Axial Magma Chamber Research Articles

Seismic Constraints on Hydrothermal Circulation and Magmato‐Tectonic Interactions Beneath Lucky Strike Volcano, Mid‐Atlantic Ridge

AbstractThe Lucky Strike volcano is the central edifice of the Lucky Strike segment on the slow spreading Mid‐Atlantic Ridge. The volcano summit hosts one of the largest known deep‐sea hydrothermal fields, and overlies an axial magma chamber (AMC) whose summit reflector lies 3–3.8 km beneath the seafloor. We present a 12‐year microearthquake catalog beneath the volcano, which we constructed using data collected from 2007 to 2019 as part of the EMSO‐Azores observatory. The catalog reveals continuous low magnitude seismicity (85% with ML < 0), focused mainly north‐northwest of the hydrothermal field and 0–2 km above the AMC reflector. Focal mechanisms estimated only from the 2010–2011 deployment show a mixture of thrust, normal and strike/slip faulting mechanisms. We propose that the observed microearthquake activity is due to thermal cracking, tectonic cracking, and possible small dike injections occurring at the base of hydrothermal downflow zones. We document a ∼800 m eastward shift of the seismicity sometime during a catalog gap from June 2013 to April 2015, accompanied by a change in the seismicity pattern from a patch just above the AMC to a more vertically aligned structure. We also identify several higher seismicity periods, often including relatively large magnitude events (ML > 0.8). We interpret the spatio‐temporal variations in seismicity patterns and the correlation between larger magnitude events and higher seismicity periods to indicate that the hydrothermal domain is a dynamic interface, where the variations are caused by changes in permeability and/or temperature induced by tectonic and small episodic melt injections.

Trans‐Lithospheric Ascent Processes of the Deep‐Rooted Magma Plumbing System Underneath the Ultraslow‐Spreading SW Indian Ridge

AbstractProcesses of magma generation and transportation in global mid‐ocean ridges are key to understanding lithospheric architecture at divergent plate boundaries. These magma dynamics are dependent on spreading rate and melt flux, where the SW Indian Ridge represents an end‐member. The vertical extent of ridge magmatic systems and the depth of axial magma chambers (AMCs) are greatly debated, in particular at ultraslow‐spreading ridges. Here we present detailed mineralogical studies of high‐Mg and low‐Mg basalts from a single dredge on Southwest Indian Ridge (SWIR) at 45°E. High‐Mg basalts (MgO = ∼7.1 wt.%) contain high Mg# olivine (Ol, Fo = 85–89) and high‐An plagioclase (Pl, An = 66–83) as phenocrysts, whereas low‐Mg basalts contain low‐Mg# Ol and low‐An Pl (Fo = 75–78, An = 50–62) as phenocrysts or glomerocrysts. One low‐Mg basalt also contains normally zoned Ol and Pl, the core and rim of which are compositionally similar to those in high‐Mg and low‐Mg basalts, respectively. Mineral barometers and MELTS simulation indicate that the high‐Mg melts started to crystallize at ∼32 ± 7.8 km, close to the base of the lithosphere. The low‐Mg melts may have evolved from the high‐Mg melts in an AMC at a depth of ∼13 ± 7.8 km. Such great depths of magma crystallization and the AMC are likely the result of enhanced conductive cooling at ultraslow‐spreading ridges. Combined with diffusion chronometers, the basaltic melts could have ascended from the AMC to seafloor within 2 weeks to 3 months at average rates of ∼0.002–0.01 m/s, which are the slowest reported to date among global ridge systems and may characterize mantle melt transport at the slow end of the ridge spreading spectrum.

Gravity Data Reveal New Evidence of an Axial Magma Chamber Beneath Segment 27 in the Southwest Indian Ridge

