Melt Lens Research Articles

Ultrasonic velocity drops and anisotropy reduction in mica-schist analogues due to melting with implications for seismic imaging of continental crust

Melt generation in the continental crust is thought to significantly influence seismic bulk velocities and anisotropy, although existing laboratory data provide limited constraints on such seismic attributes. In this study we measured ultrasonic compressional wave speeds (Vp) and anisotropy during sample compaction, mica-breakdown and melt-generation in synthetic, foliated quartz–muscovite aggregates. Measurements were performed at peak conditions of 300 MPa hydrostatic confining pressure and 750 °C, over a six hour time period, for three separate samples where wave propagation directions were aligned at 0°, 45° and 90° with respect to the foliation plane. The experiments are initially marked by sample compaction and rapid reduction in porosity (from 25% to <2 vol%), with a corresponding increase in Vp. During initial stages of the experiment Vp ranges from 4.65 to 5.45 km/s, depending on the direction of the P wave propagation, resulting in up to ∼16% anisotropy, which originates mainly from the preferred orientation of muscovite and possible incipient melt lenses oriented parallel to foliation. As the experiment progresses the bulk Vp decreases gradually and Vp anisotropy drops from ∼16% to ∼2% after five hours into the experiment. The drop in anisotropy is caused by muscovite-breakdown and increasing amount of partial melting of the aggregate. The bulk velocities are considerably lower than predicted for a mixture of muscovite and β-quartz (stable at 300 MPa and 750 °C). Thermodynamic modeling indicates that mica breakdown and production of melt with ∼2 wt% dissolved water in the aggregate can explain the low Vp, even though β-quartz is stable; the modeled Vp is ∼4.75 km/s at 300 MPa and 750 °C. These ultrasonic attributes signify movement of the melt to fill vacant pores and coating grain boundaries, effectively connecting melt films and reducing seismic anisotropy. The results may apply to regions where crustal melting is on-going, in particular regions of elevated temperature that are only weakly deforming, with coupled anomalously low bulk velocities and seismic anisotropy.

Anatexis at the roof of an oceanic magma chamber at IODP Site 1256 (equatorial Pacific): an experimental study

Replenished axial melt lenses at fast-spreading mid-oceanic ridges may move upward and intrude into the overlying hydrothermally altered sheeted dikes, resulting in high-grade contact metamorphism with the potential to trigger anatexis in the roof rocks. Assumed products of this process are anatectic melts of felsic composition and granoblastic, two-pyroxene hornfels, representing the residue after partial melting. Integrated Ocean Drilling Program Expeditions 309, 312, and 335 at Site 1256 (eastern equatorial Pacific) sampled such a fossilized oceanic magma chamber. In this study, we simulated magma chamber roof rock anatectic processes by performing partial melting experiments using six different protoliths from the Site 1256 sheeted dike complex, spanning a lithological range from poorly to strongly altered basalts to partially or fully recrystallized granoblastic hornfels. Results show that extensively altered starting material lacking primary magmatic minerals cannot reproduce the chemistry of natural felsic rocks recovered in ridge environments, especially elements sensitive to hydrothermal alteration (e.g., K, Cl). Natural geochemical trends are reproduced through partial melting of moderately altered basalts from the lower sheeted dikes. Two-pyroxene hornfels, the assumed residue, were reproduced only at low melting degrees (<20 vol%). The overall amphibole absence in the experiments confirms the natural observation that amphibole is not produced during peak metamorphism. Comparing experimental products with the natural equivalents reveals that water activity (aH2O) was significantly reduced during anatectic processes, mainly based on lower melt aluminum oxide and lower plagioclase anorthite content at lower aH2O. High silica melt at the expected temperature (1000–1050 °C; peak thermal overprint of two-pyroxene hornfels) could only be reproduced in the experimental series performed at aH2O = 0.1.

