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This paper identifies documented collections of red-sulphur gem casts by Christian Dehn (1696–1770) that have survived unrecognized in three British country houses: Burton Constable in Yorkshire, Burghley House in Lincolnshire, and Mountstuart on the Isle of Bute in Scotland. All three collections resulted from Grand Tours to Italy, where, in Rome, Dehn's workshop supplied bespoke collections of casts representing engraved gemstones. [ABSTRACT FROM AUTHOR]

This paper explores the interest of Charles IV of Luxembourg (1316–1378), King of Bohemia and Holy Roman Emperor, in collecting gemstones, which has hitherto been little studied. It analyses this interest in connection with the fourteenth-century French royal court, and especially with the collecting activities of Charles V of Valois (1338–1380), King of France. Both of these sovereigns had an interest in ancient gems and they used them in the same manner; their practices prove to have been inspired by the reinvented traditions of the saintly Capetian monarch Louis IX, King of France, and of the ancient Roman Empire. This study shows how Charles of Luxembourg and Charles of Valois sought out and used precious and semi-precious stones as instruments of their royal self-images and claims to power. [ABSTRACT FROM AUTHOR]

The genus Barbaria , recently established to accommodate the former Echiniscus bigranulatus group, is a tardigrade group emblematic for the South American tardigrade fauna. This unappendaged echiniscid lineage is widely recognized for the so-called 'double' sculpturing composed of endocuticular pillars and pseudopores or pores in the dorsal cuticle. The phylogenetic relationships in the genus have so far been completely unknown, but the discovery of two new species (B. paucigranulata sp. nov. and B. weglarskae sp. nov.), together with new genetic data for further six species (B. bigranulata , B. charrua comb. nov. , B. danieli , B. jenningsi , B. madonnae and B. ollantaytamboensis), create an opportunity not only to uncover phyletic relationships, but also to reconstruct morphological evolution in the genus. To achieve this, we sequenced five genetic markers (18S rRNA, 28S rRNA, ITS1, ITS2, COI) for multiple populations of eight species of Barbaria (two-thirds of all known species) collected in Alabama (USA), Argentina and the Antarctic, and we analysed them in tandem with detailed morphological data. Our phylogentic analysis and the reconstruction of evolution of morphological traits suggests that the ancestor of the genus inhabited the Neotropics, and it was morphologically most similar to B. bigranulata. We also analyse literature records of Barbaria and conclude that the genus is most likely limited to the Neotropics, Antarctica and southern parts of the Nearctic. The findings are discussed in the context of the phylogeny of the Echiniscus evolutionary line. [ABSTRACT FROM AUTHOR]

Agate was highly regarded in antiquity and through the early modern period; when sliced, images seemingly inscribed by nature could be identified on its surfaces, and its layered colours were exploited for cameo carving. However, agate is also widely recorded in the rough in contemporaneous natural history collections where the 'eye' that could be cut from the core was valued in Aristotelian medicine. Taking as its starting point the hitherto neglected 1622 correspondence about a piece of agate between Peter Paul Rubens and the French humanist Nicholas‐Claude Fabri de Peiresc, this essay brings together collecting, friendship and artistic production within a context of early mineralogy. Through an investigation of the material, tactile, aesthetic and para‐scientific attributes of agate, and drawing on further evidence from Rubens's life, I propose a sensibility shaped by a predilection for the lithic whose influence is discernible in the artist's work as more widely understood.

Why do some rebel groups resort to terrorism tactics while others refrain from doing so? How rebel organizations finance their rebellion creates variation in the extent to which terrorism undermines their legitimacy. Rebel organizations pay attention to the legitimacy costs associated with terrorism. Organizations that rely primarily on civilian support, and to a lesser extent on foreign support, exercise more restraint in their use of terrorism. Rebels who finance their fight with lootable resources such as gems or drugs are least vulnerable to the costs of alienating domestic supporters. Thus, they are more likely to resort to terrorism and to employ more of it. The article elaborates this legitimacy-cost theory and tests it using new data on Terrorism in Armed Conflict from 1970 to 2007. We find robust support for the hypothesis that groups who finance their fight with natural resources are significantly more likely to employ terrorism (though not necessarily to conduct more deadly attacks) relative to those who rely on local civilian support. Groups with external sources of financing, such as foreign state support, may be more likely to engage in terrorism than those who rely on local civilians, but not significantly so. [ABSTRACT FROM AUTHOR]

The increasing availability of event data has led to a surge in micro-level research investigating civil war violence. Event data, however, introduces bias due to systemic underreporting of events. This study introduces a method that accounts for this bias by making a spatial prediction of unreported events based on the known locations of rebels and their preferences for local-level factors estimated using habitat analysis. I then combine this prediction with known locations of rebel activity and use it to predict rebel territory and relocation patterns. I next perform a regression analysis on 119 African rebel groups that links preferences to conflict behavior. Preferences for gold, petroleum, gemstones, and cobalt are associated with civilian victimization, whereas rebels who prefer steeper slopes are more successful in battles with government forces. Further, government forces tend to be successful when battling rebels with preferences for petroleum and international borders. [ABSTRACT FROM AUTHOR]

Recent research has brought back to prominence a remarkable collection of glyptic objects housed in the National Museum in Krakow. It was assembled by Constantine Schmidt-Ciążyński (1818-1889) in the second half of the nineteenth century, but has been little known since that time and remains largely unpublished. This extensive collection (over 2,500 pieces) sheds light in a unique manner on the European gem trade and on gem collecting in the mid-to late-1800s. The gems were engraved in a variety of minerals; only one object is made from an artificial material, namely glass paste. The collection offers an opportunity to investigate the collector's aims for his collection and the circumstances that shaped its special character. It seems very likely that the entire set was designed to have particular socio-cultural and educational functions. [ABSTRACT FROM AUTHOR]

