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 r ocks in response to the emplacement of similar to 1.1Ga mafic plutonic rock s during intracontinental rifting in the midcontinent region. Oxygen and hy drogen isotopic studies of the North Shore Volcanic Group (NSVG) (northeast ern Minnesota) and related hypabyssal sills demonstrate the importance of b oth permeability and temperature in controlling isotopic exchange in the ri ft-related hydrothermal system. Oxygen isotopic values of mineral separates and volcanic whole rocks range from 5.5 parts per thousand to 17.7 parts p er thousand, and show a strong correlation with whole rock water content an d intensity of hydrothermal alteration. In the shallow portion of the strat igraphic sequence, elevated delta O-18 values and mineralogical alteration are confined principally to highly permeable amygdaloidal flow tops and bot toms. Massive flow interiors show little or no perturbation from delta O-18 values considered normal for fresh basaltic rocks. With increased depth an d temperature, isotopic exchange becomes more pervasive, delta O-18 values remain elevated, and oxygen isotopic profiles show no difference between fl ow margins and interiors. At depths where greenschist facies mineral assemb lages are found, flow margins show a reversal to delta O-18 values as low a s 6.6 parts per thousand and deltaD values decrease to < - 70<parts per tho usand>. Both increased temperature and the involvement of a relatively D-de pleted fluid at depth have played a role in lowering previously elevated de lta O-18 and deltaD values. Enhanced rates for hydrogen isotopic exchange r elative to oxygen are suggested by whole rock deltaD values that tend to be uniform throughout flows that are less than similar to 7 m thick. deltaD p rofiles that parallel those of delta O-18 values are found in thicker flows . The correlation between degree of hydrothermal alteration and delta O-18 va lues suggests that solution-recrystallization was the primary mechanism of isotopic exchange in the hydrothermal system. Individual feldspar phenocrys ts from a single lava flow may show a range of delta O-18 values from 10.5 parts per thousand to 17.7 parts per thousand, and ranges in individual alt ered phenocrysts may be as large as 4.8 parts per thousand. Feldspar isotop ic heterogeneity suggests that permeability varied on a grain to grain scal e, and that flow through microfractures was important in controlling isotop ic variations. Coexisting plagioclase and pyroxene from hypabyssal sills sh ow a very steep slope on a delta O-18(plagioclase)-delta O-18(pyroxene) plo t, also suggestive of kinetically dominated exchange. Assuming a fluid-buff ered 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 compa red to the lifetime of the intrusive activity in the rift, and suggests tha t fluid flow was either episodic in nature, or confined to the late stages of magmatic evolution. (C) 2001 Elsevier Science B.V. All rights reserved.