Peter J. Michael

Regionally distinctive sources of depleted MORB: Evidence from trace elements and H2O

Abstract The sources of depleted mid-ocean ridge basalts (N-MORB) worldwide are regionally distinctive in that they share incompatible element ratios with spatially associated, enriched mid-ocean ridge basalts (E-MORB). The ratio of H 2 O Ce is uniform for N-MORB and E-MORB within a region, suggesting that the order of incompatibility during the evolution of MORB sources is La > H2O ≈ Ce > Nd. However, there are significant regional variations in H 2 O Ce . N-MORB and E-MORB from the American-Antarctic Ridge (AAR), Southwest Indian Ridge (SWIR), southern Mid-Atlantic Ridge (SMAR), Pacific-Nazca Ridge 27–34° S (PNR), East Pacific Rise 10–12° N (EPR), Explorer Ridge, Mid-Cayman Rise Spreading Center (MCR) and Galapagos Spreading Center (GSC), as well as basalts from Loihi Seamount, have H 2 O Ce ratios that average about 155–213 (±40 for each region). N-MORB through E-MORB from the Mid-Atlantic Ridge north of about 22° N (NMAR) have higher H 2 O Ce ratios, averaging 240–280 (±50 for each region). There are no correlations of H 2 O Ce with spreading rate or extent or depth of melting, indicating that variations in H 2 O Ce are not related to MORB melting but are a characteristic of the source. K Nb ratios are also regionally variable but interpretation is complicated by a slight dependence of K Nb on source enrichment. H 2 O Ce is not correlated with K Nb , 3 He 4 He or Pb isotopic parameters but may be correlated with high 87 Sr 86 Sr at a given 143 Nd 144 Nd . Data from other regions are needed before a correlation between H 2 O Ce and 87 Sr 86 Sr can be established. The poor correlation of H 2 O Ce with 3 He 4 He makes it unlikely that H 2 O Ce variations are related to variations in juvenile H2O in the source. It is more likely that H2O in MORB is derived from recycled, subducted, altered oceanic crust. High H 2 O Ce in MORB from the NMAR might be related to a period of rapid subduction in the past that resulted in depressed isotherms and less dehydration in the slab. The constancy of H 2 O Ce within regions despite differences between regions indicates that N-MORB and E-MORB sources may share a common heritage. This constraint on the evolution of the depleted mantle is not easily reconciled with most conventional models of mantle evolution. A model in which the sources of N-MORB have been influenced by inputs of regionally distinctive plume material that has been previously depleted by small extents of melting could account for the trace element variations but is physically implausible. It is possible that high H 2 O Ce may be a regional characteristic of the mantle that is unrelated to plumes.

Magmatic and amagmatic seafloor generation at the ultraslow-spreading Gakkel ridge, Arctic Ocean

A high-resolution mapping and sampling study of the Gakkel ridge was accomplished during an international ice-breaker expedition to the high Arctic and North Pole in summer 2001. For this slowest-spreading endmember of the global mid-ocean-ridge system, predictions were that magmatism should progressively diminish as the spreading rate decreases along the ridge, and that hydrothermal activity should be rare. Instead, it was found that magmatic variations are irregular, and that hydrothermal activity is abundant. A 300-kilometre-long central amagmatic zone, where mantle peridotites are emplaced directly in the ridge axis, lies between abundant, continuous volcanism in the west, and large, widely spaced volcanic centres in the east. These observations demonstrate that the extent of mantle melting is not a simple function of spreading rate: mantle temperatures at depth or mantle chemistry (or both) must vary significantly along-axis. Highly punctuated volcanism in the absence of ridge offsets suggests that first-order ridge segmentation is controlled by mantle processes of melting and melt segregation. The strong focusing of magmatic activity coupled with faulting may account for the unexpectedly high levels of hydrothermal activity observed.

Influence of spreading rate and magma supply on crystallization and assimilation beneath mid‐ocean ridges: Evidence from chlorine and major element chemistry of mid‐ocean ridge basalts

