Mid-ocean Ridge Basalt Glasses Research Articles

Recycled ocean crust and sediment in Indian Ocean MORB

Mid-ocean ridge basalts (MORB) from the Indian Ocean are characterized by higher 87Sr/ 86Sr and lower 206Pb/ 204Pb, on average, than Atlantic and Pacific MORB. These distinct isotopic characteristics are thought to be the result of large-scale mantle contamination, either by ancient recycled sediments or delaminated subcontinental lithosphere. To constrain the upper mantle mixing end-members, we have analyzed the concentrations of Nb, U, Th, Pb and 17 other incompatible elements (by HPLC and ID-TIMS methods), as well as Sr, Nd, and Pb isotope ratios in nine hand picked MORB glasses from the Central Indian and Carlsberg Ridge. Nb/U and Ce/Pb ratios are sensitive indicators of crustal contaminants in the mantle, and such contaminations have been identified in EM-type ocean island basalts. Here we show the first case of highly significant correlations of Nb/U, Ba/Nb and Ba/Th with 87Sr/ 86Sr for Indian Ocean MORB and we confirm and strengthen a previously detected correlation with Ce/Pb (or Nd/Pb). These correlations are interpreted as residual heterogeneities that have not been completely rehomogenized by convective stirring of the mantle. The results are explained by a quantitative model involving contamination of an Atlantic-Pacific type upper mantle with a heterogeneous, recycled component consisting of ∼ 1.5 Ga old altered oceanic crust with small, but variable, amounts of ancient pelagic sediment added. This contamination causes higher 87Sr/ 86Sr, lower 143Nd/ 144Nd, generally low but variable 206Pb/ 204Pb and the distinct trace element correlations seen in the Indian Ocean MORB data. Mass balance calculations show that only small amounts of pelagic sediment are necessary to satisfy the criteria of the contamination model, and these can be subducted within as little as 200 Ma. Lead isotopic systematics constrain the recycled sediment component to be older than about 1.25 Ga. Apart from this, the model does not constrain the age of the contamination event since the results are equally valid for (1) “slow” mixing of the recycled component subsequent to subduction at ∼ 1.5 Ga or (2) storage of the material after subduction in a mantle boundary layer followed by relatively recent reinjection into the MORB source and “fast” mixing.

An Experimental Study of Water and Carbon Dioxide Solubilities in Mid-Ocean Ridge Basaltic Liquids. Part II: Applications to Degassing

Degassing processes in basaltic magmas rich in both water and carbon dioxide can be modeled using the solubilities of the endmember systems and the assumption of Henry's law. Suites of vapor-saturated basaltic melts having a range of initial CO_2/H_2O ratios and erupted over a narrow depth interval will define negatively sloped arrays on an H_2O vs CO_2 plot. It is important that all of the major volatile species be considered simultaneously when interpreting trends in dissolved volatile species concentrations in magmas. Based on measured concentrations of water and carbon dioxide in basaltic glasses, the composition of the vapor phase at 1200°C that could coexist with a basaltic melt and the pressure at which it would be vapor saturated can be calculated. The range in vapor compositions in equilibrium with submarine basalts reflects the range in water contents in the melts characteristic of each environment. The ranges in the molar proportion of CO_2 in vapor phases (X^ν_(CO2)) calculated to be in equilibrium with submarine tholeiitic glasses are 0⋅93–1⋅00 for mid-ocean ridge basalts (MORB), 0⋅60–0⋅99 for glasses from Kilauea [representative of ocean island basalts (OIB)] and 0–0⋅94 for glasses from back-arc basins (BABB). MORB glasses from spreading centers ranging from slow (e.g. the Mid-Atlantic Ridge) to fast (e.g. East Pacific Rise, 9–13°N) are commonly supersaturated with respect to CO_2-rich vapor, resulting from magma ascent rates so rapid that magmas erupt on the sea-floor without having been fully degassed by bubble nucleation and growth during ascent. In contrast to the MORB glasses, volatile contents in submarine glasses from Kilauea are consistent with having been in equilibrium with a vapor phase containing 60–100 mol% CO_2 at the pressure of eruption, reflecting differences in average magma transport rates during eruptions at mid-ocean ridges and hotspot volcanoes. Degassing during decompression of tholeiitic basaltic magma is characterized by strong partitioning of CO_2 into the vapor phase. During open system degassing, CO_2 is rapidly removed from the melt with negligible loss of water, until a pressure is reached at which the melt is in equilibrium with nearly pure water vapor. From this pressure downward, the water content of the melt follows the water solubility curve. During closed system degassing, water and CO_2 contents in vapor-saturated basaltic magmas will depend strongly on the vapor composition as determined by the initial volatile concentrations. Deviation from open system behavior, toward lower dissolved H_2O and CO_2 saturation concentrations at a given pressure, will be greatest in melts having high total volatile concentrations and high CO_2:H_2O ratios. Closed system degassing of basaltic melts having the low initial H_2O and CO_2 contents typical of MORB and OIB, however, are similar to the open system case.

