Etch pit formation on iron silicate surfaces during siderophore-promoted dissolution

Understanding the effects of microbiota on mineral alteration requires the ability to recognize evidence of bacteria-promoted dissolution on mineral surfaces. Although siderophores are known to promote mineral dissolution, their effects on mineral surfaces are not well known. We have utilized atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Mirau vertical scanning interferometry (VSI) to investigate surfaces after incubation with the siderophore desferrioxamine-B mesylate (DFAM) and under colonies of bacteria. Iron-silicate glass planchets chemically similar to hornblende were incubated in buffered growth medium with siderophore-producing bacteria (Bacillus sp.) for 46 days with parallel abiotic experiments conducted with and without 240 μM DFAM, with and without 0.01 g l^− 1 of microbially produced extracellular polysaccharides (EPS, alginate or xanthan gum). Some glass planchets were protected by dialysis tubing from direct contact with the EPS. Weekly sampling and analysis of all filtered sample solutions showed negligible Fe and Al release in the control experiments and significant release of Fe and Al in the presence of DFAM, with negligible changes in pH. Concentration of Fe in the filtered solutions after incubation with bacteria was below detection, consistent with uptake of Fe by cells. Release of Fe, Al, and Si in control, xanthan-only, and alginate-only experiments was negligible. Release of these elements was enhanced in all experiments containing DFAM, and greatest in alginate + DFAM experiments.

AFM and VSI analyses reveal widespread, small etch pits and greater root mean squared roughness on siderophore-exposed surfaces and fewer, localized, larger etch pits on bacteria-exposed surfaces. This is the first documented case of etch pit development during siderophore-promoted dissolution. Roughness was not affected by the growth medium, alginate, or xanthan gum alone. The roughness trends among samples correlate with trends in Fe depletion documented by XPS. Enhanced dissolution and roughness cannot be attributed to direct contact with EPS because no significant chemical or physical differences were observed between surfaces directly exposed to EPS and those protected by dialysis tubing. Acetate released from the EPS may have enhanced the siderophore-promoted dissolution. Siderophores produced by Bacillus sp. may be responsible for some of the ‘biopits.’ The difference in size and distribution of the biopits may be related to colonization.     

The origin of basic rocks of the Korosten AMCG complex, Ukrainian shield: Implication of Nd and Sr isotope data

The Korosten complex is a Paleoproterozoic gabbro–anorthosite–rapakivi granite intrusion which was emplaced over a protracted time interval — 1800–1737 Ma. The complex occupies an area of about 12 000 km² in the north-western region of the Ukrainian shield. About 18% of this area is occupied by various mafic rocks (gabbro, leucogabbro, anorthosite) that comprise five rock suites: early anorthositic A₁ (1800–1780 Ma), main anorthositic A₂ (1760 Ma), early gabbroic G₃ (between 1760 and 1758 Ma), late gabbroic G₄ (1758 Ma), and a suite of dykes D₅ (before 1737 Ma). In order to examine the relationships between the various intrusions and to assess possible magmatic sources, Nd and Sr isotopic composition in mafic whole-rock samples were measured. New Sr and Nd isotope measurements combined with literature data for the mafic rocks of the Korosten complex are consistent and enable construction of Rb–Sr and Sm–Nd isochronous regressions that yield the following ages: 1870 ± 310 Ma (Rb–Sr) and 1721 ± 90 Ma (Sm–Nd). These ages are in agreement with those obtained by the U–Pb method on zircons and indicate that both Rb–Sr and Sm–Nd systems have remained closed since the time of crystallisation. In detail, however, measurable differences in isotopic composition of the Korosten mafic rock depending on their suite affiliation were revealed. The oldest, A₁ rocks have lower Sr (⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ = 0.70233–0.70288) and higher Nd (εNd₍₁₇₆₀₎ = 1.6–0.9) isotopic composition. The most widespread A₂ anorthosite and leucogabbro display higher Sr and lower Nd isotopic composition: ⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ = 0.70362, εNd₍₁₇₆₀₎ varies from 0.2 to − 0.7. The G₃ gabbro–norite has slightly lower εNd₍₁₇₆₀₎ varying from − 0.7 to − 0.9. Finally, G₄ gabbroic rocks show relatively high initial ⁸⁷Sr/⁸⁶Sr (0.70334–0.70336) and the lowest Nd isotopic composition (εNd₍₁₇₆₀₎ varies from − 0.8 to − 1.4) of any of the mafic rocks of the Korosten complex studied to date. On the basis of Sr and Nd isotopic composition we conclude that Korosten initial melts may have inherited their Nd and Sr isotopic characteristics from the lower crust created during the 2.05–1.95 Ga Osnitsk orogeny and 2.0 Ga continental flood basalt event. Indeed, εNd₍₁₇₆₀₎ values in Osnitsk rocks vary from 0.0 to − 1.9 and from 0.2 to 3.4 in flood basalts. We suggest that these rocks being drawn into the upper mantle might melt and give rise to the Korosten initial melts. ⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ values also support this interpretation. We suggest that the Sr and Nd isotopic data currently available on mafic rocks of the Korosten complex are consistent with an origin of its primary melts by partial melting of lower crustal material due to downthrusting of the lower crust into upper mantle forced by Paleoproterozoic amalgamation of Sarmatia and Fennoscandia.     

