Soret separation of mid-ocean ridge basalt magma

Chemical differentiation of an initially homogeneous mid-ocean ridge basalt (MORB) liquid has been experimentally observed in a temperature gradient above the liquidus. The magnitude of this effect in producing differences in composition is comparable to that of crystal-liquid fractionation for a given temperature difference. However the chemical changes produced by the two processes, Soret and crystal-liquid fractionation, are different. Soret separations resemble those observed in a third process — silicate liquid immiscibility. This similarity is a reflection of the fact that the Soret-separable components are the same network-former/network-modifier structural components which segregate during silicate liquid immiscibility.The differences between Soret and crystal-liquid separations allow the recognition of Soret processes as anomalies in MORB suites which are not compatible with normal crystal fractionation processes. The common occurrence of primary, cumulus, magnesian orthopyroxene in MOR gabbros and the absence of such orthopyroxene as phenocrysts in the coeval erupted MORBS is one such anomaly. The peculiar covariation of plagioclase and olivine compositions in some 3-phase olivine-plagioclase-clinopyroxene MOR gabbros, when compared with normal crystal fractionation results, is another anomaly which may be understood in terms of Soret processes operating in conjunction with normal crystal fractionation. These Soret processes must operate through some sort of convective thermal boundary layer at the margin of a MOR magma chamber to allow Soret diffusive exchange to occur before the temperature contrasts which drive it are dissipated. These driving temperature contrasts are also maintained in part by the periodic replenishment of hot, fresh magma into the MOR magma chamber.     

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.     

A theoretical approach to understanding the isotopic heterogeneity of mid-ocean ridge basalt

We have developed an idealized mathematical model to understand the isotopic variability of the mantle and its relation to the observed variations in isotopic ratios ¹⁴³Nd/¹⁴⁴Nd, ⁸⁷Sr/⁸⁶Sr, ¹⁷⁶Hf/¹⁷⁷Hf, ²⁰⁸Pb/²⁰⁴Pb, ²⁰⁶Pb/²⁰⁴Pb, and ²⁰⁷Pb/²⁰⁴Pb measured on mid-ocean ridge basalt (MORB). We consider a simple box model of mantle processes. A single melt region produces a melt fraction F of melt, and the average time since a given parcel of mantle material last visited this region is given by the time scale τ_melt. The melt region fractionates the parent/daughter ratios. Over time this leads to variations in the mantle isotopic ratios as the parent decays to the daughter. Key assumptions are that the half-life of the parent isotope is large compared with τ_melt, that the flow is strongly stirring, and that the mantle has reached a statistical steady state. This enables us to neglect the specifics of the underlying flow. Sampling from our model mantle is dealt with by averaging over a large number N of samples to represent the mixing after melting.The model predicts a probability density for isotopic ratios in MORB which, with exception of the Pb isotopes, are consistent with measurements. Fitting the MORB data to this model gives estimates of the model parameters F, τ_melt, and N. Small melt fractions with F around 0.5% are essential for a good fit, whereas τ_melt and N are less well constrained. τ_melt is estimated at around 1.4 to 2.4 Ga, and N is of the order of hundreds. The model predicts a larger variability for the Pb isotopes than that observed. As has been stated by many previous authors, it appears that fundamental differences exist between the dynamics of Pb isotopes and those of Nd, Sr and Hf isotopes.     

The influence of primary magma composition,H2O and pressure on mid-ocean ridge basalt differentiation

