Gem corundum deposits of Madagascar: A review

Madagascar is one of the most important gem-producing countries in the world, including ruby and sapphires. Gem corundum deposits formed at different stages in the geological evolution of the island and in contrasting environments. Four main settings are identified: (1) Gem corundum formed in the Precambrian basement within the Neoproterozoic terranes of southern Madagascar, and in the volcano-sedimentary series of Beforona, north of Antananarivo. In the south, high-temperature (700 to 800 °C) and low-pressure (4 to 5 kbar) granulites contain deposits formed during the Pan-African orogenesis between 565 and 490 Ma. They accompany mafic and ultramafic complexes (ruby deposits of the Vohibory group), skarns at the contact between Anosyan granites and the Proterozoic Tranomaro group (sapphire deposits of the Tranomaro–Andranondambo district), and shear-zone corridors cross-cutting feldspathic gneisses, cordieritites and clinopyroxenites in the Tranomaro, Vohimena and Androyan metamorphic series (biotite schist deposits of Sahambano and Zazafotsy, cordieritites of Iankaroka and Ambatomena). The circulation of fluids, especially along discontinuities, allowed in-situ alkaline metasomatism, forming corundum host rocks related to desilicified granites, biotitites, “sakenites” and “corundumites”. (2) Gem corundum also occurs in the Triassic detrital formations of the Isalo group, as giant palaeoplacers in the Ilakaka–Sakaraha area. Here, sapphires and rubies may come from the metamorphic granulitic terranes of southern Madagascar. (3) Gem corundum deposits occur within the Neogene-Quaternary alkali basalts from Ankaratra (Antsirabe–Antanifotsy area) and in the Ambohitra Province (Nosy Be, Ambato and Ambondromifehy districts). Primary deposits are rare, except at Soamiakatra where ruby in gabbroic and clinopyroxenite xenoliths within alkali-basalts probably derive from mantle garnet peridotites. The blue-green-yellow sapphires typical of basaltic fields are always recovered in palaeoplacer (in karst formed upon Jurassic limestones from the Montagne d'Ambre, Antsiranana Province) and alluvial and soil placers (Ankaratra volcanic massif). (4) Deposits occur within Quaternary eluvial, colluvial and alluvial concentrations, such as high-quality rubies from the Andilamena and Vatomandry deposits.     

Neogene marine sediments and biota encapsulated between lava flows on Santa Maria Island (Azores,north-east Atlantic): An interplay between sedimentary,erosional and volcanic processes

Sedimentary rocks are rarely preserved on reefless volcanic oceanic islands because their sediments are mostly exported from coastal areas towards the deep sea and such islands typically undergo subsidence. In contrast, the exceptional geological record of the uplifted Santa Maria Island (Azores) provides a unique opportunity to gain insight on such coastal systems. This study focuses on a locality at Ponta do Cedro (eastern Santa Maria Island), which features a series of marine fossiliferous sediments wedged between steep lava deltas. As demonstrated by local structure, these sediments correspond to clinoforms deposited on the steep submarine slope of an active volcanic island, implying transport from shallow waters to greater depths and subsequent colonization by benthic communities. Rapid volcanic progradation eventually sealed the deposits, allowing for their preservation and providing a rare snapshot of the ecology during those intervals, in addition to insights on sedimentary dynamics along submarine island slopes. This study reveals spatial relationships between wedges of sedimentary bodies encapsulated by lavas in the Ponta do Cedro section, and interprets depositional processes preserved in those strata based on sedimentological and palaeontological data. The dynamics of the environment are mostly related to relative sea-level changes, intense volcanic activity and regional uplift during the Neogene.     

