221 



the basalt in the alkah oUvine basalt series. Deep sea 

 basalts may have "alkaline affinities"^, and may contain 

 abundant normative and modal olivine, but rarely con- 

 tain normative nepheline in the amount recorded here. 

 The high barium (300 p. p.m.), strontium (500 p. p.m.) and 

 zirconium (200 p. p.m.) further confirm the alkaline nature 

 of this basalt, although the high nickel (270 p. p.m.) and 

 chromiuna (250 p. p.m.) are anomalous for alkali basalts. 



Table 1. composition of alkali olivine basalt*, ^ 11-20, 43-49, dredged 



A FEW KIL0METRES NORTH-EAST OF ST. PAUL'S ROCKS 



Norm 



99-97 Analyst: E. Jarosewich 



* The analysed basalt is extremely fine grained and contains olivine phene- 

 crysts in a matrix of microlites of plagioclase (about Anso), titan-augite, 

 olivine and light brown barkevikitic hornblende. Accessories include an 

 iron-titanium oxide, biotite, apatite and possibly haiiyne. Small amounts 

 of clear glass and alkali feldspar also occur. Modal nepheline is not present. 



The predominance of low potash tholeiitic basalt on 

 the ocean floor and the apparent restriction of alkali 

 basalts to the top of high volcanic edifices has been 

 noted by Engel et al.^. In the Hawaiian Islands, alkali 

 basalts occur mainly as late, quantitatively minor extru- 

 sives. These relationships led some petrologists® to postu- 

 late, first, that alkali basalts are derived by low pressure 

 differentiation of sub-alkaline olivine basalts of the 

 "oceanic tholeiite" type in near surface magma chambers, 

 and second, that "oceanic tholeiite" magma is the only 

 basaltic magma derived from the mantle. Experimental 

 data indicate that low pressure differentiation of tholeiitic 

 magma should not yield normative nepheline liquids', 

 and thus argue against the first view. 



The alkali basalt described here also suggests mantle 

 derivation of alkali basalts. The basalt is clearly not part 

 of a thick "oceanic tholeiite" pile (Fig. 1), and thus based 

 on field occurrence alone is an unlikely low pressure 

 differentiate of "oceanic tholeiites". It would appear that 

 "parental" alkali basalt magma originates by some other, 

 more deep-seated phenomena, such as partial fusion of 

 eclogite of tholeiitic basalt composition — a process out- 

 lined by Yoder and Tilley'. 



The St. Paul's Rocks situation clearly indicates that it 

 is not always true that oceanic alkaline basalts occur as 

 late eruptives in otherwise tholeiitic sequences. Thus 

 caution should be used in tying the origin of all oceanic 



