Glassy Rinds Research Articles

Development of major element proxies for magmatic H2O content in oceanic basalts

Analysis of H2O concentrations in quenched glass has enabled significant improvements in our understanding of its role in mantle melting, magma differentiation, eruption dynamics, and the origin of mantle heterogeneity. Direct measurements of dissolved H2O in glass, however, are not always possible, and we lack robust methods of constraining magmatic H2O contents in aphyric, bulk rock samples that lack glass. Here, we present a major element hygrometer for mid-ocean ridge (MOR) and back-arc basin (BAB) basalt magmas based on the sensitivity of phenocryst phase assemblages to magmatic H2O contents, which translate into resolvable differences in liquid lines of descent (LLDs) as a function of magmatic H2O concentrations. Existing hygrometers lack sufficient resolution to be useful at the low H2O concentrations typical of MOR and BAB basalts (<1.0 wt%). We develop the major element proxy, Al2O3/FeO*(7.0) (fractionation-corrected to 7 wt% MgO), for determining magmatic H2O contents using cogenetic suites of oceanic basalts with well-defined LLDs and well-constrained H2O contents. H2O(7.0) positively correlates with Al2O3/FeO*(7.0) in the mid-ocean ridge basalt dataset, and this relationship is maintained in back-arc basin basalts with a broader range of water contents (up to 2.0 wt%). The main petrological control over this covariation is the role of H2O in suppressing plagioclase crystallization, while crystallization pressure and magmatic oxygen fugacity play lesser roles. Herein, we present an empirical model that uses Al2O3/FeO*(7.0) to estimate the magmatic water content in plagioclase-saturated oceanic basaltic magmas: H2O(7.0) = 1.109Al2O3/FeO∗(7.0) − 1.111. This model enables the estimation of magmatic H2O content using whole-rock major element data, which can be readily determined for aphyric or crystalline lavas that lack quenched glassy rinds, melt inclusions, or appropriate phenocryst assemblages.

Multi-tiered, disrupted crust of a sheet lava flow from the Diveghat Formation of Deccan Traps: Implications on emplacement mechanisms

The crust is a vesicular layer that caps the compact core of sheet lava flows. We describe for the first time, a crust composed of multiple layers (each distinguished by a chilled glassy rind) from the Diveghat Formation in the western Deccan Volcanic Province. The multiple layers of crust developed over a single compact core of a single sheet lava flow, are shown to have been sequentially deformed in multiple phases. This is interpreted to have resulted from the endogenous emplacement of lava in successive pulses (rather than as a continuous stream) during the extrusion of the sheet lava flow. This model has several implications on the mechanism of emplacement of sheet flows in continental flood basalt provinces.

Geochemical variability of major and trace elements and their role in abiotic stresses

Geochemical variability in rocks collected from profile section, drilling core and random samples from NIASM site has been studied using x-ray fluorescence spectrometry. The sum total of all the major oxide analyses show variable degrees of weathering and range from highly weathered samples to fresh rock. Analyses were used on an anhydrous basis in the SINCLAS programme to recalculate major oxides and normalise the geochemical analyses to 100. The programme also gives a rock name following the TAS diagram as well as fixes the Fe2O3: FeO ratio and then calculates the norm. Based on the normative mineralogy and SiO2 vs. Na2O+K2O content in the TAS diagram the present samples were classified into sub-alkaline basalts, basaltic andesites and andesite. Major oxide variation diagram in the Main Pit indicated silica and alumina oxides tend to accumulate towards the upper parts whereas TiO2 and FeO tend to be mobile in an oxidizing environment and get leached towards the lower parts of the weathering profile. The unique lobate geometry of the lava flow does not weather uniformly resulting in a rather jagged oxide variation diagram which is predominantly a function of the lobe geometry and the porosity and permeability of the lobe sub-units. The Mg# of the subalkaline basalt indicates moderately evolved magmas. The trace element Cu and Ni have a distinctly opposite signature as compared to Zr, V, Cr, Zn, and Co. Concentration of Ba, Sr and Rb are invariably high in the upper and lower parts of the Main Pit which is due to modal variations in the plagioclase content or due to the variable mobility of these elements in response to differential weathering across the Main Pit profile. Anomalous concentration of different major oxides and trace elements at 50 cm and 70 cm depth is due to presence of the thin clay horizons related to weathering of glassy rind of a single 20 cm thick pahoehoe toe. Such anomalous accumulations could suggest that the clays provide suitable sites for their adsorption or that considerable enrichment of these elements takes place due to deposition of soluble salts in an oxidizing environment.