Chlorine and major elements in >400 mid-ocean ridge basalt (MORB) glasses from 20 suites are used to examine how spreading rate, magma flux, tectonics, and hydrothermal activity influences assimilation and crystallization beneath MOR. Crystallization depths were determined for fractionated glasses using published models that describe liquids saturated with olivine+ clinopyroxene+ plagioclase. Calculated depths are minima for the onset of crystallization and maxima for the completion of crystallization for each liquid. Glasses from fast spreading ridges and from medium and slow spreading ridges with low Na8.0 define low-pressure liquid lines of descent (LLDs). Higher crystallization pressures and greater variability are obtained from slow and medium spreading ridges with high Na8.0. Crystallization pressures do not vary regularly along individual segments. The correlation between average crystallization pressure and Na8.0 suggests that magma supply (and perhaps mantle temperature) plays an important role in determining magma ascent and crystallization depths. Cl/K in glasses is an indicator of assimilation of hydrothermally influenced material. Suites of MORB with high crystallization pressures have low Cl/K: from below detection limits (≈0.01) in normal MORB (NMORB) to about 0.05–0.08 in enriched MORB (EMORB). We propose that this trend defines the mantle limit of Cl/K and that higher values are related to assimilation. Cl/K is highest (up to 1.1) and is negatively correlated with MgO along the superfast spreading southern East Pacific Rise (EPR) and the propagating, low-Na8.0 Galapagos Spreading Center (GSC) at 85°W. Cl/K is also above mantle values, but is not well correlated with MgO, in MORB from fast and medium spreading ridges and from slow spreading ridges that have low Na8.0 and low crystallization pressures, e.g., Reykjanes Ridge. Cl/K is not correlated with crystallization pressure for individual samples within any suite. We propose that the spreading rate and the extent of melting act together to determine the total magma flux to a ridge, which influences crustal temperatures and determines how magmas ascend. At the highest magma fluxes, Cl/K is correlated with MgO, consistent with continuous assimilation of material that has a uniform Cl content: crystallization and assimilation are steady state processes that occur in crustal magma bodies that are larger than the scale of crustal heterogeneity in Cl. The lowest magma fluxes occur on slow spreading ridges that have formed by small extents of melting. In this cooler environment, magma crystallizes at the base of the strong lithosphere, below the level of alteration, and then ascends rapidly with little crystallization at shallow levels, so Cl contamination is avoided. On slow and medium spreading ridges with high extents of melting, magma flux is intermediate and forms small or transient crustal magma bodies: whether a magma batch becomes enriched in Cl depends upon the particular crust that it encounters.

The concentration, behavior and storage of H2O in the suboceanic upper mantle: Implications for mantle metasomatism

Mid-ocean ridge basalt glasses from the Pacific-Nazca Ridge and the northern Juan de Fuca Ridge were analyzed for H2O by gas chromatography. Incompatible element enriched (IEE) glasses have higher H2O contents than depleted (IED) glasses. H2O increases systematically with decreasing Mg/Mg + Fe2+ within each group. Near-primary IED MORBs have an average of about 800 ppm H2O, while near-primary IEE MORBs (with chondrite normalized Nb/Zr or La/Sm ≈2) have about 2100 ppm H2O. If these basalts formed by 10–20% partial melting then the IED mantle source had 100–180 ppm H2O, while the IEE source had 250–450 ppm H2O. The ratio H2O/(Ce + Nd) is fairly constant at 95 ± 30 for all oceanic basalts from the Pacific. During trace element fractionation in the suboceanic upper mantle, H2O behaves more compatibly than K, Rb, Nb, and Cl, but less compatibly than Sm, Zr and Ti. H2O is contained mostly in amphibole in the shallow upper mantle. At pressures greater than the amphibole stability limit, it is likely that a significant proportion of H2O is contained in a mantle phase which is more refractory than phlogopite at these pressures. The role of H2O in mantle enrichment processes is examined by assuming that an enriched component was added. The modelled concentrations of K, Na, Ti and incompatible trace elements in this component are high relative to H2O, indicating that suboceanic mantle enrichment is caused by silicate melts such as basanites and not by aqueous fluids.

Mantle peridotites from continental rifts to ocean basins to subduction zones

Abstract Some key parameters for mantle-derived spinel peridotites from the North Atlantic, such as reconstructed primary modal and bulk composition, Al2O3 content of orthopyroxene (opx), Fo content of olivine, and 100 Cr/(Cr + Al) of spinel, have been compared with the same parameters for peridotite bodies from preoceanic rifts (Zabargad Island in the Red Sea), passive ocean margins (Iberian and Spitsbergen margins in the North Atlantic and southwest Australian margin in the Pacific-Antarctic Ocean) and subduction-related active margins (Mariana, Tonga and Puerto Rico trenches). Going from preoceanic rifts to passive margins to a mature ocean to subduction zones, peridotite bulk and opx Al2O3 content and modal clinopyroxene (cpx) content decrease, while olivine Fo, bulk and opx 100 Mg/(Mg + Fe), and spinel 100 Cr/(Cr + Al) increase. These systematic changes, compared with experimental data, suggest an increase of the degree of depletion of the peridotites going from a preoceanic rift to passive margins to mature oceans to subduction-related active margins. Data on Al2O3 partition between spinel and opx and application of Wells (1977) cpx-opx geothermometry suggest that peridotites from a preoceanic rift (Red Sea) and from passive margins equilibrated in the mantle under cooler conditions than oceanic peridotites. This is in agreement with the concept of a cooler thermal regime and slower asthenospheric upwelling beneath preoceanic, northern Red Sea-type rifts than beneath mid-ocean ridges. Subduction zone peridotites are more depleted than even the most refractory oceanic peridotites, though they show relatively low temperatures of equilibration, supporting the idea of water-induced melting of slivers of oceanic upper mantle near subduction zones.

