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8. Sediment Petrology of the Hydrothermal Mounds1

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8. Sediment Petrology of the Hydrothermal Mounds1 8. SEDIMENT PETROLOGY OF THE HYDROTHERMAL MOUNDS1 Peter E. Borella,2 Deep Sea Drilling Project, Scripps Institution of Oceanography, La Jolla, California and Richard Myers3 and Bill Mills, Deep Sea Drilling Project, Scripps Institution of Oceanography, La Jolla, California ABSTRACT Sediments in the area of the Galapagos hydrothermal mounds are divided into two major categories. The first group, pelagic sediments, are nannofossil oozes with varying amounts of siliceous microfossils. The second group are hydrothermal sediments consisting of manganese-oxide crust fragments and green nontronitic clay granules. Hydro- thermal sediments occur only in the upper half to two-thirds of the cores and are interbedded and mixed with pelagic sediments. Petrologic evidence indicates that hydrothermal nontronite forms as both a primary precipitate and as a re- placement mineral of pre-existing pelagic sediment and hydrothermal manganese-oxide crust fragments. In addition, physical evidence supports chemical equations indicating that the pelagic sediments are being dissolved by hydrother- mal solutions. The formation of hydrothermal nontronite is not merely confined to the surface of mounds, but also oc- curs at depth within their immediate area; hydrothermal nontronite is very likely forming today. Geologically speaking, the mounds and their hydrothermal sediments form almost instantaneously. The Galapagos mounds area is a unique one in the ocean basins, where pelagic sediments can be diagenetically transformed, dissolved, and replaced, possibly within a matter of years. INTRODUCTION In addition to the DSRV Alvin dives and these sur- face-sediment and near-surface studies, Leg 54 of the The Galapagos Spreading Center and hydrothermal Deep Sea Drilling Project (DSDP) drilled four holes in mounds area located in the Panama Basin (Fig. 1) has a mounds field located on crust approximately 0.62 become one of the most densely surveyed and studied m.y. old. The purpose was to conduct "drilling trials" areas in the oceans today. First discovered by Klitgord along the Galapagos Spreading Center and to determine and Mudie in 1972 on a "deep tow" survey, the mounds whether a proposed two-leg drilling program in the Gala- field occurs 18 to 32 km south of the Galapagos Rift at pagos was feasible in light of the difficulties already en- 86°W longitude, on crust 0.5 to 0.9 million years old countered in drilling young ocean crust with very thin (Klitgord and Mudie, 1974). The field extends approxi- sediment cover in the immediate area. Aside from this, mately 27 km in an east-west direction. Most mounds major questions addressed by Leg 54 scientists regard- occur in rows or chains and are parallel, or nearly so, to ing the mounds were as follows: (1) the origin of the the axial trend of the Galapagos Rift. However, isolated Galapagos mounds and their relationship to hydrother- mounds also exist. Many mounds are located above small mal circulation patterns in young crust, and (2) the in- vertical displacements in the basement (Lonsdale, 1977). ternal stratigraphy and composition of the mounds (Ros- They range in height from less than 1 meter to more endahl, Hekinian, et al., 1980). In addition to the Initial than 20 meters, with diameters between 5 and 50 meters. Reports of DSDP Leg 54, Natland et al. (1979), and The smaller mounds (<4 m) have slopes less than 30°, Schrader et al. (1980) have published articles concerning and the higher mounds (>4 m) have slopes up to verti- the mineralogy, stratigraphy, and chemistry of the hy- cal (Williams et al., 1979). A shallow moat less than 5 drothermal and pelagic sediments of the Galapagos meters deep surrounds many of the mounds (Lonsdale, mounds field. 1977). Utilizing all of this previous work, along with a de- Heat-flow measurements (Sclater and Klitgord, 1973; tailed drilling plan using the newly developed hydraulic Williams et al., 1974; Crane and Normark, 1977; and piston corer, DSDP Leg 70 drilled four sites (Sites 506- Williams et al., 1979), bathymetric and deep-tow sur- 509) in the Galapagos hydrothermal mounds area (Fig. veys (Klitgord and Mudie, 1974; Lonsdale, 1977), and 1). Our major intent was to address specific questions geochemical studies of the hydrothermal-vent solutions regarding the origin and spatial relationships of the and dredge hauls (Corliss et al., 1978, 1979; Williams et mounds field and sediments. The sites we drilled ranged al., 1979; Edmond et al., 1979) have shown that the from high heat-flow areas (8-23 HFU) with and without mounds field and adjacent regions are areas of intense mounds to a relatively low heat-flow area (3-5 HFU). hydrothermal activity. (For a detailed analysis and discussion of heat-flow stud- ies, see Sites 