Geochemistry of ODP Leg 115 basalts

Melting-phase relations at high pressures and Sr-Nd isotopic compositions are reported for basalts collected from the western Indian Ocean during Ocean Drilling Program Leg 115. Based on the concentrations of high-field-strength elements, we have subdivided the basalts into eight groups. A tholeiitic primary magma estimated using an olivine maximum fractionation model is representative of depleted lavas. This melt is in equilibrium with lherzolite minerals at 1.3 GPa and 1330°C under dry conditions. Also, an alkaline primary magma, representative of enriched lavas, is not saturated with orthopyroxene under dry conditions, but it is saturated with lherzolite minerals under CO2-saturated conditions at 1.7 GPa and 1350°C. These results imply that the tholeiitic magmas were segregated from mantle diapirs at shallower levels than the alkaline magmas. The highest 143Nd/144Nd value is obtained for the most depleted tholeiitic basalts, and the lowest value corresponds to the enriched alkaline basalt. The Sr isotopes of the basalts range from 0.70378 to 0.70449 and are inversely correlated with the Nd isotopic values. The present experimental and geochemical data suggest that depleted mantle material is underlain by the enriched material in the upper mantle beneath the region.

Strontium- and lead-isotope composition of some basalts at DSDP Hole 69-504B

Seventeen whole-rock samples, generally taken at 25- to 50-meter intervals from 5 to 560 meters sub-basement in Deep Sea Drilling Project Hole 504B, were analyzed for 87Sr/86Sr ratios, and rubidium and strontium concentrations. Ten of these samples also were analyzed for Pb-isotope composition. Strontium-isotope ratios for eight samples from the upper 260 meters of the hole range from 0.70287 to 0.70377, with a mean of 0.70320. In the interval 330 to 560 meters, five samples have a restricted range of 0.70259 to 0.70279, with a mean of 0.70266, almost identical to the average value of fresh mid-ocean-ridge basalts. In the interval 260 to 330 meters, approximately intermediate strontium- isotope ratios are found. The higher 87Sr/86Sr ratios in the upper part of the hole can be interpreted in terms of strontium-isotope alteration during basalt-sea-water interaction. Relative to average fresh mid-ocean ridge basalts, the upper 260 meters of basalts are enriched by an average of about 9% in sea-water strontium 87Sr/86Sr = 0.7091). This Sr presumably is located in the smectites, which, as the main secondary minerals throughout the hole, replace olivine and matrix glass and locally fill vesicles (analyzed samples contained no veins). The strontium-isotope data strongly suggest that the integrated flux of sea water through the upper part of the Hole 504B crust has been greater than through the lower part. This is also suggested by (1) the common occurrence of Feoxide- hydroxide minerals as alteration products above 270 meters, but their near absence below 320 meters, (2) the presence of vein calcite above 320 meters, but its near absence below this level, and (3) the occurrence of vein pyrite only below a depth of 270 meters. Sea-water circulation in the lower basalts may have been partly restricted by the greater number of relatively impermeable massive lava flows below 230 meters sub-basement. Although sufficient sea water was present within the lower part of the hole to produce smectitic alteration products, the overall water /rock ratio was low enough to prevent significant modification of strontium-isotope ratios. Lead-isotope ratios of Hole 504B basalts form approximately linear arrays in plots of 208Pb/204Pb and 207Pb/204Pb versus 206Pb/204Pb. The arrays are similar to those reported for basalts from other mid-ocean ridges. There is no trend in Hole 504B lead-isotope ratios with vertical position in the basement. The arrays indicate that the lead-isotope composition of the upper mantle from which the Hole 504B basaltic melts were derived was inhomogeneous.

Composition and properties of rocks and sediments from the Cape Verde fracture zone

The book presents results of comprehensive geological and geophysical studies, carried out in the Cape Verde fault zone in the 3-rd cruise of R/V Akademik Nikolaj Strakhov (1986). Detailed characterization of bottom relief, thickness and structure of the sedimentary cover, magnetic field, crust structure, lithology and stratigraphy of sediments, petrography and geochemistry of magmatic rocks. Conclusions about tectonic layering of the crust and upper mantle in the fault zone, and about a concurrent structural section of large mantle inhomogeneities have been done. The book is the first monographic description of a major fault structure of the ocean floor.

