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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 EDTA (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. The following ratios were calculated after peak areas were measured on diffractometer patterns of glycolated samples: kaolinite/chlorite (3.58 A.U./3.54 A.U.) chlorite/illite (7 A.U . chlorite contribution/ 10 A.U.) montmorillonite/illite (17 A.U./ 10 A.U.) quartz/illite (4.26 A.U./5.0 A.U .) plagioclase/K-feldspar (3.12-3. 23 A.U./3.21-3.28 A.U.) W e have presented the analyses in this way to emphasize, as did Biscaye, the quantities actually measured. If the relative abundances had been given

in percentages, totaling 100° / 0 , much more accuracy than is warranted would have been implied. M ethods now in common use for semiquantitati ve estima- tion of clay-mineral abundances are probably accurate, but only to ± 10°/o.

Fine-fraction Mineralogy. The results of x-ray diffraction analyses on core and dredge samples are given in T ables II and III. All of the common mineral constituents that are characteristic of sediment cores in this general area of the Atlantic Ocean are found in the Ridge sedi- ments. In most samples, the fo ur major clay-mineral groups (illite, mont- morillonite, chlorite, and kaolinite) as well as quartz and feldspar are present in roughly the same general proportions as those reported by Biscaye (1965). The CHAIN 44-dredge 3 sample is obviously different in composition from the other samples ; also, its montmorillonite is much better crystallized than in other samples. The dredge THV-1 5 sample, which is unusual in having only 268 Journal of Marine Research [ 2 5,3

Table II. Mineral composition of fine fraction of dredge and free-core samples. Sample no. Depth K.* Chi.* Mont.* Q.* Plag.* Q. (cm) Chi. Ill. I. I. K-spar. total 22 °N folds. Median Valley WASHINGTON J 965- J dredge THY -4 3.6 0.4 1.2 0.3 1.5 0.3 dredge THY -11 0.4 1.4 2.9 I.I 1.8 0.4 dredge THV-15 •• •• ...... 2.7 0.7 CHAIN 44, free-core 14 3282 20-28 2.3 0.4 0.9 1.5 9.3 0.4 3284 30-38 2.8 0.4 1.4 4.8 2.4 0.5 CHAIN 44, dredge 3 0.5 2.9 5.3 0.5 3.9 0.1

South Pond CHAIN 44, piston-core 22t 44-1030 1-6 3.7 0.4 0.2 1.1 0.8 0.9 44-1047 138-146 3.3 0.3 0.5 0.9 0.9 0.6 44-1072 327-333 2.5 0.4 0.5 1.2 0.8 0.5 44-1111 542-548 2.8 0.5 0.7 2.2 0.8 0.6 44-1140 670-675 7.6 0.2 3.5 0.9 0.8 0.4

North Pond CHAIN 44, dredge 6 2.2 0.5 I.I 1.3 2.1 0.6 CHAIN 44, dredge 7 4.1 0.5 0.5 1.2 2.1 0.7 CHAIN 44, piston core 11 t 44-678 12-17 2.7 0.3 1.8 I.I 1.6 0.5 44-698 42-47 3.5 0.3 1.7 1.2 1.1 0.6 44-748 155-159 2.5 0.4 1.7 1.1 1.5 0.7 44-789 249-254 2.9 0.4 1.7 1.2 1.9 0.6 44-850 389-394 3.6 0.2 1.2 1.1 I.I 0.6 44-1020 776-781 3.7 0.3 1.3 0.9 1.5 0.6

Western edge of Central Ridge Province CHAIN 17, pipe dredge 3.9 0.5 1.6 0.6tt

Equator West of Ridge Province CHAIN 35, dredge 19 3.8 0.4 2. 1 0.6 1.3 0.5

* K. = Kaolinite, Chi. = Chlorite, I. = Illite, Q. = Quartz, Plag. = Plagioclase, K-spar. = Potassium feldspar, Mont. = Montmorillonite. ** Too poorly crystallized for semiquantitative estimation; high iron-oxide content. t Analyses by R. Siever, M. Kastner, W. Sabine, and P. Berneburg. tt No montmorillonite, no plagioclase. Siever and Kastner: Mid-Atlantic Ridge Sediments

