Earth and Planetary Sciences ETDs

Publication Date



The composition of major mineral phases in four rocks from the equatorial Mid-Atlantic Ridge, as determined by electron microprobe analysis and supplemented by additional mineralogic and petrographic study, have provided data for comparison with continental mafic assemblages. The compositional range of coexisting minerals in the samples: a nepheline-normative gabbro (teschenite), an olivine tholeiite, and two uralitized norites, is similar to the ranges of comparable continental suites. The teschenite contains pyroxene (Wo48-46 En37-23Fs15-31) with high amounts of CaO (20.3-22.2 wt %), Al2O3 (2.9-6.8 wt. %), and Ti02 (1.2-2.9 wt. %) characteristic of continental teschenitic intrusives such as the Black Jack and Shiant Isles sills. Plagioclase compositions (An75-10) are also comparable to plagioclase in those assemblages. Secondary analcite is present in miarolitic cavities where it formed prior to the growth of natrolite and gonnardite. Sample At90A, a PL-group olivine tholeiite contains euhedral phenocrysts of plagioclase (An86-40) and olivine (Fo85-75) in a groundmass suggestive of eutectic crystallization of plagioclase (An81-32), olivine (Fo76-70), and augite (Wo42-35En45-38Fs13-27). The two uralitized norites are very similar texturally and compositionally. Both are high-iron, high titanium rocks (FeO + .9Fe203 = 17.3 and 18.2 wt. %; TiO2 = 4.0 and 5.9 wt. %). The latter is due largely to the presence of nearly 8 modal percent ilmenite. Plagioclase and pyroxene in these rocks have compositions similar to corresponding phases in continental layered intrusives such as Skaergaard and Bushveld. The data suggest that the petrogenetic differentiation processes which produced these low K, abyssal oceanic rocks are not drastically different from those which obtain in continental areas.

Degree Name

Earth and Planetary Sciences

Level of Degree


Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Klaus Keil

Second Committee Member

Martin Prinz

Third Committee Member

Douglas Gridley Brookins

Fourth Committee Member

Jonathan Ferris Callender



Document Type


Included in

Geology Commons