Structural contribution from the Oman ophiolite to processes of crustal accretion at the East Pacific Rise

AbstractA comprehensive model for the activity of the elementary accretion segment at fast‐spreading ridges relies on integration of structural data from the Oman ophiolite and geophysical results from the East Pacific Rise (EPR) around 9°N, which are of comparable size and spreading rates. The axial melt lens at shallow crustal level provides a link between Deval segmentation at the seafloor and a lower melt sill at Moho level, imaged at the EPR as a crustal melt zone (CMZ) and mapped in Oman as the Moho transition zone (MTZ). Both are attached to a mantle upwelling at the EPR, and to a frozen diapir in Oman. The physical link between diapiric mantle uprising at the Moho and Devals segmentation at the seafloor is the melt being injected from the mantle into the lower MTZ, ponding there, and then being released by powerful injections into the upper melt lens. The magma chamber covers the diapir at a distance of 5 km from the ridge axis.

Origin of Basalts by Hybridization in Andesite-dominated Arcs

Mafic magmas are common in subduction zone settings, yet their high density restricts their ascent to the surface. Once stalled in the crust, these magmas may differentiate, and assimilate crust and other melts and crystal mushes to produce hybridized intermediate magmas. The Soufriere Hills Volcano on Montserrat is a ‘type locality’ for such hybridization processes and yet, just 3?km south of the crater, voluminous basalts have erupted from the South Soufriere Hills volcano within the same time period as the Soufriere Hills Volcano was erupting hybrid andesites (131–128?ka). Basaltic South Soufriere Hills magmas have 48–53?wt % SiO2 and 4–6?wt % MgO. They were hot (970–1160°C), volatile-rich (melt inclusions contain up to 6·2?wt % H2O) and were stored at 8–13?km depth prior to eruption (based on olivine- and pyroxene-hosted melt inclusion volatile geochemistry). Melt inclusions do not preserve basaltic liquids: they are andesitic to rhyolitic in composition, related to one another by a line of descent controlled by simple closed-system fractionation. Whole-rock compositions, however, are best described by a hybridization model involving ‘back-mixing’ of andesitic to rhyolitic melts with mafic crystal phases such as magnetite, olivine, orthopyroxene and clinopyroxene. Phenocryst zoning illustrates repeated mixing events between evolved melts and mafic phenocrysts; this feature, when coupled with the heterogeneity of crystal compositions, strongly suggests that although the bulk compositions are basaltic (containing Fo80 olivine), they were assembled from disparate ingredients, probably derived from mafic crystal mushes and more evolved melt lenses of variable composition. The mixing events occur days to weeks prior to eruption. We propose that the South Soufriere Hills basaltic magmas, with their higher bulk density relative to andesites from neighbouring volcanoes, ultimately may have been eruptible owing to both the transtensional tectonics imposed by offshore grabens (related to oblique subduction in the Lesser Antilles arc) and surface unloading caused by large-scale edifice collapse. Our observations support the idea that compositional changes in arcs might reflect not only changes in source compositions, but also effects caused by variations in crustal strain and tectonics.

Inside the magma chamber of a dying ridge segment in the Oman ophiolite

AbstractFor the first time, the crystallized remnant of an oceanic ridge magma chamber is documented in the Oman ophiolite. It exists in the centre of a 40 km long monoclinal ridge (Jebel Dihm, Wadi Tayin massif), exposing a full crustal section perpendicular to the spreading direction. New detailed mapping supported by U‐Pb zircon geochronology suggests that the active, fast‐spreading ridge that died just prior to detachment of the ophiolite is preserved and largely intact. Our observations provide insights into the crystallizing mush zone of a magma chamber, before it crosses the external walls and solidifies as deformed gabbros. Our data provide new constraints on the shape and internal dynamics of a magma chamber, including gabbro subsidence from the floor of a perched melt lens and the limited contribution of sills to crustal accretion. By locating precisely the palaeo‐ridge axis, prior full spreading rate estimates can be increased to ~140 km Ma−1.