Table III. Minor constituents of fine fraction. Clinoptilolite 22°N heulandite Phillipsite Amp hi bole Talc Sepiolite Median Valley CHAIN 44 free-core I 4t XX X xx x x CHAIN 44

dredge 3 xx XX X

South Pond CHAIN 44 piston-core 22t xx xx xx xx North Pond CHAIN 44

dredge 6 xx X X CHAIN 44

piston-core lit xx X xx X

Western edge of Central Ridge Province CHAIN 17 pipe dredge xx XX

Equator West of Ridge Province CHAIN 35 dredge 19 XX X X xx xx x Abundant. xx Present. x Trace. t Average for core samples shown in Table II. feldspars identifiable from coherent scattering peaks on the x-ray pattern, contains only one identifiable clay- indicated by a weak and diffuse scattering maximum in the range of montmorillonite. The bottom sawple from piston core 22 appears to be distinctive in having high kaolinite-chlorite and mont- morillonite-illite ratios as well as a montmorillonite peak (as glycolated) resolv- able into two distinct reflections. Minor constituents of the fine fractions (Table III)- phillipsite, clinop- tilolite-heulandite, talc, and sepiolite- are not all characteristic of Atlantic sediment cores. Hathaway and Sachs (1965) have noted the occurrence of sepiolite elsewhere on the Ridge in samples from CHAIN 35-dredge I I. Biscaye (1965) has noted the presence of clinoptilolite-heulandite in many samples, but he did not find phillipsite or talc.

Coarse-fraction Mineralogy: Carbonate. Quantitatively, the most important constituent in the coarse fraction is foraminiferal carbonate. Many of the samples studied were foraminiferal sands, chosen because the large grain size 270 'Journal of M arine Research [ 2 5,3

Table IV. X -ray diffraction spacings and composition of carbonates from CHAIN 44 cores and dredge hauls.

Core II Core II Dr. 3 F-C 14 Core 22 Core 22 Core 22 44-859 44-745 44-3282 44-1111 44-1139 composite: 620-830cm d(rn)A 3.034 3.032 3.032 3.032 3.033 3.034 3.033 Mol 0 /o MgC03 0.5 1.5 1.5 1. 5 1.0 0.5 1.0 in CaC03

made possible the finding of recognizable silicate-rock fragments. The coarser foraminiferal sands were graded. One section of piston core 22, from 776 to 7 8 1 cm depth, appeared to be cross-bedded. Some foraminiferal tests contain clay that is released as a core or internal mold when the carbonate is dissolved. The mineral composition of the filling in the tests is identical to that of the free fine-clay fraction associated with the ooze in fine-clay beds or laminae. The smaller tests are empty, the larger ones are filled, and the intermediate sizes are partially filled, probably because of the size of the foraminiferal aperture. The entrance of clay into some of the tests and the correlation of test size with the amount of clay in them can be used to infer current action. T ogether with a consideration of graded bedding and cross-bedding of the foraminiferal sand and mixed fauna! populations (R. Cifelli, personal communication), the clay fillings suggest the existence of currents that were strong enough to dis- perse and suspend clay from previously deposited sediment. That suspended clay could then pass through the small apertures of the tests. 2 Judgi ng by the well-preserved intact nature of the shell, it seems unlikely that bottom-scav- enger activity caused the clay fillings. Furthermore, the composition of the clay inside and outside the shells suggests that the organic matter of the forami- nifera was completely decayed prior to transport and burial; otherwise we would expect to find some incipient glauconitization in the shell filling. It seems reasonable to conclude that (i) the foraminifera settled on some relati vely unstable slope, where organic decomposition took place (oxidized), (i i) the shells were picked up by a current that also carried stirred-up clay from the surrounding sediment, (iii) clay settled in the shells in varying amounts during the period of transportation, and (iv) the shells were laid down in the sedi- ment pond as a current-bedded graded foraminiferal sand. Foraminifera from seven samples were crushed and then analyzed by x-ray diffraction, using criteria given by Goldsmith and Graf (1958) for the amount of MgC03 present in solid solution. All samples were composed of calcite and very small amounts of M gC03 (Table IV). Extrapolating from higher temperature data, the amounts of MgC0 3 present are probably comparable to

