Ion Probe Analyses Research Articles

Negative δ 18O values in Allan Hills 84001 carbonate: Possible evidence for water precipitation on mars

The Martian meteorite ALH84001 contains ∼1% by weight of carbonate formed by secondary processes on the Martian surface or in the shallow subsurface. The major form of this carbonate is chemically and isotopically zoned rosettes which have been well documented elsewhere. This study concentrates upon carbonate regions ∼200 μm across which possess previously unobserved magnesium rich inner cores, interpreted here as rosette fragments, surrounded by a later stage cement containing rare Ca-rich carbonates (up to Ca 81Mg 07Fe 04Mn 07) intimately associated with feldspar. High spatial resolution ion probe analyses of Ca-rich carbonate surrounding rosette fragments have δ 18O V-SMOW values as low as −10 ‰. These values are not compatible with deposition from a global Martian atmosphere invoked to explain ALH84001 rosettes. The range of δ 18O values are also incompatible with a fluid that has equilibrated with the Martian crust at high temperature or from remobilisation of carbonate of rosette isotopic composition. At Martian atmospheric temperatures, the small CO 2(gas)-CO 2(ice) fractionation makes meteoric CO 2 an unlikely source for −10 ‰ carbonates. In contrast, closed system Rayleigh fractionation of H 2O can generate δ 18O H2O −30 ‰, as observed at high latitudes on Earth. We suggest that atmospheric transport and precipitation of H 2O in a similar fashion to that on Earth provides a source of suitably 18O depleted water for generation of carbonate with δ 18O V-SMOW = −10 ‰.

Correlations between chemical and age domains in monazite, and metamorphic reactions involving major pelitic phases: an integration of ID-TIMS and SHRIMP geochronology with Y–Th–U X-ray mapping

Chemical mapping and in situ U–Th–Pb analyses reveal a link between age domains and zones of relative yttrium (Y) depletion or enrichment within monazite crystals and are correlated with metamorphic reactions involving garnet. Conventional, small-fraction isotope dilution thermal ionization mass spectrometry (ID-TIMS) and sensitive high-resolution ion microprobe (SHRIMP) techniques were utilized to measure U–Th–Pb isotopic compositions in metamorphic monazite from pelitic rocks of the southern Canadian Cordillera. Monazite ID-TIMS data from individual samples commonly demonstrate a 2 to 25 Ma range in U-Pb ages. This is difficult to reconcile using conventional regression techniques due to complexities such as excess 206Pb or bulk mixing of discrete age domains. Backscattered electron (BSE) imaging and X-ray elemental mapping for Y, Th, and U revealed complex internal zonation within many of the monazite crystals and served as a guide for detailed, in situ (∼30 μm) isotopic analysis using the SHRIMP. The Y maps generally provided the clearest indication of growth or recrystallization domains and were critical for targeting SHRIMP analysis because these relationships were not always clear in BSE, U, and Th images. Moreover, the Y domains consistently correlated with distinct age domains, with up to three or more in some crystals. These data clearly illustrate the cause of age dispersion within the analyzed monazite grains and demonstrate the significance of multiple age domains in metamorphic monazite that may be irreconcilable or misinterpreted when using conventional dating techniques such as ID-TIMS. Recent studies have investigated the interaction between accessory minerals such as monazite and major pelitic phases throughout a metamorphic event. Researchers have begun to focus on the partitioning of Y between monazite and garnet because it is highly compatible in both phases. Due to its greater volume, garnet exerts considerable control over the Y budget available during metamorphism in pelitic rocks. Consequently, this is reflected in the production and consumption of monazite, as it is sensitive to the availability of Y, and is preserved as internal zones of relative Y enrichment or depletion. Thus, precise ages of contrasting Y domains within monazite provided by in situ ion probe analysis may be correlated with metamorphic reactions involving garnet and assigned to points along the P– T path.

The fate of B, Cl and Li in the subducted oceanic mantle and in the antigorite breakdown fluids

