RESULTS

The downcore presence or absence of each mineral, or mineral group, identified from the diffractogram patterns is shown in Figure F1. Example diffractogram patterns of six samples are shown in Figure F2. Mineral peaks were generally sharp and well defined. The occurrence of broader peaks, such as at 5°-7°2, reflect the presence of various expandable clays, and at 32°-37°2, reflect the presence of serpentine minerals. The minerals present in the Site 1068 breccia matrix include: calcite, quartz, expandable clays, illite, plagioclase, chlorite, hornblende, apatite, analcime, serpentine, and andradite garnet. Distinguishing between albite and anorthite varieties of plagioclase was not possible given their similar 2 peak positions. Clinochlore is the variety of chlorite that generally best matches the diffractogram pattern; however, the blue ferriferous chloride mineral aërinite was identified in Sample 173-1068A-20R-7, 24-26 cm. Hornblende varieties include ferrorichterite and ferropargasite. Apatite is the F-rich variety, fluorapatite. Serpentine varieties include lizardite and chrysotile. In addition, although the presence of the mineral brucite was indicated from shipboard XRD analyses (Unit I; Shipboard Scientific Party, 1998), it could not be identified in this study with any certainty because of an overlap in peak position with the higher 2 boehmite peaks. A comparison between downcore trends in breccia matrix mineralogy determined from this study and the downcore trends in breccia bulk mineralogy as determined aboard ship are given in Table T1.

Downcore clay mineral abundance could not be quantified given the difficulty in distinguishing among the many possible clay mineral varieties when the clay minerals are not oriented, as is the case with pressed-powder samples. Further complicating the identification and quantification of clay minerals based on diffractogram patterns is the fact that the 14-Å chlorite peak lies at ~6.1°2, partially overlapping clay peaks with similarly high d-spacings. However, downcore changes in the shape of the 5°-7°2 peak(s) indicate that a variety of clay types exist in the breccia matrix and that their abundance varies downcore. Figure F3 shows a sampling of the downcore change in the shape of the 5°-7°2 peak and also the presence (or absence) of the 10-Å illite peak (8.8°2). Based on the position and shape of the 5°-7°2 peak, montmorillonite (smectite) group minerals including, but not restricted to, 15-Å saponite and 15-Å nontronite are probably present in the matrix (Carroll, 1970). Saponite and nontronite can be produced from the hydrothermal alteration of mafic and ultramafic rocks including lherzolite (Velde, 1995), of which the basement rock underlying the breccia succession is partially composed (Shipboard Scientific Party, 1998). Alternatively, saponite and nontronite can form diagenetically (Velde, 1995).

Relative variations in mineral abundances of the 3.04-Å calcite, 4.26-Å quartz, 4.03-Å plagioclase, 7.12-Å chlorite, 8.5-Å hornblende, 2.80-Å apatite, 5.6-Å analcime, 7.27-Å serpentine, and 3.06-Å garnet peaks are listed in Table T2. The mineral/boehmite peak area ratios for these minerals are plotted as a function of depth downcore in Figure F4. Matrix calcite generally decreases in abundance downcore, whereas the siliciclastic minerals (quartz and plagioclase), and minerals of diagenetic or hydrothermal origin (chlorite, analcime, and serpentine) show an increase in downcore abundance within the breccia matrix. The downcore transition from Maastrichtian Unit II marine sediments to the top of the Lower Cretaceous Unit IVA breccia (at 853 meters below seafloor [mbsf]) is marked by a sharp decrease in quartz, plagioclase, and chlorite abundances and a sharp increase in the abundance of calcite. Subunit IVA is generally characterized by a matrix with high calcite abundances and moderate abundances of quartz.

A change to generally lower and more variable calcite abundances, along with an increase in the noncalcareous minerals, occurs below 864 mbsf. This abundance change in the matrix mineralogy corresponds to (1) the transition from Subunit IVA to IVB, (2) an increase in fine rock fragments in the matrix (Shipboard Scientific Party, 1998), and (3) a color change of the powdered breccia matrix from predominantly moderate orange pink to variable browns, oranges, and grays (see Table T2). Apatite was present in only one sample (173-1068A-17R-2, 106-107 cm)(see the middle photo in Fig. F1 and also Fig. F2C) taken from a uniquely clast-poor, moderately sorted, 5-cm reddish brown to light brown layer within Subunit IVB that has a graded contact with the breccias above and below it. The clasts that are present in this interval are silt- to sand-size subrounded grains (Shipboard Scientific Party, 1998). The lithologic characteristics of the interval in which apatite is present do not support a hydrothermal origin for this mineral; shipboard description does not indicate the presence of a mineral vein from which the apatite could be derived.

Matrix chlorite, plagioclase, and quartz abundances are variable but generally increase downcore within Subunit IVB. The presence of these minerals may be attributed to hydrothermal alteration of the breccia matrix that increases in intensity with depth downcore. Alternatively, some portion of these minerals may be attributed to the inclusion of small rock fragments within the matrix or may have formed during diagenesis.

The downcore transition into Subunit IVC at 885 mbsf is marked by a decrease in quartz and plagioclase abundances. Other minerals do not show distinct abundance trends at this subunit boundary. However, below ~889.5 mbsf, a new suite of matrix minerals are present: analcime, serpentine, and andradite garnet. Analcime is a sodic zeolite of hydrothermal or diagenetic origin and the only zeolite stable in deeper, older rocks (Velde, 1995). Serpentine and garnet were identified shipboard in serpentinized peridotite immediately underlying the breccia succession (Shipboard Scientific Party, 1998). This study shows, however, that these minerals are also present in the deepest portion of the Subunit IVC breccia matrix.