(Table 1) Physical properties of basalts from DSDP Holes 69-504B and 70-504B

A detailed study of physical properties was made on core samples from Deep Sea Drilling Project Hole 504B. The measured properties are density, porosity, sonic velocity, electrical resistivity, and fluid permeability. Basalts from this young oceanic crust have higher density and sonic velocity than the average DSDP basalts. Porosity (and temperature) dependences of physical properties are given by V = Vo - a-phi; roo = roo-0 exp(E*/RT)phi**-q; k = k0' phi**2q-1; where V is the sonic velocity (km/s), Vo = 6.45 (km/s), a = 0.111 (km/s %), phi is the porosity (%), roo is the electrical resistivity (ohm m), roo-0 = 0.002 (ohm m), E* = 2.7 (Kcal/mol) for fresh basalts, RT has its usual meaning, q = 1.67 ± 0.27, k is the permeability, k0' = (1 to about 10) x 10**-12 (cm**2). Porosity distribution in the crust in this area is estimated by combining the seismic velocity distribution and velocity-porosity relation. Because of the rapid decrease in porosity with depth, resistivity increases and permeability decreases rapidly with depth. The decreasing rate of permeability with increasing depth is approximately given by k(cm**2) = 2 x 10**-10 exp(-z (km)/0.3).

Geochemistry and petrology of ODP Site 136-843 basalts

About 13 m of Cretaceous, tholeiitic basalt, ranging from normal (N-MORB) to transitional (T-MORB) mid-ocean-ridge basalts, was recovered at Ocean Drilling Program Site 843 west of the island of Hawaii. These moderately fractionated, aphyric lavas are probably representative of the oceanic basement on which the Hawaiian Islands were built. Whole-rock samples from parts of the cores exhibiting only slight, low-temperature, seawater alteration were analyzed for major element, trace element, and isotopic composition. The basalts are characterized by enrichment in the high field strength elements relative to N-MORB, by a distinct positive Eu anomaly, and by Ba/Nb and La/Nb ratios that are much lower than those of other crustal or mantle-derived rocks, but their isotope ratios are similar to those of present-day N-MORB from the East Pacific Rise. Hole 843A lavas are isotopically indistinguishable from Hole 843B lavas and are probably derived from the same source at a lower degree of partial melting, as indicated by lower Y/Nb and Zr/Nb ratios and by higher concentrations of light and middle rare earth elements and other incompatible elements relative to Hole 843B lavas. Petrographic and trace-element evidence indicates that the Eu anomaly was the result of neither plagioclase assimilation nor seawater alteration. The Eu anomaly and the enrichments in Ta, Nb, and possibly U and K relative to N-MORB apparently are characteristic of the mantle source. Age-corrected Nd and Sr isotopic ratios indicate that the source for the lavas recovered at ODP Site 843 was similar to the source for Southeast Pacific MORB. An enriched component within the Cretaceous mantle source of these basalts is suggested by their initial 208Pb/204Pb-206Pb/204Pb and epsilon-Nd-206Pb/204Pb ratios. The Sr-Pb isotopic trend of Hawaiian post-shield and post-erosional lavas cannot be explained by assimilation of oceanic crust with the isotopic composition of the Site 843 basalts.

Velocities and elastic moduli of basalts from Pacific DSDP sites

Compressional (Vp) and shear (Vs) wave velocities have been measured to 10 kb in 32 cores of basalt from 14 Pacific sites of the Deep Sea Drilling Project. Both Vp and V s show wide ranges (3.70 to 6.38 km/sec for Vp and 1.77 to 3.40 km/sec for V s at 0.5 kb) which are linearly related to density and sea floor age, confirming earlier findings by Christensen and Salisbury of decreasing velocity with progressive submarine weathering based on studies of basalts from five sites in the Atlantic. Combined Pacific and Atlantic data give rates of decreasing velocity of -1.89 and -1.35 km/sec per 100 my for Vp and Vs respectively. New analyses of oceanic seismic refraction data indicate a decrease in layer 2 velocities with age similar to that observed in the laboratory, suggesting that weathering penetrates to several hundred meters in many regions and is largely responsible for the extreme range and variability of layer 2 refraction velocities.

