Vesicle Cylinders Research Articles

Formation of segregation structures in Hafnarhraun p\u0101hoehoe lobe, SW Iceland: a window into crystal\u2013melt separation in basaltic magma

To gain insights into crystal–melt separation processes during basalt differentiation, we have studied an 8-m-thick pāhoehoe lava lobe from the Hafnarhraun lava flow field in SW Iceland. The lobe has abundant melt segregations, porous cylindrical and sheet-like structures, generally interpreted as separated residual melts of a lava lobe. We divide these melt segregations into three types based on morphology and composition: vesicle cylinders (VC), type 1 horizontal vesicle sheets (HVS1), and type 2 horizontal vesicle sheets (HVS2). Remarkably, the studied VC are not simple residual melts generated by fractional crystallization, but their composition points to removal of plagioclase from the parental lava. HVS1 resemble VC, but have fractionated more olivine (ol) + plagioclase (plg) ± augite and have lost most, if not all, of their olivine phenocrysts. HVS2 are Fe-rich and evolved, corresponding to residual melts after 50–60% fractional crystallization of the lobe. We suggest that the Hafnarhraun VC formed in a two-stage process. Firstly, VC forming residual melt and vapor detached as rising diapirs from ol+plg+melt+vapor mush near the lava base, and later, these VC diapirs accumulated ol phenocrysts and minor plg microphenocrysts in the lava core. HVS1 represent accumulations of VC to the viscous base of the solidifying upper crust of the lobe, and HVS2 formed as evolved vapor-saturated residual melts seeped into voids within the upper crust. Such vapor-aided differentiation, here documented for the Hafnarhraun lava, may also apply to shallow crustal magma storage zones, contributing to the formation of evolved basalts.

Characterization of subaerial volcanic facies using acoustic image logs: Lithofacies and log-facies of a lava-flow deposit in the Brazilian pre-salt, deepwater of Santos Basin

Abstract Volcanic rock facies characterization in subsurface log data have always being challenging. Even though considerable types of well logs are acquired, the results achieved on facies characterization with the conventional log suits are very limited. Conversely, high-resolution borehole image logs calibrated with side wall core samples can provide the necessary structural and textural information for facies definitions. In this case study, an integration of good quality acoustic image log data, side wall core petrography and geochemical analyses provided a good understanding of volcanic facies and stratigraphic relationships. Additionally, outcrop data from the analogous Serra Geral Formation and other Large Igneous Provinces were used for comparison. In the studied well, from Santos basin, Brazil, it was possible to identify several kinds of subaerial basaltic lava flow units, such as compound pahoehoe, sheet pahoehoe and rubbly pahoehoe lava flows. Vesicles, amygdales, vesicle cylinders, sub-horizontal vesicle sheets, autobreccias and entablature are some of the structures described in this study. As a result, 2 image catalogues of subaerial volcanic rocks were produced characterizing facies and flow units along with a stratigraphic model of the history of this volcanism. This is the first time that pahoehoe lava flow units could be characterized at an offshore Brazilian basin. The results achieved are important for the understanding of the Cretaceous volcanism events in the pre-salt layer and also provide support for the evaluation and geological modelling of the volcanic rocks in Santos Basin oil fields.

Control of early-formed vesicle cylinders on upper crustal prismatic jointing in compound pāhoehoe lavas of Elephanta Island, western Deccan Traps, India

Upper crustal prismatic joints and vesicle cylinders, common in pāhoehoe lava flows, form early and late, respectively, and are therefore independent features. However, small-scale compound pāhoehoe lava lobes on Elephanta Island (western Deccan Traps, India), which resemble S-type (spongy) pāhoehoe in some aspects, contain vesicle cylinders which apparently controlled the locations of upper crustal prismatic joints. The lobes are decimeters thick, did not experience inflation after emplacement, and solidified rapidly. They have meter-scale areas that are exceptionally rich in vesicle cylinders (up to 68 cylinders in 1 m2, with a mean spacing of 12.1 cm), separated by cylinder-free areas, and pervasive upper crustal prismatic jointing with T, curved T, and quadruple joint intersections. A majority (≥76.5%) of the cylinders are located exactly on joints or at joint intersections, and were not simply captured by downward growing joints, as the cylinders show no deflection in vertical section. We suggest that large numbers of cylinders originated in a layer of bubble-rich residual liquid at the top of a basal diktytaxitic crystal mush zone which was formed very early (probably within the first few minutes of the emplacement history). The locations where the rising cylinders breached the crust provided weak points or mechanical flaws towards which any existing joints (formed by thermal contraction) propagated. New joints may also have propagated outwards from the cylinders and linked up laterally. Some cylinders breached the crust between the joints, and thus formed a little later than most others. The Elephanta Island example reveals that, whereas thermal contraction is undoubtedly valid as a standard mechanism for forming upper crustal prismatic joints, abundant mechanical flaws (such as large concentrations of early-formed, crust-breaching vesicle cylinders) can also control the joint formation process.

