Petrological Considerations Research Articles

Pseudoleucite syenites at Loch Borralan, Scotland: Petrology and a genetic model

ABSTRACT The alkaline Loch Borralan intrusion (Assynt Region, NW Highlands of Scotland) consists of a composite arrangement of several ultramafic to felsic plutonic rock bodies which were emplaced around 430 Ma into the Moine Thrust Zone during the Caledonian Orogeny. Some of the Loch Borralan rocks are ultrapotassic and contain pseudoleucite, i.e., a pseudomorph of alkali feldspar and nepheline after leucite. In total, 25 samples have been investigated, representing garnet-bearing pseudoleucite syenites and accompanying rock types such as nepheline-garnet-bearing syenites, alkali feldspar syenites, an amphibole syenite, a biotite-clinopyroxene syenite, and calcite-bearing glimmerites. Pseudoleucite is always associated with garnet, biotite, orthoclase, and minor clinopyroxene and titanite. Mineral chemical data indicate rather primitive magma compositions with no major differences between the various investigated main rock units. The abundant occurrence of up to 2 cm large, mostly euhedral pseudoleucite crystals and petrological phase considerations suggest that magmatic leucite physically separated from its host magma as a flotation cumulate. Based on our data and a comparison with previous field-based and experimental work, K-rich basanitic to tephriphonolitic melts that originated from a K-enriched mantle source may be parental to these rocks. The high liquidus temperatures at low pressures (e.g., ∼1100 °C at 1 bar PH2O) required to crystallize leucite could have resulted from the ascent of successive melt batches in a composite intrusion. Later melt batches would increase the temperature in earlier, already partially cooled batches, causing an increase in temperature and a decrease in pressure during ascent. The subsequent decomposition of leucite to pseudoleucite is interpreted to result from either dry breakdown or autometasomatism, i.e., involvement of late-magmatic fluids.

Origin of Triassic magmatism of the Southern Alps (Italy): Constraints from geochemistry and Sr-Nd-Pb isotopic ratios

During Middle-early Late Triassic (∼243–235?Ma) a diffuse igneous activity developed in the Southern Alps (Italy). Sparse lava flow and pyroclastic succession remnants of such Southern Alps Triassic Igneous Rocks (SATIR) crop out in the Brescian Prealps, the Vicentinian Alps (Recoaro-Schio-Posina), Non Valley, Dolomites and Julian Alps. Plutonic rocks are found in two main plutonic complexes (Monzoni and Predazzo) and a small stock (Cima di Pape). Coeval igneous products can be traced eastward to Austria and Dinarides, for a total length of ∼450?km.The coeval formation of major late Anisian-late Ladinian carbonate platforms in the subsiding eastern Dolomites, and significant uplift and subaerial erosion in the western Dolomites suggest, for these areas, the occurrence of large-scale strike-slip and extensional tectonics with the development of horst-and-graben structures.This study reports the first complete review of the SATIR activity, including new mineral chemical data on 14 samples, 61 major and trace element whole rock analyses and 7 Sr-Nd-Pb isotopic ratios for volcanic and plutonic samples from Dolomites (lavas plus Monzoni and Predazzo plutonic rocks) and Vicentinian Alps lavas. Despite the variable post-magmatic modifications, the large areal distribution of the products and their wide spectrum of chemical compositions, these samples show rather common geochemical and mineralogical characteristics and define major and trace element trends that can be associated with nearly close-system magmatic evolution. Minor upper crustal contaminations can be observed in specific cases, mostly in the most differentiated products (SiO2 >70?wt%).A specific characteristic of SATIR is their calcalkaline to shoshonitic affinity, resembling the derivation from subduction-modified mantle sources, a feature at odds with the coeval strike-slip and extensional tectonics. Geochemical modelling, petrological considerations and geological constraints allow us to propose a model in which the SATIR mantle sources reflect previous subduction metasomatism (likely occurred during Variscan times). These metasomatised mantle sources were reactivated ∼90–100 Myr after the end of the subduction, when continental rifting caused a raise of the geotherms and passive upwelling of asthenospheric mantle.

