Mid-ocean Ridge Basalt Compositions Research Articles

Geochemical and Nd–Pb isotopic characteristics of the Tethyan asthenosphere: implications for the origin of the Indian Ocean mantle domain

It is unclear why the Pb, Nd, and Sr isotopic composition of the modern mid-ocean ridge basalts (MORB) from the Indian Ocean is different from that of the North Atlantic and Pacific Oceans. A possible explanation for this is that the Indian MORB-type isotopic signature is a long-lived regional feature of the mantle, as evidently shown by the isotopic composition of the ∼350 Ma MORB-like Mian-Lue northern ophiolite, which was formed in the same region presently occupied by the Indian Ocean. However, this hypothesis is in conflict with the lack of Indian MORB-type isotopic signature in a number of ∼150 Ma Tethyan and Indian Ocean crusts. To further constrain the origin of the Indian MORB-type isotopic signature, we analyze the geochemical and Pb, Nd, and Sr isotopic composition of representative mafic rocks from four Tethyan ophiolites ranging in age from 90 to 360 Ma. The Sr isotopic composition of the samples is unreliable due to alteration, but the age-corrected Nd and Pb isotopic ratios and geochemical data indicate that these Tethyan rocks were derived from a geochemically depleted asthenospheric source that had a clear Indian MORB-type isotopic signature. We therefore conclude that the bulk of the Indian suboceanic mantle was most probably inherited from the Tethyan asthenosphere. A few regions in both the Tethyan and Indian Oceans, however, are most probably underlain by Pacific and North Atlantic MORB-type mantle (and vice-versa) because of the flow of the asthenosphere in response to tectonic plate reorganizations that lead to openings and closures of ocean basins. The Indian MORB-type isotopic signature of the western Pacific marginal basin crusts could be due to either flow of the Indian Ocean mantle into the western Pacific or to endogenous production of such an isotopic signature from delaminated East-Asian sublithospheric materials during closure of the Tethys Ocean.

Osmium isotopic constraints on the nature of the DUPAL anomaly from Indian mid-ocean-ridge basalts.

The isotopic compositions of mid-ocean-ridge basalts (MORB) from the Indian Ocean have led to the identification of a large-scale isotopic anomaly relative to Pacific and Atlantic ocean MORB. Constraining the origin of this so-called DUPAL anomaly may lead to a better understanding of the genesis of upper-mantle heterogeneity. Previous isotopic studies have proposed recycling of ancient subcontinental lithospheric mantle or sediments with oceanic crust to be responsible for the DUPAL signature. Here we report Os, Pb, Sr and Nd isotopic compositions of Indian MORB from the Central Indian ridge, the Rodriguez triple junction and the South West Indian ridge. All measured samples have higher (187)Os/(188)Os ratios than the depleted upper-mantle value and Pb, Sr and Nd isotopic compositions that imply the involvement of at least two distinct enriched components in the Indian upper-mantle. Using isotopic and geodynamical arguments, we reject both subcontinental lithospheric mantle and recycled sediments with oceanic crust as the cause of the DUPAL anomaly. Instead, we argue that delamination of lower continental crust may explain the DUPAL isotopic signature of Indian MORB.

Trace element partitioning in Earth’s lower mantle and implications for geochemical consequences of partial melting at the core–mantle boundary

Trace element partitioning data between CaSiO 3-perovskite (CaPv), MgSiO 3-perovskite (MgPv), calcium–aluminum silicate (CAS-phase), and coexisting melts in peridotite and mid-ocean ridge basalt (MORB) compositions were obtained at 25–27 GPa and 2400–2530 °C using multi-anvil apparatus and ion microprobe. Results clearly show that CaPv is the predominant host for large ion lithophile elements (LILE) in the lower mantle. Because of the overwhelmingly high CaPv/melt partition coefficients (>10 for many of the LILE), partial melting in the lower mantle causes strong enrichment of LILE in the CaPv-bearing solid phase residue. CaPv has the following partitioning characteristics: (1) uniformly high partition coefficients for heavy rare earth elements (HREE) (e.g. 15 for Yb), decreasing toward light REE (e.g. 7 for La), (2) systematically lower partition coefficients for high field strength elements (Nb, Zr, Ti) and Sr relative to neighboring REE, (3) high Th and U, and systematically low Pb partition coefficients. Previous high-pressure studies have shown that the stability field of CaPv above solidus temperature is much wider in basaltic composition than in peridotite, indicating that melting of subducted oceanic crust in the lower mantle could produce significant geochemical CaPv signatures. Strong enrichment in Th and U relative to Pb in CaPv would result in radiogenic Pb isotopic compositions of the CaPv-bearing solid residue. Some clinopyroxenes in plume mantle peridotite xenoliths possess trace element patterns closely resembling those of natural CaPv found in diamonds and CaPv from the present experiments, suggesting that they were inherited from the CaPv-bearing precursor. In contrast, CaPv is the first phase to disappear during partial melting of peridotite above 24 GPa, and its geochemical signature may not be observable in nature. (MgPv + CaPv) fractional crystallization from a magma ocean has previously been put forward as a mechanism for Si depletion of the upper mantle. However, our data show that the magma ocean Sm/Ba ratio would deviate significantly from observed chondritic values when fractionation exceeds only 6%, if the crystallizing mixture were 10% CaPv and 90% MgPv. A significant hidden perovskite reservoir in the lower mantle can therefore be excluded.