We present an inventory of B, Cl and Li concentrations in (a) key minerals from a set of ultramafic samples featuring the main evolutionary stages encountered by the subducted oceanic mantle, and in (b) fluid inclusions produced during high-pressure breakdown of antigorite serpentinite. Samples correspond to (i) nonsubducted serpentinites (Northern Apennine and Alpine ophiolites), (ii) high-pressure olivine-bearing antigorite serpentinites (Western Alps and Betic Cordillera), (iii) high-pressure olivine–orthopyroxene rocks recording the subduction breakdown of antigorite serpentinites (Betic Cordillera). Two main dehydration episodes are recorded by the sample suite: partial serpentinite dewatering during formation of metamorphic olivine, followed by full breakdown of antigorite serpentine to olivine+orthopyroxene+fluid. Ion probe and laser ablation ICP-MS (LA ICP-MS) analyses of Cl, B and Li in the rock-forming minerals indicate that the hydrous mantle is an important carrier of light elements. The estimated bulk-rock B and Cl concentrations progressively decrease from oceanic serpentinites (46.7 ppm B and 729 ppm Cl) to antigorite serpentinites (20 ppm B and 221 ppm Cl) to olivine–orthopyroxene rocks (9.4 ppm B and 45 ppm Cl). This suggests release of oceanic Cl and B in subduction fluids, apparently without inputs from external sources. Lithium is less abundant in oceanic serpentinites (1.3 ppm) and the initial concentrations are still preserved in high-pressure antigorite serpentinites. Higher Li contents in olivine, Ti-clinohumite of the olivine–orthopyroxene rocks (4.9 ppm bulk rock Li), as well as in the coexisting fluid inclusions, suggest that their budget may not be uniquely related to recycling of oceanic Li, but may require input from external sources. Laser ablation ICP-MS analyses of fluid inclusions in the olivine–orthopyroxene rocks enabled an estimate of the Li and B concentrations in the antigorite breakdown fluid. The inclusion compositions were quantified using a range of salinity values (0.4–2 wt.% NaCl equiv) as internal standards, yielding maximum average fluid/rock D B∼5 and fluid/rock D Li∼3.5. We also performed model calculations to estimate the B and Cl loss during the two dehydration episodes of serpentinite subduction. The first event is characterized by high fluid/rock partition coefficients for Cl (∼100) and B (∼60) and by formation of a fluid with salinity of 4–8 wt.% NaCl equiv. The antigorite breakdown produces less saline fluids (0.4–2 wt.% NaCl equiv) and is characterized by lower partition coefficients for Cl (25–60) and B (12–30). Our calculations indicate that the salinity of the subduction fluids decreases with increasing depths. fluid/rock D B/ fluid/rock D Cl<1 (∼0.5) indicates that Cl preferentially partitions into the evolved fluids relative to B and that the B/Cl of fluids progressively increases with increasing depths and temperatures. Despite light element release in fluids, appreciable B, Cl and Li are still retained in chlorite, olivine and Ti-clinohumite beyond the antigorite stability field. This permits a bulk storage of about 10 ppm B, 45 ppm Cl and 5 ppm Li, i.e., concentrations much higher than in mantle reservoirs. Chlorite is the Cl repository and its stability controls the Cl and H 2O budget beyond the antigorite stability; B and Li are bound in olivine and clinohumite. The subducted oceanic mantle thus retains light elements beyond the depths of arc magma sources, potentially introducing anomalies in the upper mantle.

Provenance ages and alteration histories of shales from the Middle Archean Buhwa greenstone belt, Zimbabwe: Nd and Pb isotopic evidence

Shales of the ca. 3.0 Ga Buhwa Greenstone Belt, Zimbabwe, were derived from a compositionally diverse provenance whose ages, determined by ion probe analyses of detrital zircons in interbedded sandstones, range from 3.8 to 3.1 Ga. Geochemical data for the shales were previously interpreted to indicate that sediments had been derived from an intensely weathered source. REE concentrations in the shales were interpreted to suggest that the provenance was compositionally mixed, with components of felsic (tonalite and alkalic granitoid) and mafic rocks. Sm/Nd and Nd isotopic compositions of these rocks can be used to model initial Nd isotopic ratios at the time of sedimentation (ε Ndsed), as well as model crustal formation ages (T DM). The former, at the age of sedimentation, range from +0.6 to −10.8, consistent with a range of provenance ages. The latter range from 4.46 Ga to 2.99 Ga. The oldest crustal formation ages, up to 0.7 Ga older than known detrital components, are interpreted here to indicate that the Sm-Nd system of the sediments experienced open system behavior. The implied alteration would have included an increase in Sm/Nd by about 20–25 percent, probably in the form of preferential loss of Nd with respect to Sm. The Pb isotopic compositions of whole rock samples are quite radiogenic, with a range of 206Pb/ 204Pb from 25.5 to 154. An array of ten samples lies scattered about a line with a 207Pb/ 204Pb - 206Pb/ 204Pb slope age of about 2.73 Ga. Five individual samples were sequentially leached to further test the timing and characteristics of this U-Th-Pb alteration event. These arrays of a whole rock, three leach steps, and a residue also form linear Pb-Pb arrays (one is more scattered) with ages ranging from 2260 ± 360 Ma to 2824 ± 170 Ma, suggesting that all samples experienced a latest Archean to earliest Proterozoic enrichment in U/Pb. This age range also may be the approximate age of Sm/Nd enrichment for the shales. All samples, both whole rocks and leached samples, lie grouped on a 208Pb/ 204Pb - 206Pb/ 204Pb diagram around a line with 232Th/ 238U = 3.5 (2.9 to 3.9). Because of the lack of large differences in the Th/U of the samples through large ranges of U/Pb, we interpret this consistency in Th/U to mean that the shales of the Buhwa belt experienced Pb loss, rather than U and Th gain. Circumstances that may be responsible for Pb loss in a sedimentary basin include loss of saline fluids during basin dewatering. Such an event would likely have been related to folding associated with the thrusting and magmatic intrusion of the adjacent Limpopo Belt, suggesting that uplift, dewatering, and geochemical and isotopic alteration can be genetically related.

