Samuel Adam Soule

Geochemistry of lavas from the 2005–2006 eruption at the East Pacific Rise, 9°46′N–9°56′N: Implications for ridge crest plumbing and decadal changes in magma chamber compositions

Detailed mapping, sampling, and geochemical analyses of lava flows erupted from an ∼18 km long section of the northern East Pacific Rise (EPR) from 9°46′N to 9°56′N during 2005–2006 provide unique data pertaining to the short-term thermochemical changes in a mid-ocean ridge magmatic system. The 2005–2006 lavas are typical normal mid-oceanic ridge basalt with strongly depleted incompatible trace element patterns with marked negative Sr and Eu/Eu* anomalies and are slightly more evolved than lavas erupted in 1991–1992 at the same location on the EPR. Spatial geochemical differences show that lavas from the northern and southern limits of the 2005–2006 eruption are more evolved than those erupted in the central portion of the fissure system. Similar spatial patterns observed in 1991–1992 lavas suggest geochemical gradients are preserved over decadal time scales. Products of northern axial and off-axis fissure eruptions are consistent with the eruption of cooler, more fractionated lavas that also record a parental melt component not observed in the main suite of 2005–2006 lavas. Radiogenic isotopic ratios for 2005–2006 lavas fall within larger isotopic fields defined for young axial lavas from 9°N to 10°N EPR, including those from the 1991–1992 eruption. Geochemical data from the 2005–2006 eruption are consistent with an invariable mantle source over the spatial extent of the eruption and petrogenetic processes (e.g., fractional crystallization and magma mixing) operating within the crystal mush zone and axial magma chamber (AMC) before and during the 13 year repose period. Geochemical modeling suggests that the 2005–2006 lavas represent differentiated residual liquids from the 1991–1992 eruption that were modified by melts added from deeper within the crust and that the eruption was not initiated by the injection of hotter, more primitive basalt directly into the AMC. Rather, the eruption was driven by AMC pressurization from persistent or episodic addition of more evolved magma from the crystal mush zone into the overlying subridge AMC during the period between the two eruptions. Heat balance calculations of a hydrothermally cooled AMC support this model and show that continual addition of melt from the mush zone was required to maintain a sizable AMC over this time interval.

Interplay between faults and lava flows in construction of the upper oceanic crust: The East Pacific Rise crest 9°25′–9°58′N

The distribution of faults and fault characteristics along the East Pacific Rise (EPR) crest between 9°25′N and 9°58′N were studied using high-resolution side-scan sonar data and near-bottom bathymetric profiles. The resulting analysis shows important variations in the density of deformational features and tectonic strain estimates at young seafloor relative to older, sediment-covered seafloor of the same spreading age. We estimate that the expression of tectonic deformation and associated strain on “old” seafloor is ∼5 times greater than that on “young” seafloor, owing to the frequent fault burial by recent lava flows. Thus the unseen, volcanically overprinted tectonic deformation may contribute from 30% to 100% of the ∼300 m of subsidence required to fully build up the extrusive pile (Layer 2A). Many longer lava flows (greater than ∼1 km) dam against inward facing fault scarps. This limits their length at distances of 1–2 km, which are coincident with where the extrusive layer acquires its full thickness. More than 2% of plate separation at the EPR is accommodated by brittle deformation, which consists mainly of inward facing faults (∼70%). Faulting at the EPR crest occurs within the narrow, ∼4 km wide upper crust that behaves as a brittle lid overlying the axial magma chamber. Deformation at greater distances off axis (up to 40 km) is accommodated by flexure of the lithosphere due to thermal subsidence, resulting in ∼50% inward facing faults accommodating ∼50% of the strain. On the basis of observed burial of faults by lava flows and damming of flows by fault scarps, we find that the development of Layer 2A is strongly controlled by low-relief growth faults that form at the ridge crest and its upper flanks. In turn, those faults have a profound impact on how lava flows are distributed along and across the ridge crest.