Geochemistry of Kerguelen-Heard Plateau basalts

During Leg 120 basalts were recovered at four drill holes on the Kerguelen-Heard Plateau. This paper reports the trace element and Sr, Nd, Hf, and Pb isotopic characteristics of these basalts and compares these basalts with Indian Ocean basalts and Kerguelen and Heard island volcanics. Kerguelen-Heard Plateau basalts are extremely heterogeneous in character. Intersite variations are larger than intrasite variations. Part of the chemical variations of the plateau volcanics overlap with those characteristics of Kerguelen Island volcanics, which indicates tapping of the same mantle source during the two different periods of activity. The estimates of the degree of melting for the plateau basalts (smaller degree of melting than for mid-ocean ridge basalts) and the heterogeneous character of the plateau exclude an origin that requires large degrees of melting or more rigorous convection than at ocean ridges. However, all characteristics indicate an oceanic origin for the Kerguelen-Heard Plateau.

Geochemistry of oceanic crust of the Cocos Plate

New geochemical data from the Cocos Plate constrain the composition of the input into the Central American subduction zone and demonstrate the extent of influence of the Galápagos Hotspot on the Cocos Plate. Samples include sediments and basalts from Ocean Drilling Program (ODP) Site 1256 outboard of Nicaragua, gabbroic sills from ODP Sites 1039 and 1040, tholeiitic glasses from the Fisher Ridge off northwest Costa Rica, and basalts from the Galápagos Hotspot Track outboard of Central Costa Rica. Site 1256 basalts range from normal to enriched MORB in incompatible elements and have Pb and Nd isotopic compositions within the East Pacific Rise MORB field. The sediments have similar 206Pb/204Pb and only slightly more radiogenic 207Pb/204Pb and 208Pb/204Pb isotope ratios than the basalts. Altered samples from the subducting Galápagos Hotspot Track have similar Nd and Pb isotopic compositions to fresh Galápagos samples but have significantly higher Sr isotopic composition, indicating that the subduction input will have a distinct geochemical signature from Galápagos-type mantle material that may be present in the wedge beneath Costa Rica. Gabbroic sills from Sites 1039 and 1040 in East Pacific Rise (EPR) crust show evidence for influence of the Galápagos Hotspot ?100 km beyond the morphological hotspot track.

Primary mineralogy of ODP Leg 115 basalts

Basement rocks were recovered at four sites on Leg 115 along the Reunion hotspot track in the western Indian Ocean. Plate tectonic reconstructions indicate that the drilled structures formed in three different volcanic environments. Sites 706 and 713 from the eastern side of the Saya de Malha Bank and the northern end of the Chagos Bank, respectively, are on a large volcanic platform analogous to Iceland on the Mid-Atlantic Ridge. Lavas at Site 707 on the northwestern side of the Saya de Malha Bank erupted during the early stages of rifting of the Seychelles from India. Basalts from Site 715 were erupted onto an isolated oceanic island that was distant from ocean ridges and continents much as Reunion Island is today. Many of the rocks were examined in thin section and found to be primarily augite-plagioclase basalts with minor olivine and rare opaque oxides. Site 715 is unusual in that it contains a variety of basalts including olivine-rich and aphyric Fe-Ti basalts. At each of the four sites the rocks were grouped into chemical types (units) on the basis of ship- board bulk-rock analyses and at least one thin section from each chemical unit was analyzed by electron microprobe. The plagioclase and augite chemistry reflects the bulk-rock chemistry and, in general, these minerals were in equilibrium with their host magmas at the time the basalts were quenched. Olivine was rarely preserved, but where it is still present it also appears to have crystallized in equilibrium with the host magma. At three of the drill sites plagioclase phenocrysts or megacrysts that crystallized from a primitive magma are also present. The one site (715) that does not contain these primitive plagioclase phenocrysts is also the site that appears to have been influenced the least by ocean- ridge or Deccan-type magmas. Site 715, furthermore, has a mineralogy that is dominated by olivine as compared with the plagioclase-rich lavas of the other sites.