Constraints from melt inclusions on depths of magma residence at intermediate magma supply along the Galápagos Spreading Center

Shallow, seismically imaged melt lenses are a ubiquitous feature of mid-ocean ridges with high magma supply; melt lenses deepen and become less continuous along axis as the rate of magma supply decreases. Despite compelling petrologic evidence for evolution of magma within the crust prior to eruption at lower magma supply, melt lenses are rarely detected along ridge segments with rates of magma supply less than 0.3×106 m3/yr/km, and the depths of sub-axial magma reservoirs are therefore poorly known. We use ion microprobe measurements of H2O and CO2 concentrations of olivine-hosted melt inclusions to calculate vapor saturation pressures that constrain crystallization depths at two locations along the Galápagos Spreading Center (94.2°W and 95°W). These sites were chosen to examine crystallization pressures in the presence (94.2°W) and absence (95°W) of a seismically imaged melt lens. At 95°W, where magma supply is too low to sustain a seismically resolvable melt lens, samples were selected from each of the three most recent eruptive units, allowing us to document temporal variations in vapor saturation pressures and the depth of magma residence at this location. Clusters in melt inclusion entrapment depths for these eruptions range from 3.0 to 3.4 km below the seafloor, indicating that magmas at 95°W resided at a narrow range of mid-crustal depths prior to eruption, generally consistent with the global trend of increasing melt lens depth with decreasing rate of magma supply. A discrepancy between seismic data and the peak in melt inclusion entrapment depths at 94.2°W may reflect temporal variability of magmatic systems at this location. This study demonstrates the potential for using measurements of the concentrations of H2O and CO2 in olivine-hosted melt inclusions to determine the depths of crustal magmatic systems that feed mid-ocean ridge eruptions, even in locations where seismic studies have not detected melt lenses.

Magmatic plumbing at Lucky Strike volcano based on olivine‐hosted melt inclusion compositions

AbstractHere 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 &gt; 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.

Seismological imaging of ridge–arc interaction beneath the Eastern Lau Spreading Center from OBS ambient noise tomography

The Lau Basin displays large along-strike variations in ridge characters with the changing proximity of the adjacent subduction zone. The mechanism governing these changes is not well understood but one hypotheses relates them to interaction between the arc and back-arc magmatic systems. We present a 3D seismic velocity model of the shallow mantle beneath the Eastern Lau back-arc Spreading Center (ELSC) and the adjacent Tofua volcanic arc obtained from ambient noise tomography of ocean bottom seismograph data. Our seismic images reveal an asymmetric upper mantle low velocity zone (LVZ) beneath the ELSC. Two major trends are present as the ridge-to-arc distance increases: (1) the LVZ becomes increasingly offset from the ridge to the north, where crust is thinner and the ridge less magmatically active; (2) the LVZ becomes increasingly connected to a sub-arc low velocity zone to the south. The separation of the ridge and arc low velocity zones is spatially coincident with the abrupt transition in crustal composition and ridge morphology. Our results present the first mantle imaging confirmation of a direct connection between crustal properties and uppermost mantle processes at ELSC, and support the prediction that as ELSC migrates away from the arc, a changing mantle wedge flow pattern leads to the separation of the arc and ridge melting regions. Slab-derived water is cutoff from the ridge, resulting in abrupt changes in crustal lava composition and crustal porosity. The larger offset between mantle melt supply and the ridge along the northern ELSC may reduce melt extraction efficiency along the ridge, further decreasing the melt budget and leading to the observed flat and faulted ridge morphology, thinner crust and the lack of an axial melt lens.

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.

Rapid hydrothermal cooling above the axial melt lens at fast-spreading mid-ocean ridge.

Axial melt lenses sandwiched between the lower oceanic crust and the sheeted dike sequences at fast-spreading mid-ocean ridges are assumed to be the major magma source of oceanic crust accretion. According to the widely discussed “gabbro glacier” model, the formation of the lower oceanic crust requires efficient cooling of the axial melt lens, leading to partial crystallization and crystal-melt mush subsiding down to lower crust. These processes are believed to be controlled by periodical magma replenishment and hydrothermal circulation above the melt lens. Here we quantify the cooling rate above melt lens using chemical zoning of plagioclase from hornfelsic recrystallized sheeted dikes drilled from the East Pacific at the Integrated Ocean Drilling Program Hole 1256D. We estimate the cooling rate using a forward modelling approach based on CaAl-NaSi interdiffusion in plagioclase. The results show that cooling from the peak thermal overprint at 1000–1050°C to 600°C are yielded within about 10–30 years as a result of hydrothermal circulation above melt lens during magma starvation. The estimated rapid hydrothermal cooling explains how the effective heat extraction from melt lens is achieved at fast-spreading mid-ocean ridges.