This article deals with the history of the State Hermitage Museum’s collection of gem casts, with a focus on the dactyliotheca stored in the Department of Classical Antiquity containing over 25,000 pieces and currently kept in storage. This collection of plaster impressions has never been displayed to the public and its contents have not yet been published. Nevertheless, it forms a unique example of a collection of casts made from cameos and intaglios, both antique and modern. It is important in a number of ways, including its usefulness in studying the evolution of engraving techniques and its value in contributing to the repertoire of images encountered on gems. It is also an important resource for the study of gems that have not survived in original form to present day. [ABSTRACT FROM PUBLISHER]

Appended to the royal wardrobe accounts, the jewel accounts of King Henry III of England are the earliest records of their type. Describing gifts that were given and received by the king, the purchase of textiles, precious metal objects and specie, as well as the regalia, the Henrician jewel accounts provide valuable information about the aesthetics and material culture of English kingship during the thirteenth century. This article explains how the royal jewel accounts were created and structured, considers their utility, and shows how they can be used to shed new light on Henry III's character and kingship. [ABSTRACT FROM AUTHOR]

The first Duchess of Marlborough has been recognized as a powerful figure in court politics under Queen Anne. Her patronage of artists, sculptors, and architects – Laguerre, Rysbrack, Talman, Wren, Vanbrugh – has been examined by scholars. In this essay I take a different tack. I focus on a series of artefacts that played an important part in the Duchess's life: the jewels she amassed, the Turkish tent that her husband the Duke of Marlborough had used on the battlefield, and a sculpture of Queen Anne that she erected at Blenheim. Drawing on a wide range of sources from her own correspondence and contemporary biographies to caricature and popular print, I ask how an elite woman of immense wealth, but little formal education, strategically employed material things to exert influence socially and politically, and what were the unpredictable consequences thereof. [ABSTRACT FROM AUTHOR]

This article discusses the usage of gems in the 1920s and 1930s Soviet material culture. Material culture is a term in archeological literature to describe the tools, buildings and other artefacts that comprise the material remains of a former society. Architecture like the building of public monuments and mausoleums in the Soviet era has political and cultural meaning. Gemology has reached a high standard of production in the Tsarist regime of 1917. In the Urals, the Russkiye Samotsvety or Russian Gemstone has a host of experienced craftsman. The Russian Gemstone survived through the changing regimes from Tsarist period to Soviet era, and up to the post-Soviet period.

Garnets in continentally derived high-pressure (HP) rocks of the Sesia Zone (Western Alps) exhibit three different chemical zonation patterns, depending on sample locality. Comparison of observed garnet zonation patterns with thermodynamically modelled patterns shows that the different patterns are caused by differences in the water content of the subducted protoliths during prograde metamorphism. Zonation patterns of garnets in water-saturated host rocks show typical prograde chemical zonations with steadily increasing pyrope content and increasing XMg, together with bell-shaped spessartine patterns. In contrast, garnets in water-undersaturated rocks have more complex zonation patterns with a characteristic decrease in pyrope and XMg between core and inner rim. In some cases, garnets show an abrupt compositional change in core-to-rim profiles, possibly due to water-undersaturation prior to HP metamorphism. Garnets from both water-saturated and water-undersaturated rocks show signs of intervening growth interruptions and core resorption. This growth interruption results from bulk-rock depletion caused by fractional garnet crystallization. [ABSTRACT FROM PUBLISHER]

Petrogenetic grids in the system NCKFMASH (Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O) and the subsystems NCKMASH and NCKFASH calculated with the software THERMOCALC 3.1 are presented for the P–T range 7–30 kbar and 450–680°C, for assemblages involving garnet, chloritoid, biotite, carpholite, talc, chlorite, kyanite, staurolite, paragonite, glaucophane, jadeite, omphacite, diopsidic pyroxene, plagioclase, zoisite and lawsonite, with phengite, quartz/coesite and H2O in excess. These grids, together with calculated compatibility diagrams and P–T and T–XCa and P–XCa pseudosections for different bulk-rock compositions, show that incorporation of Ca into the NKFMASH system leads to many of the NKFMASH invariant equilibria moving to lower pressure and/or lower temperature, which results, in most cases, in the stability of jadeite and garnet being enlarged, but in the reduction of stability of glaucophane, plagioclase and AFM phases. The effect of Ca on the stability of paragonite is dependent on mineral assemblage at different P–T conditions. The calculated NCKFMASH diagrams are powerful in delineating the phase equilibria and P–T conditions of natural pelitic assemblages. Moreover, contours of the calculated phengite Si isopleths in P–T and P–XCa pseudosections confirm that phengite barometry in NCKFMASH is strongly dependent on mineral assemblage. [ABSTRACT FROM AUTHOR]

The effects of small amounts of H2O (<4 wt % in the melt) on the multiply saturated partial melting of spinel lherzolite in the system CaO–MgO–Al2O3–SiO2 ± Na2O ± CO2 have been determined at 1·1 GPa in the piston-cylinder apparatus. Electron microprobe analysis and Fourier transform infrared spectroscopy were used to analyse the experimental products. The effects of H2O are to decrease the melting temperature by 45°C per wt % H2O in the melt, to increase the Al2O3 of the melts, decrease MgO and CaO, and leave SiO2 approximately constant, with melts changing from olivine- to quartz-normative. The effects of CO2 are insignificant at zero H2O, but become noticeable as H2O increases, tending to counteract the H2O. The interaction between H2O and CO2 causes the solubility of CO2 at vapour saturation to increase with increasing H2O, for small amounts of H2O. Neglect of the influence of CO2 in some previous studies on the hydrous partial melting of natural peridotite may explain apparent inconsistencies between the results. The effect of small amounts of H2O on multiply saturated melt compositions at 1·1 GPa is similar to that of K2O, i.e. increasing H2O or K2O leads to quartz-normative compositions, but increasing Na2O produces an almost opposite trend, towards nepheline-normative compositions. [ABSTRACT FROM AUTHOR]