Equations of state of Na?K?Al host phases and implications for MORB density in the lower mantle

We report equations of state (EoS) of two important silicate phases known to store Na, K, and Al, at pressures and temperatures prevailing in the Earth’s lower mantle. Using laser heated diamond anvil cell, we produced the Ca-ferrite type (CF) and the new aluminum rich (NAL) phases using different synthetic and natural starting materials, at pressures ranging from 23 to 65 GPa. Unit cell volumes were investigated by means of angle dispersive X-ray diffraction at the ID30 beamline of the ESRF synchrotron ring (Grenoble, France). CF-phases were synthesized in NaAlSiO 4 –MgAl 2 O 4 and NaAlSiO 4 –(Mg,Fe) 2 SiO 4 systems, and for a natural mid-ocean ridge basalt (MORB). The insertion of MgAl 2 O 4 in the NaAlSiO 4 -based lattice has reduced effects on the CF-phase EoS, whereas that of (Mg,Fe) 2 SiO 4 is mainly to increase the room pressure volume. CF-phases most relevant to the Earth’s mantle show compression behavior comparable to the pure NaAlSiO 4 -CF-phase, with bulk moduli values close to K 0 =190 GPa and K 0 ′=4. We also synthesized (NAL) phases in NaAlSiO 4 –MgAl 2 O 4 system, and using CaNa- and K-rich silicate glasses as starting materials. All NAL-compounds investigated show similar compression behavior, except for one anomalously NaAlSiO 4 -rich composition. For NAL-phase with the most “natural” conditions relevant composition, we refine bulk moduli of K 0 =182 GPa and K 0 ′=6. We then model the density profiles expected for the CF and NAL-phases, using our newly determined EoS and the most accurate chemical compositions reported in the literature. These two phases appear significantly less dense than a pyrolitic type material at all lower mantle pressures and 300 K. However, the density profiles calculated for mineral assemblages corresponding to typical MORB compositions point out to denser MORBs compared to the pyrolite. This is due to the high Fe-content in MORB, yielding a densest Fe-rich perovskite that counterbalances the lower densities of the CF and NAL-phases. The density gap is calculated between 0.4 and 4.0% depending on the starting MORB composition and pressure.

Composition of the depleted mantle

We present an estimate for the composition of the depleted mantle (DM), the source for mid‐ocean ridge basalts (MORBs). A combination of approaches is required to estimate the major and trace element abundances in DM. Absolute concentrations of few elements can be estimated directly, and the bulk of the estimates is derived using elemental ratios. The isotopic composition of MORB allows calculation of parent‐daughter ratios. These estimates form the “backbone” of the abundances of the trace elements that make up the Coryell‐Masuda diagram (spider diagram). The remaining elements of the Coryell‐Masuda diagram are estimated through the composition of MORB. A third group of estimates is derived from the elemental and isotopic composition of peridotites. The major element composition is obtained by subtraction of a low‐degree melt from a bulk silicate Earth (BSE) composition. The continental crust (CC) is thought to be complementary to the DM, and ratios that are chondritic in the CC are expected to also be chondritic in the DM. Thus some of the remaining elements are estimated using the composition of CC and chondrites. Volatile element and noble gas concentrations are estimated using constraints from the composition of MORBs and ocean island basalts (OIBs). Mass balance with BSE, CC, and DM indicates that CC and this estimate of the DM are not complementary reservoirs.

