O-isotopic Compositions Research Articles

The formation and alteration of the Renazzo-like carbonaceous chondrites II: Linking O-isotope composition and oxidation state of chondrule olivine

To better understand the formation conditions of type-I and type-II chondrules in the Renazzo-like carbonaceous (CR) chondrites, an in situ major- and minor-element and O-isotope study was conducted. Twenty-one ferromagnesian chondrules from three CR chondrites (GRA 95229, GRA 06100, and QUE 99177) were analyzed to establish an internally-consistent data set. From this study we infer that type-II chondrule precursors contained enhanced S-bearing dust and ice abundances relative to type-I chondrules. There is a relationship between the O-isotope composition and oxidation state of olivine, which may be related to the amount of 16O-poor ice and reduced carbon accreted by chondrule precursors before melting. Type-II chondrules formed under H2O/H2 ratios of ∼230–740 times solar. In contrast, type-I chondrules formed under more reducing conditions with lower H2O/H2 ratios of ∼10–100 times solar. We find a relationship between type-II chondrule petrology (relict free vs. relict grain-bearing) and O-isotope composition, which is due to degree of melting and exchange with a 16O-poor gas reservoir. The 16O-poor gas that interacted with both type-I and type-II chondrules is estimated to have an isotopic composition between ∼δ18Og=13–27‰ and δ17Og=10–22‰, different from the O-isotope composition of the water accreted by the CR chondrite parent body. Due to partial melting, type-I chondrules and relict grain-bearing type-II chondrules exchanged with the 16O-poor gas to a lower degree than relict-free type-II chondrules.

A modified procedure for measuring oxygen-18 content of nitrate

Summary Mass spectrometric analysis of O-isotopic composition of nitrate has many potential applications in studies of environmental processes. Through this work, rapid, reliable, precise, broadly applicable, catalyst-free, low-priced and less labor intensive procedure for measuring δ 18 O of nitrate using Isotope Ratio Mass Spectrometer has been developed and implemented. The conditions necessary to effect complete nitrate recovery and complete removal of other oxygen containing anions and dissolved organic carbon (DOC) without scarifying the isotopic signature of nitrate were investigated. The developed procedure consists of two main parts: (1) wet chemistry train for extraction and purification of nitrate from the liquid matrix; (2) off-line pyrolysis of extracted nitrate salt with activated graphite at 550 °C for 30 min. The conditions necessary to effect complete nitrate recovery and complete removal of other oxygen containing compounds were investigated. Dramatic reduction in processing times needed for analysis of δ 18 O of nitrate at natural abundance level was achieved. Preservation experiments revealed that chloroform (99.8%) is an effective preservative. Isotopic contents of some selected nitrate salts were measured using the modified procedure and some other well established methods at two laboratories in Egypt and Germany. Performance assessment of the whole developed analytical train was made using internationally distributed nitrate isotopes reference materials and real world sample of initial zero-nitrate content. The uncertainty budget was evaluated using the graphical nested hierarchal approach. The obtained results proved the suitability for handling samples of complicated matrices. Reduction of consumables cost by about 80% was achieved.

Distribution of nitrate in groundwater affected by the presence of an aquitard at an agricultural area in Chiba, Japan

Geological and geographical parameters including land use, stratigraphic structure, groundwater quality, and N- and O-isotopic compositions of nitrate in groundwater were investigated to elucidate the mechanism of groundwater pollution by NO3− in the agricultural area of Katori, Chiba, Japan. An aquitard distributed in the western part of the study area has produced two unconfined aquifers. The average concentrations of NO3− and dissolved oxygen (DO) were high in the aquifer above the aquitard (7.5 and 7.6 mg/L, respectively) and in the aquifer of the eastern part of the study area that was not influenced by the aquitard (11.9 and 7.8 mg/L, respectively); however, the levels in the aquifer under the aquitard were relatively low (2.2 and 3.7 mg/L, respectively). The δ15N and δ18O values of NO3− generally increased exponentially in the groundwater that flowed into the aquifer under the aquitard as the concentration of NO3− decreased, although this decrease in NO3− also occasionally occurred without isotopic changes. These results indicated that the aquitard prevented the penetration of NO3−, DO, and gaseous O2. Under the aquitard, denitrification and dilution with unpolluted water that entered from natural forested areas reduced the NO3− concentrations in the groundwater. The major sources of NO3− in groundwater in the study area were estimated to be NH4-chemical fertilizer, livestock waste, and manure.