Melting of the subcontinental lithospheric mantle by the Emeishan mantle plume; evidence from the basal alkaline basalts in Dongchuan, Yunnan, Southwestern China

The Emeishan continental flood basalt (ECFB) sequence in Dongchuan, SW China comprises a basal tephrite unit overlain by an upper tholeiitic basalt unit. The upper basalts have high TiO₂ contents (3.2–5.2 wt.%), relatively high rare-earth element (REE) concentrations (40 to 60 ppm La, 12.5 to 16.5 ppm Sm, and 3 to 4 ppm Yb), moderate Zr/Nb and Nb/La ratios (9.3–10.2 and 0.6–0.9, respectively) and relatively high _{Nd (t)} values, ranging from − 0.94 to 2.3, and are comparable to the high-Ti ECFB elsewhere. The tephrites have relatively high P₂O₅ (1.3–2.0 wt.%), low REE concentrations (e.g., 17 to 23 ppm La, 4 to 5.3 ppm Sm, and 2 to 3 ppm Yb), high Nb/La (2.0–3.9) ratios, low Zr/Nb ratios (2.3–4.2), and extremely low _{Nd (t)} values (mostly ranging from − 10.6 to − 11.1). The distinct compositional differences between the tephrites and the overlying tholeiitic basalts cannot be explained by either fractional crystallization or crustal contamination of a common parental magma. The tholeiitic basalts formed by partial melting of the Emeishan plume head at a depth where garnet was stable, perhaps > 80 km. We propose that the tephrites were derived from magmas formed when the base of the previously metasomatized, volatile-mineral bearing subcontinental lithospheric mantle was heated by the upwelling mantle plume.     

Provenance of the Neoproterozoic Maricá Formation (Sul-rio-grandense Shield, Southern Brazil): Petrographic and Sm–Nd isotopic constraints

This paper reports the integrated application of petrographic and Sm–Nd isotopic analyses for studying the provenance of the Neoproterozoic Maricá Formation, southern Brazil. This unit encompasses sedimentary rocks of fluvial and marine affiliations. In the lower fluvial succession, sandstones plot in the “craton interior” and “transitional continental” fields of the QFL diagram. Chemical weathering probably caused the decrease of the ¹⁴⁷Sm/¹⁴⁴Nd ratios to 0.0826 and 0.0960, consequently lowering originally > 2.0 Ga T_DM ages to 1.76 and 1.81 Ga. ¹⁴³Nd/¹⁴⁴Nd ratios are also low (0.511521 to 0.511633), corresponding to negative ε_Nd present-day values (− 21.8 and − 19.6). In the intermediate marine succession, sandstones plot in the “dissected arc” field, reflecting the input of andesitic clasts. Siltstones and shales reveal low ¹⁴³Nd/¹⁴⁴Nd ratios (0.511429 to 0.511710), ε_Nd values of − 18.1 and − 23.6, and T_DM ages of 2.16 and 2.37 Ga. Sandstones of the upper fluvial succession have “dissected arc” and “recycled orogen” provenance. ¹⁴³Nd/¹⁴⁴Nd isotopic ratios are also relatively low, from 0.511487 to 0.511560, corresponding to ε_Nd values of − 22.4 and − 21.0 and T_DM of 2.07 Ga. A uniform granite–gneissic basement block of Paleoproterozoic age, with subordinate volcanic rocks, is suggested as the main sediment source of the Maricá Formation.     

Volatiles in a peralkaline system: Abiogenic hydrocarbons and F–Cl–Br systematics in the naujaite of the Ilímaussaq intrusion, South Greenland

Agpaitic rocks comprise most of the exposed part of the 1.16 Ga old, 8 × 17 km large and about 1700 m thick Ilímaussaq intrusion in South Greenland. Within these, more than 600 m thick sequence of sodalite-rich “naujaites” (mainly sodalite + arfvedsonite + alkali feldspar + nepheline + eudialyte + aenigmatite) are interpreted as a sodalite flotation cumulate. Sodalites show two to three different zones in cathodoluminescence (CL) and at least two zones in thin sections. The CL zones can be related to chemical differences detectable by electron microprobe, whereas relations with optical zonations are less obvious. Compositional trends in sodalite reflect trends in the evolution of volatile contents in the melt. The sodalite at Ilímaussaq is almost free of Ca and closely corresponds to the pure Na–Cl sodalite endmember with about 7 wt.% of Cl; S contents reach up to 0.9 wt.%. Cl/Br ratios range from 500 to 1700. Raman spectroscopy shows that S is present as [SO₄]²⁻ in sodalite, although sphalerite (ZnS) is a stable phase in naujaites. Peralkalinity and fO₂ conditions allow S²⁻ and [SO₄]²⁻ to be present contemporaneously.

The whole naujaite sequence is divided into two parts, an upper part with low, homogeneous S contents and Cl/Br ratios in the sodalite cores, and a lower part with strongly variable and higher S contents and with Cl/Br ratios, which are decreasing downwards. The details of the S content and the Cl/Br ratio evolution show that sodalite strongly influences the halogen contents of the melt by scavenging Cl and Br.

The naujaites were formed from a highly reduced, halogen-rich magma in equilibrium with magmatic methane at about 800 °C, which, upon ascent, cooling and fractionation, exsolved an aqueous fluid phase. Both fluids were trapped in separate inclusions indicating their immiscibility.

