Chemical and isotopic compositions of rocks and minerals from the Palenyi Island, Por'ya Guba (Bay), White Sea

Hypersthene-garnet-sillimanite-quartz enclaves were studied in orthopyroxene-plagioclase and orthopyroxene-clinopyroxene crystalline schists and gneisses from shear zones exposed in the Palenyi Island within the Early Proterozoic Belomorian Mobile Belt. Qualitative analysis of mineral assemblages indicates that these rocks were metamorphosed to the granulite facies (approximately 900°C and 10-11 kbar). Oxygen isotopic composition was determined in rock-forming minerals composing zones of the enclaves of various mineral and chemical composition. Closure temperatures of the isotopic systems obtained by methods of oxygen isotopic thermometry are close to values obtained with mineralogical geothermometers (garnet-orthopyroxene and garnet-biotite) and correspond to the high-temperature granulite facies (860-900°C). Identified systematic variations in d18O values were determined in the same minerals from zones of different mineral composition. Inasmuch as these zones are practically in contact with one another, these variations in d18O cannot be explained by primary isotopic heterogeneity of the protolith. Model calculations of the extent and trend of d18O variations in minerals suggest that fluid-rock interaction at various integral fluid/rock ratios in discrete zones was the only mechanism that could generate the zoning. This demonstrates that focused fluid flux could occur in lower crustal shear zones. Preservation of high-temperature isotopic equilibria of minerals testifies that the episode of fluid activity at the peak of metamorphism was very brief.

Isotopic and chemical compositions of rocks and minerals from the TAG hydrothermal mound, ODP Site 957

Strontium- and oxygen-isotopic measurements of samples recovered from the Trans-Atlantic Geotraverse (TAG) hydrothermal mound during Leg 158 of the Ocean Drilling Program provide important constraints on the nature of fluid-rock interactions during basalt alteration and mineralization within an active hydrothermal deposit. Fresh Mid-Ocean Ridge Basalt (MORB), with a 87Sr/86Sr of 0.7026, from the basement beneath the TAG mound was altered at both low and high temperatures by seawater and altered at high temperature by near end-member black smoker fluids. Pillow breccias occurring beneath the margins of the mound are locally recrystallized to chlorite by interaction with large volumes of conductively heated seawater (>200°C). The development of a silicified, sulfide-mineralized stockwork within the basaltic basement follows a simple paragenetic sequence of chloritization followed by mineralization and the development of a quartz+pyrite+paragonite stockwork cut by quartz-pyrite veins. Initial alteration involved the development of chloritic alteration halos around basalt clasts by reaction with a Mg-bearing mixture of upwelling, high-temperature (>300°C), black smoker-type fluid with a minor (<10%) proportion of seawater. Continued high-temperature (>300°C) interaction between the wallrock and these Mg-bearing fluids results in the complete recrystallization of the wallrock to chlorite+quartz+pyrite. The quartz+pyrite+paragonite assemblage replaces the chloritized basalts, and developed by reaction at 250-360°C with end-member hydrothermal fluids having 87Sr/86Sr ~0.7038, similar to present-day vent fluids. The uniformity of the 87Sr/86Sr ratios of hydrothermal assemblages throughout the mound and stockwork requires that the 87Sr/86Sr ratio of end-member hydrothermal fluids has remained relatively constant for a time period longer than that required to change the interior thermal structure and plumbing network of the mound and underlying stockwork. Precipitation of anhydrite in breccias and as late-stage veins throughout most of the mound and stockwork, down to at least 125 mbsf, records extensive entrainment of seawater into the hydrothermal deposit. 87Sr/86Sr ratios indicate that most of the anhydrite formed from ~2:1 mixture of seawater and black smoker fluids (65%±15% seawater). Oxygen-isotopic compositions imply that anhydrite precipitated at temperatures between 147°C and 270°C and require that seawater was conductively heated to between 100°C and 180°C before mixing and precipitation occurred. Anhydrite from the TAG mound has a Sr-Ca partition coefficient Kd ~0.60±0.28 (2 sigma). This value is in agreement with the range of experimentally determined partition coefficients (Kd ~0.27-0.73) and is similar to those calculated for anhydrite from active black smoker chimneys from 21°N on the East Pacific Rise. The d18O (for SO4) of TAG anhydrite brackets the value of seawater sulfate oxygen (~9.5?). Dissolution of anhydrite back into the oceans during episodes of hydrothermal quiescence provides a mechanism of buffering seawater sulfate oxygen to an isotopically light composition, in addition to the precipitation and dissolution of anhydrite within the oceanic basement during hydrothermal recharge at the mid-ocean ridges.

