Axial Magma Chamber Research Articles

Three‐dimensional geometry of axial magma chamber roof and faults at Lucky Strike volcano on the Mid‐Atlantic Ridge

AbstractWe present results from three‐dimensional (3‐D) processing of seismic reflection data, acquired in June 2005 over the Lucky Strike volcano on the Mid‐Atlantic Ridge as a part of the Seismic Study for Monitoring of the Mid‐Atlantic Ridge survey. We use a 3‐D tomographic velocity model derived from a coincident ocean bottom seismometer experiment to depth convert the poststack time‐migrated seismic volume and provide 3‐D geometry of the axial magma chamber roof, fault reflectors, and layer 2A gradient marker. We also generate a high‐resolution bathymetric map using the seismic reflection data. The magma chamber roof is imaged at 3.4 ± 0.4 km depth beneath the volcano, and major faults are imaged with dips ranging between 33° and 50°. The magma chamber roof geometry is consistent with a focused melt supply at the segment center and steep across‐axis thermal gradients as indicated by the proximity between the magma chamber and nearby faults. Fault scarps on the seafloor and fault dip at depth indicate that tectonic extension accounts for at least 10% of the total plate separation. Shallow dipping reflectors imaged in the upper crust beneath the volcano flanks are interpreted as buried lava flow surfaces.

Constraints on the accretion of the gabbroic lower oceanic crust from plagioclase lattice preferred orientation in the Samail ophiolite

Oceanic crust represents more than 60% of the earth's surface and despite a large body of knowledge regarding the formation and chemistry of the extrusive upper oceanic crust, there still remains significant debate over how the intrusive gabbroic lower oceanic crust is accreted at the ridge axis. The two proposed end-member models, the Gabbro Glacier and the Sheeted Sills, predict radically different strain accumulation in the lower crust during accretion. In order to determine which of these two hypotheses is most applicable to a well-studied lower crustal section, we present data on plagioclase lattice preferred orientations (LPO) in the Wadi Khafifah section of the Samail ophiolite. We observe no systematic change in the strength of the plagioclase LPO with height above the crust-mantle transition, no dominant orientation of the plagioclase a-axis lineation, and no systematic change in the obliquity of the plagioclase LPO with respect to the modal layering and macroscopic foliation evident in outcrop. These observations are most consistent with the Sheeted Sills hypothesis, in which gabbros are crystallized in situ and fabrics are dominated by compaction and localized extension rather than by systematically increasing shear strain with increasing depth in a Gabbro Glacier. Our data support the hypothesis of MacLeod and Yaouancq (2000) that the rotation of the outcrop-scale layering from sub-horizontal in the layered gabbros to sub-vertical near the sheeted dikes is due to rapid vertical melt migration through upper gabbros close to the axial magma chamber. Additionally, our results support the hypothesis that the majority of extensional strain in fast spreading ridges is accommodated in partially molten regions at the ridge axis, whereas in slow and ultra-slow ridges large shear strains are accommodated by plastic deformation.

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

AbstractThe 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.

Permeability of the Lucky Strike deep-sea hydrothermal system: Constraints from the poroelastic response to ocean tidal loading

We use the time delay between tidal loading and the induced subsurface flow response to constrain the poroelastic behavior and permeability of the Lucky Strike hydrothermal field on the Mid-Atlantic Ridge. We demonstrate that high-temperature (T>200°C) exit-fluid discharge records from four hydrothermal sites across the field are highly coherent with contemporaneously acquired bottom pressure records at tidal periods, with the thermal response lagging pressure by ∼155° (5.3 h) on average across all sites for the semi-diurnal (M2) frequency over a three-year observation period. In a one-dimensional poroelastic model of ocean tidal loading this phase lag corresponds to a high-permeability system where pore pressure perturbations at the seafloor rapidly propagate downward from the seafloor interface until they encounter a permeability boundary. Our results suggest that at the Lucky Strike field this tidal pumping is largely restricted to the ∼600 m thick extrusive layer (i.e., seismic layer 2A). Under a plausible set of matrix elastic parameters, the ∼5.3 h lag between pressure and exit-fluid temperature is consistent with an effective matrix permeability of ∼10−10 m2 and an average vertical flow velocity of ∼0.02 m/s within the extrusive layer. Our results argue against tidal pumping of the entire crustal section between the seafloor and the axial magma chamber (at ∼3.4 kmbsf) because this scenario requires unrealistically high effective permeabilities (∼10−9 m2) and average vertical flow velocities (∼0.15 m/s) over this depth range. Our effective permeability estimate for the extrusive layer is broadly consistent with previous results, and indicates that flow must be channeled in discrete permeable pathways (e.g., faults, fissures) that cut through the extrusive volcanic layer.