MORB suites display variations in their chemical differentiation trends which are closely related to the incompatible element enrichment of the basalts. We examine suites of primitive to evolved basalts from the Pacific-Nazca Ridge at 28° S (mostly depleted); from the Juan Fernandez microplate region (depleted) and from the Explorer Ridge, northeast Pacific (mostly enriched). Trends for incompatible element enriched MORBs consistently show less depletion of Al₂O₃ and less enrichment of FeO when plotted on MgO variation diagrams.Least squares modeling indicates that enriched basalts have undergone less plagioclase crystallization than depleted basalts especially in the early stages of differentiation. Using thermodynamic modelling, we show that variations between MORB differentiation trends result largely from differences in the major element chemistry and H₂O content of primary magmas. Our chosen enriched and depleted near-primary magmas are similar in major element chemistry but the enriched near-primary magma has higher H₂O and lower Al₂O₃ than the depleted near-primary magma. The MORB crystallization sequence is: olivineolivine+plagioclase olivine+plagioclase+high-Ca pyroxene; and the separate and combined effects of lower Al₂O₃ and higher H₂O are to cause plagioclase to crystallize later (lower temperature), and to make the interval of olivine+plagioclase crystallization shorter. As a result, enriched differentiates have higher Al₂O₃ and lower FeO than depleted MORBs at a given MgO content, even though their parents' Al₂O₃ is lower. Crystallization of enriched basalts at higher pressure than depleted basalts is not able to account for differences between the differentiation trends because the proportion of plagioclase is higher during three-phase crystallization at high pressure.The variations in trends do not depend on geographic location and thus are superimposed on any regional variations in MORB chemistry or mantle source. Nor are they related to spreading rate. Depleted basalts from the fast-spreading 28° S and Juan Fernandez ridges have differentiation trends similar to depleted basalts from the medium-spreading Galapagos Spreading Center, whereas differentiation trends for enriched basalts from the medium-spreading Explorer Ridge are quite different. Fe³⁺/Fe_total is similar (and quite low) for enriched and depleted basalts, indicating that neither oxidation state nor early magnetite crystallization are important.     

Search for the proverbial mantle osmium sources to the oceans: Hydrothermal alteration of mid-ocean ridge basalt

We present Os and Sr isotopes and Mg, Os, and Sr concentrations for ridge-crest high-temperature and diffuse hydrothermal fluids, plume fluids and ridge-flank warm spring fluids from the Juan de Fuca Ridge. The data are used to evaluate the extent to which (1) the high- and low-temperature hydrothermal alteration of mid-ocean ridge basalts (MORBs) provides Os to the deep oceans, and (2) hydrothermal contributions of non-radiogenic Os and Sr to the oceans are coupled. The Os and Sr isotopic ratios of the high-temperature fluids (265-353 °C) are dominated by basalts (¹⁸⁷Os/¹⁸⁸Os = 0.2; ⁸⁷Sr/⁸⁶Sr = 0.704) but the concentrations of these elements are buffered approximately at their seawater values. The ¹⁸⁷Os/¹⁸⁸Os of the hydrothermal plume fluids collected ∼1 m above the orifice of Hulk vent is close to the seawater value (=1.05). The low-temperature diffuse fluids (10-40 °C) associated with ridge-crest high-temperature hydrothermal systems on average have [Os] = 31 fmol kg⁻¹, ¹⁸⁷Os/¹⁸⁸Os = 0.9 and [Sr] = 86 μmol kg⁻¹, ⁸⁷Sr/⁸⁶Sr = 0.709. They appear to result from mixing of a high-temperature fluid and a seawater component. The ridge-flank warm spring fluids (10-62 °C) on average yield [Os] = 22 fmol kg⁻¹, ¹⁸⁷Os/¹⁸⁸Os = 0.8 and [Sr] = 115 μmol kg⁻¹, ⁸⁷Sr/⁸⁶Sr = 0.708. The data are consistent with isotopic exchange of Os and Sr between basalt and circulating seawater during low-temperature hydrothermal alteration. The average Sr concentration in these fluids appears to be similar to seawater and consistent with previous studies. In comparison, the average Os concentration is less than seawater by more than a factor of two. If these data are representative they indicate that low-temperature alteration of MORB does not provide adequate non-radiogenic Os and that another source of mantle Os to the oceans must be investigated. At present, the magnitude of non-radiogenic Sr contribution via low-temperature seawater alteration is not well constrained. If non-radiogenic Sr to the oceans is predominantly from the alteration of MORB, our data suggest that there must be a different source of non-radiogenic Os and that the Os and Sr isotope systems in the oceans are decoupled.     