Formation of a spatter-rich pyroclastic density current deposit in a Neogene sequence of trachytic-mafic igneous rocks at Mason Spur, Erebus volcanic province, Antarctica

Erosion has revealed a remarkable section through the heart of a volcanic island, Mason Spur, in the southwestern Ross Sea, Antarctica, including an unusually well-exposed section of caldera fill. The near-continuous exposure, 10 km laterally and > 1 km vertically, cuts through Cenozoic alkalic volcanic rocks of the Erebus volcanic province (McMurdo Volcanic Group) and permits the study of an ancient volcanic succession that is rarely available due to subsequent burial or erosion. The caldera filling sequence includes an unusual trachytic spatter-rich lapilli tuff (ignimbrite) facies that is particularly striking because of the presence of abundant black fluidal, dense juvenile spatter clasts of trachytic obsidian up to 2 m long supported in a pale cream-coloured pumiceous lapilli tuff matrix. Field mapping indicates that the deposit is an ignimbrite and, together with petrological considerations, it is suggested that mixing of dense spatter and pumiceous lapilli tuff in the investigated deposit occurred during emplacement, not necessarily in the same vent, with the mixed fragmental material emplaced as a pyroclastic density current. Liquid water was not initially present but a steam phase was probably generated during transport and may represent water ingested during passage of the current as it passed over either wet ground, stream, shallow lake or (possibly) snow. Well-exposed caldera interiors are uncommon and that at Mason Spur is helping understand eruption dynamics associated with a complex large island volcano. The results of our study should help to elucidate interpretations of other, less well exposed, pyroclastic density current deposits elsewhere in Antarctica and globally.

Petrological Considerations for the Demineralization of Rajmahal Coals with Pseudomonas mendocina B6-1

ABSTRACT The present study entails the results of the petrological coniderations for demineralization of Rajmahal Gondwana coals with Psudomonas mendocina B6-1. Inertinite group macerals are the dominant constituents of these coals, followed by vitrinite group, while liptinite occurs in low concentration. The amount of Mineral matter is moderately high. The concentration of major, minor and trace elements is high when compared with Clarke values. After the treatment of these coals with Pseudomonas mendocina strain B6-1, a significant reduction in the elemental content of oxygen, hydrogen and sulphur was observed. A gradual reduction of pyrite phase due to bioleaching was identified and its signatures were reflected in the XRD spectra and FTIR absorption bands. Over 5 % reduction in the ash content and decrease of major, minor and trace element to variable degrees were also noticed. Fe, As, and Ca positively correlate with the ash removal percentage indicating that the samples with high concentration of these elements were prone to demineralization with Pseudomonas mendocina strain B6-1 whereas Ni, Zn, Cr and Cu, negatively correlate with the ash removal percentage and shows that their enrichment impeded the process of demineralization. With increasing concentration of vitrinite the removal of major/minor/trace elements also increased which is attributed to the possible association of these elements with the mineral matter occurring as superficial mounting and superficial blanketing over the vitrinite macerals. This could facilitate the bacterial access to the elements to act upon and remove it to the variable extent. Some minerals occur intergrown with inertinites causing restricted bacterial action owing to the non-exposure of the mineral particles and less surface area available to the bacteria for bioleaching. Sulfur removal strongly relates with increase in inertinite content and decrease in liptinite content. This appears that sulfur associated with liptinites have shown difficult removal condition. The maximum removal of the trace elements like Cd, Cu, Co, Zn and Pb was observed from the ‘banded dull coal’ samples of the Rajmahal basin while maximum removal of major/minor elements like Fe, Ca and Mg was noticed from the ‘banded bright coal’ samples. However, maximum removal of Mn and As was also observed in the samples of ‘banded coal’.