Diverse magma flow directions during construction of sheeted dike complexes at fast- to superfast-spreading centers

Dike intrusion is a fundamental process during upper oceanic crustal accretion at fast- to superfast-spreading ridges. Based on the distribution of magma along fast-spreading centers inferred from marine geophysical data, models predict systematic steep flow at magmatically robust segment centers and shallow magma flow toward distal segment ends. Anisotropy of magnetic susceptibility (AMS) fabrics from 48 fully-oriented block samples of dikes from upper oceanic crust exposed at Hess Deep Rift and Pito Deep Rift reveal a wide range of magma flow directions that are not consistent with such simple magma supply models. The AMS is interpreted to arise from distribution anisotropy of titanomagnetite crystals based on weak shape-preferred orientation of opaque oxide and plagioclase crystals generally parallel to AMS maximum eigenvectors. Most dike samples show normal AMS fabrics with maximum eigenvector directions ranging from subvertical to subhorizontal. The distributions of inferred magma flow lineations from maximum eigenvectors show no preferred flow pattern, even after structural correction. We use a Kolmogorov–Smirnov test (KS-test) to show that the distribution of bootstrapped flow lineation rakes from Pito Deep are not statistically distinct from Hess Deep, and neither are distinguishable from Oman and Troodos Ophiolite AMS data. Magma flow directions in sheeted dikes from these two seafloor escarpments also do not correlate with available geochemistry in any systematic way as previously predicted. These results indicate distinct compositional sources feed melt that is injected into dikes at fast- to superfast-spreading ridges with no preference for subhorizontal or subvertical magma flow. Collectively, results imply ephemeral melt lenses at different along-axis locations within the continuous axial magma chamber and either direct injection or intermingling of melt from other deeper ridge-centered or off-axis sources.

Repeat ridge jumps and microcontinent separation: insights from NE Arabian Sea

Microcontinents separate due to ridge jumps associate either asymmetric sea floor spreading or plume–ridge interactions. India separated from Seychelles at ∼64 Ma by asymmetric sea floor spreading initially when the spreading centre in the Mascarene Basin jumped towards the Indian sub-continent between magnetic chrons C29 and C28. The subsequent tectonics is difficult to comprehend since Laxmi Ridge-another microcontinent-formed during the later phase. Most of the studies considered the Laxmi Ridge as a sliver. Others considered it to be oceanic crust. High resolution, deep (∼25 km) seismic data reveals that (i) the ridge possesses > 15 km deep sea-ward dipping reflector (SDR) packages; (ii) normal faulted rift valleys devoid of syn-rift sedimentary packages; and (iii) axial magma chambers 5–7 km beneath the ridge top. Additionally, from 2D forward gravity models we deduce that the ridge most possibly comprises of high density (oceanic) crust. We conclude the Laxmi Ridge to be indeed composed of oceanic crust and a fossil spreading centre. We thus identified the ridge jumps and their relation to the Seychelles microcontinent separation.Previous numerical models suggest that the time required for a ridge jump is controlled by magmatic heating, spreading rate at the ridge, and plate ages. For repeated ridge jumps, the additional factor is the dynamic relation between the plume and lithosphere in terms of melt transfer and heating. We find that the medium spreading rates and high magmatic heating due to the Réunion plume and young plates favoured rapid and repeated ridge jumps towards the plume.