(Table 1) Lead isotopic composition and concentration at DSDP Leg 82 Holes

The aim of DSDP Leg 82 was to decipher the temporal and spatial evolution of Azores Plume. The Pb-isotopic results of this leg are rather complex, and can be summarized as follows: 1. At a given site (561, 558), variations of Pb isotopic compositions are generally accompanied by major changes in trace-element ratios, indicating significant heterogeneity of the source region. There is a correlation between Pb isotopes and trace elements. 2. In contrast, if all the data (i.e., all studied sites) of Leg 82 are considered together, no correlation can be discerned between Pb isotopes and trace elements. Site 556, especially, shows abnormal behavior. 3. Leg 82 samples not only cover the entire range of Pb isotopic composition previously established for the Atlantic Ocean, but extend this field to more radiogenic values. 4. The data are compatible with the hot-spot model proposed by Schilling (1975), if one considers that the Azores Plume itself is isotopically heterogeneous, and that it has been progressively contaminated to various degrees by upper mantle material.

Chemical composition of basalts and basaltic glasses from the Sierra Leone Fracture Zone region

Major and trace element (including REE) geochemistry of basalts and chilled basaltic glasses from the MAR axial zone in the vicinity of the Sierra Leone FZ (5-7°10'N) has been studied. Associations of basalts of various compositions with particular ocean-floor geological structural features have been analyzed as well. Three basaltic varieties have been discriminated. Almost ubiquitous are high-Mg basalts (Variety 1) that are derivatives of N-MORB tholeiitic melts and that are produced in the axial zone of spreading. Variety 2 is alkaline basalts widespread on the southwestern flank of the MAR crest zone in the Sierra Leone region, likely generated through deep mantle melting under plume impact. Variety 3 is basalts derivative from T- and P-MORB-like tholeiitic melts and originating through addition of deeper mantle material to depleted upper mantle melts. Magma generation parameters, as calculated from chilled glass compositions, are different for depleted tholeiites (44-55 km, 1320-1370°C) and enriched tholeiites (45-78 km, 1330-1450°C). Mantle plume impact is shown to affect not only tholeiitic basalt compositions but also magma generation conditions in the axial spreading zone, resulting in higher Ti and Na concentrations in melts parental to rift-related basalts occurring near the plume. T- and P-MORBs are also developed near areas where mantle plumes are localized. High-Mg basalts are shown to come in several types with distinctive Ti and Na contents. Nearly every single MAR segment (bounded by sinistral strike slips and the Bogdanov Fracture Zone) is featured by its own basalt type suggesting that it has formed above an asthenospheric diapir with its unique magma generation conditions. These conditions are time variable. Likely causes of temporal and spatial instability of the mantle upwelling beneath this portion of the MAR are singular tectonic processes and plume activity. In sulfide-bearing rift morphostructures (so-called "Ore area'' and the Markov Basin), basalts make up highly evolved suites generated through olivine and plagioclase fractionation, which is suggestive of relatively long-lived magma chambers beneath the sulfide-bearing rift morphostructures. Functioning of these chambers is a combined effect of singular geodynamic regime and plume activity. In these chambers melts undergo deep differentiation leading to progressively increasing concentration of sulfide phase, eventually to be supplied to the hydrothermal plumbing system.

(Table 1) Osmium concentrations and isotopic ratios for ODP Leg 125 peridotites

Mantle peridotites drilled from the Izu-Bonin-Mariana forearc have unradiogenic 187Os/188Os ratios (0.1193 to 0.1273), which give Proterozoic model ages of 820 to 1230 million years ago. If these peridotites are residues from magmatism during the initiation of subduction 40 to 48 million years ago, then the mantle that melted was much more depleted in incompatible elements than the source of mid-ocean ridge basalts (MORB). This result indicates that osmium isotopes record information about ancient melting events in the convecting upper mantle not recorded by incompatible lithophile isotope tracers. Subduction zones may be a graveyard for ancient depleted mantle material, and portions of the convecting upper mantle may be less radiogenic in osmium isotopes than previously recognized.

