Geological significance of anisotropy of seismic velocities in Earth's crust

Anisotropism of seismic velocities is at its minimum in rocks consisting mainly of feldspar and quartz, is appreciable in pyroxenes and amphiboles (chain-structured silicates), and is at its maximum in mica-rich rocks (sheef-structured silicates). Inasmuch as the difference between the formation and boundary velocities has been established by the DSS for the Earth's crust, as a whole, and is recognizable also in the tops of the mantle, the crust and the tops of the mantle probably consist of seismically anistropic rocks. -- IGR Staff.     

Secondary Ca–Al silicates in plutonic rocks: implications for their cooling history

Secondary Ca-Al silicates are used to constrain the P-T-x conditions of the very early post-magmatic stage of the intermediate to basic Hercynian plutonic complexes of Charroux-Civray (NW Massif Central, France) and Fichtelgebirge (NE Bavaria, Germany). The secondary Ca-Al silicates hydrogarnet, prehnite, pumpellyite, epidote and laumontite form lenses within unaltered or only slightly chloritized biotite. Hydrogarnet as the first occurring Ca-Al silicate phase crystallizes at temperatures above 340 °C. The common paragenesis prehnite + pumpellyite post-dates hydrogarnet and indicates rather narrow ranges of temperature (200-280 °C) and pressure (2-3 kbar). Laumontite is formed at the end of Ca-Al silicate crystallization (180-260 °C, 1-3 kbar), mostly in small fractures in association with prehnite and adularia. The observed crystallization sequence of the Ca-Al silicates and their stabilities define a retrograde alteration path for the plutonic rocks. The Ca-Al silicate assemblage results from an early pervasive alteration of the plutonic rocks by low X_CO2 fluids during post-magmatic cooling. Subsolidus cooling starts at about 4 kbar at solidus temperatures as indicated by magmatic epidote stability, hornblende barometry and fluid inclusion data, and continues under slightly decreasing pressure (uplift) down to 2-3 kbar at 200-280 °C (prehnite-pumpellyite paragenesis). This shows that Ca-Al silicate assemblages may be a unique tool to constrain the P-T conditions of the subsolidus cooling of intermediate to basic plutonic bodies.     

Compositional variations of olivine from the Jinchuan Ni–Cu sulfide deposit,western China: implications for ore genesis

The Jinchuan Ni–Cu sulfide deposit is hosted by an elongated, olivine-rich ultramafic body that is divided by subvertical strike-slip faults into three segments (central, eastern, and western). The central segment is characterized by concentric enrichments of cumulus olivine crystals and interstitial sulfides (pyrrhotite–pentlandite–chalcopyrite intergrowth), whereas the eastern and western segments are characterized by an increase of sulfides toward the lower contacts. In all segments sulfides are concentrated at the expense of intercumulus silicates. Olivine re-crystallization is found to be associated with actinolite alteration in some samples. The compositional variations of primary olivine from the sulfide-poor samples can be explained by a small degree of olivine crystallization (<5%) from a basaltic magma followed by local re-equilibration of the olivine with up to 30% trapped silicate liquid. In the sulfide-bearing samples the compositions of primary olivine record the results of olivine-sulfide Fe–Ni exchange that occurred after the trapped silicate liquid crystallized. Our olivine data indicate that Ni in the original sulfide liquids increased inward in the central segment and laterally away from the lower contact in the eastern segment. Variations in the compositions of sulfide liquids are thought to result from fractional segregation of immiscible sulfide liquid from a basaltic magma in a staging chamber instead of in situ differentiation. High concentrations of olivine crystals (mostly >50 modal%) and sulfide (averaging ~5 wt%) in the rocks are consistent with the interpretation that the Jinchuan deposit was formed by olivine- and sulfide-laden magma successively ascending through a conduit to a higher, now-eroded, level. Sulfide enrichment toward the center in the central segment and toward the lower contact in the eastern and western segments may have, in part, resulted from flow differentiation and gravitational settling during magma ascent, respectively.Editorial handling: P. Lightfoot     

Experimental and thermodynamic study of the crystallization of diamond and silicates in a metal-silicate-carbon system