Hydrothermal systems are integral to mid-ocean ridge activity; they form massive seafloor sulfide (SMS) deposits rich in various metallic elements, which are potential mineral resources. Since 2007, many hydrothermal fields have been discovered along the ultraslow-spreading Southwest Indian Ridge (SWIR). The Duanqiao hydrothermal field is located at segment 27’s axis between the Indomed and Gallieni transform faults; tomography models reveal an obvious low-velocity anomaly beneath it, indicating a possible axial magma chamber (AMC). However, confirmation of an AMC’s existence requires further study and evidence. In this study, we first calculated the gravity effect to identify the heterogeneous distribution of crustal density beneath segment 27 and the surrounding area. Next, we used the gravity-inversion method to obtain the crustal density structure beneath the study area. The results indicate that a thickened crust and low-density crustal materials exist beneath segment 27. The low-density anomaly in the lower crust beneath the Duanqiao hydrothermal field suggests the existence of an AMC covered with a cold and dense upper crust. The density results identify several faults, which provide potential channels for magma migration. In addition, the melt migrates westward and redistributes laterally toward the segment’s western end. However, when migrating toward the segment’s eastern end, the melt is affected by a rapid cooling mechanism. Therefore, the segment’s ends present different density features and morphologies of nontransform discontinuities (NTDs).

Geochemistry, sulfur and lead isotopic composition of hydrothermal sulfide from the Duanqiao hydrothermal field on the Southwest Indian Ridge: implications for ore genesis

ABSTRACT Hydrothermal activities on ultraslow-spreading ridges exhibit diverse characteristics, long histories with multiple participants, and might form large-scale, high-grade sulfide deposits. The Duanqiao hydrothermal field (DHF) is located at the segment with the thickest oceanic crust and a large axial magma chamber on the Southwest Indian Ridge, providing unique perspective of sulfide metallogenesis on ultraslow-spreading ridges. Previous studies revealed that DHF sulfide exhibits distinct features of enrichment of ore-forming elements in comparison with those of hydrothermal fields on sediment-starved mid-ocean ridges. However, the genesis and processes responsible for such differences remain poorly constrained. In this study, mineralogical, geochemical and S and Pb isotopic analyses were performed on relict sulfide mound samples to characterize DHF formation. The samples show clear concentric mineral zonation from the interior to the exterior wall. Assemblages of chalcopyrite, sphalerite, and pyrite are distributed mainly in the interior wall, whereas pyrite and marcasite are distributed mainly in the exterior wall. The low Cu content and Pb isotopic composition of the sulfide indicate that the metals are derived mainly from basement basalts. The δ34S values exhibit positive values distributed over a reasonably narrow range (2.42‰–7.97‰), which suggests approximately 62.1%–88.5% of S with basaltic origin. Compared with most hydrothermal fields along the sediment starved mid-ocean ridges, the DHF sulfide shows particularly high contents of Pb (263–2630 ppm), As (234–726 ppm), Sb (7.32–44.3 ppm), and Ag (35.2 to >100 ppm). The δ34S values exhibit an increasing tendency from the sample exterior to the interior. We propose that these features probably reflect the existence of a subsurface zone refining process. Our results provide new insight into the sulfide formation process and contribute to understanding the metallogenic mechanism of hydrothermal sulfides on ultraslow-spreading ridges.

Time series of hydrothermal vent fluid chemistry at Main Endeavour Field, Juan de Fuca Ridge: Remote sampling using the NEPTUNE cabled observatory

