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Journal of Marine Research, Sears Foundation for Marine Research, Yale University PO Box 208118, New Haven, CT 06520-8118 USA (203) 432-3154 fax (203) 432-5872 [email protected] www.journalofmarineresearch.org il1ineralogy and Petrology ofSome Mid-Atlantic Ridge Sediment/ Raymond Si ever and Miriam Kastner Department of Geologfral Sciences, Harvard University Cambridge, Massachusetts and W oods Hole Oceanographic Institution W oods Hole, Massachusetts ABSTRACT Coarse and fine fractions of samples from dredge hauls and cores taken in a detailed sun'ey in the area of 22°N on the Mid-Atlantic Ridge were studied by x-ray diffraction and petrographic microscopy. Coarse fractions recovered as insoluble residues of foraminiferal sands consist of rock and mineral fragments identifiable as coming from two major sources: (i) an ultrabasic terrain and (ii) a " normal" basic volcanic and metamorphic greenstone terrain. Although the bulk of the detritus is derived from continental so urces, fin e-fraction mineralogy indicates so me contribution from rocks of the Ridge that have undergone marine weathering. Extensive variability in mineralogy supports the conclusion that (i) local so urces do contribute to the sediment around the Ridge, (i i) mixing during transportation has been slight, and (iii) the study of coarse fractions of foraminiferal oozes is a useful way to chart heterogeneous rock terrains on the ocean floor. Introduction. This paper is one of a series of reports on the morphology of parts of the Mid-Atlantic Ridge and on the mineralogy and petrology of the igneous, metamorphic, and sedimentary rocks of the Ridge province (van Andel et al. 1965a, Cifelli et al. 1966, Melson et al. 1966, Melson and van Andel 1966 ). In this report we present the results of studies on piston cores and dredge hauls obtained in the region of 22°N. L ocations of the samples are given in T able I, and core logs have been given by van Andel et al. ( 1965 b). General and detailed maps of the area are presented in Figs. 1 and 2. The questions sought in this work relate to the source of the sedimentary detritus found in ponded basins on the Ranks of the central part of the Ridge; this detritus occurs either as a thin mantle partly covering higher topographic areas or as scattered pockets on the Boor of the M edian Valley. There are two relevant questions, one being quantitati ve, the other qualitative: What 1 Contribution No. 1903 of the Woods H ole Oceanographic Institution. Accepted for publication and submitted to press 6 May 1967. 263 Journal of Marine Research [ 2 5,3 Table I. Location of samples. Station Latitude and Longitude Depth (m) 22°N Median Valley WASHINGTON 1965-1 22°3 i .5'N--45°00.2'W to 1985-1735 dredge THV-4 44°59.8'W WASHINGTON 1965-J 22°22.5'N--45°14.2'W to 2720-2475 dredge THV-11 22°2 1. 7'N--45° I 5.3'W WASHINGTON 1965-1 22°47.5'N--45°1 l.4'W to 2630-2460 dredge THV-15 45°12 .2'W CHAIN 44, free-core 14* 22°46. 7'N--45°06.2'W 3245 CHAIN 44, dredge 3 22°38.0'N--45°0.7'W to 3400-2400 44°58.8'W South Pond CHAIN 44, piston-core 22 22° I 5.6'N--46°23 .2'W 4049 North Pond CHAIN 44, dredge 6 22°46.8'N--46°10.8'W to 4400-4050 22°4 7 .O'N--46°12 .0'W CHAIN 44, dredge 7 22°48.8' N--46°05.3'W to 4040-3765 22°49.3'N--46°04.2'W CHAIN 44, piston-core 11 22°46.5'N--46°09.2'W 4480 Western edge of Central Ridge Province CHAIN 17, pipe dredge 22°56'N--46°35'W 3125 Equator West of Ridge Province CHAIN 35, dredge 19 00°44'N-34°47'W to 4000 00°50'N-34°45.5'W • Free core indicates cores taken with an unattached free-fall gravity corer described by Sachs and Raymond (1965). proportion of the sediment is of general pelagic derivation and what proportion is indigeneous - that is, derived from the Ridge? Can contributions from any distinctive rock types that may crop out on the Ridge be recognized? Also, the small