Instantaneous velocity of mid-ocean ridges

Intuition suggests that all points on the same mid-ocean ridge should rotate around the relative pole of the two-plate system at the same instantaneous angular velocity. Contrary to intuition, the instantaneous angular velocity of a ridge varies from one point to another along the ridge, given the general case in which two plates move around different plate-specific poles of rotation. The variation in the instantaneous angular velocity of a ridge is a function of the motion characteristics of the plates and the position of the ridge relative to the poles of plate motion. The length or orientation of individual ridge segments is predicted to vary over time, leading to local changes in the shape of the ridge. The gradient in instantaneous angular velocity for the fast-spreading East Pacific Ridge, between the Cocos and Pacific plates, is an order of magnitude greater than the gradient along the Mid-Atlantic Ridge, between the North American and African plates. This great contrast in ridge instantaneous velocity gradients may be reflected in the contrasting ridge geometries of the East Pacific and Mid-Atlantic Ridges.     

Regularities of ore formation in mid-ocean ridges

The regularities of the concentration of ore matter in the mid-ocean ridges are considered, and the mechanisms of hydrothermal and cumulative treatment of the crust by ore elements are substantiated.     

Serpentinization of abyssal peridotites at mid-ocean ridges

Serpentinites are an important component of the oceanic crust generated in slow to ultraslow spreading settings. In this context, the MOHO likely corresponds to a hydration boundary, which could match the 500 °C isotherm beneath the ridge axis. Textures from serpentinites sampled in ridge environments demonstrate that most of the serpentinization occurs under static conditions. The typical mineralogical association consists of lizardite ± chrysotile + magnetite ± tremolite ± talc. Despite the widespread occurrence of lizardite, considered as the low temperature serpentine variety, oxygen isotope fractionation suggests that serpentinization starts at high temperature, in the range of 300–500 °C. The fluid responsible for serpentinization is seawater, possibly evolved by interaction with the crust. Compared with fresh peridotites, serpentinites are strongly hydrated (10–15% H₂O) and oxidized. Serpentinization, however, does not seem to be accompanied by massive leaching of major elements, implying that it requires a volume increase. It results in an increase in chlorine, boron, fluorine, and sulfur, but its effect on other trace elements remains poorly detailed. The presence of serpentinites in the oceanic crust affects its physical properties, in particular by lowering its density and seismic velocities, and modifying its magnetic and rheological properties. Serpentinization may activate hydrothermal cells and generate methane and hydrogen anomalies which can sustain microbial communities. Two types of hydrothermal field have been identified: the Rainbow type, with high temperature (360 °C) black smokers requiring magmatic heat; the low temperature (40–75 °C) Lost City type, by contrast, can be activated by serpenintization reactions. To cite this article: C. Mével, C. R. Geoscience 335 (2003).     

大洋中脊下地壳结晶作用与熔融演化的地球化学证据

2012年8月19日,《自然——地球科学》（Nature Geoscience）发表了题为《大洋中脊下地壳结晶作用与熔融演化》的文章。文章给出了确定大洋中脊下地壳结晶作用发生位置及熔融演化的地球化学方法．,该研究提供了位于太平洋的2个扩张中心（快速扩张的东太平洋隆起和中速扩张的胡安·德富卡洋中脊）大洋中脊轴下方结晶深度...     

慢速-超慢速扩张洋中脊热液活动及其机理

海底热液系统是地球热量平衡的重要组成,也是地球化学循环和成矿作用发生的主要场所,与洋中脊系统在空间上具有很强的联系.慢速-超慢速扩张洋中脊中确认的活跃热液喷口数量约占全球总数量的三分之一,查明热液发育位置及发育岩性与岩浆-构造活动的耦合关系,对于研究海底热液活动演化过程和海底找矿具有很好的指示意义.本文将全球慢速-超慢...     