207Pb/206Pb and 238U/230Th dating of uranium migration in carbonate fractures from the Palmottu uranium ore (southern Finland)

Precambrian U ore deposits, such as Oklo, Cigar-Lake or Palmottu, constitute invaluable analogues of nuclear waste repositories that provide direct evidence of U mobility or sequestration, over very large time intervals in geological formations. In this study, pervasive millimeter thick calcite veins filling microfractures in gneiss and granite surrounding the Palmottu U ore (Finland) were analysed, as fingerprints of past fluid circulation through the site. Stable Pb isotopes and short halflife 234U-230Th isotopes, all products of the U decay chain, have been chosen to investigate long term (over a Ga) and recent (within the last 500 ka) U migrations, respectively. Lead, U and Th isotopes have been analysed by thermo-ionisation mass spectrometry on mg-size bulk carbonate samples, and ion probe analyses of Pb isotopes performed on microinclusions of pyrite, monazite and coffinite. A striking contrast was found between the preservation of a well defined Pb–Pb isochron (1925±70 Ma, MSDW=2.47, N=17) on the one hand, and fractionated 230Th/238U ratios (230Th/238U<1) on the other hand. This shows that although the veins were formed simultaneously to the ore itself, during the Fennoscandian orogeny, and behaved mainly as a closed system for several billion years, U migrations did occur in the last 0.5 Ma. The recent U mobility is probably related to changes in the groundwater circulation through the bedrock. Nowadays, the dispersion of the U is, however, restricted to within a few meters around the U ore. It is shown that complex processes of dissolution and re-adsorption (or re-crystallisation) took place within these tiny carbonate fracture fillings. Relatively unradiogenic 206Pb/204Pb ratios measured by SIMS in coffinite micro-inclusions demonstrate recent precipitation of this mineral. Coffinite being the main U-bearing phase in the carbonate fracture fillings, its crystallization probably played a major role in U redistribution within the fractures, and in the relatively restricted dispersion of U through the bedrock.

Constraints on the pre-metamorphic trace element composition of Barberton komatiites from ion probe analyses of preserved clinopyroxene

The trace element compositions of igneous augite preserved in metamorphosed komatiite samples from the lower Komati Formation, Barberton Mountainland, South Africa (3.49 Ga), were analyzed in order to estimate the pre-metamorphic bulk composition of the samples. The chemical effects of metamorphism are evaluated by calculating the trace element (TE) composition of a hypothetical augite in equilibrium with the bulk sample composition, and comparing it with ion probe analyses of the cores of augite crystals preserved in that sample. Differences between the calculated and measured composition of the augite indicate chemical gain or loss from the bulk sample during metamorphism. The data indicate that the rare earth elements (REE) and Y were immobile during metamorphism. Immobility is also indicated for Ti and Zr, though chromite fractionation makes this conclusion less firm. Sr and Eu were highly mobile (>50% loss) in all samples studied. Because the REE were immobile, the light REE (LREE) enrichments [(La/Sm)n>1] present in some Barberton komatiite samples are a primary igneous feature, and not the result of post-emplacement metamorphic processes. The variations in (La/Sm)n correlate with Ti/Zr in a strikingly similar manner to variations present in modern boninites, which are high Mg, hydrous subduction zone magmas. Plume-based models that appeal to majorite garnet fractionation cannot produce the low Ti/Zr of komatiites or their wide variation in La/Sm. Thus, we propose that a subduction origin for the Barberton komatiites and basaltic komatiites is more consistent with the available trace element data.

Hydromagmatic amphibole in komatiitic, tholeiitic and ferropicritic units, Abitibi greenstone belt, Ontario and Qu�bec: evidence for Archaean wet basic and ultrabasic melts

¶Detailed petrographic, electron microprobe and ion probe studies of Archaean hydromagmatic amphiboles from the Abitibi greenstone belt, Canada, yield new insights into the origin of Al-undepleted komatiitic and Al-depleted tholeiitic and ferropicritic melts. The amphiboles are present in peridotite layers and basal chill zones of thick differentiated basic and ultrabasic sills and flows, and are titanian pargasite–hastingsite in composition. They can be grouped into two petrographic types: (1) amphibole in the groundmass; and (2) amphibole-bearing melt inclusions. The groundmass amphiboles are oikocrysts, rims and interstitial grains, present in minor to major amounts. The oikocrysts host cumulus olivines (Fo83–84) that are rounded in shape, embayed, and smaller in size. The amphibole-bearing melt inclusions are hosted in cumulus olivines (Fo83–84 in komatiitic rocks and Fo79 in tholeiitic rocks), spherical to ovoid in shape, 50–500 µm in size, and dominated modally by amphibole. The melt inclusions also contain euhedral chromite and aluminous spinel and micrometric clinopyroxene and glass, and sub-micrometric iron–nickel sulphide, chloro-apatite and ilmenite. In-situ ion probe analyses indicate the amphibole is: (1) enriched in Nb, LREE and Zr and depleted in Sr and HREE relative to primitive mantle; (2) contains up to 1–3 wt% H2O; and (3) overall displays δD values from 50‰ to −140‰, including many values in the accepted magmatic range of −60‰ to −90‰.