Petrophysical variations within the basaltic lava flows from Tural-Rajawadi hot springs, Western India and their bearing on the viability of low-enthalpy geothermal systems

The Tural-Rajawadi hot springs (Temp. 30-62 °C) are a part of the West Coast Geothermal Province of India, located at the foothills of the Western Ghats in Maharashtra state. The hot springs manifest as several small pools/puddles along lineaments within the valley that exposes five (5) surficial lava flows belonging to the Cretaceous Deccan Traps. In addition, six (6) distinct lava flows were identified based on their internal structure, textural variations, vesiculation and presence of glassy rinds in the 203m deep exploratory borehole drilled at Rajawadi. These lavas are of pahoehoe sheet flow and rubbly pahoehoe morphologies. Temperature of water increased from 51 at the top to62 °C at the bottom. XRD studies of vesicle and joint fillings within the lava flows reveal peculiar assemblage of calcite, nontronite, clinopitolite, stilbite, apophyllite, cristobalite, etc. indicating mineralisation related to circulating hot geothermal fluids. The detailed petrophysical properties like textural diversity, crystal size distribution, specific gravity (2.766 to 2.999), resistivity (295.5 to 1790Ωm at 50Hz) and thermal conductivity were measured and their variations within two (2) thick subsurface lava flows is presented. These variations can be attributed to the crystallisation, cooling history, post emplacement hydrothermal alteration and mineralisation the lava flows have undergone. Understanding these variations is essential for assessing viability and commercial feasibility in using and exploiting low-enthalpy geothermal systems.

Composition of the slab-derived fluids released beneath the Mariana forearc: Evidence for shallow dehydration of the subducting plate

In cold subduction systems, the downgoing plate is thought to mostly dehydrate (≥90% water) beneath the arc front. However, the composition of aqueous fluids released from the shallow subducting slab is poorly constrained because the occurrence of melt inclusions is rare in forearcs. The Southeast Mariana Forearc Rift (SEMFR) is a recently discovered site where extensionally induced magmatism occurred in Pliocene time unusually close to the trench. SEMFR basalts sampled fluids derived from the subducted plate at ∼30 to 100 km deep. Here, we use SEMFR basalts to obtain new insights into the composition of shallow slab-derived fluids by evaluating major element, trace element and volatile contents of olivine-hosted melt inclusions and glassy rinds. Olivine-hosted melt inclusions and their host glassy rinds both contain at least ∼2 wt% H2O, representing minimum estimates for magmatic water content. Melt inclusions have the highest volatile and alkali ratios (i.e., H2O/Ce=6000–19000; Rb/Th=200–2600; Cs/Th=0.4–20) yet recorded in glasses from subduction zones (arc magmas have mean H2O/Ce<2700; Rb/Th<40; Cs/Th<2). Our results indicate that shallow slab-derived fluids are water- and alkali-rich as compared to the deeper fluids released beneath the arc system. Shallow subduction outfluxes peak at ∼70–80 km slab depth, demonstrating that significant slab dehydration occurs beneath the forearc.