The mode of emplacement of Neogene flood basalts in Eastern Iceland: Facies architecture and structure of the Hólmar and Grjótá olivine basalt groups

Hólmar and Grjótá are two stratigraphically distinct transitional alkaline olivine basalt lava groups within the westward-dipping Neogene flood basalts of eastern Iceland. The Hólmar olivine basalt group, separated from the overlying Grjótá olivine basalt group by only a few tholeiite flows, can be traced over 80km north–south, with thicknesses varying from ~250m where thickest to ~30m where thinnest. The Grjótá group can be traced over 50km also north–south, reaching thicknesses of ~250m and thinning down-dip to ~10m. In contrast to other groups in eastern Iceland that thicken down-dip, the studied olivine basalt groups thicken up-dip. The groups filled topographic confinements and formed aprons around central volcanoes. We have estimated the minimum volumes to be ~119km3 for Hólmar and ~86km3 for Grjótá. Scoria cones are found in the Hólmar group, and two thick olivine dolerite sills cross-cut the Hólmar group and probably belong to the plumbing system that fed the Grjótá group. The architecture of the lava groups are near identical. The architecture is compound, with lobes stacked horizontally and vertically, varying from 1–15m thick and 2–200m long, but do also encompass a number of thicker (15–20m) and more extensive (>1km long) lava lobe in the stacks. Filled lava tubes are commonly observed within the lava flows. The constituent lobes of the flows are often directly emplaced or welded together, suggesting rapid buildup, but are also found interbedded with redbeds and thicker tuff deposits, and occasionally preserve tree molds. The internal structure follows the characteristics for lava lobe morphology in general, with an upper vesicular crust forming half to one third of the total thickness, a massive core with abundant vesicle cylinders, and a thin basal vesicular crust. Flow tops are of the pahoehoe type, seldom found with scoria or clinker. Inflation structures such as tumuli and inflation clefts were identified in the flows. The eruptions were dominantly fissure-fed, with a few vents evolving to short-lived point source volcanism. The pre-existing topography exerted control on the advancement of the flows and influenced the final architecture of the groups at each location. It is evident that the large and dense spatial distribution of these groups, including numerous thick lava units, suggests large volcanic episodes. Together, the groups imply a period with higher production of olivine basalts in Iceland and form well-defined stratigraphic markers. The transitional alkaline olivine basalt groups are interpreted to delineate a fossil southward propagating rift flank volcanic zone located east of a now extinct and buried mature rift zone.

Internal differentiation of Kutsugata lava flow from Rishiri Volcano, Japan: Processes and timescales of segregation structures' formation

Internal differentiation processes in a solidifying lava flow were investigated for the Kutsugata lava flow from Rishiri Volcano in northern Japan. In a representative 6-m thick lava flow that was investigated in detail in this study, segregation products darker than the host lavas manifested mainly in the form of pipes (vesicle cylinders) and layers (vesicle sheets), occurring around 0.5–2.3 m and 2.0–4.0 m above the base, respectively. Both the cylinders and sheets are significantly richer in incompatible elements such as TiO 2 and K 2O than the host lavas, which suggest that these products essentially represent residual melt produced during solidification of the lava flow. Field observation and the geochemical features of the lavas suggest that the vesicle cylinders grew upward from near the base of the flow by continuous feeding of residual melt from the neighboring host lavas to the heads of the cylinders. On the other hand, the vesicle sheets were produced in situ in the solidifying lava flow as fracture veins caused by horizontal compression. The vesicle cylinders have a remarkably higher MgO content (up to 8 wt.%) than the host lava (< 6 wt.%), whereas the vesicle sheets display MgO depletion (as low as 3.5 wt.%). The relatively high MgO content of the vesicle cylinders cannot be explained solely by the mechanical mixing of olivine phenocrysts with the residual melt. It is suggested that the vesicle cylinders were produced by the extraction of olivine-bearing interstitial melt from an augite-plagioclase network in the host lava, whereas the vesicle sheets were formed by the migration of the residual melt from a crystal network consisting of plagioclase, augite, and olivine in the host lava into platy fractures. We infer that this selective crystal fractionation for forming the vesicle cylinders resulted from processes in which abundant vesicles rejected from the upward-migrating floor solidification front prevented olivine crystals from being incorporated into the crystal network in the host lava. The vesicle cylinders are considered to have formed in ∼ 1 day after the lava flow came to rest, while relatively large vesicle sheets (> 1 cm thick) appeared much later (after ∼ 9 days). The formation of these segregation products was essentially complete within 20 days after the lava emplacement.