Eruptive and tectonic history of the Endeavour Segment, Juan de Fuca Ridge, based on AUV mapping data and lava flow ages

AbstractHigh‐resolution bathymetric surveys from autonomous underwater vehicles ABE and D. Allan B. were merged to create a coregistered map of 71.7 km2 of the Endeavour Segment of the Juan de Fuca Ridge. Radiocarbon dating of foraminifera in cores from three dives of remotely operated vehicle Doc Ricketts provide minimum eruption ages for 40 lava flows that are combined with the bathymetric data to outline the eruptive and tectonic history. The ages range from Modern to 10,700 marine‐calibrated years before present (yr BP). During a robust magmatic phase from >10,700 yr BP to ∼4300 yr BP, flows erupted from an axial high and many flowed >5 km down the flanks; some partly buried adjacent valleys. Axial magma chambers (AMCs) may have been wider than today to supply dike intrusions over a 2 km wide axial zone. Summit Seamount formed by ∼4770 yr BP and was subsequently dismembered during a period of extension with little volcanism starting ∼4300 yr BP. This tectonic phase with only rare volcanic eruptions lasted until ∼2300 yr BP and may have resulted in near‐solidification of the AMCs. The axial graben formed by crustal extension during this period of low magmatic activity. Infrequent eruptions occurred on the flanks between 2620–1760 yr BP and within the axial graben since ∼1750 yr BP. This most recent phase of limited volcanic and intense hydrothermal activity that began ∼2300 yr BP defines a hydrothermal phase of ridge development that coincides with the present‐day 1 km wide AMCs and overlying hydrothermal vent fields.

Upper crustal seismic structure of the Endeavour segment, Juan de Fuca Ridge from traveltime tomography: Implications for oceanic crustal accretion

The isotropic and anisotropic P wave velocity structure of the upper oceanic crust on the Endeavour segment of the Juan de Fuca Ridge is studied using refracted traveltime data collected by an active-source, three-dimensional tomography experiment. The isotropic velocity structure is characterized by low crustal velocities in the overlapping spreading centers (OSCs) at the segment ends. These low velocities are indicative of pervasive tectonic fracturing and persist off axis, recording the history of ridge propagation. Near the segment center, velocities within the upper 1 km show ridge-parallel bands with low velocities on the outer flanks of topographic highs. These features are consistent with localized thickening of the volcanic extrusive layer from eruptions extending outside of the axial valley that flow down the fault-tilted blocks that form the abyssal hill topography. On-axis velocities are generally relatively high beneath the hydrothermal vent fields likely due to the infilling of porosity by mineral precipitation. Lower velocities are observed beneath the most vigorous vent fields in a seismically active region above the axial magma chamber and may reflect increased fracturing and higher temperatures. Seismic anisotropy is high on-axis but decreases substantially off axis over 5–10 km (0.2–0.4 Ma). This decrease coincides with an increase in seismic velocities resolved at depths ≥1 km and is attributed to the infilling of cracks by mineral precipitation associated with near-axis hydrothermal circulation. The orientation of the fast-axis of anisotropy is ridge-parallel near the segment center but curves near the segment ends reflecting the tectonic fabric within the OSCs.

Off-axis structures of spreading zones according to results of experimental modeling

The off-axis topography of spreading ridges is a result of tectonic and magmatic processes occurring in the axial zone and operating off the ridge axis during further evolution of the crust. The results of physical and numerical simulations have shown that differences in topography roughness, rift valley depth, frequency and amplitude of normal faults, and geometric stability of the rift axis are determined by (a) the rate of extension and accretion of the new crust, (b) the thickness of the brittle lithospheric layer, and (c) the temperature of the underlying asthenosphere. Under conditions of the fast spreading, the stationary axial magma chamber in the crust predetermines the existence of the thinner and weakened lithosphere. As a result, the axis jumps for a short distance and the axis geometry remains almost rectilinear. The destruction of the thin axial lithosphere with a low mechanical strength results in formation of frequent and low-amplitude normal faultings. All these factors lead to the formation of the characteristic poorly dissected topography of fast-spreading ridges. Without a stationary axial magmatic chamber in the crust of slow-spreading ridges and with a thick and strong lithosphere, a deeply dissected axial and off-axis topography arises. The axis jumps for a significant distance within the rift valley, giving rise to geometric instability of the axis and development of transform and nontransform offsets.