Iron Age silicate slags from Val Malenco (Italy): the role of textural and compositional studies in the reconstruction of smelting conditions

A petrographic study of silicate smelt-slags from several localities of Val Malenco (Italy) provides a key for unravelling the technologies used in reduction of copper ores during the Iron Age. Thick “tap type” slags with “rope surface” structures and thin, flat slags, were analysed. All slags are made of olivine (up to 60%), oxides, sulphides, minor interstitial Fe-rich pyroxenes (esseneite and augite) and glass. Olivine is zoned, with Mg-rich cores, Fe-rich inner rims and thin Ca-rich outer rims. Mesh textures, the composition of olivine cores and relic minerals, such as Cr-rich spinel, suggest that the mineralised levels of the Malenco ophiolitic complex were the source of the exploited ore. Ca-rich olivine rims and Ca-rich interstitial phases suggest a Ca-rich material was supposedly added during smelting to lower the melting point of the ore. The material used for this task could have been marbles or calcschists of ophicarbonates outcropping nearby Lanzada. Based on petrological considerations, the smelting conditions were constrained to a furnace in which, for temperatures around 1080-1200°C, oxygen fugacity was 10 -14 -10 -9 atm.

Fluid-triggered earthquake swarms in the Rwenzori region, East African Rift—Evidence for rift initiation

The Rwenzori Mountains are located within the Albertine Rift Valley in western Uganda. To monitor the microseismic activity in the area we have deployed a seismic network of up to 35 stations for a period of about 20months. The analysis of the recordings revealed several earthquake clusters within a restricted area NE of the mountain block. The clusters form elongated pipes with 1–2km diameter and vertical extensions of 3–5km. Most of them are located in 5–16km depths; however one cluster reaches down to 22km. Each cluster is composed of a series of single earthquake swarms with durations between a few days and more than a week, interrupted by intervals of inactivity of up to several months. Some of the swarm events exhibit vertical migration tendencies with estimated velocities between 0.3 and 1km/day. Local magnitudes range from ML=0.5 to ML=4.0 with b-values between 0.96 and 1.2. The source mechanisms of the swarm earthquakes are dominated by normal faulting with tension-axes orientations perpendicular to the rift axis. There are only few strike-slip events and no reverse mechanisms. From petrological considerations we presume that the earthquake swarms are triggered by fluids and gases which originate from a magmatic source below the crust. Melt and/or CO2 are guided along the intersection lines of two steep fault sets that were identified by shear-wave splitting analysis and fault mapping in the Rwenzori area. The existence of a magmatic source within the lithosphere is supported by the detection of a shear-wave velocity reduction in 55–80km depth from receiver-function analysis and the location of mantle earthquakes at about 60km. We interpret these observations as indication for an initial rifting process that may eventually lead to the complete detachment of the Rwenzori block from the surrounding rift flanks.

Geological and Petrological Considerations for Coal Bed Methane Exploration: A Review

Coal bed methane occupies pores and cleats of coal. The cleats are generated during the bituminous rank stage while their abundance decreases at high rank due to an increase in the thermal maturation. It is the organic component of coal that stores methane, whereas the inorganic component reduces the gas sorption capacity. At a given rank, the vitrinite-rich bright coals have a higher sorption capacity. This phenomenon is not prominent at the low rank stage. The moisture content of coal hinders the coal bed methane production. Recovery of coal bed methane from low rank coals has opened new windows for the future of coal bed methane exploration programs.

“SIGMELTS”: A web portal for electrical conductivity calculations in geosciences

Electrical conductivity measurements in the laboratory are critical for interpreting geoelectric and magnetotelluric profiles of the Earth's crust and mantle. In order to facilitate access to the current database on electrical conductivity of geomaterials, we have developed a freely available web application (SIGMELTS) dedicated to the calculation of electrical properties. Based on a compilation of previous studies, SIGMELTS computes the electrical conductivity of silicate melts, carbonatites, minerals, fluids, and mantle materials as a function of different parameters, such as composition, temperature, pressure, water content, and oxygen fugacity. Calculations on two-phase mixtures are also implemented using existing mixing models for different geometries. An illustration of the use of SIGMELTS is provided, in which calculations are applied to the subduction zone-related volcanic zone in the Central Andes. Along with petrological considerations, field and laboratory electrical data allow discrimination between the different hypotheses regarding the formation and rise from depth of melts and fluids and quantification of their storage conditions.