Cooling rates of mid-ocean ridge lava deduced from clinopyroxene spherulites

We present a compositional analysis of spherulites and their surrounding groundmass in basaltic andesite lava samples collected from the Pacific–Antarctic Ridge. The spherulites consist of radiating dendritic branches of clinopyroxene, and contain abundant vesicles that are stretched parallel to the branches. Stretched vesicles require that the spherulites grew when the groundmass was liquid, and thus while it was hotter than its glass transition. Although chemical components incompatible in clinopyroxene should have been enriched in the melt by spherulite growth, no compositional variations occur at a scale larger than the 10μm-wide slit aperture used for laser ablation ICP-MS. The absence of large-scale gradients implies that the spherulites grew faster than the elements could diffuse ~10μm. Because diffusion is temperature controlled, cooling must have been rapid enough to prevent movement. If the spherulites nucleated when the melt was at between 1380 and 1223K (i.e., undercooled by 0 to 157K), then the constant concentrations of Li, Na, and K, the fastest diffusing elements analyzed, require temperature to have cooled faster than 106–7Kmin−1. If, instead, the spherulites nucleated at undercoolings of 357 to 465K (or as cold as 915K, equal to the glass transition temperature), then the cooling rate could have been as slow as those inferred for glassy rinds on submarine lavas by relaxation geospeedometry. At all possible cooling rates, however, the spherulites must have grown as fast as ~103–5μms−1 to reach their final sizes of 1.4 to 3.2mm. We speculate that the rapid growth resulted from low kinetic barriers to clinopyroxene growth in the basaltic andesite melt.

Nature and distribution of slab‐derived fluids and mantle sources beneath the Southeast Mariana forearc rift

Subduction zone magmas are produced by melting depleted mantle metasomatized by fluids released from the subducted slab. In most subduction zones, formation of backarc basin (BAB) and arc magmas depletes the mantle source toward the trench, resulting in more depleted mantle beneath the forearc. Slab‐derived fluids are aqueous beneath the forearc where the slab dehydrates, and the deeper subduction component is increasingly dominated by sediment melt at ≥100 km depth. In this study, we present new data for the Southeast Mariana forearc rift (SEMFR), an unusual region of forearc igneous activity, where 2.7–3.7 Ma lavas were recovered by Shinkai 6500 diving and dredged during the TN273 Thomas Thompson cruise. SEMFR is divided into SE (near the trench) and NW (near the arc) sectors. NW SEMFR lavas and glassy rinds are more depleted in melt‐mobile elements (e.g., Nb and Yb) and more enriched in fluid‐mobile elements (e.g., Cs, Rb, and Ba). SEMFR lavas were produced by partial melting of a BAB‐like mantle source, metasomatized by sediment melt and aqueous fluids released from dehydrating the subducted oceanic crust, and the forearc serpentinized peridotites. Evidence of sediment melt, even in SE SEMFR lavas, could be explained by inheritance of BAB‐like Th/Nb in the SEMFR mantle source. Geochemical mapping demonstrates that the subduction components and mantle depletion increase towards the arc, suggesting (i) input of a less‐depleted mantle beneath SE SEMFR that flowed toward the arc and (ii) aqueous slab‐derived fluids become increasingly important at ∼50–100 km depth, reflecting that phengite and barite from the downgoing plate and forearc serpentinite broke down beneath the arc volcanoes.

An experimental study of hydromagmatic fragmentation through energetic, non-explosive magma–water mixing