Hydrous magmatism triggered by assimilation of hydrothermally altered rocks in fossil oceanic crust (northern Oman ophiolite)

Mid‐ocean ridges magmatism is, by and large, considered to be mostly dry. Nevertheless, numerous works in the last decade have shown that a hydrous component is likely to be involved in ocean ridges magmas genesis and/or evolution. The petrology and geochemistry of peculiar coarse grained gabbros sampled in the upper part of the gabbroic sequence from the northern Oman ophiolite (Wadi Rajmi) provide information on the origin and fate of hydrous melts in fast‐spreading oceanic settings. Uncommon crystallization sequences for oceanic settings (clinopyroxene crystallizing before plagioclase), extreme mineral compositions (plagioclase An% up to 99, and clinopyroxene Mg # up to 96), and the presence of magmatic amphibole, imply the presence of a high water activity during crystallization. Various petrological and geochemical constraints point to hydration, resulting from the recycling of hydrothermal fluids. This recycling event may have occurred at the top of the axial magma chamber where assimilation of anatectic hydrous melts is recurrent along mid‐ocean ridges or close to segments ends where fresh magma intrudes previously hydrothermally altered crust. In ophiolitic settings, hydration and remelting of hydrothermally altered rocks producing hydrous melts may also occur during the obduction process. Although dry magmatism dominates oceanic magmatism, the dynamic behavior of fast‐spreading ocean ridge magma chambers has the potential to produce the observed hydrous melts (either in ophiolites or at spreading centers), which are thus part of the general mid‐ocean ridges lineage.

The petrogenesis of plagioclase‐phyric basalts at mid‐ocean ridges

Plagioclase ultraphyric basalts (PUBs) have been sampled along most mid‐ocean ridges with ultraslow to intermediate spreading rates. Over the past 40 years, the prevalent models for their origin assume positive buoyancy of plagioclase in basaltic liquids resulting in selective concentration of plagioclase phenocrysts by floatation. However, when the global population of PUB lavas is examined, this hypothesis becomes less compelling. PUB host lavas demonstrate a large range of compositions and densities, similar to aphyric glasses from the same ridge segments. Most importantly, the majority of PUB host liquids are less dense than their phenocryst cargo, meaning that plagioclase floatation within a magma chamber cannot be the driving force for phenocryst enrichment. Furthermore, PUB lavas have never been sampled on axis at fast‐spreading centers or from locations with noted contemporaneous axial magma chambers, where PUBs should be abundant if plagioclase is buoyant in mid‐ocean ridge basalt (MORB). Instead, we argue that the high modal abundance of plagioclase results from interaction between magma and preexisting zones of crystal cumulates within the lower crust, possibly followed by loss of olivine during magma ascent. PUBs erupt when the magma maintains an ascent velocity greater than the settling rate of the plagioclase phenocrysts, which precludes long crustal residence times for these magmas. In addition to being a proxy for lower spreading rates, our findings also suggest that PUB eruption can also be used as a proxy for the absence of a magma chamber or transport through a conduit system that bypassed an axial chamber.

Hydrothermal seismicity beneath the summit of Lucky Strike volcano, Mid-Atlantic Ridge

We present twoyears (July 2007–July 2009) of earthquake locations from a local seismological network on the Lucky Strike volcano (Mid-Atlantic Ridge), whose summit hosts one of the most active and largest deep-sea hydrothermal fields known. Two clusters of small (ML<0.8) but continuous seismicity with well-defined lower depth limits extend north and south along-axis from the hydrothermal field. The lower limit of the northern cluster (79% of events) is a few hundred meters above the axial magma chamber (AMC) reflector, whereas the lower limit of the southern cluster (7% of events) is 600m shallower. During a 3-month long event swarm in April–June 2009, the northern cluster's lower limit deepened by 50–100m and two other event clusters were activated: one to the east of the hydrothermal vent fields (5% of events) and another beneath the fields (2% of events). We interpret the continuous background events in the northern and southern clusters as adjustments within a narrow weak axial region due to stress created by thermal contraction events at the bottom of the hydrothermal circulation zone or in the AMC. We interpret the swarm as an episode of hydrothermal penetration toward the AMC that may have been activated by a broader, AMC-level, contraction event. We propose that a narrow axial region of lower upper crustal porosity, combined with the sloped top of the AMC, generates along-axis hydrothermal circulation towards the vent fields. The hydrothermal circulation may in turn modify the AMC topography through enhanced cooling. The AMC reflector's complex topography suggests that the volcano is in a waning phase in which the AMC has already been significantly altered since the latest magma emplacement.

