Oxygen isotope systematics of emerald: relevance for its origin and geological significance

Oxygen isotopic composition of emerald from 62 occurrences and deposits in the world reveals a wide range in δ¹⁸O (SMOW) between +6.2 and +24.7‰. The δ¹⁸O-values for each deposit are restricted and can be used to determine the origin of emerald from the world's most important producers. The δ¹⁸O-value of emerald appears to be a fingerprint of its origin, especially for gems of exceptional quality from Colombia (eastern emerald zone, δ¹⁸O = +16.8 ± 0.1‰; western emerald zone, δ¹⁸O = +21.2 ± 0.5‰), Afghanistan (δ¹⁸O = +13.5 ± 0.1‰), Pakistan (Swat-Mingora districts, δ¹⁸O = +15.7 ± 0.1‰), Brazil (Santa Terezinha de Goiás, δ¹⁸O = +12.2 ± 0.1‰; Quadrilatero Ferrifero, δ¹⁸O = +6.9 ± 0.4‰) and Zimbabwe (Sandawana, δ¹⁸O = +7.5 ± 0.5‰). Furthermore, the ¹⁸O-composition of emerald appears to be a good marker of its geological environment because the data suggest that host-rock-buffering of fluid δ¹⁸O is considerable during fluid-rock interaction. Received: 29 January 1998 / Accepted: 25 March 1998     

Environmental control of the oxygen isotope composition of Porites coral microatolls

Understanding the influence of climatic and non-climatic factors on geochemical signals in corals is critical for assessing coral-derived records of tropical climate variability. Porites microatolls form large, disk-shaped colonies constrained in their upward growth by exposure at or close to mean spring low water level, and occur on Indo-Pacific reefs. Microatolls appear suitable for paleoclimate reconstruction, however the systematics of the microatoll chemistry-climate relationship are yet to be characterized. In this study, the δ¹⁸O signal in Porites microatolls from well-flushed reef flats on Kiritimati (Christmas) Island, central Pacific was investigated for intra-coral (growth aspect and extension rate effects) and between-coral effects, and to explore the climate signal contained within their skeletons. Samples for δ¹⁸O analysis were taken from six individual transects from different positions within Porites microatoll XM22. The results show that: (1) the average standard deviation for the mean δ¹⁸O values of transects that represent the same time periods is 0.03‰, and is within measurement error for a single analysis (0.04‰); (2) the average standard deviation for time-equivalent, near-monthly samples along the transects within the same microatoll is 0.07‰ and; (3) comparison of the average δ¹⁸O values of records for different microatolls from across Kiritimati Island show only a small between-coral differences of 0.04‰ and 0.11‰ for different time periods. These differences in mean δ¹⁸O are within the range for intra- and inter-colony differences in seasonal and interannual δ¹⁸O reported for dome-shaped Porites. Based on these results, a stacked microatoll δ¹⁸O record was constructed for the period 1978-2007 for comparison with published coral δ¹⁸O records for nearby dome-shaped Porites. There is a systematic offset between the two types of records, which is probably due to variations in δ¹⁸O seawater across Kiritimati Island. Despite the offset, all records show similar amplitudes for the seasonal-cycle of δ¹⁸O, and there is a strong correlation (r = −0.71) between microatoll δ¹⁸O and local sea surface temperature (SST). The δ¹⁸O-SST slope relationship for microatolls is −0.15‰/°C, very similar to that reported for fast-growing domed corals (−0.18‰ to −0.22‰/°C). Statistical analysis of the stacked microatoll δ¹⁸O record shows that it is correlated with both local and large-scale climate variables (primarily SST) at semiannual, annual and interannual timescales. Our results show that the signal reproducibility and fidelity of skeletal δ¹⁸O in coral microatolls is comparable to that observed for more conventional coral growth forms. Longer-lived, and fossil, Porites microatolls, where they have grown in suitably flushed environments, are likely to contain δ¹⁸O signals that can significantly extend instrumental records of tropical climate variability.     

