Mineral and chemical composition of hydrothermally altered rocks and sediments from the Middle Valley, ODP Leg 139 data

The Middle Valley segment at the northern end of the Juan de Fuca Ridge is a deep extensional rift blanketed with 200-500 m of Pleistocene turbiditic sediment. Sites 857 and 858 were drilled during Ocean Drilling Program Leg 139 to determine whether these two sites were hydrologically linked end members of an active hydrothermal circulation system. Site 858 was placed in an area of active hydrothermal discharge with fluids up to 270°C venting through anhydrite-bearing mounds on top of altered sediment. The shallow basement of fine-grained basalt that underlies the vents at Site 858 is interpreted as a seamount that was subsequently buried by turbidites. Site 857 was placed 1.6 km south of the Site 858 vents in a zone of high heat flow and numerous seismically imaged ridge-parallel faults. Drilling at Site 857 encountered sediments that are increasingly altered with depth and that overlie a series of mafic sills at depths of 460-940 m below sea floor. Sill margins and adjacent baked sediment are highly altered to magnesian chlorite and crosscut with veins filled with quartz, chlorite, sulfides, epidote, and wairakite. The sill interiors vary from slightly altered, with unaltered plagioclase and clinopyroxene in a mesostasis replaced by chlorite, to local zones of intense alteration and brecciation. In these latter zones, the sill interiors are pervasively replaced by chlorite, epidote, quartz, pyrite, titanite, and rare actinolite. The most complete replacement is associated with brecciated horizons with low recovery and slickensides on fracture surfaces, which we interpret as intersections between faults and the sills. Geochemically, the alteration of the sill complex is reflected in significant whole-rock depletions in Ca, Sr, and Na with corresponding enrichments in Mg, Al, and most metals. The latter results from the formation of conspicuous sulfide poikiloblasts. In contrast, metamorphism of the Site 858 seamount includes incomplete albitization of plagioclase phenocrysts and replacement of sparse mafic phenocrysts. Much of the basement alteration at Site 858 is confined to crosscutting veins except for a highly altered and veined horizon at the contact between basaltic basement and the overlying sediment. The sill complex at Site 857 is more highly depleted in 18O (d18O = 2.4 per mil - 4.7 per mil) and more pervasively replaced by secondary minerals relative to the extrusives at Site 858 (d18O = 4.5 per mil - 5.5 per mil). There is no evidence of significant albitization of the plagioclase at Site 857, suggesting high Ca/Na in the pore fluids. Fluid-inclusion data from hydrothermal minerals in altered mafic rocks and veins at Sites 857 and 858 show a consistency of homogenization temperatures, varying from 245 to 270°C, which is within the range of temperatures observed for the fluids venting at Site 858. The consistency of the fluid inclusion temperatures, the lack of albitization within the Site 857 sills, and the apparently low water/rock ratio collectively suggest that the sill complex at Site 857 is in thermal equilibrium and being altered by a highly evolved Ca-rich fluid similar to the fluids now venting at Site 858. The alteration evident in these two deep crustal drillsites is a result of the ongoing hydrothermal circulation and is consistent with downhole logging results, instrumented borehole results, and hydrothermal fluid chemistry. The pervasive alteration of the laterally extensive sill-sediment complex at Site 857 determines the chemistry of the fluids that are venting at Site 858. The limited alteration of the Site 858 lavas suggests that this basement edifice acts as a penetrator or ventilator for the regional hydrothermal reservoir with much of the flow focussed at the highly altered and veined sediment-basalt contact.