Micrometer-sized aegirine crystals and primary hydrocarbon-bearing inclusions are abundant in the crystal cores. The inclusions were trapped at pressures up to 4 kbar, although the emplacement pressure of the intrusion is about 1 kbar. This indicates growth of the sodalite during melt ascent and a very effective mechanism of trace element scavenging during sodalite growth. Sodalite rims are devoid of aegirine or primary hydrocarbon inclusions and probably reflect the emplacement stage.     

Early diagenetic influences on iron transformations in a freshwater lake sediment

Iron transformations in a calcium carbonate rich fresh-water sediment were studied by analyzing the relevant constituents of both interstitial water and solid matter. Analysis of interstitial water shows that the observed redox sequence NO⁻₃/NH⁺₄, MnO₂/Mn(II), FeOOH/Fe(II), SO²⁻₄/S(−II) is roughly in agreement with that predicted by the Gibbs Free Energy for the corresponding reactions. In contrast to marine sediments, these redox transitions occur in the uppermost sediments, i.e., at depths of 0–4 cm.

Deeper in the sedimentary sequence, the depth profile for dissolved iron exhibits a steady non-linear increase up to 400 μmol dm⁻³. In this anoxic zone, according to thermodynamic predictions, iron (II)-minerals such as iron sulfide, siderite, and vivianite should precipitate while Fe(III) oxides should be completely dissolved. However, microscopic analysis showed that Fe(III) oxides were present throughout the studied sediment. Furthermore, scanning electron microscope/energy dispersive spectroscopy analysis suggests the presence of iron sulfide could be verified but not that of siderite or vivianite. These observations indicate kinetic control of iron transformations.

We have investigated the importance of kinetic control of iron distribution in anoxic sediments using a diagenetic model for dissolved iron(II). A rough estimate of time scales for dissolution and precipitation rates was made by imposing limiting boundary conditions. Using the calculated rate constant, we established that more than 1000 years would be required for the complete dissolution of Fe(III) oxides, which is agreement with our observations and experimental data from the literature. Calculated precipitation rates of Fe(II) for a given mineral phase such as siderite yield a maximum value of 3 μg_(FeCO3) g⁻¹_{(dry sediment)} yr⁻¹. Such low rates would explain the absence of siderite and vivianite.

Finally, it can be inferred from the Mn_T/Fe_T ratio in the sediments that this ratio depends on the redox conditions of the sediment-water interface at the time of deposition. Thus, this ratio can be used as “paleo-redox indicator” in lacustrine sediments.     

Geochemistry and petrology of Tertiary volcanic rocks and related ultramafic xenoliths from the central and eastern Oman Mountains

The Tertiary volcanic rocks of the central and the eastern parts of the Oman Mountains consist mainly of basanites with abundant upper mantle ultramafic xenoliths. The lavas are alkaline (42–43 wt.% SiO₂; 3.5–5.5 wt.% Na₂O + K₂O). They include primitive (11–14 wt.% MgO) features with strong OIB-like geochemical signatures. Trace element and Sr–Nd isotope data for the basanites suggest mixing of melts derived from variable degrees of melting of both garnet- and spinel lherzolite-facies mantle source. The associated xenolith suite consists mainly of spinel and Cr-bearing diopside wehrlite, lherzolite and dunite with predominantly granuloblastic textures. No significant difference in chemistry was found between the basanites and xenoliths from the central and eastern Oman Mountains, which indicate a similar mantle source. Calculated oxygen fugacity indicates equilibration of the xenoliths at − 0.43 to − 2.2 log units above the fayalite–magnetite–quartz (FMQ) buffer. Mantle xenolith equilibration temperatures range from 910–1045 + 50 °C at weakly constrained pressures between 13 and 21 kbar. Xenolith data and geophysical studies indicate that the Moho is located at a depth of  40 km. A geotherm substantially hotter (90 mW m^− 2) than the crust–mantle boundary (45 mW m^− 2) is indicated and probably relates to tectonothermal events associated with the local and regional Tertiary magmatism. The petrogenesis of the Omani Tertiary basanites is explained by partial melting of an asthenospheric mantle protolith during an extension phase predating opening of the Gulf of Aden and plume-related alkaline volcanic rocks.     

Geochemical and Sr–Nd–Pb isotopic compositions of the Eocene Dölek and Sariçiçek Plutons, Eastern Turkey: Implications for magma interaction in the genesis of high-K calc-alkaline granitoids in a post-collision extensional setting