Matching satellite-derived gravity signatures and seismicity patterns along mid-ocean ridges

A world-wide correlation between satellite-derived gravity signatures and the relative abundance of teledetected earthquakes over mid-ocean ridges has yielded some unexpected results. Rift valley disappearances along slow-spreading centres and attendant excess volcanism coincide with seismicity gaps, at times related to nearby hotspots, whereas earthquake clusters along virtually aseismic, faster-spreading centres systematically indicate the presence of active propagating ridge tips. Therefore, at the world scale of investigation, seismicity fairly well predicts ridge morphology and 2nd order axial discontinuities. The occurrence of a certain degree of seismicity along the 'ductile' Reykjanes ridge south of the Iceland hotspot is tentatively explained in terms of prevailing shear stresses due to oblique spreading which accumulate on the available brittle volume on the flanks of the ridge rather than on its crest.     

Hydrothermal processes along mid-ocean ridges: An experimental investigation

An experimental investigation of high-temperature seawater/basalt interactions has been conducted in order to better evaluate the geochemical and economic implications of hydrothermal circulation of seawater in the oceanic crust along active mid-ocean ridges. The results indicate that, as seawater reacts with basalt between 200 ° C and 500 ° C at 500–800 bars, the fluid tends to change from an oxygenated, slightly alkaline, Na⁺, Mg⁺⁺, SO₄ ⁼, Cl^? solution to a reducing, acidic, Na⁺, Ca⁺⁺, Cl^?, solution with Fe, Mn and Cu concentrations up to 1500, 190 and 0.3 ppm respectively. Silica concentrations in the fluid reach concentrations of 200–600 ppm; however, Al abundances remain very low (～0.5 ppm). Gray and green smectites, anhydrite, albite, tremolite-actinolite, chalcopyrite, pyrrhotite and hematite were the dominant alteration products formed. These data imply that large-scale circulation of seawater in the oceanic crust could account for the Al-deficient metalliferous sediments associated with mid-ocean ridges and could be important in the genesis of certain Fe-Cu sulfide ore deposists. The process could also affect the geochemical budgets of certain elements and exert substantial control of the steady-state composition of seawater by removing excess Na and Mg and adding Ca, Si, and H to the oceans.     

Segmentation and seismicity of the ultraslow Knipovich and Gakkel mid-ocean ridges

Comprehensive analysis of detailed bathymetric data obtained during legs 24–27 of the R/V Akademik Nikolai Strakhov has been carried out on the Knipovich Ridge. The revealed variations of magmatic activity along the axis supplement the available information on segmentation of this ridge [7, 19, 33]. The new statistical data characterize seismic activity under settings of ultraslow oblique extension. As follows from the seismic data, the Knipovich Ridge belongs to structural units with intermediate geodynamics between the spreading ridge and transform fault. Magmatic and amagmatic segments of the Arctic ultraslow Knipovich and Gakkel mid-ocean ridges are compared.     

Experimental modeling of structure-forming deformations in rift zones of mid-ocean ridges

The modern methods of physical modeling of structure-forming deformations in extension zones of oceanic lithosphere are discussed; the methods differ in their experimental equipment, model material, and experimental techniques. The simulation performed with an elastic-ductile model has demonstrated that extension of a brittle lithospheric layer results in disruption of its continuity and in formation of a rift valley according to the mechanism of running fracture propagation. The modeling results provide insights into qualitative pattern of faulting and fracturing within a rift zone, specific features of rift segmentation, and development of various structural elements (axis bends, echelons of fractures, nontransform offsets, small and large overlaps, etc.) under various geodynamic conditions of spreading. The modeling has shown that origination and evolution of structures of various types depend on the lithosphere’s thickness beneath the rift axis; the width of the lithosphere’s heating zone; the spreading orientation; and, to a lesser degree, on the spreading rate. A relatively rectilinear rift broken into particular segments bounded by small-amplitude offsets with or without minor overlaps arises in the case of both a small width of the heating zone, closely related to the axial magma chamber, and a small thickness of the lithosphere (fast-spreading conditions). In the case of a wide heating zone caused by ascent of an asthenospheric wedge or a mantle plume, offsets of rift are more pronounced and deformations embrace a wider region. If, as a result, the thickness of the lithosphere increases, the rift will be less linear and the structural heterogeneity will become more contrasting. In addition to the thickness of the lithosphere, the angle between the rift zone and the extension axis also controls the rift configuration: the greater the angle, the more conspicuous the en echelon arrangement of fractures. For any spreading type, the propagating front of linear microfractures that disrupt the upper brittle layer of the lithosphere predates the origin of mesoscopic fractures and predetermines a general trend of the rift zone. This indicates that the fractures of various sizes propagate simultaneously.     