Termination of a 6 year ridge‐spreading event observed using a seafloor seismic network on the Endeavour Segment, Juan de Fuca Ridge

AbstractWe present automatically determined epicenters and magnitudes for 36,523 earthquakes recorded along the Endeavour segment between August 2003 and October 2006 using a local ocean‐bottom seismometer (OBS) network. The catalog is dominated by two swarm sequences in January and February 2005 in the vicinity of the Endeavour overlapping spreading center, which included earthquakes in West Valley, the northern portion of the Endeavour segment, southwest Endeavour Valley, and the Endeavour vent fields. These swarms are attributed to volcanism including a dike intrusion on the northern Endeavour in February 2005 and smaller diking events on the propagating tip of the West Valley segment in both swarms. The dike on the northern Endeavour propagated to the south, which is inconsistent with magma sourced from the axial magma chamber beneath the elevated central portion of the segment. Following the swarms, seismic activity on the Endeavour segment decreased on average to ~15% of pre‐swarm values and almost ceased at the segment ends. We infer that a 6 year non‐eruptive event that started with a swarm in 1999 and finished with the 2005 swarms ruptured the entire segment and relieved plate‐spreading stresses. The inferred coupling between the 1999 and 2005 events, the observation of extensive precursory activity prior to the 2005 swarms, and the interaction between seismically active regions during the swarms is consistent with static triggering with delays influenced by viscoelastic relaxation, hydraulic diffusion, and magma withdrawal and replenishment.

Subseafloor phase equilibria in high-temperature hydrothermal fluids of the Lucky Strike Seamount (Mid-Atlantic Ridge, 37°17′N)

As part of an integrated study conducted at the Lucky Strike Seamount (Mid-Atlantic Ridge, 37°17′N) in 2008, gas-tight sampling devices were used to collect high-temperature (∼300°C) hydrothermal fluids issuing from sulfide structures distributed throughout the vent field located in the summit depression. Compared with previous observations from 1993 to 1997, the most substantial changes in vent fluid compositions are dramatically increased CO2 concentrations (∼5×, up to 133mmol/L) and the observation of vent fluids enriched in dissolved chloride relative to seawater. Combined with an increase in δ13CCO2 values by ∼4‰ in 2008, the elevated CO2 indicates replenishment of the magmatic heat source and may be indicative of a recent magmatic event. The additional supporting fluid chemistry is, however, similar to that of the previous sampling intervals, necessitating a reassessment of the subseafloor controls on vent fluid chemistry at Lucky Strike in the context of recently obtained geophysical data that provides the depth/extent of a steady-state magma chamber. Two-phase behavior is indicated by the chloride variability in the vent fluids; and comparison with experimental data for the associated chloride-dependent partitioning of minor/trace elements suggests the possibility of a similar source fluid for all the vent structures, while limiting the likelihood of shallow phase separation and subseafloor mixing for the hydrothermal end-members. A recently calibrated Fe/Mn geothermometer indicates minimum subseafloor equilibration temperatures of 350–385°C. However, constraints imposed by dissolved Si/Cl in conjunction with geophysical observations are consistent with peak reaction conditions at temperatures of 430–475°C and pressures near the top of the axial magma chamber (∼410–480bars), where magmatic CO2 becomes entrained in the circulating fluids. The distance between the magma chamber and the seafloor at Lucky Strike is substantially greater than at most faster spreading ridges; and we propose the resulting increased residence time in the up-flow zone leads to the re-equilibration of temperature sensitive transition metals at conditions less extreme than those associated with peak reaction. Agreement between experimental data, thermodynamic model calculations, and dissolved concentrations of Fe, Cu, Zn, H2, and H2S in the Lucky Strike fluids reinforce the hypothesis of pH-redox equilibria for transition metals at relatively oxidizing conditions and temperatures predicted by the empirical Fe/Mn geothermometer. In-situ pH measurements of the high-temperature fluids exiting the seafloor are also consistent with the model calculations.

Shallow axial magma chamber at the slow-spreading Erta Ale Ridge

Thermal models predict that spreading velocity at mid-ocean rifts should influence the geometry of the underlying magma chamber. InSAR data from the slow-spreading Ethiopian Rift identify a shallow, elongated magma chamber—a feature usually associated with fast-spreading rifts—beneath the Erta Ale segment, implying that spreading velocity may not be so important after all.

