Petrology of basalts from DSDP Hole 83-504B

Basalts from Hole 504B, Leg 83, exhibit remarkable uniformity in major and trace element composition throughout the 1075.5 m of basement drilled. The majority of the basalts, Group D', have unusual compositions relative to normal (Type I) mid-ocean ridge basalts (MORB). These basalts have relatively high mg values (0.60-0.70) and CaO abundances (11.7-13.7%; Ca/Al = 0.78-0.89), but exhibit a marked depletion in compatible trace elements (Cr and Ni); moderately incompatible trace elements (Zr, Y, Ti, etc.); and highly incompatible trace elements (Nb, LREE, etc.). Petrographic and compositional data indicate that most of these basalts are evolved, having fractionated significant amounts of plagioclase, olivine, and clinopyroxene. Melting experiments on similar basalt compositions from the upper portion of Hole 504B (Leg 70; Autio and Rhodes, 1983) indicate that the basalts are co-saturated with olivine and plagioclase and often clinopyroxene on the 1-atm. liquidus. Two rarely occurring groups, M' and T, are compositionally distinct from Group D' basalts. Group T is strongly depleted in all magmaphile elements except the highly incompatible ones (Nb, La, etc.), while Group M' has moderate concentrations of both moderately and highly incompatible trace elements and is similar to Type I MORB. Groups M' and T cannot be related to Group D' nor to each other by crystal fractionation, crystal accumulation, or magma mixing. The large differences in magmaphile element ratios (Zr/Nb, La/Yb) among these three chemical groups may be accounted for by complex melting models and/or local heterogeneity of the mantle beneath the Costa Rica Ridge. Xenocrysts and xenoliths of plagioclase and clinopyroxene similar in texture and mineral composition to crystals in coarse-grained basalts from the lower portion of the hole are common in Hole 504B basalts. These suggest that addition of solid components either from conduit or magma chamber walls has occurred and may be a common source of disequilibrium crystals in these basalts. However, mixing of plagioclase-laden depleted melts (similar to the Costa Rica Ridge Zone basalts) with normal MORB magmas could provide an alternate source for some refractory plagioclase crystals found out of equilibrium in many phyric MORB. The uniformity of major element compositions in Hole 504B basalts affords an ideal situation for investigating the effects of alteration on some major and trace elements in oceanic basalts. Alteration observed in whole-rock samples records primarily two events - a high-temperature and a low-temperature phase. High-temperature phases include: chlorite, talc, albite, actinolite, sphene, quartz, and pyrite. The low-temperature phases include smectite (saponite), epistilbite or laumontite, and minor calcite. Laumontite may actually straddle the gap between the low- and high-temperature mineral assemblages. Alteration is restricted primarily to partial replacement of primary phases. Metamorphic grade, in general, increases from the top to the bottom of Hole 504B (Legs 69, 70, and 83) as seen in the change from a smectiteto- chlorite-dominated secondary mineral assemblage. However, a systematic progression for the interval recovered during Leg 83 is not apparent. Rather, the extent of alteration appears to be a function of the initial texture and fracture density. Variations in whole-rock major and trace element concentrations cannot be attributed convincingly to any differences in alteration observed. Compositional characteristics of the secondary minerals indicated that extensive remobilization of elements has not occurred; local redistribution is suggested in most cases. Thus, the major and trace element signature of these basalts remains effectively the same as the original composition prior to alteration.

Geochemistry and petrology of basalts from Celebes Sea

Major-, trace-, and rare-earth element analyses are presented from a suite of basaltic rocks from the basement of the Celebes Sea. The major elements and trace-elements were determined by X-ray fluorescence techniques, and the rare-earth elements were analyzed by instrumental neutron activation analysis. Compositionally the Celebes Sea basalts are very similar to typical normal mid-ocean ridge basalts, such as those described from the Indian Ocean triple junction. Petrogenetic modeling shows that all of the basalts analyzed can be formed by 10% to 20% partial melting of a light rare-earth element-depleted spinel lherzolite followed by fractional crystallization of mixtures of olivine, Plagioclase, and iron oxide. The Celebes Sea is interpreted as a fragment of the basement of the Jurassic Argo abyssal plain trapped during the Eocene to the north of Australia.

