The composition of carbonaceous chondrite matrix

Matrix compositions of 32 carbonaceous chondrites have been analyzed by an electron microprobe defocussed-beam technique. Except in those chondrites that show evidence of metamorphism, matrices are compositionally similar and have correlation coefficients of +0.96 or greater. Weight per cent Mg/Si in matrices is constant (0.82 ± 0.05) but less than ratios derived from bulk analyses. Matrices in metamorphosed meteorites are Mg-depleted relative to those of other chondrites. Al Rais and Renazzo (anomalous by any classification scheme) have Mg-enriched matrices. Average matrix compositions cluster into chemical subgroups similar to those based on bulk chemical and petrographie criteria [C1, C2, C3(0), C3(V)]. C1 matrices are particularly variable in composition from point to point within the same meteorite, but points within individual breccia clasts appear to be more compositionally uniform. Cl matrices are depleted in Na, S, and Ca relative to solar and C2 matrix values, probably as a result of leaching. Matrix Ca/A1 ratios are highly variable and generally fall below the accepted meteoritic value. The only strong interelement correlation is for Fe, Ni, and S in C2 matrices, suggesting mixing of variable proportions of two components: Mg-rich phyllosilicate and a Ni-bearing chalcophile phase. The amount of magnetite associated with C2 matrix appears to vary systematically with matrix composition. Isotopic, chemical, and mineralogical constraints suggest that matrix, although appreciably altered in some meteorites, is chiefly a solar system condensation product which contains an admixture of unprocessed interstellar dust.     

The monsoon imprint during the ‘atypical’ MIS 13 as seen through north and equatorial Indian Ocean records

Previous studies have suggested that Marine Isotope Stage (MIS) 13, recognized as atypical in many paleoclimate records, is marked by the development of anomalously strong summer monsoons in the northern tropical areas. To test this hypothesis, we performed a multi-proxy study on three marine records from the tropical Indian Ocean in order to reconstruct and analyse changes in the summer Indian monsoon winds and precipitations during MIS 13. Our data confirm the existence of a low-salinity event during MIS 13 in the equatorial Indian Ocean but we argue that this event should not be considered as “atypical”. Taking only into account a smaller precession does not make it possible to explain such precipitation episode. However, when considering also the larger obliquity in a more complete orbitally driven monsoon “model,” one can successfully explain this event. In addition, our data suggest that intense summer monsoon winds, although not atypical in strength, prevailed during MIS 13 in the western Arabian Sea. These strong monsoon winds, transporting important moisture, together with the effect of insolation and Eurasian ice sheet, are likely one of the factors responsible for the intense monsoon precipitation signal recorded in China loess, as suggested by model simulations.     

The New CLOCIT Irradiation Facility for 40Ar/39Ar Geochronology: Characterisation,Comparison with CLICIT and Implications for High‐Precision Geochronology

The Cadmium‐Lined Outer‐Core Irradiation Tube (CLOCIT) is a new irradiation facility for ⁴⁰Ar/³⁹Ar geochronology at the Oregon State University TRIGA^® reactor. We report fluence (i.e., time‐integrated flux) parameters from the first four CLOCIT irradiations and compare them with the existing Cadmium‐Lined Inner‐Core Irradiation Tube (CLICIT). CLOCIT provides an average neutron flux equivalent of 1.45–1.53 × 10^?4 J h^?1; about 55% of CLICIT. Radial fluence gradients were on the order of 0.2–4.2% cm^?1. A planar fit of J‐values results in residuals in the range of uncertainty in the J‐value, but systematic deviations resolve a non‐planar component of the neutron flux field, which has also been observed in CLICIT. Axial neutron fluence gradients were 0.6–1% cm^?1, compared with 0.7–1.6% cm^?1 for the CLICIT. Production rate ratios of interfering reactions were (⁴⁰Ar/³⁹Ar)_K = (4 ± 6) × 10^?4 and (³⁸Ar/³⁹Ar)_K = (1.208 ± 0.002) × 10^?2, (³⁶Ar/³⁷Ar)_Ca =  (2.649 ± 0.014) × 10^?4, (³⁸Ar/³⁷Ar)_Ca =  (3.33 ± 0.12) × 10^?5 and (³⁹Ar/³⁷Ar)_Ca =  (9.1 ± 0.28) × 10^?4, similar to the CLICIT values.     

