g)
is 2.32 g/cm3, and the average wet bulk density (b)
is 2.81 g/cm3. The average porosity (
)
of all Unit II samples is 27.1%.The results of shore-based Leg 169 index properties measurements and calculations are shown in tabular form in Table T2. Figures F1, F2, and F3 are graphical representations of wet bulk density, grain density, porosity, and compressional wave velocity results for samples from Hole 856H (Fig. F1), Hole 1035F (Fig. F2), and Hole 1035H (Fig. F3), the three deepest holes drilled in the vicinity of the BHMS deposit. Figure F4 is a compilation plot for Hole 856H that includes both shipboard and shore-based index properties data from Leg 169 and the massive sulfide section, to 93.8 mbsf, drilled during Leg 139. Identical symbols, defined in Figure F1, are used in all four figures to represent sample lithologies encountered at different depths in each hole. Characteristics of each lithologic unit and subunit used in Leg 169 core descriptions are summarized in Table T1.
Shore-based index
properties analyses were performed on 20 Unit II samples from Holes 856H, 1035D,
1035E, 1035F, and 1035H (Tables T1,
T2; Figs. F1,
F2, F3,
F4). The average grain
density (g)
is 2.32 g/cm3, and the average wet bulk density (b)
is 2.81 g/cm3. The average porosity ()
of all Unit II samples is 27.1%.
Thirteen samples are from Subunit IIA, described as interbedded, laminated hemipelagic claystone and turbiditic siltstone and sandstone with generally <2% sulfide (Shipboard Scientific Party, 1998d). These rocks have an average wet bulk density of 2.29 g/cm3, an average grain density of 2.80 g/cm3, and an average porosity of 28.4%. Seven samples are from Subunit IID. These sediments are, in general, finer grained than those of Subunit IIA and mainly consist of chloritic siltstone and mudstone with occasional interbeds of fine-grained sandstone (Shipboard Scientific Party, 1998d). The rocks are characterized by a slightly higher average wet bulk density of 2.37 g/cm3, an average grain density of 2.83 g/cm3, and an average porosity of 24.9%.
Shore-based index properties analyses were performed on 31 Unit VI samples from Holes 856H, 1035D, 1035F, and 1035H (Tables T1, T2; Figs. F1, F2, F3, F4). The average grain density is high (3.03 g/cm3), as is the average wet bulk density (2.57 g/cm3). The average porosity of all Unit VI samples is 23.2%.
Ten samples are from Subunit VIA, which is described as moderately to intensely indurated and hydrothermally altered mudstone, siltstone, and sandstone with 10%-50% sulfide, commonly present in veins (Shipboard Scientific Party, 1998d). These rocks are characterized by a high average wet bulk density of 2.86 g/cm3 and average grain density of 3.17 g/cm3, and an average porosity of 14.9%. Sixteen samples are from Subunit VIB, described as siltstone with 2%-10% disseminated and vein-hosted sulfide (Shipboard Scientific Party, 1998d). These sulfide-poor rocks have a lower average wet bulk density of 2.28 g/cm3 and average grain density of 2.82 g/cm3. The average porosity is 29.5%. Five samples are from Subunit VIC, characterized by sulfide-banded sandstone with 10%-50% sulfide (Shipboard Scientific Party, 1998d). Subunit VIC rocks have a lower average wet bulk density of 2.77 g/cm3, and a higher average grain density of 3.40 g/cm3 than Subunit VIA samples. The average porosity is 19.9%.
Shore-based index properties analyses were performed on four Unit VII samples from Hole 856H (Tables T1, T2; Fig. F1). This unit, also penetrated in Hole 857D during Leg 139 and interpreted as "hydrothermal basement," consists of slightly to intensely altered fine-grained basaltic sills alternating with highly indurated hemipelagic and turbiditic sediment (Shipboard Scientific Party, 1992c). Two samples are from veined, fractured sill rock, and two samples are from silty claystone to siltstone interbeds. The average grain density is 2.90 g/cm3, and the average wet bulk density is 2.70 g/cm3. The average porosity is 10.2%.