Petrology of high-MgO bronzite andesite resembling boninite at DSDP Hole 60-458

A high-MgO andesite which is texturally similar to boninite and a variolitic basalt collected from Site 458, about 100 km west of the Mariana Trench, have been studied through microprobe analyses and melting experiments at high water pressures. The boninite-type andesite is very similar in composition and texture to a boninite from Bonin Islands, except that the former is more calcic than the latter. The variolitic basalt contains magnesian pigeonite (Ca12Mg74Fe14) in cores of augite microphenocrysts. This pigeonite crystallized at temperatures above 1200°C. In the melting experiments of the boninite-type rock, clinopyroxene crystallizes as a liquidus phase at pressures at least above 8 kbar. No olivine crystallizes near the liquidus temperatures, indicating that the magma of this rock cannot be in equilibrium with the upper mantle periodotite (lherzolite) at depths at least greater than 25 km. The boninite-type rock is probably a product of fractional crystallization of a more primitive magma (e.g., olivine-bearing boninite magma) by separation of olivine and orthopyroxene. The magma of the variolitic basalt also cannot be in equilibrium with the upper mantle peridotite, and may be a product of fractional crystallization of a more primitive basaltic magma.

Geochemistry of serpentenites from DSDP Hole 84-566C

We studied a unique chrysotile-antigorite serpentinite, drilled on Deep Sea Drilling Project Leg 84 (Site 566) in the Guatemala forearc. Our in situ major and trace element data provide new constraints on possible reactions and associated trace element mobilisation during shallow serpentinite subduction. Chrysotile of the studied serpentinite, formed by the hydration of an upper mantle peridotite precursor, is partially replaced by antigorite (alone) which also occurs in 0.5 mm wide unoriented veins crosscutting the rock. Based on textural relationships and the P-T-X stability of the rock forming phases, the replacement of chrysotile by antigorite occurred at T < 300 °C, due to interaction between the chrysotile-serpentinite and an aqueous fluid. A comparison of the chemical compositions of reactant and product phases reveals that about 90% of the Cl, more than 80% of the B and about 50% of the Sr hosted originally by chrysotile was lost during fluid-assisted chrysotile-to-antigorite transformation and accompanying partial dehydration, and documents the much lower affinity of antigorite for trace element uptake than that of chrysotile. The fluid-assisted chrysotile-to-antigorite transformation and associated trace element loss documented here can occur in the shallow (< 30 km) region of subduction zones. This transformation decreases notably the Cl and B inventory of subducting serpentinites, which are regarded as one of the most important carriers of these elements into subduction zones. The evolution of serpentinites during initial subduction stages thus appears to be critical in the recycling of specific trace elements such as B or Cl from forearc to subarc depths.