Noble gas state in the mantle

Noble gas elemental and isotopic data on mantle‐derived materials such as mid‐ocean ridge basalt (MORB), hotspot volcanics, diamonds, and mantle xenoliths published up to August 1993 are reviewed critically. Characteristic features of the mantle‐derived materials, which are important in evaluating the significance of the data, are discussed. We choose MORB glasses, hotspot volcanics, mantle xenoliths, and diamonds as the sources to infer the noble gas state in the mantle: MORB and hotspot volcanics to represent the modern depleted and the less degassed mantle, and diamonds to represent the ancient mantle. Because of various fractionation processes, the elemental abundance data obtained from the mantle‐derived materials is unlikely to constrain the noble gas composition of the mantle, except for the systematic enrichment of the heavier noble gases relative to air. On the other hand, apart from the secondary addition of radiogenic components such as 4He, 21Ne, 40Ar, 129Xe, and 131–136Xe, the noble gas isotopic compositions deduced from the mantle‐derived materials can provide useful information as to the values in the mantle; a best estimate of the mantle noble gas state is presented.

226Ra- 230Th disequilibrium in axial and off-axis mid-ocean ridge basalts

We describe 226Ra- 230Th disequilibrium in mid-ocean ridge basalt (MORB) glasses from the Juan de Fuca, Gorda, and East Pacific ridges. These first mass spectrometric measurements of 226Ra in MORB glasses at sub-picogram abundance levels confirm the large excesses over 230Th determined by radon-emanation techniques and alpha spectrometry. All off-axis MORB glasses have 226Ra- 230Th and 234U- 238U in secular equilibrium. This suggests that magmatic processes, not secondary post-eruption alteration, generate 238U-series disequilibrium in these MORB. Least evolved, N-MORB from axial valleys have ( 226Ra 230Th ) between 2.2 –2.3. Differentiated and enriched E-type MORB have consistently low ( 226Ra 230Th ) ratios compared with N-MORB from the same ridge sections. Ra-Th fractionation may be less pronounced, or magma residence-transit periods may be long for differentiated MORB. Also, E-MORB may be generated by different melt extraction volumes and rates. Estimated 226Ra- 230Th ages for N-MORB agree with location on and off ridge segments, and with Th-U model ages. These preliminary results show that 226Ra- 230Th disequilibrium could be used to quantify volcanic episodicity at ocean ridges.

238U/ 204Pb in MORB and open system evolution of the depleted mantle

Concentrations of U, Th and Pb were determined by isotope dilution on 30 mid-ocean ridge basalt (MORB) glasses. The mean values of 238U/ 204Pb (μ) and 232Th/ 238U (κ) in these glasses are 11.20 and 2.67 respectively. There are significant differences in these ratios between ocean basins. Mean κ values increase in the order Atlantic < Pacific < Indian; mean μ is higher in Pacific MORB than in Indian and Atlantic MORB, but differences between the Atlantic and Indian are not significant. μ correlates strongly with U concentration, but not with Pb concentration or with 206Pb/ 204Pb . The correlation of μ with U but not with 206Pb/ 204Pb reflects the effect of partial melting and fractional crystallization on μ. Since the maximum value of U in the depleted mantle must be less than the bulk silicate earth value of 0.018 ppm, the μ-U correlation can be used to estimate the maximum mean value of μ in the depleted mantle ( μ DM ). This yields a maximum μ DM of 4.67. Similarly, the correlation between μ and Rb/Sr suggests a maximum μ DM of 6.3. These results are consistent with values for μ DM derived using mass balance from the crustal and depleted mantle values of κ, the depleted mantle Pb/Ce ratio, and the crustal abundance of Ce. The value of μ DM thus seems to be less than the bulk earth value, estimated to be 8 ± 2. This is consistent with the incompatible-element-depleted character of this reservoir, but inconsistent with Pb isotope ratios, which record time-integrated values for μ of 8–9, and which indicate that μ has increased over time. This inconsistency requires that the depleted mantle be an open system with a residence time for Pb of 1 ± 0.5 Ga. Neither the continental crust nor a primitive lower mantle have the appropriate isotopic composition to be the source of Pb in the depleted mantle. Mantle plumes do have the appropriate isotopic composition, and as little as 10% of the observed plume flux is needed to mix with the depleted mantle to supply the required flux of Pb.