Equations of state of Na–K–Al host phases and implications for MORB density in the lower mantle

We report equations of state (EoS) of two important silicate phases known to store Na, K, and Al, at pressures and temperatures prevailing in the Earth’s lower mantle. Using laser heated diamond anvil cell, we produced the Ca-ferrite type (CF) and the new aluminum rich (NAL) phases using different synthetic and natural starting materials, at pressures ranging from 23 to 65 GPa. Unit cell volumes were investigated by means of angle dispersive X-ray diffraction at the ID30 beamline of the ESRF synchrotron ring (Grenoble, France). CF-phases were synthesized in NaAlSiO 4–MgAl 2O 4 and NaAlSiO 4–(Mg,Fe) 2SiO 4 systems, and for a natural mid-ocean ridge basalt (MORB). The insertion of MgAl 2O 4 in the NaAlSiO 4-based lattice has reduced effects on the CF-phase EoS, whereas that of (Mg,Fe) 2SiO 4 is mainly to increase the room pressure volume. CF-phases most relevant to the Earth’s mantle show compression behavior comparable to the pure NaAlSiO 4-CF-phase, with bulk moduli values close to K 0=190 GPa and K 0′=4. We also synthesized (NAL) phases in NaAlSiO 4–MgAl 2O 4 system, and using CaNa- and K-rich silicate glasses as starting materials. All NAL-compounds investigated show similar compression behavior, except for one anomalously NaAlSiO 4-rich composition. For NAL-phase with the most “natural” conditions relevant composition, we refine bulk moduli of K 0=182 GPa and K 0′=6. We then model the density profiles expected for the CF and NAL-phases, using our newly determined EoS and the most accurate chemical compositions reported in the literature. These two phases appear significantly less dense than a pyrolitic type material at all lower mantle pressures and 300 K. However, the density profiles calculated for mineral assemblages corresponding to typical MORB compositions point out to denser MORBs compared to the pyrolite. This is due to the high Fe-content in MORB, yielding a densest Fe-rich perovskite that counterbalances the lower densities of the CF and NAL-phases. The density gap is calculated between 0.4 and 4.0% depending on the starting MORB composition and pressure.

The terrestrial Li isotope cycle: light-weight constraints on mantle convection

The two stable isotopes of Li are significantly fractionated by exchange reactions with clays near the Earth’s surface. The isotopic legacy of this process provides a robust tracer of surface material that is returned (recycled) to the mantle. Altered oceanic crust has a heavy Li isotopic composition (high 7Li/ 6Li). Heterogeneous distribution of subducted, altered oceanic crust in the mantle will result in variations in Li isotope ratios. A rapidly accumulating dataset of Li isotope analyses on mantle-derived materials indeed indicates a significant range in Li isotope ratios. This observation provides powerful evidence for the widespread distribution of recycled material in the convecting mantle. There is substantial overlap in Li isotopic compositions of ocean island basalts (OIB) and mid-ocean ridge basalts (MORB). Some OIB, however, have slightly heavier Li compositions than typical, depleted MORB. At face value this suggests a larger contribution of recycled oceanic crust in the sources of some OIB than in the upper mantle. Yet recent evidence implies that heavy Li is lost from the slab at subduction zones and the recycled residual crust is left with an isotopically light signature. Extremely light Li isotope ratios are observed in some continental mantle xenoliths but not in OIB proposed to contain large proportions of recycled crust. Studies of subduction zone lavas imply the heavy Li isotope signature of altered oceanic crust is transferred to the mantle above the subducting plate. Thus cycling of sub-arc mantle may be an important process in forming chemical heterogeneities sampled by OIB. Inferences from initial Li isotope results are thus notably different from those from the longer established but more equivocal radiogenic isotope tracers. Li isotopes promise to provide significant new constraints on the distribution of recycled material in the mantle and its implications for mantle convection.