The major and trace elements and Sr–Nd–Pb isotopes of the host rocks and the mafic microgranular enclaves (MME) gathered from the Dölek and Sariçiçek plutons, Eastern Turkey, were studied to understand the underlying petrogenesis and geodynamic setting. The plutons were emplaced at  43 Ma at shallow depths ( 5 to 9 km) as estimated from Al-in hornblende geobarometry. The host rocks consist of a variety of rock types ranging from diorite to granite (SiO₂ = 56.98–72.67 wt.%; Mg# = 36.8–50.0) populated by MMEs of gabbroic diorite to monzodiorite in composition (SiO₂ = 53.21–60.94 wt.%; Mg# = 44.4–53.5). All the rocks show a high-K calc-alkaline differentiation trend. Chondrite-normalized REE patterns are moderately fractionated and relatively flat [(La/Yb)_N = 5.11 to 8.51]. They display small negative Eu anomalies (Eu/Eu = 0.62 to 0.88), with enrichment of LILE and depletion of HFSE. Initial Nd–Sr isotopic compositions for the host rocks are ε_Nd(43 Ma) = − 0.6 to 0.8 and mostly I_Sr = 0.70482–0.70548. The Nd model ages (T_DM) vary from 0.84 to 0.99 Ga. The Pb isotopic ratios are (²⁰⁶Pb/²⁰⁴Pb) = 18.60–18.65, (²⁰⁷Pb/²⁰⁴Pb) = 15.61–15.66 and (²⁰⁸Pb/²⁰⁴Pb) = 38.69–38.85. Compared with the host rocks, the MMEs are relatively homogeneous in isotopic composition, with I_Sr ranging from 0.70485 to 0.70517, ε_Nd(43 Ma) − 0.1 to 0.8 and with Pb isotopic ratios of (²⁰⁶Pb/²⁰⁴Pb) = 18.58–18.64, (²⁰⁷Pb/²⁰⁴Pb) = 15.60–15.66 and (²⁰⁸Pb/²⁰⁴Pb) = 38.64–38.77. The MMEs have T_DM ranging from 0.86 to 1.36 Ga. The geochemical and isotopic similarities between the MMEs and their host rocks indicate that the enclaves are of mixed origin and are most probably formed by the interaction between the lower crust- and mantle-derived magmas. All the geochemical data, in conjunction with the geodynamic evidence, suggest that a basic magma derived from an enriched subcontinental lithospheric mantle, probably triggered by the upwelling of the asthenophere, and interacted with a crustal melt that originated from the dehydration melting of the mafic lower crust at deep crustal levels. Modeling based on the Sr–Nd isotope data indicates that  77–83% of the subcontinental lithospheric mantle involved in the genesis. Consequently, the interaction process played an important role in the genesis of the hybrid granitoid bodies, which subsequently underwent a fractional crystallization process along with minor amounts of crustal assimilation, en route to the upper crustal levels generating a wide variety of rock types ranging from diorite to granite in an extensional regime.     

Diamonds and their mineral inclusions from the A154 South pipe, Diavik Diamond Mine, Northwest territories, Canada

Mineral inclusions recovered from 100 diamonds from the A154 South kimberlite (Diavik Diamond Mines, Central Slave Craton, Canada) indicate largely peridotitic diamond sources (83%), with a minor (12%) eclogitic component. Inclusions of ferropericlase (4%) and diamond in diamond (1%) represent “undetermined” parageneses.

Compared to inclusions in diamonds from the Kaapvaal Craton, overall higher CaO contents (2.6 to 6.0 wt.%) of harzburgitic garnets and lower Mg-numbers (90.6 to 93.6) of olivines indicate diamond formation in a chemically less depleted environment. Peridotitic diamonds at A154 South formed in an exceptionally Zn-rich environment, with olivine inclusions containing more than twice the value (of  52 ppm) established for normal mantle olivine. Harzburgitic garnet inclusions generally have sinusoidal rare earth element (REE_N) patterns, enriched in LREE and depleted in HREE. A single analyzed lherzolitic garnet is re-enriched in middle to heavy REE resulting in a “normal” REE_N pattern. Two of the harzburgitic garnets have “transitional” REE_N patterns, broadly similar to that of the lherzolitic garnet. Eclogitic garnet inclusions have normal REE_N patterns similar to eclogitic garnets worldwide but at lower REE concentrations.

Carbon isotopic values (δ¹³C) range from − 10.5‰ to + 0.7‰, with 94% of diamonds falling between − 6.3‰ and − 4.0‰. Nitrogen concentrations range from below detection (< 10 ppm) to 3800 ppm and aggregation states cover the entire spectrum from poorly aggregated (Type IaA) to fully aggregated (Type IaB). Diamonds without evidence of previous plastic deformation (which may have accelerated nitrogen aggregation) typically have < 25% of their nitrogen in the fully aggregated B-centres. Assuming diamond formation beneath the Central Slave to have occurred in the Archean [Westerlund, K.J., Shirey, S.B., Richardson, S.H., Gurney, J.J., Harris, J.W., 2003b. Re–Os systematics of diamond inclusion sulfides from the Panda kimberlite, Slave craton. VIIIth International Kimberlite Conference, Victoria, Canada, Extended Abstracts, 5p.], such low aggregation states indicate mantle residence at fairly low temperatures (< 1100 °C). Geothermometry based on non-touching inclusion pairs, however, indicates diamond formation at temperatures around 1200 °C. To reconcile inclusion and nitrogen based temperature estimates, cooling by about 100–200 °C shortly after diamond formation is required.     

Late Triassic granitoids of the eastern margin of the Tibetan Plateau: Geochronology, petrogenesis and implications for tectonic evolution