Silicate inclusions are widespread in natural diamonds, which also may contain rare inclusions of native iron. This suggests that some natural diamonds crystallized in metal-silicate-carbon systems. We experimentally studied the crystallization of diamond and silicate phases from the starting composition Fe_0.36Ni_0.64 + silicate glass + graphite and calculated the Fe mole fractions of the silicate phases crystallizing under these conditions. The silicates synthesized together with diamond had low Fe mole fractions [Fe/(Fe + Mg + Ca)] in spite of strong Fe predominance in the system. The Fe mole fractions of the silicates decreased in the sequence garnet-pyroxene-olivine, which is consistent with the results of our thermodynamic calculations. The Fe mole fraction of silicates under various redox conditions under which metal-carbon melts are stable drastically decreases with decreasing fo₂. The low Fe mole fractions of silicate inclusions in diamond from the Earth’s mantle can be explained by the highly reducing crystallization conditions, under which Fe was concentrated as a metallic phase of the magmatic melts and could be only insignificantly incorporated in the structures of silicates.     

Pressure solution as a metamorphic process in deformed terrigenous sedimentary rocks

The most widespread record that terrigenous sedimentary rocks have deformed by a pressure solution mechanism is seen in the development of spaced cleavages and transposition structures under conditions of low grade metamorphism. Such structures are most common in immature sandstones and siltstones. Mineral reactions, involving modification of detrital grains and diagenetic minerals, and forming a logical extension to diagenetic processes, are an integral part of the deformation mechanism, and the cleavage stripes represent accumulations not just of inert particles, but mostly of newly crystallized micas, the products of these reactions. The mechanism of deformation by pressure solution is now seen to involve mmetamorphic reactions, change in volume of solid phases during reaction, removal of some silica from the rock, rearrangement of reaction products to produce fabrics, solution of some detrital grains in cleavage stripes.Formal reactions have been written to describe the alteration of detrital felspar and epidote to white mica, the modification of greywacke matrix to white mica, and the transformation of diagenetic chlorite to white mica, all of which are observed to occur during formation of the pressure solution cleavages. These reactions emphasize the importance of metamorphic processes during pressure solution deformation, suggest that pressure solution may involve removal of silica released as a product of the reactions, indicate that the pH of the aqueous phase may be buffered to a level that silica solubility is increased, involve a volume reduction that contributes to the overall shortening during deformation, and also involve dehydration, the large scale circulation of released water possibly being important to the removal and redistribution of silica during pressure solution.     

Silicate melt removal and sulfide liquid retention in ultramafic rocks of the Duke Island Complex,Southeastern Alaska

Magmatic Ni-Cu-PGE sulfide mineralization occurs within olivine clinopyroxenite, hornblende-bearing clinopyroxenite, and magnetite-hornblende-rich rocks in the Ural-Alaskan-Type Duke Island Complex in Southeast Alaska. The addition of large amounts of sulfur from country rocks occurred during fractional crystallization of the parental magma when clinopyroxene was becoming a liquidus mineral. Textural interfaces between sulfide and silicate minerals are strongly interlobate, and differ significantly from net-textures that are developed in many Ni-Cu-PGE deposits. Sulfide-free olivine clinopyroxenite is an adcumulate; residual liquid was efficiently expelled from the accumulating crystal pile. A significant interstitial liquid component is observable only in the form of interstitial sulfide in the S-rich rocks. Rounded sulfide inclusions and blebby to vermicular sulfide-silicate intergrowths indicate that silicate crystallization occurred under conditions of sulfide saturation. The presence of dense sulfide liquid inhibited the growth of silicate minerals and led to the development of interlobate grain boundaries. Strong, localized wetting of sulfide liquids on crystallizing silicates, and downward percolation of sulfide liquid through a crystallizing mush may have contributed to the evolution of these textures. Residual silicate liquid was removed from the system due to a combination of buoyant advection and compaction, but dense sulfide liquid remained.     

