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

摘要

Hydrothermal flow systems developed in volcanic and interflow sedimentary rocks in response to the emplacement of similar to 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 parts per thousand to 17.7 parts per thousand, and show a strong correlation with whole rock water content and intensity of hydrothermal alteration. In the shallow portion of the stratigraphic sequence, elevated delta O-18 values and mineralogical alteration are confined principally to highly permeable amygdaloidal flow tops and bottoms. Massive flow interiors show little or no perturbation from delta O-18 values considered normal for fresh basaltic rocks. With increased depth and temperature, isotopic exchange becomes more pervasive, delta O-18 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 delta O-18 values as low as 6.6 parts per thousand and deltaD values decrease to < - 70arts per thousand>. Both increased temperature and the involvement of a relatively D-depleted fluid at depth have played a role in lowering previously elevated delta O-18 and deltaD values. Enhanced rates for hydrogen isotopic exchange relative 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 profiles that parallel those of delta O-18 values are found in thicker flows. The correlation between degree of hydrothermal alteration and delta O-18 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 delta O-18 values from 10.5 parts per thousand to 17.7 parts per thousand, and ranges in individual altered phenocrysts may be as large as 4.8 parts per thousand. 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 delta O-18(plagioclase)-delta O-18(pyroxene) 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. (C) 2001 Elsevier Science B.V. All rights reserved. [References: 45]