Whole rock analytical techniques and glas analytical techniques of silicic magmas from the South East Rift, Manus Basin

There has been much recent interest in the origin of silicic magmas at spreading centres away from any possible influence of continental crust. Here we present major and trace element data for 29 glasses (and 55 whole-rocks) sampled from a 40 km segment of the South East Rift in the Manus Basin that span the full compositional continuum from basalt to rhyolite (50-75 wt % SiO2). The glass data are accompanied by Sr-Nd-Pb, O and U-Th-Ra isotope data for selected samples. These overlap the ranges for published data from this part of the Manus Basin. Limited increases in Cl/K ratios with increasing SiO2, La-SiO2 and Yb-SiO2 relationships, and the oxygen isotope data rule out models in which the more silicic lavas result from partial melting of altered oceanic crust or altered oceanic gabbros. Rather, the data form a coherent array that is suggestive of closed-system fractional crystallization and this is well simulated by MELTS models run at 0.2 GPa and QFM (quartz-fayalite-magnetite buffer) with 1 wt % H2O, using a parental magma chosen from the basaltic glasses. Although some assimilation of altered oceanic crust or gabbro cannot be completely ruled out, there is no evidence that this plays an important role in the origin of the silicic lavas. The U-series disequilibria are dominated by 238U and 226Ra excesses that limit the timescale of differentiation to less than a few millennia. Overall, the data point to rapid evolution in relatively small magma lenses located near the base of thick oceanic crust; we speculate that this was coupled with relatively low rates of basaltic recharge. A similar model may be applicable to the generation of silicic magmas elsewhere in the ocean basins.

(Table 1) Ooze/chalk cycles: frequency of occurrence at DSDP Leg 74 Sites

Shallow- to deep-water environments are represented by the sediments and rocks recovered from the Walvis Ridge- Angola Basin transect. These calcareous oozes, chalks, limestones, and volcaniclastic sedimentary rocks are used to define and correlate four lithostratigraphic units. The sediments were deposited in cycles which represent recurring tectonic or Oceanographic events and may be related to climatic fluctuations and orbital perturbations. Turbidites are the most common and easily identified sedimentary cycle. They are Late Cretaceous to Paleocene in age and are repeated in intervals ranging from thousands to tens of thousands of years. They are also found interbedded between basalt layers. Turbidites are easily distinguished from the other cycles present by their sedimentary structures, mineral composition, alteration products, and physical properties (GRAPE) data. Large-scale turbidites, debris, or slump breccias are found at or just above the Cretaceous/Tertiary boundary and indicate an event of considerable energy possibly related to intense tectonic activity. Diagenetic cycles, interpreted as small-scale dissolution cycles or sequences produced by biogenic activity, occur in early Paleocene chalks. The recurrence intervals average -20,000 y. but have a wide range of values. Variations in CaCO3 content, color, gradational boundaries, and trace fossil content characterize these sediments. These cycles reflect bottom-water conditions. Ooze-chalk cycles occur in upper Oligocene to upper Paleocene sediments and represent conditions that once existed at the sediment/water interface where they obtained their diagenetic potential. These oscillations are repeated over tens of thousands of years and may have no modern analogs. Color variations in sediments at the Cretaceous/Tertiary boundary indicate local fluctuations in oxygen content within the sediments or the water column. This situation lasted for several hundred thousand years and is not repeated elsewhere in the sequence. Large dissolution cycles are recorded in the sediments at Site 527 that are of middle Miocene and early Oligocene to middle Eocene age. During this time the seafloor at this site appears to have been located at or subsided to a depth occupied by a fluctuating CCD and lysocline.

(Table 1) Paleomagnetic properties of specimens from DSDP Holes 69-504B and 70-504B