Diffusive fractionation of noble gases and helium isotopes during mantle melting

The large differences in He and Ar diffusivities in silicate minerals could result in fractionation of the He/Ar ratio during melting of the mantle, producing He/Ar ratios in the primary mantle melts that are higher than those of the bulk mantle. Modeling noble gas diffusion out of the bulk mantle into fast diffusion pathways (such as fractures or melt channels) suggests that significant (order of magnitude) He/Ar fractionation will occur if the fast diffusion channels are spaced several meters apart and the noble gas residence in these diffusion channels is of the order days to weeks. In addition, the 15% difference in 3He and 4He diffusivities could also produce isotopic fractionation between the melt and its solid source. Modeling the behavior of He and Ar during melting shows that small increases (few %) in 3He/ 4He should be correlated with larger variations (factor of 5) in 4He/ 40Ar. However, in order to test this hypothesis the effects of subsequent He–Ar fractionation that occur during degassing have to be corrected. I describe a scheme that can separate He/Ar variations in the primary melt from overprinted fractionation during magmatic degassing. Using the degassing-corrected data, there is a correlation between the primary melt’s 4He/ 40Ar and 3He/ 4He in mid-ocean ridge basalts (MORBs). The slope of the correlation is consistent with the models of preferential diffusion of 3He relative to 4He and of 4He relative to 40Ar from the solid mantle into the melt. Diffusive fractionation of noble gases during melting of the mantle can also account for low 4He/ 40Ar ratios commonly found in residual mantle xenoliths: preferential diffusion of He relative to Ar will produce some regions of the mantle with low 4He/ 40Ar, the complement of the high 4He/ 40Ar ratios in basalts. Diffusive fractionation cannot, however, account for differences between the He and Ne isotopic compositions of MORBs compared with ocean island basalts (OIBs); not only are the extremely high 3He/ 4He ratios of OIBs (up to 50 Ra) difficult to produce at reasonable mantle time and lengthscales, but also the Ne isotopic compositions of MORBs and OIBs do not lie on a single mass fractionation line, therefore cannot result from diffusive fractionation of a single mantle Ne source. If preferential diffusion of He from the solid mantle into primary melts is a significant process during generation of MORBs, then it is difficult to constrain the He concentration of the mantle: He concentrations in basalts and the He flux to the ocean essentially result from extraction of He from a larger (and unknown) volume of mantle than that that produced the basalts themselves. The He concentration of the mantle cannot be constrained until more accurate estimates of the diffusion contribution are available.

The statistical upper mantle assemblage

A fundamental challenge in modern mantle geochemistry is to link geochemical data with geological and geophysical observations. Most of the early geochemical models involved a layered mantle and the concept of geochemical reservoirs. Indeed, the two layer mantle model has been implicit in almost all geochemical literature and the provenance of oceanic island basalt (OIB) and mid-ocean ridge basalt (MORB) [van Keken et al., Annu. Rev. Earth Planet. Sci. 30 (2002) 493–525]. Large-scale regions in the mantle, such as the ‘convective’ (i.e. well-stirred, homogeneous) upper mantle, sub-continental lithosphere, and the lower mantle were treated as distinct and accessible geochemical reservoirs. Here we discuss evidence for a ubiquitous distribution of small- to moderate-scale (i.e. 10 2–10 5 m) heterogeneity in the upper mantle, which we refer to as the statistical upper mantle assemblage (SUMA). This heterogeneity forms as the result of long-term plate tectonic recycling of sedimentary and crustal components. The SUMA model does not require a convectively homogenized MORB mantle reservoir, which has become a frequently used concept in geochemistry. Recently, Kellogg et al. [Earth Planet. Sci. Lett. 204 (2002) 183–202] modeled MORB and OIB Sr and Nd isotopic compositions as local mantle averages of random distributions of depleted residues and recycled continental crustal material. In this model, homogenization of the MORB source region is achieved by convective stirring and mixing. In contrast, in the SUMA model, the isotopic compositions of MORB and OIB are the outcome of homogenization during sampling, by partial melting and magma mixing (e.g. [Helffrich and Wood, Nature 412 (2001) 501–507]), of a distribution of small- to moderate-scale upper mantle heterogeneity, as predicted by the central limit theorem. Thus, the ‘SUMA’ acronym also captures what we consider the primary homogenization process: sampling upon melting and averaging. SUMA does not require the participation of distinct (e.g. lower mantle) reservoirs to explain OIB compositions. The complementary methods outlined in this work and in e.g. Kellogg et al. and Helffrich and Wood, provide a strong link between geochemical data and potential geophysical observables, such as the degree of partial melting, size of the melting regime, the characteristic length scales of the chemical heterogeneity and bathymetry, seismic scattering and anisotropy. We support the view that statistical distributions of lithologic components and sampling theory should replace the concept of distinct, isolated geochemical reservoirs, and extensive convective stirring prior to sampling.