Xenoliths of subducted crustal origin hosted by Miocene ultrapotassic igneous rocks in the southern Pamir provide important new information regarding the geological processes accompanying tectonism during the Indo-Eurasian collision. Four types have been studied: sanidine eclogites (omphacite, garnet, sanidine, quartz, biotite, kyanite), felsic granulites (garnet, quartz, sanidine and kyanite), basaltic eclogites (omphacite and garnet), and a glimmerite (biotite, clinopyroxene and sanidine). Apatite, rutile and carbonate are the most abundant minor phases. Hydrous phases (biotite and phengite in felsic granulites and basaltic eclogites, amphiboles in mafic and sanidine eclogites) and plagioclase form minor inclusions in garnet or kyanite. Solid-phase thermobarometry reveals recrystallization at mainly ultrahigh temperatures of 10001100C and near-ultrahigh pressures of 2528?GPa. Textures, parageneses and mineral compositions suggest derivation of the xenoliths from subducted basaltic, tonalitic and pelitic crust that experienced high-pressure dehydration melting, K-rich metasomatism, and solid-state re-equilibration. The timing of these processes is constrained by zircon ages from the xenoliths and 40Ar/39Ar ages of the host volcanic rocks to 5711?Ma. These xenoliths reveal that deeply subducted crust may undergo extensive dehydration-driven partial melting, density-driven differentiation and disaggregation, and sequestration within the mantle. These processes may also contribute to the alkaline volcanism observed in continent-collision zones. [ABSTRACT FROM AUTHOR]

A late Mesozoic belt of volcanicintrusive complexes occurs in SE China. Volcanic activity at Xiangshan in the NW of the belt took place mainly in the Late Jurassic (158135?Ma). The volcanic rocks from the Xiangshan volcanic complex include rhyolitic crystal tuffs, welded tuffs, rhyolite lavas, porphyritic lavas, and associated subvolcanic rocks. Mineral assemblages in these magmatic rocks include K-feldspar, plagioclase, quartz, Fe-rich biotite and minor amphibole, orthopyroxene and almandine. Mineral geothermometry indicates a high crystallization temperature (>850C) for the Xiangshan magmas. The volcanic rocks are generally peraluminous; SiO2 contents are between 654% and 768% and the samples have high alkalis, rare earth elements (REE), high field strength elements and Ga contents and high Ga/Al ratios, but are depleted in Ba, Sr and transition metals. Trace element geochemistry and SrNdO isotope systematics imply that the Xiangshan magmas were probably derived from partial melting of Middle Proterozoic metamorphic lower-crustal rocks that had been dehydrated during an earlier thermal event. These features suggest an A-type affinity. Quenched mafic enclaves, hosted by the subvolcanic rocks, consist mainly of alkali feldspar, plagioclase, clinopyroxene, phlogopite and amphibole. Geothermometry calculations indicate that the primary magmas that chilled to form the quenched enclaves had anomalously high temperatures (>1200C). The quenched enclaves have boninitic affinities; for example, intermediate SiO2 contents, high MgO and low TiO2 contents, high Mg-numbers and high concentrations of Sc, Ni, Co and V. However, they also have shoshonitic characteristics, e.g. enrichment in alkalis, high K2O contents with high K2O/Na2O ratios, high light REE and large ion lithophile element contents, low initial ?Nd values (-42) and high initial 87Sr/86Sr ratios (07081). We suggest a phlogopite-bearing spinel harzburgitic lithospheric mantle source for these high-Mg potassic magmas. Underplating of such anomalously high-temperature magmas could have induced granulite-facies lower-crustal rocks to partially melt and generate the Xiangshan A-type volcanic suite. A back-arc extensional setting, related to subduction of the Palaeo-Pacific plate, is favoured to explain the petrogenesis of the Xiangshan volcanic complex and quenched enclaves. [ABSTRACT FROM AUTHOR]

Tourmaline in the Martinamor antiform occurs in tourmalinites (rocks with >1520% tourmaline by volume), clastic metasedimentary rocks of the Upper Proterozoic Monterrubio formation, quartz veins, pre-Variscan orthogneisses and Variscan granitic rocks. Petrographic observations, back-scattered electron (BSE) images, and microprobe data document a multistaged development of tourmaline. Overall, variations in the Mg/(Mg + Fe) ratios decrease from tourmalinites (036075), through veins (038066) to granitic rocks (023046), whereas Al increases in the same order from 584665 to 622688?apfu. The incorporation of Al into tourmaline is consistent with combinations of x?Al(NaR)-1 and AlO(R(OH))-1 exchange vectors, where x? represents X-site vacancy and R is (Mg + Fe2+ + Mn). Variations in x?/(x? + Na) ratios are similar in all the types of tourmaline occurrences, from 010 to 053, with low Ca-contents (mostly <010?apfu). Based on field and textural criteria, two groups of tourmaline-rich rocks are distinguished: (1) pre-Variscan tourmalinites (probably Cadomian), affected by both deformation and regional metamorphism during the Variscan orogeny; (2) tourmalinites related to the synkinematic granitic complex of Martinamor. Textural and geochemical data are consistent with a psammopelitic parentage for the protolith of the tourmalinites. Boron isotope analyses of tourmaline have a total range of d11B values from -156 to 68; the lowest corresponding to granitic tourmalines (-156 to -117) and the highest to veins (19 to 68). Tourmalines from tourmalinites have intermediate d11B values of -80 to +20. The observed variations in d11B support an important crustal recycling of boron in the Martinamor area, in which pre-Variscan tourmalinites were remobilized by a combination of mechanical and chemical processes during Variscan deformation, metamorphism and anatexis, leading to the formation of multiple tourmaline-bearing veins and a new stage of boron metasomatism. [ABSTRACT FROM AUTHOR]

