216 



(Reprinted from Nature, Vol. 209, No. 5023, pp. 604-605, 

 February 5, 1966) 



Greenstones from the Central Valley of the 

 Mid- Atlantic Ridge 



In a preliminary report ^ of a recent study of the mor- 

 phology of the Mid- Atlantic Ridge about 22° N. latitude, 

 attention was directed to the dredging of a suite of 

 greenstones from the western face of the central valley. 

 Although siinilar rocks have been recently reported from 

 the Carlsberg Ridge^ in the Indian Ocean, rocks of this 

 type have not been previously recorded from the Mid- 

 Atlantic Ridge. Preliminary petrographic examinatioix 

 and chemical analysis of the Mid-Atlantic Ridge green- 

 stones justify the present interim report. 



The collection from the upper slope (Dredge 2, 2,670- 

 2,050 m) consists, as noted^, of fragments of greenstone, 

 basalt, and partly metamorphosed basaltic tuff. The 

 fragments exhibit a thin manganiferous coating, usually on 

 all sides. This obsei-vation and the angularity of the frag- 

 ments suggest that they were collected from a slowly 

 accumulating talus slope. The collection from the base 

 of the slope (Dredge 3, 4,000-3,200 m) consists of green- 

 stones, and rare fragments of diabase. The greenstone 

 fragments exhibit only very thiii and spotty mangani- 

 ferous coatings, rarely on more tliau two faces. The 

 distribution of these coatings, and the fresh uneroded 

 aspect of the uncoated faces, indicate that these rocks 

 had been only recently exposed. The preservation, 

 unbroken, of thin platey fragments in Dredge 2 we take 

 to indicate that the fresh surfaces of the Dredge 3 frag- 

 ments are unlikely to have been produced by fracturing 

 within the dredge. 



The greenstones consist of actinolite, epidote, chlorite 

 and plagioclase in various proportions. The latter is 

 mainly albito although some specimens contain relics of 

 more calcic plagioclase. Dredge 2 contains greenstones 

 which are essentially free of epidote whereas in those in 

 Dredge 3 epidote is abundant. Although searched for 

 specifically, zeolites and puinpellyite have not been noted 

 in the 13 greenstones which have so far been examined in 

 detail . 



Most of the greenstones are not deformed. In some of 

 the specimens shear planes occur but they are widely 

 spaced and give no well-defined fabric. Because of this 

 lack of intense penetrative deformation, the pre-meta- 

 morphism textures are preserved. These show that some 

 of the greenstone fragments were derived from submarine 

 basaltic flows and others from basaltic tuffs. 



One of the greenstones analysed has unusually high 

 sodium content (Table 1, No. 1). This specimen is textur- 

 ally characterized by albite veins, dispersed pyrite and 

 rare relict labradorite. These features suggest that the 

 'spilitic' composition is a result of hydrothermal albitiza- 

 tiori during metamorphism. Except with regard to their 

 water content, many of the greenstones (for example. 



This is Contribution Number 1697 from the Woods Hole Oceanogrupbic 

 institution and a contribution of Scripi)s Institution of Oceanograpiiy. 