Chemical evolution of mid-ocean ridge hot springs

Chemical analyses yielding elemental concentrations of major and minor elements of four hot springs on the East Pacific Rise at 21°N and 10 hot springs from the southern trough of the Guaymas Basin, Gulf of California provide a basis for thermodynamic modeling of conductive cooling of the hydrothermal endmembers, mixing of the hydrothermal endmember with seawater, and reaction of an EPR-type fluid with sediment of bulk chemical composition corresponding to unaltered sediment from DSDP hole 477 in the Guaymas Basin. Results of the calculations indicate that conductive cooling of endmember fluids within closed chimneys and conduits accounts for the solution composition of one vent on the East Pacific Rise and several vents in Guaymas Basin. Mixing of the hydrothermal fluids with seawater yields a prediction of mineral assemblages closely approximating those observed in samples of chimneys from the East Pacific Rise and drill cores in Guaymas Basin. The hypothesis that Guaymas Basin hydrothermal fluids result from interaction of an EPR-type fluid with sediment cover in Guaymas Basin is supported by calculations which predict an increase in pH to a value similar to Guaymas Basin fluids, an order-of-magnitude decrease in metal concentrations, and an excellent agreement between predicted mineral assemblages as a function of extent of interaction with sediment, and observed mineral assemblage distribution with depth.     

The very depleted nature of certain primary mid-ocean ridge basalts

Many mid-ocean ridge basalts (MORBs) might be ultimately derived from primary magmas that are very depleted in Na₂O and TiO₂. These very depleted primary magmas have 0.60 to 1.50 wt.% Na₂O and 0.10 to 0.50 wt.% TiO₂ compared to MORBs, which typically have > 1.90% Na₂O and >0.60% TiO₂. Evidence for these depleted primary magma compositions is obtained from megacrysts in MORBs, from glass inclusions within these megacrysts, and from the highly calcic plagioclases (An_91–96) and depleted clinopyroxenes (Na₂O mostly between 0.10 and 0.35) in certain abyssal peridotites.Cumulate ultramanfi and gabbroic rocks from the North Arm Mountain Massif of the Bay of Islands ophiolite complex show a progressive increase in the Na₂O and TiO₂ abundances in clinopyroxene crystals with stratigraphic height in the ophiolite. The use of mineral-liquid distribution coefficients and cumulate mineral compositions indicate that the liquids from which these minerals crystallized had 0.10 to 0.20 wt.% TiO₂ and 0.60 to 0.80 wt.% Na₂O for the lowermost cumulate ultramafic rocks, with TiO₂ and Na₂O abundances of liquids increasing progressively to normal MORB abundances during crystallization of higher-level gabbroic cumulates. These data clearly demonstrate that primary basalts that are very depleted in Na₂O and TiO₂ can differentiate to form residual magmas that are indistinguishable from MORBs.     

Lithium isotopes in global mid-ocean ridge basalts

The lithium isotope compositions of 30 well-characterized samples of glassy lavas from the three major mid-ocean ridge segments of the world, spanning a wide range in radiogenic isotope and elemental content and sea floor physical parameters, have been measured. The overall data set shows a significant range in δ⁷Li (+1.6 to +5.6), with no global correlation between Li isotopes and other geochemical or tectonic parameters. The samples with the greatest lithophile element depletion (N-MORB: K₂O/TiO₂ < 0.09) display an isotopic range consistent with the extant database. Samples with greater trace element enrichment display a greater degree of isotopic variability and trend toward heavier compositions (δ⁷Li = +2.4 to +5.6), but are not distinct on average from N-MORB. Together with published data, N-MORB is estimated to have mean δ⁷Li = +3.4 ± 1.4‰ (2σ), consistent with the estimate for an uncontaminated MORB source based on pristine peridotite xenoliths. Locally, where sampling density permits, sources of Li isotope heterogeneity may be evaluated. Sample sets from the East Pacific Rise show correlation of δ⁷Li with halogen concentration ratios. This is interpreted at 15.5°N latitude to represent incorporation of <5 weight percent recycled subduction-modified mantle in the MORB source. At 9.5°N latitude the data are more consistent with shallow level magma chamber contamination by seawater-derived components (<0.5 wt.%).     

