Contributions to the mineral chemistry of Hawaiian rocks

Abstract

Phenocryst and groundmass pyroxenes in 24 rocks of the tholeiitic, alkalic, and nephelinic suites from Haleakala and West Maui volcanoes, Maui, Hawaii, were analyzed quantitatively by electron microprobe. Results and conclusions: i) Tholeiites contain augite, pigeonite, and bronzite; alkalic rocks contain salite, augite, and ferroaugite; and nephelinic rocks have salite, sometimes of Wo>50 mole %. ii) The three suites can be distinguished by Ca contents of pyroxenes: High-Ca pyroxenes of tholeiitic rocks have Wo_30–40; those of alkalic rocks have Wo_38–48; and those of the nephelinic rocks have Wo_47–51; i.e. Wo in clinopyroxene increases from tholeiitic, to alkalic, to nephelinic suites, iii). In the alkalic suite, rock types can be distinguished on the basis of clinopyroxene composition: Alkalic olivine and alkalic basalts have Wo_38–45, hawaiites and mugearites have Wo_45–48. Trachytes can be distinguished from both groups by higher Fe (Fs_22–30) and Ca contents (Wo_43–47). iv) Pyroxenes in tholeiitic rocks show higher intrarock variability (e.g. Fs₁₂Wo₄₀-Fs₃₇Wo₃₀) than those of the alkalic and nephelinic suites, v) Na₂O bulk-rock content affects Na₂O content of the precipitating high-Ca pyroxene; e.g. Na₂O in groundmass pyroxene increases from tholeiitic, to alkalic (mafic members only), to nephelinic suites; a similar relationship is present within the differentiated alkalic suite, vi) In tholeiites, changes in groundmass high-Ca pyroxene compositions are related to changes in bulk rock compositions, e.g. FeO/FeO+MgO+CaO in clinopyroxene increases as this ratio increases in the bulk rock; this is not true for alkalic and nephelinic rocks, vii) In groundmass high-Ca pyroxene, Al₂O₃, Na₂0, and TiO₂ contents increase and MnO content decreases with increasing Wo content from tholeiitic, to alkalic (mafic members only), to nephelinic suites, viii) Groundmass high-Ca pyroxenes are richer in MnO and Na₂O and poorer in Cr₂O₃ compared to coexisting phenocrysts. High-Ca pyroxene phenocrysts in nephelinic rocks and in one mugearite are depleted in SiO₂ and enriched in Al₂O₃ relative to coexisting groundmass clinopyroxene, indicating increased SiO₂ activity during crystallization. Some tholeiites show the reverse; this Si—Al relationship is not clear in other samples.