Significant cooling during exhumation of UHP eclogite from the Taohang area in the Sulu region of eastern China and its tectonic significance

AbstractThe decompressional pressure–temperature (P–T) path was estimated for ultrahigh‐pressure (UHP) eclogite from the Sulu region of eastern China by applying geo‐thermobarometers to well‐preserved equilibrium mineral pairs. The sample studied is a kyanite‐bearing eclogite that was collected from the Taohang area of the Sulu region. Garnet is relatively homogeneous in chemical composition, but omphacite has a clear chemical zoning with decreasing jadeite content from core to rim. Assuming that peak‐P equilibrium compositions are preserved in the cores of garnet (Grt) and omphacite (Omp), P–T conditions were calculated to be about 700°C and 3.4 GPa. On the other hand, the jadeite content of omphacite rims varies from 0.35 to 0.46 mol.%. Nevertheless, the variation in Fe/Mg ratios of omphacite rims is very small. Temperatures of 566 ± 54°C were obtained at 1.5 GPa for garnet rim and omphacite rim pairs. These petrological considerations indicate that temperatures should have significantly declined during the early decompression stage of this eclogite. In other areas of the Sulu region, isothermal decompression paths were proposed, and it was concluded that the UHP rocks were exhumed as a large mass tens of kilometers in thickness to avoid thermal effects from the surrounding materials. However, the newly identified decompression path accompanying the significant cooling may indicate that the Taohang outcrop was located at the margin of the Sulu UHP terrane. Thus, the decompressional P–T path is not unique in the Sulu region and varies depending on the location.

Radial anisotropy and prior petrological constraints: A comparative study

Radial seismic anisotropy models are traditionally obtained using empirical constraints based on laboratory experiments and petrological considerations. We tested the hypothesis that such petrological constraints affect the uppermost mantle models of S wave anisotropy using a statistical approach. In addition, we were able to determine which model features are constrained by the data and which are dominated by the prior. We focused on large‐scale models and found that the most likely models obtained in both cases are highly correlated. This demonstrates that for the best data‐fitting solution, the geometry of uppermost mantle radial anisotropy is not strongly affected by prior petrological constraints. The amplitude of the anomalies, however, can change significantly: The best data‐fitting model obtained without petrological constraints displays stronger amplitudes than the one obtained with prior. This could become an issue when quantitatively interpreting seismic anisotropy models, and thus emphasizes the importance of accurately accounting for parameter uncertainties and trade‐offs, and of understanding whether the seismic data or the prior constraints the model. We showed that model uncertainties are strongly affected by the prior as the relative RMS uncertainties were reduced by a factor of 2. In addition, we showed that while the model distributions are not necessarily Gaussian a priori, imposing petrological constraints can force the distributions to be narrower and more Gaussian‐like, as expected from inverse theory. Finally, we demonstrated that the age dependence of seismic wave velocities is robust and independent of prior constraints. A similar age signal exists for anisotropy, but with larger uncertainties without prior constraints.

Evidence for a short-lived stratified magma chamber: petrology of the Z-To5 tephra layer ( c . 5.8 ka) at Zao volcano, NE Japan

