Glassy Margins Research Articles

Fluidal-clast breccia generated by submarine fire fountaining, Trooper Creek Formation, Queensland, Australia

A distinctive monomictic breccia, composed of fluidal and blocky basaltic andesite clasts, occurs in a Cambro-Ordovician submarine volcanic succession in northern Queensland, Australia. Associated with this fluvial-clast buccia facies are coherent facies and coarse and fine breccia facies of the same composition. The fluidal-clast breccia facies is internally massive and >250 m thick, varying only in the ratio of fluidal clasts to blocky clasts. Fluidal clasts range in size from 2 cm to 170 cm, and have moderately to highly vesicular cores and thick (up to 1 cm), non-vesicular, formerly glassy rims. Blocky clasts are highly vesicular to non-vesicular, <2 cm, angular, dominantly equant or splintery in shape and identical in composition to the fluidal clasts. The fluidal clasts strongly resemble subaerial volcanic bombs and are interpreted to be the products of submarine fire fountaining of relatively low-viscosity lava. The blocky clasts were mainly derived from disintegration of the fluidal clasts, by means of quench fragmentation. Coherent basaltic andesite intercalated with the fluidal-clast breccia represents co-genetic lavas, dykes and irregular shallow intrusions. The coarse and fine breccia facies is very thickly bedded, monomictic (basaltic andesite), poorly sorted and clast supported. This facies is interpreted to have been generated by periodic gravitational collapse of unstable accumulations of the fluidal-clast breccia facies. Subaqueous fire-fountain breccias are distinguished from subaerial fire-fountain breccias by thick glassy margins on fluidal clasts, the lack of welding and agglutination, and the distinctive association of highly vesicular, fluidal clasts with non-vesicular, angular, blocky clasts. Recognition of submarine fire-fountain breccias in volcanic successions constrains the eruption style, proximity (tens of metres) to source and environment of deposition.

Variable Ti-content and grain size of titanomagnetite as a function of cooling rate in very young MORB

Transmission electron microscopy observations and rock magnetic measurements on a pillow from the ‘New Flow’, extruded in 1993 at the Juan de Fuca Ridge, demonstrate that Ti-content in large (up to 40 μm) titanomagnetite grains varies as a function of the cooling rate. Large grains in the interior of the pillow have a narrow composition range of approximately x=0.6, where x represents the ulvöspinel content in the solid solution series with magnetite. In contrast, the titanomagnetite grains near the pillow rim are progressively smaller and have a broad composition range (average x=∼0.45). Within 0.5 cm of the rim a significant portion of the grains is single domain (SD) to superparamagnetic (SP) and appears to have little or no Ti. This lower Ti-content provides a ready explanation for the higher Curie temperature near the glassy margin of the pillow. Moreover, determination of the oxidation state of the titanomagnetite does not show low temperature oxidation anywhere within the pillow, indicating no rapid alteration to titanomaghemite as previously suggested for this very young mid-ocean ridge basalt (MORB) in order to explain the Curie temperatures. Submicrometer titanomagnetite in interstitial glass in the interior of the pillow also shows variable x values. These grains make contributions to susceptibility that show up as high (up to ∼580°C) Curie temperatures in thermomagnetic analyses and they appear to be SD to SP. The variation of x values for the titanomagnetite in the MORB is consistent with thermodynamic and kinetic considerations, because crystallization of titanomagnetite in the MORB is controlled by processes depending on cooling rate and crystal-melt fractionation.

Facies architecture and origin of a submarine rhyolitic lava flow‐dome complex, Ponza, Italy