Oxygen isotope systematics of gem corundum deposits in Madagascar: relevance for their geological origin

The oxygen isotopic composition of gem corundum was measured from 22 deposits and occurrences in Madagascar to provide a gemstone geological identification and characterization. Primary corundum deposits in Madagascar are hosted in magmatic (syenite and alkali basalt) and metamorphic rocks (gneiss, cordieritite, mafic and ultramafic rocks, marble, and calc-silicate rocks). In both domains the circulation of fluids, especially along shear zones for metamorphic deposits, provoked in situ transformation of the corundum host rocks with the formation of metasomatites such as phlogopite, sakenite, and corundumite. Secondary deposits (placers) are the most important economically and are contained in detrital basins and karsts. The oxygen isotopic ratios (¹⁸O/¹⁶O) of ruby and sapphire from primary deposits are a good indicator of their geological origin and reveal a wide range of δ¹⁸O (Vienna Standard Mean Ocean Water) between 1.3 and 15.6‰. Metamorphic rubies are defined by two groups of δ¹⁸O values in the range of 1.7 to 2.9‰ (cordieritite) and 3.8 to 6.1‰ (amphibolite). “Magmatic” rubies from pyroxenitic xenoliths contained in the alkali basalt of Soamiakatra have δ¹⁸O values ranging between 1.3 and 4.7‰. Sapphires are classified into two main groups with δ¹⁸O in the range of 4.7 to 9.0‰ (pyroxenite and feldspathic gneiss) and 10.7 to 15.6‰ (skarn in marble from Andranondambo). The δ¹⁸O values for gem corundum from secondary deposits have a wide spread between −0.3 and 16.5‰. The ruby and sapphire found in placers linked to alkali basalt environments in the northern and central regions of Madagascar have consistent δ¹⁸O values between 3.5 and 6.9‰. Ruby from the placers of Vatomandry and Andilamena has δ¹⁸O values of 5.9‰, and between 0.5 and 4.0‰, respectively. The placers of the Ilakaka area are characterized by a huge variety of colored sapphires and rubies, with δ¹⁸O values between −0.3 and 16.5‰, and their origin is debated. A comparison with oxygen isotope data obtained on gem corundum from Eastern Africa, India, and Sri Lanka is presented. Giant placer deposits from Sri Lanka, Madagascar, and Tanzania have a large variety of colored sapphires and rubies with a large variation in δ¹⁸O due to mingling of corundum of different origin: mafic and ultramafic rocks for ruby, desilicated pegmatites for blue sapphire, syenite for yellow, green, and blue sapphire, and skarn in marbles for blue sapphire.     

Stable isotope chemistry of fossil bone as a new paleoclimate indicator

During fossilization, bone is thought to recrystallize and alter chemically on timescales of kyr to a few tens of kyr, i.e., similar to the timescale for formation of soils. Therefore, C- and O-isotope compositions of bone apatite should correlate with trends in soil water composition and aridity, and serve as paleoclimate indicators. This hypothesis was tested by analyzing C- and O-isotope compositions of the CO₃ component of fossil bone apatite from mid-Oligocene through late Pleistocene units in Oregon and western Idaho, including the John Day (19.4-30.0 Ma), Mascall (15.2-15.8 Ma), and Rattlesnake (7.2-7.8 Ma) Formations, whose paleosol sequences have been studied in detail, and the Juntura (10-11 Ma), Hagerman (3.2 Ma), and Fossil Lake (<23-650 ka) fossil localities. Tooth enamel δ¹⁸O values provide a baseline of meteoric water compositions. Stable isotope compositions of bone CO₃ do change in response to broad climatic trends, but show poor correlation with compositions of corresponding paleosol CO₃ at specific horizons. Instead, compositional deviations between bone and paleosol CO₃ correlate with compositional deviations with the next higher paleosol; this suggests that the timescale for fossilization exceeds one paleosol cycle. Based on stratigraphic evidence and simple alteration models, fossilization timescales are estimated at 20-50 kyr, indicating that bone CO₃ will prove most useful for sequences spanning >100 kyr. C-isotopes show negative and strong positive deviations during wet and dry climates respectively, and short-term trends correspond well with changes in aridity within the Mascall and Rattlesnake Formations, as inferred from paleosols. A proposed correction to δ¹⁸O values based on δ¹³C anomalies implies a small, ∼1.5‰ increase in meteoric water δ¹⁸O during the late Oligocene global warming event, consistent with a minimum temperature increase of ∼4 °C. A strong inferred decrease in δ¹⁸O of 4-5‰ after 7 Ma closely parallels compositional changes in tooth enamel, and reflects a doubling in the height of the Cascade Range.     