Abstract Volcanic rocks from the Z-To5 tephra layer of Zao volcano, NE Japan, preserve petrological information that reflects the magmatic processes under the volcano. The Z-To5 rocks were formed by the mixing of three magmas that differed in composition and phenocryst assemblage: basalt (1150–1200 °C), with high Mg (Fo c . 81 ) olivine; basaltic andesite (1020–1100 °C), with Mg-rich orthopyroxene (Mg#= c . 78) and clinopyroxene (Mg#= c . 78), lower Mg olivine (Mg#= c . 78), and calcic (An c . 85 ) plagioclase; and andesite (900–1000 °C) with Mg-poor orthopyroxene (Mg#=61–66) and clinopyroxene (Mg#=64–68), and An-poor plagioclase. The basaltic magma was formed through fractionation of Fo c . 85 olivine from a less differentiated basaltic magma during its fast ascent from the depths. The andesitic magma, which occupied a shallow magma chamber, was heated by underplating of the basaltic magma, resulting in dissolution of some minerals. Subsequently, the basaltic andesite magma was formed by mixing of the basaltic and andesitic magmas in the chamber. Petrological evidence for the rapid growth of phenocrysts in the basaltic andesite magma suggests that the magma residence time was short. The basaltic andesite magma, mixing with a small portion of the andesitic magma, was withdrawn upon eruption. The rates of these processes are inferred to be rapid based on petrological considerations.

A three-dimensional model of the Pyrenean deep structure based on gravity modelling, seismic images and petrological constraints

Summary The present study aims at building a 3-D density model of a whole collisional mountain belt at the lithospheric scale. The geometry of internal structures is constrained by seismic studies, while density values are imposed by petrological considerations. The model must fit both the Bouguer and geoid anomalies. This approach is applied to the Pyrenees, for which numerous geophysical data are available. The crustal structure is constrained from the results of seismic profiles. The most important crustal feature is the large Moho jump which coincides with the surface expression of the North Pyrenean Fault, considered as the suture between the Iberian and Eurasian plates. The Eurasian crust is about 30 km thick, whereas the Iberian crust is 50 km thick in the central part of the range. Thick sedimentary basins at the Piedmont, and two large heavy intracrustal bodies located in the western and central parts of the range, are also important features of the crustal model. At the lithospheric scale, a low-velocity strip under the North Pyrenean Fault down to 80–100 km depth, interpreted as the subduction of the Iberian lower crust, has been revealed by tomographic models. The density of this structure has been constrained using petrological arguments, which assume that the crustal material is transformed into eclogite at about 12–15 kbar (40–50 km depth). An eclogite-type density value, much higher than the mantle density, is thus imposed from 50 to 100 km depth. Theoretical Bouguer and geoid anomalies are computed for this 3-D density model using a summation of density block anomalies. A good agreement is found with observations, giving strong support to the proposed model. The crustal structure, as seen by seismic profiles, explains most of the observed long-wavelength negative Bouguer anomaly, as well the local anomalies related to the intracrustal bodies. A possible mantle origin of these bodies is ruled out. On the other hand, the transformation of the subducted lower crust into dense eclogite provides a successful fit to the long-wavelength positive geoid anomalies. In addition, the water released during eclogite formation may help to explain the high conductivity zone detected down to 80 km south of the North Pyrenean Fault. The 3-D model proposed here thus reconciles petrological considerations and geophysical data.

Seismic attenuation tomography of the Tonga‐Fiji region using phase pair methods

The anelastic structure of the region surrounding the Tonga slab and Lau back arc spreading center in the southwest Pacific is studied using data from 12 broadband island stations and 30 ocean bottom seismographs. Two differential attenuation methods determine δt* over the frequency band 0.1 to 3.5 Hz for earthquakes in the Tonga slab. The S‐P method measures the difference in spectral decay between P and S waves arriving at the same station. The P‐P method measures the difference in spectral decay for P waves with different paths through the upper mantle. Eight hundred sixty phase pairs are used to invert for two‐dimensional 1/Qα structure using a nonnegative least squares algorithm. A grid search method determines the Qα/Qβ ratio most compatible with both the S‐P and P‐P differential measurements. The highest attenuation (Qα = 90) is found within the upper 100 km beneath the active portions of the Lau Basin extending westward to the Lau Ridge. These regions probably delineate the source region for the back arc spreading center magmas, expected to be within the upper 100 km based on petrological considerations. The high attenuation regions also correlate well with zones of low P wave velocity determined by regional velocity tomography. Somewhat lower attenuation is found beneath the Fiji Plateau than beneath the Lau Basin. The entire back arc is characterized by a gradual decrease in attenuation to a depth of 300 to 400 km. The slab is imaged as a region of low attenuation (Qα > 900) material. A Qα/Qβ ratio of 1.75 provides the best fit between the S‐P and P‐P data sets upon inversion. Spectral stacking shows no frequency dependence within the frequency band analyzed.