The Islands of Ponza and Gavi (western Pontine Archipelago, Italy) preserve parts of a subaqueously emplaced, high‐silica (75–77% SiO2) rhyolitic lava flow that overlies the margins of three older domes. Exposures of the lava alone cover 24 km2, and with a thickness of 150 m exposed in the cliffs, the preserved volume of the lava flow is 3.6 km3, and the original volume is likely to have exceeded 7 km3. The lava flow consists of coherent obsidian and several hyaloclastite facies, including in situ and clast‐rotated breccias and sandstones, which all exhibit gradational relationships. The coherent obsidian occurs as separate domains and is often flow banded. Although flow folds occur, the banding is generally subhorizontal, dipping gently to the NNE. The hyaloclastite, representing 90% of the lava by volume, has a diffuse layering varying from 0.5 to 2 m in thickness. This layering also dips gently to the NNE and is marked in places by alternation of some coherent horizons, coarse and finely fragmented in situ hyaloclastite, pumiceous hyaloclastite, and clast‐rotated hyaloclastite. The layering is laterally continuous and consistent in orientation over most of the 13 km length of Ponza: the subhorizontal orientation of the flow banding and presence of resedimented deposits in the upper parts of the sequence indicate that the unit is a lava flow. The resedimented deposits comprise both proximal and distal deposits of debris flows and turbidity currents initiated by gravitational instability of the upper and marginal parts of the lava flow and include stratified monomictic obsidian breccias and bedded vitric siltstones and sandstones. The lava flow has been intruded and crosscut by rhyolitic bodies of various ages, including discordant, coherent to internally fractured contemporaneous rhyolite bodies, and late, penecontemporaneous dikes with glassy margins and alteration haloes.

Evidence for microbial activity at the glass-alteration interface in oceanic basalts

A detailed microbiological and geochemical study related to the alteration of basaltic glass of pillow lavas from the oceanic crust recovered from Hole 896A on the Costa Rica Rift (penetrating 290 m into the volcanic basement) has been carried out. A number of independent observations, pointing to the influence of microbes, may be summarized as follows: (1) Alteration textures are reminiscent of microbes in terms of form and shape. (2) Altered material contains appreciable amounts of C, N and K, and the N/C ratios are comparable to those of nitrogen-starved bacteria. (3) Samples stained with a dye (DAPI) that binds specifically to nucleic acids show the presence of DNA in the altered glass. Further, staining with fluorescent labeled oligonucleotide probes that hybridize specifically to 16S-ribosomal RNA of bacteria and archaea demonstrate their presence in the altered part of the glass. (4) Disseminated carbonate in the glassy margin of the majority of pillows shows δ 13C values, significantly lower than that of fresh basalt, also suggests biological activity. The majority of the samples have δ 18O values indicating temperatures of 20–100°C, which is in the range of mesophilic and thermophilic micro-organisms.

Interlayered sill-sediment structure at the base of a Miocene basaltic andesite sheet intrusion, Rebun Island, Hokkaido, Japan.

A unique, interlayered sill-sediment structure occurs along the lower contact of a Miocene basaltic andesite sheet intrusion on Rebun Island, northern Hokkaido, Japan. The intrusion is a sill or laccolith 1,200×800 m across and more than 150 m thick, which has intruded alternating diatomaceous mudstone and siltstone. The interlayered sill-sediment structure is 25 m high and 70 m wide in cross section and comprises alternating fluidally shaped basaltic andesite sills and sediment layers. Each basaltic andesite sill forms a thin wedge-like projection, 10-20 m long and 30-60 cm thick, which has quenched glassy margins. The sediment layers are 10-25 m long and 30-100 cm thick, and consist of massive diatomaceous mudstone. Contacts between the basaltic andesite sills and sediment layers are mostly sharp. In places, the margins of the basaltic andesite sills are brecciated, forming peperite zones 50×100 cm wide. The peperite consists of polyhedral clasts of basaltic andesite 5-20 cm across, separated by massive, indurated mudstone. Some large clasts show a jigsaw-fit texture. The interlayered sill sediment structure is inferred to have formed by parallel, sheet-like injections of basaltic andesite magma along bedding planes of wet and poorly consolidated sediment in an upper bathyal marine environment (several hundred metres deep). During parallel injections of the basaltic andesite magma into the wet sediment, a large amount of water vapour was generated in the wet sediment, and thin water vapour films were maintained continuously at the magma-sediment interface, insulating the magma from the host sediment. Following injections of the magma into the wet sediment, the temperature of the magma declined, so that the water-vapour films on the magma gradually disappeared. The breakdown of the insulating vapour films at the magma-sediment interface resulted in quench fragmentation of the magma, generating small zones of peperite around the basaltic andesite sills.