Chemical and isotopic compositions of rocks and minerals from the Franz Josef Land Archipelago

Rb, Sr, Sm, Nd, U, and Pb contents and Sr, Nd, and Pb isotopic composition were determined in tholeiite and subalkaline basalts (in both whole-rock samples and individual minerals) from the Franz Josef Land Archipelago. Isotopic data obtained for the Arctic basin are similar to those for islands from the Pacific, Atlantic, and Indian oceans. The assimilation of crustal (sedimentary) rocks by primary depleted material makes isochron determination of basalt age difficult or impossible. The subalkaline basalts (basaltic andesites) were presumably formed by the metasomatic introduction of incompatible elements in tholeiitie basalts and, only partially, through crustal contamination and fractional crystallization.

Chemical and isotopic composition of rocks and minerals at ODP Site 209-1275

This paper reports results of an investigation of a representative collection of samples recovered by deep-sea drilling from the oceanic basement 10 miles west of the rift valley axis in the crest zone of the Mid- Atlantic Ridge at 15°44'N (Sites 1275B and 1275D). Drilling operations were carried out during Leg 209 of the Drilling Vessel JOIDES Resolution within the framework of the Ocean Drilling Program (ODP). The oceanic crust was penetrated to depth of 108.7 m at Site 1275B and 209 m at Site 1275D. We reconstructed the following sequence of magmatic and metamorphic events resulting in the formation of a typical oceanic core complex of slow-spreading ridges: (1) formation of strongly fractionated (enriched in iron and titanium) tholeiitic magmatic melt parental to gabbroids under investigation in a large magma chamber located in a shallow mantle and operating for a long time under steady-state conditions; (2) transfer of the parental magmatic melt of the gabbroids to the base of the oceanic crust, its interaction with host mantle peridotites, and formation of troctolites and plagioclase peridotites; (3) intrusion of enriched trondhjemite melts as veins and dikes in the early formed plutonic complex, contact recrystallization of the gabbro, and development in the peridotite-gabbro complex of enriched geochemical signatures owing to influence of trondhjemite injections; (4) emplacement of dolerite dikes (transformed to diabases); (5) metamorphism of upper epidoteamphibolite facies with participation of marine fluids; and (6) rapid exhumation of the plutonic complex to the seafloor accompanied by greenschist-facies metamorphism. Distribution patterns of Sr and Nd isotopes and strongly incompatible elements in the rocks suggest contributions from two melt sources to the magmatic evolution of the MAR crest at 15°44'N: a depleted reservoir responsible for formation of the gabbros and diabases and an enriched reservoir, from which trondhjemites (granophyres) were derived.

Chemical composition of basalts and minerals from basalts of the Bouvet triple junction

This study focuses on mafic volcanic rocks from the Bouvet triple junction, which fall into six geochemically distinct groups: (1) N-MORB, the most widespread type, encountered throughout the study area. (2) Subalkaline volcanics, hawaiites and mugearites strongly enriched in lithophile elements and radiogenic isotopes and composing the Bouvet volcanic rise, and compositionally similar basalts and basaltic andesites from the Spiess Ridge, generated in a deeper, fertile mantle region. (3) Relatively weakly enriched basalts, T-MORB derived by the mixing of Type 1 and 2 melts and exposed near the axes of the Mid-Atlantic, Southwest Indian, and America-Antarctic Ridges. (4) Basalts with a degree of trace lithophile element enrichment similar to the Spiess Ridge and Bouvet Island rocks, but higher in K, P, Ti, and Cr. These occur within extensional structures: the rift valley of the Southwest Indian Ridge, grabens of the East Dislocation Zone, and the linear rise between the Spiess Ridge and Bouvet volcano. Their parental melts presumably separated from plume material that spread from the main channels and underwent fluid-involving differentiation in the mantle. (5) A volcanic suite ranging from basalt to rhyolite, characterized by low concentrations of lithophile elements, particularly TiO2, and occurring on the Shona Seamount and other compressional features within the Antarctic and South American plates near the Bouvet triple junction. Unlike Types 1 to 4, which display tholeiitic differentiation trends, this suite is calc-alkaline. Its parental melts were presumably related to the plume material as well but, subsequently, they underwent a profound differentiation involving fluids and assimilated surrounding rocks in closed magma chambers in the upper mantle. Alternatively, the Shona Seamount might be a fragment of an ancient oceanic island arc. (6) Enriched basalts, distinguished from the other enriched rock types in very high P and radiogenic isotope abundances and composing a tectonic uplift near the junction of the three rifts. It thus follows that the main factors responsible for the compositional diversity of volcanic rocks in this region include (i) mantle source heterogeneity, (ii) plume activity, (iii) an intricate geodynamic setup at the triple junction giving rise to stresses in adjacent plate areas, and (iv) the geological prehistory. The slow spreading rate and ensuing inefficient mixing of the heterogeneous mantle material result in strong spatial variations in basaltic compositions.