THE MONTGENEVRE OPHIOLITE MANTLE SECTION. FROM GENESIS TO LATER PROCESSES: PETROLOGICAL AND STRUCTURAL CONSTRAINTS

The Alpine Piedmont Ligurian ophiolites are considered to be remnants of the Jurassic Tethyan ocean. Numerous similarities with northern Apennines ophiolites have been emphasized, particularly with the external belt. The most typical features of these ophiolites are: (1) a lherzolitic mantle section, (2) a reduced cumulate high level-type gabbroic portion, without well defined layering, (3) a lack of a sheeted- dyke complexes; (4) a reduced and discontinuous MORB crustal section. An upwelling of lherzolitic mantle with a low degree of partial melting, in a slow-spreading ridge environment, is generally proposed for the genesis of such a succession. The minor proportion of gabbros together with their high-level type characteristics are considered to be the result of crystallization in small magma chambers dispersed within the lherzolitic upper mantle. Except for minor exceptions, primary contacts between upper mantle and gabbro sections are not observed. This is due to modifications during oceanic and alpine tectonism. The Montgenevre ophiolite complex (MOC) is well known for having largely escaped Alpine metamorphic overprinting. Comprising several tectono-lithological units, it presents a complete pseudo-stratigraphy. Thus it is a favourable context for the study of primary features and oceanic transformations. Peridotitic rock-types are intensely serpentinized (> 90% most often, locally up to 70% only). Mantle peridotites are mainly lherzolites, but minor associated wehrlitic, harzburgitic, and dunitic zones are associated. Plagioclase-bearing peridotites are common. Serpentinites crop out either: (1) in the basal complex of the MOC, which is characterized by ophiolitic masses, and detritus, incorporated within the supra-ophiolite sedimentary cover and the schistes lustres, or, (2) mainly, in the upper tectonic unit, or klippe, essentially composed of the ophiolitic succession. The serpentinized peridotites are dominantly exposed at the eastern part of the upper unit. At the western part, with dominant submarine volcanites, serpentinites, often crushed, are localized only at the contact zone with the basal complex or along the contacts between lithological units, particularly between gabbro and serpentinized peridotite, testifying for a tectonic relationship. Several primary characteristics are still recognizable within the serpentinites: (1) type of original peridotite, (2) impregnation or not of basaltic liquid, (3) mantle tectonite or ultrabasic cumulate textures, (4) reworked serpentinites sometimes capped by a supra-ophiolite sedimentary cover. Moreover, as classically observed, pyroxenitic and troctolitc- gabbroic levels, gabbro and dolerite more or less rodingitic dykes, and albitite are associated within the mantle section. Petrological and structural considerations are obtained from the study of such relationships. The macroscopic and microscopic structures observed within the Montgenevre ophiolite upper mantle ultrabasic rocks, in a context apparently close to the petrological Moho, are mostly of tectonite type with high temperature plastic deformation of olivine and orthopyroxene, typical of asthenospheric conditions. Sometimes mylonitization developed in the oceanic environment, certainly during transverse or longitudinal fracturing of the lithosphere. The latter observation is typical of slow spreading ridge environments as observed for instance in the MAR Vema fracture zone. Magmatic liquid percolation locally modified the peridotitic rocks (e.g., impregnated lherzolites) and can, in some cases, when very close to the Moho, produce troctolitic lithologies belonging to the lowermost part of the cumulate section. Such rocks are sometimes difficult to differentiate from mantle peridotitic types exhibiting major melt impregnation, or even more or less diffuse melt “pockets”, and also caracterized by cumulate textures. There is no difference in primary mineralogical composition or in textural features, but the troctolitic rocks in the mantle suffered from rodingitic recrystallizations, whereas they are saussuritized in the lowermost crustal section. Another discriminant way is structural, based on lattice study of olivine crystals which should be undeformed (or very slighly plastically deformed) with low concentration of intracrystalline defects when related to the lowermost cumulate crustal section. For comparable mantle related ultrabasic cumulates, olivine is indeed strongly plastically deformed (with high intracrystalline defects concentration). Other presented petrological considerations are mainly based on geochemical data (major, trace and rare earth elements, and lead isotopes analyses).