The eclogitized pillows of the Betic Ophiolitic Association: relics of the Tethys Ocean floor incorporated in the Alpine chain after subduction

ABSTRACTIn the volcanic sequence of the Betic Ophiolitic Association (BOA) volcanic structures and textures are preserved in spite of being metamorphosed to the edogite facies. The original quenched glassy margins of the pillows and lava flows are still recognizable by the darker colour, fine‐grained textures and scarcity of phenocrysts. The BOA eclogitized pillows have chemical compositions very similar to the basalts enriched in LIL elements erupted nowadays at the mid‐oceanic ridges. Magmatism which generated the BOA most likely began under continental rift conditions at the Triassic‐Jurassic boundary and continued under ocean‐floor extensional conditions during the lower and middle Jurassic. In age and petrological characteristics this magmatism is equivalent to that of the western Tethys ophiolites. During the Late Cretaceous, due to the collision of African and Iberian Plates, the BOA ophiolites were subducted and underwent a metamorphism in the eclogite fades whose climax in the Lugros outcrop can be estimated at 650–700°C and about 20 kb.

Styles of Eruptive Activity on Intraplate Volcanoes in the Society and Austral Hot Spot Regions: Bathymetry, Petrology, and Submersible Observations

Submersible observations, carried out in the Society and Austral hot spot regions (South Pacific) during the Teahitia II cruise (1988–1989), enable us to propose a model relating The style of volcanism to the depth of eruption on large intraplate seamounts. (1) The bases of volcanic edifices are characterized by broad flat flows formed during a high rate of discharge of a very fluid magma on the sea floor. (2) The edifices' flanks deeper than 500 m are mainly made up of bulbous, tubular pillows and large drained lava tunnels occurring on slopes steeper than 60°–70°. These lava flows occur on lateral volcanic vents, about 200–300 m high, along linear rift zones which are formed on the flank of the major edifices. (3) Volcanic ejecta due to hydromagmatic eruptions occur at shallow depths (&lt;500 m). Mixed explosive and quiet eruptions are marked by volcanic ejecta interbedded with pillows and intrusives (sills and dikes). (4) Volcanic shards, produced by the shattering of the vesiculated glassy margins of the flows, are observed on the edifices' slopes at all depths (from 3000 m depth to the sea surface), concentrated between pillows or covering large areas, and forming thin indurated layers of mixed shard, sediment, and hydrothermal deposits. On the basis of their morphological appearances, the volcanic edifices scattered on the hot spot‐affected seafloor are divided into two groups: (1) The small edifices (less than 500 m tall) consist of conical, linear, and truncated circular abyssal hills made up of old mid‐ocean ridge basalt (MORB) type basalts (depleted in K2O) inherited from volcanic activity near the East Pacific Rise and more recent conical vents made up of alkali‐enriched lavas extruded during “hot spot” magmatism. (2) The larger edifices (&gt;500 m in height) are individual conical features built at a high rate of volcanic discharge localized along radial linear eruptive fissures called rift zones. The slopes located between the rift zones are covered by talus material are limited, and unimportant mass‐wasting (sector collapse) features were observed on the edifices. The conical‐shaped features are the primary morphology of the intraplate volcanoes formed during edifice growth which is closely linked to the hydrostatic pressure of the magmatic column. The mean values of the slopes (11°–17°) of intraplate seamounts are the same as those of subaerial intraplate shield volcanoes, when the seawater hydrostatic pressure is taken into account.

Water, carbon dioxide, and hydrogen isotopes in glasses from the ca. 1340 A.D. eruption of the Mono Craters, California: Constraints on degassing phenomena and initial volatile content

Water and molecular carbon dioxide concentrations, the speciation of water, and hydrogen isotope ratios have been measured in a series of obsidians from the ca. 1340 A.D. eruption of the Mono Craters chain in central California. Obsidians were collected from domes and pyroclastic deposits. The maximum water contents of the obsidian clasts from the pyroclastic deposits generally declined as the eruption proceeded, but at most stratigraphic levels the obsidians display a range of water contents. The water contents of the obsidians from domes are lower than those from the pyroclastic beds. The D/H ratio varies monotonically with the total water content. The proportions of water dissolved in these glasses as hydroxyl groups and as molecules of water vary smoothly with total water content. Dissolved molecular carbon dioxide contents are low, but roughly proportional to total water content. The proportions of molecular water and hydroxyl groups in the obsidians from the pyroclastic phase of this eruptive cycle indicate temperatures of 500–600°C, much lower than magmatic temperatures. We propose that these glasses are samples of the cool glassy margins (e.g., roof, walls) of the feeder systems of the volcanoes caught up in the explosive eruption. The water contents of the glasses are interpreted in terms of the pressure (i.e., depth) at which they were frozen into the glassy rind. Modelling of the hydrogen isotopes and CO 2/H 2O ratios of the obsidians suggests that degassing of the magma initially approached that of a closed system, but changed to an open system when explosive eruptions ceased and the domes were extruded quiescently. These models also suggest that the parental rhyolitic magmas from which the erupted glasses were derived contained significant amounts of both water and carbon dioxide and imply that a vapor phase is present throughout the evolution of silicic magmas. Basaltic magma underplating rhyolitic magma in a crustal magma chamber could be the source of the CO 2.