Three Unit VIII samples from Hole 856H were measured. The highly altered and veined fine-grained pillow basalts are characterized by an average wet bulk density of 2.69 g/cm3, an average grain density of 2.89 g/cm3, and an average porosity of 10.8%.
Shore-based index properties analyses were performed on 10 Unit VII samples from Hole 1035H (Tables T1, T2). These sulfides were described according to the sulfide classification scheme originally defined aboard ship during Leg 139 (Shipboard Scientific Party, 1992b). The average grain density of these 10 samples is high (4.31 g/cm3), as is the average wet bulk density (3.69 g/cm3). The average porosity of all Unit V samples is 18.7%.
Five samples from Hole
1035F are from Subunit VD, which corresponds to the Type 5 sulfide
classification of massive colloform and vuggy pyrite (Table T2).
These rocks are characterized by higher average wet bulk density (4.00 g/cm3),
higher average grain density (4.56 g/cm3), and an average porosity of
13.9%. Variable proportions of pyrite (g
= 4.92 g/cm3), pyrrhotite (g
= 4.55 g/cm3), and magnetite (g
= 5.15 g/cm3) (Horai, 1971), 10%-25% of which are present in Type 4
and 5 rocks, result in high grain density and wet bulk density values.
Four samples from Hole 1035H cores are described as Subunit VC. A lithologic summary for this subunit is presented in Table T1; this category does not correspond to the Leg 139 classification scheme for Hole 856H sulfides. These rocks have an average wet bulk density of 3.40 g/cm3, an average grain density of 4.09 g/cm3, and an average porosity of 22.8%.
The results of 29 elevated-pressure velocity experiments conducted on minicores from Holes 856H, 1035D, 1035F, 1035H, and 1037B are presented in Table T3. Compressional wave velocities vs. depth are shown with index properties data in the multiplots of Figure F1 for Hole 856H, Figure F2 for Hole 1035F, and Figure F3 for Hole 1035H. The symbols used to represent different lithologies are defined in Figure F1; the units and subunits used in Leg 169 core descriptions are summarized in Table T1.
Velocities at elevated pressures were measured for 11 Unit VI samples obtained from a sulfide feeder zone and mineralized sediment interval beneath the massive sulfide deposit at Site 1035, and seven Unit II samples of hemipelagic and turbiditic sediment encountered beneath Unit VI sediments in Holes 856H, 1035F, and 1035H. Results are given in Table T3 and graphically shown in Figure F1 (Hole 856H), Figure F2 (Hole 1035F), and Figure F3 (Hole 1035H). Descriptions of lithologic subunits are summarized in Table T1 (Shipboard Scientific Party, 1998d). Two samples represent Subunit VIA, eight samples are from Subunit VIB, one sample is from Subunit VIC, five samples represent Subunit IIA, and two samples are from Subunit IID.
Variations in compressional wave velocities among Subunit IIA samples may be linked to the presence or absence of thinly bedded, coarser grained turbidite layers with minor sulfide (Samples 169-856H-45R-1, 16-18 cm, and 47R-1, 118-120 cm) and parallel laminae in siltstone samples (Sample 169-856H-50R-1, 132-134 cm). Velocity differences between samples from Subunits VIA, VIB, and VIC are likely related to percentages of sulfide minerals present in the mudstone, siltstone, and sandstone. The fastest velocities, including Vp of 5114 m/s at 90 MPa, are observed in Sample 169-1035H-21R-1, 88-91 cm, of Subunit VIC (Fig. F3). This altered, laminated sandstone contains 50%-70% bed-parallel sulfide. In contrast, Sample 169-856H-26R-1, 91-93 cm, a siltstone of Subunit VIB with small sulfide blebs comprising <10% of the sample, has a Vp of 3603 m/s at 90 MPa (Fig. F1).
Two "hydrothermal basement" (Unit VII) samples from Hole 856H were measured. One is a basalt/diabase recovered from near the base of sill VIIE, the fifth and last sill drilled above oceanic basement, and the other was sampled from the interbed of indurated sediment immediately above oceanic basement (Shipboard Scientific Party, 1998d).