海洋地殻の構成と火成活動

Volumetrically, magmatism at spreading axes occupies more than 60% of present day volcanism, and oceanic crust generated there covers more than two thirds of earth surface. Therefore, studies of the oceanic crust and magmatism at spreading axes yield significant insights to understanding earth evolution. Recent studies on oceanic crust through drilling, submersible surveys, dredging and geophysical exploration indicates that there is considerable variation in lithologic composition and stratigraphic sequences in oceanic crust that is dependent on the spreading rate. Typical ophiolitic stratigraphic sequences are expected to occur at fast-spreading ridges, while complicated and variable stratigraphic successions are encountered at slow-spreading ridges. Mid-Ocean Ridge Basalts (MORBs) are generally regarded as having comparatively uniform compositions. Major element compositions of MORB corrected for low-pressure fractionation show a global systematic variation correlated to axial highs, which can be ascribed to the degree of partial melting. However, the composition of MORB should represent a final product of magmas whose compositions have been greatly modified during ascent through mantle peridotite, by crustal fractionation that produced lower crust (ultramafic to gabbroic cumulates), and by mixing in a melt lens in shallow crust. Therefore, to fully understand MORB petrogenesis it is necessary to study the total magmatic system, which includes surface lavas, dike complexes, plutonic cumulates extracted from melts, and residual mantle. Recent results from the MARK area of the Mid-Atlantic Ridge, the Atlantis bank of the Southwest Indian Ridge and Hess Deep near the East Pacific Rise strongly suggest that source mantle is heterogeneous and that the magmatic system differs significantly depending on the spreading rate.

Geodynamic Implications of Jurassic Ophiolites Associated with Island-Arc Volcanics, South Apuseni Mountains, Western Romania

The South Apuseni Mountains are located in the inner zone of the Carpathian belt. This area is characterized by a complex assemblage of nappes, in which Jurassic igneous associations are well represented. New geological and geochemical data on these igneous associations document the occurrence of Middle Jurassic ophiolites overlain by Late Jurassic calc-alkaline volcanic rocks. The ophiolite sequence is characterized by: (1) an intrusive section mainly represented by small gabbroic bodies showing both layered and isotropic textures, as well as scarce ultramafic cumulates, melagabbros, gabbronorites, and ferrogabbros; (2) a basaltic sheeted dike complex; (3) a volcanic sequence including massive and pillow-lava basalts and rare pillow breccias; and (4) very rare Callovian-Oxfordian radiolarian cherts. Although chemically variable, gabbros show a clear high-Ti magmatic affinity. Basaltic rocks display N-MORB-normalized incompatible-element patterns consistent with the compositions of present-day mid-ocean ridge basalts. Their high-Ti magmatic affinity is indicated by the chondrite-normalized REE patterns, which are rather flat or slightly enriched in light REE. All these features suggest that ophiolites of the South Apuseni Mountains were generated in a mid-ocean ridge setting. Evidence of ductile deformation and synkinematic metamorphism are lacking, suggesting that ophiolites were dismembered in multiple slices at shallow structural levels during the orogenic phases. Calc-alkaline rocks are represented by massive lava flows including basalts, basaltic andesites, andesites, dacites, and rhyolites, as well as by some granitoid complexes intruded into the ophiolitic sequence. Volcanics show highly porphyritic textures, with clinopyroxene, orthopyroxene, hornblende, and plagioclase phenocrysts, and incompatible-element compositions characterized by Ta-Nb, P, and Ti depletion, as well as by Rb-Ba-Th and La-Ce enrichment. These rocks also show marked light REE enrichments, commonly interpreted to be a consequence of mantle source enrichment by subduction-derived components. Ophiolites represent the remnants of an oceanic basin, whereas calc-alkaline rocks represent a magmatic island-arc setting developed over the previously formed oceanic lithosphere. The geological and petrological data suggest that the South Apuseni Mountains ophiolites can be correlated with the mid-ocean ridge-type ophiolites of the Vardar zone. They are interpreted to have assumed their present location in the inner zone of the Carpathian belt during the tectonic escape of Adria-related microplates during the Late Paleogene-Early Neogene.