Carbonate-derived CO 2 purging magma at depth: Influence on the eruptive activity of Somma-Vesuvius, Italy

Mafic phenocrysts from selected products of the last 4 ka volcanic activity at Mt. Vesuvius were investigated for their chemical and O-isotope composition, as a proxy for primary magmas feeding the system. 18O/ 16O ratios of studied Mg-rich olivines suggest that near-primary shoshonitic to tephritic melts experienced a flux of sedimentary carbonate-derived CO 2, representing the early process of magma contamination in the roots of the volcanic structure. Bulk carbonate assimilation (physical digestion) mainly occurred in the shallow crust, strongly influencing magma chamber evolution. On a petrological and geochemical basis the effects of bulk sedimentary carbonate digestion on the chemical composition of the near-primary melts are resolved from those of carbonate-released CO 2 fluxed into magma. An important outcome of this process lies in the effect of external CO 2 in changing the overall volatile solubility of the magma, enhancing the ability of Vesuvius mafic magmas to rapidly rise and explosively erupt at the surface.

HETEROGENEOUS DISTRIBUTION OF 26 Al AT THE BIRTH OF THE SOLAR SYSTEM

It is believed that Al-26, a short-lived (t1/2 = 0.73 Ma) and now extinct radionuclide, was uniformly distributed in the nascent Solar System with the initial Al-26/Al-27 ratio of ~5.2\times10-5, suggesting its external stellar origin. However, the stellar source of Al-26 and the manner in which it was injected into the solar system remain controversial: the Al-26 could have been produced by an asymptotic giant branch star, a supernova, or a Wolf-Rayet star and injected either into the protosolar molecular cloud or protoplanetary disk. Corundum (Al2O3) is thermodynamically predicted to be the first condensate from a cooling gas of solar composition. Here we show that micron-sized corundum condensates from O-16-rich gas (Big Delta O-17 ~ -25%) of solar composition recorded heterogeneous distribution of Al-26 at the birth of the solar system: the inferred initial Al-26/Al-27 ratio ranges from ~6.5x10-5 to <2x10-6; ~50% of the corundum grains measured are Al-26-poor. Other Al-26-poor, O-16-rich refractory objects include grossite (CaAl4O7)- and hibonite(CaAl12O19)-rich calcium-aluminum-rich inclusions (CAIs) in CH chondrites, platy hibonite crystals in CM chondrites, and FUN (fractionation and unidentified nuclear isotopic anomalies) CAIs in CV, CO, and CR chondrites. Considering the apparently early and short duration (<0.3 Ma) of condensation of refractory O-16-rich solids in the solar system, we infer that Al-26 was injected into the collapsing protosolar molecular cloud and later homogenized in the protoplanetary disk. The apparent lack of correlation between Al-26 abundance and O-isotope compositions of corundum grains put important constraints on the stellar source of Al-26 in the solar system.

Strontium-, carbon- and oxygen-isotope compositions of marbles from the Cycladic blueschist belt, Greece

AbstractThe Cycladic blueschist belt, Greece, is mostly submerged below sea level and regional correlations are difficult to establish. Marbles are widespread within the belt and locally used as marker horizons to subdivide monotonous schist sequences. However, owing to the lack of distinctive petrographic characteristics, the marbles have not been used for island-to-island correlations. This study aims to investigate the potential of Sr-, C- and O-isotope compositions of marbles as a tool for unravelling the litho- and/or tectonostratigraphic relationships across the Cycladic islands, and as a proxy for the time of sediment formation. For this purpose, we have studied metamorphic carbonate rocks from the islands of Tinos, Andros, Syros, Sifnos and Naxos. Identical87Sr/86Sr values for certain marble horizons occurring on Tinos, Andros and Sifnos are interpreted to document coeval regional carbonate precipitation. The87Sr/86Sr values of the apparently least altered samples intersect the seawater curve multiple times within the most likely time interval of original carbonate precipitation (&lt; 240 Ma; as indicated by previously published ion probe U–Pb zircon data) and thus an unequivocal age assignment is not possible. Very broad temporal correlations are possible, but more subtle distinctions are not feasible. On Andros, the overlapping Sr-isotope values of marbles representing the lowest and highest parts of the metamorphic succession are in accordance with a model suggesting isoclinal folding or thrusting of a single horizon, or very fast sedimentation. In contrast, distinct87Sr/86Sr values for samples from Tinos, representing different levels of the metamorphic succession, suggest that these rocks represent a temporal succession and not the tectonic repetition of a single horizon. Based on Sr-, O- and C-isotope characteristics alone the time equivalence of marbles occurring on different islands could not be documented unambiguously. However, by using various combinations of these parameters, some occurrences can be discriminated from the overall sample population. The new data further accentuate the general potential of coupled Sr-, C- and O-isotope characteristics for identification of archaeological provenance and complement existing datasets for Aegean marbles.