Late Triassic granitoids in the Songpan-Garzê Fold Belt (SGFB), on the eastern margin of the Tibetan Plateau, formed at 230 to 220 Ma and can be divided into two groups. Group 1 are high-K calc-alkaline rocks with adakitic affinities (K-adakites), with Sr > 400 ppm, Y < 11 ppm, strongly fractionated REE patterns ((La/Yb)_N = 32–105) and high K₂O/Na₂O (≈ 1). Group 2 are ordinary high-K calc-alkaline I-types with lower Sr (< 400 ppm), higher Y (> 18 ppm) and weakly fractionated REE patterns ((La/Yb)_N < 20). Rocks of both groups have similar negative Eu anomalies (Eu/Eu = 0.50 to 0.94) and initial ⁸⁷Sr/⁸⁶Sr (0.70528 to 0.71086), but group 1 rocks have higher ε_Nd(t) (− 1.01 to − 4.84) than group 2 (− 3.11 to − 6.71). Calculated initial Pb isotope ratios for both groups are: ²⁰⁶Pb/²⁰⁴Pb = 18.343 to 18.627, ²⁰⁷Pb/²⁰⁴Pb = 15.610 to 15.705 and ²⁰⁸Pb/²⁰⁴Pb = 38.269 to 3759. Group 1 magmas were derived through partial melting of thickened and then delaminated TTG-type, eclogitic lower crust, with some contribution from juvenile enriched mantle melts. Group 2 magmas were generated by partial melting of shallower lower crustal rocks. The inferred magma sources of both groups suggest that the basement of the SGFB was similar to the exposed Kangding Complex, and that the SGFB was formed in a similar manner to the South China basement. Here, passive margin crust was greatly thickened and then delaminated, all within a very short time interval ( 20 Myr). Such post-collisional crustal thickening could be the tectonic setting for the generation of many adakitic magmas, especially where there is no spatial and temporal association with subduction.     

Continental margin magmatism and migmatisation in the west-central Fennoscandian Shield

The Ljusdal Batholith (LjB) is a major component of the central Svecofennian Domain in Sweden. It is separated from the Bothnian Basin to the north by the 1.82–1.80 Ga crustal-scale Hassela Shear Zone (HSZ). The LjB has emplacement ages of 1.86–1.84 Ga, is mainly alkali-calcic, metaluminous, has _Nd values between − 0.3 and + 1.2 and was formed in a magmatic arc setting.

During the Svecokarelian orogeny the LjB was affected by at least three fold episodes. Large-scale folded screens of migmatised metasedimentary rocks occur in the eastern part of the batholith, and to the north of the HSZ, there is a 50 km wide diatexite belt. The Transition Belt (TrB), consisting of 1.88–1.85 Ga granitoids, is located at the northwestern extension of this belt. A calc-alkaline and peraluminous composition combined with negative _Nd values (− 1.7 to − 0.8) indicates a large proportion of metasediments in the source for these granitoids.

U–Pb SIMS data on zircon rims from migmatites and leucogranites to the north and east of LjB yield ages of 1.87–1.86 Ga, i.e. coeval with the granitoids of the LjB and the TrB. There is thus a close relationship between the LjB, the TrB and the migmatites in both space and time. Syn-migmatitic shearing along the HSZ indicates that a proto-HSZ was initiated already at c. 1.86 Ga, and the location of the proto-HSZ is inferred to be controlled by two older nuclei present in the lower parts of the crust. As crustal-scale shear zone systems are known to act as ascent pathways for sheet-like flow in active orogenies the TrB may represents accumulations of melts that were attracted and extracted by the proto-HSZ and intruded in a block that was not pervasively affected by subsequent shear along the HSZ.

An active continental margin setting for the LjB implies subduction at c. 1.86 Ga, and provides a heat source for both the migmatites and the TrB.

A later migmatisation at 1.82 Ga has been recorded to the south of the HSZ. Within the LjB the 1.82 Ga stromatic migmatites are folded by F₂ folds, and the fabric is truncated by 1.80 Ga pegmatites.     

Perspectives on basaltic magma crystallization and differentiation: Lava-lake blocks erupted at Mauna Loa volcano summit, Hawaii

Explosive eruptions at Mauna Loa summit ejected coarse-grained blocks (free of lava coatings) from Moku'aweoweo caldera. Most are gabbronorites and gabbros that have 0–26 vol.% olivine and 1–29 vol.% oikocrystic orthopyroxene. Some blocks are ferrogabbros and diorites with micrographic matrices, and diorite veins (≤ 2 cm) cross-cut some gabbronorites and gabbros. One block is an open-textured dunite.

The MgO of the gabbronorites and gabbros ranges  7–21 wt.%. Those with MgO > 10 wt.% have some incompatible-element abundances (Zr, Y, REE; positive Eu anomalies) lower than those in Mauna Loa lavas of comparable MgO; gabbros (MgO < 10 wt.%) generally overlap lava compositions. Olivines range Fo_83–58, clinopyroxenes have Mg#s  83–62, and orthopyroxene Mg#s are 84–63 — all evolved beyond the mineral-Mg#s of Mauna Loa lavas. Plagioclase is An_75–50. Ferrogabbro and diorite blocks have  3–5 wt.% MgO (TiO₂ 3.2–5.4%; K₂O 0.8–1.3%; La 16–27 ppm), and a diorite vein is the most evolved (SiO₂ 59%, K₂O 1.5%, La 38 ppm). They have clinopyroxene Mg#s 67–46, and plagioclase An_57–40. The open-textured dunite has olivine  Fo_83.5. Seven isotope ratios are ⁸⁷Sr/⁸⁶Sr 0.70394–0.70374 and ¹⁴³Nd/¹⁴⁴Nd 0.51293–0.51286, and identify the suite as belonging to the Mauna Loa system.

