TABLE AND FIGURE CAPTIONSTable Captions
Table 2. Site summary information, Leg 193.
Figure 1. Major active hydrothermal sites at convergent margins of the western Pacific Ocean. Large arrows = plate movements.
Figure 2. Regional tectonic setting of the PACMANUS site drilled during Leg 193. The Manus Basin occupies a backarc position relative to present-day subduction on the New Britain Trench to its south. Creation of new oceanic crust occurs at the Manus spreading center and at smaller segments to its west. Major transform faults are somewhat oblique to the spreading segments. The eastern Manus rift zone is a pull-apart structure between two of the major transform faults. It is underlain by thinned older Tertiary arc crust, equivalent to exposures on New Ireland to the north and New Britain to the south. This older crust was generated during subduction on the now inactive Manus Trench. Active volcanoes of the Bismarck arc, above the New Britain subductionBenioff zone are indicated; submarine volcanism in the eastern Manus rifts lies well off the trend of this chain. Yellow boxes = known hydrothermal sites. Thick blue arrows = plate motions. Curved blue arrows = the sense of rotation on microplates as defined by Global Positioning System geodesy (Tregoning et al., 1998) or by the opening and westward propagation of the Woodlark Basin (Taylor et al., 1995).
Figure 3. Tectonic model for the Manus Basin (adapted from Martinez and Taylor, 1996). About 80 km of extension by low-angle normal faulting and crustal thinning has occurred in the Eastern Manus Basin between the Weitin and Djaul transform faults. The same amount of movement occurred on the Willaumez transform fault, where a slight obliquity between extension direction and fault strike allowed volcanism in the extensional transform zone. Between the Willaumez and Djaul transforms, equivalent movement was accommodated by wedge-shaped opening of the Manus spreading center and compensating counter-clockwise rotation of the Manus microplate. Mid-ocean-ridge basin (MORB)-type basaltic volcanism dominates the Manus spreading center, the extensional transform zone, the East Sherburne Zone that overlies a sediment basin, and limited activity in the Southern Rifts. By contrast, the Eastern Manus Basin is dominated by arc type volcanism.
Figure 4. Bathymetry of the Eastern Manus Basin from multibeam data compiled by Institut Francais de Reserche pour l'Exploration de la Mer (IFREMER). The southeast-trending Djaul transform is conspicuous. The northeast-trending deep on the western side is a failed spreading segment. The PACMANUS site lies at the crest of a northeast-trending ridge of dacite (Pual Ridge).
Figure 5. High-resolution single channel seismic profile of (A) the section across Pual Ridge at the position of the PACMANUS hydrothermal field and (B) interpretation. All Leg 193 sites lie on the crest of Pual Ridge. Data from the Sonne cruise 94, Leg 2, single channel seismic line 07.
Figure 6. Seafloor geology of the Eastern Manus Basin. Edifices of the Eastern Manus Volcanic Zone, which extends between the active ends of the Djaul and Weitin transform faults, range from picritic basalt to rhyodacite in composition. Red dots = known hydrothermal sites, including the three main active sites of PACMANUS, Deep Sea Monitoring System (DESMOS), and Susu Knolls. Red lines = extensional fault scarps.
Figure 7. Distribution of hydrothermal deposits within the PACMANUS field along the crest of Pual Ridge, with the names assigned to active sites containing massive sulfide chimneys. Based on bottom-tow photography and submersible dive observations from the PACMANUS cruises (Frankin, 1991, 1993, 1996, 1997), EDISON-I cruise (Sonne, 1994), ManusFlux cruise (Yokosuka, 1995), BIOACCESS cruises (Natsushima, 1996, 1998), and KODOS'99 cruise (Onnuri, 1999).
