In order to investigate magnetic processes operative within magma chambers underlying oceanic spreading centers, a suite of 59 samples was collected over an 8-m stratigraphic interval at the base of the cumulate ultramafic unit exposed on the North Arm Mountain Massif in the Bay of Islands ophiolite complex, Newfoundland. Interlayered dunites and wehrlites are present in the 8-m section. Low abundances of incompatible elements (Ti, Y, Zr) and other evidence suggest that these rocks originally formed as adcumulates. Olivine compositions are in the range Fo84.5--89.9 and clinopyroxene Mg ♯ (=Mg/Mg+Fe2+)¿100) range from 89.7 to 93.3. Spinel compositions are highly variable, and Mg ♯ and Al2O3 contents of spinels have been modified by subsolidus reequilibration with olivine and clinopyroxene. Cryptic variation patterns defined by the Mg ♯ of olivine and clinopyroxene are characterized by successive cryptic regressions that have a repeat interval averaging ~1 m. The cryptic regressions are believed to record influxes of fresh, relatively unfractionated magma into a magma chamber or conduit. The gradual decrease in the Mg ♯ of olivine and clinopyroxene and the decrease in olivine Ni contents between successive cryptic regressions indicate that ~10% crystallization of each batch of magma occurred before a new batch of magma entered the crystallization site of these cumulate rocks. In some cases, the cryptic regressions in mineral chemistry correlate with abrupt lithologic changes (from wehrlite to dunite), suggesting rapid displacement of the fractionated magma by more primitive incoming magma or rapid mixing of fresh incoming magma and resident magma. In other cases, inverse fractionation trends and fine scale interlayering of the cumulate rocks appear to represent zones of less rapid mixing between fractionated and fresh magmas.

Comparison of cryptic variation patterns at the base and the top of the cumulate ultramafic section on North Arm Mountain indicates that in some cases the liquids that crystallized to form the upper part of the unit were more primitive (higher MgO/FeO) than the liquids that crystallized to form the base of the unit. This feature is an indication of the complexity of processes that may operate in magma chambers underlying present-day oceanic spreading centers.