An unusual manganese silicate occurrence at the Hoskins mine,Grenfell district,New South Wales

Stratiform manganese silicate rocks overlie jasper and metabasah in the ?Middle Silurian Hoskins Formation at the Hoskins manganese mine near Grenfell, NSW. Two dominant mineral assemblages occur in the Mn silicate rocks: (1) a “reduced’ assemblage, probably gradational into underlying jasper, containing abundant rhodonite and/or tephroite, plus subordinate carbonates, quartz, hausmannite, spessartine and Ba minerals, and (2) a well‐laminated ‘oxidized’ assemblage rich in red Mn‐rich alkali pyroxene and amphibole, braunite, manganoan pectolite and minor Mn‐rich mica, alkali feldspars, carbonates, quartz and barite. Several Mn silicates implicitly contain trivalent Mn. The Mn silicate rocks are rich in Mn, Ba and Sr, and also contain anomalously high Co, Cu, As and W; oxidized assemblages are alkali‐rich. Bulk compositions and geological setting suggest a submarine volcanic exhalative origin for the precursors of the Mn silicate rocks and jasper. Metamorphism has occurred at upper greenschist facies with original high oxygen fugacity conditions in the exhalative sediments being largely reflected in the resulting assemblages. Although analogues of the reduced Mn silicate rocks are widespread in metamorphosed Mn deposits, equivalents of the oxidized assemblages appear to be particularly uncommon.     

Syntectonic Mobility of Supergene Nickel Ores of New Caledonia (Southwest Pacific). Evidence from Garnierite Veins and Faulted Regolith

Supergene nickel deposits of New Caledonia that have been formed in the Neogene by weathering of obducted ultramafic rocks are controlled by fracture development. The relationship of tropical weathering and tectonic structures, faults and tension gashes, have been investigated in order to determine whether fractures play a passive role only, as previously thought; or alternatively, if brittle tectonics was acting together with alteration. Observation of time‐relationship, textures, and mineralogy of various fracture fills and fault gouges shows that active faulting has played a prominent role not only in facilitating drainage and providing room for synkinematic crystallization of supergene nickel silicate, but also in mobilizing already formed sparse nickel ore, producing the very high grade ore nicknamed “green gold”.     

Radiocaesium dispersion and fixation in the lagoon systems of “Ria de Aveiro”, Portugal

Radiocaesium (¹³⁷Cs) dispersion and Cs⁺ fixation were studied in the sediments collected from the lagoon systems of “Ria de Aveiro”. The Cs⁺ sorption was tested for the fine mica grains and for the < 2 μm clay fractions extracted from silty clays. The Cs⁺ exchange is found strongly onto mica-rich fractions than smectite-rich fractions. The distribution coefficient increases if the silty material is constituted by rich-mica clay fractions or if the non-clay minerals are removed from the silty-clay material. The samples studied behave as multisite ion exchange, where Cs⁺ engages in ion-exchange reactions with hydrated cations on planar sites on expansible layer silicates. Higher concentrations of the ¹³⁷Cs were found associated with mica-rich silty clays. The ¹³⁷Cs ranges from 3.2 to 3.9 Bq kg^− 1 in the < 38 μm fractions and from 2.9 to 3.3 Bq kg^− 1 in the < 64 μm fractions.     

Micas from the Khaluta carbonatite deposit,western Transbaikal region

The Khaluta carbonatite deposit located in the western Transbaikal region was formed during the Late Mesozoic rifting in the southern framework of the Siberian Craton. Carbonatite is associated with shonkinite and syenite and is accompanied by fenitization. The composition of mica in more than 160 samples of country rocks, carbonatites, silicate rocks, and fenites was studied. The Fe³⁺ and Fe²⁺ contents, as well as oxygen isotopic composition, were determined. The Mg and Fe contents increase, whereas the Ti and Al contents decrease in micas when passing from silicate rocks and fenites to carbonatites. Micas from carbonatites are depleted in Al, enriched in Fe³⁺, and distinguished by high Si and F contents. According to our calculations, in some cases Al replaces Si in the tetrahedral site instead of replacement of Fe³⁺ as is characteristic of tetraferriphlogopite. Formally, the mica from carbonatites falls within the tetraferriphlogopite field, but typical inverse pleochroism is not always observable. The δ¹⁸O values of micas from carbonatite, shonkinite, syenite, and fenite are similar to those of mantle-derived silicate minerals. The δ¹⁸O values in the minerals coexisting with phlogopite testify to their isotopic equilibrium and make it possible to calculate the crystallization temperature of carbonatite.     