Thermal Evolution of the Lithosphere in a Rift Environment as Inferred from the Geochemistry of Mantle Cumulates, Northern Victoria Land, Antarctica

Among the abundant mantle xenoliths carried by the Cenozoic alkaline basalts of northern Victoria Land, Antarctica, we have studied a suite of clinopyroxene-rich cumulates collected at Browning Pass (Mt. Melbourne Volcanic Province), ranging in composition from wehrlites to clinopyroxenites. Clinopyroxenes belonging to both the Cr-diopside (wehrlites) and Al-augite series (ol-clinopyroxenites and clinopyroxenites) all show convex-upward REE patterns. Modal and cryptic metasomatism has variably affected the xenoliths, accounting for amphibole replacement of clinopyroxene and/or selective enrichment in incompatible elements. Chemical features, along with O–Sr–Nd isotopic data, indicate that both the parental magmas and the metasomatizing melts are related to the Cenozoic magmatic activity and imply the role of at least two mantle components with distinct isotopic fingerprints. The positive covariation between δ18Oolivine and the amount of modal olivine, and between δ18Oolivine and olivine Fo content, suggest that during the fractionation of olivine and pyroxene, the parent magma experienced a change in O-isotope composition; a low-δ18O melt component was not only added to the minerals during the metasomatic event but was also involved in the genesis of the parental melts. The Browning Pass cumulates are used to constrain the origin of the Antarctic Cenozoic magmatism from a heterogeneous mantle source whose depleted end-member is inferred to be the local lithospheric mantle, whereas the enriched end-member is represented by early metasomatic veins or domains emplaced into the depleted mantle during an amagmatic phase of rifting at the beginning of Ross Sea opening. Thermobarometric analysis of the process shows that the respective contribution to magma generation of the two end-members is related to the change of local thermal regime induced by an ‘edge effect’ in the mantle circulation at the lithospheric step between the thick East Antarctic craton and the thinned Ross Sea crust.

Isotopic evidence for the infiltration of mantle and metamorphic CO 2–H 2O fluids from below in faulted rocks from the San Andreas Fault system

To characterize the origin of the fluids involved in the San Andreas Fault (SAF) system, we carried out an isotope study of exhumed faulted rocks from deformation zones, vein fillings and their hosts and the fluid inclusions associated with these materials. Samples were collected from segments along the SAF system selected to provide a depth profile from upper to lower crust. In all, 75 samples from various structures and lithologies from 13 localities were analyzed for noble gas, carbon, and oxygen isotope compositions. Fluid inclusions exhibit helium isotope ratios ( 3He/ 4He) of 0.1–2.5 times the ratio in air, indicating that past fluids percolating through the SAF system contained mantle helium contributions of at least 35%, similar to what has been measured in present-day ground waters associated with the fault (Kennedy et al., 1997). Calcite is the predominant vein mineral and is a common accessory mineral in deformation zones. A systematic variation of C- and O-isotope compositions of carbonates from veins, deformation zones and their hosts suggests percolation by external fluids of similar compositions and origin with the amount of fluid infiltration increasing from host rocks to vein to deformation zones. The isotopic trend observed for carbonates in veins and deformation zones follows that shown by carbonates in host limestones, marbles, and other host rocks, increasing with increasing contribution of deep metamorphic crustal volatiles. At each crustal level, the composition of the infiltrating fluids is thus buffered by deeper metamorphic sources. A negative correlation between calcite δ 13C and fluid inclusion 3He/ 4He is consistent with a mantle origin for a fraction of the infiltrating CO 2. Noble gas and stable isotope systematics show consistent evidence for the involvement of mantle-derived fluids combined with infiltration of deep metamorphic H 2O and CO 2 in faulting, supporting the involvement of deep fluids percolating through and perhaps weakening the fault zone. There is no clear evidence for a significant contribution from meteoric water, except for overprinting related to late weathering.

OXYGEN ISOTOPE COMPOSITION OF GARNET IN THE PENINSULA GRANITE, CAPE GRANITE SUITE, SOUTH AFRICA: CONSTRAINTS ON MELTING AND EMPLACEMENT MECHANISMS