Gold in porphyry copper deposits: its abundance and fate

Porphyry copper deposits are among the largest reservoirs of gold in the upper crust and are important potential sources for gold in lower temperature epithermal deposits. Whether gold remains in porphyry copper deposits is important both to their economic attractiveness and to the distribution of gold in the upper crust. Cu/Au atomic ratios of porphyry copper ore deposits form a continuous range from about 5000 to 5,000,000 with a median near 40,000, which separates gold-rich and gold-poor deposits. Gold is found in porphyry copper deposits in solid solution in Cu–Fe and Cu sulfides and as small grains of native gold, usually along boundaries of bornite. SIMS (ion probe) analyses of ore minerals from the gold-rich Batu Hijau, Kingking and Skouries porphyry copper deposits show that bornite contains about 1 ppm Au, whereas chalcopyrite contains about an order of magnitude less. Chalcocite and covellite contain 10–20 ppm Au, but are not abundant enough to account for a significant part of the gold endowment in many porphyry copper deposits. The amount of gold presently in solid solution in Cu–Fe sulfides is not adequate to account for all the gold in porphyry copper deposits, and the remainder is present as micron-scale grains of native gold. Experiments in the Cu–Fe–S–Au system show that bornite and chalcopyrite can contain about 1000 ppm gold at typical porphyry copper formation temperatures of 600–700°C, and indicate that bornite and chalcopyrite in porphyry copper deposits were saturated with respect to gold at temperatures of only 200–300°C. In contrast, Cu/Au ratios of bulk ore in porphyry copper deposits would require bornite and chalcopyrite to be saturated with respect to gold at temperatures similar to those at which primary (potassic) ore and alteration are thought to form. This indicates that the maximum gold endowment of porphyry copper deposits is probably fixed by the amount of gold that will go into solid solution in Cu–Fe sulfides when the deposit forms at high temperature, and that gold is not commonly added later from other sources, although it can be redistributed during cooling or later events. The experimental data also suggest that high-temperature (600–700°C) vapors can extract considerably more gold from porphyry copper systems than can low-temperature (300°C) alteration. Comparison to Cu/Au ratios of volcanic emissions suggests further that high-temperature processes remove copper (relative to gold) from porphyry copper systems, whereas low-temperature processes remove gold preferentially and that this can account for deposits with extremely low and high Cu/Au ratios, respectively. Deposits with Cu/Au ratios between about 20,000 and 100,000, however, probably reflect different degrees of removal of gold or copper by immiscible sulfides.

The anhydrous amphibole ungarettiite from the Woods mine, New South Wales, Australia

We report ungarettiite from the Woods mine, New South Wales, Australia; this is the second known occurrence. Electron-microprobe and ion-probe analyses show it to be very close to end-member composition, NaNa 2 Mn 2+ 2 Mn 3+ 3 Si 8 O 22 O 2 . We record the first direct determination of H 2 O (

Location and quantitative analysis of OH in coesite

The incorporation of hydrogen (deuterium) into the coesite structure was investigated at pressures from 3.1 to 7.5 GPa and temperatures of 700, 800, and 1100 °C. Hydrogen could only be incorporated into the coesite structure at pressures greater 5.0 GPa and 1100 °C . No correlation between the concentration of trace elements such as Al and B and the hydrogen content was observed based on ion probe analysis (1335 ± 16 H ppm and 17 ± 1 Al ppm at 7.5 GPa, 1100 °C). The FTIR spectra show three relatively intense bands at 3575, 3516, and 3459 cm−1 (ν1 to ν3, respectively) and two very weak bands at 3296 and 3210 cm−1 (ν4 and ν5, respectively). The band at 3516 cm−1 is strongly asymmetric and can be resolved into two bands, 3528 (ν2a) and 3508 (ν2b) cm−1, with nearly identical areas. Polarized infrared absorption spectra of coesite single-crystal slabs, cut parallel to (0 1 0) and (1 0 0), were collected to locate the OH dipoles in the structure and to calibrate the IR spectroscopy for quantitative analysis of OH in coesite (ɛi,tot=190 000 ± 30 000 l mol−1H2O cm−2). The polarized spectra revealed a strong pleochroism of the OH bands. High-pressure FTIR spectra at pressures up to 8 GPa were performed in a diamond-anvil cell to gain further insight into incorporation mechanism of OH in coesite. The peak positions of the ν1, ν2, and ν3 bands decrease linearly with pressure. The mode Gruneisen parameters for ν1, ν2, and ν3 are −0.074, −0.144 and −0.398, respectively. There is a linear increase of the pressure derivatives with band position which follows the trend proposed by Hofmeister et al. (1999). The full widths at half maximum (FWHM) of the ν1, ν2, and ν3 bands increase from 35, 21, and 28 cm−1 in the spectra at ambient conditions to 71, 68, and 105 in the 8 GPa spectra, respectively. On the basis of these results, a model for the incorporation of hydrogen in coesite was developed: the OH defects are introduced into the structure by the substitution Si4+(Si2)+4O2−= [4]□(Si2) + 4OH−, which gives rise to four vibrations, ν1, ν2a, ν2b, and ν3. Because the OH(D)-bearing samples do contain traces of Al and B, the bands ν4 and ν5 may be coupled to Al and/or B substitution.