Fabric controls on strain accommodation in naturally deformed mylonites: The influence of interconnected micaceous layers

We present microstructural analyses demonstrating how the geometrical distribution and interconnectivity of mica influences quartz crystallographic preferred orientation (CPO) development in naturally deformed rocks. We use a polymineralic (Qtz + Pl + Kfs + Bt + Ms ± Grt ± Tur) mylonite from the Zanskar Shear Zone, a section of the South Tibetan Detachment (NW Himalaya), to demonstrate how quartz CPO intensity decreases from quartz-dominated domains to micaceous domains, independently of whether or not quartz grains are pinned by mica grains. We then use a bimineralic (Qtz + Ms) mylonite from the Main Central Thrust (NW Himalaya) to show how increasing mica grain connectivity is concomitant with a systematic weakening of quartz CPO. Our results draw distinctions between CPO weakening due to: (i) second phase drag, leading to ineffective recovery in quartz; and (ii) increased transmission and localisation of strain between interconnected mica grains. In the latter case, well-connected micaceous layers take up most of the strain, weakening the rock and preventing straining of the stronger quartz matrix. Our findings suggest that rock weakening in quartz-rich crustal rocks is influenced not only by the presence of mica-rich layers but also the degree of mica grain connectivity, which allows for more effective strain localization through the entire rock mass.     

Late-stage magmatic to deuteric/metasomatic accessory minerals from the Cerro Boggiani agpaitic complex (Alto Paraguay Alkaline Province)

This work describes rare accessory minerals in volcanic and subvolcanic silica-undersaturated peralkaline and agpaitic rocks from the Permo-Triassic Cerro Boggiani complex (Eastern Paraguay) in the Alto Paraguay Alkaline Province. These accessory phases consist of various minerals including Th-U oxides/silicates, Nb-oxide, REE-Sr-Ba bearing carbonates-fluorcarbonates-phosphates-silicates and Zr-Na rich silicates. They form a late-stage magmatic to deuteric/metasomatic assemblage in agpaitic nepheline syenites and phonolite dykes/lava flows made of sodalite, analcime, albite, fluorite, calcite, ilmenite-pyrophanite, titanite and zircon. It is inferred that carbonatitic fluids rich in F, Na and REE percolated into the subvolcanic system and metasomatically interacted with the Cerro Boggiani peralkaline and agpaitic silicate melts at the thermal boundary layers of the magma chamber, during and shortly after their late-stage magmatic crystallization and hydrothermal deuteric alteration.     

The Structure and Stratigraphic Position of the Agoi Olistostrome,Northwest Caucasus

A sequence of clays and clayey siltstones with olistostrome horizons and olistoplaques of the Upper Cretaceous rocks occur in the coastal section northwest of the mouth of the Agoi River. The rocks of this sequence are mostly gray sandy calcareous clays with rare interlayers of siltstones. The horizons and lenses of olistostromes contain small clasts, large blocks, and olistoplaques of rocks of various ages. The microfauna from matrix and clasts of olistostromes of different horizons of the sequence was sampled and identified. The large blocks and olistoplaques are tectonized with formation of slickensides and cracks, which do not travel beyond the country rocks. The thickness of the sequence is 280 m.     