Gabbronorites and gabbros originated in solidification zones of Moku'aweoweo lava lakes where they acquired orthocumulate textures and incompatible-element depletions. These features suggest deeper and slower cooling lakes than the lava lake paradigm, Kilauea Iki, which is basalt and picrite. Clinopyroxene geobarometry suggests crystallization at < 1 kbar P. Highly evolved mineral Mg#s, < 75, are largely explained by cumulus phases exposed to evolving intercumulus liquids causing compositional ‘shifts.’ Ferrogabbro and diorite represent segregation veins from differentiated intercumulus liquids filter pressed into rigid zones of cooling lakes. Clinopyroxene geobarometry suggests < 300 bar P. Open-textured dunite represents olivine-melt mush, precursor to vertical olivine-rich bodies (as in Kilauea Iki). Its Fo_83.5 identifies the most primitive lake magma as  8.3 wt.% MgO. Mass balancing and MELTS show that such a magma could have yielded both ferrogabbro and diorite by ≥ 50% fractional crystallization, but under different fO₂: < FMQ (250 bar) led to diorite, and FMQ (250 bar) yielded ferrogabbro. These segregation veins, documented as similar to those of Kilauea, testify to appreciable volumes of ‘rhyolitic’ liquid forming in oceanic environments. Namely, SiO₂-rich veins are intrinsic to all shields that reached caldera stage to accommodate various-sized cooling, differentiating lava lakes.     

Upper Cambrian carbonate sequences of the Argentine Precordillera and the Steptoean C-Isotope positive excursion (SPICE)

Carbon and Sr-isotope profiles in Upper Cambrian platformal carbonate Formations in the Precordillera, western Argentina (Zonda, La Flecha and La Silla Formations), were constructed for three representative sections: (a) Quebrada de la Flecha, Eastern Precordillera, (b) Cerro La Silla, Central Precordillera and (c) Quebrada de La Angostura, northern part of the Central Precordillera.

At Quebrada de La Angostura, upper part of the La Flecha Formation, δ¹³C_carb varies continuously up-section from − 2.0 to + 5.6‰ (PDB) and records the SPICE anomaly (+ 5‰) reported for the first time in South America. The peak of this excursion is characterized by intercalated 2 m thick beds of black shale with marl and limestone that record the onset of a sea-level change.

The Steptoean Zonda Formation dolomites at the Quebrada de la Flecha exhibit a total δ¹³C range from − 2.7 to + 0.6‰ with discrete positive anomaly about 200 m from the transition to the overlying Sunwaptan La Flecha Formation. Pronounced C-isotope anomaly (− 5.6‰) is observed in the La Flecha Formation at about 300 m below the transition to the La Silla Formation.

At the Cerro La Silla section, the Zonda Formation exhibit δ¹³C values of  − 1‰, increasing slightly at the transition to the La Flecha Formation (− 1 to 0‰). The transition of the La Flecha to the La Silla Formations is characterized by alternation of black shales and dolomitic limestone with a discrete positive C-isotope excursion, probably corresponding to the SPICE.

At the Quebrada de La Flecha, ⁸⁷Sr/⁸⁶Sr for the Zonda Formation varies from 0.70924 to 0.70955 and for the La Flecha Formation from 0.70908 to 0.70942. At Cerro La Silla this ratio varies from 0.70914 to 0.70923 for the La Flecha Formation, and from 0.70898 to 0.70980 for the La Silla Formation. At the Quebrada de La Angostura, ratios for the La Flecha carbonates range from 0.70918 to 0.70993. The overall variation of ⁸⁷Sr/⁸⁶Sr is consistent with globally reported Upper Cambrian seawater values at ca. 500 Ma.

The unambiguous record of SPICE in the La Flecha Formation at the Quebrada de La Angostura supports a Steptoean age for its deposition and allows precise local, regional, and global stratigraphic correlation. The pronounced negative C-isotope excursion recorded in the La Flecha Formation carbonates at the Quebrada de La Flecha is likely equivalent to that registered in Sunwaptan carbonates of North America and Australia, and might be tied to a global event, as a valuable tool in stratigraphic correlation (SNICE, acronym for Sunwaptan negative isotope carbon excursion).     

The origin of medium-K ankaramitic arc magmas from Lombok (Sunda arc, Indonesia): Mineral and melt inclusion evidence

High-calcium, nepheline-normative ankaramitic basalts (MgO > 10 wt.%, CaO/Al₂O₃ > 1) from Rinjani volcano, Lombok (Sunda arc, Indonesia) contain phenocrysts of clinopyroxene and olivine (Fo_85–92) with inclusions of spinel (Cr# 58–77) and crystallised melt. Olivine crystals have variable but on average low NiO (0.10–0.23 wt.%) and high CaO (0.22–0.35 wt.%) contents for their forsterite number. The CaO content of Fo_89–91 olivine is negatively correlated with the Al₂O₃ content of enclosed spinel (9–15 wt.%) and positively correlated with the CaO/Al₂O₃ ratios of melt inclusions (0.9–1.5). Major and trace element patterns of melt inclusions are similar to that of the host rock, indicating that the magma could have formed by accumulation of small batches of melt, with compositions similar to the melt inclusions. The liquidus temperature of the magma was  1275 °C, and its oxygen fugacity ≤ FMQ + 2.5. Correlations between K₂O, Zr, Th and LREE in the melt inclusions are interpreted to reflect variable degrees of melting of the source; correlations between Al₂O₃, Na₂O, Y and HREE are influenced by variations in the mineralogy of the source. The melts probably formed from a water-poor, clinopyroxene-rich mantle source.     