sediment ponds in this tectonic milieu serve as small "experiments" in sedimentology that may give us the means to generalize about the mechanics of marine sedimentation in the deep sea, far from continents. The extensive and detailed bathymetric, geophysical, and sampling data that are available from the 22°N region of the Ridge make possible a much more intensive analysis of the sedimentary regime than has been possible heretofore. Sample Preparation. Six to eight grams of a sample were used. The samples from CHAIN 44-piston-core I I were taken from compressed cakes that had the interstitial water removed (Siever 1965). Samples were suspended in distilled water without mechanical disaggregation and then wet-sieved through Siever and Kastner : Mid-Atlantic Ridge Sediments a 270-mesh screen (53 micrometer = 53 µ). The coarse fraction was washed and dried. To dissolve all carbonate, the fine fraction was treated with 200 ml of saturated EDT A (ethylenedinitrilotetraacetic acid tetrasodium salt) solution ~pH 5.5-6.0); this treatment no doubt affected the clay minerals slightly, but 1t was chosen as the mildest way to remove carbonate. Samples were then 30° t ,(:-c., A-." .. .. , 30° 0 AREA OF if •.,...-INVESTIGATION o• 6 o· 30° o• Figure 1. General map of Atlantic Ocean showing area investigated. washed at least three times with distilled water. Oriented aggregate slides for x-ray spectrometer analysis were prepared from the washed suspension of the < 53 µ fraction. The magnetic grains of the coarse fraction ( > 53 µ) were segregated with a magnetic separator. The magnetic and nonmagnetic fractions were examined with a binocular microscope, and mineral constituents were hand picked for identification by x-ray diffraction and petrographic micro- scopy. X-ray Identification of Fine Fractions. Oriented aggregate slides of the fine fractions, glycolated and unglycolated, were analyzed on a Phillips diffracto- meter at 1°-2 0/minute, copper radiation, nickel filter. Resolution of the 3.54 Iv 0' 0' 46° 45° 2 0 \! 2 0 0 0 •o g 0 ! o..__ lj "'i:: "I 0 ;,i 0 D ...__I::) Gt {j oU 0 ... .,, ;:;"I · I; ... "' /0\ .a "'.... "'I::) "I "';:,- (l"o"::H~44 PC - 22 o•I\ I} I ~· 0 km , 0 15 46 ° 45 ° no 1,1 t . ml I•• 22° T . ~a n Andel 1966 22° r-, Figure 2 . Loca tion of cores and dredge hauls taken on CHAIN cruise 44 to the Mid-Atlantic Ridge in the area of 22° N. See Fig. 1, Iv wV\ Siever and Kastner : Mid-Atlantic Ridge Sedim ents A.U. (004) chlorite peak and the 3.58 A.U . (002) kaolinite peak was obtained at 0.25°/minute in much the same fas hi on described by Biscaye (1965). The precision of a semiquantitative anal ys is of clay minerals, feldspars, and quartz by x-ray diffraction methods depends on unifo rm quality of the slides, uniform crystallinity of the clay minerals, and a relatively constant grain-size di stribution. A prel iminary appraisal showed very little difference among several fin e-size fractions within the < 5 3 µ group (exce pting the fin est frac- ti ons - those of less than o. I µ) ; this confirmed the findings of Biscaye ( I 96 5: 828) that the < 2 µ and 2-20 µ fracti ons are generally similar. W e also de- termined that the grain-size distribution in the silt-clay range did not vary much from sample to sample. W e have fo und some differences in crystallinity of the montmorillonites in a few samples, but in most of these the materials did not vary signi fica ntly. From tests on the uniformity of slide preparation we have fo und that the error introduced by nonreproducibility of slide prepara- tion is well wi thin the precision limits of repeated analyses of diffraction patterns in the same preparation. That precision justi fies the presentation of onl y two significant fi gures for peak area ratios. Accuracy is affected by the fact that oriented aggregates tend to be unhomogeneous so that montmorillonite is overestimated.