Oxygen isotope ratios of cellulose-derived phenylglucosazone: An improved paleoclimate indicator of environmental water and relative humidity

Oxygen atoms within fossil wood provide high-resolution records of climate change, particularly for the Quaternary. However, current analysis methods of fossil cellulose do not differentiate between different positions of the oxygen atoms. Here, we propose a refinement to tree-cellulose paleoclimatology modeling, using the cellulose-derived compound phenylglucosazone as the isotopic substrate. Stem samples from trees were collected at northern latitudes as low as 24°37′N and as high as 69°00′N. We extracted stem water and cellulose from each stem sample and analyzed them for their ¹⁸O content. In addition, we derived the cellulose to phenylglucosazone, a compound which lacks the oxygen attached to the second carbon of the cellulose-glucose moieties. Oxygen isotope analysis of phenylglucosazone allowed us to calculate the ¹⁸O content of the oxygen attached to the second carbon of the cellulose-glucose moieties. By way of these analyses, we tested two hypotheses: first, that the ¹⁸O content of the oxygen attached to second carbon will more closely reflect the ¹⁸O content of the stem water, and will not resemble the ¹⁸O content of either cellulose or its derivative phenylglucosazone. Second, tree-ring models that incorporate the variable oxygen isotope fractionation shown here and elsewhere are more accurate than those that do not. Our first hypothesis was rejected on the basis that the oxygen isotope ratios of the oxygen attached to the second carbon of the glucose moieties had a noisy isotopic signal with a large standard deviation and gave the poorest correlation with the oxygen isotope ratios of stem water. Related to this isotopic noise, we observed that the correlation between oxygen isotope ratios of phenylglucosazone with both stem water and relative humidity were higher than those observed for cellulose. Our hypothesis about tree-ring models which account for changes in the oxygen isotopic fractionation during cellulose synthesis was consistent only for the ¹⁸O content of phenylglucosazone. We showed that the tree-ring model based on the ¹⁸O content of phenylglucosazone was an improvement over existing models that are based on whole cellulose. Additionally, this approach may be used in other cellulose based archives such as peat deposits and lacustrine sediments.     

Oxygen isotope systematics of chondrules in the Allende CV3 chondrite: High precision ion microprobe studies

The oxygen three-isotope systematics of 36 chondrules from the Allende CV3 chondrite are reported using high precision secondary ion mass spectrometer (CAMECA IMS-1280). Twenty-six chondrules have shown internally homogenous Δ¹⁷O values among olivine, pyroxene, and spinel within a single chondrule. The average Δ¹⁷O values of 19 FeO-poor chondrules (13 porphyritic chondrules, 2 barred olivine chondrules, and 4 chondrule fragments) show a peak at −5.3 ± 0.6‰ (2SD). Another 5 porphyritic chondrules including both FeO-poor and FeO-rich ones show average Δ¹⁷O values between −3‰ and −2‰, and 2 other FeO-poor barred olivine chondrules show average Δ¹⁷O values of −3.6‰ and 0‰. These results are similar to those for Acfer 094 chondrules, showing bimodal Δ¹⁷O values at −5‰ and −2‰. Nine porphyritic chondrules contain olivine grains with heterogeneous Δ¹⁷O values as low as −18‰, indicating that they are relict olivine grains and some of them were derived from precursors related to refractory inclusions. However, most relict olivine grains show oxygen isotope ratios that overlap with those in homogeneous chondrules. The Δ¹⁷O values of four barred olivine chondrules range from −5‰ to 0‰, indicating that not all BO chondrules plot near the terrestrial fractionation line as suggested by previous bulk chondrule analyses. Based on these data, we suggest the presence of multiple oxygen isotope reservoirs in local dust-rich protoplanetary disk, from which the CV3 parent asteroid formed.A compilation of 225 olivine and low-Ca pyroxene isotopic data from 36 chondrules analyzed in the present study lie between carbonaceous chondrite anhydrous mineral (CCAM) and Young and Russell lines. These data define a correlation line of δ¹⁷O = (0.982 ± 0.019) × δ¹⁸O − (2.91 ± 0.10), which is similar to those defined by chondrules in CV3 chondrites and Acfer 094 in previous studies. Plagioclase analyses in two chondrules plot slightly below the CCAM line with Δ¹⁷O values of −2.6‰, which might be the result of oxygen isotope exchange between chondrule mesostasis and aqueous fluid in the CV parent body.     