Comparison and interpretation of graphitization in contact and regional metamorphic rocks

Abstract X‐ray diffraction (XRD) analyses of carbonaceous materials were carried out in conjunction with petrological studies for selected metamorphic rocks in order to compare the structural state of carbonaceous materials between contact and regional metamorphic rocks. The most extensive study was done for the Daimonji contact aureole in the eastern part of Kyoto city, Japan. The Daimonji contact aureole can be divided into three mineral zones using mineral parageneses of pelitic rocks: chlorite, biotite and cordierite zones. The cordierite zone can be further subdivided into lower‐ and higher‐grade subzones. Petrological considerations allow the two isograd reactions that define the lower‐ and higher‐grade cordierite subzones to be determined and suggest these reactions occurred at 510–560°C and 560–590°C per 2.0‐2.3 kbar, respectively. A combination of the petrological studies and the XRD data of carbonaceous materials suggest that fully ordered graphite (FG; defined by d(002) ≤ 3.360 Å following the convention used by many workers), appears around 560–590°C in the Daimonji contact aureole. This data and refinement of geothermometer for published data confirmed that the FG appears at 400–500°C in regional metamorphic rocks, but at higher than 530°C in contact aureoles. One possible explanation for such a temperature difference is the duration of heating. However, the width at half height (WH) of the graphite peak attains a similar value of 0.30° at around 500°C both in contact and regional metamorphic rocks, suggesting that WH value is a more reliable indicator of metamorphic grade than the change of d(002) value. Furthermore, the depressed d(002) data of graphite was observed locally in the higher grade part (≥ 500°C) of the Ryoke regional metamorphic belt, where granitic intrusions exist within a few km distance. These facts indicate that the duration of heating is not an important factor controlling the change of d(002) value. It is possible that interlayered impurities, such as chlorine, which was derived from igneous intrusions, may be an important factor in suppressing the reduction in d(002) at temperatures greater than 500°C.

Acfer 094, a uniquely primitive carbonaceous chondrite from the Sahara

Abstract— The Saharan meteorite Acfer 094 is a unique type of carbonaceous chondrite. Mineralogical and petrological considerations and O isotopes are unable to distinguish unambiguously between a CO3 vs. CM2 classification. The other important light elements, C and N, have systematics that do not match any previously recognised meteorite group. Particularly important in this respect is the very low C/N ratio and δ13C of the macromolecular C. Acfer 094 has more diamond and SiC, especially X type grains, than any other specimen studied, suggesting minimal thermal or aqueous processing to decrease its very primitive status.

Trace elements in diamond inclusions from eclogites reveal link to Archean granites

We report trace element data for rare garnet and clinopyroxene inclusions trapped within diamonds occurring in eclogite xenoliths from the Siberian craton. These inclusions are more depleted in incompatible trace elements and have lower Mg# than the eclogite host minerals, reflecting metasomatic enrichment of the eclogites after diamond formation by high Mg# melts that are enriched in incompatible trace elements (such as kimberlites). A reconstructed whole rock composition (using inclusion data) is severely depleted in light rare earth elements and niobium but shows a marked strontium enrichment. The melt in equilibrium with this eclogite is calculated to have had very high La Yb and Sr Nd and is Nb depleted (if rutile is present), characteristics similar to those of some convergent margin magmas but most especially Archean tonalitic rocks. Experimental petrologic studies demonstrate that eclogites of the type described here are in equilibrium with silicic melts such as tonalites or trondhjemites. Thus, both our trace element data and petrological considerations are consistent with the eclogites forming in equilibrium with Archean tonalitic or trondhjemitic magmas. Our data may therefore comprise the first evidence for the fate of these voluminous residues. A corollary of our findings is that trace element and isotopic data from eclogite minerals must be interpreted with caution because of the eclogite's complex evolutionary history. Specifically, models of early Earth differentiation based on isotopic data from Siberian eclogites [1,2] require re-evaluation.

