Comparison of Chemical and Magnetic Stratigraphy of Holes 396 and 396B

Natland, James H.

doi:10.2973/dsdp.proc.46.134.1979

Cited by 3 publications

(5 citation statements)

References 8 publications

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“…In nearby Hole 504B, a similar variation in magnetic inclination occurs beneath the 56-m-thick Unit 1, where inclinations fit those predicted for the site, and this likewise may reflect tilting of the deeper section before extrusion of the uppermost units (Furuta and Levi, 1983;Allerton et al, this volume). This record of tilting of the uppermost crust during accretion at the axis is commonly observed in other DSDP drill cores (e.g., Dmitriev, Heirtzler, et al, 1978;Melson et al, 1978;Natland, 1979;Donnelly, Francheteau, Bryan, Robinson, Flower, Salisbury, et al, 1979). General decreases of natural remanent magnetization intensity and magnetic susceptibility below -350 mbsf in Hole 896A are similar to trends observed in the volcanic section of Hole 504B, and reflect a general increase in the abundance of massive units having larger grain sizes downward in both holes (Al- Adamson (1985) and Alt, Kinoshita, Stokking, et al (1993).…”

Section: Lithostratigraphy Of Holes 504b and 896asupporting

confidence: 69%

Ridge-Flank Alteration of Upper Ocean Crust in the Eastern Pacific: Synthesis of Results for Volcanic Rocks of Holes 504B and 896A

Alt¹,

Teagle²,

Laverne³

et al. 1996

Proceedings of the Ocean Drilling Program, 148 Scientific Results

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The lithostratigraphy and alteration of volcanic basement from Holes 504B and 896A, located in different parts of a ridge flank circulation cell, are summarized and compared. The 290-m-thick volcanic section of Hole 896A is located on a basement high that coincides with high heat flow and upwelling fluids. The 571.5-m-volcanic section of Hole 504B is located ~l km away in an area of ambient heat flow. Subtle differences in lithostratigraphy include slightly greater proportions of massive units and fewer pillow basalts in Hole 896A than in Hole 504B. The volcanic sections are geochemically similar, but there is no direct correlation of lithologic or geochemical units between the two sites. Veins are comparable in abundance in the two sections (-30 veins/m, mean vein width < 1 mm), but carbonate veins and thick (>2 mm) saponite and carbonate veins are more abundant in Hole 896A than in Hole 504B. Permeabilty values of the upper basement sections in Holes 896A and 504B are similar (~IO~1 3 to 10~1 4 m 2 ), suggesting that the upper -200 m of basement is sufficiently permeable on a regional scale to support circulation of seawater through basement.Alteration effects in basement from Hole 896A are similar to those in the upper 320 m of volcanic rocks in Hole 504B. These include celadonitic phyllosilicates in fractures and alteration halos and reddish, Fe-oxyhydroxide-rich alteration halos along fractures. Dark gray rocks, characterized by the presence of saponite ± carbonate ± pyrite occur throughout Hole 896A and the entire Hole 504B volcanic section. Alteration reflects evolution from open circulation of cold, oxidizing seawater, to more restricted circulation of seawater caused by burial of the crust by sediments and sealing of fractures with saponite. Latestage carbonates and minor zeolites formed in veins throughout both holes from reacted seawater fluids (decreased fluid Mg/ Ca). Oxygen and strontium isotopic evidence indicate an early generation of carbonates in both holes that formed at relatively low temperatures (~25°-35°C) during open circulation, whereas later carbonates in Hole 896A formed at slightly higher temperatures (~50°-70°C) during more restricted circulation, possibly similar to the present ridge flank hydrothermal upflow conditions (~50°-80°C).Chemical changes in altered upper crust include oxidation, increased alkalis, Mg, CO 2 , and H 2 O; local uptake of P; elevated δ l8 θ, δD, δ"B, and 87 Sr/ 86 Sr; and lower S contents and δ 34 S. The greatest chemical changes occur in alteration halos and breccias, and the smallest chemical changes occur in the lower volcanic section of Hole 504B. Secondary minerals filling fractures and cementing breccias are sites of uptake of Mg, CO 2 , and H 2 O, in addition to changes occurring in altered rocks.

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Section: Lithostratigraphy Of Holes 504b and 896asupporting

confidence: 69%

Ridge-Flank Alteration of Upper Ocean Crust in the Eastern Pacific: Synthesis of Results for Volcanic Rocks of Holes 504B and 896A

Alt¹,

Teagle²,

Laverne³

et al. 1996

Proceedings of the Ocean Drilling Program, 148 Scientific Results

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“…The effective permeability of particular fault or fracture zones within crystalline rock also depends on fracture roughness [e.g., Brown, 1987], the relation between fault geometry and ambient stress field [e.g., Tsang and Witherspoon, 1981;Bruhn, 1994;Barton et al, 1995Barton et al, , 1996 [Peterson et al, 1986;Gallahan and Duncan, 1994]. While the lateral extent of variability in alteration is difficult to assess from seafloor cores, except in the few locations where upper crustal sections have been sampled at adjacent sites [Natland, 1979;Muehlenbachs, 1980;Alt et al, 1996], ophiolites provide opportunities to map out two-and three-dimensional alteration patterns [e.g., Robertson, 1988, 1990;Haymon et al, 1989; Valsami-Jones and Cann, 1994] and, together with seafloor samples, help to define consistent trends in upper crustal alteration and associated water-rock interaction.…”