The effect of Ca-Tschermaks component on trace element partitioning between clinopyroxene and silicate melt

We have studied the influence of Ca-Tschermaks (Calcium Tschermaks or CaTs) content of clinopyroxene on the partitioning of trace elements between this phase and silicate melt at fixed temperature and pressure. Ion probe analyses of experiments carried out in the system Na 2O–CaO–MgO–Al 2O 3–SiO 2, at 0.1 MPa and 1218°C, produced crystal-melt partition coefficients ( D) of 36 trace elements (Li, Cl, Sc, Ti, V, Cr, Fe, Co, Ge, Sr, Y, Zr, Nb, Mo, Ru, Rh, In, Sn, Sb, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta and W), for clinopyroxene compositions between 10 and 32 mol% CaTs. Partition coefficients for 2+ to 5+ cations show, for each charge, a near parabolic dependence of log D on ionic radius of the substituting cation, for partitioning into both the M1 and M2 sites of clinopyroxene. Fitting the results to the elastic strain model of Blundy and Wood [Blundy, J.D., Wood, B.J., 1994. Prediction of crystal-melt partition coefficients from elastic moduli. Nature 372, 452–454] we obtain results for the strain-free partition coefficients of theoretical cations ( D 0), with site radius r 0, and for the site's Young's Modulus ( E). In agreement with earlier data our results show that increasing ivAl concentration in cpx is matched by increasing D, E M1, E M2 and D 0 for tri-, tetra- and pentavalent cations. The degree of fractionation between chemically similar elements (i.e. Ta/Nb, Zr/Hf) also increases. In contrast, D values for mono-, di- and hexavalent cations decrease with increasing ivAl in the cpx. The large suite of trace elements used has allowed us to study the effects of cation charge on D 0, r 0 and E. We have found that D 0 and r 0 decrease with increasing cation charge, e.g. r 0=0.66 Å for 4+ cations and 0.59 Å for 5+ cations substituting into M1. Values of E M1 and E M2 increase with cation charge as well as with increasing ivAl content. The increase in E M2 is linear and close to the trend set by Hazen and Finger [Hazen, R.M., Finger, L.W., 1979. Bulk modulus-volume relationship for cation–anion polyhedra. J. Geophys. Res. 84 (10) 6723–6728] for oxides. E M1 values are much higher and do not fit the trend predicted by the Hazen and Finger relationship.

Application of Electron Microprobe Age Mapping and Dating of Monazite

Abstract High resolution X-ray mapping and dating of monazite (Th, REE-phosphate) using the electron microprobe is an exceptionally powerful technique for structural, metamorphic, and tectonic analysis in geology. Age determination of geologic materials has been conventionally accomplished by mass spectrometry-based analysis of radioisotopic ratios in minerals from hand-picked mineral separates, careful sampling of individual grains out of petrographic thin sections, or by detailed ion probe analysis. Recently, use of the electron microprobe for dating purposes has been attempted. In principal, the concentrations of Th, U and Pb uniquely define the age if non-radiogenic Pb is either not initially present or can be subtracted from the total Pb. Monazite contains high concentrations of Th and U and does not appear to incorporate significant non-radiogenic Pb during mineral growth. Furthermore, monazite is a ubiquitous accessory phase in many metamorphic and igneous rocks, making it ideal for microprobe dating.

319 ± 7 Ma crystallization age for the Blond granite (northwest Limousin, French Massif Central) obtained by U/Pb ion-probe dating of zircons

Zircons from the Blond leucogranite (NW Limousin, French Massif Central) have been dated by U/Pb isotope analysis on the CAMECA IMS 1270 ion-probe facility at CRPG-CNRS. The obtained age of 319 ± 7 Ma, different from previous Rb/Sr whole rock isochron age determination (301 ± 5 Ma), integrates this granite in the Late Hercynian history of the Limousin, together with other Namuro-Westphalian granitoids. U/Pb isotope ion-probe analysis of zircons appears to be a particularly useful method for dating relatively recent highly fractionated granites.