Composition of rocks and minerals from the Bolshoy Lyakhovsky Island (New Siberian Islands)

In the southeast of the Bolshoi Lyakhovsky Island there are outcrops of tectonic outliers composed of low-K medium-Ti tholeiitic basic rocks represented by low altered pillow basalts, as well as by their metamorphosed analogs: amphibolites and blueschists. The rocks are depleted in light rare-earth elements and were melted out of a depleted mantle source enriched in Th, Nb, and Zr also contributed to the rock formation. The magma sources were not affected by subduction-related fluids or melts. The rocks were part of the Jurassic South Anyui ocean basin crust. The blueschists are the crust of the same basin submerged beneath the more southern Anyui-Svyatoi Nos arc to depth of 30-40 km. Pressure and temperature of metamorphism suggest a setting of "warm" subduction. Mineral assemblages of the blueschists record time of a collision of the Anyui-Svyatoi Nos island arc and the New Siberian continental block expressed as a counter-clockwise PT trend. The pressure jump during the collision corresponds to heaping of tectonic covers above the zone of convergence 12 km in total thickness. Ocean rocks were thrust upon the margin of the New Siberian continental block in late Late Jurassic - early Early Cretaceous and mark the NW continuation of the South Anyui suture, one of the main tectonic sutures of the Northeastern Asia.

Chemical composition of rocks and minerals from the Karymsky volcanic center, Eastern Kamchatka

The powerful eruption in the Akademii Nauk caldera on January 2, 1996 marked a new activity phase of the Karymsky volcano and became a noticeable event in the history of modern volcanism in Kamchatka. The paper reports data obtained by studying more than 200 glassy melt inclusions in phenocrysts of olivine (Fo82-72), plagioclase (An92-73), and clinopyroxene (Mg# 83-70) in basalts of the 1996 eruption. The data were used to estimate composition of the parental melt and physicochemical parameters of the magma evolution. According to our data, the parental melt corresponded to low magnesium, high aluminum basalt (SiO2 = 50.2%, MgO = 5.6%, Al2O3 = 17%) of the mildly potassium type (K2O = 0.56%) and contained much dissolved volatile components (H2O = 2.8%, S = 0.17%, and Cl = 0.11%). Melt inclusions in the minerals are similar in chemical composition, a fact testifying that the minerals crystallized simultaneously with one another. Their crystallization started at pressure ~1.5 kbar, proceeded within a narrow temperature range of 1040+/-20°C, and continued until near-surface pressure ~100 bar was reached. Degree of crystallization of the parental melt during its eruption was close to 55%. Massive crystallization was triggered by H2O degassing under pressure <1 kbar. Magma degassing in an open system resulted in escape of 82% H2O, 93% S, and 24% Cl (of their initial contents in the parental melt) to the fluid phase. Release of volatile compounds to the atmosphere during the eruption that lasted for 18 h was estimated as 1.7x10**6 t H2O, 1.4x10**5 t S, and 1.5x10**4 t Cl. Concentrations of most incompatible trace elements in the melt inclusions are close to those in the rocks and to the expected fractional differentiation trend. Melt inclusions in plagioclase were found to be selectively enriched in Li. The Li-enriched plagioclase with melt inclusions thought to originate from cumulate layers in the feeding system beneath Karymsky volcano, in which plagioclase interacted with Li-rich melts/brines and was subsequently entrapped and entrained by the magma during the 1996 eruption.

Composition of rocks and minerals from the transition zone of the Reykjanes Ridge

Investigations of petrography, mineralogy, and chemical composition of gases and fluids in tuffs and lavas were carried out on samples dredged in the transition zone from the shelf and slope of Iceland to the Reykjanes Ridge. The samples were collected from the depths of 950-720 m during different expeditions of R/V Akademik Kurchatov and Mikhail Lomonosov. Mantle ultrabasite inclusions were first recognized in the region of Iceland. It can be assumed that they are related to eruptive structures formed on the ocean floor during Pliocene and are associated with the Iceland hot spot.

Composition of rocks and minerals from the Cape Verde Fault Zone

Results of comprehensive geological, geophysical and geochemical studies carried out in the Cape Verde Fracture Zone (Central Atlantic) during Cruise 9 of R/V ''Antares'' (1990-1991) are published in the book. Detailed characterization of various bedrock complexes (ultrabasites, gabbroids, dolerites, basalts, metamorphic rocks) is given. Geological conditions of newly found hydrothermal mineralization in the area are described. Problems of ore melts are under consideration. New data on hydrochemical anomalies and heat flow are given. The book contains original materials on sedimentary formations of the area.