Breakup between Australia and Antarctica: A brief review in the light of new data

The arguments justifying the revised timing of breakup between Australia and Antarctica (Cande and Mutter, 1982) and the reconstruction of Broken Ridge and Kerguelen Plateau (Mutter and Cande, 1983) are reviewed and considered with respect to new subsidence data. The age of breakup was revised from anomaly 22 time (55 My B.P.) to anomaly 34 time (85 My B.P.). The rough topography of the Diamantina Zone can be attributed to very slow spreading (−5 mm/yr.) beginning between the times of anomaly 34 and anomaly 19. The reconstruction of Broken Ridge and Kerguelen Plateau at anomaly 34 time shows overlap of these two features, but the overlap problem is nearly resolved by anomaly 18 time ( ~ 42 My B.P.). Normal seafloor spreading rates (22 mm/yr.) commenced at anomaly 19 time ( ~ 43 My B.P.). Subsidence patterns calculated from biostratigraphic data from wells drilled along Australia's southern margin are interpreted as more consistent with the revised age of Australia-Antarctic breakup. Subsidence curves systematically show rapid subsidence associated with the rift phase of margin development followed by much slower thermally-controlled subsidence during the drift phase. The timing of the rift-to-drift transition is believed to coincide with the age of breakup ( ~ 60 to 110 My B.P.). In addition, the subsidence curves indicate a west-to-east propagation of breakup along the southern margin. Magnetic anomaly patterns and stratigraphie observations are consistent with this hypothesis.     

Partitioning and transport of metals across the O2H2S interface in a permanently anoxic basin: Framvaren Fjord,Norway

The geochemical processes operating on metals in anoxic marine waters influence metal mobility and mode of transport to the sediments in a manner different from that observed in oxic regimes. In order to better understand these processes, dissolved and particulate Mn, Fe, Co, Ni, Cu, Zn, and Cd concentrations were determined in the water column of a permanently anoxic basin, Framvaren Fjord, Norway. Class specific behavior determines the degree to which these metals are involved in the processes of redox cycling at the $\text{O}{}_2\text{H}{}_2\text{S}$ interface and metal sulfide precipitation in the sulfidic water. Metal sulfide precipitation influences the magnitude of metal enrichment in the sediments. The transition metals, Mn, Fe, and Co, show active involvement in redox cycling, characterized by dissolved maxima just below the $\text{O}{}_2\text{H}{}_2\text{S}$ interface. Nickel concentrations appear unaffected by processes influencing the profiles of the other metals. The metals, Cu, Zn, and Cd, display a dramatic solubility decrease across the interface, are not involved in redox cycling, and are enriched in the sediments relative to a lithogenic component by factors of 11, 105, and 420, respectively. Ion activity products of the metals and sulfide provide evidence that chemical equilibria with a pure metal sulfide solid phase is not the dominant process controlling dissolved metal concentrations in the sulfide containing waters.     

Comparison of modern and ancient barite‐bearing acid‐sulphate soils using micromorphology,geochemistry and field relationships

Although pedogenic barite has been documented in many modern soils and palaeosols, no actualistic studies on its formation have been reported. Because barite is stable over the entire range of pressure and temperature of the Earth's crust, it preserves reliable data about the original environment in which it formed. Pedogenic barite and barite‐bearing soils have been used as indicators of landscape stability, environmental conditions, climate and microbial acti‐vity. This study compares field data, micromorphology and stable isotope geochemistry of a barite‐bearing palaeosol from the Morrison Formation (Jurassic) and a modern analogue soil in south‐central Texas, USA. Morrison barite‐bearing palaeosols are over‐thickened cumulic palaeosols that developed in subaerially exposed lacustrine sediments during an extended lake contraction event. Lateral facies relationships document changes in hydrology and duration of episaturated conditions (perched water table above the Btg horizons) that correspond to differences in barite nodule morphology and abundance. Barite precipitation occurred at a redox boundary higher on the landscape after organic matter was completely oxidized. Sulphur isotope data indicate that the initial source of sulphur was soil organic matter. Meteoric water is the likely source of oxygen for the sulphate. Barium sourced from weathering feldspars and clays. The modern analogue displays similar catenary relationships, redox features and micromorphological characteristics compared to the Morrison palaeosols, suggesting that similar pedogenic processes led to barite precipitation. Synthesized data suggest that conditions favourable to barite‐bearing soil formation are low‐gradient basins that have received feldspar‐rich sediments (i.e. volcanically influenced basins), soils that developed near salt domes, soils that developed in exposed wetland or lacustrine sediments and coastal plain deposits. When studied in a well‐documented palaeogeographic context, barite‐bearing soils are valuable to palaeoclimate, palaeoenvironmental and palaeohydrological studies. Combined with regional interfluve palaeosols, barite‐bearing palaeosols may document temporal changes in drainage, surface stability, and accommodation consistent with sequence boundaries/maximum flooding surfaces and climate changes.     