Seismic evidence of a complex multi‐lens melt reservoir beneath the 9° N Overlapping Spreading Center at the East Pacific Rise

AbstractThe crustal structure at fast spreading ridges is characterized by the presence of an axial melt lens along a significant part of the ridge axis over a partially molten lower crust. Using three‐dimensional seismic reflection data and waveform inversion, here we show the existence of two to three stacked melt lenses beneath both limbs of the 9°N overlapping spreading center on the East Pacific Rise, which we suggest are the source of the very evolved lavas locally observed on the seafloor. The melt lenses are 2–4 km wide and lie in a 900 m depth range. We suggest that the presence of a complex multi‐lens melt reservoir in the crust is due to the reduced ambient stress regime, hence cracking, resulting from the overlap of the two segments of the ridge, allowing melt to stay longer in the crust at different depths.

Anatomy of an active submarine volcano

Research Article| August 01, 2014 Anatomy of an active submarine volcano A.F. Arnulf; A.F. Arnulf 1Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California–San Diego, La Jolla, California 92093, USA Search for other works by this author on: GSW Google Scholar A.J. Harding; A.J. Harding 1Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California–San Diego, La Jolla, California 92093, USA Search for other works by this author on: GSW Google Scholar G.M. Kent; G.M. Kent 2Nevada Seismological Laboratory, 0174, University of Nevada–Reno, Reno, Nevada 89557, USA Search for other works by this author on: GSW Google Scholar S.M. Carbotte; S.M. Carbotte 3Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA Search for other works by this author on: GSW Google Scholar J.P. Canales; J.P. Canales 4Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02540, USA Search for other works by this author on: GSW Google Scholar M.R. Nedimović M.R. Nedimović 3Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA5Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H4J1, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information A.F. Arnulf 1Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California–San Diego, La Jolla, California 92093, USA A.J. Harding 1Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California–San Diego, La Jolla, California 92093, USA G.M. Kent 2Nevada Seismological Laboratory, 0174, University of Nevada–Reno, Reno, Nevada 89557, USA S.M. Carbotte 3Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA J.P. Canales 4Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02540, USA M.R. Nedimović 3Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA5Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H4J1, Canada Publisher: Geological Society of America Received: 28 Feb 2014 Revision Received: 12 May 2014 Accepted: 13 May 2014 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2014 Geological Society of America Geology (2014) 42 (8): 655–658. https://doi.org/10.1130/G35629.1 Article history Received: 28 Feb 2014 Revision Received: 12 May 2014 Accepted: 13 May 2014 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A.F. Arnulf, A.J. Harding, G.M. Kent, S.M. Carbotte, J.P. Canales, M.R. Nedimović; Anatomy of an active submarine volcano. Geology 2014;; 42 (8): 655–658. doi: https://doi.org/10.1130/G35629.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Most of the magma erupted at mid-ocean ridges is stored in a mid-crustal melt lens that lies at the boundary between sheeted dikes and gabbros. Nevertheless, images of the magma pathways linking this melt lens to the overlying eruption site have remained elusive. Here, we have used seismic methods to image the thickest magma reservoir observed beneath any spreading center to date, which is principally attributed to the juxtaposition of the Juan de Fuca Ridge with the Cobb hotspot (northwestern USA). Our results reveal a complex melt body, which is ∼14 km long, 3 km wide, and up to 1 km thick, beneath the summit caldera. The estimated volume of the reservoir is 18–30 km3, more than two orders of magnitude greater than the erupted magma volumes of either the A.D. 1998 or 2011 eruption. Our images show a network of sub-horizontal to shallow-dipping (<30°) features that we interpret as pathways facilitating melt transport from the magma reservoir to the eruption sites. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Correlated patterns in hydrothermal plume distribution and apparent magmatic budget along 2500 km of the Southeast Indian Ridge