Seafloor hydrothermal fluids play a critical role in regulating a wide range of geochemical and biological processes. Chemical and physical changes to venting fluids undoubtedly occur in response to geological events, at depth or near the seafloor, with corresponding effects on vent fluid chemistry, mineralization, and biological activity. However, the uncertain timing of such events makes it impractical to anticipate their occurrence. Thus, the temporal evolution of seafloor vent fluids and coexisting mineral deposits is most often inferred from observations made using conventional deep submergence assets that may be non-continuous for a specific vent field. The recent development of submarine volcanic observatories operated by Ocean Networks Canada (ONC) and the Ocean Observatory Initiative (OOI) in the U.S. that deliver power to instruments on the seafloor and communications via high-speed fiber optic cable permits real-time communication and monitoring of seafloor vents. Here, we describe the development and application of a remotely operated hydrothermal fluid sampler that enables repeat on-demand sampling of high-temperature vent fluids triggered through the internet. The sampler was deployed from September 2019 to June 2020 at the S&M vent area, in the southern portion of Main Endeavour Field (MEF), Juan de Fuca Ridge, on ONC's NEPTUNE ocean observatory. Nine vent fluid samples were acquired over a period of nine months. Analyses of these samples confirm the moderately high-temperature origin of the source fluid, as indicated, for example, by dissolved chloride depletion relative to seawater (vapor-rich) and moderately high silica concentrations. These and other dissolved species, such as methane, a known hallmark of MEF vent fluids, are in excellent agreement with the reported composition of S&M vent fluids over the past 15 years or more, suggesting overall chemical stability and rock-dominated alteration processes. On the other hand, short-term variability of dissolved Mg, sulfate and barium indicate entrainment of conductively heated and partially reacted seawater. Both before and after the incursion of the “secondary” seawater derived fluid, however, samples reveal the lowest Mg and sulfate concentrations yet reported for seafloor hydrothermal vent fluid samples. Accordingly, these data provide new insight on the solubility of these and other elements coexisting with minerals close to the Axial Magma Chamber (AMC), from which the primary source fluid is ultimately derived. A separate noteworthy event characterized by ∼20 °C decrease in temperature and associated 24–77% decrease in dissolved Fe concentration was observed toward the end of the deployment, with implications for hydrothermal transition metal fluxes and linked biogeochemical processes. On-demand remote acquisition of a continuous series of high-temperature vent fluid samples from a single vent permits the temporal evolution of heat and mass transfer processes to be studied with a heretofore unavailable perspective. Furthermore, deployment of this sampler within the NEPTUNE ocean observatory opens the door to future studies based on comparative analysis of contemporaneous datasets.

Extending the dataset of fluid geochemistry of the Menez Gwen, Lucky Strike, Rainbow, TAG and Snake Pit hydrothermal vent fields: Investigation of temporal stability and organic contribution

The Menez Gwen, Lucky Strike, Rainbow, TAG (Transatlantic Geotraverse) and Snake Pit hydrothermal vent fields on the Mid-Atlantic Ridge were revisited and resampled for geochemical investigation during the BIOBAZ 2013 and BICOSE 2014 research cruises. Geochemical analysis of the major and minor elements of the hydrothermal fluid and concentrations of gases extends and complements the existing dataset. Our results are consistent with values previously reported and fall within the range of the analytical error. This indicates that the hydrothermal vent field system has remained relatively stable over the last few decades. However, some differences were observed and (i) suggested a recent eruption at Menez Gwen, (ii) supported the occurrence of low-temperature serpentinisation in this same site, (iii) supported a change in the reaction zone or axial magma chamber (AMC) depth at Lucky Strike, (iv) an increase of the temperature at depth at Snake Pit and (v) supported the hypothesis of large seawater entrainment through the TAG hydrothermal mound. Besides, it is possible that small temporal and spatial scale processes may control a significant part of the geochemistry, owing to the fact that some variations in the data could not be interpreted. However, our investigation of the organic geochemistry represents a pioneering addition to research for Menez Gwen, Snake Pit and TAG and a much more comprehensive study for Lucky Strike and Rainbow. Concentrations for a wide variety of semi volatile organic compounds (SVOCs) were obtained for the first time at all sites. Our results showed that a great part of the total organic carbon (TOC) could not be allocated by the total SVOCs studied here, suggesting that other processes/sources of organic carbon remain to be identified. The TAG organic geochemistry seemed entirely based on thermogenic processes whereas mixed processes may occur at the other vent field. The presence of n-alkanes suggested the contribution of a low-temperature fluid at all sites. An additional high-temperature organic matter degradation component was likely present at Menez Gwen and Lucky Strike. Our results also indicated that both abiogenic and biogenic processes produced organic compounds. Therefore, we suggest that a portion of the fatty acids at Menez Gwen and polyaromatic hydrocarbons (PAHs) at Rainbow may be derived from abiogenic processes, whereas biogenic processes could be responsible for the presence of n-fatty acids (n-FAs) at Lucky Strike and Rainbow. Moreover, organic geochemistry data appeared to be helpful in understanding some inorganic processes.