In this paper we report the first experimental investigation of non-explosive hydromagmatic fragmentation during energetic mixing with water. We mix magma and water by two methods: (1) pouring a basaltic melt between two converging water sprays; and (2) jetting basaltic melt at high pressure (3 MPa) through a nozzle into a tank of stagnant water. These experiments involved shear at relative velocities of ~ 5–16 m/s and vigorous mixing for less than a second, providing sufficient time for glassy rinds to grow but insufficient time for clot interiors to cool. In resulting fragments, we examined the gross morphology, which reflects fluid deformation during mixing, and surface textures, which reflect the growth and disruption of glassy rinds. We find major differences in both fragment morphology and surface texture between experiments. Water-spray experiments produced Pele's hair, thin bubble shards, melt droplets, and angular, fracture-bound droplet pieces. Melt-jet experiments produced mostly coarse (> 1 mm diameter), wavy fluidal fragments with broken ends. Fluidal surfaces of fragments produced by water-spray experiments were generally shiny under reflected light and, in microscopic examination, smooth down to micron scale, implying no disruption of glassy rinds, except for (a) rare flaking on Pele's hair that was bent prior to solidification; or (b) cracking and alligator-skin textures on segments of melt balls that had expanded before complete cooling. In contrast, textures of fluidal surfaces on fragments produced by melt-jet experiments are dull in reflected light and, in scanning electron images, exhibit ubiquitous discontinuous skins (“rinds”) that are flaked, peeled, or smeared away in stripes. Adhering to these surfaces are flakes, blocks, and blobs of detached material microns to tens of microns in diameter. In the water-spray fragments, we interpret the scarcity of disrupted surface rinds to result from lack of bending after surfaces formed. In the melt-jet fragments, the ubiquity of partially detached rinds and rind debris likely reflects repeated bending, scraping, impact, and other disruption through turbulent velocity fluctuations. When extrapolated to jets of Surtseyan scale, where velocity fluctuations reach tens of meters per second and turbulent mixing persists for tens of seconds, rind disintegration could fragment a large fraction of the erupted material.

Enumeration of Archaea and Bacteria in seafloor basalt using real-time quantitative PCR and fluorescence microscopy

A SYBR Green real-time quantitative PCR (Q-PCR) assay for the detection and quantification of Bacteria and Archaea present in the glassy rind of seafloor basalts of different ages and water depths is presented. Two sets of domain-specific primers were designed and validated for specific detection and quantification of bacterial and archaeal 16S rRNA genes in DNA extracted from basaltic glass. Total cell numbers were also estimated by fluorescence microscopy analysis of SYBR Gold-stained samples. The results from the two different approaches were concurrent, and Q-PCR results showed that the total number of cells present in basalts was in the range from 6 x 10(5) to 4 x 10(6) cells g(-1) basaltic glass. Further, it was demonstrated that these cells were almost exclusively from the domain Bacteria. When applying the same methods on samples of different ages (22 years-0.1 Ma) and water depths (139-3390 mbsl), no significant differences in cell concentrations or in the relative abundance of Archaea and Bacteria were detected.

Morphological Biosignatures in Early Terrestrial and Extraterrestrial Materials

Biosignatures in early terrestrial rocks are highly relevant in the search for traces of life on Mars because the early geological environments of the two planets were, in many respects, similar and, thus, the potential habitats for early life forms were similar. However, the identification and interpretation of biosignatures in ancient terrestrial rocks has proven contentious over the last few years. Recently, new investigations using very detailed field studies combined with highly sophisticated analytical techniques have begun to document a large range of biosignatures in Early Archaean rocks. Early life on Earth was diversified, widespread and relatively evolved, but its traces are generally, but not always, small and subtle. In this contribution I use a few examples of morphological biosignatures from the Early-Mid Archaean to demonstrate their variety in terms of size and type: macroscopic stromatolites from the 3.443 Ga Strelley Pool Chert, Pilbara; a meso-microscopic microbial mat from the 3.333 Ga Josefsdal Chert, Barberton; microscopic microbial colonies and a biofilm from the 3.446 Ga Kitty’s Gap Chert, Pilbara; and microscopic microbial corrosion pits in the glassy rinds of 3.22–3.48 Ga pillow lavas from Barberton. Some macroscopic and microscopic structures may be identifiable in an in situ robotic mission to Mars and in situ methods of organic molecule detection may be able to reveal organic traces of life. However, it is concluded that it will probably be necessary to return suitably chosen Martian rocks to Earth for the reliable identification of signs of life, since multiple observational and analytical methods will be necessary, especially if Martian life is significantly different from terrestrial life.