Oxygen Thermobarometry of Orogenic Lherzolite Massifs

The oxidation state has been determined for spinel peridotites from 13 orogenic lherzolite massifs including Beni Bousera, Ronda, and 11 smaller massifs in the French Pyrenees. The oxygen fugacity (fo2) was calculated for 67 samples from microprobe analyses using a set of secondary spinel standards to correct the ferric iron content in the spinels. The utility of this method is confirmed by the good agreement between the calculated values and those determined by Mossbauer spectroscopy on 28 samples. The spinel peridotites of Ronda and Beni Bousera are relatively reduced, averaging -11 and -1.5 log units relative to fayalite-magnetite-quartz (FMQ) respectively, which is in agreement with values from abyssal peridotites and mid-ocean ridge basalt (MORB) glasses. The Pyrenean massifs are relatively oxidized [ΔlogfO2(FMQ)=-0.4] and are intermediate between the abyssal peridotites and continental xenolith suites. No systematic gradients are observable. Instead, variations of up to 2 log units in fo2 occur at a localized scale. This type of variation is also observed for trace elements and radiogenic isotopes. The harzburgites at Beni Bousera record the most reduced conditions. Local oxidation coincides with the appearance of amphibolc, indicating that metasomatizing fluids or melts are generally oxidized compared with the host peridotites. Partial re-equilibration in the plagioclase peridotite fades has occurred at Ronda, causing the spinels to become Cr rich. Re-equilibration is extremely heterogeneous. Mild oxidation appears to attend the crystallization of fine-grained plagioclase. The similarity in fo2 values at Beni Bousera and Ronda indicates a fairly uniform oxidation state at a scale of ˜ 200 km. This scale of homogeneity is also observed in the Pyrenees, where no significant variation in fo2 is apparent along 200 km of strike in the Northern Pyrenean Zone.

Mantle Oxidation State and Its Relationship to Tectonic Environment and Fluid Speciation

The earth's mantle is degassed along mid-ocean ridges, while rehydration and possibly recarbonaton occurs at subduction zones. These processes and the speciation of C-H-O fluids in the mantle are related to the oxidation state of mantle peridotite. Peridotite xenoliths from continental localities exhibit an oxygen fugacity (fo(2)) range from -1.5 to +1.5 log units relative to the FMQ (fayalite-magnetite-quartz) buffer. The lowest values are from zones of continental extension. Highly oxidized xenoliths (fo(2) greater than FMQ) come from regions of recent or acive subduction (for example, Ichinomegata, Japan), are commonly amphibole-bearing, and show trace element and isotopic evidence of fluid-rock interaction. Peridotites from ocean ridges are reduced and have an averae fo(2) of about -0.9 log units relative to FMQ, virtually coincident with values obtained from mid-ocean ridge basalt (MORB) glasses. These data are further evidence of the genetic link between MORB liquids and residual peridotite and indicate that the asthenosphere, although reducing, has CO(2) and H(2)O as its major fluid species. Incorporation of oxidized material from subduction zones into the continental lithosphere produces xenoliths that have both asthenospheric and subduction signatures. Fluids in the lithosphere are also dominated by CO(2) and H(2)O, and native C is generally unstable. Although the occurrence of native C (diamond) in deep-seated garnetiferous xenoliths and kimberlites does not require reducing conditions, calculations indicate that high Fe(3+) contents are stabilized in the garnet structure and that fo(2) deareases with increasing depth.