Geochemistry and petrogenesis of basalts from DSDP Leg 92 holes

Basalts recovered on DSDP Leg 92 include all the major basalt types so far recovered from the ocean crust of the eastern Pacific. Basalts from Holes 597, 597A, 597B, 597C, and 599B are tholeiites exhibiting all the mineralogical and geochemical characteristics of N-type mid-ocean ridge basalts (MORB). Fragments of ferrobasalts and alkali basalts were also obtained, however, from Holes 60IB and 602B, respectively. Hole 597C, which penetrated 91 m into basement and is the deepest hole so far drilled in fast-spreading crust, yielded basalts that can be divided into three major lithologic units. The lowest unit, Unit III, contains modal olivine and comprises basalts which, at about 8 to 10% MgO, are as basic as any sampled from fast-spreading crust. The middle unit, Unit II, is the most evolved; its basalts are olivine free and contain between 6 and 7.5% MgO. The upper unit, Unit I, is intermediate in composition between Units II and III; it is characterized by both modal olivine and glomerocrysts made up of plagioclase and rare olivine. Unit I is probably a massive flow, whereas Units II and III may be massive flows or sills. The basalts appear to have undergone three stages of alteration ("deuteric," "relatively reducing," and "oxidizing"), the intensity of alteration decreasing markedly downcore. Hole 597B, at 26.4 m of basement penetration the only other "deep" hole, contains just one lithologic unit, which closely resembles Unit I of Hole 597C. Petrogenetic modeling reveals that the three lithologic units in Hole 597C are cogenetic and that they were derived from a depleted mantle source similar to the source of the tholeiites and ferrobasalts sampled in other holes; the alkali basalts are the only rocks derived from enriched mantle. Lavas of Unit III probably lay on the olivine-plagioclase cotectic, whereas the other lavas lay on an olivine-plagioclase-clinopyroxene peritectic. Some 60% of closed-system crystallization is needed to generate the most-evolved from the last-fractionated tholeiite, and a further 50% crystallization (80% overall) is needed to generate the ferrobasalts. Xenocrysts of calcic plagioclase and pseudomorphosed olivine in tholeiites from Hole 597B and Unit I of Hole 597C, and in the ferrobasalts from Hole 601B, provide evidence, however, that some magma mixing may have taken place.

(Table 1) Os and Sr isotopes in low-temperature hydrothermal fluids from the Juan de Fuca Ridge at ODP Site 168-1026B

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, 187Os/188Os = 0.9 and [Sr] = 86 µmol/kg, 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, 187Os/188Os = 0.8 and [Sr] = 115 µmol/kg, 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.

Geochemistry of ODP Hole 111-504B basalts (Table 2)

Fifty samples of basalt recovered during ODP Leg 111 from the dikes (Layer 2C) of Hole 504B (1350.0-1562.3 m below seafloor) were analyzed by X-ray-fluorescence techniques. All of the samples are highly depleted in magmaphile elements relative to other mid-ocean ridge basalts, with TiO2 = 0.75-1.24 wt%, Na2O = 1.59-2.22 wt%, Zr = 38-64 ppm, Nb = 0.3-1.5 ppm, and Y = 20-30 ppm (for samples containing 0%-2% phenocrysts), but have ratios of highly incompatible elements similar to normal Type I mid-ocean ridge basalts (e.g., Zr/Nb > 30). Abundances of compatible elements are similar to those of typical mid-ocean ridge basalts, with MgO = 7.2-9.2 wt%, Fe2O3* = 9.3-12.5 wt%, Ni = 55-164 ppm, and Cr = 26-388 ppm. Approximately 2% of the samples recovered from the top part of Hole 504B are similar to normal Type I or Type II ocean floor basalts. However, all of the analyzed Leg 111 samples from Hole 504B are depleted basalts. Aphyric dike rocks from Leg 111 are virtually identical to the depleted aphyric samples recovered from the pillow lavas and dikes in the upper 1075 m of Hole 504B during DSDP Legs 69, 70, and 83, with the exception of elements readily altered by seawater (Sr, Rb, and K). These elements reach a maximum in both abundance and variability in the pillow lavas of the upper 571.5 m of Hole 504B and decline to more constant values in the dike system sampled on Legs 83 and 111, apparently as a result of a decrease in porosity and increase in alteration temperatures relative to the pillow lavas. Based on compositional similarities to the vast majority of the pillows and flows, the dikes sampled on Leg 111 appear to be the feeder system for the pillow lavas in the upper part of Hole 504B. The incompatible-element-depleted compositions of the Costa Rica Rift Zone basalts are consistent with multistage melting of a normal mid-ocean ridge source.