Melt segregations in a Columbia River Basalt lava flow: A possible mechanism for the formation of highly evolved mafic magmas

The Levering lava lies within the Grande Ronde Formation of the Columbia River Basalt Group. At Sentinel Gap it consists of a 14–20 m-thick pahoehoe sheet lobe featuring the threefold vertical division typical of inflated pahoehoe lobes into basal zone, lava core and lava crust. Segregation structures such as horizontal vesicle sheets are exceptionally well developed at the lava core to lava crust interface. The lava core contains numerous well-developed vesicle cylinders which extend from the basal zone up through the core and transform into horizontal vesicle sheets at the interface with the upper crust. The host lava samples show no statistically significant change in composition with stratigraphic height within the Sentinel Gap lobe. However, material in the segregation structures is markedly more evolved, with MgO content of ∼ 3 wt.% compared to ~5 wt.% in the host lava. The segregations are enriched in incompatible elements (e.g. TiO 2, K 2O, Ba, Zr) by an average factor of 1.6. Modelling, constrained by whole-rock chemical composition, indicates generation of segregated material by 30 to 40% fractional crystallisation of the host lava. Field relations show that vesicle cylinders are fed by pipe vesicles originating in the basal zone. The segregations are thus rooted in the basal crust, implying that the melt is derived by extraction of volatile-enriched inter-crystal melt from a mush horizon near the base of the flow. The Grande Ronde Formation is divided into five distinct chemical groups; three high-MgO (> 4.5 wt.% MgO) and two low-MgO (< 4.5 wt.% MgO). The Levering host lava belongs to one of the high-MgO groups, while its segregations are identical to one of the low-MgO groups. This correlation leads us to propose that the high-MgO and low-MgO groups are related by fractional crystallisation and the process by which segregations are formed within the Levering lava emulates the subsurface process that forms the highly evolved, aphyric Grande Ronde lavas.

On the edge of CAMP: Geology and volcanology of the Jurassic North Mountain Basalt, Nova Scotia

The North Mountain Basalt (NMB) is a sequence of continental tholeiitc basalts that are part of the larger, 200 Ma Central Atlantic Magmatic Province (CAMP).The NMB occur within the Fundy Basin, one of several Mesozoic extensional basins that formed during the incipient rupture of Pangea. In addition to the NMB, the Fundy Basin was also filled by fluvial, alluvial, playa, lacustrine and aeolian sediments under arid to semi-arid conditions. Mapping of a 200 km long segment of the NMB along the southern coast of the Bay of Fundy indicates for the first time the lateral continuity of the three recognized lava units, the East Ferry, Margaretsville and Brier Island members (EFM, MM, BIM, respectively), that comprise the NMB. The EFM (< 180 m) and BIM (> 150 m) are massive and characterized by pervasive development of columnar jointing, both colonnade and entablature. In addition, whereas the EFM contains abundant layers of mafic pegmatite, the BIM is characterized by abundant mesostasis, possibly reflecting widespread quenching of this flow, and locally abundant segregation pipes due to mobilization of a late-stage felsic liquid. These two units represent expansive eruption of low viscoity flows onto a low slope surface, which encouraged uniform thickness and their massive nature, but the role of faulted basins cannot be ruled out. In contrast, the MM (170 m) consists of numerous (> 16) inflated pahoehoe sheet flow lobes. Evidence that these flows formed as pahoehoe flows is represented by exceptional development of the vesicle zonation (i.e., upper crust, vesicle sheets, vesicle cylinders, pipe vesicles) as observed in modern Hawaiian flows, historical Icelandic flows and ancient flows of the Columbia River Basalt Group. The thickness of upper crust zones indicates that the individual flows formed over months to years, whereas inclined basal pipe vesicles indicate the MM and BIM flowed to the south. The similar age and nature of the NMB to other basalt sequences in CAMP suggest that similar processes operated throughout this LIP, thereby having implications for such phenomena as atmospheric contamination and extinction events caused by eruption of these voluminous basaltic lava flows.