Mechanism of frictional fusion in fault zones

Spherulitic pseudotachylytes from the Arunta Block formed by frictional fusion of mylonitic parent rocks during high-level reactivation of a previously ductile fault zone. Fusion occurred preferentially in mica-rich domains due to release of water through disruption of the mica lattice by frictional sliding. This generated selective localised melting of mica during frictional heating, with the production of initial pseudotachylyte melts enriched in water and ferromagnesian components. Subsequent fusion of adjacent salic phases, promoted by the high water content of the existing melt, would tend to shift the trend of later melts towards a total melt composition. Therefore, under conditions of frictional sliding, fusion appears to be favoured in crystalline quartzofeldspathic rocks possessing both a high shear strength, and a significant water content locked up in the lattices of hydrous minerals, principally biotite.The presence or pre-existence of glass in many pseudotachylytes demands that they cooled in a near-surface environment, i.e. at depths of less than about 5 km. Thus glassy pseudotachylytes must postdate associated mylonite series rocks, generally forming subsequent to exhumation of the mylonites to a higher level in the crust. Some non-glassy pseudotachylytes, however, may possibly form towards the end of movements in a ductile regime, as strain hardening sets in.     

A Preliminary Research on Skarns of Magmatic Origin

ＩＮＴＲＯＤＵＣＴＩＯＮＴｈｅｍｉｄｄｌｅａｎｄｌｏｗｅｒｒｅａｃｈｅｓｏｆＣｈａｎｇｊｉａｎｇＲｉｖｅｒａｒｅｏｎｅｏｆｔｈｅｍａｉｎｒｅｇｉｏｎｓｉｎＣｈｉｎａｗｈｉｃｈｉｓｃｈａｒａｃｔｅｒｉｓｔｉｃｏｆｔｈｅｗｉｄｅｓｐｒｅａｄｄｉｓｔｒｉｂ...     

Theory of metamorphic segregation and related processes

Metamorphic segregation is defined here as the formation and growth of bands or domains of different bulk compositions within an originally unbanded rock. It can result from an instability arising in some deforming rocks when diffusion transfer is significant. The nature of this instability is demonstrated separately for a differentiation associated with a crenulation (Type C) and for one without crenulation (Type L). However, the continuous gradation between the two types observed in nature is also expected from the models.A requirement for both types is that one chemical component of the rock, typically silica, diffuse more rapidly than others, e.g. phyllosilicate components, in response to stress-induced pressure gradients. In addition, Type L requires that, at least at an early stage, portions of the rock rich in this mobile component, e.g. rich in quartz, be more competent than those portions which are poor in quartz. By a process akin to the development of pressure shadows, silica diffuses toward domains which are already the richest in quartz. Alternatively, if the rock is chemically open to loss of silica, this silica preferentially originates from dissolution of quartz in the more mica-rich domains. In either case the compositional contrast between domains is increased and metamorphic segregation results. Type C differentiation is best explained if we accept the suggestion made by many petrographers that layer silicates catalyse the pressure-induced transfer of silica. This suggestion can in fact account for other features of metamorphic segregation bands.Metamorphic segregation should also proceed during the development of quartz segregations, quartz rods, slaty cleavage and mylonite banding. The stress distribution argued for Type L bands may also occur in many banded migmatites. In migmatites. however, the assemblage crystallizing in the leucosome may be derived from a circulating hydrothermal fluid as well as from diffusion transfer.     

Siderophile-rich particles in the melt rocks at the E. Clearwater impact structure,Quebec: Their characteristics and relationship to the impacting body

The composition of the sampled melt rocks at the 22 km diameter E. Clearwater impact structure indicates the presence of ～8% C-1 material. The meteoritic component is fractionated with refractory siderophiles, up to 30 times C-1 abundances, concentrated in ten to hundred micron-sized, magnetic particles. These particles consist of the Ni-sulphide, millerite, and what is assumed to be a mixture of refractory silicates and magnetite with grain sizes of <1 μm. The larger particles have a core-rim structure with millerite and occasionally very minor galena and possibly pentlandite in the core. An origin as a combination of altered meteoritic metal and condensed meteoritic silicate is favored for the origin of the siderophile-rich particles. If 8% meteoritic material is taken as the average meteoritic contamination in the melt, then the E. Clearwater projectile may have impacted with a velocity of 17 km s^?1. Peak shock pressures would have been of the order of 300 GPa, sufficient to vaporize the silicate component but only melt the metal component of the projectile. As the meteoritic material was being driven down into vaporized/ melted target rocks during the initial stages of impact, the melted Fe, Ni metal underwent oxidation, Fe was removed, and meteoritic silicate material recondensed on the cooler, essentially Ni metal. As cavity excavation proceeded, these Ni metal, silicate-oxide particles were incorporated in the melt, their refractory nature prevented thermal digestion and sulphur in the melt reacted with the metal to produce millerite on final equilibration. If this hypothesis is correct, it suggests that the E. Clearwater projectile was a C-2 or C-3 chondrite, both of which are compatible with the trace element composition of the melt rocks. Clearwater Lake is a twin impact structure formed by an asteroid pair. It is still not clear, however, what type of projectile formed the 32 km diameter western structure, where the surface melt rocks contain no identifiable meteoritic signature.     

