Distribution of Rb, Sr and Ni in igneous rocks, central and western Aleutian Islands, Alaska

Details Distribution of Rb, Sr and Ni in igneous rocks, central and western Aleutian Islands, Alaska Distribution of Rb, Sr and Ni in igneous rocks, central and western Aleutian Islands, Alaska

Feb 1974

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1974gecoa..38..245d&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 38, Issue 2, pp.245-266

Computer Science

8

Scientific paper

New analyses of Rb, Sr and Ni in 64 igneous rocks from the Aleutian Islands reveal the following geochemical characteristics: 1. (1) trends of increasing Rb content, Rb/Sr, and K/Sr and decreasing K/Rb in the sequence basalt-andesite-dacite; 2. (3) low Rb content and high K/Rb (always > 300); 3. (3) Sr content approximately equal in the three major rock types on the average; and 4. (4) generally low Ni content ( < 20 ppm in most of the basalts and all but one of the more siliceous rocks). These results, together with the known major-element chemistry of the rocks, indicate serious quantitative difficulties in the derivation of the Aleutian andesites from basalt magma by fractional crystallization or by contamination with sea-floor sediment. Calculations of the fractionation of K, Rb, Sr and Ni during partial melting of several ultramafic and mafic assemblages indicate the following: 1. (1) if andesite is derived directly from mantle peridotite by hydrous partial melting, the extent of melting must be small ( < 10 per cent). 2. (2) The average composition of six Aleutian basalts with tholeiitic affinities can be closely matched by about 30 per cent partial melting of a high-pressure, hydrous assemblage (garnet amphibolite) with the major- and trace-element chemistry of oceanic tholeiite. 3. (3) The K---Rb---Sr distribution observed in the entire spectrum of Aleutian rocks can be approximated by 1-30 per cent partial melting of altered oceanic tholeiite in a high pressure, anhydrous assemblage (garnet granulite). The exact means of deriving Ni-poor rocks from this assemblage is not clear, but several possibilities exist.

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