Geology of Lonar Crater, India

Lonar Crater, India, is one of the youngest and best preserved impact structures on Earth. The 1.88-km-diameter simple crater formed entirely within the Deccan traps, making it a useful analogue for small craters on the basaltic surfaces of the other terrestrial planets and the Moon. In this study, we present a meter-scale–resolution digital elevation model, geological map of Lonar Crater and the surrounding area, and radiocarbon ages for histosols beneath the distal ejecta. Impact-related deformation of the target rock consists of upturned basalt fl ows in the upper crater walls and recumbent folding around rim concentric, subhorizontal, noncylindrical fold axes at the crater rim. The rim-fold hinge is preserved around 10%– 15% of the crater. Although tearing in the rim-fold is inferred from fi eld and paleomagnetic observations, no tear faults are identifi ed, indicating that large displacements in the crater walls are not characteristic of small craters in basalt. One signifi cant normal fault structure is observed in the crater wall that offsets slightly older layer-parallel slip faults. There is little fl uvial erosion of the continuous ejecta blanket. Portions of the ejecta blanket are overlain by aerodynamically and rotationally sculpted glassy impact spherules, in particular in the eastern and western rim, as well as in the depression north of the crater known as Little Lonar. The emplacement of the continuous ejecta blanket can be likened to a radial groundhugging debris fl ow, based on the preserved thickness distribution of the ejecta, the effi cient exchange of clasts between the ejecta fl ow and the underlying histosol, and the lack of sorting and stratifi cation in the bulk of the ejecta. The ejecta profi le is thickened at the distal edge and similar to fl ejecta structures observed on Mars.

How lava flows: New insights from applications of lidar technologies to lava flow studies

Mafic lava flows are common; for this reason, they have long been a focus of volcanological studies. However, field studies of both older and active flows have been hampered by difficulties in field access; active flows are hot, whereas older flows have rough and jagged surfaces that are difficult to traverse. As a result, morphometric studies of lava flows have generally lagged behind theoretical studies of flow behavior. The advent of laser scanning (LS) (i.e., lidar, light detection and ranging) technologies, both airborne mapping (ALSM) and terrestrial (TLS), is promoting detailed studies of lava flows by generating data suitable for production of high-resolution digital elevation models (DEMs). These data are revolutionizing both the visual and quantitative analysis of lava flows. First and foremost, this technology allows accurate mapping of flow boundaries, particularly in vegetated areas where bare earth imaging dramatically improves mapping capabilities. Detailed imaging of flow surfaces permits mapping and measurement of flow components, such as channels, surface folds, cracks, blocks, and surface roughness. Differencing of preeruptive and posteruptive DEMs allows analysis of flow thickness variations, which can be related to the dynamics of lava emplacement. Multitemporal imaging of active flows provides information not only on the rates and locations of individual flow lobes, but also measurement of pulsed lava transport. Together these new measurement capabilities can be used to test proposed models of channel formation, lava tube formation, rates of flow advance, and flow conditions within lava channels; they also provide new ways to assess the hazard and risk posed by lava flow inundation. Early published studies illustrate the potential of applying lidar to volcanic terrain; it is clear, however, that the availability of high-resolution digital topography is poised to revolutionize the study of mafic lava flows.

Magmatic Plumbing at Lucky Strike Volcano Based on Olivine-Hosted Melt Inclusion Compositions

Here we present volatile, major, and trace element concentrations of 64 olivine-hosted melt inclusions from the Lucky Strike segment on the mid-Atlantic ridge. Lucky Strike is one of two locations where a crustal melt lens has been seismically imaged on a slow-spreading ridge. Vapor-saturation pressures, calculated from CO2 and H2O contents of Lucky Strike melt inclusions, range from approximately 300–3000 bars, corresponding to depths of 0.5–9.9 km below the seafloor. Approximately 50% of the melt inclusions record crystallization depths of 3–4 km, corresponding to the seismically imaged melt lens depth, while an additional ∼35% crystallize at depths > 4 km. This indicates that while crystallization is focused within the melt lens, significant crystallization also occurs in the lower crust and/or upper mantle. The melt inclusions span a range of major and trace element concentrations from normal to enriched basalts. Trace element ratios at all depths are heterogeneous, suggesting that melts are not efficiently homogenized in the mantle or crust, despite the presence of a melt lens. This is consistent with the transient nature of magma chambers proposed for slower-spreading ridges. To investigate the petrogenesis of the melt inclusion compositions, we compare the measured trace element compositions to theoretical melting calculations that consider variations in the melting geometry and heterogeneities in the mantle source. The full range of compositions can be produced by slight variations in the proportion of an Azores plume and depleted upper mantle components and changes in the total extent of melting.