506-509 reports and Becker et al., this vol- ume.) In these four localities sediment thickness ranged Honnorez, J., Von Herzen, R. P., et al., Init. Repts. DSDP, 70: Washington (U.S. from 30 to 52 meters. A fifth site (Site 510) was drilled Govt. Printing Office). in an area of relatively high heat flow on crust estimated 2 Saddleback College, Mission Viejo, California. ' Present address: Geological Research Division, Scripps Institution of Oceanography. to be 2.7 m.y. old. Sediment thickness here was 115 me- 197 P. E. BORELLA, R. MYERS, B. MILLS 86°06'W N H E Site 506 12 kHz pinger survey Mounds crossed on pinger survey Chain of cones and ridges Major fault 20-50 m throw Scanning sonar track 86°07"W Site 507 (m) (ft.) ' 914- 3000 / _ / i-U u '"IMU I 507C L 610- 2000 U-LU.L14j.ljjj, ? 507-4 « - 1-507A,B i I. li'"~ -= ^ j^uT 07Hs 1 Beacon - - 507 507AV>- I 305- 1000 - \ \ 1•': ?-'.'.- 0°35'N - pit -5θff ç>07D' • 507E \ - - ^•5071 Beacon( 0 0 Enlarc ed 1 km view of Site Survey Track \ C D Figure 1. A. General site locality map, DSDP Leg 70. B. Detailed survey, Site 506. C. Detailed survey, Site 507. D. Detailed survey, Site 509. 198 PETROLOGY OF MOUNDS SEDIMENTS ters. Honnorez et al. (1981) discussed the preliminary 7) The remainder of the vial is filled with med-cast. results of our findings concerning the nature and origin 8) The sample and resin remain at room temperature for 12 hr. to of the hydrothermal mounds. allow penetration. 9) The vial is placed in an oven at 60 to 62°C for 12 hr. to cure the PURPOSE sample. The second method, which has proved to be less time consuming To date, no detailed petrologic or petrographic analy- and just as useful, is very simple. sis of hydrothermal-mounds sediments has been per- 1) A small amount of sample is placed in distilled water and agitated with an ultrasonic vibrator until most of the grains are in formed. Most work has concentrated on bulk chemical, suspension. X-ray diffraction, electron microprobe, and electron 2) Prepared, plastic-coated, nonmetal-bearing nylon grids used in microscope analyses (e.g., Hoffert et al., 1980; Schrader TEM work are inserted into the top of the suspension momentarily et al., 1980; Donnelly, 1980; and Dymond et al., 1980, and then removed. to name a few). The purpose of this chapter is to de- 3) Excess water is drawn off the grit. 4) Grids with the fine-grained sediment are allowed to dry at room scribe the petrologic and petrographic characteristics of temperature. the mounds sediments primarily from hand specimens 5) The samples are ready for viewing on the TEM. and thin sections. Scanning transmission electron mi- croscopy (STEM) and elemental analysis, along with X- RESULTS ray diffraction analysis, will be used to support our Sediments in the Galapagos hydrothermal mounds findings and suppositions. In addition, we hope to pre- area are assigned to two major categories. The first sent sufficient physical evidence to support or refute ex- group, pelagic sediments, are essentially nannofossil isting and proposed models of the origin and distribu- oozes with varying amounts of siliceous microfossils tion of the mounds sediments. (diatoms and radiolarians) and foraminifers. The sec- ond sediment group includes hydrothermal sediments PROCEDURES which are subdivided into manganese-oxide and/or oxy- A total of 253 sediment samples were selected for study. The sam- hydroxide-crust fragments and green nontronitic clay pling program consisted of taking samples at 10-cm intervals across transitional and stratigraphic boundaries. Each set of samples con- granules. sisted of two pure end members, hydrothermal sediments and pelagic ooze with transitional and/or mixed samples in between. Pelagic Sediments From these sets, 125 petrographic thin sections were made. The Megascopically, the pelagic sediments are very fine- technique developed to impregnate these sediment samples, especially the expandable hydrothermal clays, though painstaking and unique, grained biogenic oozes. The colors of the oozes vary, proved to be quite successful. For this reason, the procedure is out- with the most dominant colors being a light greenish lined below: gray (5GY 8/1), greenish gray (5GY 6/1), and light olive 1) Samples are placed in a small aluminum disposable dish and gray (5Y 6/1). However, the mudline in every core is dried in an oven at 72 °C for 72 hr. marked by a grayish brown (5YR 3/2) to moderate 2) Samples are impregnated and potted with Petropoxy #154 and placed in an oven at 105 °C overnight. brown (5YR 3/4) pelagic ooze. The brown color of this 3) The resulting billet is ground flat, first with 120 and then with uppermost layer is attributed to the presence of Fe and 400 silicon carbide grit. Mn which have migrated upward through the sediments 4) The flat side of the billet is mounted to a glass slide with Devcon and become oxidized (Lynn and Bonatti, 1965).
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