Fluid compositions and mineralogy of precipitates from Mid Atlantic Ridge hydrothermal vents at 4°48'S

The effect of volcanic activity on submarine hydrothermal systems has been well documented along fast- and intermediate-spreading centers but not from slow-spreading ridges. Indeed, volcanic eruptions are expected to be rare on slow-spreading axes. Here we report the presence of hydrothermal venting associated with extremely fresh lava flows at an elevated, apparently magmatically robust segment center on the slow-spreading southern Mid-Atlantic Ridge near 5°S. Three high-temperature vent fields have been recognized so far over a strike length of less than 2 km with two fields venting phase-separated, vapor-type fluids. Exit temperatures at one of the fields reach up to 407°C, at conditions of the critical point of seawater, the highest temperatures ever recorded from the seafloor. Fluid and vent field characteristics show a large variability between the vent fields, a variation that is not expected within such a limited area. We conclude from mineralogical investigations of hydrothermal precipitates that vent-fluid compositions have evolved recently from relatively oxidizing to more reducing conditions, a shift that could also be related to renewed magmatic activity in the area. Current high exit temperatures, reducing conditions, low silica contents, and high hydrogen contents in the fluids of two vent sites are consistent with a shallow magmatic source, probably related to a young volcanic eruption event nearby, in which basaltic magma is actively crystallizing. This is the first reported evidence for direct magmatic-hydrothermal interaction on a slow-spreading mid-ocean ridge.

Geochemistry of basalts at DSDP Site 89-462

At Site 462 in the Nauru Basin, western Pacific Ocean, 56 lithologic units have been recovered from an extensive flood basalt province. Fossil evidence suggests that the lavas were emplaced during the interval 100-115 Ma, some 30 m.y. after formation of the underlying Jurassic ocean crust. The lithologic units can be broadly divided into three chemical units, the lowermost two of which are chemically monotonous, suggesting rapid eruption of basalt from a compositionally homogeneous magma chamber. All the basalts are hypersthene- (hy-) rich tholeiites, with approximately chondritic La/Sm, La/Yb, Zr/Nb, La/Ta, and Th/Hf ratios. Chemically they resemble, in part, "transitional" mid-ocean ridge basalts (MORB) from areas such as the Reykjanes Ridge, although Rb, Ba, and K contents are very low and similar to those of "normal" MORB. Their 87Sr/86Sr ratios are higher than in N-type MORB (Fujii et al., 1981). The chemistry of the Nauru basalts differs from that of continental flood basalts, which tend to be strongly enriched in large-ion lithophile (LIL) elements, although the extent to which the differences result from sialic contamination or source variability is not clear.