Garnet is an accessory mineral in the Cape Granite Suite, and garnet δ 18 O values in the Peninsula Granite range in from 10.0 to 11.4‰ (mean 10.6 ± 0.6‰, n = 15). These values are consistent with the garnet being produced during incongruent melting of a metapelitic source that has a similar O-isotope composition to the Malmesbury Group. Peninsula Granite quartz δ 18 O values range from 13.2 to 14.0‰ (mean 13.6 ± 0.3‰, n = 17), at the high end of the range previously observed for the Cape Granite Suite. These high δ 18 O values are consistent with the source of the Peninsula Granite magma having a greater component of clay minerals, which have inherently high δ 18 O values. Garnet has a high closure temperature (>800 o C) to oxygen diffusion and its δ 18 O value should, therefore, correlate closely with that of the source. Quartz has a significantly lower closure temperature (~550 o C) than garnet, and sub-solidus oxygen isotope re-equilibration between quartz and feldspar during slow cooling ought to result in a greater variation in quartz δ 18 O values compared to that of garnet. That the reverse is the case suggests that granite magmas were derived from a moderately heterogeneous source, as expected for metasedimentary rocks. This source underwent melting to produce different batches of granitic magma containing entrained garnets of slightly different δ 18 O value. Magma batches were subsequently mixed and homogenized before and/or during the emplacement process, resulting in a narrower spread of quartz δ 18 O values.

Detection of oxygen isotopic anomaly in terrestrial atmospheric carbonates and its implications to Mars.

The debate of life on Mars centers around the source of the globular, micrometer-sized mineral carbonates in the ALH84001 meteorite; consequently, the identification of Martian processes that form carbonates is critical. This paper reports a previously undescribed carbonate formation process that occurs on Earth and, likely, on Mars. We identified micrometer-sized carbonates in terrestrial aerosols that possess excess (17)O (0.4-3.9‰). The unique O-isotopic composition mechanistically describes the atmospheric heterogeneous chemical reaction on aerosol surfaces. Concomitant laboratory experiments define the transfer of ozone isotopic anomaly to carbonates via hydrogen peroxide formation when O(3) reacts with surface adsorbed water. This previously unidentified chemical reaction scenario provides an explanation for production of the isotopically anomalous carbonates found in the SNC (shergottites, nakhlaites, chassignites) Martian meteorites and terrestrial atmospheric carbonates. The anomalous hydrogen peroxide formed on the aerosol surfaces may transfer its O-isotopic signature to the water reservoir, thus producing mass independently fractionated secondary mineral evaporites. The formation of peroxide via heterogeneous chemistry on aerosol surfaces also reveals a previously undescribed oxidative process of utility in understanding ozone and oxygen chemistry, both on Mars and Earth.

Diel cycling and stable isotopes of dissolved oxygen, dissolved inorganic carbon, and nitrogenous species in a stream receiving treated municipal sewage

Diel and synoptic studies were undertaken in Silver Bow Creek, a small, highly eutrophic stream receiving municipal sewage from the city of Butte, Montana, USA. During mid-summer baseflow conditions, oxidation of ammonium from the Butte wastewater treatment plant created a 2-km long reach marked by nightly hypoxia and extreme growth of aquatic plants and algae. Large diel cycles in the concentrations and isotopic compositions of dissolved oxygen (DO) and dissolved inorganic carbon (DIC) were inversely related, and are explained by the daily cycle of photosynthesis and respiration, modified by the effects of nitrification. Nitrification rates were higher during the day than at night due to a combination of higher water temperatures and higher DO concentrations. Changes in nitrification rate imparted diel cycles in the concentrations and N-isotopic compositions of NO 3 − and NH 4 +, whereas total dissolved inorganic nitrogen (DIN) concentrations and δ 15N-DIN values showed minimal diel variation. δ 15N-NH 4 steadily increased with distance downstream over a 5 km reach, whereas δ 15N-NO 3 showed a more complex spatial pattern. Plant assimilation caused downstream decreases in DIN and soluble reactive phosphorous: however, it was not possible to determine whether the plants assimilated NO 3 −, NH 4 +, or both. An important new finding of this study is the recognition that synoptic and diel changes in δ 18O-DO in the zone of active nitrification imparted a corresponding change in δ 18O-NO 3. Future studies examining the O-isotope composition of nitrate need to consider whether DO has a constant or shifting isotopic composition at the source of nitrification. The results of this study underscore the complex and dynamic interactions between biologically-active solutes in nutrient-rich streams, and also emphasize the importance of recognizing these cycles when collecting samples for scientific or monitoring purposes.

