Mechanisms and patterns of O and H isotopic exchange during hydrothermal alteration of the North Shore Volcanic Group and related hypabyssal sills, Midcontinent Rift System, Minnesota

Hydrothermal flow systems developed in volcanic and interflow sedimentary rocks in response to the emplacement of ∼1.1Ga mafic plutonic rocks during intracontinental rifting in the midcontinent region. Oxygen and hydrogen isotopic studies of the North Shore Volcanic Group (NSVG) (northeastern Minnesota) and related hypabyssal sills demonstrate the importance of both permeability and temperature in controlling isotopic exchange in the rift-related hydrothermal system. Oxygen isotopic values of mineral separates and volcanic whole rocks range from 5.5‰ to 17.7‰, and show a strong correlation with whole rock water content and intensity of hydrothermal alteration. In the shallow portion of the stratigraphic sequence, elevated δ18O values and mineralogical alteration are confined principally to highly permeable amygdaloidal flow tops and bottoms. Massive flow interiors show little or no perturbation from δ18O values considered normal for fresh basaltic rocks. With increased depth and temperature, isotopic exchange becomes more pervasive, δ18O values remain elevated, and oxygen isotopic profiles show no difference between flow margins and interiors. At depths where greenschist facies mineral assemblages are found, flow margins show a reversal to δ18O values as low as 6.6‰ and δD values decrease to <−70‰. Both increased temperature and the involvement of a relatively D-depleted fluid at depth have played a role in lowering previously elevated δ18O and δD values. Enhanced rates for hydrogen isotopic exchange relative to oxygen are suggested by whole rock δD values that tend to be uniform throughout flows that are less than ∼7 m thick. δD profiles that parallel those of δ18O values are found in thicker flows. rrelation between degree of hydrothermal alteration and δ18O values suggests that solution-recrystallization was the primary mechanism of isotopic exchange in the hydrothermal system. Individual feldspar phenocrysts from a single lava flow may show a range of δ18O values from 10.5‰ to 17.7‰, and ranges in individual altered phenocrysts may be as large as 4.8‰. Feldspar isotopic heterogeneity suggests that permeability varied on a grain to grain scale, and that flow through microfractures was important in controlling isotopic variations. Coexisting plagioclase and pyroxene from hypabyssal sills show a very steep slope on a δ18Oplagioclase–δ18Opyroxene plot, also suggestive of kinetically dominated exchange. Assuming a fluid-buffered system in the permeable flow margins, a duration of isotopic exchange on the order of 40,000 years is calculated. This time period is short compared to the lifetime of the intrusive activity in the rift, and suggests that fluid flow was either episodic in nature, or confined to the late stages of magmatic evolution.