207 



Composiiton oj source rocks 



The very narrow range in composition of fresh abyssal tholeiitic basalts so far analy- 

 zed has been pointed out by se\eral authors (e.g., Engel et al., 1965). Thus, it is likely 

 that the compositions of the source rocks were probably within the range of compo- 

 sition of previously analyzed abyssal basalts. Tiiis conclusion is in accord with pctro- 

 graphic features, such as the relict calcic plagioclase microlitcs and phenocrysts, 

 relict brown spinel, and chloritized olivine phenocrysts (Fig. 6). 



Comparison with Mid-Atlantic Riclgc abyssal basalts 



Table VII gives the average composition and standard deviations of published 

 analyses available to the writers of abyssal basalts from the Mid-Allantic Ridge. 

 These include two analyses of fresh abyssal tholeiitic basalts in dredge 2 from 22'N 



1 AHi.i; Ml 



COMI'OSI ,1(1N 0\ GUI 1 NSliiM S COMrARI 1) TO A\ 1 R \t'.l 0\ 1 Ki:.SH Ain SSAL DASALIS DIU DGLD liiOM 1 HFi 

 Min-Ail AMIC Kint.l 



bastili ' 



Shiihlaiil ^. , , , . , 



, . . Di/ crciiccs t'ciuccn ''icciisloiic aiui avcra<'. ascitl 



ilCMCUWII •'■' '^ "^ 



3-2 



3-3 



3-6 



3-7 



^ Average compositioit of 15 fresh basalts dredged from or near the Mid- Atlantic Ridge. Analyses 

 taken from Correns (1930, 1 analysis); Nicholls (1964, 3 analyses; analyses of rind and interior of 

 same specimen were averaged); Mum et al. (1964, 5 analyses); Engel and Engel (1964, 4 analyses); 

 and two analyses of fresh abyssal basalts from the 22'N area (dredge 2). 



which will be discussed in detail elsewhere. Also included are the difterences between 

 this average and the compositions of the greenstones. Several of these are significantly 

 larger than the standard deviations of the average Mid-Atlantic Ridge abyssal basalt. 

 These are CaO, Fe203, FeO and MgO, the alkalis, and combined water (H2O"). 