Osmium isotope systematics of ureilites

The ¹⁸⁷Os/¹⁸⁸Os for 22 ureilite whole rock samples, including monomict, augite-bearing, and polymict lithologies, were examined in order to constrain the provenance and subsequent magmatic processing of the ureilite parent body (or bodies). The Re/Os ratios of most ureilites show evidence for a recent disturbance, probably related to Re mobility during weathering, and no meaningful chronological information can be extracted from the present data set. The ureilite ¹⁸⁷Os/¹⁸⁸Os ratios span a range from 0.11739 to 0.13018, with an average of 0.1258 ± 0.0023 (1σ), similar to typical carbonaceous chondrites, and distinct from ordinary or enstatite chondrites. The similar mean of ¹⁸⁷Os/¹⁸⁸Os measured for the ureilites and carbonaceous chondrites suggests that the ureilite parent body probably formed within the same region of the solar nebula as carbonaceous chondrites. From the narrow range of the ¹⁸⁷Os/¹⁸⁸Os distribution in ureilite meteorites it is further concluded that Re was not significantly fractionated from Os during planetary differentiation and was not lost along with the missing ureilitic melt component. The lack of large Re/Os fractionations requires that Re/Os partitioning was controlled by a metal phase, and thus metal had to be stable throughout the interval of magmatic processing on the ureilite parent body.     

Oxygen and sulfur isotope systematics of sulfate produced by bacterial and abiotic oxidation of pyrite

To better understand reaction pathways of pyrite oxidation and biogeochemical controls on δ¹⁸O and δ³⁴S values of the generated sulfate in acid mine drainage (AMD) and other natural environments, we conducted a series of pyrite oxidation experiments in the laboratory. Our biological and abiotic experiments were conducted under aerobic conditions by using O₂ as an oxidizing agent and under anaerobic conditions by using dissolved Fe(III)_aq as an oxidant with varying δ¹⁸O_H2O values in the presence and absence of Acidithiobacillus ferrooxidans. In addition, aerobic biological experiments were designed as short- and long-term experiments where the final pH was controlled at ∼2.7 and 2.2, respectively. Due to the slower kinetics of abiotic sulfide oxidation, the aerobic abiotic experiments were only conducted as long term with a final pH of ∼2.7. The δ³⁴S_SO4 values from both the biological and abiotic anaerobic experiments indicated a small but significant sulfur isotope fractionation (∼−0.7‰) in contrast to no significant fractionation observed from any of the aerobic experiments. Relative percentages of the incorporation of water-derived oxygen and dissolved oxygen (O₂) to sulfate were estimated, in addition to the oxygen isotope fractionation between sulfate and water, and dissolved oxygen. As expected, during the biological and abiotic anaerobic experiments all of the sulfate oxygen was derived from water. The percentage incorporation of water-derived oxygen into sulfate during the oxidation experiments by O₂ varied with longer incubation and lower pH, but not due to the presence or absence of bacteria. These percentages were estimated as 85%, 92% and 87% from the short-term biological, long-term biological and abiotic control experiments, respectively. An oxygen isotope fractionation effect between sulfate and water (ε¹⁸O_SO4-H2O) of ∼3.5‰ was determined for the anaerobic (biological and abiotic) experiments. This measured value was then used to estimate the oxygen isotope fractionation effects between sulfate and dissolved oxygen in the aerobic experiments which were −10.0‰, −10.8‰, and −9.8‰ for the short-term biological, long-term biological and abiotic control experiments, respectively. Based on the similarity between δ¹⁸O_SO4 values in the biological and abiotic experiments, it is suggested that δ¹⁸O_SO4 values cannot be used to distinguish biological and abiotic mechanisms of pyrite oxidation. The results presented here suggest that Fe(III)_aq is the primary oxidant for pyrite at pH < 3, even in the presence of dissolved oxygen, and that the main oxygen source of sulfate is water-oxygen under both aerobic and anaerobic conditions.     