Geochemistry of basalts fropm DSDP Leg 65 basalts

Samples of basalt collected on Leg 65 near 22°N on the East Pacific Rise all display the depleted light rare-earth pattern of "normal" oceanic crust. Consequently the La/Ta ratio is close to 18, as opposed to the value of 9 associated with the flat or enriched patterns found along parts of the Mid-Atlantic Ridge and the Emperor Seamount chain. The Leg 65 samples are chemically similar to those from the CYAMEX area at 21 °N and to the Leg 54 samples from 9°N, suggesting homogeneity of the upper mantle under the northern part of the East Pacific Rise over a minimum distance of about 1500 km. The geochemistry of the rocks and their field relationships with respect to depth and distance from the axis of the Rise show no pattern of distribution linked to the degree of fractional crystallization and thus cast doubt on any possible model involving large, long-lived magma chambers at the axis of the Rise.

Petrology and geochemistry of basalts from the Vavilov Basin

During ODP Leg 107, two holes were drilled in the basement of Vavilov Basin, a central oceanic area of the Tyrrhenian sea. Hole 655B is located near the Gortani ridge in off-axis position at the western rim of the basin; Hole 651A is located on a basement swell at the axis of the basin. This paper deals with mineral chemistry, major and trace element geochemistry, and petrogenesis of the basalts recovered in the two holes. The mineralogy of the basalts is broadly homogeneous, but all of them have suffered important seawater alteration. Their major-element compositions are similar to both normal-mid-ocean-ridge-basalts (N-MORB) and back-arc-basalts (BAB) except for Na2O contents (BAB-like), and K2O which is somewhat enriched in upper unit of Hole 651 A. Their affinity with N-MORB and BAB is confirmed by using immobile trace elements such as Zr, Y, and Nb. However, basalts from the two sites present contrasting geochemical characteristics on spidergrams using incompatible elements. Hole 655B basalts are homogeneous enriched tholeiites, similar to those from DSDP Hole 373 (located on the opposite side of the basin near the eastern rim), and show affinities with enriched MORB (E-MORB). At Hole 651 A, the two basalt units are chemically distinct. One sample recovered in lower unit is rather similar to those from Hole 655B, but basalts from upper unit display calc-alkaline characteristic evidenced by the increase of light-ion-lithophile-element (LILE)/high-field-strength-element (HFSE) ratio, and appearance of a negative Nb-anomaly, making them comparable with orogenic lavas from the adjacent Eolian arc. The observed chemical compositions of the basalts are consistent with a derivation of the magmas from a N-MORB type source progressively contaminated by LILE-enriched fluids released from dehydration of the bordering subducted plate. Implications for evolution of the Tyrrhenian basin are tentatively proposed taking into consideration geochemical and chronological relationships between basalts from Leg 107 Holes 655B and 651 A, together with data from Leg 42 Site 373 and Vavilov Seamount. These data illustrate back-arc spreading in ensialic basin closely associated with the maturation of the adjacent subduction, followed by the growth of late off-axis central volcano, whereas the active subduction retreats southeastward.

Geochemistry and isotopic ratios of basalts obtained during R/V Akademik Nikolaj Strakhov cruise ANS25

Based on the investigation of samples recovered during Cruise 25 of the R/V ''Akademik Nikolai Strakhov'', the character of magmatism was determined in the flank parts of the rift zone at the 74°05'N and 73°50'N region, where the direction of the rift valley changes from the north-northwest in the Knipovich Ridge to the northeast-trending structures of the Mohns Ridge. It was shown that the tholeiitic magmas of this region shows all the geochemical characteristics of TOR-2, which is typical of the Mohns Ridge and most oceanic rift zones worldwide, and differ from the basalts of the Knipovich Ridge, which are assigned to a shallower type of tholeiitic magmatism (Na-TOR). The persistent depletion of the magmas in terms of lithophile element contents and radiogenic isotope ratios of Sr, Nd, and Pb reflects the conditions of their formation during the ascent of the depleted oceanic mantle, which has occurred without significant complications since the early stages of the formation of the Mohns Ridge.