Experimental constraints on major and trace element partitioning during partial melting of eclogite

Isobaric partial melting experiments were performed on an Fe-free synthetic composition to simulate partial melting of subducted oceanic crust. Nominally anhydrous experiments at 3.0 GPa yielded melts in equilibrium with garnet (13 to 16 mol.% grossular) and aluminous clinopyroxene (14 to 16 wt.% Al 2O 3). Melt compositions show decreasing Si and alkalis and increasing Ca, Mg, and Ti contents with increasing temperatures. Experiments at 1200 and 1300°C were rutile saturated, whereas experiments at 1400°C contained no residual rutile. We argue that during the initial stages of subduction, accessory rutile is likely to be stable in subsolidus eclogites of average midocean ridge basalt composition and that only large degrees of partial melting will eradicate rutile from an eclogitic source. At 3 GPa, any eclogites with a bulk TiO 2 content of ≥1.5 wt.% rutile will produce rutile-saturated partial melts, except at very high degrees of melting. At higher pressures, all bulk Ti may dissolve in clinopyroxene and garnet, leaving no accessory rutile. Trace element partition coefficients for 24 trace elements between clinopyroxene, garnet, and melt were determined by secondary-ion mass spectrometry analysis of experimental run products at 1400°C and 3 GPa. Partition coefficients for the rare earth elements agree well with previous studies and have been evaluated using the lattice strain model. Partitioning data for high–field strength elements indicate complementary D Zr/D Hf for clinopyroxene and garnet. Partial melting of an eclogitic component of different modal compositions may therefore explain both subchondritic and superchondritic Zr/Hf ratios. Superchondritic Zr/Hf has recently been observed in some ocean island basalts (OIB), and this may be taken as further evidence for components of recycled oceanic crust in OIB. The data also indicate slight Nb/Ta fractionation during partial melting of bimineralic eclogite, which is not, however, sufficient to explain some recently observed Nb/Ta fractionation in island arc rocks. Accessory rutile, however, can explain such fractionation.

Subsolidus and melting phase relations of basaltic composition in the uppermostlower mantle

The phase relations and the element partitioning in a mid-oceanic ridge basalt composition were determined for both above-solidus and subsolidus conditions at 22 to 27.5 GPa by means of a multianvil apparatus. The mineral assemblage at the solidus changes remarkably with pressure; majorite and stishovite at 22 GPa, joined by Ca-perovskite at 23 GPa, further joined by CaAl 4Si 2O 11-rich CAS phase at 25.5 GPa, and Mg-perovskite, stishovite, Ca-perovskite, CF phase (approximately on the join NaAlSiO 4-MgAl 2O 4), and NAL phase ([Na,K,Ca] 1[Mg,Fe 2+] 2[Al,Fe 3+,Si] 5.5–6.0O 12) above 27 GPa. The liquidus phase is Ca-perovskite, and stishovite, a CAS phase, a NAL phase, Mg-perovskite, and a CF phase appear with decreasing temperature at 27.5 GPa. Partial melt at 27 to 27.5 GPa is significantly depleted in SiO 2 and CaO and enriched in FeO and MgO compared with those formed at lower pressures, reflecting the narrow stability of (Fe,Mg)-rich phases (majorite or Mg-perovskite) above solidus temperature. The basaltic composition has a lower melting temperature than the peridotitic composition at high pressures except at 13 to 18 GPa (Yasuda et al., 1994) and therefore can preferentially melt in the Earth’s interior. Recycled basaltic crusts were possibly included in hot Archean plumes, and they might have melted in the uppermost lower mantle. In this case, Ca-perovskite plays a dominant role in the trace element partitioning between melt and solid. This contrasts remarkably with the case of partial melting of a peridotitic composition in which magnesiowüstite is the liquidus phase at this depth.

Calculation of Peridotite Partial Melting from Thermodynamic Models of Minerals and Melts, IV. Adiabatic Decompression and the Composition and Mean Properties of Mid-ocean Ridge Basalts

Composition, mean pressure, mean melt fraction, and crustal thickness of model mid-ocean ridge basalts (MORBs) are calculated using MELTS. Polybaric, isentropic batch and fractional melts from ranges in source composition, potential temperature, and final melting pressure are integrated to represent idealized passive and active flow regimes. These MELTS-derived polybaric models are compared with other parameterizations; the results differ both in melt compositions, notably at small melt fractions, and in the solidus curve and melt productivity, as a result of the self-consistent energy balance in MELTS. MELTS predicts a maximum mean melt fraction (~0·08) and a limit to crustal thickness (<=15 km) for passive flow. For melting to the base of the crust, MELTS requires an ~200°C global potential temperature range to explain the range of oceanic crustal thickness; conversely, a global range of 60°C implies conductive cooling to ~50 km. Low near-solidus productivity means that any given crustal thickness requires higher initial pressure in MELTS than in other models. MELTS cannot at present be used to model details of MORB chemistry because of errors in the calibration, particularly Na partitioning. Source heterogeneity cannot explain either global or local Na–Fe systematics or the FeO–K2O/TiO2 correlation but can confound any extent of melting signal in CaO/Al2O3.