Using precious metal probes to quantify mid-ocean ridge magmatic processes

Basalts from the East Pacific Rise (EPR), Siqueiros transform zone and Mid-Atlantic Ridge (MAR) area have been analyzed for the platinum-group elements (PGE) and a wide range of incompatible elements. The low PGE content of the most primitive mid-ocean ridge basalts (MORB) suggests that they leave the mantle sulfide-saturated but become sulfide under-saturated, as a consequence of decompression, during their ascent from the mantle to the MOR magma chamber. Because the pressure drop is small, the ascending magma enters the magma chamber slightly sulfide under-saturated and requires only a small amount of fractional crystallization to return it to sulfide-saturation. Sulfide-saturation is marked by the Pd content of the melt falling by over an order of magnitude at ca. 9.5 wt.% MgO. However, once the magma has become sulfide-saturated, it shows no evidence of a further decline in Pd with decreasing MgO as the system evolves. This must result from regular replenishment of the underlying axial magma chamber by fresh batches of primitive magma. Most replenishments occur when the MgO content of the resident melt in the magma chamber is between ∼9 to 6.5 wt.% MgO. We combine the tight constraints imposed by highly compatible Pd abundances, with those imposed by strongly incompatible elements, to produce the first model that successfully accounts for the variations of both classes of elements in open system mid-ocean ridges magma chambers for normal-type MORB (N-MORB). We show that the lack of decline in the Pd content of sulfide-saturated mid-ocean ridge basalts, with decreasing MgO, requires frequent small replenishments (between 1% and 4%) of similar magnitude, rather than large initial inputs that systematically decline from near 100% to near zero during the life of an individual ridge system.Assuming the spreading rate of ∼110 mm/yr for the EPR at 9°N, and 14 yr between the 1991–1992 and 2005–2006 eruptions, the calculated volume of erupted and intruded magma is 2.3 × 103 m3/m of ridge length. If the expelled melt represents 0.5% of the accessible magma in the chamber, as suggested by the Pd modeling, the volume of the magma accessible during tapping is 4.6 × 105 m3/m of ridge length.

Origin of oceanic ferrodiorites by injection of nelsonitic melts in gabbros at the Vema Lithospheric Section, Mid Atlantic Ridge

Oxide gabbros are a minor but diffuse component of the lower oceanic crust. Their presence poses questions on lower crust exhumation processes and magma differentiation at mid ocean ridges because they are systematically associated with shear zones and are hardly explained by classical fractionation and melt migration models. Here, we report on a study of lower-crust gabbros recovered from the Vema Lithospheric Section at 11°N along the Mid Atlantic Ridge, where oxide gabbros are abnormally abundant relative to ridge centred magmatic intrusives and where we found a peculiar lithological occurrence represented by deformed diorites extremely enriched in Fe-Ti-oxides and apatites. Their complex genetic history reveals a hybrid nature consistent with derivation from high pressure injections of Fe-Ti-P saturated nelsonitic melts in a primitive gabbroic groundmass that induced fracturing, de-compaction, mineral resorption and chemical re-equilibration. Melt injections may occur after intense ductile shearing at the edges of the axial magma chamber following lateral differentiation of primitive melts injected at the centre of the ridge axis segment. We propose a regime of lateral, instead of vertical, melt differentiation along the ridge axis and a possible role for melt immiscibility in the formation of Fe-Ti-P melt pockets in oceanic domains.

The Origin of Plagiogranites: Coupled SIMS O Isotope Ratios, U–Pb Dating and Trace Element Composition of Zircon from the Troodos Ophiolite, Cyprus

AbstractU–Pb ages, trace element content and oxygen isotope ratios of single zircons from five plagiogranite intrusions of the Troodos ophiolite were measured to determine their crystallization age and assess the importance of fractional crystallization versus crustal anatexis in their petrogenesis. The results indicate that oceanic magmatism in Troodos took place at 94·3 ± 0·5 Ma, about 3 Myr earlier than previously recognized. Later hydrothermal alteration has affected most of the Troodos plagiogranitic rocks, resulting in growth of new zircon and/or partial alteration of zircon domains, causing slightly younger apparent crystallization ages. The new age inferred for seafloor spreading and ocean crust accretion in Troodos nearly overlaps that of the Semail ophiolite in Oman (95–96 Ma), strengthening previous indications for simultaneous evolution of both ophiolites in similar tectonic settings. Average δ18O(Zrn) values in the Troodos plagiogranites range between 4·2 and 4·8 ‰. The lower values in this range are lower than those expected in equilibrium with mantle-derived melt (5·3 ± 0·6 ‰), indicating variable contribution from hydrothermally altered, deep-seated oceanic crust in most of the Troodos plagiogranite intrusions. The inferred substantial involvement of crustal component is consistent with the existence of a shallow axial magma chamber, typical of fast-spreading mid-ocean ridge settings, within the Troodos slow-spreading ridge environment. This apparent contradiction may be reconciled by episodically intense magmatism within an otherwise slow, magmatically deprived spreading axis.