Olivine-rich rocks containing olivine + orthopyroxene + spinel + Ca-amphibole clinopyroxene garnet are present in the central tztalStubai crystalline basement associated with eclogites of tholeiitic affinity. These rocks contain centimetre-sized garnet layers and lenses with garnet + clinopyroxene corundum. Protoliths of the olivine-rich rocks are thought to be olivine + orthopyroxene + spinel dominated cumulates generated from an already differentiated Fe-rich (XMgmelt = 057065) tholeiitic magma that was emplaced into shallow continental crust. Protoliths of the garnet-rich rocks are interpreted as layers enriched in plagioclase and spinel intercalated in a cumulate rock sequence that is devoid of, or poor in, plagioclase. UPb sensitive high-resolution ion microprobe dating of zircons from a garnet layer indicates that emplacement of the cumulates took place no later than 517 7?Myr ago. After their emplacement, the cumulates were subjected to progressive metamorphism, reaching eclogite-facies conditions around 800C and >2?GPa during a Variscan metamorphic event between 350 and 360?Ma. Progressive high-P metamorphism induced breakdown of spinel to form garnet in the olivine-rich rocks and of plagioclase + spinel to form garnet + clinopyroxene corundum in the garnet layers. Retrogressive metamorphism at T = 650680C led to the formation of Ca-amphibole, chlorite and talc in the olivine-rich rocks. In the garnet layers, hgbomite formed from corundum + spinel along with Al-rich spinel, Ca-amphibole, chlorite, aspidolitepreiswerkite, magnetite, ilmenite and apatite at the interface between olivine-rich rocks and garnet layers at P < 08?GPa. Progressive desiccation of retrogade fluids through crystallization of hydrous phases led to a local formation of saline brines in the garnet layers. The presence of these brines resulted in a late-stage formation of Fe- and K-rich Ca-amphibole and Sr-rich apatite, both characterized by extremely high Cl contents of up to 35 and 65?wt % Cl, respectively. [ABSTRACT FROM AUTHOR]

Migmatitic granulites and arc-related felsic intrusives of Pan-African age form the bedrock in the Rio de Janeiro area, SE Brazil. These rocks preserve a partial record of three parageneses. The earliest assemblage (M1) grew during fabric formation in the rocks (D1) and is characterized by the mineral assemblage Pl + Bt + Sil + Kfs + Qtz. Peak metamorphic conditions (M2) are characterized by the assemblage Bt + Crd + Kfs + Pl + Grt + liq + Qtz and are inferred to have developed during D2 folding of the rocks at T = 750800C and P = 7?kbar. M3 reaction textures overprint the M2 assemblage and comprise symplectitic intergrowth of cordierite(II) and quartz that formed after garnet, whereas secondary biotite formed as a result of reactions between garnet and K-feldspar. By comparing the observed modal abundances with modal contours of garnet, cordierite and quartz on the relevant pseudosection a post M2PT vector indicating contemporaneous cooling and decompression can be deduced. The inferred equilibrium assemblage and reaction textures are interpreted to reflect a clockwise PT path involving heating followed by post-peak decompression and associated cooling. We infer that metamorphism occurred in response to advective heating by the abundant syn-collisional (arc-related) I-type granitoids in the region, consistent with the unusually high peak T/P ratio. [ABSTRACT FROM AUTHOR]

Chromium as Cr3+ substitutes for octahedrally coordinated Al in upper-mantle minerals, thereby reducing the activity of Al2O3 in the system and hence the concentration of Al2O3 in partial melts. The effect of Cr2O3 on melt compositions multiply saturated with the spinel lherzolite phase assemblage has been quantified in the system CaOMgOAl2O3SiO2Cr2O3 at 11?GPa as a function of 100 Cr/(Cr + Al) in the spinel (Cr#sp). The decrease of Al2O3 in the melt with increasing Cr#sp is accompanied by increasing MgO and SiO2, whereas CaO remains almost constant. Consequently, the CaO/Al2O3 ratio of the melt increases with Cr#sp, and the melt becomes richer in normative diopside, hypersthene and quartz. The effect may explain certain mantle melts with unusually high CaO/Al2O3 ratios. The concentration of Cr2O3 in the melt remains low even at high Cr#sp, which means that the strong effect of Cr2O3 on partial melting equilibria is not readily apparent from its concentration in the melt itself. The existence of a highly refractory major component such as Cr2O3 nullifies simplified conclusions from the inverse approach in the experimental study of basalt petrogenesis, as there is insufficient information in the composition of the partial melt to reconstruct the conditions of melting. [ABSTRACT FROM AUTHOR]