What processes at mid-ocean ridges tell us about volcanogenic massive sulfide deposits

Episodic seafloor spreading, ridge topography, and fault movement at ridges find (more extreme) analogs in the arc and back-arc setting where the volcanogenic massive sulfide (VMS) deposits that we mine today were formed. The factors affecting sulfide accumulation efficiency and the extent to which sulfides are concentrated spatially are the same in both settings, however. The processes occurring at mid-ocean ridges therefore provide a useful insight into those producing VMS deposits in arcs and back-arcs. The critical observation investigated here is that all the heat introduced by seafloor spreading at mid-ocean ridges is carried out of the crust within a few hundred meters of the ridge axis by ??350°C hydrothermal fluids. The high-temperature ridge hydrothermal systems are tied to the presence of magma at the ridge axis and greatly reduce the size and control the shape of axial magma intrusions. The amount of heat introduced to each square kilometer of ocean crust during its formation can be calculated, and its removal by high-temperature convection allows calculation of the total base metal endowment of the ocean basins. Using reasonable metal deposition efficiencies, we conclude that the ocean floor is a giant VMS district with metal resources >600 times the total known VMS reserves on land and a copper resource which would last >6,000?years at current production rates.     

Oceanic plagiogranites as a result of interaction between magmatic and hydrothermal systems in the slow-spreading mid-ocean ridges

The paper considers some petrological and geochemical aspects of the formation of oceanic plagiogranites (OPG)—felsic intrusive rocks, which were found in the plutonic complexes of modern mid-ocean ridges (MOR) and ophiolites of paleo-collisional zones. Based on the multi-equilibrium clinopyroxene-orthopyroxene-amphibole-plagioclase geothermobarometry, typical OPG found in gabbros and peridotites were formed at temperatures of 820–850°C and pressure of 2–2.5 kbar. Close temperature estimates (825 ± 50°C) were obtained from literature data on Ti content in zircon, with allowance for lowered TiO₂ activity in the rock. Under these P-T parameters, OPG can be generated only in the presence of fluid of water activity $ \left( {a_{H_2 O} } \right) $ \left( {a_{H_2 O} } \right) close to 0.9. OPG and associated recrystallized gabbroids contain high-temperature hornblende with significant Cl content (0.5–2 wt %). In addition, the plagiogranites are characterized by particular geochemical features such as extremely high Na₂O/K₂O (up to 135), sharp LREE enrichment ((Ce/Yb)_cn and (La/Sm)_cn up to 10 and 4, respectively), and elevated ⁸⁷Sr/⁸⁶Sr ratio relative to DMM. All these facts point to the key role of hydrothermal fluid, the seawater derivative, in the OPG formation. The fluid with $ a_{H_2 O} = 0.9 $ a_{H_2 O} = 0.9 (approximately 28 wt % NaCl) could be produced from seawater due to hydration reactions at the higher lower temperature horizons of oceanic crust in the course of its percolation to the OPG generation areas. The formation of plagiogranites in the MOR oceanic core complexes possibly reflects the fundamental feature of oceanic accretion: practically simultaneous (at the geological time scale) proceeding of exogenic (neptunic) and endogenous (plutonic) processes.     