Interpreting H2O and CO2 Contents in Melt Inclusions: Constraints from Solubility Experiments and Modeling

Research Article| January 01, 2008 Interpreting H2O and CO2 Contents in Melt Inclusions: Constraints from Solubility Experiments and Modeling Gordon Moore Gordon Moore Department of Chemistry & Biochemistry, Arizona State University Tempe, Arizona, 85287-1604, U.S.A., gordon.moore@asu.edu Search for other works by this author on: GSW Google Scholar Reviews in Mineralogy and Geochemistry (2008) 69 (1): 333–362. https://doi.org/10.2138/rmg.2008.69.9 Article history first online: 13 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Gordon Moore; Interpreting H2O and CO2 Contents in Melt Inclusions: Constraints from Solubility Experiments and Modeling. Reviews in Mineralogy and Geochemistry 2008;; 69 (1): 333–362. doi: https://doi.org/10.2138/rmg.2008.69.9 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search Due to their volatile nature and low solubility in silicate melts at the surface of the Earth, the direct measurement of the volatile components H2O and CO2 in magmatic systems is dependent on the presence of glass inclusions trapped in a crystal host prior to eruption. These inclusions, along with the glassy rinds of submarine pillow lavas, represent one of the few windows researchers have into the pre-eruptive chemical characteristics of magmatic systems (e.g., Anderson et al. 2000; Roggensack 2001a; Metrich and Wallace 2008). This information is critically important, as it is volatile exsolution... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Ice-rich terrain in Gusev Crater, Mars?

The morphology of materials on the floor of Gusev Crater (14° S, 175° W), Mars, imply a history of volcanism and subsequent removal of an ice-rich deposit. Fluid lava flows observed in the western portion of Gusev Crater paradoxically terminate in a steep, thick ( < 60 m ) flow front adjacent to hummocky terrain. The hummocky terrain is morphologically similar to deglaciated terrain on Earth, generated when glacial debris are left behind after the glacier has retreated. We propose the following scenario for the floor of Gusev Crater. First, ice-rich material was deposited adjacent to Thira Crater. Second, fluid lavas were emplaced and ponded against the ice-rich deposits. At some later time, the ice within the deposit sublimated, leaving hummocky terrain. Current age estimates for the Gusev flows are Hesperian, suggesting that the ice removal occurred in the upper Hesperian or more recently. If this hypothesis is correct, quench features (glassy rinds, columnar jointing) should be observed at the lava flow margin; the hummocky deposit should be poorly sorted, angular debris.

Generation of fine hydromagmatic ash by growth and disintegration of glassy rinds

The deposits of mafic hydromagmatic eruptions are more fine grained and variable in vesicularity than dry magmatic deposits. Blocky, equant shapes of many hydromagmatic clasts also contrast with droplet, thread, and bubble wall morphology of dry magmatic fragments. Small (&lt;∼180 μm), blocky hydromagmatic pyroclasts have traditionally been interpreted to result from discrete vapor explosions, although such explosions tend to occur only under certain conditions. This paper considers a process of hydromagmatic ash formation that involves repeated growth and disintegration of glassy rinds on pyroclast surfaces as they deform within turbulent flows. This process, termed “turbulent shedding”, may occur during the expansion phase of vapor explosions or during turbulent but nonexplosive mixing of magma with water, steam, or water sprays. The occurrence of turbulent shedding and the resulting fragment sizes depend on the timescale for rind growth and the timescale between disturbances that remove or disintegrate glassy rinds. Turbulent shedding is directly observable in some small littoral jets at Kilauea. Calculations suggest that, in the presence of liquid water or water sprays, glassy rinds having a thickness of microns to millimeters should form in milliseconds to seconds. This is similar to the timescale between turbulent velocity fluctuations that can shred lava globules and remove such rinds. The fraction of a deposit consisting of fine ash should increase with the duration of this process: Large‐scale Surtseyan jets generate hundreds or thousands of shedding events; bubble bursts or tephra jets at Kilauea's coast may produce only a few.