Melt aggregation within the crust beneath the Mid-Atlantic Ridge: evidence from plagioclase and clinopyroxene major and trace element compositions

Ocean Drilling Program Hole 923A, located on the western flank of the Mid-Atlantic Ridge south of the Kane Fracture Zone, recovered primitive gabbros that have mineral trace element compositions inconsistent with growth from a single parental melt. Plagioclase crystals commonly show embayed anorthitic cores overgrown by more albitic rims. Ion probe analyses of plagioclase cores and rims show consistent differences in trace element ratios, indicating variation in the trace element characteristics of their respective parental melts. This requires the existence of at least two distinct melt compositions within the crust during the generation of these gabbros. Melt compositions calculated to be parental to plagioclase cores are depleted in light rare earth elements, but enriched in yttrium, compared to basalts from this region of the Mid-Atlantic Ridge, which are normal mid-ocean ridge basalt (N-MORB). Clinopyroxene trace element compositions are similar to those predicted to be in equilibrium with N-MORB. However, primitive clinopyroxene crystals are much more magnesian than those produced in one-atmosphere experiments on N-MORB, suggesting that the major element composition of the melt was unlike N-MORB. These data require that the diverse array of melt compositions generated within the mantle beneath mid-ocean ridges are not always fully homogenised during melt extraction from the mantle and that the final stage of mixing can occur efficiently within crustal magma chambers. This has implications for the process of melt extraction from the mantle and the liquid line of descent of MORB.

REACTIVE CRACK FLOW IN THE OCEANIC MANTLE: AN ION PROBE STUDY ON CPX FROM VEIN-BEARING ABYSSAL PERIDOTITES

REACTIVE CRACK FLOW IN THE OCEANIC MANTLE: AN ION PROBE STUDY ON CPX FROM VEIN-BEARING ABYSSAL PERIDOTITES

Elemental redistribution in Tieschitz and the origin of white matrix

Abstract— Two types of mesostasis coexist within some porphyritic chondrules in Tieschitz. One type is smooth. The other, confined to chondrule margins, is blocky on a 5–10 μm scale. Mesostases in one porphyritic olivine‐pyroxene (POP) chondrule and one porphyritic olivine (PO) chondrule were analysed by scanning electron microscopy (SEM) and energy‐dispersive x‐ray spectrometry (EDS), as was white matrix nearby. Mesostases in the PO chondrule and in four others were analysed by ion probe. Pyroxene phenocrysts or dendrites extend across contacts between smooth and blocky mesostasis with no compositional change. Relative to smooth mesostasis, blocky mesostasis is enriched in Al, alkalis, Ba, F, and Cl but depleted in Si, Fe, and Ca. White matrix fills channels between the chondrules. It is physically and chemically similar to blocky mesostasis, but three ion probe analyses indicate that, unlike the mesostases, it is poor in Sc and has variable and fractionated rare earth elements (REEs).Smooth mesostasis is interpreted as solidified primary chondrule liquid; whereas blocky mesostasis is its alteration product or, less likely, a precipitate replacing smooth mesostasis leached out by aqueous fluid. White matrix may have formed by secondary alteration or replacement of mesostases that had been expelled from chondrules during accretion, or as a precipitate filling interchondrule voids. Iron may have been lost from the bulk meteorite, but most other elements merely underwent internal redistribution. Disturbed isotopic systems indicate that aqueous fluid may have been active on the Tieschitz parent body only 2 Ga ago. If correct, this would be the first evidence that an ordinary chondrite parent body underwent internal reprocessing significantly later than 4.5 Ga ago.

Origin of Geochemical Heterogeneity in the Mantle Peridotites from the Bay of Islands Ophiolite, Newfoundland, Canada: Ion Probe Study of Clinopyroxenes

Representative samples from a 6 km thick ophiolitic mantle section were selected for ion probe analysis of clinopyroxene (cpx). The purpose was to evaluate the trace element heterogeneity occurring on a kilometer-scale in the geologically well constrained uppermost mantle beneath the ancient Bay of Islands ophiolite spreading center. The range of REE patterns in the mantle section can be explained by 7–20% near-fractional melting of a weakly depleted mantle source followed by varying degrees of trace element chromatographic exchange between an interstitial liquid and the refractory residue and/or precipitation of phases from the interstitial liquid. Peridotites from the basal mantle section are the least depleted ones. They require little to no overprint by melt circulation and can be modeled as residues of fractional melting with a low (0.1%) residual porosity. Their presence in the mantle section is possibly related to ductile accretion of less depleted peridotite during ophiolite obduction. Peridotites from the central, but mainly uppermost mantle section require stronger interaction with percolating melts. Uppermost peridotites also show the highest spatial variability in the degree of depletion. A preferred, but nonunique model for their formation is refertilization of initially more refractory residues due to cpx ± plagioclase addition from liquids depleted relative to N-MORB. Such depleted liquids are also inferred from wehrlitic rocks occurring in the crust-mantle transition zone. They may have originated as melts emerging from mantle columns or mixtures of such melts, i.e., melts whose compositions were affected by reaction with a large volume of mantle rocks.