The geochemical cycle of boron: Constraints from boron isotope partitioning experiments between mica and fluid

The fractionation of boron isotopes between synthetic boromuscovite and fluid was experimentally determined at 3.0 GPa/500 °C and 3.0 GPa/700 °C. For near-neutral fluids Δ¹¹B_(mica-fluid) = δ¹¹B_(mica) − δ¹¹B_(fluid) is − 10.9 ± 1.3‰ at 500 °C, and − 6.5 ± 0.4‰ at 700 °C. This supports earlier assumptions that the main fractionation effect is due to the change from trigonal coordination of boron in neutral fluids to tetrahedrally coordinated boron in micas, clays and melts. The T-dependence of this effect is approximated by the equation Δ¹¹B_{(mica,clay,melt–neutral fluid)} = − 10.69 · (1000/T [K]) + 3.88; R² = 0.992, valid from 25 °C for fluid–clay up to about 1000 °C for fluid–silicate melt. Experiments at 0.4 GPa that used strongly basic fluids produced significantly lower fractionations with Δ¹¹B_{(mica–fluid)} of − 7.4 ± 1.0‰ at 400 °C, and − 4.8 ± 1.0‰ at 500 °C, showing the reduced fractionation effect when large amounts of boron in basic fluids are tetrahedrally coordinated. Field studies have shown that boron concentrations and ¹¹B/¹⁰B-ratios in volcanic arcs systematically decrease across the arc with increasing distance from the trench, thus reflecting the thermal structure of the subducting slab. Our experiments show that the boron isotopic signature in volcanic arcs probably results from continuous dehydration of micas along a distinct P–T range. Continuous slab dehydration and boron transport via fluid into the mantle wedge is responsible for the boron isotopic signature in volcanic arcs.     

Late Quaternary basin evolution of the Gulf of Corinth: Sequence stratigraphy, sedimentation, fault–slip and subsidence rates

The Gulf of Corinth is a graben, which has undergone extension during the Late Quaternary. The subsidence rate is rapid in the currently marine part whereas uplift now affects a large part of the initially subsiding area in the North Peloponnese. In this paper, we document the rates of subsidence/uplift and extension based on new subsurface data, including seismic data and long piston coring in the deepest part of the Gulf. Continuous seismic profiling data (air gun) have shown that four (at least) major oblique prograding sequences can be traced below the northern margin of the central Gulf of Corinth. These sequences have been developed successively during low sea level stands, suggesting continuous and gradual subsidence of the northern margin by 300 m during the Late Quaternary (last 250 ka). Subsidence rates of 0.7–1.0 m kyr^− 1 were calculated from the relative depth of successive topset to foreset transitions. The differential total vertical displacement between the northern and the southern margins of the Corinth graben is estimated at about 2.0–2.3 m kyr^− 1.

Sequence stratigraphic interpretation of seismic profiles from the basin suggests that the upper sediments (0.6 s twtt thick) in the depocenter were accumulated during the last 250 ka at a mean rate of 2.2–2.4 m kyr^− 1. Long piston coring in the central Gulf of Corinth basin enabled the recovery of lacustrine sediments, buried beneath 12–13.5 m of Holocene marine sediments. The lacustrine sequence consists of varve-like muddy layers interbedded with silty and fine sand turbidites. AMS dating determined the age of the marine–lacustrine interface (reflector Z) at about 13 ka BP. Maximum sedimentation rates of 2.4–2.9 m kyr^− 1 were calculated for the Holocene marine and the last glacial, lacustrine sequences, thus verifying the respective rates obtained by the sequence stratigraphic interpretation. Recent accumulation rates obtained by the ²¹⁰Pb-radiometric method on short sediment box cores coincide with the above sedimentation rates. Vertical fault slip rates were measured by using fault offsets of correlated reflector Z. The maximum subsidence rate of the depocenter (3.6 m kyr^− 1) exceeds the maximum sedimentation rate by 1.8 m kyr^− 1, which, consequently, corresponds to the rate of deepening of the basin's floor. The above rates indicate that the 2.2 km maximum sediment thickness as well as the 870 m maximum depth of the basin may have formed during the last 1 Ma, assuming uniform mean sedimentation rate throughout the evolution of the basin.     

Aridic crusts and vein mineralisations in the playa Areg el Makrzene, South Tunisia

The playa Areg el Makrzene is situated in the north of the escarpment of the Djebel Rehach in southern Tunisia. It is characterised by more than 70 spring mounds which are bound to faults in the Triassic sandstone, and by (sub-) recent calcareous and iron hydroxide precipitations. The artesian spring water contains high amounts of Na⁺, Ca²⁺, Mg²⁺ and Cl⁻, SO₄²⁻, and HCO₃, shows slightly alkaline character and a temperature of 25 °C. The spring mounds consist of porous travertine (calcite), goethite, gypsum and of Na–Mg evaporites and are up to 10 m high. Few metres away from the springs the amorphous iron hydroxides have altered to badly ordered goethite and ferrihydrite and finally to well ordered goethite. The muddy precipitate has altered to hard crusts (calcrete, ferricrete). The source of the precipitated material is discussed.

The faults in the sandstone are filled with (hydr-)oxides of iron and manganese, e.g. with goethite, hollandite and pyrolusite. The amount of Mn-oxides may reach 60 mass%.     