Element analyses and isotopic composition of basalts from the western Pacific

New Sr- Nd- and Pb-isotopic and trace element data are presented on basalts from the Sulu and Celebes Basins, and the submerged Cagayan Ridge Arc (Western Pacific), recently sampled during Ocean Drilling Program Leg 124. Drilling has shown that the Sulu Basin developed about 18 Ma ago as a backarc basin, associated with the now submerged Cagayan Ridge Arc, whereas the Celebes Basin was generated about 43 Ma ago, contemporaneous with a general plate reorganisation in the Western Pacifc, subsequently developing as an open ocean receiving pelagic sediments until the middle Miocene. In both basins, a late middle Miocene collision phase and the onset of volcanic activity on adjacent arcs in the late Miocene are recorded. Covariations between 87Sr/86Sr and 143Nd/144Nd show that the seafoor basalts from both the Sulu and Celebes Basins are isotopically similar to depleted Indian mid-ocean ridge basalts (MORB), and distinct from East Pacifc Rise MORB, defining a single negative correlation. The Cagayan Arc volcanics are different, in that they have distinctly lower epsilon-Ne(T) for a given epsilon-Sr(T), compared to Sulu and Celebes basalts. In the 207Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb diagrams, the Celebes, Sulu and Cagayan rocks all plot distinctly above the Northern Hemisphere Reference Line, with high Delta 7/4 Pb (5.3-9.3) and Delta 8/4 Pb (46.3-68.1) values. They define a single trend of radiogenic lead enrichment from Celebes through Sulu to Cagayan Ridge, within the Indian Ocean MORB data field. The data suggest that the overall chemical and isotopic features of the Sulu, Cagayan and Celebes rocks may be explained by partial melting of a depleted asthenospheric N-MORB-type ("normal") mantle source with isotopic characteristics similar to those of the Indian Ocean MORB source. This asthenospheric source was slightly heterogeneous, giving rise to the Sr-Nd isotopic differences between the Celebes and Sulu basalts, and the Cagayan Ridge volcanics. In addition, a probably slab-derived component enriched in LILE and LREE is required to generate the elemental characteristics and low Ne(T) of the Cagayan Ridge island arc tholeiitic and calcalkaline lavas, and to contribute to a small extent in the backarc basalts of the Sulu Sea. The results of this study confirm and extend the widespread Indian Ocean MORB signature in the Western Pacifc region. This signature could have been inherited by the Indian Ocean mantle itself during the rupture of Gondwanaland, when fragments of this mantle could have migrated towards the present position of the Celebes, Sulu and Cagayan sources.

Lead isotope ratios of basalts from DSDP Leg 37 (Table 1)

Analyses of Pb from mid-ocean ridge basalts obtained from DSDP Leg 37 lie on a line of near-zero-age slope. The data seem consistent with an evolutionary model of three stages, the beginning of the second stage being at about 600 m.y. - the model age obtained for the least radiogenic Pb. The beginning of the third stage of evolution at the time of formation of the rocks apparently did not change the average Th/U ratio since measured values are consistent with the ratio deduced from Pb isotope ratios. Ratios of U/Pb are not consistent with Pb isotope ratios, however, thus ruling out a simple two-stage evolutionary model.

Hydrothermal activity along the slow-spreading Lucky Strike ridge segment (Mid-Atlantic Ridge): Distribution, heatflux, and geological controls

We have reviewed available visual information from the seafloor, and recently acquired microbathymetry for several traverses across the Lucky Strike segment, to evaluate the distribution of hydrothermal activity. We have identified a new on-axis site with diffuse flow, Ewan, and an active vent structure ∼1.2 km from the axis, Capelinhos. These sites are minor relative to the Main field, and our total heatflux estimate for all active sites (200–1200 MW) is only slightly higher than previously published estimates. We also identify fossil sites W of the main Lucky Strike field. A circular feature ∼200 m in diameter located on the flanks of a rifted off-axis central volcano is likely a large and inactive hydrothermal edifice, named Grunnus. We find no indicator of focused hydrothermal activity elsewhere along the segment, suggesting that the enhanced melt supply and the associated melt lenses, required to form central volcanoes, also sustain hydrothermal circulation to form and maintain large and long-lived hydrothermal fields. Hydrothermal discharge to the seafloor occurs along fault traces, suggesting focusing of hydrothermal circulation in the shallow crust along permeable fault zones.