Ion Probe Oxygen Isotope Analysis of Quartz and Magnetite from the Biwabik Iron Formation, Minnesota

The oxygen isotopic compositions of coexisting quartz and magnetite have previously been measured, using conventional fluorination techniques, in samples from a number of Precambrian iron formations (e.g. Perry and Bonnischen, 1966; Perry et al., 1973). Their aim was to utilize equilibrium mineral pair geothermometry to infer temperatures of metamorphism and diagenesis of both recrystallized and relatively unaltered Precambrian chemical sediments. The present study has acquired in situ, high spatial resolution, ion probe oxygen isotope measurements of samples from the relatively undeformed Biwabik Iron Formation, Minnesota, in conjunction with detailed petrographic observation. This had the aim of testing the extent of isotopic equilibrium between this mineral pair to investigate the interplay of depositional signatures and modifications due to diagenetic recrystallization of initial precipitates. Methods . Detailed petrographic observation and compositional analyses were obtained of seven samples from cores 2, 5 and 7 from the Biwabik Iron Formation of the Mesabi Deep Drilling Project (details in Pfleider et al., 1968). Oxygen isotope analyses of these samples were then obtained using the Isolab 54 ion microprobe at the University of Manchester, UK (Saxton et al., 1996). The 1So/160 ratios were determined by secondary ionization mass spectrometry using a Cs + primary beam and the 81SOsMow values determined by calibration with standard materials (Lyon et al., 1995). Analysis transects of between 11 and 19 spots were made across quartz and magnetite in the selected areas within the chosen samples with crater sizes ranging from 20 to 60 Ixm in diameter and a few gm in depth. Results and discussion. Petrographic observation at high resolution identified magnetite as a late stage mineral, generally with a subhedral to euhedral crystal habit from 10 to 100 gm, which exhibited a cross-cutting relationship with all the other constituent phases. It was present in a number of textural associations including granule rims, replaced granules, disseminated crystals and crystalline aggregates. Quartz was present as a microcrystalline matrix mineral between the disseminated and aggregated magnetite crystals. Mean values for quartz and magnetite within each sample were calculated to enable the determination of the oxygen isotopic fractionation (AQM) between this coexisting mineral pair. These values are shown in Table 1. Mean quartz values range from 18.3 to 24.2 %0 and mean magnetite values range from -10.7 to -3 .6 %0. The calculated isotopic fractionations range from 25.6 to 33.6 %0. Our data have been plotted in Fig. 1 in conjunction with previously published oxygen isotope analyses of the quartz-magnetite mineral pair from the Biwabik Iron Formation (Perry and Bonnischen, 1966; Perry et al, 1973). Oxygen isotope ratios of quartz and magnetite from the metamorphosed Duluth contact zone and the transition zone between this and the relatively undeformed iron formation are shown by open circles (Perry and Bonnischen, 1966). Data from samples taken from the same cores as the present study (Perry et al., 1973) are plotted as solid diamonds and lie in a steeply dipping array towards lower quartz and higher magnetite values than the present results. An average fractionation of 24%0 was determined from this data. This is almost 10%o lower than the highest fractionation shown by the present study (open diamonds). This can be explained by a comparison of ion microprobe analysis and the conventional fluorination techniques that were used to generate the other data sets. Firstly, ion probe analysis eliminates the effects of contamination of

Cosmogenic 3He Accumulation in Alluvial Garnets in Bedload from Central Nepal Rivers: Implications for Cosmic-Ray Exposure Dating of Himalayan Erosional Processes