Mineralogical and strontium isotopic record of hydrothermal processes in the lower ocean crust at and near the East Pacific Rise

Tectonic exposures of upper plutonics (>800 m) that are part of a contiguous section of young East Pacific Rise (EPR) crust at the Hess Deep Rift provide the first regional-scale constraints on hydrothermal processes in the upper plutonic crust formed at a fast-spreading ridge. Submersible-collected samples recovered over a 4-km-wide region show that the sheeted dike complex is largely underlain by a 150- to 200-m-thick gabbro unit, followed by a more primitive gabbronorite unit. Gabbroic samples are variably altered by pervasive fluid flow along fracture networks to amphibole-dominated assemblages. The gabbroic rocks are significantly less altered (average 11% hydrous phases) than the overlying sheeted dike complex (average 24%), and the percentage of hydrous alteration diminishes with depth. Incipient, pervasive fluid flow occurred at amphibolite facies conditions (average 720°C), with slightly higher temperatures in the lower 500 m of the section. The extent of subsequent lower-temperature alteration is generally low and regionally variable. The gabbroic samples are slightly elevated in 87Sr/86Sr relative to fresh rock values (0.7024) and less enriched than the overlying sheeted dike complex. 87Sr/86Sr for the pervasively altered gabbroic samples ranges from 0.70244 to 0.70273 (mean 0.70257), tonalites is 0.7038, and pyroxene hornfels ranges from 0.70259 to 0.70271. 87Sr/86Sr does not vary with depth, and there is a strong positive correlation with the percentage of hydrous phases. Strontium contents of igneous and hydrothermal minerals, combined with bulk rock 87Sr/86Sr, indicate that Sr-isotopic exchange is largely controlled by the uptake of fluid 87Sr/86Sr in hydrous minerals and does not require Sr gain or loss. The minimum, time-integrated fluid–rock ratio for the sheeted dike complex and upper plutonics is 0.55–0.66, and the fluid flux calculated by mass balance is ~2.1 to 2.5 × 106 kg m−2, 30–60% higher than fluid fluxes calculated in the same manner for sheeted dike complexes on their own at Hess and Pito Deeps, and Ocean Drilling Program Hole 504B. Alteration patterns within the upper plutonics evolved in response to axial magma chamber (AMC) dynamics at the EPR, such that magma replenishment led to assimilation and thermal metamorphism of the country rock, and the position of the hydrothermal root-zone tracked the vertical migration of the AMC. The freshness of the lowermost gabbroic rocks suggests that pervasive fluid flow does not lead to significant fluid and heat fluxes at and near fast-spreading ridges.

Network of off-axis melt bodies at the East Pacific Rise

At faster-spreading mid-ocean ridges, the creation of new oceanic crust through magmatism usually occurs within a narrow zone on the ridge axis. Three-dimensional seismic images of the fast-spreading East Pacific Rise reveal a network of magmatic bodies 4–8 km away from the ridge axis that seem to be connected to the axial magma chamber.

Modeling heat transfer from a convecting, crystallizing, replenished silicic magma chamber at an oceanic spreading center

[1] Most hydrothermal systems at oceanic spreading centers are underlain by basaltic magma bodies; however, some are underlain by higher-silica magmas such as andesite or dacite. The different viscosity of the latter magmas, which results from their higher SiO2 content, lower liquidus and solidus temperatures, and higher water contents, affects the rate of heat transport from these magmas, and the behavior of the overlying hydrothermal system. We construct viscosity models for andesite and dacite melts as a function of temperature and water content and incorporate these expressions into a numerical model of thermal convective heat transport from a high Rayleigh number, crystallizing, replenished axial magma chamber (AMC) beneath a hydrothermal circulation system. Simulations comparing the time-dependent heat flux from dry basalt, 0.1 wt.% H2O andesite, 3 wt.% H2O andesite, and 4 wt.% H2O dacite indicate that higher-viscosity magmas convect less vigorously, resulting in lower heat flux, possibly lower vent temperatures, and a slower decay rate of the heat flux. Hydrothermal systems driven by unreplenished high-silica melts may have slightly longer lifetimes than those driven by basalt; however, vent temperature and heat output decay on decadal time scales. Magma replenishment at a rate of ∼10−8–10−7 m/s across the base of the AMC can maintain relatively stable heat output between ∼107–109 Watts and typical hydrothermal vent temperatures. Such replenishment rates are not likely to result from buoyancy-driven melt transport by porous flow through the lower crust, especially for high-viscosity magmas such as andesite and dacite.

