Low-temperature fluid–rock interaction—an isotopic and mineralogical perspective of upper crustal evolution, eastern flank of the Juan de Fuca Ridge (JdFR), ODP Leg 168

Abstract

During ODP Leg 168, 10 sites were drilled across the eastern flank of the Juan de Fuca Ridge (JdFR), to examine the conditions of fluid–rock interaction in three distinct hydrothermal regimes (referred to as the Hydrothermal Transition (HT), Buried Basement (BB) and Rough Basement (RB) transects), extending over a ∼120 km linear transect perpendicular to the spreading ridge. This was carried out in an attempt to constrain the conditions and processes that control the location, style and magnitude of low temperature (<150°C) fluid–rock interaction within this setting. This paper presents new data on the petrology, mineral chemistry and whole rock strontium and oxygen isotopic compositions of basalts from the eastern flank of the JdFR, in order to investigate the extent, style and sequence of low-temperature hydrothermal alteration and to establish how the hydrothermal regime evolved with time. Throughout the flank, a progressive sequence of low-temperature hydrothermal alteration has been identified, marked by changes in the dominant secondary mineral assemblage, changing from: chlorite+chlorite/smectite; to iron oyxhydroxide+celadonite; to saponite±pyrite; culminating at present with Ca- to CaMg(±Fe,Mn)-carbonate. The changes in secondary mineralogy have been used to infer a series of systematic shifts in the conditions of alteration that occurred as the basement moved off-axis and was progressively buried by sediment. In general, hydrothermal alteration of the uppermost oceanic crust commenced under open, oxidative conditions, with interaction between unmodified to slightly modified seawater and basaltic crust, to a regime in which circulation of a strongly modified seawater-derived fluid was more restricted, and alteration occurred under non-oxidative conditions. Across the flank, petrological observations and microprobe analyses indicate that the observed ranges in secondary mineral composition are directly related to changes in the geochemical and textural characteristics of the basement, as well as to interaction between fluids and phases from the four stages of alteration. This is suggestive of an increase in fluid–rock increased with time. Whole rock ${}^{87}\text{Sr}\text{/}{}^{86}\text{Sr}$ and $\text{δ}{}^{18}\text{O}$ analyses of basalts from across the eastern flank of the JdFR reinforce petrological observations, with ${}^{87}\text{Sr}\text{/}{}^{86}\text{Sr}$ and $\text{δ}{}^{18}\text{O}$ values slightly elevated above accepted pristine MORB values for this region. These results are consistent with an increase in the amount of fluid–rock interaction with time. Across the flank, enrichment in the ${}^{87}\text{Sr}\text{/}{}^{86}\text{Sr}$ and $\text{δ}{}^{18}\text{O}$ relative to MORB, is influenced by a number of factors, including: local and regional variations in the crustal lithology and structure; the age of the crust; the extent of bulk rock alteration; and theoretically, the relative abundance of different isotopically-enriched secondary mineral phases in the crust.

Introduction

Although the effects of hydrothermal interaction at mid-ocean ridge axes are extremely important, theoretical modelling and direct observations have shown that mid-ocean ridge flanks lose more than three times as much heat as active mid-ocean ridge axes. This in turn equates to more than 10 times as much seawater being able to flux through the flanks, as at the ridge axes (Stein and Stein, 1994). Furthermore, as more than one third of the present oceanic crust is actively involved in low-temperature hydrothermal alteration within a flank setting (Anderson et al., 1977; Stein and Stein, 1994), these areas play an important role in controlling the global distribution of elements and heat in the Earth's geochemical and thermal budgets.

In general, the extent and type of low-temperature alteration (i.e., <150°C; Honnorez, 1981) that occurs in young oceanic crust is influenced by a number of factors related to the structure of the basement (e.g., rock type, permeability, primary and secondary structure) and the nature of the fluid involved in alteration (e.g., salinity, temperature, f_O2). This results in the products of low-temperature alteration varying considerably on both a local and regional scale, making it difficult to constrain the primary processes that influence the extent, location and type of alteration.

In an attempt to restrain these processes, a series of holes were drilled across three distinct hydrothermal regimes on the eastern flank of the Juan de Fuca Ridge (JdFR), during ODP Leg 168 (Fig. 1), in which the different hydrothermal regimes had been recognised during previous seismic and geophysical surveys and shallow drilling expeditions (Davis et al., 1992; Wheat and Mottl, 1994; Thomson et al., 1995). The primary aim of ODP Leg 168 was to use a variety of petrological, geochemical, hydrologic and geophysical methods to establish what controlled the extent of fluid–rock interaction across the flank, noting how the fluid chemistry, as well as the fluid flux and nature of hydrothermal alteration evolved with time.

This paper uses a combination of petrological, mineral chemistry and whole rock Sr- and O-isotope analyses to examine the extent, style and sequence of low-temperature hydrothermal alteration across the eastern flank of the JdFR. The effects of a varying basement lithology and changes in the hydrothermal fluid composition on the evolution of the low-temperature hydrothermal regime, are also examined. Throughout the eastern flank of the JdFR, the sequence of alteration phases found corresponds with that recorded by a number of other workers on both young and old oceanic crust, including the East Pacific Ocean (e.g., Bass, 1976; Honnorez et al., 1983; Alt et al., 1986, Alt et al., 1992; Laverne et al., 1996; Teagle et al., 1996), West Pacific Ocean (e.g., Natland and Mahonney, 1981; Alt, 1993); and Mid-Atlantic Ocean (e.g., Böhlke et al., 1980; Alt and Honnorez, 1984; Adamson and Richards, 1990). By examining how the secondary mineral assemblage varies over time, a sequence of changing conditions of alteration can be proposed.