AbstractMultiple geological processes affect the distribution of hydrothermal venting along a mid‐ocean ridge. Deciphering the role of a specific process is often frustrated by simultaneous changes in other influences. Here we take advantage of the almost constant spreading rate (65–71 mm/yr) along 2500 km of the Southeast Indian Ridge (SEIR) between 77°E and 99°E to examine the spatial density of hydrothermal venting relative to regional and segment‐scale changes in the apparent magmatic budget. We use 227 vertical profiles of light backscatter and (on 41 profiles) oxidation‐reduction potential along 27 first and second‐order ridge segments on and adjacent to the Amsterdam‐St. Paul (ASP) Plateau to map ph, the fraction of casts detecting a plume. At the regional scale, venting on the five segments crossing the magma‐thickened hot spot plateau is almost entirely suppressed (ph = 0.02). Conversely, the combined ph (0.34) from all other segments follows the global trend of ph versus spreading rate. Off the ASP Plateau, multisegment trends in ph track trends in the regional axial depth, high where regional depth increases and low where it decreases. At the individual segment scale, a robust correlation between ph and cross‐axis inflation for first‐order segments shows that different magmatic budgets among first‐order segments are expressed as different levels of hydrothermal spatial density. This correlation is absent among second‐order segments. Eighty‐five percent of the plumes occur in eight clusters totaling ∼350 km. We hypothesize that these clusters are a minimum estimate of the length of axial melt lenses underlying this section of the SEIR.

Hybrid shallow on-axis and deep off-axis hydrothermal circulation at fast-spreading ridges.

Hydrothermal flow at oceanic spreading centres accounts for about ten per cent of all heat flux in the oceans and controls the thermal structure of young oceanic plates. It also influences ocean and crustal chemistry, provides a basis for chemosynthetic ecosystems, and has formed massive sulphide ore deposits throughout Earth's history. Despite this, how and under what conditions heat is extracted, in particular from the lower crust, remains largely unclear. Here we present high-resolution, whole-crust, two- and three-dimensional simulations of hydrothermal flow beneath fast-spreading ridges that predict the existence of two interacting flow components, controlled by different physical mechanisms, that merge above the melt lens to feed ridge-centred vent sites. Shallow on-axis flow structures develop owing to the thermodynamic properties of water, whereas deeper off-axis flow is strongly shaped by crustal permeability, particularly the brittle-ductile transition. About 60 per cent of the discharging fluid mass is replenished on-axis by warm (up to 300 degrees Celsius) recharge flow surrounding the hot thermal plumes, and the remaining 40 per cent or so occurs as colder and broader recharge up to several kilometres away from the axis that feeds hot (500-700 degrees Celsius) deep-rooted off-axis flow towards the ridge. Despite its lower contribution to the total mass flux, this deep off-axis flow carries about 70 per cent of the thermal energy released at the ridge axis. This combination of two flow components explains the seismically determined thermal structure of the crust and reconciles previously incompatible models favouring either shallower on-axis or deeper off-axis hydrothermal circulation.

Processes in Caldera-Forming High-Silica Rhyolite Magma: Rb-Sr and Pb Isotope Systematics of the Otowi Member of the Bandelier Tuff, Valles Caldera, New Mexico, USA

The 1·60Ma Otowi Member of the BandelierTuff is a chemically and isotopically zoned high-silica rhyolite ignimbrite with subchondritic concentrations of Sr. Sanidine phenocrysts and glasses from early erupted to late-erupted tuff exhibit systematic large variations in Sr/Sr (0·70519^0·70947) and small ranges in Pb isotope ratios (Pb/Pb1⁄417·795^17·835). In all but the earliest-erupted tuff, sanidine phenocrysts are zoned with relatively Srand Ba-rich overgrowths on Srand Ba-poor cores.We estimate sodic sanidine/ melt partition coefficients for Rb, Cs, Sr, Ba and Pb to aid in understanding the origins of the elemental and isotopic variations. Isotopic heterogeneity is observed within single pieces of pumice, single quartz^sanidine glomerocrysts and single crystals. At the observed high values of Rb/Sr ( 1000) caused by the extreme Sr depletion, radiogenic ingrowth or contamination of the magma by small amounts of country-rock could both be responsible for elevated Sr/Sr.The Pb isotope variations cannot be due to ingrowth and unequivocally indicate open-system behavior. Among the sanidine phenocrysts, positive correlations of Sr/Sr vs Rb/Sr superficially resemble isochrons but are better modelled as mixing arrays; at least three such arrays, corresponding to three distinct magmatic recharge events, can be identified.The latest event may have played a role in destabilizing the system and triggering eruption. In all cases the recharging magma was another high-silica rhyolite only slightly less depleted in Sr.These relationships are explained by a model that starts with a crystal-poor high-silica rhyolite melt lens overlying a sanidineþ quartz crystal pile. Successive melting events within the crystal pile and mixing of new and old melts (recharge) causes isotopic and chemical zoning in the main melt lens and short length scale disequilibrium in the crystal pile. A late episode of quartz crystallization may have captured inclusions of melt that had been additionally contaminated by Precambrian granitoid country-rock as the magma began the upward journey that culminated in eruption.