Paleomagnetic and rock magnetic measurements of basalt specimens from DSDP Hole 504B, associated with the Costa Rica Rift, have a mean natural remanence intensity (Jn) between 5 and 10 x 10**-3 gauss, consistent with the presence of a magnetized layer that is 0.5 to 1 km thick, which produces the observed magnetic anomalies. A mean Koenigsberger ratio (Qn) greater than 10 indicates that the remanence dominates the magnetic signal of the drilled section. The susceptibility (x) increases with depth, and the median demagnetizing field (MDF) decreases with increasing depth in Hole 504B, congruent with the downhole increase in the relative abundance of massive flow units. Hole 504B is composed of at least 12 units with distinct stable average inclinations (Is), which probably represent extrusion at times of different geomagnetic field directions and possibly also the effects of faulting. The thickness of basalt associated with these inclination units varies from less than 9 meters to possibly as much as 160 meters. Two relatively thick magnetic units (40 m and 45 m, separated by 100 m) have anomalously high Is values of -53° and -63°, in contrast with the near zero inclinations expected for the equatorial latitude of Site 504. For this reason and because the average inclination of all the magnetic units is skewed to a negative value, it might be that the entire section at Hole 504B was tilted by approximately 30°.

Rb-Sr systematics of volcanic rocks and alteration minerals of ODP Hole 104-642E

Subaerially erupted tholeiites at Hole 642E were never exposed to the high-temperature seawater circulation and alteration conditions that are found at subaqueous ridges. Alteration of Site 642 rocks is therefore the product of the interaction of rocks and fluids at low temperatures. The alteration mineralogy can thus be used to provide information on the geochemical effects of low temperature circulation of seawater. Rubidium-strontium systematics of leached and unleached tholeiites and underlying, continentally-derived dacites reflect interactions with seawater in fractures and vesicular flow tops. The secondary mineral assemblage in the tholeiites consists mainly of smectite, accompanied in a few flows by the assemblage celadonite + calcite (+/- native Cu). Textural relationships suggest that smectites formed early and that celadonite + calcite, which are at least in part cogenetic, formed later than and partially at the expense of smectite. Smectite precipitation occurred under variable, but generally low, water/rock conditions. The smectites contain much lower concentrations of alkali elements than has been reported in seafloor basalts, and sequentially leached fractions of smectite contain Sr that has not achieved isotopic equilibrium. 87Sr/86Sr results of the leaching experiments suggest that Sr was mostly derived from seawater during early periods of smectite precipitation. The basalt-like 87Sr/86Sr of the most readily exchangeable fraction seems to suggest a late period of exposure to very low water /rock. Smectite formation may have primarily occurred in the interval between the nearly 58-Ma age given by the lower series dacites and the 54.5 +/- 0.2 Ma model age given by a celadonite from the top of the tholeiitic section. The 54.5 +/- 0.2 Ma Rb-Sr model age may be recording the timing of foundering of the Voring Plateau. Celadonites precipitated in flows below the top of the tholeiitic section define a Rb-Sr isochron with a slope corresponding to an age of 24.3 +/- 0.4 Ma. This isochron may be reflecting mixing effects due to long-term chemical interaction between seawater and basalts, in which case the age provides only a minimum for the timing of late alteration. Alternatively, inferrential arguments can be made that the 24.3 +/- 0.4 isochron age reflects the timing of the late Oligocene-early Miocene erosional event that affected the Norwegian-Greenland Sea. Correlation of 87Sr/86Sr and 1/Sr in calcites results in a two-component mixing model for late alteration products. One end-member of the mixing trend is Eocene or younger seawater. Strontium from the nonradiogenic endmember can not, however, have been derived directly from the basalts. Rather, the data suggest that Sr in the calcites is a mixture of Sr derived from seawater and from pre-existing smectites. For Site 642, the reaction involved can be generalized as smectite + seawater ++ celadonite + calcite. The geochemical effects of this reaction include net gains of K and CO2 by the secondary mineral assemblage. The gross similarity of the reactions involved in late, low-temperature alteration at Site 642 to those observed in other sea floor basalts suggests that the transfer of K and C02 to the crust during low-temperature seawater-ocean crust interactions may be significant in calculations of global fluxes.