Oxygen,carbon and strontium isotope systematics in two profiles across the Glarus thrust: implications for fluid flow

. The Glarus thrust is a prominent tectonic feature in the eastern Helvetic Alps. It has been recognized as a potential major pathway for syntectonic crustal scale fluid flow. The oxygen, carbon and strontium isotope patterns obtained from two vertical profiles across the thrust indicate fundamentally different flow regimes in the southern section of the thrust, where the footwall is represented by Mesozoic limestones, and in the northern section, where the footwall is represented by Tertiary flysch. At the Grauberg locality in the south, the observed isotope patterns give evidence of a net mass transport component from the hanging wall Verrucano to the footwall limestone with a maximum time-integrated volumetric fluid flux of 6.1 m³/m² In the south, the hydration of the lowermost 10 to 20 m of the hanging wall Verrucano requires introduction of an aqueous fluid by subhorizontal flow along the thrust with a minimum time integrated flux of 240 m³/m². At the Lochseite locality in the north, the isotope patterns indicate a vertical mass transport component from the footwall flysch to the hanging wall Verrucano with a time-integrated fluid flux of 2.6 m³/m². In the north, the fluids were probably derived from compaction and dehydration of the footwall flysch during thrusting. The ascending fluids were ponded below the Verrucano and 'lubricated' the thrust. Short-term pressure drops associated with seismic motion along the thrust led to the precipitation of calcite in veins at the thrust surface contributing material to the Lochseiten calc-mylonite, a thin calc-mylonite layer at the thrust contact. Although cross thrust fluid flow may have been two to three orders of magnitude smaller than flow along the thrust, it had a major impact on the isotopic composition of the Lochseiten calc-mylonite. In particular, it buffered the oxygen isotope composition of the calc-mylonite towards the relatively ¹⁸O-depleted composition of the hanging wall Verrucano in the south and towards the relatively ¹⁸O-enriched compositions of the footwall flysch in the north. By this mechanism a regional south to north ¹⁸O-enrichment trend was simulated within the Lochseiten calc-mylonite.     

Hydrogen isotope systematics of submarine basalts

The D/H ratios and water contents in fresh submarine basalts from the Mid-Atlantic Ridge, the East Pacific Rise, and Hawaii indicate that the primary D/H ratios of many submarine lavas have been altered by processes including (1) outgassing, (2) addition of seawater at magmatic temperature, and (3) low-temperature hydration of glass. Decreases in δD and H²O⁺ from exteriors to interiors of pillows are explained by outgassing of water whereas inverse relations between δD and H₂O⁺ in basalts from the Galapagos Rise and the FAMOUS Area are attributed to outgassing of CH₄ and H₂. A good correlation between δD values and H₂O is observed in a suite of submarine tholeiites dredged from the Kilauea East Rift Zone where seawater (added directly to the magma), affected only the isotopic compositions of hydrogen and argon. Analyses of some glassy rims indicate that the outer millimeter of the glass can undergo lowtemperature hydration by hydroxyl groups having δD values as low as ?100.δD values vary with H₂O contents of subaerial transitional basalts from Molokai, Hawaii, and subaerial alkali basalts from the Society Islands, indicating that the primary δD values were similar to those of submarine lavas.Extrapolations to possible unaltered δD values and H₂O contents indicate that the primary δD values of most thoteiite and alkali basalts are near ?80 ± 5: the weight percentages of water are variable, 0.15–0.35 for MOR tholeiites, about 0.25 for Hawaiian tholeiites, and up to 1.1 for alkali basalts. The primary δD values of ?80 for most basalts are comparable to those measured for deep-seated phlogopites. These results indicate that hydrogen, in marked contrast to other elements such as Sr, Nd, Pb, and O, has a uniform isotopic composition in the mantle. This uniformity is best explained by the presence of a homogeneous reservoir of hydrogen that has existed in the mantle since the very early history of the Earth.     