Chemical composition of rocks and minerals from the ophiolite complex in the Hunter Fracture Zone

A collection of dredge samples from the Hunter Fracture Zone includes holocrystalline massive and cumulose basic and ultrabasic rocks and volcanites of the ophiolite complex: from basalts to rhyolites. The ultrabasic rocks are largely serpentinized harzburgites and lherzolites; their relict mineralogy is typical of peridotite considered to be the refractory residue of partial melting of the mantle. Cumulate textured ultramafic rocks probably are related to the cumulate gabbro and granodiorite rather than to the residual mantle material. The gabbroic rocks are dominantly cumulate textured Pl-Opx-Cpx±Ol gabbronorite and Pl-Cpx±Ol gabbros; the mineral features of these rocks are the result of their crystallization at moderate pressure (in a moderate level magma chamber). The massive Pl-Cpx±Ol gabbros are less common. Green and brown-green Ca-amphibole has partially or totally replaced the clinopyroxene in many samples. There is an overlap in mineral chemistry between the cumulate rocks and the Opx-Cpx-Pl volcanic rocks and boninites. It is interpreted as an indication that the cumulate rocks were co-genetic with Opx-Cpx-Pl volcanic rocks and that they both constitute remnants of an island arc volcanic-plutonic series. The petrologic evidence indicates that ophiolite gabbroic rocks were derived from an island-arc rather than from a mid-ocean ridge.

Composition of melts and minerals from volcanic rocks of the Kuril-Kamchatka Arc

In this paper allivalites, coarse- and giant-textured olivine-anorthite rocks occurring as separate blocks in the eruption products of many volcanoes from the frontal part of the Kuril-Kamchatka Arc are under consideration. New data are reported on petrography, mineralogy, and composition of melt inclusions in minerals from ten allivalite samples collected at Ksudach, Ilinsky, Zavaritsky, Kudryavy, and Golovnin Volcanoes. Crystallization temperatures of allivalite minerals were estimated as 970-1080°C at melt water content of 3.0-3.5 wt % and oxygen fugacity NNO=1-2. Genetic connection was established between compositions of melt inclusions and interstitial glasses in allivalites and volcanic rocks. Cumulate nature of allivalites was demonstrated. Using mass balance calculations, degree of fractionation of primary melts during formation of cumulate layers was estimated for various volcanoes as 22-46%.

Ages of rocks and minerals from the Okhotsk-Chukotka volcanic belt

Current geochronological data on the Okhotsk-Chukotka volcanic belt (OCVB) and relevant problems are discussed. The belt evolution is suggested to be modeled based on 40Ar/39Ar and U-Pb dates more useful in several aspects than common K-Ar or Rb-Sr dates and methods of paleobotanical correlation. Based on new40Ar/39Ar and U-Pb dates obtained for volcanic rocks in the OCVB northern part, the younger (Coniacian) age is established for lower stratigraphic units in the Central Chukotka segment of the belt, and eastward migration of volcanic activity is shown for terminal stages of this structure evolution.

Chemical composition of rocks and minerals from the Mid-Atlantic Ridge near 15°20'N

The monograph summarizes results of petrological and geochemical studies of rocks from the ocean floor collected by the authors during expeditions to the Central Atlantic. Detailed work in the Capa Verde transform fault zone gave a large amount of new information about magmatic and hydrothermal systems of the Mid-Atlantic Ridge.

Chemical and isotopic compositions of rocks and minerals from the Ashadze and Logachev hydrothermal fields, Mid-Atlantic Ridge

This study was aimed at reconstructing a sequence of events in the magmatic and metamorphic evolution of peridotites, gabbroids, and trondhjemites from internal oceanic complexes of the Ashadze and Logachev hydrothermal vent fields. Collections of plutonic rocks from Cruises 22 and 26 of R/V "Professor Logachev", Cruise 41 of R/V "Akademik Mstislav Keldysh", and from the Serpentine Russian-French expedition aboard R/V "Pourquoi pas?" were objects of this study. Data reported here suggest that the internal oceanic complexes of the Ashadze and Logachev fields formed via the same scenario in these two regions of the Mid-Atlantic Ridge. On the other hand, an analysis of petrological and geochemical characteristics of the rocks indicated that the internal oceanic complexes of the MAR axial zone between 12°58'N and 14°45'N show pronounced petrological and geochemical heterogeneity manifested in variations in degree of depletion of mantle residues and in Nd isotopic compositions of rocks from the gabbro-peridotite association. Trondhjemites from the Ashadze hydrothermal field can be considered as partial melting products of gabbroids under influence of hydrothermal fluids. It was supposed that presence of trondhjemites in internal oceanic complexes of MAR can be used as a marker for the highest temperature deep-rooted hydrothermal systems. Perhaps, the region of the MAR axial zone, in which petrologically and geochemically contrasting internal oceanic complexes are spatially superimposed, serves as an area for development of large hydrothermal clusters with considerable ore-forming potential.