Magnetotelluric exploration of the Wagner Basin, Gulf of California, Mexico: Evidence for an axial magma chamber and hydrothermal circulation

Different geological and geophysical disciplines have suggested the Wagner Basin as a promising location of geothermal resources. As a reconnaissance tool Magnetotelluric (MT) data were measured at 10 sites along a profile over this basin in the northern Gulf of California. The subsurface distribution of electric resistivity was estimated with two-dimensional smooth inversion using the apparent resistivities and phases from both field polarizations. We found a deep conductor underneath the center of the basin, interpreted as a region of partial melt feeding the accretion zone of this incipient oceanic crust. From six MT sites with close-by heat flow measurements, we also found a positive correlation between the interpreted shallow (first 100 m) low resistivities and high heat flows. This shows that the MT method can be used to map submarine heat sources.

Evolution, hydrothermal assimilation, and ascent of magma inferred from volatile contents in MORB glasses: An example from thick lava pile at IODP Site 1256

A continuous section through an 811 m-thick lava pile of mid-ocean ridge basalts at Integrated Ocean Drilling Program Site 1256 provides important constraints on the variation with depth of volatile element (H2O, CO2, S and Cl) contents in fresh matrix glasses to understand evolution of magma, condition of magma ascent, and degree of hydrothermal assimilation. The lava pile is divided into two groups based on eruption location: off-axis basalts of upper portion (250–534 m beneath seafloor, mbsf) and on-axis basalts of lower portion (534–1061 mbsf). The lava pile is composed of pillow, sheet, and massive lavas that include thin (<2 m) hyaloclastite intervals.Variations of H2O and major element contents in Site 1256 matrix glasses can be explained by fractional crystallization of one primitive basalt (MgO, 7.7 wt%) at the pressure of a melt lens (top of axial magma chamber). CO2/Nb and S/Dy in a large majority of fresh matrix glasses are lower than undegassed ratios, and these facts indicate that CO2 and S losses occurred during differentiation within the melt lens and/or magma transport to the seafloor. Saturation pressures calculated from dissolved CO2 and H2O contents give a wide range of pressures from seafloor (~25 MPa) to axial magma chamber (~60 MPa). Many off-axis glasses have saturation pressures of seafloor, but on-axis glasses have high saturation pressures up to axial magma chamber depths. The saturation pressures of off-axis glasses decrease with decreasing depth (i.e., with increasing distance from the axis). These observations indicate that the many off-axis glasses were highly degassed because their source magmas travelled for long time periods from melt lens to off-axis seafloor locations. On the other hand, on-axis glasses were less degassed because magma transport times to on-axis locations were shorter than those for off-axis locations.The most distinctive characteristic for the Site 1256 glasses is their higher Cl/Nb than any other mid-ocean ridge basalt glasses. The strong Cl enrichment is explained by assimilation of hydrothermal brines. Beneath Site 1256, the melt lens was very shallow (<1.2 km) and hydrothermal circulation of high-salinity brines with variable salinities would reach easily to the melt lens.

Seafloor hydrothermal activity along mid-ocean ridge with strong melt supply: study from segment 27, southwest Indian ridge

Continuous tow investigations have shown that the present vent field inventory along fast to intermediate spreading ridges may be underestimated by at least 3–6 times, while the limited towed line investigations of venting sites along slow to ultra-slow spreading ridges make it impossible to determine their distribution. The Chinese Dayang cruise has conducted detailed towed line surveys of hydrothermal activity on segment 27 of the ultra-slow spreading southwest Indian ridge in 2015. The results have identified as many as 9 hydrothermal fields along 85-km-long segment, including one confirmed hydrothermal field, three inferred hydrothermal fields and five suspected fields. Hydrothermal activities are not only limited along-axis but also found approximately 10 km away from the axis. These vent fields are likely powered by a seismically identified axial magma chamber, including melt migration along normal faults to flank areas. The calculated hydrothermal activity frequency on segment 27 is approximately 3.6–8 times higher than that calculated from the Interridge database, suggesting that careful system exploration can reveal more hydrothermal activities even on ultra-slow spreading ridges effected by hotspot.