Chemical composition of Ninetyeast Ridge basalts

On Leg 121 of the Ocean Drilling Program, we recovered basaltic rocks from a total of three basement sites in the southern, central, and northern regions of Ninetyeast Ridge. These new sites complement the previous four basement holes drilled during Legs 22 and 26 of the Deep Sea Drilling Project, and confirm the predominantly tholeiitic, light rare earth element-enriched character of the basalts that cap the ridge. The basalts show marked iron enrichment; ferrobasalts occur at Sites 214 and 216 and oceanic andesites at Site 253. All of the basalts recovered during Leg 121 are altered, and range from aphyric olivine tholeiites (Site 756), to strongly plagioclase-phyric basalts (Site 757). Basalts from Site 758, which were clearly erupted in a submarine environment (pillow basalts are present in the section), are sparsely to strongly plagioclase-phyric. The basalts recovered at any one hole are isotopically homogeneous (except for the basalts from Site 758, which show a range of Pb isotopes), and it is possible to relate the magmas at any one site by high-level fractionation processes. However, there are significant variations in isotope ratios and highly incompatible element ratios between sites, which suggest that the mantle source for the ridge basalts was compositionally variable. Such variation, in view of the large volume of magmatic products that form the ridge system, is not surprising. There is not, however, a systematic variation in basalt composition along the ridge. We agree with previous models that relate Ninetyeast Ridge to a mantle plume in the southern Indian Ocean. The tholeiitic, iron-enriched, and voluminous character of the ridge basalts is typical of oceanic islands associated with plumes on or near a mid-ocean ridge (e.g., Iceland, Galapagos Islands, and St. Paul/Amsterdam islands). The absence of recovered alkalic suites is inconsistent with an intraplate setting, such as the Hawaiian Islands or Kerguelen Island. Thus, the major element data, like the gravity data, strongly suggest that the ridge was erupted on or very close to an active spreading center. Isotopically, the most likely plume that created the excess magmatism on the Ridge is the Kerguelen-Heard plume system, but the Ninetyeast Ridge basalts do not represent a simple mixing of the Kerguelen plume and mid-ocean Ninetyeast Ridge basalt mantle.

Comparisons of z values between fine-grained titanomagnetites within interstitial glass and large-grained titanomaghemtes in older ocean-floor basalts

Transmission electron microscopy observations and rock magnetic measurements reveal that alteration of fine- and large-grained iron-titanium oxides can occur at different rates. Fine-grained titanomagnetite occurs as a crystallization product within interstitial glass that originated as an immiscible liquid within a fully differentiated melt; in several samples with ages to 32 Ma it displays very little or no oxidation (z = ca. 0). In contrast, samples with ages of 10 Ma or older are observed to also contain highly oxidized (z >/= 0.66) large-grained titanomaghemite. These large grains, having originated by direct crystallization from melt, are associated with pore space. Such pore space can serve as a conduit for fluids that promote alteration, whereas fine grains may have been "armored" against alteration by the glass matrix in which they are embedded. Apparently, alteration of oceanic crust is a heterogeneous process on a microscopic scale. The existence of pristine, fine-grained titanomagnetite in the interstitial glass of older ocean-floor basalts that have undergone significant alteration implies that such glassy material is capable of carrying original thermal remanent magnetization and may be suitable for paleointensity determinations.

Composition of basalts from the Romanche Fracture Zone

New magnetometric, petrological, and geochemical data on basalts from the central Romanche Fracture Zone allow to classify these rocks into two groups. Igneous rocks from the active part of the fracture zone that have undergone transtension are referred to alkaline rocks. According to some indications, they are younger that oceanic tholeiites of the southern fault-line ridge, which were affected by elevated pressure in the past. These data indicate with a high probability that the Romanche Fracture Zone belongs to a rare group of magmatically active demarcation transform lines that separate large oceanic domains different in structural and geochemical features.