Perspectives on the origin of plagiogranite in ophiolites from oxygen isotopes in zircon

The formation of oceanic plagiogranite has been attributed primarily to either 1) extreme fractional crystallization of a mantle melt, or 2) partial melting of hydrated mafic crust, with support for the latter from field evidence and recent melting experiments. Remelting of hydrothermally-altered ocean crust could yield rocks (and minerals) with diverse primary magmatic δ18O values forming proximal to the magmatic center where crustal growth is occurring. To constrain the magmatic δ18O of a wide range of silicic rocks in oceanic crust and evaluate their petrogenesis, we characterized the δ18O of zircons in 22 plagiogranite samples (tonalite and trondhjemite) from 8 different ophiolites and one dacite sampled along the East Pacific Rise using Secondary Ion Mass Spectrometry (SIMS). The δ18O values of 202 magmatic zircons from ophiolites range from 3.9 to 5.6‰ (n=244 spots; average 4.9±0.6‰; 2SD), extending ~1‰ below typical zircon in equilibrium with mantle and from gabbroic massifs along slow-spreading mid-ocean ridges (4.7–5.9‰). East Pacific Rise dacite zircons range from 4.6 to 5.0‰ (n=12 spots). Plagiogranite from the dike-gabbro transition zone of the northern Oman Ophiolite yield the lowest δ18O(Zrn), with rock-average values of 4.3–5.0‰. The low-δ18O values are best explained by remelting of crust altered by hydrothermal fluids with seawater-like isotopic compositions at high temperatures, possibly due to vertical migration of the boundary between an active magma chamber and a vigorous high-temperature hydrothermal system in the overlying crust. If the partial melt was assimilated into a fractionating melt lens with MORB-like δ18O, as envisioned for km-scale plagiogranite bodies in Oman, up to 20% contamination by a protolith with δ18O=2‰ would be required. Previous oxygen isotope constraints from quartz in Oman plagiogranite suggested melting of both high and low δ18O crust had occurred; comparison of quartz–zircon pairs indicates that quartz has been modified in most samples, and we find no evidence for the involvement of high-δ18O rocks during plagiogranite formation.

Oxygen isotope evidence for the formation of andesitic–dacitic magmas from the fast-spreading Pacific–Antarctic Rise by assimilation–fractional crystallisation

Andesitic to dacitic lavas occur along a 300km long portion of the Pacific–Antarctic Rise close to the intersection of the spreading axis with the Foundation seamount chain. The fresh silicic glasses have low δ18O isotope values between 5.6‰ and 5.1‰ whereas basaltic glasses from the same ridge section have normal MORB δ18O values. Additionally, two FeTi basaltic and all silicic glasses have high Cl (up to 1.1wt.%) and K2O (up to 1.6wt.%) contents, indicating assimilation of hydrothermally altered material. Modelling suggests that the fractionating magma assimilated up to 30% of hydrothermally altered material after 57% fractional crystallisation of the basaltic magma in a melt lens at less than 2km depth. In contrast, the basaltic glasses show little assimilation and clinopyroxene-melt barometry indicates crystal fractionation in deeper melt sills. Relatively low H2O/Ce and Li/Ce ratios as well as the low δ18O values and high Cl and K concentrations in the silicic glasses suggest assimilation of altered crustal rock rather than a brine. While some of the variability in highly incompatible element ratios is best explained by crystal fractionation processes of FeTi-oxides (e.g., decreasing Nb/U) others require a reaction of the melt with residual amphibole and clinopyroxene (Cl/K, Tb/Yb, Hf/Sm, Ce/Yb). The initial onset of FeTi oxide crystallisation is associated with a reduced oxygen fugacity causing sulphide saturation and significant loss of S, Cu, and Co from the evolved melts. This change to more reducing melts as a result of oxide crystallisation is supported by the strong development of a negative Eu anomaly in the andesites and dacites indicating stronger partitioning of Eu into plagioclase. Reverse mineral zoning in the evolved lavas also indicates that replenishment by mafic melts occurs in the shallow melt lens.