Unit VII in the vicinity of Sample 169-856H-60R-2, 26-28 cm, is described as altered, fine- to medium-grained greenish gray pyroxene-phyric basalt/diabase. A thin section made from a sample 20 cm downcore (58-60 cm) from this physical properties sample is described as a medium-grained, "spectacularly fresh," subophitic to ophitic diabase with 40% plagioclase phenocrysts, 35% clinopyroxene phenocrysts, and 15% olivine(?) phenocrysts (Shipboard Scientific Party, 1998d). The sample is slightly altered, with quartz, chlorite, and titanite alteration minerals; pyrrhotite and ilmenite are opaque constituents. Vp at 90 MPa was 6319 m/s (Table T3; Fig. F1). This is comparable with velocities measured for slightly altered diabase samples, with similar densities and porosities, obtained from cores below 762.5 mbsf in Hole 857D during Leg 139 (Gröschel-Becker et al., 1994b).
Sample 169-856H-62R-1, 72-74 cm, is an interbedded mudstone/siltstone with ilmenite, quartz, and pyrrhotite mineralization along bedding planes and fractures that result from its proximity to the sediment/sill contact zone. The Vp at 50 MPa of 5255 m/s (Table T3; Fig. F1) is fast for a sedimentary rock and is likely caused by thermal alteration and mineralization.
Velocity data for four flow samples of basalt interpreted as oceanic basement from Hole 856H in Middle Valley and Hole 1037B, Escanaba Trough, are given in Table T3. The Vp at 90 MPa of 4358 m/s for Sample 169-856H-64R-1, 91-93 cm, is also shown graphically in Figure F1.
This sample was described as a fine-grained, greenish gray pillow basalt with chlorite-filled variolites and populated with veins and blebs of chlorite and quartz with chalcopyrite. The slowness of the velocity data when compared to fresh basalts is explained by the high degree of alteration observed in the pillow basalt and by the presence of low-velocity chlorite.
The fine- to medium-grained, relatively unaltered basalts sampled from Hole 1037B cores have higher velocities at 80 and 90 MPa than the Hole 856H sample (Table T3). The lower velocity at 80 MPa of 5953 m/s of Sample 169-1037B-58R-1, 105-108 cm, compared to velocities of 6360 and 6365 m/s, respectively, for the two deeper samples, can be linked to the extent of alteration and the resulting differences in mineralogy. Sample 169-1037B-58R-1, 105-108 cm, is a fine-grained basalt with plagioclase and pyroxene phenocrysts altered to actinolite and cut by a <1-mm-wide vein of calcite with some chlorite (Shipboard Scientific Party, 1998b). In contrast, Sample 169-1037B-61R-1, 75-77 cm, is only slightly vesicular with vesicles filled with smectite/chlorite, and Sample 169-1037B-62R-1, 75-77 cm, is similarly homogenous except for a few irregular veins of chlorite and zeolite.
Shore-based velocity measurements were conducted on three Hole 1035F samples assigned to Unit VD and equivalent to Leg 139 Type 5 massive colloform and vuggy pyrite (Table T2). The minicores taken for velocity measurements from Hole 1035H cores are both assigned to Subunit VC. Sample 169-1035H-2R-1, 50-53 cm, was described as a sulfide breccia with moderately indurated, pyrite-dominated clasts in a fine-grained matrix of pyrite, marcasite, and sphalerite. Sample 169-1035H-16R-2, 109-111 cm, was described as a fine- to medium-grained massive to semimassive, pyrite- and magnetite-rich sulfide with a partly oxidized, mottled texture caused by neoblastic pyrite.
Vp at 40 MPa for Samples 169-1035F-4R-1, 70-72 cm, 5R-2, 126-128 cm, and 6R-1, 16-18 cm, are 5467, 6478, and 5892 m/s, respectively (Table T3; Fig. F2). These data are comparable with velocities measured for four Type 5 massive sulfide samples, with similar densities and porosities, obtained from cores above 93.8 mbsf in Hole 856H, and from nearby Hole 856G, during Leg 139 (Gröschel-Becker et al., 1994a). The variability in velocities is likely caused by textural variations in the samples (i.e., the ratio of pore spaces, or vugs, to pyrite).