CONSTRAINTS ON THE LITHOSPHERIC STRUCTURE OF MID OCEAN RIDGES FROM OCEANIC CORE COMPLEX MORPHOLOGY

The Mid-oceanic ridge system is a feature unique to Earth. It is one of the fundamental components of plate tectonics and reflects interior processes of mantle convection within the Earth. The thermal structure beneath the mid-ocean ridges has been the subject of several modeling studies. It is expected that the elastic thickness of the lithosphere is larger near the transform faults that bound mid-ocean ridge segments. Oceanic core complexes (OCCs), which are generally thought to result from long-lived fault slip and elastic flexure, have a shape that is sensitive to elastic thickness. By modeling the shape of OCCs emplaced along a ridge segment, it is possible to constraint elastic thickness and therefore the thermal structure of the plate and how it varies along-axis. This thesis builds upon previous studies that utilize thin plate flexure to reproduce the shape of OCCs. I compare OCC shape to a suite of models in which elastic thickness, fault dip, fault heave, crustal thickness, and axial infill are systematically varied. Using a grid search, I constrain the parameters that best reproduce the bathymetry and/or the slope of ten candidate OCCs identified along the 12°—15°N segment of the Mid-Atlantic Ridge. The lithospheric elastic thicknesses that explains these OCCs is thinner than previous investigators suggested and the fault planes dip more shallowly in the subsurface, although at an angle compatible with Anderson’s theory of faulting. No relationships between model parameters and an oceanic core complexes location within a segment are identified with the exception that the OCCs located less than 20km from a transform fault have slightly larger elastic thickness than OCCs in the middle of the ridge segment.

Major, minor, trace element, Sm-Nd and Sr isotope compositions of mafic rocks from the earliest oceanic crust of the Alpine Tethys

Recent isotopic and biochronologic dating has demonstrated that the Gets nappe contains remnants of the oldest part of the oceanic crust of the Alpine Tethys. The ophiolites are associated with deep sea sediments, platform carbonates and continental crustal elements suggesting a transitional environment between continental and oceanic crust. Therefore, the ophiolites from the Gets nappe provide the opportunity to assess the nature of mantle source and the magma evolution during the final rifting stage of the European lithosphere. Trace clement analyses of mafic rocks can he divided into two sets: (1) P, Zr and Y contents are consistent with those of mid-ocean ridge basalts and REE patterns have a P-MORB affinity. (2) P,Zr Ti and Y contents are compatible with within-plate basalts and are characterized by REE spectra similar to that of T-MORB. Both have Nd isotopic compositions similar to those of synrift magma of the Red Sea and to the Rhine Graben. The model ages are in agreement with an LREE-enriched subcontinental mantle source derived from depleted mantle 800 to 900 Ma ago. Minor, trace element and Sm-Nd compositions suggest that these rocks are basaltic relies of an earliest stage of oceanic spreading i.e. an embryonic ocean. Comparison between REE patterns, Nd and Sr isotope compositions, isotopic and biochronologic ages from different Alpine Tethys ophiolites shows that samples with enriched LREE are from the older ophiolitic suites and are relies of the embryonic ocean floor. Later phases of ocean spreading are characterized by basalts that are depleted in LREE.

Multiple plume events in the genesis of the peri-Caribbean Cretaceous oceanic plateau province

The oceanic crust fragments exposed in central America, in north-western South America, and in the Caribbean islands have been considered to represent accreted remnants of the Caribbean-Colombian Oceanic Plateau (CCOP). On the basis of trace element and Nd, Sr, and Pb isotopic compositions we infer that cumulate rocks, basalts, and diabases from coastal Ecuador have a different source than the basalts from the Dominican Republic. The latter suite includes the 86 Ma basalts of the Duarte Complex which are light rare earth element (REE) -enriched and display (relative to normal mid-ocean ridge basalts, NMORB) moderate enrichments in large ion lithophile elements, together with high Nb, Ta, Pb, and low Th contents. Moreover, they exhibit a rather restricted range of Nd and Pb isotopic ratios consistent with their derivation from an ocean island-type mantle source, the composition of which includes the HIMU (high U-238/Pb-204) component characteristic of the Galapagos hotspot. In contrast, the 123 Ma Ecuadorian oceanic rocks have flat REE patterns and (relative to NMORB) are depleted in Zr, Hf, Th, and U. Moreover, they show a wide range of Nd and Pb isotopic ratios intermediate between those of ocean island basalts and NMORB. It is unlikely, on geochemical grounds, that the plume source of the Ecuadorian fragments was similar to that of the Galapagos. In addition, because of the NNE motion of the Farallon plate during the Early Cretaceous, the Ecuadorian oceanic plateau fragments could not have been derived from the Galapagos hotspot but were likely formed at a ridge-centered or near-ridge hotspot somewhere in the SE Pacific.