The effect of compression on noble gas solubility in silicate melts and consequences for degassing at mid-ocean ridges

The effect of compression on noble gas solubility in silicate melts is still badly understood due to a lack of theoretical guidance. In the experimental literature, noble gases dissolving in liquid silicates are found to concentrate almost linearly with increasing pressure up to several tens of kbar, suggesting that Henry’s law could be valid up to very high pressures, although this law stipulates that the gaseous phase in contact with the liquid must be ideal. Recently, new experiments dealing with the dissolution of argon in synthetic and natural silicate melts have pointed out that the evolution of concentration with pressure exhibits a departure from linearity in the 50-100 kbar range, leading either to a levelling off or to a sudden collapse of the argon concentration above 50 kbar. Here, we investigate by means of liquid state physics how volatile species dissolve into silicate melts under pressure. We use a hard sphere model (the reference fluid in liquid state physics) to describe silicate melts and gas at high pressures. One of our main results is that, when pressure increases, the concurrent compaction of gas and melt explains the almost-linear behaviour of the noble gas concentration up to several tens of kbars, before melt compaction dominates and concentration either levels off or decreases gradually in the 50-100 kbar range. In spite of the existence of a quasi-linear regime over a large pressure range, our work disqualifies the use of the Henry law when dealing with high pressures. The implication of these findings to provide an understanding of degassing at mid-ocean ridges is next investigated. Applying our model to the scenario where CO₂ vesicle generation occurs in the magma at mantle depths during its ascent from melting regions, we evaluate magma vesicularity as well as noble gas concentrations in the basalt melt and in vesicles as a function of pressure at depth. It is stressed that the variable and usually strong noble gas elemental fractionation observed in mid-ocean ridge basalts can be explained by assuming a sequence of several vesiculation stages interrupted by vesicle loss during magma ascent.     

A new view of the He-Ar-CO2 degassing at mid-ocean ridges: Homogeneous composition of magmas from the upper mantle

Deep-sea exploration is rapidly improving our understanding of volatiles geochemistry in mid-ocean-ridge igneous products. It is also placing greater constraints on degassing processes of the Earth’s mantle, with the result that degassing models based on vapour-melt equilibrium are no longer able to explain the increasing number of data. In fact, such models force to postulate an upper mantle strongly heterogeneous at any scale, and cannot account for the widespread carbon supersaturation of the recovered igneous products. Here we review the global He-Ar-CO₂ dataset of fluid inclusions in mid-ocean-ridge glasses using the framework of advanced modelling of multicomponent bubble growth in magmas. We display that non-equilibrium fractionations among He, Ar and CO₂, driven by their different diffusivities in silicate melts, are common in most of the natural conditions of magma decompression and their signature strongly depends on pressure of degassing. Due to the comparable Ar and CO₂ diffusivity, magma degassing at low pressure fractionates both the He/Ar and He/CO₂ ratio by a similar extent, while the slower CO₂ diffusion at high pressure causes early kinetic effects on Ar/CO₂ ratio and dramatically changes the degassing path. On this ground, the very different geochemical signatures among suites of data coming from different ridge segments mainly depend on the depth of the magma chamber where the melt was stored. Besides, the variations inside a single suite highlight variable ascent speed and cooling rate of the emplaced lava. The large variations in both the He/CO₂ and Ar/CO₂ ratios at almost constant He/Ar, displayed in glasses coming from the Mid-Atlantic Ridge 24-30°N segment and the Rodriguez Triple Junction, are therefore interpreted as a high-pressure signature. In contrast, the simultaneous increase in both He/CO₂ and He/Ar of the East Pacific Rise, Pito Seamount and South-East Indian Ridge data sets suggests the dominance of low-pressure fractionation, implying that the shallow magma chambers are at a lower depth than those of the Mid-Atlantic Ridge 24-30°N and Rodriguez Triple Junction. Our conclusions support the presence of a relationship between spreading rate and depth of high-temperature zones below ridges, and are consistent with the depth of magma chambers as suggested from seismic studies. Non-equilibrium degassing explains the volatile systematics of mid-ocean-ridge basalts by starting from a single mantle-derived magma, dispensing with the supposed need for heterogeneities in abundance ratios of volatiles in the mantle below oceanic ridges.     