Sources of plutonium to the tropical Northwest Pacific Ocean (1943-1999) identified using a natural coral archive

The Pu isotopes, ²³⁹Pu and ²⁴⁰Pu, were determined in annually-banded skeletons of an accurately dated (1943-1999) modern coral (Porites lobata) from Guam Island to identify historical Pu sources to the tropical Northwest Pacific Ocean. Activity concentrations of ^239+240Pu and ²⁴⁰Pu/²³⁹Pu atom ratios were determined in the dated coral bands using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Close-in fallout from the former US Pacific Proving Grounds (PPG) in the Marshall Islands and global fallout were identified as the two main sources. The Guam site was dominated by PPG close-in fallout in the 1950s, with an average ²⁴⁰Pu/²³⁹Pu atom ratio of 0.315 ± 0.005. In addition, a higher ²⁴⁰Pu/²³⁹Pu atom ratio (0.456 ± 0.020) was observed that could be attributed to fallout from the “Ivy Mike” thermonuclear detonation in 1952. The atom ratio decreased in the 1960s and 1970s due to increase in the global fallout with a low ²⁴⁰Pu/²³⁹Pu atom ratio (∼0.18). Recent coral bands (1981-1999) are dominated by the transport of remobilised Pu, with high ²⁴⁰Pu/²³⁹Pu atom ratios, from the Marshall Islands to Guam Island along the North Equatorial Current (NEC). This remobilised Pu was estimated to comprise 69% of the total Pu in the recent coral bands, although its contribution was variable over time.     

Oxygen isotope heterogeneity of the mantle deduced from global 18O systematics of basalts from different geotectonic settings

Based upon a compilation and analysis of O-isotope data for Neogene volcanic rocks worldwide, the ¹⁸O variation for 743 basalts (historic lavas, submarine glasses, and lavas with <0.75 wt% H₂O) is +2.9 to +11.4. Mid-ocean-ridge basalt (MORB) has a uniform O-isotope composition with ¹⁸0=+5.7±0.2. Basalts erupted in different tectonic settings have mean ¹⁸O/¹⁶O ratios that are both lower and higher than MORB, with continental basalts enriched in ¹⁸O by ca. 1 over oceanic basalts. The ¹⁸O range for the subset of 88 basalts with Mg# [100·Mg(Mg+Fe²⁺)] 75–68, considered to be unmodified primary mantle partial melts, is +3.6 to +8.7. These features are a clear indication that: (1) the Earth's upper mantle is heterogeneous with respect to its O-isotope composition; (2) that both low-¹⁸O and high-¹⁸O reservoirs have contributed to basalt petrogenesis. Large-ion lithophile element-enriched basalts associated with subduction at convergent plate margins are slightly enriched in ¹⁸O, a characteristic that is considered to be an intrinsic feature of the subduction process. Intraplate oceanic and continental basalts have highly variable ¹⁸O/¹⁶O ratios, with individual localities displaying ¹⁸O ranges in excess of 1.5 to 2. Systematic co-variations between O-, Sr-, Nd-, and Pb-isotope ratios reflect the same principal intramantle end-member isotopic components (DMM, HIMU, EM-I, EM-II) deduced from radiogenic isotope considerations and, therefore, imply that a common process is responsible for the origin of upper mantle stable and radiogenic isotope heterogeneity, namely the recycling of lithospheric material into the mantle.     

铬同位素体系及其在铬铁矿成矿过程中的应用

近年来铬(Cr)同位素体系在示踪地幔部分熔融、岩浆结晶分异及俯冲带流体相关的地质过程中均取得了重要进展。本文通过实例研究综述了Cr同位素在铬铁矿成矿作用方向的主要进展,包括:(1) Stillwater层状岩体橄榄岩带和K?z?ldaˇg蛇绿岩壳幔过渡带内铬铁矿及共生硅酸盐矿物的Cr同位素研究,揭示了层状/似层状铬铁矿成矿过程中可能发生明显的Cr同位素分馏,且主要受结晶分异和岩浆补给过程控制;(2) K?z?lda?蛇绿岩豆荚状铬铁矿床的Cr同位素研究,证明了俯冲带地幔橄榄岩中尖晶石的部分熔融,可能是豆荚状铬铁矿床主要的成矿物质来源之一,同时俯冲带流体也可能直接参与成矿;(3)层状岩体及蛇绿岩中普遍存在矿物间的Cr同位素不平衡分馏现象,不仅可用以记录岩浆作用的冷却时间,同时也证明了铬铁矿成矿过程中释放的流体对矿物间的元素交换具有明显促进作用。     