Mid-ocean ridge popping rocks: implications for degassing at ridge crests

The vesicle size distribution (VSD) and rare gas abundances in popping rocks from 14°N on the Mid-Atlantic Ridge provide constraints on the behavior of volatiles during ridge crest volcanism. These popping rocks, which contain 16–18 volume percent vesicles, are rare mid-ocean ridge basalt (MORB) magmas which appear to have retained much of their volatile inventory. The logarithm of vesicle population density displays the same linear correlation with decreasing size in two of the samples studied. This implies that continuous and simultaneous nucleation and bubble growth have occurred during magma ascent, with no significant perturbations due to accumulation, coalescence or loss of bubbles. In contrast, most MORB magmas display low vesicularities and we suggest that they have suffered some degree of pre-eruptive vesicle loss. We tentatively propose that large vesicles are produced by coalescence when MORB melt is at rest in chambers and conduits, and may be lost during early gas-rich episodes. Most MORB would represent residual liquids which erupt after vesicle loss has occurred, whereas popping rocks would represent a rare case where physical sorting of vesicles from melt did not occur, because storage in a magma chamber did not occur. The rare gas concentrations in the studied popping rocks are the highest yet measured in glassy ridge basalts ([He] > 50 μccSTP/g). The rare gas abundance pattern of these popping rocks probably resembles the pattern for non-vesiculated MORB magma and potentially reflects that of the depleted mantle source. This pattern is similar to the “mean MORB” pattern (computed from MORB glasses with 40Ar/ 36Ar > 10,000) although a higher enrichment in He (and possibly Ne) compared to the heavier rare gases is observed in MORB. The overall similarity in abundance patterns for MORB and popping rocks indicates that vesiculation and vesicle loss do not fractionate the Ar Kr Xe relative abundances from those in non-vesiculated magma, and that the modern flux ratios of these gases at ridges are similar to their elemental ratios in the depleted mantle. The degassing flux of He at ridge crests estimated from the MORB He deficit relative to popping rocks is comparable to the flux derived from the 3He budget for the abyssal ocean. This suggests that degassing at ridges may be strongly influenced by the dynamics and style of submarine volcanism.

The oxidation state of the Earth's sub-oceanic mantle from oxygen thermobarometry of abyssal spinel peridotites

THE oxygen fugacity (fO2), or redox state, of the Earth's mantle is an important parameter in the evolution of mantle-derived magmatic rocks. Most estimates of the upper-mantle redox state have been based on samples of the sub-continental mantle and terrestrial lavas, and estimates of fO2 obtained by thermo-barometric methods tend to cluster within ±1.5 log units of the FMQ (fayalite–magnetite–quartz) reference oxygen fugacity buffer1–3. Much less is known about the redox state of the sub-oceanic mantle, the ultimate source of mid-ocean-ridge basalts (MORBs). Here we use oxygen thermobarometry of abyssal spinel peridotites, representative of most of the Earth's mid-ocean-ridge systems, to show that the redox state of the sub-oceanic mantle lies at values of up to 4 log units (average of 1.35 log units) more reduced than FMQ. Our results are in excellent agreement with oxygen fugacities of MORBs obtained from Fe3+/Fe2+ ratios of quenched glass in pillow basalts4. This agreement confirms the redox state of the MORB source region and suggests that MORB glasses (as opposed to the cores of pillow basalts) have not undergone significant oxidation (hydrogen degassing) during their ascent.

230Th/ 238U and 226Ra/ 230Th fractionation in young basaltic glasses from the East Pacific Rise

Mid-ocean ridge basalt (MORB) glasses treated to remove manganese and iron oxyhydroxide coatings containing U and Th decay chain nuclides sequestered from the ambient aqueous environment show little fractionation of ( 230Th) from ( 238U), but extensive enrichment of ( 226Ra) over ( 230Th). Chronometry based on ( 230Th)-( 238U) disequilibrium yields a mean age of 151,000 ± 55,000 (2σ) years ago for Th—U fractionation, from a source region with Th/U (atom ratio) = 1.58 ± 0.43 (2σ). ( 226Ra) excesses over ( 230Th) indicate an enrichment event more recent than the fractionation of Th from U. ( 226Ra/ 230Th) correlates well with K/U, providing a rough means of estimating the age since eruption of MORB glasses.