Vesiculation and vesicle loss in mid-ocean ridge basalt glasses: He, Ne, Ar elemental fractionation and pressure influence

Sarda and Graham (1990) proposed that in mid-ocean ridge basalts (MORBs), degassing occurs through equilibrium vesiculation followed by various extents of vesicle loss. This model predicts that in a bulk sample of MORB glass with vesicles, the rare gases represent a binary mixture between a vesicle component and a component dissolved in the melt. As vesiculation is expected to produce very different rare gas concentrations and elemental ratios in gas and melt, binary mixing systematics should be recorded in the MORB rare gas abundance data. Indeed, a large range of ⁴He/⁴⁰Ar∗ ratios was known to exist, but these binary mixing systematics remained elusive because helium was used as a proxy for rare gas abundance because helium is not affected by air addition. Here we show that using Ar instead of He, the ⁴He/⁴⁰Ar∗ ratio is higher where the Ar concentration is lower, as expected from simple binary mixing systematics.Taking advantage of the growing Ne database, we further show that the predicted binary mixing is recorded by the He-Ar and He-Ne couples, provided He concentration is not used to trace vesicle abundance. This is because a significant part of helium remains in the melt due to its higher solubility. In contrast, Ar or Ne concentrations, which can both be corrected for air addition, clearly trace vesicles and yield binary mixing patterns that hold for ridges worldwide. The model of vesiculation and vesicle loss thereby finds geochemical support in the rare gas abundance data.The He-Ne-Ar concentration data is best explained by assuming the ratio of helium to neon or argon solubility is about 5 to 15 times higher than values measured in 1 bar laboratory experiments, due to higher He and lower Ne and Ar solubilities. We propose that this is a pressure effect, and vesiculation mainly occurs during magma ascent in the mantle after melting.     

Seafloor hydrothermal alteration at an Archaean mid-ocean ridge

A hydrothermally metamorphosed/altered greenstone complex capped by bedded cherts exposed in the North Pole, Pilbara Carton, Western Australia, is interpreted as an accretionary complex. It is distinctive in being characterised by both duplex structure and an oceanic crust stratigraphy. This complex is shown to represent an Archaean upper oceanic crust with a mid‐ocean ridge hydrothermal metamorphism that increases in grade stratigraphically downward. Three mineral zones have been defined; Zone A of the zeolite facies, the prehnite‐pumpellyite facies or the lower‐greenschist facies at high‐X_CO2 condition, Zone B of the greenschist facies, and Zone C of the greenschist/amphibolite transition facies. In Zone A metabasites, Ca‐Al silicates including Ca‐zeolites, prehnite and pumpellyite are absent and epidote/clinozoisite is extremely rare. Instead, abundant carbonates are present with chlorite suggesting high‐X_CO2 composition in the fluid. On the other hand, in Zones B and C metabasites, where Ca‐amphibole + epidote/clinozoisite + chlorite + Ca‐Na plagioclase are the dominant assemblages, carbonate is not identified. The metamorphic conditions boundary of Zones B/C were estimated to be about 350 °C at a pressure of <0.5 kbar. Fluid compositions coexisting with Archaean greenstones at the transition between Zones B and C were estimated by thermodynamic calculation in the CaFMASCH system (T = 350–370 °C, P = 150–1000 bar) at X_CO2 of 0.012–0.140, such values are higher than present‐day vent fluids collected near mid‐ocean ridges with low‐X_CO2 values, up to 0.005. The Archaean seawater depth at the mid‐ocean ridge was estimated to be 1600 m at X_CO2 = 0.06 using a depth‐to‐boiling point curve for a fluid. The carbonation due to high‐X_CO2 hydrothermal fluids occurred near the ridge‐axis before or was coincident with ridge metamorphism.     