Geochemistry of garnetiferous Ti–clinohumite rock and talc–kyanite–phengite–almandine schist from the Kokchetav UHP terrane, Kazakhstan: An insight to possible origins of some chemically unusual UHP rocks

Major-element, trace-element and O-isotope analyses were carried out for garnetiferous Ti–clinohumite rocks and eclogites from the Kumdy Kol area, as well as talc–kyanite–phengite–almandine schists (previously named as whiteschists) and eclogites from the Kulet area of the Kokchetav ultrahigh-pressure (UHP) terrane, to explore their possible origins. Both garnetiferous Ti–clinohumite rocks and talc–kyanite–phengite–almandine schists are possibly genetically related to eclogites, as they occur in close association with eclogites enclosed within gneiss/metapelite in these two areas. Most eclogites from the Kumdy Kol and the Kulet areas exhibit chemical characteristics similar to that of ocean floor basalt, although a few show arc affinity with slight Ta and Nb depletions. Garnetiferous Ti–clinohumite rocks from the Kumdy Kol region are characterized by very high Mg, low Si, Ca, Na, and K contents compared with eclogites. They also exhibit LREE depleted patterns with negative Eu anomalies. Their O-isotope compositions are less than + 5.6‰. On the other hand, the rocks show comparable Al, Ni, Cr, Co and HREE contents as eclogites. The Mg enrichment of these garnetiferous Ti–clinohumite rocks is suggested to have originated from their mafic protoliths that had been subjected to mafic rock–ultramafic rock–fluid interactions. Talc–kyanite–phengite–almandine schists from the Kulet area are typically low in Ca and Na, but high in Mg, K, Rb, Ba, Cs, Th, U and O-isotope compositions (+ 6 to + 10‰) relative to eclogites. This peculiar chemical composition can be accounted for if the basaltic protoliths had been subjected to sequential high-temperature (enrichment of Mg) and low-temperature (enrichment of K, Rb, Ba, Cs, Th, U and O-isotope compositions) seawater–basalt interactions. One talc–kyanite–phengite–almandine schist sample KLW 7 is distinctly high in Si and K, but low in Mg and Fe, which can be attributed to more severe low-temperature hydrothermal alterations. However, except for Mg, the enrichment of Rb, Ba, Cs, Th, U and O-isotope compositions in these schist samples could also be enhanced by, or otherwise be interpreted as a result of, fluid influx sourced from enclosing metapelite during subduction.

Matrix and whole-rock fractionations in the Acfer 094 type 3.0 ungrouped carbonaceous chondrite

Abstract– We used the electron microprobe to study matrix in the ungrouped type 3.0 carbonaceous chondrite Acfer 094 using 7 × 7-point, focused-beam arrays; data points attributable to mineral clasts were discarded. The grid areas show resolvable differences in composition, but differences are less pronounced than we observed in studies of CR2 LaPaz Icefield (LAP) 02342 (Wasson and Rubin [2009]) and CO3.0 Allan Hills A77307 (Brearley [1993]). A key question is why Acfer shows an anomalously uniform composition of matrix compared with these other carbonaceous chondrites. Both whole-rock and matrix samples of Acfer 094 show enhancements of Ca and K; it appears that these reflect contamination during hot desert weathering. By contrast, the whole-rock abundance of Na is low. Although weathering effects are responsible for some fractionations, it appears that nebular effects are also resolvable in matrix compositions in Acfer 094. As with LAP 02342, we infer that the observed differences among different areas were inherited from the solar nebula and may have been carried by porous chondrules that experienced low (about 20%) degrees of melting. Acfer 094 has been comminuted by one or more impact events that may also have caused volatile loss. Thus, despite preserving evidence (e.g., an exceptionally high content of presolar SiC) implying a high degree of pristinity, Acfer 094 is far from pristine in other respects. This evidence of comminution and an O-isotopic composition similar to values measured in metamorphosed CM chondrites suggest that Acfer was hydrated before being outgassed by the inferred impact event. Convection within the plume associated with the impact event probably also contributed to the homogenization of the Acfer 094 matrix.

Oxygen isotope evidence for sorption of molecular oxygen to pyrite surface sites and incorporation into sulfate in oxidation experiments

Abstract Experiments were conducted to investigate (i) the rate of O-isotope exchange between SO4 and water molecules at low pH and surface temperatures typical for conditions of acid mine drainage (AMD) and (ii) the O- and S-isotope composition of sulfates produced by pyrite oxidation under closed and open conditions (limited and free access of atmospheric O2) to identify the O source/s in sulfide oxidation (water or atmospheric molecular O2) and to better understand the pyrite oxidation pathway. An O-isotope exchange between SO4 and water was observed over a pH range of 0–2 only at 50 °C, whereas no exchange occurred at lower temperatures over a period of 8 a. The calculated half-time of the exchange rate for 50 °C (pH = 0 and 1) is in good agreement with former experimental data for higher and lower temperatures and excludes the possibility of isotope exchange for typical AMD conditions (T ⩽ 25 °C, pH ⩾ 3) for decades. Pyrite oxidation experiments revealed two dependencies of the O-isotope composition of dissolved sulfates: O-isotope values decreased with longer duration of experiments and increasing grain size of pyrite. Both changes are interpreted as evidence for chemisorption of molecular O2 to pyrite surface sites. The sorption of molecular O2 is important at initial oxidation stages and more abundant in finer grained pyrite fractions and leads to its incorporation in the produced SO4. The calculated bulk contribution of atmospheric O2 in the dissolved SO4 reached up to 50% during initial oxidation stages (first 5 days, pH 2, fine-grained pyrite fraction) and decreased to less than 20% after about 100 days. Based on the direct incorporation of molecular O2 in the early-formed sulfates, chemisorption and electron transfer of molecular O2 on S sites of the pyrite surface are proposed, in addition to chemisorption on Fe sites. After about 10 days, the O of all newly-formed sulfates originates only from water, indicating direct interaction of hydroxyls from water with S at the anodic S pyrite surface site. Then, the role of molecular O2 is as proposed in previous studies: acting as electron acceptor only at the cathodic Fe pyrite surface site for oxidation of Fe(II) to Fe(III).

