168 Preliminary Report

Hydrothermal Transition Transect (Sites 1023, 1024, and 1025)

Setting of the Hydrothermal Transition Transect
A fundamental change in the nature of hydrothermal circulation occurs as sediment thins between the ridge crest and the low basement ridges that approach the seafloor 20 km to the east of the ridge. Within 20 km of the ridge, basement is exposed to the overlying ocean or covered by a relatively thin (less than 1 m to a few tens of meters), discontinuous veneer of hemipelagic sediment through which fluids can pass with little hydrologic impedance. To the east of this area of extensive igneous outcrop, the igneous crust is blanketed continuously by turbidite sediments that create a hydrologic barrier. Heat flow and estimated upper crustal temperatures, seismic velocities in the upper crust, and estimates of basement pore-fluid compositions all indicate lateral gradients associated with the transition from open to sealed hydrothermal circulation. Heat flow and estimated basement temperatures increase systematically to the east, away from the area of exposed basement. Estimated temperatures in the upper igneous crust increase from less than 10°C near where basement rocks outcrop to about 40°-45°C another 20 km to the east (40 km from the ridge). Over this same distance the average heat flow increases from less than 15% to more than 80% of the value expected for the underlying lithosphere.

Basement pore-fluid compositions, estimated from sediment pore-fluid studies, are close to seawater at 20 km from the ridge, but are strongly depleted in magnesium and enriched in calcium at 40 km from the ridge. The basement fluids 20 km east of the point of burial have considerably higher inferred chlorinities than seawater. Part of the increased chlorinity may be due to ongoing hydration reactions in the crust. This is consistent with the increase in basement temperature and with the seismic data, which also reveal a systematic change. Interval velocities determined for the upper crustal seismic Layer 2A increase from values ranging from 3000-3500 m/s to values exceeding 5000 m/s over the same spatial interval of about 20 km. Although these velocities were determined for a layer known to have strong vertical velocity gradients, they probably indicate a significant increase in velocities throughout Layer 2A. The increase is believed to indicate a decrease in porosity and bulk modulus resulting from alteration. A similar magnitude of change occurs on other ridge flanks, although at a much slower rate, probably because the hydrologic isolation of the upper igneous crust is normally much more gradual.

Goals of the Hydrothermal Transition Transect
To document changes in basement fluid temperatures and compositions, the physics of fluid flow, and alteration of the crustal rocks, a transect of holes was completed across this hydrothermal transition. Drilling was directed toward solving numerous fundamental questions about lateral fluid and heat transport and about the physical and chemical alteration of the crust that results from water/rock interaction. Specific questions include the following:

1. How does chemical and thermal transport take place over distances of 10-20 km in sediment-covered igneous crust? If there is a net horizontal transport of fluid, what is the rate?
2. What is the source and magnitude of the pressure gradient that drives the flow?
3. How do the changes in fluid chemistry and temperature with distance from sediment free areas affect the nature of rock alteration?
4. What is the dominant factor responsible for the increase in upper crustal velocity?
5. Is there an accompanying decrease in permeability?
6. At what rate does the alteration take place?

It was anticipated that if relationships could be determined between basement temperature, fluid chemistry, and crustal alteration, then the results of studying this simple hydrothermal transition zone could be generalized to other ocean basins.

Scientific Results
Drilling at Sites 1023, 1024, and 1025 (
Fig. 3) recovered similar sequences of rhythmic sand and silt turbidites (lithologic Unit I) (Fig. 4). Most individual beds are a few centimeters to 1 m thick and are interbedded with hemipelagic mud typically tens of centimeters to 1 m thick. These sequences were deposited above an interval of turbidite-free hemipelagic mud, ranging from 3 to 20 m thick (Unit II), which was, in turn, deposited directly over igneous basement (Unit III). The total thickness of Unit I is 190, 150, and 87 m at Sites 1023, 1024, and 1025, respectively. Calcareous nannofossils sampled from the hemipelagic layers show the following age correlations: base of Emiliania huxleyi (~0.28 Ma) at roughly 43 meters below seafloor (mbsf) (Site 1023), 30 mbsf (Site 1024), and 34 mbsf (Site 1025); top of Pseudoemiliania lacunosa (~0.46 Ma) at base of sediments (192.46) mbsf at Site 1023; and Site 1025 does not contain P. lacunosa, thus the basal sediments are <0.46 Ma. Site 1024 contains the Brunhes/Matuyama magnetic polarity reversal (0.78 Ma) at 166.2 mbsf. Basement ages of Sites 1023, 1024, and 1025 from seasurface magnetic anomalies are 0.87, 0.99, and 1.27 Ma, respectively, which indicates that sedimentation did not begin for several thousand years after basement formation.