Mantle components in Iceland and adjacent ridges investigated using double-spike Pb isotope ratios

High precision Sr-Nd isotope ratios together with Pb isotope ratios corrected for mass fractionation using a double spike are reported for an extensive suite of late Quaternary to Recent lavas of Iceland, the Kolbeinsey and Reykjanes Ridges, and a small number of basalts from further south on the Mid-Atlantic Ridge. Compared with global MORB, the Icelandic region is distinguished by having low ²⁰⁷Pb/²⁰⁴Pb for any given ²⁰⁶Pb/²⁰⁴Pb, expressed by negative Δ²⁰⁷Pb (−0.8 to −3.5) in all but four Icelandic samples. Most samples also have elevated ²⁰⁸Pb/²⁰⁴Pb (strongly positive Δ²⁰⁸Pb), which combined with their negative Δ²⁰⁷Pb is very unusual in MORB worldwide. The negative Δ²⁰⁷Pb is interpreted as a consequence of evolution in high-μ mantle sources for the last few hundred Ma. The region of negative Δ²⁰⁷Pb appears to correspond with the region of elevated ³He/⁴He, suggesting that both lithophile and volatile elements in melts from the whole region between 56 and 70°N are dominantly sourced in a plume that has incorporated recycled Palaeozoic ocean crust and unradiogenic He, probably from the deep mantle. At least four mantle components are recognized on Iceland, two with an enriched character, one depleted and one that shows some isotopic affinities to EM1 but is only sampled by highly incompatible-element-depleted lavas in this study. Within restricted areas of Iceland, these components contribute to local intermediate enriched and depleted components that display near binary mixing systematics. The major depleted Icelandic component is clearly distinct in Pb isotopes from worldwide MORB, but resembles the depleted mantle source supplying the bulk of the melt to the Kolbeinsey and southern Reykjanes Ridges. However, an additional depleted mantle source is tapped by the northern Reykjanes Ridge, which with very negative Δ²⁰⁷Pb and less positive Δ²⁰⁸Pb is distinct from all Icelandic compositions. These components must mostly mix at mantle depths because a uniform mixture of three Icelandic components is advected southward along the Reykjanes Ridge.Despite strong covariation with isotope ratios, incompatible trace element ratios of Icelandic magmas cannot be representative of old mantle sources. The observed parent-daughter ratios in depleted and enriched Icelandic lavas would yield homogeneous Sr, Nd, Hf and ²⁰⁶Pb isotope signatures ∼170 Ma ago if present in their sources. The heterogeneity in ²⁰⁷Pb/²⁰⁴Pb is not however significantly reduced at 170 Ma, and the negative present day Δ²⁰⁷Pb cannot be supported by the low μ observed in depleted lavas from Iceland or the adjacent ridges. Since μ is higher in melts than in their sources, it follows that all the depleted sources must be residues from <170 Ma partial melting events. These are thought to have strongly affected most incompatible trace element ratios.     

Compositional controls on vent fluids from ultramafic-hosted hydrothermal systems at mid-ocean ridges: An experimental study at 400°C, 500 bars