Section: Additional Geological Geophysical and Crustal Alteration Cmentioning

confidence: 99%

Permeability within basaltic oceanic crust

Fisher¹

1998

Reviews of Geophysics

340

320

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Abstract.Water-rock interactions within the seafloor are responsible for significant energy and solute fluxes between basaltic oceanic crust and the overlying ocean. Permeability is the primary hydrologic property controlling the form, intensity, and duration of seafloor fluid circulation, but after several decades of characterizing shallow oceanic basement, we are still learning how permeability is created and distributed and how it changes as the crust ages. Core-scale measurements of basaltic oceanic crust yield permeabilities that are quite low (generally 10 -22 to 10 -•:7 m2), while in situ measure-

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“…Elsewhere I have defined "rules of stratigraphic order" (Natland, 1978) to deal with basalt drill-hole chemical and magnetic stratigraphy, but Site 332 presents an unusual array of problems. Without biostratigraphic information, we cannot be talking rigorously about timestratigraphic units.…”

Section: Correlations Between Holes and The Problem Of Replication Ofmentioning

confidence: 99%

“…Subtly but distinctly different magma types may have been part of the same eruptive event, may even have had different parents, may have been hybrids. Elsewhere (Natland, 1978), I have stated that "similar but not identical chemical units may have been part of the same eruptive sequence if, for example, magmas were being supplied simultaneously from more than one source, or if they reached the surface through different conduit systems. Factors other than crystal fractionation (such as magma mixing, fractional resorp-tion of minerals, especially Plagioclase, zone refining, local source heterogeneities, leaving various proportions of residual liquids behind, etc.)…”

Section: Correlations Between Holes and The Problem Of Replication Ofmentioning

confidence: 99%

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Comparison of the Chemical and Magnetic Stratigraphy of DSDP Sites 332 and 395

Natland¹

1979

Initial Reports of the Deep Sea Drilling Project

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Deep drilling into oceanic basement at Sites 332 (36° 53 'N, 33°3 9 "W; 583 m) and 395 (22 ° 45 'N, 46 ° 05 'W; 572 m) provides an opportunity to compare the origin of a major portion of Layer 2 at two places on the Mid-Atlantic Ridge. The sites are similar in that basement at each consists dominantly of relatively few chemically distinct basaltic units (18 in Holes 332A and 332B; 9 in Holes 395 and 395A). These units occur as composites of basaltic pillow lavas, thin flows, and glassy breccia zones as thin as a few meters and as thick as 209 meters. Only two closely spaced dolerite units, totaling 15 meters thick (in Hole 395A), have been confirmed as intrusive at either site. Holes 395 and 395A also contain gabbroic and ultramafic cobbles and boulders, interpreted as talus, both above and below the topmost aphyric basal unit. No plutonic rocks were cored at Site 332.The chemical groups at both sites can be divided into strongly phyric types and moderately, sparsely, or aphyric types (less phyric types). The first group contains 15 to 25 per cent or more of phenocrysts of olivine, Plagioclase, clinopyroxene, or combinations of these minerals. Many of the phenocrysts appear to have accumulated by mechanical processes (gravity settling or floating, and flow differentiation). The proportion of strongly phyric basalts in both Hole 395A and Hole 332B, curiously, is 38 per cent. Among the phyric basalt types in Hole 332B are olivine-rich basalts (oceanites), representing about 5 per cent of the interval cored. All other phyric basalts at both sites have Plagioclase as the predominant phenocryst, with lesser olivine and/or clinopyroxene.For the most part, the less phyric basalt types cannot be related to each other, or to phyric basalts, by crystal fractionation involving phases present in the rocks. Instead, chemical differences appear to have resulted largely from variations in the extent of melting in the mantle. All basalts are low-K and -Ti mid-ocean ridge basalts, but those of Site 332 are typically lower in Ti, Zr, and other low-partitioncoefficient elements than those of Site 395. They thus appear to represent greater partial melts of the mantle than those of Site 395. Basalts of Site 395, however, appear to derive from a distinctly more depleted mantle than those of Site 332, and have La/Sm of about 0.8. At Site 332, less phyric basalts and most strongly plagioclasephyric basalts have La/Sm of about 2.2. The olivine-rich basalts and oceanites have La/Sm of about 1.5.At Site 332, there is evidence for minor crystal fractionation and accumulation between some less phyric types that erupted more or less simultaneously. Less phyric and strongly phyric types are interbedded repeatedly at intervals of tens of centimeters to meters in the deeper parts of Hole 332B. Apparently, either these came from different sources and magma chambers at essentially the same time, or the interval has been repeated numerous times by faulting. A third possibility is that the phyric basalts are closely spaced narrow intrusive...

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Comparison of Chemical and Magnetic Stratigraphy of Holes 396 and 396B

Cited by 3 publications

References 8 publications

Ridge-Flank Alteration of Upper Ocean Crust in the Eastern Pacific: Synthesis of Results for Volcanic Rocks of Holes 504B and 896A

Ridge-Flank Alteration of Upper Ocean Crust in the Eastern Pacific: Synthesis of Results for Volcanic Rocks of Holes 504B and 896A

Permeability within basaltic oceanic crust

Comparison of the Chemical and Magnetic Stratigraphy of DSDP Sites 332 and 395

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