Geochemistry at DSDP Site 76-533
Bulk X-ray mineralogy of 47 hemipelagic mud and clay samples from the Blake Outer Ridge has revealed that the sediments contain low magnesian calcite, calcian dolomite, ferroan dolomite, and magnesian siderite. Dolomite and siderite are authigenic and occur as rhombohedrons scattered through the sed...
| Main Author: | |
|---|---|
| Format: | Dataset |
| Language: | English |
| Published: |
PANGAEA
1983
|
| Subjects: | |
| Online Access: | https://doi.pangaea.de/10.1594/PANGAEA.809827 https://doi.org/10.1594/PANGAEA.809827 |
| Summary: | Bulk X-ray mineralogy of 47 hemipelagic mud and clay samples from the Blake Outer Ridge has revealed that the sediments contain low magnesian calcite, calcian dolomite, ferroan dolomite, and magnesian siderite. Dolomite and siderite are authigenic and occur as rhombohedrons scattered through the sediments, whereas calcite is mostly biogenic. Pliocene dolomitic lenses are made up of interlocking polyhedral grains of ferroan dolomite. The contents of authigenic dolomite and siderite are 3 to 8% in carbonate sediments and 70 to 89% in dolomitic lenses. Dolomite occurs largely in the cores above 192 m sub-bottom depth, whereas siderite occurs in the cores below 87 m. The distribution and occurrence of dolomite and siderite have determined the diagenetic zonation of carbonates as Zone I (dolomitic zone, top-90 m), Zone II (transition zone, 90-180 m), and Zone III (sideritic zone, 180 m-bottom). Measurements of major and minor elements in the untreated total sediment samples and the insoluble residues after digestion in acid-reducing solution have revealed that the soluble fraction concentrates carbonates and ferromanganese associations (Ca, Mg, Sr, Fe, and Mn). Typical "hydrogenous elements" (Co, Cu, Ni, and V) are more concentrated in the insoluble residues rather than in the soluble fraction; the concentrations of these four elements are low and comparable to modern offshore mud, probably because the Site 533 sediments were deposited at a high rate of sedimentation. The contents of Fe2O3 and MnO are somewhat high for rapidly accumulated mud, particularly in the Pliocene sediments (8.09 and 0.26%, respectively, on a Carbonate-free basis). The high Fe and Mn contents are mainly due to the high contribution of the leacheable nonlithogenous fraction; leacheable Fe and Mn originate in the ferromanganese oxide accumulated on the seafloor. Only a small amount of ferric oxide was converted to iron sulfide in the surficial part of Zone I. Most ferromanganese oxide was reduced and precipitated as ferroan dolomite and ... |
|---|