Massive and brecciated dikes in the McDougall and Despina faults, Noranda, Quebec, Canada

Abstract The McDougall and Despina faults of the central Noranda volcanic complex cut subaqueous volcanic rocks in the Archean Abitibi greenstone belt. Rhyodacitic dikes occupy the faults, along with lesser amounts of andesitic, dioritic and a mixed basaltic-rhyodacitic dike. There are two types of rhyodacitic dikes, one massive the other brecciated. Massive dikes are homogeneous and spherulitic; brecciated dikes are dominated by curved, angular fragments with a few vesicles. Both occur either alone or together in the faults. Where the two occur together they are commonly interlayered in concentric layered lobes. The faults are interpreted as fissures for pulses of nonexplosive rhyodacitic lava. Many intrusive pulses interacted with an external fluid which occupied the faults. This interaction resulted in brecciated, glassy margins and massive, crystalline pulse interiors. Magma/fluid interaction is thus invoked as the mechanism responsible both for dike brecciation and the concentric layering. The dikes are considered as intrusive analogs of extrusive rhyolitic lobe lava observed in Iceland and in Noranda.

Pipe vesicles—An alternate model for their origin

Most pipe vesicles occur near the base of basaltic flows where they are interpreted to be traces left by ascending gas bubbles. Some basaltic pillows also contain pipe vesicles, but in these pillows pipe vesicles have a radial distribution, indicating that buoyancy cannot be a factor in their formation. Pipe vesicles do not extend through chilled glassy margins at the base of flows or rims of pillows, but rather, they form only where there has been significant crystallization, indicating that the gas forming these vesicles is exsolved from the lava and is not derived from an external source. Pipe vesicles in flows and pillows are proposed to form by the exsolution of gas onto bubbles that are attached to the zone of solidification. As this zone advances into the cooling lava, continued exsolution of gas causes the bubbles to grow normal to the solidification front as pipes. Gas tubes formed in a similar manner can be seen in most ice cubes. Lava near the advancing tip of a pipe vesicle cools more rapidly than elsewhere because heat is able to transfer along the pipe by radiation. Pipe vesicles, therefore, grow into the lava as “cold fingers.” They cease growing when separate gas bubbles nucleate ahead of them in the ever broadening zone of solidification. Lava need contain no more than 0.01 wt% H 2 O for pipe vesicles to form in this way.

The volcanic‐tectonic cycle of the FAMOUS and AMAR Valleys, Mid‐Atlantic Ridge (36°47′N): Evidence from basalt glass and phenocryst compositional variations for a steady state magma chamber beneath the valley midsections, AMAR 3