Manganese-chromium isotope systematics of enstatite meteorites

We present results of a study of the ⁵³Mn-⁵³Cr isotope systematics in the enstatite chondrites and achondrites (aubrites). The goal of this study was to explore the capabilities of this isotope system to obtain chronological information on these important classes of meteorites and to investigate the original distribution in the inner solar system of the short-lived radionuclide ⁵³Mn. Our earlier work (Lugmair and Shukolyukov, 1998; Shukolyukov and Lugmair, 2000a) has shown that the asteroid belt bodies are characterized by essentially the same initial ⁵³Mn abundance. However, we have found the presence of a gradient in the abundance of the radiogenic ⁵³Cr between the earth-moon system, Mars, and the asteroid Vesta. If this gradient is considered as a function of the heliocentric distance a linear radial dependence is indicated. This can be explained either by an early, volatility controlled Mn/Cr fractionation in the nebula or by an original radially heterogeneous distribution of ⁵³Mn. The enstatite chondrites are suggested to form in the inner zones of the solar nebula, much closer to the Sun than the ordinary chondrites. Therefore, their investigation may be an important test on the hypothesis on a radial heterogeneity in the initial ⁵³Mn.We have studied the bulk samples of the EH4-chondrites Indarch and Abee and the EL6-chondrite Khairpur. Although these meteorites have essentially the same Mn/Cr ratio as the ordinary chondrites, the relative abundance of the radiogenic ⁵³Cr is three times smaller than in the ordinary chondrites. Because these meteorites are primitive (undifferentiated) and no Mn/Cr fractionation had occurred within their parent bodies, this difference is a strong argument in favor of an initially heterogeneous distribution of ⁵³Mn in the early inner solar system. This finding is also consistent with formation of the enstatite chondrites in the inner zones of the solar nebula. Using the characteristic ⁵³Cr excess of the enstatite chondrites and the observed gradient, their place of origin falls at about 1.4 AU or somewhat closer to the Sun (i.e. >1.0-1.4 AU).We also present chronological results for the enstatite chondrites and achondrites. The ‘absolute’ ⁵³Mn-⁵³Cr ages of the EH4-chondrites are old: ∼4565 Ma. The EL6-chondrite Khairpur is ∼4.5 Ma younger, which is in good agreement with the ¹²⁹I-¹²⁹Xe data from the literature. The age of the aubrite Peña Blanca Spring appears to be similar to those of the enstatite chondrites while that of the aubrite Bishopville is at least ∼10 Ma younger, which is also in agreement with the ¹²⁹I-¹²⁹Xe data. The results from bulk samples of aubrites indicate that the last Mn/Cr fractionation in their parent body occurred ∼ 4563 Ma ago and imply an evolution of the Mn-Cr isotope system in an environment with an higher than chondritic Mn/Cr ratio for several millions of years.     

40Ar/39Ar ages in deformed potassium feldspar: evidence of microstructural control on Ar isotope systematics

Detailed field and microstructural studies have been combined with high spatial resolution ultraviolet laser ⁴⁰Ar/³⁹Ar dating of naturally deformed K-feldspar to investigate the direct relationship between deformation-related microstructure and Ar isotope systematics. The sample studied is a ~1,000 Ma Torridonian arkose from Skye, Scotland, that contains detrital feldspars previously metamorphosed at amphibolite-facies conditions ~1,700 Ma. The sample was subsequently deformed ~430 Ma ago during Caledonian orogenesis. The form and distribution of deformation-induced microstructures within three different feldspar clasts has been mapped using atomic number contrast and orientation contrast imaging, at a range of scales, to identify intragrain variations in composition and lattice orientation. These variations have been related to thin section and regional structural data to provide a well-constrained deformation history for the feldspar clasts. One hundred and forty-three in-situ ⁴⁰Ar/³⁹Ar analyses measured using ultraviolet laser ablation record a range of apparent ages (317-1030 Ma). The K-feldspar showing the least strain records the greatest range of apparent ages from 420-1,030 Ma, with the oldest apparent ages being found close to the centre of the feldspar away from fractures and the detrital grain boundary. The most deformed K-feldspar yields the youngest apparent ages (317-453 Ma) but there is no spatial relationship between apparent age and the detrital grain boundary. Within this feldspar, the oldest apparent ages are recorded from orientation domain boundaries and fracture surfaces where an excess or trapped ⁴⁰Ar component resides. Orientation contrast images at a similar scale to the Ar analyses illustrate a significant deformation-related microstructural difference between the feldspars and we conclude that deformation plays a significant role in controlling Ar systematics of feldspars at both the inter- and intragrain scales even at relatively low 'bulk' strains. The data show that Ar loss and trapping within the deformed K-feldspars reflects the presence of a deformation-induced population of small diffusion domains in combination with 'short-circuit' diffusion along deformation-induced defects. The complex history of microstructures induced in the K-feldspars during their cooling, alteration, erosion and sedimentation do not appear to be as significant as deformation-induced microstructures in controlling the distribution of apparent ages at the grain scale.     