Composition and depth of origin of primary mid-ocean ridge basalts

Some workers have held that mid-ocean ridge basalts are fractionated from high pressure (15–30 kbar) picritic primary magmas whereas others have favored primary magmas generated at about 10 kbar with compositions close to those of mid-ocean ridge basalts. Of critical significance are presumed differences in composition between experimentally determined primary magmas and the least fractionated mid-ocean ridge basalts. To evaluate the significance of these differences, all based on electron microprobe analyses, we consider three sources of uncertainty: (1) analytical uncertainties for a single microprobe laboratory, (2) systematic interlaboratory analytical differences, and (3) real variations in the possible compositions of primary magmas that can be produced from a peridotite source at a given pressure. The first source of error is surprisingly large and can account for a substantial part of the total variation of normative quartz (hypersthene calculated as equivalent olivine and quartz) in FAMOUS basalts. The second is not as serious but remains undetermined for many laboratories. The third is potentially the largest but is not yet fully documented. The least fractionated FA-MOUS basalts have high mg numbers (70–73) compatible with derivation from the mantle by direct partial melting with little or no subsequent fractional crystallization. Because of the wide range of normative quartz content in these basalts, it appears necessary to consider them as representatives of multiple parental magmas. When all the sources of uncertainty are taken into account, we conclude that the experimental data by various investigators are all fairly consistent and favor derivation of the least fractionated mid-ocean ridge basalts by at most only a small amount of fractional crystallization from primary magmas having a wide range of normative quartz content and generated over a range of pressures from about 7–11 kbar. Contribution No. 420, Department of Geosciences, The University of Texas at Dallas     

New discriminant-function-based multidimensional discrimination of mid-ocean ridge and oceanic plateau

From a compilation of geochemical data for the discrimination of the tectonic settings of mid-ocean ridge(MOR;3730 samples) and oceanic plateau(OP;3656 samples),we present two new discriminant functions and diagrams obtained from censored multivariate discordant outlier-free datasets.Ten different sets of data(original concentrations as well as isometric log-ratio transformed(ilr) variables;all 10 major(M) elements as well as all 10 major and 6 trace elements MT) were used to evaluate the quality of discrimination from linear discriminant analysis(LDA) and canonical analysis.Two selected multidimensional models ilrM(9 ilr transformed variables of multi-normally distributed 10 major elements) and ilrMT(15 ilr transformed variables of multi-normally distributed combined 10 major and 6 trace elements),considered as the best or most representative(total of5650 samples for ilr_M and 2858 for ilrMT),provided percent success values,respectively,of 80.9% for the MOR and 81.1% for the OP(ilr_M) and 88.5% for the MOR and 90.1% for the OP(ilrMT).Both processes(log-ratio transformation and multi-normality) rendered the percent success values similar for both groups(MOR and OP).The respective discriminant functions were successfully used for four tests from known tectonic settings and four application cases(two for ophiolites and two for Precambrian rocks),documenting thus the utility of the new discrimination procedure for the MOR and OP tectonic settings.Furthermore,we showed that our multidimensional procedure is robust against analytical errors or uncertainties,as well as post-emplacement compositional changes caused by element mobility from both low or high temperature alteration.The robustness against the gain or loss of a single element at a time was also documented,from which the ilr_(MT) model was evaluated as more robust than the ilr_M model.A new online computer program MOROPdisc was written in Java Framework ZK,which is freely available for use at our web portal http://tlaloc.ier.unam.mx.     

The Othris Ophiolite, Greece: A snapshot of subduction initiation at a mid-ocean ridge

The mantle section of the Tethyan-type Othris Ophiolite, Greece, records tectono-magmatic processes characteristic of both mid-ocean ridges and supra-subduction zones. The Othris Ophiolite is a remnant of the Jurassic Neotethys Ocean, which existed between Eurasia and Gondwanaland. Othris peridotites range from fertile plagioclase lherzolites to depleted harzburgites. Abundances of Al₂O₃ and CaO show well-defined inverse linear correlations with MgO, suggesting that the Othris peridotites formed as residua from variable degrees of partial melting.

Peridotites from the Fournos Kaïtsa and western Katáchloron sub-massifs are similar to abyssal peridotites and can be explained by a multistage model with some melting in the garnet stability field followed by moderate degrees of anhydrous near-fractional melting in the spinel stability field. In contrast, the peridotites from the Metalleio, Eretria, and eastern Katáchloron sub-massifs, and the Vourinos ophiolite are highly depleted and have extremely low concentrations of Al₂O₃ and heavy rare earth elements. These peridotites have enriched light REE contents compared to the middle REE. These residua are best modelled by hydrous melting due to a flux of slab-derived fluid to the mantle wedge during melting.