The mechanism of tidal triggering of earthquakes at mid-ocean ridges

The strong tidal triggering of mid-ocean ridge earthquakes has remained unexplained because the earthquakes occur preferentially during low tide, when normal faulting earthquakes should be inhibited. Using Axial Volcano on the Juan de Fuca ridge as an example, we show that the axial magma chamber inflates/deflates in response to tidal stresses, producing Coulomb stresses on the faults that are opposite in sign to those produced by the tides. When the magma chamber’s bulk modulus is sufficiently low, the phase of tidal triggering is inverted. We find that the stress dependence of seismicity rate conforms to triggering theory over the entire tidal stress range. There is no triggering stress threshold and stress shadowing is just a continuous function of stress decrease. We find the viscous friction parameter A to be an order of magnitude smaller than laboratory measurements. The high tidal sensitivity at Axial Volcano results from the shallow earthquake depths.

Lithospheric Structure and Tectonic Processes Constrained by Microearthquake Activity at the Central Ultraslow‐Spreading Southwest Indian Ridge (49.2° to 50.8°E)

AbstractBeneath ultraslow‐spreading ridges, the oceanic lithosphere remains poorly understood. Using recordings from a temporary array of ocean bottom seismometers, we here report an ~17‐days‐long microearthquake study on two segments (27 and 28) of the ultraslow‐spreading Southwest Indian Ridge (49.2° to 50.8°E). A total of 214 locatable microearthquakes are recorded; seismic activity appears to be concentrated within the west median valley at Segment 28 and adjacent nontransform discontinuities. Earthquakes reach a maximum depth of ~20 km beneath the seafloor, and they mainly occur in the mantle, implying a cold and thick brittle lithosphere. The relatively uniform brittle/ductile boundary beneath Segment 28 suggests that there is no focused melting in this region. The majority of earthquakes is located below the Moho interface, and a 5‐km‐thick aseismic zone is present beneath Segment 28 and adjacent nontransform discontinuities. At the Dragon Flag hydrothermal vent field along Segment 28, the presence of a detachment fault has been inferred from geomorphic features and seismic tomography. Our seismicity data show that this detachment fault deeply penetrates into the mantle with a steeply dipping (~65°) interface, and it appears to rotate to a lower angle in the upper crust, with ~55° of rollover. There is a virtual seismic gap beneath magmatic Segment 27, which may be connected to the presence of an axial magma chamber beneath the spreading center and focused melting; in this scenario, the increased magma supply produces a broad, elevated temperature environment, which suppresses earthquake generation.

Formation of fast-spreading lower oceanic crust as revealed by a new Mg–REE coupled geospeedometer

A new geospeedometer is developed based on the differential closures of Mg and rare earth element (REE) bulk-diffusion between coexisting plagioclase and clinopyroxene. By coupling the two elements with distinct bulk closure temperatures, this speedometer can numerically solve the initial temperatures and cooling rates for individual rock samples. As the existing Mg-exchange thermometer was calibrated for a narrow temperature range and strongly relies on model-dependent silica activities, a new thermometer is developed using literature experimental data. When the bulk closure temperatures of Mg and REE are determined, respectively, using this new Mg-exchange thermometer and the existing REE-exchange thermometer, this speedometer can be implemented for a wide range of compositions, mineral modes, and grain sizes.Applications of this new geospeedometer to oceanic gabbros from the fast-spreading East Pacific Rise at Hess Deep reveal that the lower oceanic crust crystallized at temperatures of 998–1353 °C with cooling rates of 0.003–10.2 °C/yr. Stratigraphic variations of the cooling rates and crystallization temperatures support deep hydrothermal circulations and in situ solidification of various replenished magma bodies. Together with existing petrological, geochemical and geophysical evidence, results from this new speedometry suggest that the lower crust formation at fast-spreading mid-ocean ridges involves emplacement of primary mantle melts in the deep section of the crystal mush zone coupled with efficient heat removal by crustal-scale hydrothermal circulations. The replenished melts become chemically and thermally evolved, accumulate as small magma bodies at various depths, feed the shallow axial magma chamber, and may also escape from the mush zone to generate off-axial magma lenses.