Flaviramulus basaltis gen. nov., sp. nov., a novel member of the family Flavobacteriaceae isolated from seafloor basalt

Four yellow-pigmented, Gram-negative, motile strains were isolated from the glassy rind of submarine basaltic lava from the Jan Mayen area of the Norwegian/Greenland Sea at a depth of 1300 m below sea level. The four strains had identical 16S rRNA gene sequences and were indistinguishable in all phenotypic and chemotypic tests performed, indicating that they belonged to the same species. The strains had an obligately aerobic chemo-organotrophic metabolism. The strains were capable of growth at temperatures between -2 and 34 degrees C, at pH between 6.5 and 8.6, and at sea salt concentrations between 3 and 60 g l(-1). The strains were able to utilize organic acids, amino acids and sugars but not alcohols; they were also capable of hydrolysing a wide range of macromolecules. The predominant fatty acids were 15 : 0 iso, 15 : 1 iso, 15 : 0 iso 3-OH and 17 : 0 iso 3-OH. The mean DNA G+C content of the strains was 31.4 mol%. 16S rRNA gene sequence analysis indicated that the strains were affiliated to the genera Gaetbulibacter and Algibacter. However, phenotypic characteristics, especially aerobic metabolism, suggested that the strains should be placed within a new genus. On the basis of the polyphasic characterization of the four strains, it is suggested that the strains be included in the family Flavobacteriaceae as representatives of a novel species in a new genus, for which the name Flaviramulus basaltis gen. nov., sp. nov. is proposed. The type strain is H35(T) (=CIP 109091(T)=DSM 18180(T)).

Breadcrust bombs as indicators of Vulcanian eruption dynamics at Guagua Pichincha volcano, Ecuador

Vulcanian eruptions are common at many volcanoes around the world. Vulcanian activity occurs as either isolated sequences of eruptions or as precursors to sustained explosive events and is interpreted as clearing of shallow plugs from volcanic conduits. Breadcrust bombs characteristic of Vulcanian eruptions represent samples of different parts of these plugs and preserve information that can be used to infer parameters of pre-eruption magma ascent. The morphology and preserved volatile contents of breadcrust bombs erupted in 1999 from Guagua Pichincha volcano, Ecuador, thus allow us to constrain the physical processes responsible for Vulcanian eruption sequences of this volcano. Morphologically, breadcrust bombs differ in the thickness of glassy surface rinds and in the orientation and density of crack networks. Thick rinds fracture to create deep, widely spaced cracks that form large rectangular domains of surface crust. In contrast, thin rinds form polygonal networks of closely spaced shallow cracks. Rind thickness, in turn, is inversely correlated with matrix glass water content in the rind. Assuming that all rinds cooled at the same rate, this correlation suggests increasing bubble nucleation delay times with decreasing pre-fragmentation water content of the melt. A critical bubble nucleation threshold of 0.4-0.9 wt% water exists, below which bubble nucleation does not occur and resultant bombs are dense. At pre-fragmentation melt H2O contents of >~0.9 wt%, only glassy rinds are dense and bomb interiors vesiculate after fragmentation. For matrix glass H2O contents of ≥1.4 wt%, rinds are thin and vesicular instead of thick and non-vesicular. A maximum measured H2O content of 3.1 wt% establishes the maxi- mum pressure (63 MPa) and depth (2.5 km) of magma that may have been tapped during a single eruptive event. More common H2O contents of ≤1.5 wt% suggest that most eruptions involved evacuation of ≤1.5 km of the conduit. As we expect that substantial overpressures existed in the conduit prior to eruption, these depth estimates based on magmastatic pressure are maxima. Moreover, the presence of measurable CO2 (≤17 ppm) in quenched glass of highly degassed magma is inconsistent with simple models of either open- or closed-system degassing, and leads us instead to suggest re-equilibration of the melt with gas derived from a deeper magmatic source. Together, these observations suggest a model for the repeated Vulcanian eruptions that includes (1) evacuation of the shallow conduit during an individual eruption, (2) depressurization of magma remaining in the conduit accompanied by open- system degassing through permeable bubble networks, (3) rapid conduit re-filling, and (4) dome formation prior to the subsequent explosion. An important part of this process is densification of upper conduit magma to allow repressur- ization between explosions. At a critical overpressure, trapped pressurized gas fragments the nascent impermeable cap to repeat the process.