The occurrence of both styles of melting regimes within close spatial and temporal association in the same ophiolite is explained by intra-oceanic thrusting and forced subduction initiation at (or near) a mid-ocean ridge. Thus, the Othris Ophiolite, and probably Tethyan-type ophiolites in general, represent a transient phase of plate tectonic reorganisation rather than quasi-steady state plate tectonics.     

Generation of mid-ocean ridge basalts at pressures from 1 to 7 GPa

We propose a model for the generation of average MORBs based on phase relations in the CaO-MgO-Al₂O₃-SiO₂-CO₂ system at pressures from 3 to 7 GPa and in the CaO-MgO-Al₂O₃-SiO₂-Na₂O-FeO (CMASNF) system at pressures from ∼0.9 to 1.5 GPa. The MELT seismic tomography (Forsyth et al., 2000) across the East Pacific Rise shows the largest amount of melt centered at ∼30-km depth and lesser amounts at greater depths. An average mantle adiabat with a model-system potential temperature (T_p) of 1310°C is used that is consistent with this result. In the mantle, additional minor components would lower solidus temperatures ∼50°C, which would lower T_p of the adiabat for average MORBs to ∼1260°C. The model involves generation of carbonatitic melts and melts that are transitional between carbonatite and kimberlite at very small melt fractions (<0.2%) in the low-velocity zone at pressures of ∼2.6 to 7 GPa in the CMAS-CO₂ system, roughly the pressure range of the PREM low-velocity zone. These small-volume, low-viscosity melts are mixed with much larger volumes of basaltic melt generated at the plagioclase-spinel lherzolite transition in the pressure range of ∼0.9 to 1.5 GPa.In this model, solidus phase relations in the pressure range of the plagioclase-spinel lherzolite transition strongly, but not totally, control the major-element characteristics of MORBs. Although the plagioclase-spinel lherzolite transition suppresses isentropic decompression melting in the CMAS system, this effect does not occur in the topologically different and petrologically more realistic CMASNF system. On the basis of the absence of plagioclase from most abyssal peridotites, which are the presumed residues of MORB generation, we calculate melt productivity during polybaric fractional melting in the plagioclase-spinel lherzolite transition interval at exhaustion of plagioclase in the residue. In the CMASN system, these calculations indicate that the total melt productivity is ∼24%, which is adequate to produce the oceanic crust. The residual mineral proportions from this calculation closely match those of average abyssal peridotites.Melts generated in the plagioclase-spinel lherzolite transition are compositionally distinct from all MORB glasses, but do not have a significant fractional crystallization trend controlled by olivine alone. They reach the composition field of erupted MORBs mainly by crystallization of both plagioclase and olivine, with initial crystallization of either one of these phases rapidly joined by the other. This is consistent with phenocryst assemblages and experimental studies of the most primitive MORBs, which do not show an olivine-controlled fractionation trend. The model is most robust for the eastern Pacific, where an adiabat with a T_p of ∼1260°C is supported by the MELT seismic data and where the global inverse correlation of (FeO)₈ with (Na₂O)₈ is weak. Average MORBs worldwide also are well modeled. A heterogeneous mantle consisting of peridotite of varying degrees of major-element depletion combined with phase-equilibrium controls in the plagioclase-spinel lherzolite transition interval would produce the form of the global correlations at a constant T_p, which suggests a modest range of T_p along ridges. Phase-composition data for the CMASNF system are presently not adequate for quantitative calculation of (FeO)₈-(Na₂O)₈-(CaO/Al₂O₃)₈ systematics in terms of this model. The near absence of basalts in the central portion of the Gakkel Ridge suggests a lower bound for T_p along ridges of ∼1240°C, a potential temperature just low enough to miss the solidus for basalt production at ∼0.9 GPa. An upper bound for T_p is poorly constrained, but the complete absence of picritic glasses in Iceland and the global ridge system suggests an upper bound of ∼1400°C. In contrast to some previous models for MORB generation that emphasize large potential temperature variations in a relatively homogeneous peridotitic mantle, our model emphasizes modest potential temperature variations in a peridotitic mantle that shows varying degrees of heterogeneity. Calculations indicate that melt productivity changes from 0 to 24% for a change in T_p from 1240 to 1260°C, effectively producing a rapid increase to full crustal thickness or decrease to none as ridges appear and disappear.     