Cosmogenic isotopes, such as 3He, l~ 26A1, are continuously producted at the Earth's surface in rocks and soils by cosmic ray spallation of light isotopes, providing a powerful tool for dating young geomorphological processes. The datation method based on the accumulation of cosmogenic 3He is particulary appealing because: 3Hec (where suffix c refers to 3He produced by cosmic rays) is a stable isotope, its production rate is high and its natural background is very low. Moreover, its analysis is sensitive, relatively simple and rapid. However, the applicability of this method is limited by the high diffusivity of 3He. In practice, 3He~ is quantitatively retained in olivine phenocrysts, allowing datation of young basaltic flows. Dung/ and Roselieb (1996) have argued that the diffusivity of helium is lower in garnet than in olivine. The use of garnet for 3He c dating presents the interesting advantage to extend the field of this method to crustal metamorphic domains, and therefore to the case of major orogens. We have measured3Hec in alluvial garnets sampled in bedload from Central Nepal rivers (Fig. 1). They represent 3 different monolithologic catchments. By analysing fractions of several tenths of grains, we integer the complex history of each individual grain and the results obtained are mostly meaningful for comparison between different positions in a river basin. The garnets were formed in gneisses of the High Himalaya Crystalline (HHC) and schists of the Lesser Himalaya (LH) during high grade metamorphism resulting from the Main Central Thrusting (MCT) during Miocene. In this system, uplift rates estimated using the Ar/Ar method on micas, feldspars and monazite in garnet are high between 2 and 4 mm/yr depending on the position with regard to the MCT (Copeland et al., 1991). For 3Heo dating purpose, pyropes and almandines are more appropriate than grossularites et spessartites (Dung/and Roselieb, 1996). Electron probe analysis showed that the mean composition of the garnets was Aim. 70%, Pyr. 15%, Gros. 7%, Spes. 5%, Andr. 3% except for sample MO38 wich showed a larger contribution of the grossular end-member (Alm. 54%, Pyr. 12%, Gros. 26%, Spes. 6%, Andr. 2%). Helium was extracted using a CO2 laser from mgsized pure garnet fractions and its amount and isotopic ratio measured with a high sensitivity rare gas mass spectrometer in Nancy (SHe = 3.5 • 10 -4 A/Torr, He blank = 2 x 10 15 mol). Helimn-3 present in minerals is the sum of the following potential contributions: 3Her 3He c + 3Hea + 3Hen + 3He m (where suffixes t, a, n and m refer to total, atmospheric, nucleogenic, and mantle-derived). The parent rocks of metamorphic garnets are thought to be marine sediments, mostly turbidites, which makes the possible contribution of mantle-derived 3He m unlikely. In agreement with that, He isotopic ratios measured in hot springs along the MCT are show typical radiogenic values (Marty et al., 1996). 3Hea was estimated from the measured 2~ content. The contribution of nucleogenic 3He in garnet, produced by activation of Li by natural thermal neutrons, was computed from ion probe analysis of Li in garnet and from U and Th analysis &garnets (1CP-MS), or from estimate of U, Th content of parent rocks. In all cases the atmospheric and nucleogenic 3He contributions were found to be small with respect to the total 3He content of the samples.

In situ Pb Isotope Ratios in Melt Inclusions from Oceanic Island Basalts: Mangaia, a Case Study

The study of oceanic basalts has provided unequivocal evidence for a chemically heterogeneous mantle 1. However, the nature, distribution and scale of these heterogeneities remain problematic. The aggregation of melts on their way to the surface, and magma chamber processes previous to eruption, obscure the characteristics of the different mantle sources. Although trace and major element studies of melt inclusions from high Mg# olivine phenocrysts have proved to be useful in defining the composition of unaggregated melts 2, the uncertainty remains in distinguishing if they represent different extents of melting from a single source or if they originate from different source compositions. To address these problems, we have measured Pb isotopes in single melt inclusion by 1270 ionmicroprobe; we chose Mangaia Island as a case study, because its lavas are isotopically homogeneous and they represent one of the end-member mantle components (HIMU) 3. Melt inclusions, in olivine and clinopyroxene phenocrysts (Mg# 86-89) from two primitive lavas from Mangaia are partially crystallized and ellipsoidal in shape (30 to 300 lam), indicative of their primary nature. Partially resorbed clinopyroxene inclusions in the olivine also occur. Some of the melt inclusions contain FeCuNi sulphide and MgFeCa carbonate globules. The (La/Yb)cN ratios in the inclusions range from -5 to ~40. In-run precision for the Pb isotopic measurements range from 0.4 to 1% (2or), depending on the Pb content of the melt inclusion. Loihi glass 158-4 previously analysed by TIMS 4 was used as a standard during the period of measurement of the melt inclusions. The Pb content of the inclusions are roughly the same as the standard. The ion probe analyses reproduced the TIMS value within <0.2% standard errors (2or n = 17). The fractionation per amu (<0.15%) was smaller than the inrun precision. No indication of interference or hidryde was found in the Pb measurement. Ion microprobe analyses of Pb isotope ratios (2~176 and 2~176 within single melt inclusion from one sample show large and systematic variations that overlap with the whole field for HIMU lavas (Mangaia, Tubuai and St. Helena islands). The 2~176 and 2~176 ratios range from 0.706 to 0.768 and from 1.840 to 1.940 respectively, which is much larger than the range observed in Mangaia lavas (0.726 to 0.735 and 1.869 to 1.883) 5. Sulphide and carbonate globules, also analysed for Pb isotopes, have isotopic compositions typical of HIMU, suggesting a primary origin. No clear correlation was found between (La/Yb)cN ratios and Pb isotopic composition. The rough linear trend produce by the melt inclusions from one sample in 2~176 2~176 space reproduce the trend defined by the HIMU islands, indicating that lavas from Mangaia do not derive from a single homogeneous mantle source and geochemical models should consider at least two mantle sources. The HIMU Pb isotopic composition of the carbonate globules in conjunction with their textures and composition provide direct evidence for a carbonated mantle source for the Austral Islands.