The occurrence, vesiculation, and solidification of dense blue glassy pahoehoe

Dense blue glassy pahoehoe occurs on the lower Kalapana flow field of Hawaii as limited breakouts at the base of tumuli and pressure rises or in localized depressions. Dense blue glassy pahoehoe differs from the more abundant spongy pahoehoe in that it has a distinctive silvery blue color, a poorly vesiculated glassy outer selvage, and lower extrusion temperature (∼10 °C cooler). The color of blue glassy pahoehoe may be attributed to the concentration of iron and magnesium at the surface, whereas the golden color of the spongy pahoehoe is possibly the result of fluorine enrichment. The outer margin of the dense pahoehoe consists of three layers, developed in response to an increased cooling rate toward the surface. The outer glass selvage is less than 1 cm thick containing minor amounts of chromite, olivine, plagioclase, and augite. Inside the glassy rind is a ∼5 mm thick transition zone characterized by increasing numbers and sizes of quenched plagioclase crystals and augite clusters. The interior of dense blue glassy pahoehoe is largely crystalline. Crystal size distribution (CSD) theory demonstrates that the formation and textures of plagioclase, pyroxene, and oxides are related to the cooling rates, growth rates ( G), and nucleation rates ( J) present in each layer of the dense pahoehoe. In contrast to the outer selvage, the crystalline interior typically contains numerous large coalesced vesicles as compared to the evenly vesiculated spongy pahoehoe. The enigmatic contrast between the poorly vesicular outer glassy selvage and the vesicular interior has been attributed to both lower and higher vapor contents of the dense pahoehoe. The higher density, lower extrusion temperatures, degassed outer glassy selvage, well-developed crystalline interior, and fluorine-deficient surface of dense blue glassy, as compared to spongy pahoehoe, indicate that blue glassy pahoehoe was stagnant long enough to exsolve considerable vapor in the lava tube system before extrusion.

Color atlas of low‐temperature alteration features in basalts from the Southern East Pacific Rise

Digital photomicrographs from thin sections of basalts dredged on the eastern flank of the Southern East Pacific Rise are used to illustrate the features of low‐temperature alteration in oceanic crust. The rocks have been altered under seawater‐dominated conditions and show typical precipitation sequences of secondary minerals covering crustal ages from 0 to 9 Ma. Fe‐oxyhydroxide and celadonite disseminated in the groundmass or as void and vein fillings are the first alteration products formed under oxidizing conditions. They are partly replaced by saponite under more reducing conditions. In older rocks phillipsite joints the alteration assemblage in fractures within the glassy rinds, sometimes occurring as a mixture with celadonite. Microprobe analyses of secondary minerals are given, and the location of all analyzed points are indicated in each photomicrograph. Additionally an overview of all thin sections with the location of each photomicrograph is presented in the appendix.

