydrous supercritical fluids during ultrahigh-pressure metamorphism, possible at all temperatures whenever water is available, has a decisive weakening effect on the rheological properties of crustal rocks during deep subduction.

Comments on Some Misconceptions in Igneous and Experimental Petrology and Methodology: a Reply

Comments on Some Misconceptions in Igneous and Experimental Petrology and Methodology: a Reply

Comments on Some Misconceptions in Igneous and Experimental Petrology and Methodology: a Reply

rate’ of cpx and opx. Isobaric melting experiments show INTRODUCTION a > b, but the abyssal peridotite data suggest b > a, i.e. I recently showed (Niu, 1997) that proportions of residual opx would contribute more than cpx to the melt during minerals in abyssal peridotites (e.g. Dick, 1989; Johnson melting beneath ocean ridges. As abyssal peridotites et al., 1990; Johnson & Dick, 1992) exhibit systematic are apparently metamorphosed and deformed under variations with whole-rock MgO content. I also showed subsolidus conditions (e.g. Niu, 1997; Niu et al., 1997) that the whole-rock MgO content in melting residues is this could have resulted in mineral modal and comlinearly related to the overall extent of melting or melt positional re-equilibration. In this case, equation (1) could depletion [see fig. 8 of Niu (1997)]. These observations be an artifact of such a post-melting equilibration. The thus allow an explicit examination of how residual mineral effect of this equilibration may exist, but it is not petproportions change in response to varying extents of rographically evident, and it cannot explain the observed melting as shown in Fig. 1 [fig. 12 of Niu (1997)], and modal changes with increasing extent of melting (see allow an insightful understanding of the chemical and Dick et al., 1984; Niu et al., 1997). As the abyssal peridotites mineralogical consequences of actual mantle melting prostudied are from many locations (not a single location), cesses beneath ocean ridges. Figure 1 can be expressed and define much of the data range on a global scale, I in terms of a simple mathematical form [equation (8) of thus interpreted equation (1), with b > a, as representing Niu (1997)]: the net effect of most likely complex polybaric (vs isobaric) melting processes beneath ocean ridges (Niu, 1997). This 0·466 cpx+ 0·652 opx+ 0·049 spl interpretation is supported by decompression melting model results in which b > a [see fig. 6 of Niu (1997)], = 1·000 melt+ 0·167 ol. (1) and by the polybaric phase equilibria analysis which shows that opx (vs cpx) melting is further assisted by the incongruent melting of opx ⇒ ol+ SiO2 with decreasing Equation (1) says that the abyssal peridotite data (vs models) pressure [see fig. 7 of Niu (1997)]. Importantly, this tell us that in order to produce 1·000 mass unit of a interpretation is consistent with, and required by, the basaltic melt, approximately 0·466 mass unit of cpx, 0·652 fact that melting can occur beneath ocean ridges only mass unit of opx, and 0·049 mass unit of spinel are required because of fertile mantle upwelling and decompression (i.e. to melt whereas about 0·167 mass unit of olivine is required polybaric vs isobaric) in response to plate separation. to crystallize. Equation (1) is identical in form to isobaric My observation [Fig. 1 and equation (1)] and inmelting reactions determined experimentally (e.g. Kinzler terpretations have received aggressive criticisms from & Grove, 1992a; Baker & Stolper, 1994): a cpx+ b opx+ c Walter (1999) because my observation differs from isobaric spl = d ol+ 1 Melt, suggesting that mantle melting is melting experiments and his model melting reactions [see indeed incongruent. What differs between equation (1) and the isobaric melting reactions is the relative ‘melting his equations (2)–(5), table 1 and fig. 2]. In this reply, I

Kushiro receives 1999 Harry H. Hess Medal

Ikuo Kushiro was awarded the Harry H. Hess Medal at the AGU Spring Meeting Honors Ceremony, which was held on June 2, 1999, in Boston, Massachusetts. The medal recognizes outstanding achievements in the research of the constitution and evolution of Earth and its sister planets.Characterization of rock‐forming processes requires laboratory experiments to determine the physicochemical properties of materials or their analog model compounds at high temperatures and high pressures. The experimental data, however, are not particularly useful unless integrated with observational and theoretical information. Ikuo Kushiro, this year's winner of the Harry H. Hess Medal, approaches experiments with this integrated view as his guiding light. He does experimental petrology, geochemistry and geophysics, but always with the understanding that the experimental data provide only a piece of a more complex puzzle. It is perhaps in part because of this attitude that Ikuo and his colleagues have provided the fundamental experimental data on phase equilibria, element partitioning, and equation‐of‐state of molten silicate applicable to most major rock‐forming processes in the Earth. All too often we ask what we can do with our instrumentation and proceed from there. Ikuo first asks the real question and then designs the experiment.

About this title

Granite magmatism represents a major contribution to crustal growth and recycling and, consequently, is one of the most important mechanisms to have contributed to the geochemical differentiation of the Earth’s crust since Archaean time. Granites are also often associated with ore bodies, and their study therefore has direct commercial relevance. The modern view of the granite problems requires the application of many different theoretical, experimental and empirical resources provided by geophysics, geochemistry, experimental petrology, structural geology, scale modelling and field geology. Because of the complexity of the granite problem, it is necessary to integrate a variety of techniques and corroborate the findings with field observations.This is the philosophy of this book. Many chapters are review papers dealing with the development and achievements of a particular technique, whilst other chapters deal with the application of a number of techniques to a specific problem. This volume brings together papers that would otherwise be dispersed in different publications. The book will be of interest to igneous petrologists, geophysicists, structural geologists and geochemists.

Understanding granites: integrating new and classical techniques

Granite magmatism represents a major contribution to crustal growth and recycling and, consequently, is one of the most important mechanisms to have contributed to the geochemical differentiation of the Earth’s crust since Archaean time. Granites are also often associated with ore bodies, and their study therefore has direct commercial relevance. The modern view of the granite problems requires the application of many different theoretical, experimental and empirical resources provided by geophysics, geochemistry, experimental petrology, structural geology, scale modelling and field geology. Because of the complexity of the granite problem, it is necessary to integrate a variety of techniques and corroborate the findings with field observations.This is the philosophy of this book. Many chapters are review papers dealing with the development and achievements of a particular technique, whilst other chapters deal with the application of a number of techniques to a specific problem. This volume brings together papers that would otherwise be dispersed in different publications. The book will be of interest to igneous petrologists, geophysicists, structural geologists and geochemists.