Space weathering and the low sulfur abundance of Eros

The surprisingly low S/Si ratio of Asteroid 433 Eros measured by the NEAR Shoemaker spacecraft probably reflects a surface depletion rather than a bulk property of the asteroid. The sulfur X-ray signal originates at a depth <10 μm in the regolith. The most efficient process for vaporizing minerals at the heliocentric distance of Eros are sputtering by solar wind ions and hypervelocity impacts. These are the same processes that account for the changes in optical properties of asteroids attributed to “space weathering” of lunar surface materials, although the relative importance of sputtering and impacts need not be the same for the Moon and asteroids. Troilite, FeS, which is the most important sulfide mineral in meteorites, and presumably on S-type asteroids like Eros, can be vaporized by much less energy than other major minerals, and will therefore be preferentially lost. Within 10⁶ years either process can remove sulfide from the top 10-100 μm of regolith. Sulfur will be lost into space and some sulfur will migrate to deeper regolith layers. We also consider other possible mechanisms of surficial sulfur depletion, such as mineral segregation in the regolith and perhaps even incipient melting. Although we consider solar wind sputtering the most likely cause of the sulfur depletion on Eros, we cannot entirely rule out other processes as causes of the sulfur deficiency. Laboratory simulations of the relevant processes can address some of the open questions. Simulations will have to be carried out in such a way that potential sulfur loss processes as well as resurfacing can be studied simultaneously, requiring a large and complex environmental chamber.     

Analysing the interaction between emission trading and renewable electricity support in TIMES

As the number of instruments applied in the area of energy and climate policy is rising, the issue of policy interaction needs to be explored further. This article analyses the interdependencies between the EU Emissions Trading Scheme (EU ETS) and the German feed-in tariffs (FITs) for renewable electricity in a quantitative manner using a bottom-up energy system model. Flexible modelling approaches are presented for both instruments, with which all impacts on the energy system can be evaluated endogenously. It is shown that national climate policy measures can have an effect on the supranational emissions trading system by increasing emission reduction in the German electricity sector by up to 79 MtCO₂ in 2030. As a result, emission certificate prices decline by between 1.9 €/tCO₂ and 6.1 €/tCO₂ and the burden sharing between participating countries changes, but no additional emission reduction is achieved at the European level. This also implies, however, that the cost efficiency of such a cap-and-trade system is distorted, with additional costs of the FIT system of up to €320 billion compared with lower costs for ETS emission certificates of between €44 billion and €57 billion (cumulated over the period 2013–2020).

Policy relevance

In order to fulfil ambitious emission reduction targets a large variety of climate policy instruments are being implemented in Europe. While some, like the EU ETS, directly address CO₂ emissions, others aim to promote specific low-carbon technologies. The quantitative analysis of the interactions between the EU ETS and the German FIT scheme for renewable sources in electricity generation presented in this article helps to understand the importance of such interaction effects. Even though justifications can be found for the implementation of both types of instrument, the impact of the widespread use of support mechanisms for renewable electricity in Europe needs to be taken into account when fixing the reduction targets for the EU ETS in order to ensure a credible long-term investment signal.     

Isothermal decompression history in the “Western Granulite” terrain, central Tanzania: Evidence from reaction textures and trapped fluids in metapelites

The Mozambique Belt (MB) of the East Africa Orogen contains large areas of granulite-facies migmatitic gneisses with Archaean and Palaeoproterozoic protolith ages and that were recycled during the Neoproterozoic Pan-African orogeny. The study area is situated along the Great Ruaha River and within the Mikumi National Park in central Tanzania where migmatitic gneisses and mafic to intermediate granulites are interlayered with Neoproterozoic granulite-facies migmatitic metapelites. Mineral textures suggest isothermal decompression, with the peak mineral assemblage comprising Grt–Bt–Ky–Kfs–Pl–Qtz ± Phn ± Ti-Oxide ± melt and amphibolite-facies retrograde assemblage Grt–Bt–Sil–Ms–Kfs–Pl–Qtz ± Fe–Ti-Oxide. The near isothermal retrograde overprint is seen in well-developed formation of pseudomorphs after garnet. The HP granulite-facies assemblages record P–T conditions of 13–14 kbar at 760–800 °C. Retrogression and the release of fluids from crystallizing melts occurred at 7 kbar and 650–700 °C. A fluid inclusion study shows three types of fluid inclusion consisting of nearly pure CO₂, as well as H₂O–NaCl and H₂O–CO₂ mixtures. We suggest that a immiscible CO₂-bearing brine represents the fluid composition during high-grade peak metamorphism, and that the fluid inclusions containing H₂O–NaCl or nearly pure CO₂ represent trapped fluids from in situ crystallised melt. The results suggest strong isothermal decompression, which is probably related to a fast exhumation after crustal thickening in the central part of the Mozambique Belt in Tanzania.     