Bulk geochemistry, mass change and alteration intensity of basalts from ODP Leg 187 sites and survey dredges

Shipboard studies during Ocean Drilling Program Leg 187 (Australian Antarctic Discordance, AAD) suggested that there was no discernible coincidence between the interpreted age of rocks recovered and the intensity of alteration observed. Samples from the oldest sites occupied appeared to exhibit the least overall effects of alteration, and the intensity of alteration varied from site to site. Previous investigations of low-temperature alteration in oceanic basement samples have been restricted by the myopic perspective provided by single drill holes or dredge collections. Combining core samples from Leg 187 and dredge samples from the AAD collection at Oregon State University (USA) offers the unique opportunity to investigate mineral and bulk chemical changes attending alteration of basalt over a range of ages from 0 to 28 Ma. Results of this research indicate that there is a general increase in the intensity of alteration as the basalts age and mosve off axis, but that this relationship is somewhat veiled by the dominating control on alteration intensity dictated by variations in permeability.

Geochemistry and isotopic ratios of basalts from ODP Leg 163 sites

Voluminous, subaerial magmatism resulted in the formation of extensive seaward-dipping reflector sequences (SDRS) along the Paleogene Southeast Greenland rifted margin. Drilling during Leg 163 recovered basalts from the SDRS at 66ºN (Site 988) and 63ºN (Sites 989 and 990). The basalt from Site 988 is light rare-earth-element (REE) enriched (La(n)/Yb(n) = 3.4), with epsilon-Nd(t=60) = 5.3, 87Sr/86Sr = 0.7034, and 206Pb/204Pb = 17.98. It is similar to tholeiites recovered from the Irminger Basin during Leg 49 and to light-REE-enriched tholeiites from Iceland. Drilling at Site 989, the innermost of the sites on the 63ºN transect, was proposed to extend recovery of the earliest part of the SDRS initiated during Leg 152. These basalts are, however, younger than those from Site 917 and are compositionally similar to basalts from the more seaward Sites 990 and 915. Many of the basalts from Sites 989 and 990 show evidence of contamination by continental crust (e.g., epsilon-Nd(t=60) extends down to -3.7, 206Pb/204Pb extends down to 15.1). We suggest that the contaminant is a mixture of Archean granulite and amphibolite and that the most contaminated basalts have assimilated ~5% of crust. Uncontaminated basalts are isotopically similar to basalts from Site 918, on the main body of the SDRS, and are light-REE depleted. Consistent with previous models of the development of this margin, we show that at the time of formation of the basalts from Sites 989 and 990 (1) melting was at relatively shallow levels in a fully-fledged rift zone; (2) fragments of continental crust were present in the lithosphere above the zones of melt generation; and (3) the sublithospheric mantle was dominated by a depleted Icelandic plume component.

(Table 1) Stratigraphic distribution and abundances of Cretaceous radiolarians from ODP Holes 130-803D and 130-807C

Among the five sites drilled during Ocean Drilling Program Leg 130, two deep holes (8O3D and 807C) penetrated Cretaceous sediments overlying the basaltic pillows, flows, and possibly basement rocks. Abundant, poorly preserved radiolarians with limited diversity were recovered from a few horizons within the sediments proximal to the basalt. At Site 803, three thin layers of radiolarites interbedded with claystone and clayey siltstone yielded radiolarian assemblages of late Albian age. At Site 807, several layers of radiolarian siltstones were recovered proximal to the basalt. Among them the most significant radiolarian assemblage is an Aptian fauna, located approximately 7 m above the basaltic flows. The Aptian radiolarian age for Site 807 is at least in accord with those suggested by planktonic foraminifer and paleomagnetic evidence. These Cretaceous radiolarians are the oldest assemblages recorded from the Ontong Java Plateau region.