2. One might even ~alcu late the ~iscosity ~f the cl a_Y suspension by measuring aperture openings and volumes of shell fillmgs and guessmg the time du ration of the turbid current. Siever and Kastner: M id-Atlantic Ridge Sediments those in thermodynamic equilibrium with CaCO3 at 5-20°C. In this state- ment there is no intention to contradict the known tendency of planktonic fo raminifera to secrete calcite with low amounts of M gCO 3, i.e., shells that are in equilibrium with their surroundings. T here is no tendency toward postdep;sitional uptake of magnesium by these calcitic shells.

Coarse-fraction M ineralogy: Silicate. Analyses of the carbonate-free coarse fractions by microscopy and x-ray diffraction are given in T able V. The total amount of insoluble residue is very small ; these sediments are in no way comparable in bulk composition to some of the laye rs that have been described as pure or nearly pure " mineral sand" (Fox and Heezen 1965). The de- termination by x-ray diffraction of compositions of coarse chlorite fragments from CH AIN 44-core 22 samples was roughly made by comparing relative intensities of first- and third-order with second- and fo urth-order basal spacing reflectio ns and by determining the angular distance between first and second orders (Brindley I 961 : 2 70). Chlorites found in C HAIN 44-dredge 3 samples range from iron-rich to magnesium-rich. The fresh-looking dark-green chlorites are iron-rich and the brownish-green chlorites, which we take to be their alteration products, are magnesium-rich. This suggests extensive iron-magnesium exchange by chlorites on exposure to seawater. Chlorites from Mid-Atlantic Ridge metamorphic greenstones and basalts are ripidolites having almost equal amounts of iron oxide and magnesium oxide (M elson and van Andel 1966). O n the other hand, the chlorites from the coarse fractions seem to have a local deri vation - from the general fine-grained continental detrital chlorite that is characteristi- cally iron-rich (Biscaye 1965 : 815). The high magnesium content of oli vines in C HA IN 44-dredge 6, Forsterite 90-95, is compatible with deri vation from oceanic tholeiites (Turner and Verhoogen 1960: 206 ). All fragments identified as serpentine minerals show talc refl ections on x-ray diffraction patterns. The palagonite in most of the samples varies in color from red to brown and is found in association with fresh-looking transparent or greenish glass shards. Large glass shards are particularly abundant in free-core-1 4 samples. The refractive indices of the glasses range from 1. 55 to 1.60, indicating a silica content ranging from 48°/o to 54°/ 0 (Williams et al. 1958 : 28), as shown in T able V. Some of the glas- ses - those from dredge 3, free-core 14, and piston-core I 1- are equal to some of the most basic glasses known. One slightly pitted magnetite spherule, about 0. 1 mm, possibly of extra- terrestrial derivation, was found in the composite sample of foraminiferal sand from piston-core 22.

Sources of Detritus. W e consider the coarse rock ·and mineral fragments of the samples to be detrital minerals deri ved from nearby igneous and meta- t,.) Table V. Mineralogy of the coarse fraction. '-I t,.)

'o' -"I u " .':l 0 u .':l Cl 0" .~:a ·5.. ~]' ·.;:; -~ "' .0 ·- ·a 0 0 -~ -~u .., .., ">< 0 C C: 0 .':l bl) c:: ·c bl) 0 Cl 0 :.a0.. .., t ·.;:; .£ ..,_" 0 "bO :0 ::, ""~ :; 0 e-" :~ "' "' 0 ..c: ..c: :.a .!!< E 0 OI ::E .s cl: iii 0. c.,~ {/)" ] 0. 0. «: 0 z 22°N Median Valley CHAIN 44 free core 14 #44-3282 X XX X x t X XX Coarse foram. sand; foram. ;:: 'I ;,: 20-28 cm tests contained clay. s::, #44-3284 X XX X x t X XX ...... 30-38 cm