Sill to surface: Linking young off-axis volcanism with subsurface melt at the overlapping spreading center at 9°03′N East Pacific Rise

No young, off-axis, mid-ocean ridge lavas have yet been directly linked to underlying off-axis melt bodies. In this study, we present new measurements of 238U–230Th–226Ra–210Pb isotope compositions for a suite of lavas from the overlapping spreading center (OSC) at 9°03′N on the East Pacific Rise (EPR). These lavas span a large range of compositions, from basalt to dacite, and include both axial and off-axis samples recovered from a prominent, axis-parallel pillow ridge and a flat-topped seamount that overlie the westernmost extent of a 4-km-wide melt lens (Kent et al., 2000). We report 210Pb excesses in axial basalts and basaltic andesites, which we suggest results from gas-magma fractionation of 222Rn from 226Ra beneath dacite magmas. In addition, our U-series ages agree with visual observations, indicating that while most recent volcanic activity occurs at the spreading axis, active volcanism also occurs away from the axis. Specifically, the off-axis pillow ridge and seamount samples overlying the off-axis subsurface melt body have eruption ages of less than 8ka, and likely as young as 1ka, despite being located on crust that has a spreading age of ~75ka. The young ages of these lavas, combined with existing geological, geochemical and geophysical constraints, provide evidence for a genetic link between the pillow ridge and seamount lavas and the seismically imaged, underlying off-axis melt lens. This link demonstrates that off-axis volcanism does not necessarily come from a sub-axial magma body and can be sourced directly from off-axis magma bodies.

Pervasive reactive melt migration through fast-spreading lower oceanic crust (Hess Deep, equatorial Pacific Ocean)

Mid-ocean ridge basalt (MORB) is the most abundant magma on Earth, and provides a geochemical window into the mantle. Deriving mantle composition and melting processes from the erupted lavas requires correction to be made for their evolution as they pass through and generate the oceanic crust. This is typically done by assuming that modification of melts in crustal magma chambers occurs exclusively by fractional crystallisation. However, extensive mineral major- and trace element data from a full section of fast-spread lower crustal rocks exposed in Hess Deep (equatorial Pacific Ocean) demonstrate that their evolution is instead controlled by reactive porous flow. These reactions lead to a strong enrichment in, and fractionation of, incompatible trace elements in the melt (as recorded by clinopyroxene compositions), leading to melt compositions far outside of the compositional realm of MORB both in terms of trace element abundances and ratios. The reactive signature increases in strength up section, peaking in varitextured gabbros interpreted to represent the fossilised axial melt lens, indicating that reactive porous flow occurred on the scale of the entire lower crust. The enrichment of the melt is coupled with a strong trace element depletion of plagioclase, olivine, and, to a lesser extent, clinopyroxene cores, suggesting that these phases represent the residues of the reactions from which trace elements have been removed. The dominant role of reactive porous flow, and the resulting deviations from fractional crystallisation predictions, suggest that the lower oceanic crust plays a much more complex and significant role in modifying the compositions of MORB than previously expected, with consequent implications for models of mantle processes.

Lower crustal crystallization and melt evolution at mid-ocean ridges

Oceanic crust is formed at mid-ocean ridges, but there is little consensus on where crystallization of melt actually occurs within the crust or mantle. Geochemical analyses of melt inclusions from two Pacific Ocean mid-ocean ridges indicate that 25% of the melt crystallizes below the melt lens to form the lower oceanic crust.