Zircons from the Devils Kitchen rhyolite in the Pleistocene Coso Volcanic field, California have been analyzed by in situ Pb/U ion microprobe (SHRIMP-RG) and by detailed cathodoluminescence imaging. The zircons yield common-Pb-corrected and disequilibrium-corrected 206Pb/238U ages that predate a previously reported KAr sanidine age by up to 200?kyr, and the range of ages exhibited by the zircons is also approximately 200?kyr. Cathodoluminescence imaging indicates that zircons formed in contrasting environments. Most zircons are euhedral, and a majority of the zircons are weakly zoned, but many also have anhedral, embayed cores, with euhedral overgrowths and multiple internal surfaces that are truncated by later crystal zones. Concentrations of U and Th vary by two orders of magnitude within the zircon population, and by 1020 times between zones within some zircon crystals, indicating that zircons were transferred between contrasting chemical environments. A zircon saturation temperature of ~750C overlaps within error a previously reported phenocryst equilibration temperature of 740 25C. Textures in zircons indicative of repeated dissolution and subsequent regrowth are probably caused by punctuated heating by mafic magma input into rhyolite. The overall span of ages and large variation in U and Th concentrations, combined with calculated zircon saturation temperatures and resorption times, are most compatible with crystallization in magma bodies that were emplaced piecemeal in the crust at Coso over 200?kyr prior to eruption, and that were periodically rejuvenated or melted by subsequent basaltic injections. [ABSTRACT FROM AUTHOR]

The compositions of multiply saturated partial melts are valuable for the thermodynamic information that they contain, but are difficult to determine experimentally because they exist only over a narrow temperature range at a given pressure. Here we try a new approach for determining the composition of the partial melt in equilibrium with olivine, orthopyroxene, clinopyroxene and spinel (Ol + Opx + Cpx + Sp + Melt) in the system CaO–MgO–Al2O3–SiO2 (CMAS) at 1·1 GPa: various amounts of K2O are added to the system, and the resulting melt compositions and temperature are extrapolated to zero K2O. The ‘sandwich’ experimental method was used to minimize problems caused by quench modification, and Opx and Cpx were previously synthesized at conditions near those of the melting experiments to ensure they had appropriate compositions. Results were then checked by reversal crystallization experiments. The results are in good agreement with previous work, and establish the anhydrous solidus in CMAS to be at 1320 ± 10°C at 1·1 GPa. The effect of K2O is to depress the solidus by 5·8°C/wt %, while the melt composition becomes increasingly enriched in SiO2, being quartz-normative above 4 wt % K2O. Compared with Na2O, K2O has a stronger effect in depressing the solidus and modifying melt compositions. The isobaric invariant point in the system CMAS–K2O at which Ol + Opx + Cpx + Sp + Melt is joined by sanidine (San) is at 1240 ± 10°C. During the course of the study several other isobaric invariant points were identified and their crystal and melt compositions determined in unreversed experiments: Opx + Cpx + Sp + An + Melt in the system CMAS at 1315 ± 10°C; in CMAS–K2O, Opx + Cpx + Sp + An + San + Melt at 1230 ± 10°C and Opx + Sp + An + San + Sapph + Melt at 1230 ± 10°C, where An is anorthite and Sapph is sapphirine. Coexisting San plus An in three experiments help define the An–San solvus at 1230–1250°C. [ABSTRACT FROM AUTHOR]

The four, currently best constrained, independent thermobarometers for garnet peridotites, namely Taylor’s (Neues Jahrbuch für Mineralogie, Abhandlungen 172, 381–408, 1998) pyroxene solvus and Krogh’s (Contributions to Mineralogy and Petrology 99, 44–48, 1988) clinopyroxene–garnet Fe–Mg exchange thermometers, and Taylor’s (1998) Al-in-orthopyroxene and Nimis & Taylor’s (Contributions to Mineralogy and Petrology, 139, 541–544, 2000) Cr-in-clinopyroxene barometers, have been applied to garnet lherzolites from the Central Alps. Analyses from the literature, as well as new in-house analyses, all pertaining to core compositions of first-generation, garnet lherzolite minerals, have been selected for thermobarometric calculations. The P–T data obtained for the three known garnet lherzolite occurrences in the Central Alps are tightly constrained, consistent with one another, and summarized as follows: Alpe Arami, 3·2 GPa and 840°C; Monte Duria, 3·0 GPa and 830°C; Cima di Gagnone, 3·0 GPa and 740°C. These values are consistent with experimental data on pargasite stability and composition in peridotitic systems. Our P–T estimates, along with microstructural and field geological observations, indicate that the garnet lherzolite parageneses form part of the prograde, Alpine, high-pressure metamorphic sequence of the Adula–Cima Lunga unit. Thermobarometry shows that the garnet lherzolites reached a maximum depth of subduction of ∼100 km, leaving little room for an extraordinary high-pressure, Alpine metamorphism at Alpe Arami. The very high pressure estimates obtained by some other workers are ascribed to inaccuracies in thermometric evaluations and to the strong temperature dependence of the Al-in-Opx barometer. Uncertainties in the determination of garnet–olivine equilibrium compositions, combined with the small sensitivity of garnet–olivine chemical exchanges to temperature variations, and unreliability of garnet–orthopyroxene thermometry at low-temperature conditions may account for the observed inconsistencies. A discrepancy between thermobarometric data for the garnet lherzolites and the associated eclogites is non-existent in our example, and in other localities it may also be a thermobarometric artefact. [ABSTRACT FROM PUBLISHER]