The 1978 AMAR expedition extended the investigation of the Mid‐Atlantic Ridge that was begun by the 1973–1974 FAMOUS expedition from the northern end of the FAMOUS rift (the original FAMOUS area) to the narrow central and southern end of the FAMOUS rift (Narrowgate region) and into the broad AMAR valley south of Fracture Zone B. Available field and geochemical data allow us to characterize in detail the volcanic‐tectonic cycle for these two segments of the mid‐ocean ridge system. A dynamic model of the crust‐forming processes within these two rift valley segments is deduced from (1) variations in the chemical composition of the erupted basaltic liquids as preserved in chilled glassy margins, (2) composition and petrographie relationships of megacrysts and phenocrysts that represent cumulus crystallization onto the floor of a shallow magma chamber of a primitive magma that has reached pyroxene saturation early in its evolution, and (3) detailed stratigraphie and regional observations that characterize the periodicity of volcanic construction and superimposed tectonism related to the ongoing extension of the valley floor. Each volcanic cycle is composed of several eruptive episodes that are initially typified by rapidly extruded sheet flows of primitive composition. The decreasing extrustion rates at the end of an eruptive event are associated with more evolved liquid compositions that reflect fractionation within the shallow conduits. Intermixing of magmas is indicated by the relatively limited variability in glass composition not explicable in terms of simple fractionation processes, the common evidence of crystal resorption, and the apparent disequilibrium between megacrysts, phenocrysts, and their enclosing glass. This indicates control by a crustal magma chamber. Liquid and crystal compositions and textural relations can best be explained by a three‐stage crystallization history: (1) crystallization of olivine, plagioclase, and clinopyroxene onto the floor of a magma chamber, (2) partial resorption of mineral phases by a superheated, undersaturated liquid above the floor, (3) intratelluric crystallization of olivine and plagioclase (± clinopyroxene) during rise of melt to the surface through the conduit system above the magma chamber. A magma chamber model is suggested for the AMAR‐FAMOUS rift valleys that satisfies both seismic and geological constraints. We propose that a small steady state magma chamber is maintained beneath the topographically higher midsections of each valley segment (AMAR and Narrowgate regions) but that these thin and terminate near the intersecting fracture zones. Expansion and contracton of the central magma body is controlled by the fluctuating imbalance between magma supply and chamber crystallization. The FAMOUS‐Narrowgate rift is currently in a contraction period so that in the northern FAMOUS region, primitive liquid has erupted at Mount Venus and Mount Pluto without intercepting and mixing into the steady state chamber that supplies the mixed magmas to the Narrowgate region.

Magnetic properties of variably oxidized pillow basalt

In our study of changes in magnetic properties during low temperature oxidation of oceanic pillows, we were able to essentially eliminate variations in grain size which plagued many previous studies. This was accomplished by keeping the cores parallel to the outer glassy margin of the pillow while drilling the variably oxidized portions of the basalt. Curie temperature (Tc) measurements indicated a large range in the degree of low temperature oxidation, although most were in the early stages of maghemitization (Tc&lt;300°C). In samples with hysteresis parameters showing pseudo‐single domain behavior, the natural remanent magnetization (Jnrm) and susceptibility both decreased substantially with low temperature oxidation. Thus, the Koenigsberger ratio (Jnrm/Jinduced) did not change. The reduction in Jnrm was mirrored by comparable decreases in saturation magnetization and saturation remanence. Low temperature oxidation also increased the bulk coercivity and the coercivity of remanence in the oxidized basalt relative to the less oxidized. These later results are consistent with the theory that low temperature oxidation generally stabilizes the remanent magnetization of seafloor pillow basalts. The increase in coercivity with oxidation in these pillow basalts contrasts will synthetic samples which show an overall decrease. The behavior of natural samples reflects processes which occur during low temperature oxidation that do not normally occur in synthetic samples, specifically the loss of iron from the grain and perhaps grain cracking.

Restrictions on the compositions of mid-ocean ridge basalts: a fluid dynamical investigation

The nature of the melts parental to mid-ocean ridge basalts has been vigorously debated. Opinions vary between those considering the parental magma to be magnesia-rich (picritic basalt) with 15–20% MgO and those arguing that the parental melts are compositionally related to the most primitive liquids erupted at mid-ocean ridges (MgO∼9–11%). This difference of between 5 and 11% in MgO content represents a temperature interval of 100–250 °C. The actual MgO content places constraints on the thermal conditions at the mantle source. The high-magnesia liquid theory is supported by melting experiments on mantle and basalt samples1–3 and by petrological studies of ophiolite complexes4. However, this theory does not seem to be immediately consistent with the fact that no glasses or aphyric rocks of these compositions have ever been sampled from the sea floor. Indeed the most primitive glass compositions that occur as inclusions in olivine megacrysts and as glassy margins to pillow lavas have MgO contents in the range 9–11%. Models of generating low MgO partial melts from the mantle have been proposed. For example, Presnall et al.5 consider that melting at cusps in the peridotite mantle solidus could generate such melts. We outline here a fluid dynamical model for a basaltic mid-ocean ridge magma chamber supplied by high-magnesia melt from the mantle. The model predicts that high-magnesia extrusives will not be erupted, that ultramafic cumulates should occur at the base of the oceanic crust, that lava compositions will be restricted to MgO contents <10% and that magma mixing will be a common feature of sea-floor basalts6.