Segregation processes in vesiculating crystallizing magmas

Vesiculation of crystallising magma can produce either a mobile vesicular magma or a rigid network of crystals containing vesicular liquid. Where partially crystallized rigid mush underlies less-crystallized magma, such as near the base of a lava flow or in the cumulus pile of a magma chamber, evolved interstitial melt and/or gas may escape into the main body of magma. The consequences of this may include contamination of the overlying liquid with gas and interstitial melt, or intrusion of diapirs of vesicular evolved liquids to form vertical vesicle cylinders and other segregation features found in many basaltic lava flows and sills. Analog experiments were used to investigate some of the phenomena that can arise during vesiculation within a crystal mush, which was simulated by pumping air through a porous plate that formed the floor of a container filled with a viscous liquid floored with a layer of glass beads. Experiments used either a single liquid or two stably stratified liquids with a liquid interface either coincident with the top of the porous layer of beads or slightly above the porous layer. For a range of liquid viscosities and air flow rates (vesiculation rates), individual bubbles emerged from the top of the porous layer of beads and carried a thin trail of interstitial liquid into the overlying liquid. The number of bubble trains leaving the surface of the porous bed increased with decreasing liquid viscosity and flow rate, and with increasing bead size (and, hence, with increasing permeability). Analog vesicle cylinders, composed of diapirs of bubbly interstitial liquid, were produced only when a layer of buoyant bubbly liquid lay above the surface of the porous layer. The relative size of the bubbles and constrictions within the porous layer are argued to control whether individual bubbles (leading to bubble trains) or vesicular liquid (leading to vesicle cylinders) leaves the porous layer and hence whether vesicle cylinders can form.

Segregation vesicles, cylinders, and sheets in vapor-differentiated pillow lavas: examples from Tore-Madeira Rise and Chile Triple Junction

We conducted a detailed field and laboratory study of internal segregation structures of two hand-size pillow lavas samples. They were dredged, respectively, on the Josephine seamount, Tore-Madeira Rise (TMR), and on a small quaternary volcanic edifice located on the continental edge of the trench close to the Chile Triple Junction (CTJ). Both pillows display a combination of four types of segregation structures (spherical vesicles, pipe vesicles, vesicle cylinders, and vesicle sheets) observed so far only within subaerial basalt flows typically 2–10 m thick. In particular, the samples offer a remarkable exposure of the transition between pipe vesicles and cylinders. We show that the vesicle sheets are not generated by the same mechanism in both occurrences; they do not seem to be connected to cylinders in the CTJ pillow as they are in the TMR pillow. The two pillows are geochemically distinct, the TMR being alkaline and the CTJ calc–alkaline. Two types of internal differentiation are proposed. The first one implies the extraction of the residual liquid from the host lava and transport towards the segregation structures, whereas the other one results from in situ crystallization within one given structure. In the latter case, glass composition is highly dependant on the nature of the neighbouring crystallizing minerals. The degree of crystallization required to produce a crystal framework strong enough for generating the segregation structures seems to be lower in pillows (ca. 25% crystallization) than in vapor-differentiated basaltic lava flows (35% crystallization).