Barium silicates of the Berisal Complex, Switzerland: A study in geochronology and rare-gas release systematics

Barium silicate minerals such as celsian, ganterite, armenite, as well as Ba-bearing and Ba-free white mica from the Berisal Complex, Simplon Nappe, Swiss Alps, were dated by ³⁹Ar-⁴⁰Ar. Ages of Ba-free micas are ca. 17 Ma, while Ba and parentless ⁴⁰Ar are correlated in Ba silicates, suggesting common inheritance from the Paleozoic orthogneissic protolith. The release pattern of reactor-produced ³⁹Ar (or ³⁷Ar) from hydrated and anhydrous minerals is very similar, with apparent activation energies of ca. 180 kJ/mole and a conspicuous kink around 900 °C. White micas release Ar at higher temperature than the literature determination of their dehydroxylation. In addition to Ar, we studied the degassing of monoisotopic ¹³¹Xe produced from Ba during neutron irradiation. Xe is degassed at higher temperature than Ar, and again all analyzed silicates have the same apparent activation energy of ca. 300 kJ/mole. The decoupling of Ar and Xe rules out delamination as the dominant degassing mechanism in mica and implies that recoiled rare gas atoms mostly reside inside the T-O-T layers of the mica structure. The near-identical apparent activation energies in such diverse silicates as tecto-, phyllo- and cyclosilicate requires instead that the in-vacuo gas release kinetics are the same in all three silicates. As the only structural element common to these three silicate families are silica tetrahedra, it is possible that their well-known rotation plays a decisive role for in-vacuo Ar degassing; additional high-temperature in situ structural investigations on feldspars and micas would be needed to help constrain the mechanisms of laboratory Ar release.     

Sulphide and oxide minerals from the Ohmine granitic rocks in Kii Peninsula,central Japan,and their primary paragenetic relations

In the Ohmine granitic rocks of Kii Peninsula, central Japan, ilmenite, rutile, pyrrhotite, pyrite and chalcopyrite commonly occur, but no magnetite is present. Their primary paragenetic relations were confirmed by examining their mode of occurrences as inclusion species in major silicate minerals and the phase relations in the Cu-Fe-S system, though their parageneses changed in cooling of the rocks. This makes it possible to estimate the fugacities of oxygen and sulfur in the granitic rocks based upon their paragenetic relations, and to discuss the nature of volatiles in the granitic rocks. Significant subsolidus reactions for sulfide minerals continued until the rocks cooled below 300° C and took place in two types of mineral grains, in groundmass and as inclusions. The removal of sulfur and copper through the silicate crystals was too easy to preserve the chemistry of sulfides included in silicates.     

Yanshanian Magma-Tectonic-Metallogenic Belt in East China of Circum-Pacific Domain (Ⅰ):Igneous Rocks and Orogenic Processes

Ｙａｎｓｈａｎｉａｎｍａｇｍａ－ｔｅｃｔｏｎｉｃ－ｍｅｔａｌｏｇｅｎｉｃｂｅｌｔｏｆｔｈｅＥａｓｔＣｈｉｎａｉｓａｐａｒｔｏｆｔｈｅｃｉｒｃｕｍ－Ｐａｃｉｆｉｃｄｏｍａｉｎ．Ａｌｏｔｏｆｐａｐｅｒｓｈａｖｅｂｅｅｎｐｕｂｌｉｓｈｅｄｔｏｄｉｓｃｕｓｓ...