Figure 8. Calculated course of vibration-isolated television (VIT) survey during the site survey prior to spudding of Hole 1188A and observed seafloor character. Bathymetric contours (5-m intervals) within the survey area are based on VIT cable measurement and sonar altimetry, calibrated to the actual drill pipe measurement depth of Hole 1188A. Contours outside the survey area are derived from submersible dives and have been added to show the general shape of Snowcap Knoll. Only the track from the initial survey is shown, but additional seafloor cover information from other surveys has been added. Global Positioning System averaged locations for all of the ODP holes at Site 1188 are also shown.
Figure 9. Key to lithology symbols used in the graphic summary log.
Figure 10. A. Graphic summary log for Hole 1188A showing the lithologic characteristics of the various units including alteration. B. Graphic summary log for Hole 1188F showing the lithologic characteristics of the various units including alteration. NR = no recovery.
Figure 11. Close-up photograph of fresh, black moderately vesicular rhyodacite from the upper part of Hole 1188A (interval 193-1188A-4R-1, 013 cm).
Figure 12. Photomicrograph in cross-polarized light of a fresh plagioclase phenocryst set in groundmass containing 35 vol% fresh plagioclase microlites, 35 vol% quartz, 25 vol% brown clay, and 5 vol% pyrite (Sample 193-1188F-31Z-1, 13 cm; field of view = 2.75 mm).
Figure 13. Many amygdules in altered dacite are filled by mosaic quartz ± pyrite ± anhydrite. In this photomicrograph they are < 1 mm, and elongate amygdules define a general orientation of alignment (interval 193-1188F-1Z-3, 8689 cm; field of view = 2.75 mm).
Figure 14. Close-up photograph of a perlitic texture of aphyric dacite (Unit 2) indicates that the groundmass originally consisted of volcanic glass (interval 193-1188A-5R-1, 3549 cm). Alteration proceeded preferentially along and outward from the perlitic cracks generating a pseudoclastic texture in some domains of the sample. A. In hand specimen this unit has a sugary appearance. The arcuate perlitic cracks are enhanced by alteration generating light gray "islands" in a dark gray, irregular network of clay, silica, and minor pyrite forming an apparent matrix. B. In thin section, well-preserved perlitic texture is present within the "islands," which, hence, represent remnants of the originally glassy groundmass (field of view = 2.75 mm).
Figure 15. Flow banding is commonly present in the altered volcanic rocks. In this case, the flow banding is folded and crosscut by silica-anhydrite-pyrite veins (interval 193-1188A-12R-2, 3652 cm). A. Photograph of hand specimen. B. Line drawing sketch of the specimen highlighting folds and fractures.
Figure 16. Summary of lithostratigraphic units, alteration style, and distribution of alteration phases at Site 1188. Occurrences of relict igneous plagioclase are also represented (on the right side). Pyrite is present in all samples studied and is not represented on the figure. Clay phases are mentioned where detected by X-ray diffraction (XRD); no indication of their abundance relative to other phases is implied. EOH = end of hole.
Figure 17. Close-up photograph of pervasively bleached vesicular volcanic rock with a zoned alteration pattern toward a darker gray, less strongly bleached kernel (interval 193-1188A-9R-1, 1629 cm).
Figure 18. Close-up photograph of a green silica-clay altered rock with a remnant perlitic texture and anhydrite-(silica-pyrite) stockwork veining (interval 193-1188A-8R-1, 108140 cm). The large white patch near the center of the photographed piece is bleaching surrounding a late irregular anhydrite-pyrite vein.
Figure 19. Photomicrograph in cross-polarized light of a plagioclase phenocryst completely replaced by fine-grained illite (birefringent mineral) and possible halloysite (gray) (Sample 193 1188F-14Z-1, 102105 cm; field of view = 0.7 mm).
Figure 20. Photomicrograph in plane-polarized light of anhydrite-pyrite veining with well developed siliceous halo hosted in green silica-clay altered volcanic rock. The vein has cut and filled two vesicles (Sample 193-1188A-8R-1, 124127 cm; field of view = 0.7 mm).