铁同位素体系及其在矿床学中的应用

本文报道了世界范围内不同含铁矿物的Fe同位素组成,进一步了解了铁同位素在不同含铁矿物中的基本分布特征;系统总结了铁同位素在不同储库和不同类型矿床中的分布特征,构筑了铁同位素体系的基本框架;结合最新的研究成果,较全面地总结了铁同位素在矿床学领域的应用,得出了铁同位素可以用来示踪流体出溶、流体演化、表生蚀变作用和成矿物质来源的基本认识。在流体出溶过程中,相对于岩体,出溶的流体富集铁的轻同位素;成矿流体体系的演化过程中,矿物的结晶沉淀会导致铁同位素发生分馏,随着Fe(III)矿物的结晶沉淀,流体逐渐富集铁的轻同位素,随着Fe(II)矿物的结晶沉淀,流体逐渐富集铁的重同位素,随着矿物的结晶沉淀,流体的Fe同位素组成随时间发生演化;在成矿后的表生蚀变作用过程,高温蚀变作用形成的产物相对于原矿物富集铁重同位素,低温蚀变作用形成的产物基本保留了原矿物的铁同位素组成;Fe同位素在示踪成矿物质来源具有应用潜力,流体出溶、流体演化等重要成矿作用过程中Fe同位素组成的变化规律是利用Fe同位素示踪Fe来源的关键所在。     

岩溶洞穴次生碳酸盐沉积铀及其同位素组成古气候环境研究进展

洞穴沉积铀含量及其同位素初始234U/238U[（234U/238U)0]变化均与过去气候环境变化关系密切。文章分别对石笋中U含量和(234U/238U)0的气候意义进行统计分析发现,多数研究认为洞穴沉积U含量和(234U/238U)0可能指示土壤湿度和有效降水变化。然而,不管是对洞穴沉积的U含量还是(234U/238U)0,其气候环境意义解读还存在两种观点。但无论如何,这些研究成果都显示了洞穴沉积的U含量和(234U/238U)0是研究过去气候环境变化的重要替代指标。在未来的研究中,除了土壤环境和过程,还应关注U来源的相对贡献变化和其他微量元素与U元素的关系。这一指标可能在对东亚地区的大气粉尘活动和冬季风演化、地表生物量的变化研究等方面发挥重要作用。      

Oxygen Isotope Studies of Epithermal Systems：A Review

Numerous stable isotope studies of whole rocks and mineral separates in epithermal systems indi-cate that even though meteoric waters are dominant components in epithermal systems ,fluids of other origins,such as sedimentary or meta-sedimentary fluids,magmatic waters and even evolved meteoric waters ,may also play a role in the formation of epithermal ore deposits.Usually the more depleted the wall rocks,the larger the size of ore deposits ,and the least depletion degrees in whole rocks for economic mineralization are by about 3.5‰.The depletion in δ^18O in wall rocks,however,may be complicated by the superimposition of low temperature-hydration over high-temperature altera-tion or vice versa,the existence of primary low-^18O and high-^18O magmas ,and alteration by vol-canic gases.The depletion in δ^18O in wall rocks is controlled by the composition and nature of flu-ids,the temperature of fluids,the elevation of rocks at the time of alteration ,lithology,boiling effects of fluids ,and alteration style,as well as by water/rock ratios.In addition ,the fluids re-sponsible for epithermal deposits have experienced positive δ^18O shifts .It seems that when the above complications and controlling factors are well defined,oxygen isotope studies would be a promising and powerful exploration tool.