Olivine (Fo₈₉), orthopyroxene (En₈₅), and clinopyroxene (Di₈₉) were reacted, individually and in combinations, with NaCl-MgCl₂ at 400°C, 500 bars to better assess alteration and mass transfer in ultramafic-hosted hydrothermal systems at mid-ocean ridges. Data indicate that temperature plays a key role in mineral solubility and kinetic processes, which influence the compositional evolution of the fluid. At the temperature and pressure of the experiments, the rate of olivine hydrolysis is sluggish as indicated by the limited extent of mass transfer between the fluid and mineral and absence of hydrous alteration phases. In contrast, reactions involving pyroxenes proceed rapidly, which result in significant increases in dissolved Ca, SiO₂, Fe and H₂, and formation of SiO₂-rich secondary minerals (talc and tremolite) and magnetite. SiO₂ release from pyroxene occurs in non-stoichiometric proportions and is a critical factor governing the stability of secondary minerals, with attendant effects on fluid chemistry.Magnetite and talc-fluid equilibria were used to calculate fluid pH at elevated temperatures and pressures. In general, pH is relatively low in the orthopyroxene- and clinopyroxene-bearing experiments due to constraints imposed by talc-fluid and talc-tremolite-fluid equilibria, respectively. Even in experiments where the olivine/pyroxene ratio is as great as 3, which is typical for abyssal peridotite, the low pH and high Fe concentrations are maintained. This is in sharp contrast to theoretical predictions assuming full equilibrium in the MgO-CaO-FeO-Fe₂O₃-SiO₂-Na₂O-H₂O-HCl system at 400°C, 500 bars.Ultramafic-hosted hydrothermal systems, such as the recently discovered Rainbow system at 36°13.80′N, 33°54.12′W on the Mid-Atlantic Ridge, indicate reaction processes in keeping with results of the present experiments, as suggested by vent fluid chemistry and temperature. In particular, relatively high SiO₂, Ca, H₂, and Fe concentrations characterize the Rainbow vent fluids. Indeed, Fe concentrations are the highest of any vent system yet discovered and require a relatively low pH in the subseafloor reaction zone from which the fluids are derived. This, together with the SiO₂ concentrations of the vent fluids, strongly indicates fluid buffering by silica-rich phases produced during pyroxene dissolution, the likely abundant presence of olivine notwithstanding. Time-series observations at Rainbow are clearly needed to better constrain the temporal evolution of hydrothermal alteration processes of ultramafic rocks in subseafloor reaction zones. In the absence of events permitting fluid continuous access to fresh rock, pyroxene will ultimately be consumed and vent fluids may then reflect changes imposed by bulk compositional constraints characteristic of ultramafic bodies at depth, which would be in better agreement with theoretical phase relations for the fully equilibrated system.     

Average compositions of magmas and mantle sources of mid-ocean ridges and intraplate oceanic and continental settings estimated from the data on melt inclusions and quenched glasses of basalts

Based on the generalization of the compositions of melt inclusions and quenched glasses from basaltic rocks, the average compositions of magmas were estimated for mid-ocean ridges (MOR), intraplate continental environments (CR), and ocean islands and plateaus (OI). These compositions were used to constrain the average contents of trace and volatile elements in mantle sources. A procedure was developed for the estimation of the average contents of incompatible elements, including volatiles (H₂O, Cl, F, and S), in the mantle. A comparison of the obtained average contents for the depleted mantle (DM) with the available published estimates showed that the contents of most incompatible trace elements (H₂O, Cl, F, Be, B, Rb, Sr, Zr, Ba, La, Ce, Nd, Sm, Eu, Hf, Ta, Th, and U) can be reliably estimated from the ratio of K to the desired trace element in the MOR magmas and the average content of K in the DM. For Nb, Ti, P, S, Li, Y, and heavy REE, we used the ratios of their contents to an element with a similar degree of incompatibility in MOR magmas (U for Nb and Dy for the other elements). This approach was used to determine the average contents of incompatible elements in oceanic plume mantle (OPM) and the subcontinental mantle of intraplate settings or continental plume mantle (CPM). It was shown that the average composition of both suboceanic and subcontinental mantle plumes is moderately enriched compared with the DM in the most incompatible elements (K, U, Ba, and La) and volatile components (H₂O, Cl, and F). The extent of volatile component enrichment in the plume mantle (500–1500 ppm H₂O) is insufficient for a significant depression of the mantle solidus. Therefore, mantle plumes must be hotter than the ambient depleted mantle. The average contents of incompatible trace elements in the OPM are similar to those of the primitive mantle, which could be related either to the retention of primitive mantle material in the regions of plume generation or to DM fertilization at the expense of the deep mantle recycling of crustal materials. In the latter case, the negative anomaly of water in the trace-element distribution patterns of the OPM is explained by the participation of dehydrated crust in its formation. Variations in the compositions of magmas and their sources were considered for various geodynamic settings, and it was shown that the sources are heterogeneous with respect to trace and volatile components. The chemical heterogeneity of the magma sources and gradual transitions between them suggest that the mantle reservoirs interact with each other. Chemical variations in continental and oceanic plume magmas can be attributed to the existence of several interacting sources, including one depleted and at least two enriched reservoirs with different contents of volatiles. These variations are in agreement with the zoned structure of mantle plumes, which consist of a hot and relatively dry core, a colder outer shell with high contents of volatile components, and a zone of interaction between the plume and depleted mantle.     