U–Pb and Lu–Hf isotope analyses, obtained by laser ablation-sector field-inductively coupled plasma-mass spectrometry on zircon grains from 37 granitoid samples indicate that the Kalahari Craton consists of at least five distinct terranes—Barberton South (BS), Barberton North (BN), Murchison–Northern Kaapvaal (MNK), Limpopo Central Zone (LCZ), and Francistown—which underwent different crustal evolutions, and were successively accreted at c. 3·23 Ga, 2·9 Ga and 2·65–2·7 Ga. The investigated granitoids were emplaced over a period of c. 1.5 billion years, and are exposed along a c. 1000 km long traverse from the Barberton Mountain Land/Swaziland to the Francistown arc complex, Botswana. The presented datasets reveal that most granitoids of the BS (3·45–3·10 Ga), MNK (2·93–2·67 Ga), Francistown (2·70–2·65 Ga) and LCZ terranes (3·2–2·03 Ga) show near-chondritic to subchondritic ∊Hft (BS = –1·7 to + 0·5; MNK = –3·4 to + 0·7; Francistown = –0·5 to + 1·1; LCZ = –12·4 to –1·8), indicating that crustal recycling—perhaps by mixing of an older crust with a depleted mantle reservoir—played an important role during their formation and growth. Higher, superchondritic ∊Hft values, as indicative for an important depleted mantle influence, were obtained only from some granitoids of the BN terrane (∊Hf3·23Ga = +2·5 ± 0·8), the Gaborone Granite Suite (∊Hf2·80Ga = +2·0 ± 1·6), and from a few detrital zircons from the Mahalapye complex of the Limpopo Belt. In addition, the datasets show that the internal Hf isotope variation of magmatic zircon domains from most granitoids is commonly less than ±1·5 ∊-units, and only in rare cases up to ±3·1 ∊-units. The rare significant ∊Hft variations may be ascribed to incomplete mixing of different sources during magma crystallization. It is also shown that the combined approach of cathodoluminescence imaging with U–Pb and Lu–Hf isotope analysis provides a powerful tool to distinguish zircon domains formed and/or altered at different times. [ABSTRACT FROM AUTHOR]

Zircon in greenschist-facies metasedimentary rocks from the Scottish Highlands displays a range of complex textures that reflect low-temperature alteration of original detrital grains. In situ back-scattered electron, cathodoluminescence, electron backscatter diffraction and chemical analyses show that altered zircon is porous, weakly luminescent, enriched in non-formula elements such as Al and Fe, and is associated with fractures within the host zircon. The low-temperature zircon appears to be nano-crystalline and to replace U-rich zircon via modification of whole grains or selective alteration of parts of grains, and is linked to the development of zircon outgrowths. The altered zircon is also associated with epitaxial xenotime outgrowths and inclusions. Low-temperature zircon is abundant in slates and other mica-rich samples and its formation is linked to a dissolution–reprecipitation mechanism. Zircon within quartz-rich host rocks typically shows evidence of deformation and the resulting fractures enhance its dissolution, creating rounded embayed morphologies. In contrast, zircon from phyllosilicate-rich rocks contains more new low-temperature growth. Zircon alters during both prograde and retrograde metamorphic events and its development is controlled by both the progressive accumulation of radiation damage in the host grain and the access of metamorphic fluids to the metamict zircon. [ABSTRACT FROM AUTHOR]

Widespread bodies of garnet–spinel metaperidotites with pyroxenitic layers occur in the ultrahigh-pressure metamorphic Kimi Complex. In this study we address the origin of such peridotite–pyroxenite associations in the context of polybaric melting regimes. We conduct a detailed geochemical investigation of major and trace element relations and compare them with a range of major element modelling scenarios. With increasing bulk-rock MgO content, the garnet–spinel metaperidotites exhibit decreasing CaO, Al2O3, TiO2, and Na2O along with increasing Ni and a gradually increasing Zr/Zr* anomaly, consistent with an origin as residues after variable degrees of melt extraction. The major element modelling further suggests a polybaric adiabatic decompression melting regime beginning at high to ultrahigh pressure, with an intermediate character between pure batch and fractional melting and a mean extent of melting of 9–11%. The pyroxenites exhibit major element compositions that cannot be reproduced by experimental or calculated melts of peridotite. Moreover, the Kimi pyroxenites have highly variable Ni and Sc contents and a wide range of Mg-number (0· 76–0· 89), inconsistent with an origin as frozen melts or the products of melt–peridotite interaction. However, both the major element systematics and the observed rare earth element patterns, with both convex and concave shapes, can be explained by an origin as clinopyroxene-rich, high-pressure cumulates involving garnet and/or Cr-spinel. [ABSTRACT FROM AUTHOR]

We have determined the near-solidus melt compositions for peridotite MM-3, a suitable composition for the production of mid-ocean ridge basalt (MORB) by decompression partial melting, at 1 and 1·5 GPa. At 1 GPa the MM-3 composition has a subsolidus plagioclase-bearing spinel lherzolite assemblage, and a solidus at ∼ 1270°C. At only ∼ 5°C above the solidus, 4% melt is present as a result of almost complete melting of plagioclase. This melting behaviour in plagioclase lherzolite is predicted from simple systems and previous experimental work. The persistence of plagioclase to > 0·8 GPa is strongly dependent on bulk-rock CaO/Na2O and normative plagioclase content in the peridotite. At 1·5 GPa the MM-3 composition has a subsolidus spinel lherzolite assemblage, and a solidus at ∼ 1350°C. We have determined a near-solidus melt composition at ∼ 2% melting within 10°C of the solidus. Near-solidus melts at both 1 and 1·5 GPa are nepheline normative, and have low normative diopside contents; also they have the highest TiO2, Al2O3 and Na2O, and the lowest FeO and Cr2O3 contents compared with higher degree partial melts. Comparison of these near-solidus melts with primitive MORB glasses, which lie in the olivine-only field of crystallization at low pressure, indicate that petrogenetic models involving aggregation of near-fractional melts formed during melting at pressures of 1·5 GPa or less are unlikely to be correct. In this study we use an experimental approach that utilizes sintered oxide mix starting materials and peridotite reaction experiments. We also examine some recent studies using an alternative approach of melt migration into, and entrapment within ‘melt traps’ (olivine, diamond, vitreous carbon) and discuss optimal procedures for this method. [ABSTRACT FROM AUTHOR]