Pressure of origin of primary mid-ocean ridge basalts

Summary High-pressure phase equilibrium studies of mid-ocean ridge basalts (MORBs) have shown that some samples are in equilibrium with olivine + orthopyroxene (OPX) + clinopyroxene ± plagioclase at about 10 kbar, whereas others are not in equilibrium with OPX at this pressure. Samples that are OPX-saturated at about 10 kbar are characterized by higher SiO 2 (&gt;49.7 wt%) than most (about &gt;75%) MORB glasses with more than 9.5 wt% MgO, suggesting that only about 25% of these glasses could be derived from primary magmas generated at about 10 kbars. Orthopyroxenes in abyssal peridotites are characterized by Al 2 O 3 contents that range from 2.0 to 6.0 wt% (probably about 2.5–6.5 wt% before subsolidus equilibration), but orthopyroxenes in peridotite partial-melting experiments and liquidus orthopyroxenes for MORBs at about 10 kbar have only 3.5–4.3 wt% Al 2 O 3 . Liquidus orthopyroxenes for high-MgO basalts at 25 kbar and 20 kbar have about 6.6 wt% and 5.5 wt% Al 2 O 3 respectively. It appears that the least depleted (Al 2 O 3 -rich end) of abyssal peridotite orthopyroxenes indicate the dominance of melting and primary MORB genesis at pressures substantially greater than 10 kbar, and probably at 20–25 kbar.

The concentration, behavior and storage of H 2O in the suboceanic upper mantle: Implications for mantle metasomatism

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

Major volatiles from a North Atlantic MORB glass and calibration to He: A size fraction analysis

The analysis of major volatiles in a North Atlantic mid-ocean ridge basalt (MORB) glass, has been performed using a new procedure based on the analysis of several size fractions of one single sample. It enables us to separate the contributions of volatiles from the glass phase, from the vesicle gas phase and eventually gas adsorbed on the grain surface. The vesicle composition (mole fractions) obtained upon crushing is 0.94 CO 2, 0.002 H 2, 0.06 H 2O. Volatiles extracted from the glass at 1150°C are 9.7 · 10 −6 mol g −1 CO 2, 156 · 10 −6 mol g −1 H 2O. The gases released at 500°C are mostly adsorbed contaminants but contribute marginally to the total amount except for numerous organic compounds which are destroyed at higher temperature. When previous noble-gas data for the same sample are considered, one obtains the evidence that: (1) the He CO 2 ratio does not fractionate between glass (4.1 · 10 −5) and vesicles (4.6 · 10 −5); and (2) water is not quantitatively degassed from MORB glasses. The ratio He : CO 2 : H 2O of 10 −4 : 2.4 : 13 for primitive magma has to be confirmed by further results to be considered as a typical value for a MORB source.

Upper mantle origin for Harding County well gases

Short-lived radioisotopes such as 129I and 244Pu represent extremely powerful geochemical tools for tracing the early history of the Earth. Terrestrial 129Xe anomalies were first found in CO2-rich well gases from Harding County, New Mexico, and then in gases from mid-ocean-ridge basalts (MORB). Here I show that noble gases and CO2 from Harding County are isotopically indistinguishable from such gases in fresh MORB glasses and that both probably have the same source, namely the upper mantle.

Oxidation states of mid-ocean ridge basalt glasses

Precise new analyses of ferrous and total iron for 78 hand-picked mid-ocean ridge basalt (MORB) glasses constrain the redox states of MORB magmas, and the systematics of those redox states with respect to geography and chemistry. The data indicate that MORB magmas at the point of eruption include some of the most reduced terrestrial lavas yet analyzed. Irrespective of their tectonic setting, geographic setting or chemical type, quench glasses from the outer surfaces of MORB lavas have Fe 3+/ΣFe ratios of 0.07 ± 0.03 (2σ), equivalent to relative oxygen fugacities 1–2 log 10 units below the fayalite-magnetite-quartz buffer (FMQ). These reduced values contrast markedly with those of cogenetic whole rocks, even for samples from the same pillow. Pillow cores (including virtually all published whole rock analyses) typically have Fe 3+/ΣFe close to 0.15, equivalent to oxygen fugacities close to FMQ. The post-eruptive oxidation responsible for this contrast apparently occurs rapidly, while the lava is still partially molten, most likely as a result of hydrogen loss. The striking contrast between glass and whole rock data suggests that two common assumptions with respect to treatment of MORB are inappropriate: (1) assumption of Fe 3+/ΣFe close to 0.15 for calculating norms and magnesium numbers; and (2) assumption of FMQ conditions for experimental studies of MORB crystallization. Oxygen fugacities in the mantle source regions of MORB have also been assumed to be close to FMQ. It is likely that, redox states determined from wholly or partially crystalline lavas are generally not representative of magmatic values. The new data reported herein define a new and rigid upper limit for the oxidation state of the sub-oceanic mantle.