Hafnium isotope homogenization during metamorphic zircon growth in amphibolite-facies rocks: Examples from the Shackleton Range (Antarctica)

Hafnium isotope analyses of a large number of metamorphic zircon grains of two garnet-kyanite-staurolite schist samples from the Shackleton Range yielded ¹⁷⁶Hf/¹⁷⁷Hf of 0.28160 ± 0.00003 and 0.28142 ± 0.00003, respectively. The variations of these analyses are less than ±1.2 epsilon units and indicate that all metamorphic zircon grains in the two rocks formed in environments with nearly homogenous Hf isotopic composition. The metamorphic origin of the zircon grains is constrained by textures as well as by their low Th/U (<0.2), ¹⁷⁶Lu/¹⁷⁷Hf (<0.0003), and ¹⁷⁶Yb/¹⁷⁷Hf ratios (<0.009), indicating that they grew in the presence of garnet. Furthermore, the grains yield Pb-Pb ages of c. 1.7 Ga, which is the time of amphibolite-facies metamorphism. In combination with petrological results, it is suggested that the observed ¹⁷⁶Hf/¹⁷⁷Hf homogeneity was caused by a fluid- and deformation-assisted dissolution of detrital zircon grains, followed by new zircon re-precipitation that was accompanied by Hf transport on at least a hand-specimen scale. This interpretation is supported by results obtained from an additional paragneiss sample that contains zoned zircon grains with xenocrystic cores formed at 2.6-1.8 Ga and metamorphic rims with a U-Pb age of 1.7 Ga. The ¹⁷⁶Hf/¹⁷⁷Hf variation of the zircon rims is mostly at ±0.0003, which is much less than that of the magmatic cores (±0.0019). The metamorphic fluid for the dissolution-homogenization-re-precipitation process most likely resulted from prograde reactions among the minerals chlorite-muscovite-biotite-garnet-staurolite-apatite, in agreement with thin section observations and P-T pseudosection calculations.     

A phase diagram for mid-ocean ridge basalts: Preliminary results and implications for petrogenesis

Samples of a primitive mid-ocean ridge basalt (MORB) glass were encapsulated in a mixture of ol (Fo90) and opx (En90) and melted at 10, 15, and 20 kbar. After quenching, the basaltic glass was present as a pool within the ol+opx capsule, but its composition had changed so that it was saturated with ol and opx at the conditions of the experiment. By analyzing the quenched liquid, the location of the ol+opx cotectic in the complex, multicomponent system relevant to MORB genesis was determined.As pressure increases from 1 atm to 10 kbar, the dry ol+opx cotectic moves from quartz tholeiitic to olivine tholeiitic compositions. With further increases in pressure, the cotectic continues to move toward the ol-di-plag join (i.e., toward alkalic compositions). Between 15 and 20 kbar, ol+opx+di-saturated liquids change from tholeiitic to alkalic in character, although part of the ol+opx cotectic is still in the tholeiitic (i.e, hy-normative) part of composition space. At pressures of 10–15 kbar, tholeiitic liquids may be able to fractionate to alkalic liquids on the ol+di cotectic.Primitive MORB compositions come close to but do not actually lie on the ol+opx cotectic under any conditions studied. This suggests that not even the most primitive of known MORBs are primary melts of the mantle. The correspondence of most MORBs to the 1 atm ol+di+plag cotectic suggests that low pressure fractionation was involved in their genesis from parent liquids. Picritic liquids that have been proposed as parents to the MORB suite could equilibrate with harzburgite (or Iherzolite) at 15–20 kbar and thus could be primary. Fractionation of ol from these liquids could yield primitive MORB liquids, but other primary liquids or more complex fractionation paths involving others phases in addition to ol cannot be ruled out. The possibility that these picritic liquids could equilibrate with ol+opx at 25–30 kbar cannot be ruled out.