Cooling of the Earth: A parameterized convection study of whole versus layered models

Compositionally layered mantle models have often been invoked in order to explain the geochemistry observed at the Earth's surface, specifically the discrepancy between ocean island basalt and mid‐ocean ridge basalt compositions. One disadvantage of layered models is the reduction in cooling efficiency compared to whole‐mantle convection as a direct result of the insulating nature of the thermal boundary layer that develops between the two convecting layers. This may pose a significant problem for layered models in which the bottom layer is enriched in heat‐producing radioactive elements with respect to the top layer. One may expect that the bottom layer would become superheated over the lifetime of the Earth. We perform this study in order to test whether layered models are thermally feasible. We are interested in discovering whether it was possible, within Earth‐like constraints, to produce a bottom layer temperature that remains below the solidus without simultaneously producing a top layer that is too cool. We use parameterized convection to explore a wide parameter range of input values for several layering configurations. We study both a whole mantle convection model and the recently proposed layered convection model, which places the boundary between the layers at 1600‐km depth [Kellogg et al., 1999]. We use the present‐day heat flow and mantle viscosity as primary constraints and use the resulting average temperature as a test for the feasibility of the models. Our results reveal that for whole mantle convection, a wide parameter range produces results that satisfy our constraints. This is in contrast to the layered convection model in which we find that the parameter range that satisfies constraints is significantly reduced or perhaps, nonexistent.

The remelting of hydrothermally altered peridotite at mid-ocean ridges by intruding mantle diapirs

Most gabbroic cumulates found at ocean spreading centres are thought to have been generated by the fractional crystallization of melts with the composition of mid-ocean ridge basalt (MORB)1. There are exceptions, however, including some cumulates which appear to have come from melts that contain more silica than MORB and are much more depleted in the incompatible elements (those elements that do not readily substitute into the main mineral phases)2. These unusual rocks bear witness to relatively deep petrological processes that are not accessible through the study of melts erupted on the sea floor, and their origin is still debated. Fortunately, the same lithologies can be studied in detail in ophiolites (sections of oceanic crust accreted to a continent). In a fossil mantle diapir of the Oman ophiolite3,4, we have observed the same dichotomy between a suite of ‘normal’, MORB-type, cumulates (‘N-cumulates’) and a suite of cumulates issued from silica-enriched but incompatible-element-depleted melts (‘D-cumulates’). While the N-cumulates crystallized inside the diapir, the D-cumulates occur essentially as intrusions surrounding the diapir. The combination of silica enrichment, extreme depletion in incompatible elements, and seawater isotopic signature indicates that the D-cumulates were formed by the remelting at low pressure of hydrated residual peridotites left after MORB extraction at the ridge axis. The distribution of the D-cumulates relative to the N-cumulates suggests that such depleted melts are produced episodically at ridge axes when the lithospheric mantle is reheated by a new diapiric pulse.

Effect of lateral migration of melt on chemical compositions: Implications for the local variations of mid‐ocean ridge basalts

The decompressional melting process involving dynamically induced migration of melt in the residual country rock is analyzed to formulate the effects of melt migration on the chemical compositions of mid‐ocean ridge basalt (MORB). It is rigorously proved that if the relative motion between melt and solid is unidirectional and steady, the composition of the melt is the same as that expected from the batch melting because it is determined by equations that are separable from those concerning the relative melt motion. The present proof is more general than the previous treatments and is applicable to any system consisting of arbitrary numbers of phases and components. The effect of lateral melt motion on the concentration of a minor component is formulated more specifically using two parameters. One of the parameters is the lateral gradient of melt flux that represents degree of convergence or divergence of melt in the flow. The other parameter that specifies the gradient of the chemical composition is introduced to evaluate the so‐called chromatographic effect that separates the minor component migration paths in the flow by their partition coefficients. The parameterization gives a good tool to predict the flow field from the known chemical variations. It is shown that some local variations of MORB compositions observed in individual mid‐ocean ridges can be modeled as a result of lateral melt migration convergent in some places and divergent in others.