The phase and melting relations in subducted pelites have been investigated experimentally at conditions relevant for slabs at sub-arc depths (T = 600–1050°C, P = 2·5–4·5 GPa). The fluid-present experiments produced a dominant paragenesis consisting of garnet–phengite–clinopyroxene–coesite–kyanite that coexists with a fluid phase at run conditions. Garnet contains detectable amounts of Na2O (up to 0·5 wt%), P2O5 (up to 0·56 wt%), and TiO2 (up to 0·9 wt%) in all experiments. Phengite is stable up to 1000°C at 4·5 GPa and is characterized by high TiO2 contents of up to 2 wt%. The solidus has been determined at 700°C, 2·5 GPa and is situated between 700 and 750°C at 3·5 GPa. At 800°C, 4·5 GPa glass was present in the experiments, indicating that at such conditions a hydrous melt is stable. In contrast, at 700°C, 3·5 and 4·5 GPa, a solute-rich, non-quenchable aqueous fluid was present. This indicates that the solidus is steeply sloping in P–T space. Fluid-present (vapour undersaturated) partial melting of the pelites occurs according to a generalized reaction phengite + omphacite + coesite + fluid = melt + garnet. The H2O content of the produced melt decreases with increasing temperature. The K2O content of the melt is buffered by phengite and increases with increasing temperature from 2·5 to 10 wt%, whereas Na2O decreases from 7 to 2·3 wt%. Hence, the melt compositions change from trondhjemitic to granitic with increasing temperature. The K2O/H2O increases strongly as a function of temperature and nature of the fluid phase. It is 0·0004–0·002 in the aqueous fluid, and then increases gradually from about 0·1 at 750–800°C to about 1 at 1000°C in the hydrous melt. This provides evidence that hydrous melts are needed for efficient extraction of K and other large ion lithophile elements from subducted sediments. Primitive subduction-related magmas typically have K2O/H2O of ∼0·1–0·4, indicating that hydrous melts rather than aqueous fluids are responsible for large ion lithophile element transfer in subduction zones and that top-slab temperatures at sub-arc depths are likely to be 700–900°C. [ABSTRACT FROM AUTHOR]

In a global examination of mid-ocean ridge basalt (MORB) glass compositions, we find that Na8–Fe8–depth variations do not support modeling of MORBs as aggregates of melt compositions generated over a large range of temperature and pressure. However, the Na8–Fe8 variations are consistent with the compositional systematics of solidus melts in the plagioclase–spinel lherzolite transition in the CaO–MgO–Al2O3–SiO2–Na2O–FeO (CMASNF) system. For natural compositions, the P–T range for melt extraction is estimated to be ∼1·2–1·5 GPa and ∼1250–1280°C. This P–T range is a close match with the maximum P–T conditions for explosive pressure-release vaporization of carbonate-bearing melts. It is proposed that fracturing of the lithosphere induces explosive formation and escape of CO2 vapor. This provides the vehicle for extraction of MORBs at a relatively uniform T and P. The upper portion of the CO2-bearing and slightly melted seismic low-velocity zone flows toward the ridge, rises at the ridge axis to lower-lithosphere depths, melts much more extensively during this rise, and releases MORB melts to the surface driven by explosively escaping CO2 vapor. The residue and overlying crust produced by this melting then migrate away from the ridge axis as new oceanic lithosphere. The entire process of oceanic lithosphere creation involves only the upper ∼140 km. When lithospheric stresses shift fracture formation to other localities, escape of CO2 ceases, the vehicle for transporting melt to the surface disappears, and ridges die. Inverse correlations of Na8 vs Fe8 for MORB glasses are explained by mantle heterogeneity, and positive variations superimposed on the inverse variations are consistent with progressive extraction of melts from short, ascending melting columns. The uniformly low temperatures of MORB extraction are not consistent with the existence of hot plumes on or close to ocean ridges. In this modeling, the southern Atlantic mantle from Bouvet to about 26°N is relatively homogeneous, whereas the Atlantic mantle north of about 26°N shows significant long-range heterogeneity. The mantle between the Charlie Gibbs and Jan Mayen fracture zones is strongly enriched in FeO/MgO, perhaps by a trapped fragment of basaltic crust. Iceland is explained as the product of this enrichment, not a hot plume. The East Pacific Rise, Galapagos Ridge, Gorda Ridge, and Juan de Fuca Ridge sample mantle that is heterogeneous over short distances. The mantle beneath the Red Sea is enriched in FeO/MgO relative to the mantle beneath the northern Indian Ocean. [ABSTRACT FROM AUTHOR]