Lucky Strike seamount: Implications for the emplacement and rifting of segment‐centered volcanoes at slow spreading mid‐ocean ridges

The history of emplacement, tectonic evolution, and dismemberment of a central volcano within the rift valley of the slow spreading Mid-Atlantic Ridge at the Lucky Strike Segment is deduced using near-bottom sidescan sonar imagery and visual observations. Volcano emplacement is rapid (<1 Myr), associated with focused eruptions, and with effusion rates feeding lava flows that bury tectonic features developed prior to and during volcano construction. This volcanic phase likely requires efficient melt pooling and a long-lived crustal magma chamber as a melt source. A reduction in melt supply triggers formation of an axial graben rifting the central volcano, and the onset of seafloor spreading may eventually split it. At Lucky Strike, this results in two modes of crustal construction. Eruptions and tectonic activity focus at a narrow graben that bisects the central volcano and contains the youngest lava flows, accumulating a thick layer of extrusives. Away from the volcano summit, deformation and volcanic emplacement is distributed throughout the rift valley floor, lacking a clear locus of accretion and deformation. Volcanic emplacement on the rift floor is characterized by axial volcanic ridges fed by dikes that propagate from the central axial magma chamber. The mode of rapid volcano construction and subsequent rifting observed at the Lucky Strike seamount is common at other central volcanoes along the global mid-ocean ridge system.

Morphology and dynamics of inflated subaqueous basaltic lava flows

During eruptions onto low slopes, basaltic Pahoehoe lava can form thin lobes that progressively coalesce and inflate to many times their original thickness, due to a steady injection of magma beneath brittle and viscoelastic layers of cooled lava that develop sufficient strength to retain the flow. Inflated lava flows forming tumuli and pressure ridges have been reported in different kinds of environments, such as at contemporary subaerial Hawaiian-type volcanoes in Hawaii, La Reunion and Iceland, in continental environments (states of Oregon, Idaho, Washington), and in the deep sea at Juan de Fuca Ridge, the Galapagos spreading center, and at the East Pacific Rise (this study). These lava have all undergone inflation processes, yet they display highly contrasting morphologies that correlate with their depositional environment, the most striking difference being the presence of water. Lava that have inflated in subaerial environments display inflation structures with morphologies that significantly differ from subaqueous lava emplaced in the deep sea, lakes, and rivers. Their height is 2–3 times smaller and their length being 10–15 times shorter. Based on heat diffusion equation, we demonstrate that more efficient cooling of a lava flow in water leads to the rapid development of thicker (by 25%) cooled layer at the flow surface, which has greater yield strength to counteract its internal hydrostatic pressure than in subaerial environments, thus limiting lava breakouts to form new lobes, hence promoting inflation. Buoyancy also increases the ability of a lava to inflate by 60%. Together, these differences can account for the observed variations in the thickness and extent of subaerial and subaqueous inflated lava flows.

Segmentation and eruptive activity along the East Pacific Rise at 16°N, in relation with the nearby Mathematician hotspot

The 16 N segment of the East Pacific Rise is the most overinflated and shallowest of this fast-spreading ridge, in relation with an important magma flux due to the proximity of the Mathematician hot-spot. Here, we analyze the detailed morphology of the axial dome and of the Axial Summit Trough (AST), the lava morphology, and the geometry of fissures and faults, in regard to the attributes of the magma chamber beneath and of the nearby hotspot. The data used are 1 m resolution bathymetry combined with seafloor photos and videos. At the dome summit, the AST is highly segmented by 10 third-order and fourth-order discontinuities over a distance of 30 km. Often, two contiguous and synchronous ASTs coexist. Such a configuration implies a wide (1100 m minimum) zone of diking. The existence of contiguous ASTs, their mobility, their general en echelon arrangement accommodating the bow shape of the axial dome toward the hotspot, plus the existence of a second magma lens under the western half of the summit pla-teau, clearly reflect the influence of the hotspot on the organization of the spreading system. The different ASTs exhibit contrasted widths and depths. We suggest that narrow ASTs reflect an intense volcanic activity that produces eruptions covering the tectonic features and partially filling the ASTs. AST widening and deepening would indicate a decrease in volcanic activity but with continued dike intrusions at the origin of abundant sets of fissures and faults that are not masked by volcanic deposits.

The Final Stages of Slip and Volcanism on an Oceanic Detachment Fault at 13°48′N, Mid‐Atlantic Ridge

Author(s): Parnell-Turner, RE; Mittelstaedt, E; Kurz, MD; Jones, MR; Soule, SA; Klein, F; Wanless, VD; Fornari, DJ