On the Mass and Metallicity Distributions of the Parent AGB Stars of O-Rich Presolar Stardust Grains

AbstractPresolar grains in meteorites formed in a sample of Asymptotic Giant Branch (AGB) stars that ended their lives within ≈1 Gyr of the origin of the Solar System 4.6 Gyr ago. The O-isotopic compositions of presolar O-rich stardust reflect the masses and metallicities of their parent stars. We present simple Monte Carlo simulations of the parent AGB stars of presolar grains. Comparison of model predictions with the grain data allow some broad conclusions to be drawn: (1) Presolar O-rich grains formed in AGB stars of mass ∼1.15–2.2 M⊙. The upper-mass cutoff reflects dredge-up of C in more massive AGB stars, leading to C-rich dust rather than O-rich, but the lack of grains from intermediate-mass AGB stars (&gt;4 M⊙) is a major puzzle; (2) The grain O-isotopic data are reproduced well if the Galaxy in presolar times was assumed to have a moderate age-metallicity relationship, but with significant metallicity scatter for stars born at the same time; (3) The Sun appears to have a moderately low metallicity for its age and/or unusual 17O/16O and 18O/16O ratios for its metallicity; and (4) The Solar 17O/18O ratio, while unusual relative to present-day molecular clouds and protostars, was not atypical for the presolar disk and does not require self-pollution of the protosolar molecular cloud by supernova ejecta.

Oxygen isotopes composition of sapphires from the French Massif Central: implications for the origin of gem corundum in basaltic fields

Alluvial and colluvial gem sapphires are common in the basaltic fields of the French Massif Central (FMC) but sapphire-bearing xenoliths are very rare, found only in the Menet trachytic cone in Cantal. The O-isotope composition of the sapphires ranges between 4.4 and 13.9‰. Two distinct groups have been defined: the first with a restricted isotopic range between 4.4 and 6.8‰ (n = 22; mean δ18O = 5.6 ± 0.7‰), falls within the worldwide range defined for blue-green-yellow sapphires related to basaltic gem fields (3.0 < δ18O < 8.2‰, n = 150), and overlaps the ranges defined for magmatic sapphires in syenite (4.4 < δ18O < 8.3‰, n = 29). A second group, with an isotopic range between 7.6 and 13.9‰ (n = 9), suggests a metamorphic sapphire source such as biotite schist in gneisses or skarns. The δ18O values of 4.4–4.5‰ for the blue sapphire-bearing anorthoclasite xenolith from Menet is lower than the δ18O values obtained for anorthoclase (7.7–7.9‰), but suggest that these sapphires were derived from an igneous reservoir in the subcontinental spinel lherzolitic mantle of the FMC. The presence of inclusions of columbite-group minerals, pyrochlore, Nb-bearing rutile, and thorite in these sapphires provides an additional argument for a magmatic origin. In the FMC lithospheric mantle, felsic melts crystallized to form anorthoclasites, the most evolved peraluminous variant of the alkaline basaltic melt. The O-isotopic compositions of the first group suggests that these sapphires crystallized from felsic magmas under upper mantle conditions. The second group of isotopic values, typified for example by the Le Bras sapphire with a δ18O of 13.9‰, indicates that metamorphic sapphires from granulites were transported to the surface by basaltic magma.