Peridotite composition from the North Atlantic: regional and tectonic variations and implications for partial melting

Abstract Mineral chemical parameters and modal abundances for mantle-derived abyssal peridotite tectonites from 14 locations in the North Atlantic (0–79°N) vary over considerable ranges and are correlated together. The data indicate that there are differences in the bulk chemistry of the peridotites which correspond to the amount of basaltic melt that has been extracted from them. These differences appear to be regional in extent. Peridotites from 34° to 45°N are the most refractory. The more intense depletion could have resulted from greater extents of partial melting, possibly caused by the presence of the Azores hotspot in this region; or it could be a pre-existing depletion, due to an earlier melting event. Basalt chemical data does not rule out either possibility, but supports greater extents of partial melting occurring in this region. The regional variations in mantle peridotite composition correlate with long-wavelength variations of crustal elevation and gravity along the northern Mid-Atlantic Ridge. These correlations support the existence of regional variations of upper mantle thermal structure and composition. Peridotites collected from fracture zones are compared with those collected away from fracture zones in the same region. Based on basalt studies, there may well be a decrease in the amount of melting as certain fracture zones are approached. The peridotite data suggest that this may be true for some fracture zones but not others. In general, peridotites collected from fracture zones are representative of the suboceanic mantle.

Chlorine in mid-ocean ridge magmas: Evidence for assimilation of seawater-influenced components

Suites of depleted MORB glasses from the fast-spreading Pacific-Nazca Ridge at 28°S and 32°S and the slow-spreading eastern boundary of the Juan Fernandez microplate were analyzed for chlorine by electron microprobe. Cl contents of primitive MORB are about 20–50 ppm, similar to values reported previously for primitive MORB from the Mid-Atlantic Ridge (MAR). Cl increases steadily with decreasing MgO to 1100 ppm in evolved MORB (FeTi basalts). FeTi basalts can be related to primitive magmas by a maximum of 67% fractional crystallization based on major element modelling. The Cl concentrations in FeTi basalts exceed by a factor of 5 to 10 the amounts that can be generated by fractional crystallization of the primitive magmas. An additional process besides crystallization must be contributing the excess Cl. FeTi basalts also contain more H2O than can be produced by fractional crystallization of a primitive parent. The H2OCl ratio of the hypothetical additional component that is necessary to account for the excess Cl and H2O in FeTi basalts is 1–6 and rules out direct addition of seawater to the magma chamber. Assimilation of hydrothermally altered wall rocks of the magma chamber most likely provides the extra Cl and H2O. Selective melting or breakdown of amphibole and incorporation of Cl-rich brine contained in the wall rocks may be important processes. Bulk assimilation is less likely because the Cl content of altered crust is too low to generate the excess Cl unless unrealistically large amounts of assimilation are invoked. A magmatic source for the additional Cl and H2O cannot be ruled out on geochemical grounds but is physically unrealistic because it requires that large volumes of magma have crystallized and exsolved a Cl-rich vapor phase that has somehow migrated to a small magma chamber. Excess Cl in evolved magmas (i.e., Cl overenrichment) is best developed in evolved MORB from propagating or overlapping spreading centers such as the Galapagos Spreading Center at 85°W and 95°W and the west ridge of the Juan Fernandez microplate. Cl overenrichment has not been observed on slow-spreading ridges including the eastern ridge of the Juan Fernandez microplate, the Southwest Indian Ridge, and the mid-Atlantic Ridge. The existence of high-Cl magmas implies that some of the Cl-rich mineralization observed in deep crustal sections and ophiolites could be due to exsolved magmatic volatiles. The assimilation of hydrothermally altered material could influence the concentration and isotopic ratios of other elements which have low abundances in MORB relative to seawater.

