, Volume 31, Issue 4, pp 251-266

Crystallization of abyssal tholeiites

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In some abyssal tholeiite magmas having a relatively high content of olivine component, olivine is the first mineral to crystallize. In others having a relatively high content of plagioclase component, plagioclase is the first mineral to crystallize. The rooks of these two groups are called OL- and PL-tholeiites respectively. Continued crystallization drives the residual liquids to the cotectic curve between the olivine and plagioclase fields. Subsequent cotectic crystallization results in an increase of pyroxene component in residual magmas, eventually leading to the crystallization of clinopyroxene. Marked enrichment of iron and TiO2 takes place in later stages. The total iron/MgO ratio may be used as an indicator for the degree of fractional crystallization. Average compositions of OL- and PL-tholeiites are calculated for a series of total iron/MgO ranges (Table 7). PL-tholeiites are usually but not always higher in Al2O3 than OL-tholeiites.