Interstitial water chemistry of DSDP Leg 85 sites

The concentration changes in pore waters of dissolved calcium, magnesium, sulfate, strontium, and silica and of alkalinity are controlled by diagenetic reactions occurring within the biogenic sediments of DSDP Sites 572, 573, and 574. Downcore increases in dissolved Sr2 + indicate recrystallization of calcite, and increases in dissolved SiO2 reflect dissolution of amorphous silica. Minor gradients in dissolved Ca(2+) and Mg(2+) suggest little if any influence from reactions involving volcanic sediments or basalt. Differences in interstitial water profiles showing the downhole trends of these chemical species mark variations in carbonate and silica diagenesis, sediment compositions, and sedimentation rate histories among the sites. The location and extent of carbonate diagenesis in these sediments are determined from Sr/Ca distributions between the interstitial waters and the bulk carbonate samples. Pore water strontium increases in the upper 100 to 250 m of sediment are assumed to reflect diffusion from underlying zones where calcite recrystallization has occurred. On the basis of calculations of dissolved strontium production and comparisons between observed and calculated "equilibrium" Sr/Ca ratios of the solids, approximately 30 to 50% of the carbonate has recrystallized in these deeper intervals. These estimates agree with the observed amounts of chalk at these sites. Variations in Sr/Ca ratios of these carbonates reflect differences in calcareous microfossil content, in diagenetic history, and, possibly, in changes in seawater Sr/Ca with time. Samples of porcelanite recovered below 300 m at Site 572 suggest formation at temperatures 20 to 30° C greater than ones estimated assuming oceanic geothermal gradients from sedimentary sections similar to those recovered on Leg 85. The higher temperatures may partially account for higher Sr/Ca ratios determined for recrystallized carbonates from this site.

(Table T1) Paleomagnetic properties of ODP Hole 191-1179D basalts

During Ocean Drilling Program Leg 191, ~100 m of mid-Cretaceous igneous crust was cored at Site 1179 (41.08°N, 159.96°E), located within magnetic Anomaly M8 on the abyssal plain of the northwest Pacific Ocean near Shatsky Rise. Paleomagnetic data from this section are significant because they can constrain the mid-Cretaceous Pacific plate paleolatitude and paleomagnetic pole, both of which can be used to infer tectonic drift and other geodynamic processes. In this study, we analyzed the paleomagnetism of 122 samples from 40 flows in the Site 1179 basalt section. Comparison of inclination data among flows implies 13 independent measurements of the paleomagnetic field. Assuming a reversed magnetic polarity because of the site location within Anomaly M8, the data give a mean paleocolatitude of 88.1° ± 6.8° (corresponding to a paleolatitude of 1.9°N). The paleocolatitude is consistent with other mid-Cretaceous Pacific paleomagnetic data that indicate ~39° northward drift of the western Pacific plate since mid-Cretaceous time. Comparison of observed between-flow colatitude variance with that expected from secular variation data suggests that secular variation may not have been completely averaged with the 13 independent groups sampled at Site 1179. Colatitude scatter in the section is markedly less in the deepest 33 m of the hole, indicating a shift from rapidly erupted flows in the bottom ~33 m of the section to more slowly emplaced flows above.

Chemical composition and age of magmatic rocks from the underwater Hokkaido Rise

Geological and geophysical investigations carried out within the Hokkaido Rise showed that intrusives composing outcrops of the crystalline basement on the ocean floor form a continuous series from monzonites and diorite-monzonites to granites with prevalence of granodiorites with stable mineralogical association: biotite - hornblende - K-feldspar. Acidic volcanic rocks are characterized by a similar mineralogical association with almost complete absence of plagioclase-pyroxene species. It seems that the Hokkaido Rise, as well as the marginal oceanic Zenkevich swell as a whole are not primary oceanic structural formations and have undergone a complex and long history of geological development with intense orogenic movements that occurred in Middle Cretaceous and preceded subalkaline basalt outpouring during postorogenic subsidence of the Earth crust.