CHAIN 44 'I dredge 3 X XX X x t XX XX XXX XX X x • Many large rock fragments. N, ;,: Chlorite varying from Fe- "' rich to Mg-rich varieties. >;:i Al bite xtls. in basalt frags. "'.., South Pond "'s::, 'I..., CHAIN 44 core 22 #44-111 X X XX X X fli'it Xx # Fine-grained foram. sand; 542-548 cm much Fe crust. #44-1113 X X XXX xx Xx # Similar to #44-1 111. 549-554 cm #44-1139 X XX X Xx # XX xx Medium-to-coarse foram. 665-670 cm sand; foram. tests contained

clay. r-, X XXX Xx # XX xx t,.) #44-1140 V\ 670-675 cm Similar to #44-1139. w Foram. sand; comp. xx X XXX x•• Xx # xx Foram. sand; one magnetite 620---830 cm spherule. Phillipsite in green '°'-l rock fragment. L-J°' North Pond CHAIN 44

dredge 6 xx xx Rich in Fe oxides (no Mn c.:, oxides); olivine; silica ;;:· CHAIN 44 sponge spicules. "''"I piston core 11 t::, ;:i #44-683 xx XX Fine-grained foram. sand. i::.... 27- 32 cm #44-742 xx XX X X X X O( X X X XXX XX X Foram. sand. identical to .... 142-147 cm #44-745. ;:i "''"I #44-745 xx XX x /3 X X X O( XXX XX X XXX 2 tourmaline grains 147-151 cm (~0.5 mm) detrital foram. sand. #44---859 xx X X XX XX Fine-grained foram. sand. 404-409 cm ....;:i~- Western edge of Cen- tral Ridge Province CHAIN 17 c.:,"' pipe dredge xx xx Rich in Fe oxides (not Mn "' oxides) . Olivine same as in Equator dredge 6. t "';:i West of Ridge Province :::- CHAIN 35

dredge 19 X X Clean foram . sand; silica sponge spicules.

xxxx Very abuudant. x x x Abundant. xx Present. X Rare. N '-l t Large glass shards. • Actinolite ? •• Dark brown . # Iron-rich. a Light brown. fJ Good xtls. w 'Journal of Marine Research [ 2 5,3 morphic source rocks. They are sufficiently coarse so that they could not have been windblown or waterborne from the continents. The contrast be- tween fine and coarse fractions confirms this conclusion. In support of the idea of local derivation we note the difference between samples from piston-core 11 and other samples, a heterogeneity that would be unexpected if there were a general mixed continental derivation. Moreover, heterogeneity in the hard rocks from the Ridge has been amply demonstrated by dredging. The local derivation is not unique in this area, for Fox and Heezen (1965) have inferred that volcanic sands from elsewhere on the Ridge are of local derivation. It is obvious that the great bulk of the sediment is carbonate derived from planktonic foraminifera and fine-grained detrital clays of continental origin. The relative abundance of clay-mineral groups, quartz, and feldspar fits well with the picture of detrital petrographic provinces (Goldberg and Griffin 1964, Biscaye 1965). No doubt a part of the montmorillonite group is of oceanic rather than continental derivation. The well-crystallized montmorillonite in the clays from the CHAIN 44-dredge 3 sample implies a nearby source, for similar material elsewhere in the ocean is found only in some beds of volcanic ash. The coincidental occurrence of well-crystallized nontronite as an alteration product in the igneous and metamorphic rocks of the Ridge in this same area (Melson and van Andel 1966) seems not to be fortuitous. But in the same general locale- in the M edian Valley, dredge THV 15- the poorly crys- tallized montmorillonite may be associated with the kinetics of alteration (probably as influenced by temperature) rather than with the nature of the source material. The simplest explanation is that the well-crystallized mont- morillonite is detritus from marine weathering of the nontronitic greenstones that crop out in the Median Valley, and that the poorly crystallized ones are low-temperature alteration products of recently deposited ash. The common occurrence of minerals in this province is not sur- prising. Though quantitatively unimportant, the indicate the role of nonequilibrium alteration products of volcanic materials in the ocean as sedi- ment contributors. The identification of these minerals as common but minor components of Atlantic sediment mixtures tends to break down any idea that the Atlantic compared with the Pacific is essentially different as a kind of sedimentation province, since zeolites in the Pacific are not only common but may be dominant in the sedimentary assemblage.