Figure 21. Photomicrograph in reflected light of chalcopyrite (light gray) partially enclosing and replacing pyrite (white) (Sample 193-1188F-37Z-2 [Piece 3, 3133 cm]; field of view = 0.28 mm).
Figure 22. Photomicrograph in reflected light of pyrrhotite inclusion (diameter = 0.006 mm) with smaller magnetite inclusions in pyrite (Sample 193-1188F-1Z-3 [Piece 3, 8689 cm]; field of view = 0.14 mm).
Figure 23. Photomicrograph in plane-polarized light of anhydrite-pyrite vein with pyrite surrounding anhydrite. Note the fine-grained pyrite at the outer fringes of the gray halo of silica around the veins (Sample 193-1188A-7R-2 [Piece 2, 3941 cm]; field of view = 1.40 mm).
Figure 24. Interval 193-1188F-14Z-1 (Piece 6, 98.5110 cm). A. Close-up photograph. B. Sketch of complex vein relationships (246.86 mbsf). The veins in this piece consist predominantly of anhydrite and pyrite. The two thicker veins, Va and Vb, are surrounded by 1- to 2-mm-thick gray siliceous halos that grade outward into 1- to 2-mm-thick light gray halos more rich in clay minerals (illite?). A thinner siliceous halo is present around vein Vc. In all veins, except for the thickest vein (Va) and the thinnest veins (VjVp), pyrite occupies the center of the veins and is rimmed by anhydrite. Vein Va has coarse anhydrite in the center of the vein, rimmed by thin veinlets of pyrite, followed by a thin rim of anhydrite. The thinnest veins are either pyrite veins (VjVm) or veinlets of anhydrite (VnVp). Some of the thinner veins branch off from the thicker veins (e.g., Vk and Vl from Vb; Vi and Vm from Va; and Vg and Vj from Vc). Veins Vc and Vd are probably part of the same vein, crosscut by Vb. This is evinced by the anhydrite selvages of veins Vc and Vd, which are overprinted by the siliceous halo around Vb (Point A). Furthermore, the right-lateral offset between Vc and Vd matches the space of the extensional jog filled with pyrite of Vb (Point B). Finally, the anhydrite crystals are aligned east-west in the vein intersection at Point A, indicating an east-west extension. Microscopic examination shows that Ve cuts across the halos around both Va and Vb, indicating it to be later. Although these veins show crosscutting relationships, on the basis of their mineralogy and nature of alteration halos, most of the veins in this specimen are considered to be part of the same main veining event. On the basis of mineralogy and nature of the alteration halos, this is considered to be the event that formed most of pyrite-anhydrite veins throughout all of Hole 1188F.
Figure 25. Hole locations for Sites 1189 and 1190. Shaded area around Site 1189 (Roman Ruins) = the approximate area of outcrop of chimney structures mapped by submersible and camera tow.
Figure 26. A. Graphic summary log for Hole 1189A showing the lithologic characteristics of the various units including alteration. B. Graphic summary log for Hole 1189B showing the lithologic characteristics of the various units including alteration. See Figure 9 for the lithology key.
Figure 27. Close-up photograph in cross-polarized light of rounded plagioclase phenocrysts in altered volcanic rock from Hole 1189B (Sample 193-1189B-16R-1 [Piece 6, 3639 cm]; field of view = 2.75 mm).
Figure 28. Photograph of moderately vesicular aphyric volcanic rock. The groundmass shows a spotty hieroglyphic texture caused by incomplete hydrothermal alteration with white, angular shard-shaped domains representing remnants of less altered groundmass (interval 193-1189B 16R-1, 114130 cm).
Figure 29. Summary of lithostratigraphic units, alteration style, and distribution of major (heavy lines) and minor to trace (light lines) alteration phases for (A) Hole 1189A and (B) Hole 1189B. Intervals where remnant igneous plagioclase was detected by X-ray diffraction (XRD) are also indicated on the right side. Pyrite is present in all samples analyzed and is not included in the plot. Clay phases are indicated where detected in XRD analyses; no indication of their abundance relative to other phases is implied. Shaded intervals = no recovery.