Basement was intersected at 192.8, 167.8, 97.5 mbsf, in Holes 1023A, 1024B, and 1025B, respectively. In Holes 1023A, 1024B, and 1025B, 1 m of basement was cored in each hole with the extended core barrel (XCB) coring system and 6 to 16 unoriented pieces of fragmented pillow basalt were recovered. Basement was also recovered from reentry Holes 1024C and 1025C. Rotary core barrel (RCB) coring in Hole 1024C penetrated only 2 m and produced 6% recovery, whereas coring in reentry Hole 1025C extended from 106.1 to 147.2 mbsf and recovery was 36.9%.

Recovery from the XCB holes yielded many basalt pieces with thin unaltered glass rims. Phenocryst contents (plagioclase, olivine, and clinopyroxene) vary 1%-2% in the basalt from Hole 1023A but <1% in the aphyric basalt from Holes 1024B and 1025B. All the basalt is vesicular (<1% vesicles, diameters <1 mm). Two types of volcanic units were identified from RCB coring of the reentry holes: pillow lava and massive basalt. Pillow basalts occur in Hole 1024C. A succession of massive basalts (the total recovery from Hole 1025C) occurs structurally below the Hole 1025B pillows. Rare glass rims (separated by about 1 to 15 m) occur in the massive basalt units. Phenocryst contents are <1% plagioclase plus clinopyroxene phenocrysts in the aphyric basalts sampled in Hole 1024C and the massive basalt from Hole 1025C. The plagioclase phenocrysts range in size up to 5 mm long, whereas the mafic phenocrysts are generally is is less than or equal to 1 mm in diameter. The pillow basalts have <1% vesicles. The massive basalts have variable vesicularity, up to 12%, and are moderately altered (5%-25%).

Whole-rock major-element analyses show the basalts from Site 1024 are low-K tholeiites with Mg/(Mg + Fe2+) * 100, or Mg#, of 60-62, whereas, those from Site 1025 are ferrobasalts (Mg# = 46-49) with TiO2 equal to 2.43 to 2.70 wt% and Fe2O3 equal to 14.50 to 15.91 wt% (with all iron calculated as Fe2O3). These Site 1025 rocks are the most fractionated rocks known from the Endeavour segment of the Juan de Fuca Ridge.

All basement rocks from Sites 1023, 1024, and 1025 exhibit alteration, which is manifested in several ways and varies between sites. The presence of fresh olivines in Hole 1023A indicates that these samples have undergone the least amount of alteration. In contrast to Hole 1023A, practically all olivines from Site 1025 are completely replaced by a mixture of clay minerals (primarily saponite with some celadonite), minor carbonate, and rare talc and chlorite/smectite. Sample interiors vary from fresh (defined as <2% replacement by alteration minerals) in most of the pillows to slight tomoderate (5%-25% alteration minerals) in the massive ferrobasalts.

Many pieces from Site 1024 contain centimeter-scale alteration halos just inside their outer surface. In hand specimen these halos have orange, yellow, and dark green material filling the vesicles. Microscopic observations suggest that these vesicle fillings include celadonite, saponite, and iron oxyhydroxides. Halos are rare in rocks from Hole 1023A and Site 1025.

Nearly all basement samples from Sites 1023, 1024, and 1025 possess incomplete clay coatings on their outer surfaces. The clay minerals are usually a light blue to blue-green color in hand specimen, and X-ray diffraction indicates that these clays are trioctahedral smectites. In nearly all basement samples from Sites 1023, 1024, and 1025, the vesicles (excluding those within alteration haloes) are completely lined by a fine-grained blue clay, which is probably the same as that observed on the sample surfaces. In microscopic sections, this mineral is identified as saponite. Some basement samples from Sites 1023, 1024, and 1025 exhibit zeolites associated with the blue clay mineral on rock surfaces and within vesicles. Pyrite occurs as a trace mineral associated with saponite on many basement pieces and inside vesicles from all HT sites. Several vein types occur at Site 1025; 111 veins were logged in Hole 1025C, including: clay (90 veins), clay + talc (10), clay + carbonate (6), clay + pyrite (1), talc (1), and quartz + clay (3). Most veins are is less than or equal to 1 mm wide.