Seismic evidence that black smoker heat flux is influenced by localized magma replenishment and associated increases in crustal permeability

AbstractHydrothermal circulation at mid‐ocean ridges is responsible for ~25% of Earth's heat flux and controls the thermal and chemical evolution of young oceanic crust. The heat flux of black smoker hydrothermal systems is thought to be primarily controlled by localized magma supply and crustal permeability. Nevertheless, magma chamber characteristics and the nature of crustal permeability beneath such systems remain unclear. Here we apply three‐dimensional full‐waveform inversion to seismic data from the hydrothermally active Endeavour segment of the Juan de Fuca Ridge to image the upper crust in high resolution. We resolve velocity variations directly above the axial magma chamber that correlate with variations in seismicity, black smoker heat flux, and the depth of the axial magmatic system. We conclude that localized magma recharge to the axial magma lens, along with induced seismogenic cracking and increased permeability, influences black smoker heat flux.

Evidence of an axial magma chamber beneath the ultraslow-spreading Southwest Indian Ridge

Abstract Ultraslow-spreading ridges are a novel class of spreading centers symbolized by amagmatic crustal accretion, exposing vast amounts of mantle-derived peridotites on the seafloor. However, distinct magmatic centers with high topographies and thick crusts are also observed within the deep axial valleys. This suggests that despite the low overall melt supply, the magmatic process interacting with the tectonic process should play an important role in crustal accretion; however, this has been obscured due to the lack of seismic images of magma chambers. Using a combination of seismic tomography and full waveform inversion of ocean bottom seismometer data from the Southwest Indian Ridge at 50°28′E, we report the presence of a large low-velocity anomaly (LVA) ∼4–9 km below the seafloor, representing an axial magma chamber (AMC) in the lower crust. This suggests that the 9.5-km-thick crust here is mainly formed by a magmatic process. The LVA is overlain by a high-velocity layer, possibly forming the roof of the AMC and defining the base of hydrothermal circulation. The steep velocity gradient just below the high-velocity layer is explained by the ponding of magma at the top of the AMC; this could provide the overpressure for lateral dike propagation along the ridge axis, leading to a complex interaction between magma emplacement, tectonic, and hydrothermal processes, and creating a diversity of seafloor morphology and extremely heterogeneous crust.

Seismic structure and magmatic construction of crust at the ultraslow‐spreading Southwest Indian Ridge at 50°28'E

AbstractWe present a three‐dimensional crustal structure of a magmatically robust segment of the ultraslow‐spreading Southwest Indian Ridge at 50°28'E based on tomographic inversions of an ocean bottom seismometer data set. Our results show an upper crustal low‐velocity band in the axial zone, which is attributed to increased porosities due to active extensions, leading to anisotropy in the upper crust with a fast direction subperpendicular to the spreading direction. In the lower crust, the results reveal a round‐shaped low‐velocity anomaly at the segment center, indicative of high temperatures and/or a small amount of melt, suggestive of the presence of an axial magma chamber. At the midcrustal depth, an along‐axis asymmetry is observed with respect to the segment center. While a small low‐velocity anomaly indicates lateral magma redistribution toward the western segment end, the deep‐penetrating low velocities and high velocity gradients toward the eastern end suggest that the crust is colder and contains a thicker fractured layer. This asymmetry occurs very close to the axial magma chamber (&lt;5 km) and seems to be related to the fact that the oblique‐spreading domain at the eastern end offsets the ridge axis by a larger distance than that at the western end. We suggest that an along‐axis deep‐penetrating hydrothermal circulation develops on the east side of the axial magma chamber, in response to the rapid change from orthogonal‐ to oblique‐spreading domains and cools the crust.