Segregation structures in vapor-differentiated basaltic flows

Vesicle cylinders represent a spectacular kind of segregation structure involving residual liquids formed in situ during the cooling of lava flows. These vertical pipes, commonly found within basalt flows typically 2–10 m thick, are interpreted as the product of a vapor-driven differentiation process. The olivine phenocrysts and the earliest generation of groundmass olivines found in cylinder-bearing basalts appear to have been generally affected by magmatic oxidation, resulting in high-temperature iddingsite (HTI) alteration. This feature is also observed within cylinder-free basalt flows which exhibit other kinds of vesicular segregation structures, such as vesicle-rich pegmatoid segregation sheets and/or segregation vesicles. Detailed textural, petrological, and geochemical characteristics of two types of cylinders, three types of vesicle sheets, and five types of segregation vesicles are described, based on the study of 12 occurrences of HTI-bearing basalt flows from oceanic shield volcanoes or continental basalt plateaus. We propose a general classification of these segregation structures likely to derive from vapor differentiation. Flow thickness is probably the main factor influencing their morphology. Finally, we suggest that the concomitant occurrence of olivine oxidation and vapor-differentiation effects results from the late persistence of water oversaturation after eruption, perhaps due to a high rate of magma ascent.

Vesicle cylinders in vapor-differentiated basalt flows

Vesicle cylinders are vertical pipes filled with bubbles and residual melt that differentiate from diktytaxitic basalt flows during crystallization. They grow from about 0.25 m above the base of the flow to the bottom of the chilled flow top. Field relations limit their growth to the period between cessation of lava movement and deep penetration of columnar joints. Basalts containing vesicle cylinders show positive correlations among increasing cylinder abundance, increasing lava porosity, and increasing groundmass crystal size. These features suggest unusually high water contents in the magma before eruption. Although both vesicle cylinders and host lava are “basaltic”, the cylinders are enriched in elements not removed by the initial crystallization of the host: Fe, Mn, Ti, Na, K, P and many incompatible trace elements. The last residues to solidify within the cylinders consist of dacitic-rhyolitic glass, Fe-Ti oxides, anorthoclase, apatite ± fayalite ± aegerine. Geothermometry indicates that the cylinders began forming at ~ 1100 − 1075 °C but ceased crystallizing at ~ 950 °C. Pre-eruptive, high-temperature, iddingsite alteration of olivine phenocrysts in many lavas containing vesicle cylinders shows that the f o 2 of the magmas was extremely high at eruption (~ 10 −4). After eruption, the f o 2 of the lavas fell dramatically to values of about 10 −11 and conditions paralleled the FMQ buffer to final crystallization. Because the iddingsite forms before eruption, the magmas may become relatively oxidizing by addition of meteoric water late in their evolution. Oxygen-18 analyses of four basalt-differentiate pairs suggest that meteoric water addition has occurred in some of the magmas. Field relations and thermal profiles of cooling lava flows limit the growth period of vesicle cylinders to 1–5 days after flows of typical thickness (3–10 m) come to rest. Estimated viscosities of host lavas and frothy differentiate during cylinder growth are ≤ 10 6 and ~ 10 4 poise, respectively. Although an adequate quantitative model describing growth of vesicle cylinders does not exist, they apparently form by bubble nucleation and resulting density instability above the rising lower solidification front of the cooling flows. As the coalesced bubbles rise, residual melt and additional vapor migrate into the low-pressure, vertical discontinuity formed by the plume.

Flow directions in Columbia River Basalt flows and paleocurrents of interbedded sedimentary rocks, South-Central Washington

More than 150 flow directions measured mainly in inclined pipe vesicles and, more rarely, vesicle cylinders, spiracles, and inclined foreset beds in pillowpalagonits-complexes) of the upper 12 widespread Yakima Basalt flows on the Columbia River Plateau in south-central Washington indicate rather uniform transport directions from southeast to northwest. This strongly supports former ideas that the large Grande Ronde-Cornucopia dike swarm in northeast Oregon fed the huge lava floods. It is shown that all previous evidence indicating eruption centers for the basalts in the Cascada Range to the west is inconclusive. Cross-bedding directions, maximum pebble sizes, and distribution of heavy mineral suites in sedimentary deposits interbedded with the basalt sheets delineate in more detail two main paleoslopes whose deposits interfinger: a) the Cascade paleoslope to the west with east-southeast paleocurrents and a predominance of volcanic heavy minerals (hornblende, oxyhornblende, hypersthene, and clinopyroxene). b) the Columbia paleoslope with south-southwest paleocurrents and chiefly sphene, epidote, clinozoisite, blue-green hornblende, garnet, sillimanite, kyanite, and staurolite. Distribution of conglomerats, current directions, and heavy mineral suites support former hypotheses that the ancestral Columbia River flowed about 50 km farther west in early Pliocene time than it does today in the area of lower Yakima Valley.