Chemical and isotopic constraints on the generation and transport of magma beneath the East Pacific Rise

Abstract Interpretation of U-series disequilibria in midocean ridge basalts is highly dependent on the bulk partition coefficients for U and Th and therefore the mineralogy of the mantle source. Distinguishing between the effect of melting processes and variable source compositions on measured disequilibria (238U-230Th-226Ra and 235U-231Pa) requires measurement of the radiogenic isotopes Hf, Nd, Sr, and Pb. Here, we report measurements of 238U-230Th-226Ra and 235U-231Pa disequilibria; Hf, Nd, Sr, and Pb isotopic; and major and trace element compositions for a suite of 20 young midocean ridge basalts from the East Pacific Rise axis between 9°28′ and 9°52′N. All of the samples were collected within the axial summit trough using the submersible Alvin. The geological setting and observational data collected during sampling operations indicate that all the rocks are likely to have been erupted from 1991 to 1992 or within a few decades of that time. In these samples, 230Th excesses and 226Ra excesses are variable and inversely correlated. Because the eruption ages of the samples are much less than the half-life of 226Ra, this inverse correlation between 230Th and 226Ra excesses can be considered a primary feature of these lavas. For the lava suite analyzed in this study, 226Ra and 230Th excesses also vary with lava composition: 226Ra excesses are negatively correlated with Na8 and La/Yb and positively correlated with Mg#. Conversely, 230Th excesses are positively correlated with Na8 and La/Yb and negatively correlated with Mg#. Th/U, 230Th/232Th, and 230Th excesses are also variable and correlated to one another. 231Pa excesses are large but relatively constant and independent of Mg#, La/Yb, Th/U, and Na8. The isotope ratios 143Nd/144Nd, 176Hf/177Hf, 87Sr/86Sr, and 208Pb/206Pb are constant within analytical uncertainty, indicating that they were derived from a common source. The source is homogeneous with respect to parent/daughter ratios Lu/Hf, Sm/Nd, Rb/Sr, and Th/U; therefore, the measured variations of Th/U, 230Th, and 226Ra excesses and major and trace element compositions in these samples are best explained by polybaric melting of a homogeneous source, not by mixing of compositionally distinct sources.

Discovery of abundant hydrothermal venting on the ultraslow-spreading Gakkel Ridge in the Arctic Ocean

Submarine hydrothermal venting along mid-ocean ridges is an important contributor to ridge thermal structure, and the global distribution of such vents has implications for heat and mass fluxes from the Earths crust and mantle and for the biogeography of vent-endemic organisms. Previous studies have predicted that the incidence of hydrothermal venting would be extremely low on ultraslow-spreading ridges (ridges with full spreading rates <2 cm yr-1—which make up 25 per cent of the global ridge length), and that such vent systems would be hosted in ultramafic in addition to volcanic rocks. Here we present evidence for active hydrothermal venting on the Gakkel ridge, which is the slowest spreading (0.6–1.3 cm yr-1) and least explored mid-ocean ridge. On the basis of water column profiles of light scattering, temperature and manganese concentration along 1,100 km of the rift valley, we identify hydrothermal plumes dispersing from at least nine to twelve discrete vent sites. Our discovery of such abundant venting, and its apparent localization near volcanic centres, requires a reassessment of the geologic conditions that control hydrothermal circulation on ultraslow-spreading ridges.

Partition coefficients for rare earth elements in mafic minerals of high silica rhyolites: The importance of accessory mineral inclusions

Abstract REE concentrations of mafic mineral separates from high-silica rhyolites measured by INAA are high and variable compared to electron microprobe analyses of the minerals themselves. The mafic phases commonly contain inclusions or have adhering grains of accessory rare earth element (REE)-rich minerals. Optical and electron microscopic observation revealed discrete grains of chevkinite (rare earth titano-silicate) included within clinopyroxenes from the Sierra La Primavera (Mexico) rhyolite, and monazite grains adhering to ortho- and clinopyroxenes from the Bishop Tuff (California). During hand-picking of mineral separates, inclusions are only partly removed. As a result, the magnitude and variability of true mineral-melt partition coefficients for light REE have been overestimated. The true REE partition coefficients of La Primavera and Bishop Tuff pyroxenes obtained by microprobe are only slightly higher than they are in lower-silica rhyolites, and are not as variable as previously thought. The partitioning slope is positive, as in less silicic systems. The relative partitioning behavior of REE in high-silica rhyolites is dominated by crystal-chemical controls and not by liquid structural effects. When a partition coefficient is used for crystallization calculations, adhering phases and inclusions should be retained or the minor phases must be accounted for separately. Alternatively, a bulk distribution coefficient can be calculated using whole rock and glass compositions and the glass mode.