A Statistical Description of Concurrent Mixing and Crystallization during MORB Differentiation: Implications for Trace Element Enrichment

The pattern of trace element enrichment and variability found in differentiated suites of basalts is a simple observable, which nonetheless records a wealth of information on processes occurring from the mantle to crustal magma chambers. The incompatible element contents of some mid-ocean ridge basalt (MORB) sample suites show progressive enrichment beyond the predictions of simple models of fractional crystallization of a single primary melt. Explanations for this over-enrichment have focused on the differentiation processes in crustal magma chambers. Here we consider an additional mechanism and focus instead on the deviation from simple fractionation trends that is possible by mixing of diverse mantle-derived melts supplied to magma chambers. A primary observation motivating this strategy is that there is significant chemical diversity in primitive high-MgO basalts, which single liquid parent models cannot match. Models were developed to simulate the compositional effects of concurrent mixing and crystallization (CMC): diverse parental melts were allowed to mix, with a likelihood that is proportional to the extent of fractional crystallization. Using a simple statistical model to explore the effects of concurrent mixing and crystallization on apparent liquid lines of descent, we show how significant departure from Rayleigh fractionation is possible as a function of the diversity of trace elements in the incoming melts, their primary MgO content, and the relative proportion of enriched to depleted melts. The model was used to make predictions of gradients of trace element enrichment in log[trace element]–MgO space. These predictions were compared with observations from a compilation of global MORB and provide a test of the applicability of CMC to natural systems. We find that by considering the trace element variability of primitive MORB, their MgO contents and degree of enrichment, CMC accurately predicts the pattern of trace element over-enrichment seen in global MORB. Importantly, this model shows that the relationship between over-enrichment and incompatibility can result from mantle processes: the fact that during mantle melting maximum variability is generated in those elements with the smallest bulk Kd. Magma chamber processes are therefore filtering the signal of mantle-derived chemical diversity to produce trace element over-enrichment during differentiation. Finally, we interrogate the global MORB dataset for evidence that trace element over-enrichment varies as a function of melt supply. There is no correlation between over-enrichment and melt supply in the global dataset. Trace element over-enrichment occurs at slow-spreading ridges where extensive steady-state axial magma chambers, the most likely environment for repeated episodes of replenishment, tapping and crystallization, are very rarely detected. This supports a model whereby trace element over-enrichment is an inevitable consequence of chemically heterogeneous melts delivered from the mantle, a process that may operate across all rates of melt supply.

Rapid ascent and emplacement of basaltic lava during the 2005–06 eruption of the East Pacific Rise at ca. 9°51′N as inferred from CO2 contents

Eruption rates at the mid-ocean ridges (MORs) are believed to strongly control the morphology and length of lava flows emplaced along the ridge axis, and thus the structure and porosity of the upper oceanic crust. Eruption rate also represents one of the few tools to gain insight into the driving pressures within sub-ridge magmatic systems. As eruption rate is inferred to vary systematically along the global mid-ocean ridge system, understanding of how to assess eruption rate in submarine systems and how it maps to observable features of the ridge axis would provide a powerful tool to understand Earth's largest magmatic system. Eruption rates at MORs are poorly constrained, however, because of a lack of direct observations, preventing the duration of an eruption to be quantified. This study uses decompression experiments of MORB samples and numerical modeling of CO2 degassing to reconstruct the timescales for magma ascent and lava emplacement during the 2005–06 eruption of the East Pacific Rise at ca. 9°51′N. Samples collected from the lava flow are all supersaturated in dissolved CO2 contents, but CO2 decreases with distance from the vent, presumably as a consequence of progressive CO2 diffusion into growing bubbles. Samples collected at the vent contain ∼105 vesicles per cm3. Pieces of these samples were experimentally heated to 1225 °C at high pressure and then decompressed at controlled rates. Results, plus those from numerical modeling of diffusive bubble growth, indicate that magma rose from the axial magma chamber to the seafloor in ≤1 h and at a rate of ≥2–3 km h−1. Our modeling, as validated by experimental decompression of MORB samples with ∼106 vesicles cm−3, also suggests that CO2 degassed from the melt within ∼10–100 min as the vesicular lava traveled ∼1.7 km along the seafloor, implying a volumetric flow rate on order of 103–4 m3 s−1. Given an ascent rate of ≥0.2 m s−1, the width of a rectangular dike feeding the lava would have been ∼1–2 m wide. MORB samples from the Pacific ridge are generally more supersaturated in dissolved CO2 than those from slower spreading Atlantic and Indian ridges. Our results suggest that Pacific MORBs ascend to the seafloor faster than Atlantic or Indian MORBs.