The response of lake margin sedimentary systems to climatically driven lake level fluctuations: Middle Devonian,Orcadian Basin,Scotland

Lake margin sedimentary systems can provide highly sensitive records of sedimentary response to climate change. The Middle Old Red Sandstone of Northern Scotland comprises a thick succession of cyclic lacustrine sediments. Within this succession the deepest lake phase, the Achanarras fish bed, allows bed‐scale correlation over 160 km across the basin. This provides a unique opportunity to examine the character of synchronous lake margin deposits, and their response to climatically driven lake level fluctuations, across a large continental basin. Detailed characterization of two separate lake margin systems was carried out utilizing multiple sections in western Orkney, in the north, and Easter Ross, in the south. Seven facies have been recognized, which include upper and lower shoreface, deep lake, shallow lake, playa, turbidite and fluvial facies. Differences in vertical and lateral facies stacking patterns reflect the response of these systems to climatically driven fluctuations in lake level. Comparison of the northern and southern systems examined highlights the variable response of lake margin systems to the same climatic change and related lake level fluctuations. In the south, a greater fluvial influence is recognized on the development of the lake margin successions, whereas in the northern example, which lay on the downwind margin of the lake, shore zone facies are more commonly developed. The variability recognized can be accounted for by regional variations in sediment supply, coastal physiography, lake size, bathymetry and potential fetch. Lake level stability is also recognized as a major control on the development of lake margin sedimentary systems, as is the linked or unlinked relationship of the catchment and the lake basin climate for which a conceptual model is proposed.     

Mantle heterogeneity and crustal recycling in Archean granite-greenstone belts: Evidence from Nd isotopes and trace elements in the Rainy Lake area, Superior Province, Ontario, Canada

The 2685–2752 Ma old granite-greenstone crust in the Rainy Lake area, Ontario, consists of metaigneous and metasedimentary rocks that range in composition from tholeiite to monzogranite and include anorthosite, trachyandesite, monzodiorite and high-silica rhyodacite. Major element, rare earth and other trace element data are the basis for modelling the formation of the crust by melting of large-ionlithophile element enriched and unenriched mantle, by melting of basalt at mantle to crustal levels and by melting of monzodiorite and tonalite at crustal levels.

All metaigneous rocks lie on a ¹⁴³Nd/¹⁴⁴Nd vs. ¹⁴⁷Sm/¹⁴⁴Nd isochron with an age of 2737 ±42 Ma and an initial ¹⁴³Nd/¹⁴⁴Nd of 0.509178 ±33 (ε_Nd = +1.9). This age is consistent with U-Pb zircon ages, which suggests the Nd isotopic system has been unaffected since the crust-forming events. The positive initial ε_Nd's are further evidence for time-averaged depletion in Sm/Nd relative to CHUR for the Archean mantle. The similarity of the initial Nd isotopic composition for both mantle-derived and crustally-derived rocks suggests rapid recycling of crustal components, which were previously derived from depleted mantle sources.

Initial ¹⁴³Nd/¹⁴⁴Nd ratios on individual rocks range from ε_Nd = +3.3 to ε_Nd = −0.4. Younger granitoids have lower ε_Nd values (+1.5 to −0.1) relative to tholeiites and monzodiorites crystallized from mantle-derived melts (+3.3 to +1.0). Thus, incorporation of slightly older crust (ca. 100–200 Ma) in some of the granitoid source areas is possible. Mantle-derived rocks form an isochron of 2764 ±58 Ma that represents a minimum age for enrichment processes in the mantle sources for the Rainy Lake area. Consideration of data from the Abitibi belt suggests such enrichment processes in the mantle may have preceded crust-forming events in a wide area of the Superior Province, perhaps by as much as 50–70 Ma.