(Table 1) Oxygen isotoic composition of interstitial watres from ODP Sites 128-798 and 128-799

Oxygen isotope compositions of the interstitial waters have been measured for 21 samples taken from the depth intervals of 1.5 to 398.9 mbsf at Site 798 (Oki Ridge) and 16.5 to 435.6 mbsf at Site 799 (Kita-Yamato Trough) in Japan Sea. The d18O values decrease with depth from -0.49 to -3.38 per mil (SMOW) at Site 798 and from -0.71 to -4.36 per mil (SMOW) at Site 799 corresponding to an average depletion gradient of -0.8 per mil per 100 m. Material balance calculations reveal that the d18O-variations at Sites 798 and 799 were principally controlled by low-temperature alteration of basement basalt and andesite, resulting in negative shifts in pore water d18O values, and by the polymorphic transformations of biogenic opal-A to opal-CT and opal-CT to microquartz, which tend to increase d18O of interstitial waters. Carbonate diagenesis and ash alteration also caused weakly negative shifts in pore water d18O values.

(Table T1) Permeability, electrical resistivity and density of ODP Hole 193-1188A and Site 193-1189 minicores, PACMANUS field

Magmatic fluids, heat fluxes, and fluid/rock interactions associated with hydrothermal systems along spreading centers and convergent margins have a significant impact on the genesis of major sulfide deposits and biological communities. Circulation of hydrothermal fluids is one of the most fundamental processes associated with localized mineralization and is controlled by inherent porous and permeable properties of the ocean crust. Heat from magmatic intrusions drives circulation of seawater through permeable portions of the oceanic crust and upper mantle, discharging at the seafloor as both focused high-temperature (250°-400°C) fluids and diffuse lower-temperature (<250°C) fluids. This complex interaction between the circulating hydrothermal fluids and the oceanic basement greatly influences the physical properties and the composition of the crust (Thompson, 1983; Jacobson, 1992, doi:10.1029/91RG02811; Johnson and Semyan, 1994, doi:10.1029/93JB00717). During Ocean Drilling Program (ODP) Leg 193, 13 holes were drilled in the PACMANUS hydrothermal system (Binns, Barriga, Miller, et al., 2002, doi:10.2973/odp.proc.ir.193.2002). The hydrothermal system consists of isolated hydrothermal deposits lined along the main crest of the Pual Ridge, a 500- to 700-m-high felsic neovolcanic ridge in the eastern Manus Basin. The principal drilling targets were the Snowcap (Site 1188) and Roman Ruins (Site 1189) active hydrothermal fields. Samples from these two sites were used for a series of permeability, electrical resistivity, and X-ray computed tomography measurements.

Chemical composition of volcanic ash layers from ODP Leg 134 sites

Volcanic ash layers (1-3 cm thick) are abundant in the North Aoba Basin drill sites but less common at forearc sites. Ash deposited on the forearc slopes is liable to be redistributed as turbidites. In addition, the westerly upper winds also minimize ash-fall on the western (forearc) side of the New Hebrides Island Arc. Crystalline components in the ashes are primarily plagioclase (An90-An44), clinopyroxene (Ca46Mg49Fe5-Ca43Mg33Fe24), olivine (Fo87-Fo62), and titanomagnetite. There are also small amounts of orthopyroxene, magnetite, apatite, and quartz. Glass shards occur in most of the ashes and range in composition from basalt to rhyolite. There is often a variety of glass compositions within a single ash layer. One explanation for this is that the rate of accumulation of ash from several different eruptions or eruptive phases exceeded the background sedimentation rate: there may also have been a certain amount of reworking. The high-K and low-K trends previously recognized in volcanic rocks from the New Hebrides Island Arc are clearly represented in the Leg 134 glasses. All of the ashes investigated here are thought to have originated from the Central Chain volcanoes. The source of the high-K group was probably the Central Basin volcanoes of Santa Maria, Aoba, and Ambrym. The lower-K series includes a distinctive group of dacites and is likely to have originated from the Epi-Tongoa-Tongariki sector of the arc where major pyroclastic eruptions, associated with caldera collapse, have occurred during the Holocene, perhaps as recently as 400 yr ago.