Marble-hosted ruby deposits from Central and Southeast Asia: Towards a new genetic model

Marble-hosted ruby deposits represent the most important source of colored gemstones from Central and South East Asia. These deposits are located in the Himalayan mountain belt which developed during Tertiary collision of the Indian plate northward into the Eurasian plate. They are spatially related to granitoid intrusions and are contained in platform carbonates series that underwent high-grade metamorphism. All occurrences are located close to major tectonic features formed during Himalayan orogenesis, directly in suture zones in the Himalayas, or in shear zones that guided extrusion of the Indochina block after the collision in South East Asia. Ar–Ar dating of micas syngenetic with ruby and U–Pb dating of zircon included in ruby gives evidence that these deposits formed during Himalayan orogenesis, and the ages document the extensional tectonics that were active, from Afghanistan to Vietnam, between the Oligocene and the Pliocene. The petrography shows that ruby-bearing marbles formed in the amphibolite facies ( T = 610 to 790 °C and P ~ 6 kbar). A fluid inclusion study defines the conditions of gem ruby formation during the retrograde metamorphic path (620 < T < 670 °C and 2.6 < P < 3.3 kbar) for the deposits of Jegdalek, Hunza and northern Vietnam. Whole rock analyses of non-ruby-bearing marbles indicate that they contain enough aluminum and chromiferous elements to produce all the ruby crystals that they contain. In addition, (C, O)-isotopic analyses of carbonates from the marbles lead to the conclusion that the marbles acted as a metamorphic closed fluid system that were not infiltrated by externally-derived fluids. The carbon isotopic composition of graphite in marbles reveals that it is of organic origin and that it exchanged C-isotopes with the carbonates during metamorphism. Moreover, the O-isotopic composition of ruby was buffered by metamorphic CO 2 released during devolatilisation of marble and the H-isotopic composition of mica is consistent with a metamorphic origin for water in equilibrium with the micas. The (C, O, H)-isotopic compositions of minerals associated with marble-hosted ruby are all in agreement with the hypothesis, drawn from the unusual chemistry of CO 2–H 2S–COS–S 8–AlO(OH)-bearing fluids contained in fluid inclusions, that gem ruby formed at P ~ 3 kbar and 620 < T < 670 °C, during thermal reduction of evaporite by organic matter, at high temperature-medium pressure metamorphism of platform carbonates during the Tertiary India–Asia collision. The carbonates were enriched in Al- and chromiferous-bearing detrital minerals, such as clay minerals that were deposited on the platform with the carbonates, and in organic matter. Ruby formed during the retrograde metamorphic path, mainly by destabilization of muscovite or spinel. The metamorphic fluid system was rich in CO 2 released from devolatilisation of carbonates, and in fluorine, chlorine and boron released by molten salts (NaCl, KCl, CaSO 4). Evaporites are key to explaining the formation of these deposits. Molten salts mobilized in situ Al and metal transition elements contained in marbles, leading to crystallization of ruby.

Isotopic and element exchange during serpentinization and metasomatism at the Atlantis Massif (MAR 30°N): Insights from B and Sr isotope data

The Lost City hydrothermal system at the southern Atlantis Massif (Mid-Atlantic Ridge, 30°N) provides a natural laboratory for studying serpentinization processes, the temporal evolution of ultramafic-hosted hydrothermal systems, and alteration conditions during formation and emplacement of an oceanic core complex. Here we present B, O, and Sr isotope data to investigate fluid/rock interaction and mass transfer during detachment faulting and exhumation of lithospheric sequences within the Atlantis Massif. Our data indicate that extensive serpentinization was a seawater-dominated process that occurred predominately at temperatures of 150–250 °C and at high integrated W/R ratios that led to a marked boron enrichment (34–91 ppm). Boron removal from seawater during serpentinization is positively correlated with changes in δ 11B (11–16‰) but shows no correlation with O-isotope composition. Modeling indicates that B concentrations and isotope values of the serpentinites are controlled by transient temperature–pH conditions. In contrast to prior studies, we conclude that low-temperature marine weathering processes are insignificant for boron geochemistry of the Atlantis Massif serpentinites. Talc- and amphibole-rich fault rocks formed within a zone of detachment faulting at temperatures of approximately 270–350 °C and at low W/R ratios. Talc formation in ultramafic domains in the massif was subsequent to an early stage of serpentinization and was controlled by the access of Si-rich fluids derived through seawater–gabbro interactions. Replacement of serpentine by talc resulted in boron loss and significant lowering of δ 11B values (9–10‰), which we model as the product of progressive extraction of boron. Our study provides new constraints on the boron geochemical cycle at oceanic spreading ridges and suggests that serpentinization associated with ultramafic-hosted hydrothermal systems may have important implications for the behavior of boron in subduction zone settings.