Ultrabasic .Assemblage. An evaluation of the coarse-fraction mineralogy of CHAIN 44-core I I samples infers erosion of an ultrabasic rock terrain near this isolated sediment pond (North Pond). Major evidence is the presence of phlogopite, serpentine, antigorite, and an abundance of magnetite. The minerals have been found separately and together in rock fragments. The finding of this distinctive mineral suite is in harmony with the finding of Siever and Kastner: Mid-A tlantic Ridge Sediments 2 75 serpentine on the Mid-Atlantic Ridge (Shand 1949). The presence of phlogo- pite and biotite implies that the source rock is composed of mica peridotite or kimberlite. The broken crystals of quartz in this assemblage and the abundance of magnetite are compatible with the inference of a serpentinized mica peridotite. Palagonite is rare in the samples containing the ultrabasic as- semblage. The ultrabasic assemblage found in the coarse fractions of piston-core 11 samples shows some variation with depth; the 27-32-cm sample contains only phlogopite and serpentine whereas the deeper samples contain magnetite, palagonite, biotite, and antigorite. Though this variation may be related to shifts in the source-rock supply with time, it seems likely that the variation is a function of the size distribution and hydraulic ratio of each mineral in relation to the fo raminiferal sand size, as has been found so often in heavy- mineral studies.

Basic //olcanic A ssem blage. The coarse fractions from all samples other than those from core I I, including the dredge samples from CHAIN 35 and CHAIN I 7, are derivable from basic volcanic rocks or greenstones such as those from the Ridge described recently (Melson et al. 1966). In addition to the obvious fragments of basalt and the abundant palagonite and basic glass, we note the variety of iron-rich chlorites, the presence of magnetite and biotite, plagioclase feldspar, and the association with phillipsite.

Heterogeneity, Mixing, and M arine Erosion. The distances are small be- tween sediment samples of drastically different coarse-fraction composition. Only 30 km separate the ultrabasic from the more " normal" basic volcanic suite. In spite of this small distance there seems to be little or no mixing. Phlogopite is abundant in an ultrabasic contribution to core Ir, but no phlogo- pite has been found in the samples that contain a basic volcani c assemblage. Conversely, chlorite is absent in the coarse fraction of the ultrabasic asse mblage. Evaluation of the possibility of local origin of some fine-grained chlorite is com- plex because of the large contribution of many compositional vari eties of chlorite from co ntinental so urces. Careful area mapping of compositional varieties and crystallinity of chlorites on the Ridge will be necessary, since it is unlikely that simple traverses such as that by ZEPHYRUS will give sufficient resolution (G oldberg et al. r 963). The differences between the coarse fractions in core 11 (ultrabasic assem- blage) and dredge 6 (palagonite and oli vine) show that, even within short distances in one basin, there is a diversity in source rock. The similarity of the fin e fractions in all samples from this pond shows the overshadowing effect of relati vely large amounts of terrigenous detritus. That this overshadowing may not be total is shown by consideration of core 22. The course fracti ons of core 22 from So uth Pond are similar in all Journal of Marine Research [ 2 5,3 respects to the coarse fractions from the normal basic volcanic assemblage at North Pond and the Median Valley, but the fine fractions of core 22 are only partly similar to the fine fractions of core 11. The important differences in fine fractions are in the montmorillonite-illite and plagioclase-K-spar ratios. In all sampled areas except South Pond there is a great abundance of mont- morillonite and plagioclase; and the simplest explanation for this is that there has been at least moderately extensive marine weathering of basalt in North Pond and the Median Valley as compared with South Pond, where there is only a terrigenous contribution. The range of mineral ratios in the fine frac- tion of samples from the Median Valley area (free-core 14, dredge 3, and THV-4, 11, 15) reflects the varying amounts of chlorite, montmorillonite, and plagioclase from marine weathering of basalt and greenstone. Our present knowledge of the pattern of chemical and mechanical marine erosion cannot account for the lack of detrital mixing, the presence of coarse grains of local derivation, and the seeming absence in the fine fraction of a large contribution from local sources. Furthermore, it is not possible to con- ceive of a current that would not transport the fine material as well as the coarse material. It is concluded, therefore, that only a limited amount of fine material is produced by the marine weathering process or that there is a con- tribution to the fine material that is not diagnosed as such by present identifica- tion methods. Examination of many of the altered rock and mineral fragments of these samples suggests that fine material may be produced by submarine chemical weathering; however, the chloritic and montmorillonitic products are so similar to the terrigenous clay products that the oceanic contributions are not distinguished from the continental contributions by ordinary powder x-ray diffraction-spectrometer methods. Although we have identified small amounts of zeolites, talc, and sepiolite in the fine fraction and have inferred that they are from indigenous sources, we suspect that many more zeolites and other kinds of silicates could be identified by using automated slow-scan- ning high-counting-rate diffraction techniques. In proposing that some indigenous contribution exists, we do not dispute the contention of Goldberg and Griffin (1964) or Biscaye (1965) that the bulk of the detrital sediment is eroded from the continents. The coarse frac- tions, of local origin, constitute less than 1°/ 0 of the foraminiferal oozes. The fraction of fine material that is oflocal origin may be less than 10°/ 0 in samples such as those from core I 1, although they may be the largest part of some samples such as THV-15. The existence of currents can be inferred from the distribution of the coarse fragments of rocks that are not likely to have been distributed by ex- plosi?n breccias or v~l~nic ash falls. The currents are implied also by the grading of the foramm1feral oozes. The local nature of the coarse-fraction miner~l~gy, the size of t_he rock particles, and the foraminiferal tests suggest a turb1d1ty current. The isolated nature of the ponded sediment basins militates Siever and Kastner: Mid-Atlantic Ridge Sediments against the concept of a general current system; if such a system existed we would expect much more mixing of the detritus than we have found.