Magmatic gases extracted and analysed from ocean floor volcanics

Magmatic gases extracted and analysed from basaltic rocks collected in the FAMOUS area near 36°50′ N in the Atlantic ocean show that the total amount of gas included in the samples varies between about 500 ppm to 1600 ppm. The main gaseous phases included in the various types of basalts consist of CO2 (270–700 ppm), CO (150–800 ppm), HCl (100–1000 ppm), H2 (0–50 ppm), SO2 (up to 175 ppm), N2 (up to about 213 ppm) and traces of hydrocarbons (up to about 24 ppm). The relative amount of CO, CO2 and SO2 varies with both the degree of crystallinity of the rock and with fractional crystallization and/or fractional melting. The glassy margin of pillow lavas have a higher CO/CO2 ratio than the more crystalline interior. The most fractionated rocks of the series rich in clinopyroxene are depleted in the CO/CO2 ratio and have a higher SO2 content than do the most mafic end members rich in olivine. Early-formed olivine was crystallized in a reducing environment rich in CO and H2 with respect to later formed mineral associations. It is likely that the carbon and sulfur oxidation is taking place at a relatively shallow depth during magmatic ascent or during volcanism. The ocean floor volcanics when compared to subaerial basalts are depleted in SO2 and have on the average ten times more H2.

Preferential formation of the atmosphere–sialic crust system from the upper mantle

IN the past decade, extensive studies of the rare gas content of mantle derived rocks have led to several models of Earth degassing, all of which assume that the argon of the atmosphere degassed from the entire mantle. We give evidence here to support the concept of concurrent transfer of elements to the atmosphere and sialic crust preferentially from the upper mantle. This concept accounts for the following observations: (1) the 40Ar concentrations in the rapidly quenched glassy margins of deep sea pillow basalts are too low for many estimates of the K concentration of the mantle1,2; (2) the 40Ar/36Ar ratio of rocks derived from the deep mantle is similar to the atmospheric 40Ar/36Ar ratio3–7; (3) the upper mantle under oceanic ridges is depleted in K and other large-ion-lithophile elements. A schematic representation of the upper mantle depletion process is given in Fig. 1 which shows that partial melting associated with spreading ridges and Benioff zones exclusively in the upper mantle is responsible for the accretion of continents and associated degassing to the atmosphere.

Excess 3He and 4He in Galapagos submarine hydrothermal waters

SINCE the discovery of excess 3He in Pacific deep waters1, it has been argued that the source of this anomaly is the mid-depth injection of primordial helium from seafloor spreading centres1,2. This hypothesis is consistent with the spatial distribution of excess 3He in the deep waters3–5, its presence in the glassy (rapidly quenched) margins of extruded ocean basalts6,7, and the detection of a large 3He excess in a ‘thermal plume’ (thermal anomaly ∼0.1 °C, sampled using a deep-tow sled) over the Galapagos Spreading Centre8–10. In this report, we summarise 37 measurements of excess helium in hydrothermal waters sampled using the Alvin deep submersible at the Galapagos Spreading Centre during February and March of 1977.

Factors controlling the noble gas abundance patterns of deep-sea basalts

A number of processes may modify the noble gas composition of silicate liquids so that the composition of noble gases observed in glassy margins of deep-sea basalts is not that of the upper mantle. Differential solubility enhances the light noble gases relative to the heavier gases; however, we demonstrate that the observed abundance pattern cannot be attributed to solubility of noble gases with atmospheric proportions. Partial melting and fractional crystallization increase the noble gas content of all species relative to mantle concentrations, but do not fractionate their relative abundances. Noble gases may be lost from an ascending magma in various ways, the most important, however, may be exclusion of gas from crystals forming at the time of solidification, which is shown to result in marked loss of gas from the basalt. Small amounts of low-temperature alteration of solidified basalt can produce dramatic changes in the noble gas abundance pattern, since the adsorption coefficients for the different noble gas favor uptake of heavy species relative to the light species. Atmospheric contamination can account for observed variations in the 40Ar/ 36Ar ratio of oceanic basalts. The degree of crystallinity of glassy margins of deep-sea basalts may control the helium abundance of these samples; however, the uniform 3He/ 4He values reported apparently reflect a relatively constant proportion of radiogenic and primordial helium in the mantle.