全球洋中脊系统的构造特征及其运动学意义

本文将全球洋中脊系统作为研究整体,根据洋中脊的全球分布、运动学特征及其初始形成时与泛大陆的构造几何关系,将全球现今的洋中脊系统划分为内、外支洋中脊。外支洋中脊为探索者洋中脊-太平洋洋隆-东南印度洋中脊-西北印度洋中脊,起源于泛大洋及冈瓦纳大陆内部;内支洋中脊为西南印度洋中脊-大西洋中脊-北冰洋加科尔洋中脊,起源于泛大陆内部。两者之间通过俯冲带、转换断层以及弥散性板块边界实现全球板块构造在运动上的平衡,并保持地球的球形几何形态恒定。外支洋中脊在全球板块构造上造成泛大洋缩减,并持续被太平洋取代,直接推动了环太平洋俯冲带的形成;内支洋中脊造成大西洋盆、印度洋盆中生代以来持续扩张。中生代以来,外支洋中脊和内支洋中脊共同作用引起非洲板块、印度澳大利亚板块向北运动,新特提斯洋盆关闭,形成特提斯(阿尔卑斯山-喀尔巴阡山-扎格罗斯山-喜马拉雅山)碰撞造山带,并通过洋中脊扩张平衡了相关的岩石圈缩短。     

Geochemistry and genesis of the Nadun Nb-enriched arc basalt in the Duolong mineral district,western Tibet:Indication of ridge subduction

The Duolong mineral district in western Tibet is one of the largest porphyry Cu–Au deposit fields with significant metallogenic potential in China.Its tectonic environment relevant to Early Cretaceous Cu–Au mineralization remains controversial.Here we report new whole-rock major and trace element,and Sr-Nd-Hf-Pb isotopic data for the newly discovered basalt in the Nadun area,Duolong mineral district,to decipher their genesis and further constrain the tectonic environment.A contemporaneous rhyolite sample interbedded with the basalt in the lower part of the volcanic section in the Nadun area yields an LA-ICP-MS zircon U–Pb age of 122.5±1.2 Ma.The basalt samples exhibit high-K calc-alkaline/shoshonite properties and are enriched in high field strength elements,e.g.,high Ti O₂(1.43–1.79 wt.%)and Nb(14.6–19.5 ppm)contents,with high Nb/La ratios(0.4–0.6),which are compositionally comparable to those of Nb-enriched arc basalts(NEABs).The(⁸⁷ Sr/⁸⁶ Sr)iratios of 0.7052 to 0.7056,negative eNd(t)(-0.7 to-0.2)and eHf(t)values(+6.0 to+6.5),and high(²⁰⁶ Pb/²⁰⁴Pb)i,(²⁰⁷ Pb/²⁰⁴Pb)i,(²⁰⁸ Pb/²⁰⁴Pb)iand ratios(18.522 to 18.561,15.641 to 15.645 and 38.679 to 38.730,respectively)suggest that the Nadun NEABs are more enriched than those of the island arc basalts(IABs)in the area.The slightly enriched radiogenic isotopes for the Nadun NEABs indicate that the subducting sediments play an important role in the source.Furthermore,their high Nb,Ti,and Cu contents indicate that the source mantle wedge was metasomatized by slab melts.The Nadun NEAB and other coeval magmatic rocks in the Duolong mineral district,including adakite,OIB-like basalt,MORB-type basalt,A-type rhyolite,and common IAB,are typical rock assemblages of ridge subduction.We infer that the Duolong mineral district were formed by ridge subduction in the Early Cretaceous.     