Geochemistry and radiogenic isotopes of ODP Hole 157-953C lavas

The Canary Island primitive basaltic magmas are thought to be derived from an HIMU-type upwelling mantle containing isotopically depleted (NMORB)-type component having interacted with an enriched (EM)-type component, the origin of which is still a subject of debate. We studied the relationships between Ni, Mn and Ca concentrations in olivine phenocrysts (85.6-90.0 mol.% Fo, 1,722-3,915 ppm Ni, 1,085-1,552 ppm Mn, 1,222-3,002 ppm Ca) from the most primitive subaerial and ODP Leg 157 high-silica (picritic to olivine basaltic) lavas with their bulk rock Sr-Nd-Pb isotope compositions (87Sr/86Sr = 0.70315-0.70331, 143Nd/144Nd = 0.51288-0.51292, 206Pb/204Pb = 19.55-19.93, 207Pb/204Pb = 15.60-15.63, 208Pb/204Pb = 39.31-39.69). Our data point toward the presence of both a peridotitic and a pyroxenitic component in the magma source. Using the model (Sobolev et al., 2007, Science Vol 316) in which the reaction of Si-rich melts originated during partial melting of eclogite (a high pressure product of subducted oceanic crust) with ambient peridotitic mantle forms olivine-free reaction pyroxenite, we obtain an end member composition for peridotite with 87Sr/86Sr = 0.70337, 143Nd/144Nd = 0.51291, 206Pb/204Pb = 19.36, 207Pb/204Pb = 15.61 and 208Pb/204Pb = 39.07 (EM-type end member), and pyroxenite with 87Sr/86Sr = 0.70309, 143Nd/144Nd = 0.51289, 206Pb/204Pb = 20.03, 207Pb/204Pb = 15.62 and 208Pb/204Pb = 39.84 (HIMU-type end member). Mixing of melts from these end members in proportions ranging from 70% peridotite and 30% pyroxenite to 28% peridotite and 72% pyroxenite derived melt fractions can generate the compositions of the most primitive Gran Canaria shield stage lavas. Combining our results with those from the low-silica rocks from the western Canary Islands (Gurenko et al., 2009, doi:10.1016/j.epsl.2008.11.013), at least four distinct components are required. We propose that they are (1) HIMU-type pyroxenitic component (representing recycled ocean crust of intermediate age) from the plume center, (2) HIMU-type peridotitic component (ancient recycled ocean crust stirred into the ambient mantle) from the plume margin, (3) depleted, MORB-type pyroxenitic component (young recycled oceanic crust) in the upper mantle entrained by the plume, and (4) EM-type peridotitic component from the asthenosphere or lithosphere above the plume center.

Gravity mapping in the southern Weddell Sea region, Antarctica (Scale 1:2 000 000)

New maps of free-air and the Bouguer gravity anomalies on the Weddell Sea sector (70-81° S, 6-75° W) of Antarctica are presented. These maps are based on the first computer compilation of available gravity data collected by ''Sevmorgeologia'' in 1976-89 in the southern Weddell Sea and adjacent coasts of western Dronning Maud Land (WDML) and Coats Land. The accomplished gravity studies comprise airborne observations with a line spacing of about 20 km and conventional measurements at over-the-ice points, which were spaced at 10-30 km and supplemented by seismic soundings. Hence, anomalies on the maps represent mainly large-scale and deep crustal features. The dominant feature in free-air gravity map is a large dipolar gravity anomaly stretching along the continental margin. Following the major grain of seabed morphology this shelf-edge/slope anomaly (SESA) is clearly divided into three segments characterized by diverse anomaly amplitudes, wavelengths and trends. They are associated with continental margins of different geotectonic provinces of Antarctica surrounding the Weddell Sea. Apparent distinctions in the SESA signatures are interpreted as the gravity expression of tectonic, deep crustal structure segmentation of the continental margin. The prominent gravity highs (100-140 mGal) of the shelf edge anomaly mapped along WDML are assumed to represent high-density mantle injections intruded into the middle/lower crust during initial rifting of continental breakup. Enlarged wavelengths and diminished amplitudes of the gravity anomaly westwards, along the Weddell Sea embayment (WSE) margin, reflect a widening of the continental slope and a significant increase in thickness of underlying sediment strata. Low amplitude, negative free-air anomalies in the Filchner-Ronne Ice Shelves (FRIS) contrast sharply with the dominating positive anomalies offshore. This indicates a greater sedimentary thickness of the basin in this area. Crustal response to the enlarged sediment load is impressed in mostly positive features of the Bouguer gravity field observed here. Two pronounced positive Bouguer anomalies of 50-70 mGal and an average widths of 200 km dominate the Weddell Sea embayment margins towards the Antarctic Peninsula and the East Antarctic craton. They correlate well with very deep seabed troughs (> 1000 m below sea level). The gravity highs are most likely caused by a shallow upper mantle underneath graben-rift structures evolved at the margins of the WSE basin. A regional zone (> 100 km in width) of the prominent Bouguer and free-air negative anomalies (-40 to -60 mGal) adjacent Coats Land to the north of the ice shelf edge may indicate the presence of the thick old cratonic crust far offshore beneath the Weddell Sea Embayment.