Petrology and alteration of selected units of Mid-Atlantic Ridge basalts sampled from sites 332 and 335, DSDP

Closely spaced samples spanning 'flow units' at sub-bottom depths of 500 and 590 m in the drill hole at site 332B and a pillow at depth of 460 m, site 335, as well as a few individual samples at various depths in these holes and in the hole at site 332A, were studied with a view to determining compositional changes effected by submarine alteration. The rocks are predominantly plagioclase-olivine phyric tholeiitic basalts, with the exception of the lower unit of 332B, which is a picrite formed by the concentration of olivine megaphenocrysts in olivine tholeiitic basalt, is markedly flow-differentiated, and is assumed to be a sill. Three generations of plagioclase and olivine evident in some of the samples as corroded megaphenocrysts (An86−78, Fo89), euhedral microphenocrysts (An76−72, Fo85−84), and groundmass crystals (An72−61, Fo85−84), record a crystallization history that begins at depth, continues en route to the surface, and ends with quenching on the sea floor. Filling interstices of the groundmass are intergrowths, commonly submicroscopic, of pyroxene–plagioclase and dark, poorly resolved titanomagnetite-charged magmatic residue. The pyroxenes arc augites, ranging to subcalcic and ferroaugite. In places, particularly near pillow and flow unit margins, magma residue partially or completely in-fills vesicle cavities (segregation vesicles). Volatile-bearing phases (notably chlorophaeites, saponite, palagonite, amorphous silica-bearing hydrous iron oxides, and carbonates) tend to be characteristic of the three principal sites in which they are found: palagonite in the glassy margins of pillows and (or) flow units, complex hydrous mineraloids adjoining veins, and chlorophaeites and saponites in the interstices of the crystalline matrix of rocks remote from veins. The latter have the aspect of primary minerals. Palagonitization results in gains in K, Fe, Ti, and Cl, and losses in Ca, Mg, and Na, but net gains and losses in the other sites are less certain. Hydration and oxidation of iron invariably accompany development of volatile-bearing phases, but there is no correlation between these parameters and variation in content of the other analysed elements. Principal component analysis shows that the major part of the compositional variation can be explained by primary factors. In these samples chemical exchange with seawater appears to be limited, possibly because of their rapid isolation by burial from the main body of ocean water.

Rare gas fractionation patterns in terrestrial samples and the Earth‐atmosphere evolution model

Rare gas abundances in various terrestrial materials can be classified into three distinct fractionation patterns: type 1 shows a progressive enrichment in the heavier gases (shales, natural gases, holocrystalline submarine basalts, seawater and groundwater), type 2 shows a large enrichment of Ne and a slight apparent enrichment of Xe (glassy margins of submarine basalts, a subaerial basalt, an olivine xenolith from Hawaii), and type 3 shows large enrichments of Ne, Kr, and Xe relative to Ar (thucolite and a shale sample). The type 1 pattern is formed by the low‐temperature adsorption of rare gases usually via the rare gases dissolved in seawater. The type 2 pattern is interpreted as a combination of high‐temperature diffusion, solubility, and bulk melting in the mantle. The type 3 pattern probably represents a hybrid pattern. These patterns are consistent with an earth‐atmosphere evolution model which assumes (1) that the present rare gas abundances in the atmosphere closely approximate the rare gas inventory of the whole earth except for xenon and (2) that the average rare gas abundance pattern in the solid earth is roughly similar to that in the atmosphere except for Xe.

Rhyodacites, andesites, ferro-basalts and ocean tholeiites from the galapagos spreading center

Volcanic rocks, dredged from depths greater than 1000 m on the Galapagos spreading center, show extreme chemical diversity, including rhyodacites, andesite, ferro-basalts, and low-K oceanic tholeiite. All samples have fresh glassy margins. The ferro-basalts contain up to 18.5% total iron as FeO and up to 3.75% TiO 2, while the oceanic tholeiites are as low as 0.02% K 2O. The ferro-basalts correlate with the previously proposed zone of high magnetic anomaly amplitudes which flank the Galapagos hot spot, and are consistent with a genesis by shallow fractional crystallization.