Seismic velocity structure of the subducting Pacific Plate in the Izu‐Bonin Region

Although numerous studies have been made on the contrast between subducting slabs and surrounding mantle, little is known about the internal structure of the oceanic lithosphere after it descends into the mantle at subduction zones. The velocity structure within the slab can be expected to be heterogeneous, based on thermal and petrological considerations. In this study, observed travel time residual data from a spatially dense seismic network above deep earthquakes in the Izu‐Bonin region are compared with three‐dimensional ray tracing calculations. The data are inconsistent with a homogeneous slab model and consistent with a heterogeneous slab model with regional velocity variations. The residual data can be explained by a model that has a velocity gradient within the slab. In the subducting Pacific plate, the velocity near the center of the slab is faster than that near the upper boundary, and gradually decreases toward the bottom of the plate. A model with a velocity decrease of 3%, as predicted by a thermal profile, explains the observed data.

Steady state geothermal model of the crust and the problem of the boundary conditions: application to a rift system, the southern Rhinegraben

Seismic, gravity and magnetotelluric data are used to model the two-dimensional heat transfer beneath an E-W vertical cross section ranging from the Black Forest, Rhinegraben to the southern Vosges. The temperature field at the present time has been calculated taking into account the effect of temperature on thermal conductivity and the radiogenic heat sources. Heat flow density conditions at depth, computed by solving iteratively the heat equation and comparing the heat flow measured at the surface with the calculated one, have been used. As the resulting heat flow condition at depth is very sensitive to the interpolation method used to estimate the heat flow values at the surface, a confidence interval of the thermal field is then proposed by varying the possible mantle heat flow conditions using a Monte Carlo method. Arguments based on petrological considerations suggest that a temperature condition could be used at the Moho. The resulting model involves regional surface heat flow densities ranging from a normal crustal value of 70 mW m −2 to a highest value of 105 mW m −2, in agreement with actual measurements. The results are compatible with those obtained by the German KTB Program on the Black Forest (Gehlen et al., 1986) and on the Swiss traverse (Rybach, 1979). They suggest that the regional high heat flow (105 mW m −2) measured in the southern Vosges is caused by the high radiogenic elements content (especially thorium) of the Crêtes sub-alkaline granite and by an asymmetry in the lithospheric heating. High heat flow density values observed along the Rhinegraben give evidence of heat refraction phenomena around high conductivity rocks.

The causes of the high-grade metamorphism of the Scourie complex, NW Scotland

Summary A number of previous studies of the possible causes of Archaean high-grade metamorphism have suggested that the heat source was large bodies of (intermediate) magma emplaced within pre-existing crust. Geochronological data for the Lewisian indicate that there was probably a gap of c. 200 Ma between the cessation of significant igneous activity and the peak of high-grade metamorphism. This is sufficiently long to allow conductive dissipation of magmatic heat and to render a model of the metamorphism relying upon heat transported in by intrusions unsatisfactory. The quantity of deep-derived CO 2 that would be required to make it the primary cause of the metamorphism is larger than petrological considerations allow. Alternative models, based on enhanced basal heat-flow in a static crust or heat derived from the decay of radionuclides resident within the crust coupled with a more normal mantle heat-flow and tectonic crustal-thickening have been erected, and reveal that the P-T conditions and age data recorded by the Scourian granulites can be satisfied by such models. Thermal models of this nature, and a consideration of the available petrological and geochronological data, suggest a very slow cooling rate which would allow considerable intercrystalline diffusion to occur. This could give rise to the observed zoning profiles and other forms of re-equilibration found within the granulite-facies rocks of the Lewisian complex.