Implications of super dense carbonic and hypersaline fluid inclusions in granites from the Ranchi area, Chottanagpur Gneissic Complex, Eastern India

A combined fluid inclusion and mineral thermobarometric study in groups of synchronous inclusions in quartz within weakly foliated granites from the Chottanagpur Gneissic Complex, India, reveals super dense carbonic (CO₂ with minor CH₄ and H₂O) inclusions and hypersaline (H₂O–NaCl ± NaHCO₃) inclusions, with halite- and nahcolite daughter phases. This study documents the highest density (1.115 g cm^− 3) CO₂ fluids ever reported in granites. Fluid isochores, constructed from CO₂ (± CH₄) and halite-bearing inclusions, coupled with two-feldspar thermometry constrain the minimum P–T at 8 kbar/ 750 °C for fluid entrapment in granites. By contrast, the carbonic inclusions in quartz from granite-hosted metapelite enclaves contain substantial CH₄ (up to 30 mol%), and the entrapment pressure ( 4.3 kbar/600 °C) is considerably lower compared to those in the granites. By implication, the sillimanite-free granites were not derived from the metapelitic enclaves, and instead were formed by partial melting of fluid-heterogeneous lower crustal protoliths, with fluid entrapment at magmatic conditions.     

Evolution of mineral–fluid interfaces studied at pressure with synchrotron X-ray techniques

In situ measurements of mineral surface evolution during the process of pressure solution are possible with the high brightness of synchrotron X-ray sources. This capability has been explored through the use of newly developed reaction vessels that allow transmission of the incident and scattered X-ray beam through a low atomic weight piston. Several new vessels are described, along with details of computational algorithms that are used to simulate X-ray scattering in this unconventional geometry. Results using calcite (CaCO₃) and halite (NaCl) as reactant crystals are presented and compared to other atomic-scale measurements of surface dissolution processes. Calcite was reacted with an unsaturated fluid at 30 bars of pressure for approximately 24 h. During reaction the root mean square surface roughness (σ) evolved from 13.7 Å (± 0.5 Å) to 19.5 Å (± 1.0 Å), giving a roughening rate of: dσ/dt = +6.3 × 10^− 5 Å s^− 1. This is consistent with other measurements made with free calcite surfaces and is driven almost entirely by chemical disequilibrium. Analysis of the surface ex situ post-reaction gives an identical σ value, showing that the in situ measurements are well-constrained. Experiments also at 30 bars but in a saturated solution indicate that the calcite surface does not significantly roughen, giving the result that pressure solution of calcite at this pressure cannot be monitored in experiments of several days duration. Experiments with halite, a much more reactive phase, in saturated solutions showed the reflectivity profile to be dynamic on a time scale of hours. This experiment was left to reach equilibrium over 108 days and then re-analyzed, showing that σ had increased from 34 Å (± 2 Å) to 41 Å (± 2 Å), giving a roughening rate of: dσ/dt ≤ +6.4 × 10^− 7 Å s^− 1. This is two orders of magnitude smaller than the calcite roughening rate caused by chemical disequilibrium and provides the first direct in situ atomic-scale measurement of the rate of surface roughening due to pressure solution.     

Nd isotopic data reveal the material and tectonic nature of the Main Central Thrust zone in Nepal Himalaya

The Main Central Thrust (MCT) is a tectono-metamorphic boundary between the Higher Himalayan crystallines (HHC) and Lesser Himalayan metasediments (LHS), reactivated in the Tertiary, but which had already formed as a collisional boundary in the Early Paleozoic. To investigate the nature of the MCT, we analyzed whole-rock Nd isotopic ratios of rocks from the MCT and surrounding zones in the Taplejung–Ilam area of far-eastern Nepal, Annapurna–Galyang area of central Nepal, and Maikot–Barekot area of western Nepal. We define the MCT zone as a ductile–brittle shear zone between the upper MCT (UMCT) and lower MCT (LMCT). The protoliths of the MCT zone may provide critical constraints on the tectonic evolution of the Himalaya. The LHS is lithostratigraphically divided into the upper and lower units. In the Taplejung–Ilam area, different lithologic units and their ε_Nd (0) values are as follows; HHC (− 10.0 to − 18.1), MCT zone (− 18.5 to − 26.2), upper LHS unit (− 17.2), and lower LHS unit (− 22.0 to − 26.9). There is a distinct gap in the ε_Nd (0) values across the UMCT except for the southern frontal edge of the Ilam nappe. In the Annapurna–Galyang and Maikot–Barekot areas, different lithologic units and their ε_Nd (0) values are as follows; HHC (− 13.9 to − 17.7), MCT zone (− 23.8 to − 26.2 except for an outlier of − 12.4), upper LHS unit (− 15.6 to − 26.8), and lower LHS unit (− 24.9 to − 26.8). These isotopic data clearly distinguish the lower LHS unit from the HHC. Combining these data with the previously published data, the lowest ε_Nd (0) value in the HHC is − 19.9. We regard rocks with ε_Nd (0) values below − 20.0 as the LHS. In contrast, rocks with those above − 19.9 are not always the HHC, and some parts of them may belong to the LHS due to the overlapping Nd isotopic ratio between the HHC and LHS. Most rocks of the MCT zone have Nd isotopic ratios similar to those of the LHS, but very different from those of the HHC. The spatial patterns in the distribution of ε_Nd (0) value around the UMCT suggest no substantial structural mixing of the HHC and LHS during the UMCT activities in the Tertiary. A discontinuity in the spatial distribution of ε_Nd (0) values is laterally continuous along the UMCT throughout the Himalayas. These facts support the theory that the UMCT was originally a material boundary between the HHC and LHS, suggesting the MCT zone was mainly developed with undertaking a role of sliding planes during overthrusting of the HHC in the Tertiary.