Stability limits of hydrous minerals in sediment and mid‐ocean ridge basalt compositions: Implications for water transport in subduction zones

Phase relationships in hydrated natural sediment and mid‐ocean ridge basalt (MORB) compositions were investigated experimentally from 6 to 15 GPa and 700°C to 1400°C in order to determine the stability of hydrous phases in subducting slabs and to constrain reactions resulting in the release of water from subduction zones to the mantle. Phengite and hydrous aluminum silicates (topaz‐OH and phase egg) are stable in the sediment composition at depths greater than 150 km and constitute H2O reservoirs. Similarly, lawsonite is an H2O reservoir in MORB composition at depths greater than 150 km. According to mass balance calculations based on experimental run products, the weight proportions of phengite and H2O at 6 GPa in the sediment are about 50% and 2%, respectively. At pressures from 8 to 12 GPa, topaz‐OH (Al2SiO4(OH)2) appears as the hydrous mineral, and the sediment retains about 0.5–0.7 wt % H2O. At higher pressures, phase egg (AlSiO3(OH)) appears as the hydrous phase with a large stability field. The H2O content in the sediment is about 0.5 wt% at pressures greater than 12 GPa. In natural MORB compositions, lawsonite is the hydrous phase at pressures lower than 10 GPa. About 1 wt%, H2O in the MORB composition is retained by lawsonite at 6 GPa and 800°C. Above 6 GPa, a lawsonite‐out reaction is controlled by compositional changes of garnet and clinopyroxene. If the oceanic crust descends with an overlying sediment layer, water can be carried down into the transition zone by the sediment. The amount of transported water is controlled by the chemical compositions of the subducting sediments. Water in the MORB layer can be stored to 300 km in lawsonite for a cold subduction zone geotherm. Water release at depths greater than 200 km through progressive lawsonite breakdown can hydrate the overlying mantle, causing the generation of a peridotite region including hydrous phases (chondrodite, clinohumite, phase A, and phase E). If this hydrous peridotite is dragged down by the descending oceanic crust, water can be transported into the transition zone by the subduction process. The amount of transported water is controlled by the thermal state of the slab and the chemical composition of MORB.

Nitrogen recycling in subduction zones

The isotopic composition of nitrogen bears important information concerning the fate of this volatile in the mantle‐crust‐atmosphere system. Based on the isotopic compositions of mid‐ocean ridge basalts and pristine diamonds, a δ15N value of about ‐ 5±2‰ (where δ15N = [( 15N/ 14)sample/ ( 15N/ 14N)air − 1] × 1000) is assigned to the upper mantle. In contrast, the origin of nitrogen has not been well documented in subduction zones. We report here δ15N values, N2/36Ar, and 40Ar/ 36Ar ratios of a comprehensive suite of samples from along convergent plate boundaries measured using a newly developed static mass spectrometry method. The δ15N values and the N2/ 36Ar ratios vary significantly from −2.7‰ to 5.0‰ and from 2.28×104 to 7.15×l05, respectively. Taking a typical δ 15N value for sediments, assuming that the N2/ 36Ar ratio of mid‐ocean ridge basalts is representative of the upper mantle, and that later isotopic and elemental fractionation are not significant, it is possible to deconvolve each fraction of nitrogen on the basis of simple mixing equations. About 50% of nitrogen in back‐arc basin basalts originated from the upper mantle, whereas island arc samples contain only about 15% of the mantle‐derived nitrogen, the major fraction being derived from recycled sedimentary nitrogen.

Velocities of a natural mid-ocean ridge basalt glass

We report, for the first time, ultrasonic velocity values for a pure (>95%), natural, submarine basalt glass of mid-ocean ridge basalt composition, from 10 to 1000 MPa at room temperature. These new data show that basalt glass, abundant in the upper oceanic crust, has the lowest velocity of any primary solid component of the oceanic crust. In addition, natural basalt glass has a steeper pressure-dependence of velocity than previously measured in more crystalline samples, indicating that cracks in natural basalt glass are weaker than in more crystalline rocks. To obtain values for the pure glass phase, we correct the natural glass data for the low-pressure closure of cracks, and the presence of minor mineralogic components and vesicles. These new data provide a baseline for evaluating the effect of abundant basalt glass and glassy mesostasis in oceanic upper crust on in situ seismic velocities. In addition, data on the elastic and seismic properties of natural glasses are useful for a better understanding of glass structure, and glass relaxation, with potential applications to submarine volcanology.