K, U and Th in mid-ocean ridge basalt glasses and heat production, K/U and K/Rb in the mantle

Analyses on fresh glass samples of mid-ocean ridge basalt yield a uniform ratio K/U = 12,700 ± 200. In contrast, Th/U increases systematically with Th concentration. From these results we calculate an upper limit (1.5 pW kg−1) and a best estimate (0.6 pW kg−1) for the heat production in the depleted portion of the mantle. A new estimate is given for the terrestrial ratio of K/Rb = 513.

Comment on “MORB—A Mohole misbegotten?” by M.J. O'Hara

One can only applaud O'Hara's call for further research into magmatic processes that take place at and near oceanic spreading centers (EOS, June 15, 1982). In particular, the desirability of having deep drill holes close to active spreading centers is indisputable and needs no elaboration. One of the major reasons for encouraging further research on midocean ridge basalt (MORB) petrogenesis is the need to resolve a controversy now being debated in the literature over the composition and depth of origin of primary MORB magmas [Rhodes, 1982]. O'Hara [1968a] and, recently, several others [Green et al., 1979; Stolper, 1980; Elthon and Scarfe, 1980] have argued that primary MORB magmas are picritic in composition and generated at various pressures from 15 to 30 kbar, depending on the author. Others contend that primary MORB magmas are close in composition to the least‐fractionated MORB glasses (which are not picritic but have mg numbers as high as 73) and are generated at more moderate pressures of about 8–10 kbar [Green and Ringwood, 1967; Kushiro, 1973; Presnall et al., 1979]. O'Hara did not mention any of the recent papers that disagreed with his concept of picritic primary magmas at spreading centers. In order to restore some balance, the following brief discussion is offered.

Hydrothermal alteration of basalt by seawater under seawater-dominated conditions

Fresh mid-ocean ridge basalt glass and diabase have been reacted with seawater at 150–300°C, 500 bar, and water/rock mass ratios of 50, 62, and 125, using experimental apparatus which allowed on-line sampling of solution to monitor reaction progress. These experiments characterize reaction under what we have called “seawater-dominated” conditions of hydrothermal alteration. In an experiment at 300°C, basalt glass undergoing alteration removed nearly all Mg 2+ from an amount of seawater 50 times its own mass. In the process, the glass was converted entirely to mixed-layer smectite-chlorite, anhydrite, and minor hematite. Removal of Mg from seawater occurred as a Mg(OH) 2 component incorporated into the secondary clay. This produced a precipitous drop in solution pH early in the experiment, accompanied by a dramatic increase in the concentrations of Fe, Mn, and Zn in solution. As Mg removal neared completion and the glass was hydrolyzed, pH rose again and heavy metal concentrations dropped. At water/rock ratios of 62 and 125 and 150–300°C, the mineral assemblage produced was similar to that at a water/rock ratio of 50. Solution chemistry, however, contrasted with the earlier experiment in that Mg concentrations in solution were greater and pH lower. This caused significant leaching of heavy metals. At 300°C nearly all of the Na, Ca, Cu, Zn, and CO 2 and most of the K, Ba, Sr, and Mn were leached from the silicates. H 2S, Al, Si, and possibly Co were also significantly mobilized, whereas V, Cr, and Ni were not. Little or no seawater sulfate was reduced. Although submarine hot spring solutions sampled to date along mid-ocean ridges clearly come from rock-dominated hydrothermal systems, evidence from ocean floor metabasalts and from heat flow studies indicates that seawater-dominated conditions of alteration prevail at least locally both in axial hightemperature systems and in ridge flank systems at lower temperatures.

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.

U–Pb, Sm–Nd and Rb–Sr systematics of mid-ocean ridge basalt glasses

The measurement of Pb, Nd and Sr isotopes in basalt glasses from mid-ocean ridges reveals correlations in isotope parameters which have important implications for the differentiation history of the mantle.