Record of Low-Temperature Alteration in Asteroids

Research Article| January 01, 2008 Record of Low-Temperature Alteration in Asteroids Michael E. Zolensky; Michael E. Zolensky Astromaterials Research and Exploration Science, NASA Johnson Space Center, Houston, Texas 77058, U.S.A., michael.e.zolensky@nasa.gov Search for other works by this author on: GSW Google Scholar Alexander N. Krot; Alexander N. Krot Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Manoa, Honolulu, Hawaii 96822, U.S.A. Search for other works by this author on: GSW Google Scholar Gretchen Benedix Gretchen Benedix Department of Mineralogy, The Natural History Museum, London SW7 5BD, United Kingdom Search for other works by this author on: GSW Google Scholar Author and Article Information Michael E. Zolensky Astromaterials Research and Exploration Science, NASA Johnson Space Center, Houston, Texas 77058, U.S.A., michael.e.zolensky@nasa.gov Alexander N. Krot Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Manoa, Honolulu, Hawaii 96822, U.S.A. Gretchen Benedix Department of Mineralogy, The Natural History Museum, London SW7 5BD, United Kingdom Publisher: Mineralogical Society of America First Online: 03 Mar 2017 © The Mineralogical Society Of America Reviews in Mineralogy and Geochemistry (2008) 68 (1): 429–462. https://doi.org/10.2138/rmg.2008.68.15 Article history First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Michael E. Zolensky, Alexander N. Krot, Gretchen Benedix; Record of Low-Temperature Alteration in Asteroids. Reviews in Mineralogy and Geochemistry 2008;; 68 (1): 429–462. doi: https://doi.org/10.2138/rmg.2008.68.15 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search Abstract Most chondritic materials experienced diverse styles of secondary alteration resulting in formation of hydrous and anhydrous oxygen-bearing minerals—silicates, oxides, and carbonates (phyllosilicates, magnetite, calcite, dolomite, breunnerite, ferrous olivine, hedenbergite, wollastonite, grossular, monticellite, forsterite, andradite, nepheline, sodalite) (Brearley 2003, 2005 and references therein). Petrographic, mineralogic, oxygen and short-lived isotope systematics (26Al-26Mg, 53Mn-53Cr, 129I-129Xe) suggest that alteration occurred in the presence of aqueous solutions under variable physico-chemical conditions (temperature, water:rock ratio, pH, fO2, and fluid compositions) in an asteroidal setting; it started within 1–2 m.y. after formation of the CV CAIs, was multistage, and lasted up to 15 m.y. (Krot et al. 2006 and references therein). Here we review bulk O-isotopic compositions of chondritic materials, O-isotopic compositions of secondary minerals produced during asteroidal alteration, and possible effects of fluid-assisted thermal metamorphism on O-isotopic exchange in primary, high-temperature minerals (melilite and anorthite) in the CV and CO CAIs. The implications of these data for understanding temperatures of alteration, water:rock ratios, and oxygen isotopic composition of water, that probably accreted into chondrite parent bodies in the form of ice, are discussed. The inferred or measured O-isotopic composition of the meteoritic water is close to the terrestrial fractionation line (Δ17O = ±2‰); it is very different from the inferred O-isotopic compositions of the Sun [Δ17O ~ −25‰] (Clayton 2002; Yurimoto and Kuramoto 2004; Hashizume and Chaussidon 2005; Lyons and Young 2005), suggesting significant evolution of O-isotopic composition of the gaseous reservoir in the inner protoplanetary disk over its short (~3–5 m.y.) life time. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Size scales over which ordinary chondrites and their parent asteroids are homogeneous in oxidation state and oxygen-isotopic composition

Literature data demonstrate that on a global, asteroid-wide scale (plausibly on the order of 100 km), ordinary chondrites (OC) have heterogeneous oxidation states and O-isotopic compositions (represented, respectively, by the mean olivine Fa and bulk Δ 17O compositions of equilibrated samples). Samples analyzed here include: (a) two H5 chondrite Antarctic finds (ALHA79046 and TIL 82415) that have the same cosmic-ray exposure age (7.6 Ma) and were probably within ∼1 km of each other when they were excavated from the H-chondrite parent body, (b) different individual stones from the Holbrook L/LL6 fall that were probably within ∼1 m of each other when their parent meteoroid penetrated the Earth’s atmosphere, and (c) drill cores from a large slab of the Estacado H6 find located within a few tens of centimeters of each other. Our results indicate that OC are heterogeneous in their bulk oxidation state and O-isotopic composition on 100-km-size scales, but homogeneous on meter-, decimeter- and centimeter-size scales. (On kilometer size scales, oxidation state is heterogeneous, but O isotopes appear to be homogeneous.) The asteroid-wide heterogeneity in oxidation state and O-isotopic composition was inherited from the solar nebula. The homogeneity on small size scales was probably caused in part by fluid-assisted metamorphism and mainly by impact-gardening processes (which are most effective at mixing target materials on scales of ⩽1 m).