Relationship between submarine MORB glass textures and atmospheric component of MORBs

We investigated the relationship between noble gas signatures and sample textures among glassy fragments. Glassy MORB fragments were classified based on their texture. By handpicking under a binocular, they were divided into three groups: (1) heterogeneous “outer” part, (2) homogeneous “middle” part, and (3) “inner” neighbors of the transition zone. Their noble gas signatures were analyzed by the step heating method. The state of homogeneities of the samples identified under a microscope was confirmed semi-quantitatively by measuring refractive indices of the fragments. The “outer” heterogeneous part characterized by devitrified and/or color changed regions was significantly affected by the atmospheric component as indicated by the relatively large amount of gases released with almost atmospheric 40Ar/ 36Ar and 20Ne/ 22Ne in the 400 °C fraction. The transition “inner” part of glassy rind adjacent to the crystallized interior which is characterized by the appearance of quenched microcrystals has lost the magma-derived component and is contaminated by some kinetic processes: it showed a small amount of noble gases except for Ne with a low 20Ne/ 22Ne ratio. The homogeneous “middle” part turns out to be least affected by contamination and is regarded to mostly retain magma-derived noble gas signatures. It is characterized by the most abundant He and the highest 40Ar/ 36Ar as a total ratio with similar values in both the middle and the high temperature fractions. This part is also characterized by lower refractive index values and a small deviation compared to the nonhomogeneous parts. Hence, for noble gas analyses of MORB glasses, it is recommended to select homogeneous “clear” glasses by excluding color changed and/or devitrified weathering part and quenched microcrystals.

E-MORB glasses from the Gakkel Ridge (Arctic Ocean) at 87°N: evidence for the Earth's most northerly volcanic activity

During the ARCTIC '91 expedition aboard RV Polarstern (ARK VIII/3) to the Central Arctic Ocean, a box corer sample on the Gakkel Ridge at 87°N and 60°E yielded a layer of sand-sized, dark brown volcanic glass shards at the surface of the sediment core. These shards have been investigated by petrographic, mineralogical, geochemical and radiogenic isotope methods. The nearly vesicle-free and aphyric glass shards bear only minute microphenocrysts of magnesiochromite and olivine (Fo 88–89). Most glasses are fresh, although some show signs of incipient low-temperature alteration. From their shapes and sizes, the glass shards most likely formed by spalling of glassy rinds of a nearby volcanic outcrop. Geochemically, the glasses are relatively unfractionated tholeiites with E-MORB trace element compositions. Thus, they are quite similar to the previously investigated ARK IV/3-11-370-5 basalts from 86°N [1]. The Nd and Sr isotopic ratios of PS 2167-2 glasses are significantly lower than for ARK IV/3-11-370-5 basalts and suggest an isotopically heterogeneous mantle source of Gakkel Ridge MORB between 86° and 87°N. The positive Δ-8/4 Pb value (∼16) and high 87Sr/ 86Sr ratio (0.70270), found for PS 2167-2 glasses are similar to that of ARK IV/3-11-370-5 basalts and show the influence of the DUPAL isotopic anomaly in the high Arctic mantle. These results argue against the presence of an `anti-DUPAL anomaly' in the mantle below the North Pole region and simple models of whole-mantle convection.

Tektite cooling rates: Calorimetric relaxation geospeedometry applied to a natural glass

Enthalpy relaxation of a reheated natural tektite glass has been used to determine the coolingrate of the glass in nature. The relaxation parameters for the tektite samples were obtained by internal calibration through a series of thermal treatments applied to the sample following the first excursion into the liquid field. The tektite fulfills the criteria required for the use of calorimetric relaxation geospeedometry in that it is chemically homogeneous and pristine as well as being metastable above the glass transition during experimental reheating. Using calorimetric relaxation geospeedometry, the thermal history of the tektite has been constrained to cooling rates of a few °C s −1 at temperatures in the region of the glass transition (ca. 780 °C). The tektite, a spheroid of ~40 mm diameter, exhibits a higher cooling rate on the exterior (10 °C s −1) than the more massive interior (1 °C s −1). This difference in cooling rate is compatible with cooling of the exterior by surface radiation and a cooling of the interior by conduction through a glassy rind.