Surface changes on Io during the Galileo mission

A careful survey of Galileo SSI global monitoring images revealed more than 80 apparent surface changes that took place on Io during the 5 year period of observation, ranging from giant plume deposits to subtle changes in the color or albedo of patera surfaces. Explosive volcanic activity was discovered at four previously unrecognized centers: an unnamed patera to the south of Karei that produced a Pele-sized red ring, a patera to the west of Zal that produced a small circular bright deposit, a large orange ring detected near the north pole of Io, and a small bright ring near Io's south pole. Only a handful of Io's many active volcanoes produced large scale explosive eruptions, and several of these erupted repeatedly, leaving at least 83% of Io's surface unaltered throughout the Galileo mission. Most of the hot spots detected from SSI, NIMS and ground-based thermal observations caused no noticeable surface changes greater than 10 km in extent over the five year period. Surface changes were found at every location where active plumes were identified, including Acala which was never seen in sunlight and was only detected through auroral emissions during eclipse. Two types of plumes are distinguished on the basis of the size and color of their deposits, confirming post-Voyager suggestions by McEwen and Soderblom [Icarus 55 (1983) 191]. Smaller plumes produce near-circular rings typically 150-200 km in radius that are white or yellow in color unless contaminated with silicates, and frequently coat their surroundings with frosts of fine-grained SO₂. The larger plumes are much less numerous, limited to a half dozen examples, and produce oval, orange or red, sulfur-rich rings with maximum radii in the north-south direction that are typically in the range from 500 to 550 km. Both types of plumes can be either episodic or quasi-continuous over a five year period. Repeated eruptions of the smaller SO₂-rich plumes likely contribute significantly to Io's resurfacing rate, whereas dust ejection is likely dominated by the tenuous giant plumes. Both types of plume deposits fade on time-scales of months to years through burial and alteration. Episodic seepages of SO₂ at Haemus Montes, Zal Montes, Dorian Montes, and the plateau to the north of Pillan Patera may have been triggered by activity at nearby volcanic centers.     

Shear Wave Anisotropy of the Upper Mantle Beneath the Region from Tingri of Tibet to Golmud of Qinghai

Measurements of shear wave splitting at 43 three-component seismic stationsshow very big difference in anisotropy on both sides of the Indus-Yarlung Zangbo suture(ITS), but little difference on both sides of the older Bangong-Nujiang suture (BNS) and theJinsha River suture (JS) to its north. Obvious discrepancy exists between the anisotropy direc-tion and the superficial tectonic trends, which is not explicable directly by the coherent uppermantle deformation usually supposed to occur in consistency with the trend of a collisional belt.On the other hand, strong spatial relationships are observed from the anisotropy results, such asthe orthogonal directions of anisotropy on both sides of ITS and the good correlation betweenthe region of larger magnitude of anisotropy and the zone of inefficient Sn propagation ofQiangtang as well as the systematic rotation of the directions of anisotropy, which should testifysome much more complicated aspects of the continental convergence mechanism. To the best ofour data, we tend to suppose that the Qinghai-Tibet plateau might result from a mechanismcomplicated by the coexistence of Argand's underthrusting and Dewey's diffuse deformation.     

The phosphates of IIIAB iron meteorites

Abstract— Thirteen phosphate minerals are found in IIIAB iron meteorites. Four of these (sarcopside, graftonite, johnsomervilleite, and galileiite) constitute the majority of occurrences. The IIIB iron meteorites are confined to occurrences of only these four phosphates. The IIIA iron meteorites may contain one or more of these four phases; they may also contain other rarer phosphates, and silica (in two instances) and a silicate rock (in one instance). Thus, the IIIA lithophile chemistry is more varied than that of the IIIB meteorites. Based on petrographic relations, sarcopside appears to be the first phosphate to form. Graftonite is probably formed by recrystallization of sarcopside. Johnsomervilleite and galileiite exsolved as enclaves in sarcopside or graftonite at lower temperatures, although some of these also nucleated as separate crystals. The IIIAB phosphates are carriers of a group of incompatible lithophile elements: Fe, Mn, Na, Ca, and K, and, rarely, Mg as well as Pb. These elements (and O) were concentrated in a residual, S-rich liquid during igneous fractional crystallization of the IIIAB core mass. The phosphates formed by oxidation of P as the core solidified and excluded O, which increased its partial pressure in the residual liquid. The trace siderophile trends in bulk IIIAB metal are paralleled by a mineralogical trend of the phosphate minerals that formed. For IIIAB meteorites with low-Ir contents in the metal, the phosphates are mainly Fe-Mn phases; at intermediate Ir values, more Na-bearing phosphates appear; at the highest Ir values, the rarer Na-, K-, Mg-, Cr-, and Pb-bearing phosphates appear. The absence of significant amounts of Mg, Si, Al, and Ti suggest depletion of these elements in the core by the overlying mantle.     