Pb concentrations, Pb isotopic composition, and amount of metalliferous components in pelagic sediments from the Pacific Ocean

The amount of lead annually transferred from oceanic crust to metalliferous sediments was estimated in order to test the hypothesis that a non-magmatic flux of lead causes the Pb surplus in the continental crust. A Pb surplus has been inferred from global crust-mantle lead mass balances derived from lead concentration correlations with other trace elements and from lead isotope systematics in oceanic basalts. DSDP/ODP data on the amount of metalliferous sediments in the Pacific Ocean and along a South Atlantic traverse are used to calculate the mean worldwide thickness of 3 (+/-1) m for purely metalliferous sediment componens. Lead isotope ratios of 39 metalliferous sediments from the Pacific define mixing lines between continent-derived (seawater) and mantle-derived (basaltic) lead, with the most metal-rich sediments usually having the most mantle-like Pb isotope composition. We used this isotope correlation and the Pb content of the 39 metalliferous sediments to derive an estimate of 130 (+/-70) µg/g for the concentration of mantle-derived lead in the purely metalliferous end-member. Mass balance calculations show that at least 12 (+/-8)% of the lead, annually transferred from upper mantle to oceanic crust at the ocean ridges, is leached out by hydrothermal processes and re-deposited in marine sediments. If all of the metalliferous lead is ultimately transferred to the continental crust during subduction, the annual flux of this lead from mantle to continental crust is 2.6 (+/-2.0) * 10**6 kg. Assuming this transfer rate to be proportional to the rate of oceanic plate production, one can fit the lead transfer to models of plate production rate variations through time. Integrating over 4 Ga, hydrothermal lead transfer to the continental crust accounts for a significant portion of the Pb surplus in the continental crust. It therefore appears to be one of the main reasons for the anomalous behavior of lead in the global crust-mantle system.

(Table 1) Heat flow measurements in the Black Sea

New data show that the thermal field is definitely related to the geologic structure at depth and to other geophysical fields. Low heat-flow values along reliably established subsurface faults suggest absence of a heated zone of the earth's crust and upper mantle in these regions.

Element analyses and isotope composition of basalts from the West Valley segment, Juan de Fuca Ridge, northeast Pacific

The 50 km-long West Valley segment of the northern Juan de Fuca Ridge is a young, extension-dominated spreading centre, with volcanic activity concentrated in its southern half. A suite of basalts dredged from the West Valley floor, the adjacent Heck Seamount chain, and a small near-axis cone here named Southwest Seamount, includes a spectrum of geochemical compositions ranging from highly depleted normal (N-) MORB to enriched (E-) MORB. Heck Seamount lavas have chondrite-normalized La/Sm en -0.3, 87Sr/86Sr = 0.70235 - 0.70242, and 206Pb/204Pb = 18.22 - 18.44, requiring a source which is highly depleted in trace elements both at the time of melt generation and over geologic time. The E-MORB from Southwest Seamount have La/Sm en -1.8, 87Sr/86Sr = 0.70245 - 0.70260, and 206Pb/204Pb = 18.73 - 19.15, indicating a more enriched source. Basalts from the West Valley floor have chemical compositions intermediate between these two end-members. As a group, West Valley basalts from a two-component mixing array in element-element and element-isotope plots which is best explained by magma mixing. Evidence for crustal-level magma mixing in some basalts includes mineral-melt chemical and isotopic disequilibrium, but mixing of melts at depth (within the mantle) may also occur. The mantle beneath the northern Juan de Fuca Ridge is modelled as a plum-pudding, with "plums" of enriched, amphibole-bearing peridotite floating in a depleted matrix (DM). Low degrees of melting preferentially melt the "plums", initially removing only the amphibole component and producing alkaline to transitional E-MORB. Higher degrees of melting tap both the "plums" and the depleted matrix to yield N-MORB. The subtly different isotopic compositions of the E-MORBs compared to the N-MORBs require that any enriched component in the upper mantle was derived from a depleted source. If the enriched component crystallized from fluids with a DM source, the "plums" could evolve to their more evolved isotopic composition after a period of 1.5-2.0 Ga. Alternatively, the enriched component could have formed recently from fluids with a lessdepleted source than DM, such as subducted oceanic crust. A third possibility is that enriched material might be dispersed as "plums" throughout the upper mantle, transported from depth by mantle plumes.