Conclusions. l. There is a local contribution of detritus from mechanically broken igneous and metamorphic rocks of the Mid-Atlantic Ridge (some chemically weathered) to the ponded sediment basins of the Ridge Province. 2. The bulk of the fine detrital fraction of the sediment is derived from continental sources. 3. The heterogeneous nature of coarse sedimentary debris obtained from two sediment ponds indicates that there is a heterogeneity in the outcrop pattern of the hard rocks on the Ridge. The mineral and rock-fragment com- position indicates the presence of ultrabasic, basic volcanic, and metamorphic greenstone outcrop areas within short distances of each other. 4. Localized turbidity currents are the most likely transportation agent for the coarse and fine hard-rock detritus and foraminiferal ooze. 5. Though chemical weathering is demonstrable in the sea, its magnitude is not sufficient to obliterate differences due to primary origin, except in local tectonically active areas. 6. Study of the coarse fractions of sediments, particularly of noncarbonate fractions in the widespread foraminiferal oozes, may be a good way to trace the sedimentary and tectonic history of portions of the .

.Acknowledgments. We are indebted to all those who helped collect the samples on the various cruises. We extend special thanks to Tj. van Andel, V. T. Bowen, P. L. Sachs, K. C. Beck, and J. Hanor. V. T. Bowen and 'IJ. van Andel have been constant in their counsel and have read and criticized the manuscript. The detailed map of the area was prepared by Tj. van Andel. The samples were collected: on cruises of the Woods Hole Oceanographic Institution, supported variously by the U.S. Atomic Energy Commission under contracts AF(30-1)-2174 and AT(30-1)-3010, by the U.S. National Science Foundation under contracts GP 921 and GP 1599, and by the U.S. Office of Naval Research under contract NoNR-2196(00); and on a cruise of the Scripps Institution of Oceanography supported under various National Science Foundation grants. Laboratory work was supported by the Petroleum Research Fund of the American Chemical Society, by the U.S. Atomic Energy Commission under contract A T(30-1)-2174, and by the Committee on Experimental and Geophysics at Harvard University. 'Journal of Marine Research

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