The North Devon Basin: a Devonian passive margin shelf succession

The North Devon Basin, situated in a more proximal passive margin regime than the rift basins to the south, is not constrained but its succession is thought to represent in large part the sediments debouched from a northerly hinterland. Rather than that immediate source being South Wales an original location of the basin well to the south-east and west of the Ardennes massif is considered probable, with its present position being attained by Carboniferous displacement along the Bristol Channel-Bray Fault. The basin's thick (6000 m) succession comprises terrestrial and marine deposits that form two major sedimentary cycles, which are apparently closely linked to rift basin formation to the south. The GCR sites span a relatively straightforward shelf succession that extends from the late Early Devonian to the Carboniferous. The sedimentology, palaeontology, and depositional environments of terrestrial and marine facies lithostratigraphical units are detailed, some sites providing the macrofossil assemblages important in the identification and definition by Sedgwick and Murchison of the Devonian System.     

The sulfur isotope composition of basaltic rocks

The sulfur isotope composition of tholeiitic basalts, olivine alkali basalts and alkalirich undersaturated basalts were investigated. A method of preparation was devised

1. for the extraction of the small amounts of sulfur contained in the rock samples (about 100 ppm S),
2. for the separation of sulfide- and sulfate-sulfur.

Tholeiitic and olivine alkali basalts show a predominance of sulfide-sulfur. Alkali-rich undersaturated basalts show sulfide- and sulfate-sulfur. The oxidation potential of the magma is reflected in the proportions of sulfide- and sulfate-sulfur. Differences in the conditions of oxidation are also the cause of the sulfur isotope fractionation observed. The mean in the isotope composition of the sulfur in the olivine alkali basalts (with the exception of two samples which show extreme deviation) is δ ³⁴S= +1.3 per mil. The values for the olivine alkali basalts are concentrated around this mean in a remarkable way, showing only small deviation for the individual samples. When the tholeiitic basalts deviate from this mean, it is only with a relative enrichment in the ³²S isotope. With a pronounced variation of the individual values, the mean for the sulfide-sulfur is δ ³⁴S=?0.3 per mil. The few sulfate values of both types of basalt are without significance for the discussion of their origin. However, this does not apply to the alkali-rich undersaturated basalts. Due to the higher water content, this basaltic magma had a higher oxygen partial pressure which favoured the formation of SO₂ and SO ₄ ^2? besides H₂S while pressure was released during the ascent of the magma. The sulfur isotope fractionation connected with this oxidation led to a total enrichment of ³⁴S in the rock, (δ ³⁴S for total sulfur: +3.1 per mil) with particular favouring the sulfate (δ ³⁴S=+4.2 per mil). It is accepted that the sulfur of all three types of basalts derives directly from the mantle. The olivine alkali basalts show the least deviation from the mantle value, which, in the place of origin of the basalts from the region investigated, would probably have been δ ³⁴S=+1.3(±0.5) per mil. From this it may be concluded that the olivine alkali basalts — the most frequent type of basalt in this region — had their origin in the partial melting of the mantle without further differentiation. From the sulfur isotope data we concluded that the primary isotope composition of the continental tholeiitic basalts probably corresponds to that of the olivine-alkali basalts, and to that of the mantle. However, due to degasing in the layers near to the surface, some samples lost ³⁴S, which may be related to the formation of SO₂ during the release of pressure. There is no positive indication of a differentiation in shallow depths (<15 km — in the sense of Green and Ringwood, 1967). The reason for the obvious isotopic fractionation of the alkali-rich undersaturated basalts may be seen in their higher primary water content. This is a pronounced indication of the origin of this type of magma. Bultitude and Green (1968) proved by experiment, that the formation of alkali-rich undersaturated basaltic magma is possible in the mantle in the presence of water. Only a small amount of water is available for the formation of magma in the mantle. With a water content higher than normal for basalts, only small amounts of magma can be formed, but at lower temperatures this would allow the melting of a larger fraction of mantle material. By reaction with the wall rock, these magmas could be enriched in those components of mantle minerals which have the lowest melting point. This may help to explain their geochemical characteristics.