Mineraly of oceanic crust from the Kane Transform, Mid-Atlantic Ridge

Gabbroic cumulates drilled south of the Kane Transform Fault on the slow-spread Mid-Atlantic Ridge preserve up to three discrete magnetization components. Here we use absolute age constraints derived from the paleomagnetic data to develop a model for the magmatic construction of this section of the lower oceanic crust. By comparing the paleomagnetic data with mineral compositions, and based on thermal models of local reheating, we infer that magmas that began crystallizing in the upper mantle intruded into the lower oceanic crust and formed meter-scale sills. Some of these magmas were crystal-laden and the subsequent expulsion of interstitial liquid from them produced '"cumulus" sills. These small-scale magmatic injections took place over at least 210 000 years and at distances of ~3 km from the ridge axis and may have formed much of the lower crust. This model explains many of the complexities described in this area and can be used to help understand the general formation of oceanic crust at slow-spread ridges.

Physical properties, velocity and attenuation characteristics, and mineralogy of gabbros from ODP Hole 176-735B

Laboratory compressional wave (Vp) and shear wave (Vs) velocities were measured as a function of confining pressure for the gabbros from Hole 735B and compared to results from Leg 118. The upper 500 m of the hole has a Vp mean value of 6895 m/s measured at 200 MPa, and at 500 meters below seafloor (mbsf), Vp measurements show a mean value of 7036 m/s. Vs mean values in the same intervals are 3840 m/s and 3857 m/s, respectively. The mean Vp and Vs values obtained from log data in the upper 600 m are 6520 and 3518 m/s, respectively. These results show a general increase in velocity with depth and the velocity gradients estimate an upper mantle depth of 3.32 km. This value agrees with previous work based on dredged samples and inversion of rare element concentrations in basalts dredged from the conjugate site to the north of the Atlantis Bank. Laboratory measurements show Vp anisotropy ranging between 0.4% and 8.8%, with the majority of the samples having values less than 3.8%. Measurements of velocity anisotropy seem to be associated with zones of high crystal-plastic deformation with predominant preferred mineral orientations of plagioclase, amphiboles, and pyroxenes. These findings are consistent with results on gabbros from the Hess Deep area and suggest that plastic deformation may play an important role in the seismic properties of the lower oceanic crust. In contrast to ophiolite studies, many of the olivine gabbros show a small degree of anisotropy. Log derived Vs anisotropy shows an average of 5.8% for the upper 600 m of Hole 735B and tends to decrease with depth where the overburden pressure and the age of the crustal section suggests closure of cracks and infilling of fractures by alteration minerals. Overall the results indicate that the average shear wave splitting in Hole 735B might be influenced by preferred structural orientations and the average value of shear wave splitting may not be a maximum because structural dips are <90°. The maximum fast-wave orientation values could be influenced by structural features striking slightly oblique to this orientation or by near-field stress concentrations. However, flexural wave dispersion analyses have not been performed to confirm this hypothesis or to indicate to what extent the near-field stresses may be influencing shear wave propagation. Acoustic impedance contrasts calculated from laboratory and logging data were used to generate synthetic seismograms that aid in the interpretation of reflection profiles. Several prominent reflections produced by these calculations suggest that Fe-Ti oxides and shear zones may contribute to the reflective nature of the lower oceanic crust. Laboratory velocity attenuation (Q) measurements from below 500 m have a mean value of 35.1, which is consistent with previous vertical seismic profile (VSP) and laboratory measurements on the upper 500 m.