Figure 30. Close-up photomicrograph in plane-polarized light of gypsum partly replacing anhydrite, which has partially replaced altered volcanic clasts (Sample 193-1189B-2R-1 [Piece 2, 1114 cm]; field of view = 1.4 mm).
Figure 31. Close-up photomicrograph in cross-polarized reflected light of jasperoidal quartz showing strong red internal reflections (Sample 193-1189B-6R-1 [Piece 2, 1315 cm]; field of view = 1.4 mm).
Figure 32. Photograph of coherent flow-banded core piece, with a pseudobreccia appearance created by silica-clay alteration halos around a feeble network of silica veins (interval 193-1189B 15R-1, 130135 cm).
Figure 33. A. Photograph of semimassive sulfide consisting of pyrite and chalcopyrite grains in an anhydrite-silica matrix. Angular white fragments are altered volcanic rock (interval 193-1189A 12R-1, 120128 cm). B. Photomicrograph of volcanic clasts with fibrous, laminate internal texture, which are abundant in this sulfide-rich interval (Sample 193-1189A-12R-1 [Piece 16, 122125 cm]; field of view - 1.4 mm).
Figure 34. Close-up photograph of pyrite crystals (white) in the center, nucleated around quartz and overgrown by marcasite crystals (light gray), which, in turn, are overgrown by more pyrite.
Figure 35. Photograph of a network of quartz-pyrite veins (dark gray) crosscutting earlier patchy silica-sulfate alteration (light gray) (interval 193-1189A-10R-1 [Piece 10, 88.5100 cm]).
Figure 36. Photograph of breccia with highly altered volcanic fragments in a matrix of quartz with minor pyrite. Note the breccia is matrix supported (Sample 193-1189B-7R-1 [Piece 1, 05 cm]).
Figure 37. Close-up photograph of hand specimen shows fresh to slightly altered dacite recovered from Site 1190 is black, moderately vesicular, and superficially aphyric in hand specimen. However, careful observation and thin section petrography show that the glassy to microcrystalline groundmass contains up to 3 vol% of plagioclase, clinopyroxene, and minor magnetite phenocrysts (interval 193-1190B-2R-1, 2835 cm).
Figure 38. Photomicrograph of glomerophyric clasts of phenocrysts (here, plagioclase, clinopyroxene, and magnetite) and a nearby microvesicle, which are a common feature of Unit 1. Interstitial spaces around these aggregates typically consist of microlite-free volcanic glass (Sample 193-1190B-2R-1 [Piece 7, 4043 cm]; field of view = ~1.8 mm).
Figure 39. Photograph of fresh (interval 193-1191A-1R-1, 9.511.5 cm) to slightly altered (interval 193-1191A-1R-1, 1218.5 cm) aphyric rhyodacite from the top part of Hole 1191A. The main stretching orientation of the vesicles is normal to the surface of the sample (interval 193 1191A-1R-1, 919 cm).
Figure 40. Photograph of altered rhyodacite with a spotty appearance and a strongly elongated tabular vesicle lined with fine zeolite crystals. The spotty appearance is caused by partial alteration of the volcanic groundmass forming very fine grained silica and clay minerals (interval 1191A 1R-1, 6571 cm).
Figure 41. Photomicrograph in plane-polarized light illustrating cristobalite-clay alteration (colorless patches) of pale brown volcanic glass in a moderately altered volcanic rock from Site 1191 (interval 193-1191A-2R-2, 103106 cm). Note that microlitic plagioclase and magnetite (fine opaque spots) are unaffected by the alteration (field of view = 0.7 mm).
Figure 42. Oil-immersion photomicrograph in reflected light of framboids of pyrite (or possible greigite, Fe3S4) in a vein dominated by marcasite (Sample 193-1191A-2R-2 [Piece 16, 103106 cm]; field of view = 0.14 mm).
Table 1 |Table of Contents