Basalts from the HT transect illustrate progressive changes in alteration intensity related to increasing basement temperature from Sites 1023 to 1024 to 1025 and variations in lithology. Higher basement temperatures and coarser grain sizes combine to produce larger degrees of hydrothermal alteration, from a fraction of 1% in aphanitic pillow basalts that have a temperature of 15.5°C, up to 25% alteration in fine-grained massive basalts in basement that have a temperature of 38.2°C at the basement/sediment interface. There is no evidence from the alteration features that these rocks ever experienced higher temperatures than those measured on this leg.

Methane is the dominant volatile hydrocarbon gas in the cored sediments and shows a strong negative correlation with pore-water sulfate concentrations at all sites. Sedimentary organic matter is low in organic carbon and has C/N ratios indicating a predominately marine origin. The organic matter is hydrogen poor, reflecting selective degradation of less stable marine organic compounds.

The composition of pore water at the three sites shows clear indications of reaction and diffusion in the sediment section. Vertical advection was evident only in Hole 1025B, where slow upward seepage of water is indicated by a systematic variation in profiles of Ca, Mg, and alkalinity (Fig. 5). Some degree of superhydrostatic pressure in basement at this site was also indicated by the presence of discharge from Hole 1025C, which was drilled through the sediments and ~5 m into upper basement. This was seen with the vibration-isolated television (VIT) camera at the time of reentry with the 10-3/4" casing string.

Basement-water compositions, inferred from pore-fluid compositions in basal sediments, reveal systematic changes with increasing temperature and distance from the outcrop in some elemental concentrations. Changes are most apparent in magnesium and calcium, which are particularly reactive in basement. Concentrations at Hole 1023A were 48-mM Mg and 12-mM Ca, only slightly different from seawater at 53-mM Mg, and 10.6-mM Ca. At Site 1025, the site farthest from outcrop, Mg fell to 28 mM and Ca rose to 37 mM. Chlorinity values are perhaps most interesting. The commonly observed increase from present-day seawater values of 555 mM at the seafloor to elevated values of relic Pleistocene water at depths of a few tens of meters below seafloor was seen at all three sites. Values decreased at greater depths, however, back to values identical to present-day seawater in Holes 1023A and 1024B and to only a slightly elevated value (560 mM) in Hole 1025B. A simple interpretation of these low chlorinity values is that the water has a residence time in basement of only a few thousand years, even at the site most distant from outcrop.

Sediment physical properties along the Hydrothermal Transition Transect generally reflect increasing compaction with depth and variations in sand, silt, and clay contents. Finer grained sediments exhibit a consistent decrease in porosity and increase in bulk density with depth, whereas coarser grained units do not. Sediment thermal conductivities are dominated by differences in sand content and in the finer grained parts of the section by compaction .

Sediment magnetic susceptibility was measured continuously with the multisensor track (MST). This permitted the identification of sandy layers in cores before they were split, facilitating the collection of whole-round samples for later permeability and thermal conductivity analyses. The massive basalts recovered from Hole 1025C have porosities below 10 vol%, P-wave velocities above 5 km/s, and thermal conductivities of about 1.8 W/(mK).

Temperatures were measured in the sediment section with the goal of obtaining accurate determinations of the temperature of the sediment/basement interface, which is inferred to be a primary hydrologic contact between the low-permeability sediment cover and high-permeability extrusive igneous rocks beneath. Temperatures at the contact increased systematically with distance from the region of outcropping basement, from approximately 15.5°C at Hole 1023A, to 22.4°C at Hole 1024B, and to 38.2°C at Hole 1025B (Fig. 6). All temperature-depth profiles are approximately linear, indicating conductive heat transport through the sediment column.

Holes 1024C and 1025C were designed as reentry CORK holes. These holes were cased and cemented into basement. After curing, the cement at the base of casing was drilled out, and additional coring and drilling were continued into basement. Permeablility measurements of upper basement were attempted by setting the drillstring packer in casing in Hole 1025C, but the packer could not be set and no data were obtained. Packer measurements were successfully completed in basement in Hole 1024C, however, and preliminary examination of the data suggests that basement at this site is relatively permeable. The consistency of basement lithologies and drilling difficulties between Sites 1024 and 1025 suggest that upper basement in Hole 1025C is also permeable.

Instrumented CORK systems were deployed in both Holes 1024C and 1025C (Fig. 7)Each instrument package comprises a long-term data logger, two absolute pressure gauges (one for sealed-hole pressures and one for seafloor pressures), a continuously operating osmotic fluid sampler, and 10 thermistor temperature sensors, including several thermistors positioned within basement in each hole. The first data from these instruments will be retrieved during summer 1997 using the remotely operated vehicle Jason.

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