弧后盆地玄武岩的成分变化及其成因

弧后盆地玄武岩(BABB)是弧后盆地扩张过程中岩浆作用的主要产物,其地球化学组成是认识弧后盆地演化的关键。现今弧后盆地主要集中在西太平洋地区。本文总结了该地区弧后盆地玄武岩的元素地球化学和同位素组成特征。总体而言,相对于开阔大洋洋中脊玄武岩(MORB),弧后盆地玄武岩的主量元素成分变化范围很大,在Al_2O_3-Mg O、Ti O_2-Mg O相关图上偏离了MORB的演化趋势,在Mg O相同的情况下表现出更高的Al_2O_3含量和更低的Ti O_2含量。弧后盆地玄武岩的微量元素特征一般介于MORB和弧玄武岩之间。一方面,它们与MORB一样在中、重稀土元素之间没有明显分馏;另一方面,与弧玄武岩一样富集大离子亲石元素Rb、Ba、Th、U、K,具有Pb的正异常和Nb、Ta的负异常等。其中,劳海盆、日本海海盆和冲绳海槽有部分样品具有Nb、Ta的正异常,表现出类似于E-MORB的微量元素特征。西太平洋地区弧后盆地玄武岩的Sr-Nd-Pb同位素组成变化范围较大,相对于MORB,其富集组分更常见,总体介于亏损地幔端元(DMM)、1型富集地幔(EM1)和2型富集地幔(EM2)三者之间。不同基底属性(大陆基底和大洋基底)和不同阶段的弧后盆地玄武岩的地球化学组成也有明显区别。弧后盆地玄武岩地球化学成分上的多样性主要受控于源区(地幔楔)的物质组成、熔融程度和岩浆上升过程中的变化等因素。地幔源区的不均一性主要体现在地幔楔自身的化学性质和俯冲板片的物质贡献差异。部分弧后盆地玄武岩具有异常高的地幔潜能温度、高的3He/4He比值以及E-MORB型的微量元素特征,说明其地幔源区还可能受到了地幔柱的影响。地幔潜能温度越高,俯冲流体贡献越多,地幔楔的熔融程度越大。此外,岩浆上升过程中发生的地壳混染、岩石圈中的熔体-岩石反应以及矿物的结晶分离都会改造岩浆的成分。     

The model of whole mantle convection with formation of a long living reservoir feeding the mid-ocean ridge

This paper shows the results of numerical experiments that reproduce the main global structures in the mantle. In particular, it is shown how, under whole mantle convection, a subducting plate produces circulation currents feeding the mid-ocean ridge.     

Dependence of mid-ocean ridge morphology on spreading rate in numerical 3-D models

The morphology of natural mid-ocean ridges changes significantly with the rate of extension. Full spreading rate on Earth varies over more than one order of magnitude, ranging from less than 10 mm/yr at the Gakkel Ridge in the Arctic Ocean to 170 mm/yr at the East Pacific Rise. The goal of this study is to reproduce and investigate the spreading patterns as they vary with extension rate using 3-D thermomechanical numerical models. The applied finite difference marker-in-cell code incorporates visco-plastic rheology of the lithosphere and a crustal growth algorithm. The evolution of mid-ocean ridges from nucleation to a steady-state is modelled for a wide range of spreading rates. With increasing spreading rate, four different regimes are obtained: (a) stable alternating magmatic and amagmatic sections (≈ 10 mm/yr), (b) transient features in asymmetrically spreading systems (≈ 20 mm/yr), (c) stable orthogonal ridge-transform fault patterns (≈ 40 mm/yr) and (d) stable curved ridges (≥ 60 mm/yr). Modelled ultraslow and slow mid-ocean ridges share key features with natural systems. Abyssal hills and oceanic core complexes are the dominant features on the flanks of natural slow-spreading ridges. Numerically, very similar features are produced, both generated by localised asymmetric plate growth controlled by a spontaneous development of large-offset normal faults (detachment faults). Asymmetric accretion in our models implies a lateral migration of the ridge segment, which might help explaining the very large offsets observed at certain transform faults in nature.