Gabbros from IODP Site 1256, equatorial Pacific: Insight into axial magma chamber processes at fast spreading ocean ridges

The ODP/IODP multileg campaign at ODP Site 1256 (Cocos plate, eastern equatorial Pacific) provides the first continuous in situ sampling of fast spreading ocean crust from the extrusive lavas, through the sheeted dikes and down into the uppermost gabbros. This paper focuses on a detailed petrographic and microanalytical investigation of the gabbro section drilled during IODP Expedition 312. The marked patchy and spotty features that can be observed in many Hole 1256D gabbros is mostly due to a close association of two different lithological domains in variable amounts: (1) subophitic domains and (2) a granular matrix. Major and trace element mineral compositions, geothermometry, and petrological modeling suggest that subophitic and granular domains follow one single magma evolution trend formed by in situ fractionation. The subophitic domains correspond to the relative primitive, high-temperature end-member, compositionally similar to the basalts and dikes from the extrusive unit upsection, while the granular domains fit with a magma evolution by crystal fractionation to lower temperatures, up to a degree of crystallization of ∼80%. Our results support the following scenario for the fossilization of the axial melt lens at ODP Site 1256: relatively primitive MORB melts under near-liquidus conditions fill the melt lens and feed the upper, extrusive crust. Near the melt lens–sheeted dike boundary at lower temperatures, crystallization starts with first plagioclase before clinopyroxene in a mushy zone forming the subophitic domains. At decreasing temperatures, the subophitic domains continue to crystallize, finally forming a well-connected framework. Evolved, residual melt is finally trapped within the subophitic network, crystallizing at near-solidus conditions to the granular matrix. Another important textural feature in Hole 1256D gabbros is the presence of microgranular domains which are interpreted as relics of stoped/assimilated sheeted dikes (transformed to “granoblastic dikes” by contact metamorphism). All these different domains can be observed in close association, often at the thin section scale, demonstrating the extremely complex petrological record of combined crystallization/assimilation processes ongoing in the axial melt lens. Very similar gabbros with a marked spotty/patchy appearance, and bearing the same close association of lithological domains as observed at Site 1256, are known in the so-called “varitextured gabbro” unit from the Oman Ophiolite located at the same structural level, between cumulate gabbros and granoblastic dikes. The close petrological similarity of the gabbro/dike transition between both IODP Hole 1256D and the Oman ophiolite suggests that in situ fractionation and dike assimilation/contamination are major magmatic processes controlling the dynamics and fossilization of the axial melt lens at fast spreading oceanic ridges.

Seismogenic structure and processes associated with magma inflation and hydrothermal circulation beneath the East Pacific Rise at 9°50′N

A dense ocean bottom seismometer array recorded more than 7,300 microearthquakes between October 2003 and April 2004 on the fast spreading East Pacific Rise at 9°50′ N. A previous initial analysis of these data revealed the general structure of an along-axis oriented hydrothermal circulation cell, with an inferred down-flow zone near a 4th-order axial discontinuity, a horizontal band of seismicity overlying the axial magma chamber (AMC), and an upflow zone that correlates with activity patterns of hydrothermal vents. Here we present cross-correlation-based double-difference hypocenter locations that reveal further insight into the detailed structure and kinematics that control seismic failure. We find that the majority of events within the inferred down flow pipe occur near the surface where the entrenched seawater presumably lowers the frictional strength of faults generated by local tectonic stresses. Beneath the eastern side of the ridge axis, just above the AMC at ∼1.4 km depth, we observe repeated shear failure along well defined steeply east dipping faults. Composite focal mechanisms indicate reverse motion, suggesting that slip on these faults is caused by AMC inflation and possibly injection of magma into a narrow sill. We resolve a small counterclockwise skew in the strike of the reverse faults relative to the direction of the ridge axis, consistent with Nuvel 1A spreading direction suggesting that the regional tectonic stress field may be controlling the seismogenic structures at depth. Earthquakes on the reverse faults occur preferentially during peak extensional tidal stresses, indicating that these faults are critically stressed.