Two approaches to determining the high-temperature (∼1000°C to ∼600°C) cooling rate of the lower oceanic crust and upper mantle are presented and critically evaluated. The first is based on the down-temperature diffusive exchange of Ca between olivine and clinopyroxene. The second, less well-constrained, approach is based on the down-temperature diffusive exchange of Mg and Fe between olivine and spinel. Cooling rates based on olivine–spinel geospeedometry are approximately an order of magnitude faster than those from Ca-in-olivine geospeedometry. In contrast, cooling rates derived from thermochronology and remanent magnetism are approximately an order of magnitude slower than those derived by Ca-in-olivine geospeedometry; this is probably because they record cooling at lower temperatures. Using the Ca-in-olivine geospeedometer, the cooling rate of samples from the lower oceanic crust and upper oceanic mantle formed in the Oman ophiolite and in the three main ocean basins has been determined. Samples from the lower oceanic crust formed at fast-spreading ridges show a large decrease in cooling rate between the top and base of the gabbroic section, with most of the variation occurring within the upper kilometre. This is consistent with vertical heat loss (within the crustal frame of reference) dominating the thermal evolution at fast-spreading ridges. Samples from Ocean Drilling Program Hole 735B, which formed at the slow-spreading Southwest Indian Ridge, show no variation in cooling rate over 1500 m depth range and cooled substantially faster than rocks from the deeper portion of the gabbros in the Oman ophiolite, where the change in cooling rate with depth is limited. These observations are consistent with heat loss from small plutons emplaced in cool lithosphere at the slow-spreading ridge. Alternatively, they could be explained by cooling through the Ca-in-olivine closure interval during uplift towards the surface. [ABSTRACT FROM AUTHOR]

The Paleoproterozoic metaplutonic rocks of the Caicó Complex Basement (Seridó region, NE Brazil) provide important and crucial insights into the petrogenetic processes governing crustal growth and may potentially be a proxy for understanding the Archean–Proterozoic transition. These rocks consist of high-K calc-alkaline diorite to granite, with Rb–Sr, U–Pb, Pb–Pb and Sm–Nd ages of c. 2·25–2·15 Ga. They are metaluminous, with high YbN, K2O/Na2O and Rb/Sr, low ISr ratios, and are large ion lithophile elements (LILE) enriched. Petrographic and geochemical data demonstrate that they belong to differentiated series that evolved by low-pressure fractionation, thus resulting in granodioritic liquids. We propose a model in which the petrogenesis of the Caicó Complex orthogneisses begins with partial melting of a metasomatically enriched spinel- to garnet-bearing lherzolite (with high-silica adakite melt as the metasomatic agent), generating a basic magma that subsequently evolved at depth through fractional crystallization of olivine, followed by low-pressure intracrustal fractionation. A subduction zone setting is proposed for this magmatism, to account for both negative anomalies in high field strength elements (HFSE) and LILE enrichment. Mantle-derived juvenile magmatism with the same age is also known in the São Francisco and West Africa cratons, as well as in French Guyana, and thus the Archean–Proterozoic transition marks a very important continental accretion event. It also represents a transition from slab-dominated (in the Archean) to wedge-dominated post-Archean magmatism. [ABSTRACT FROM AUTHOR]

The focus of this study is a suite of garnet-bearing mantle xenoliths from Oahu, Hawaii. Clinopyroxene, olivine, and garnet constitute the bulk of the xenoliths, and orthopyroxene is present in small amounts. Clinopyroxene has exsolved orthopyroxene, spinel, and garnet. Many xenoliths also contain spinel-cored garnets. Olivine, clinopyroxene, and garnet are in major element chemical equilibrium with each other; large, discrete orthopyroxene does not appear to be in major-element chemical equilibrium with the other minerals. Multiple compositions of orthopyroxene occur in individual xenoliths. The new data do not support the existing hypothesis that all the xenoliths formed at 1ċ 6–2ċ2 GPa, and that the spinel-cored garnets formed as a consequence of almost isobaric subsolidus cooling of a spinel-bearing assemblage. The lack of olivine or pyroxenes in the spinel–garnet reaction zones and the embayed outline of spinel grains inside garnet suggest that the spinel-cored garnets grew in the presence of a melt. The origin of these xenoliths is interpreted on the basis of liquidus phase relations in the tholeiitic and slightly silica-poor portion of the CaO–MgO–Al2O3–SiO2 (CMAS) system at pressures from 3ċ0 to 5ċ0 GPa. The phase relations suggest crystallization from slightly silica-poor melts (or transitional basaltic melts) in the depth range ∼110–150 km beneath Oahu. This depth estimate puts the formation of these xenoliths in the asthenosphere. On the basis of this study it is proposed that the pyroxenite xenoliths are high-pressure cumulates related to polybaric magma fractionation in the asthenosphere, thus making Oahu the only locality among the oceanic regions where such deep magmatic fractional crystallization processes have been recognized. [ABSTRACT FROM AUTHOR]

A detailed in situ isotopic (U–Pb, Lu–Hf) and geochemical study of zircon populations in a composite sequence of foliated to massive Cambro-Ordovician intrusions in the Deep Freeze Range (North Victoria Land, Antarctica), has highlighted great complexity in zircon systematics. Zircons in deformed granitoids and tonalites display complex internal textures, a wide spread of concordant U–Pb ages (between 522 and 435?Ma) and unusual trace-element compositions (anomalous enrichment of light rare earth elements, U, Th and Y) within single zircon grains. In contrast, zircons from undeformed samples display a limited range of U–Pb ages and trace-element compositions. Zircons from all age and textural populations in most of the deformed and undeformed samples show a relatively narrow range of εHf values, suggesting that the Lu–Hf system remained undisturbed. Inferred emplacement ages cover a time interval of about 30?Myr: from 508 to 493?Ma for the oldest strongly foliated synkinematic Howard Peaks megacrystic monzogranites and high-K calc-alkaline mafic to intermediate rocks of the ‘Corner Tonalite’ unit; from about 489 to 481?Ma for the younger massive shoshonitic mafic dyke suite and the high-K calc-alkaline Keinath granite. The observed isotopic and chemical variations in zircon are attributed to a sub-solidus recrystallization under hydrous conditions and varying temperature, in a setting characterized by a transpressional to extensional stress regime. [ABSTRACT FROM AUTHOR]