U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications

Geological indexes of hydrothermal venting

Edward T. Baker

Pacific Marine Environmental Laboratory, NOAA, Seattle, Washington

Journal of Geophysical Research, 101(B6), 13,741–13,753 (1996)
Not subject to U.S. copyright. Published in 1996 by the American Geophysical Union.

Introduction

Hydrothermal venting occurs along submarine spreading centers of virtually every spreading rate, morphological classification, and crustal structure. Soon after its discovery in the late 1970s, investigators began searching for a systematic relation between ridge crest characteristics and the distribution of hydrothermal sources [e.g., Rona, 1978]. Francheteau and Ballard [1983] and Crane [1985] proposed an early paradigm based on the simplifying assumption that a three-dimensional pattern of melt delivery would create tectonic segments with uniform bathymetric profiles. They proposed that on average, hydrothermal venting on any segment would be greatest at the shallowest bathymetric point and decrease toward the deepening segment ends. They noted that complete hydrothermal surveys of many segments would be useful in testing this paradigm.

Several years later, Macdonald and Fox [1988] categorized ridge morphology and axial magma chamber (AMC) occurrence [Detrick et al., 1987] along the East Pacific Rise (EPR) from 9° to 13°N in an effort to develop indicators of spatial and temporal variations in the axial magmatic budget. They concluded that axial areas with a broad cross section, an axial summit graben (or a graben recently buried by fresh lava flows), and a shallow AMC reflector were enjoying a comparatively robust magmatic budget. They were unable to extend this correlation to the distribution of hydrothermal venting because of a paucity of information about the location of discharge sources. This state persisted until recently, largely because most observations of hydrothermal venting were localized and therefore unrepresentative of segment-scale geological characteristics. Moreover, the expected gross inequality between the timescales of hydrothermal fluctuations and ridge crest structural changes encouraged the assumption that the spatial pattern of hydrothermal venting would be only poorly sensitive to ridge crest structural or thermal variations. Simply put, it seemed that almost any section of intermediate to superfast spreading ridge crest could provide sufficient heat to drive hydrothermal venting.

During the last several years, coordinated and quantitative studies of ridge morphology, ridge structure, basalt petrology, and hydrothermal venting patterns have furnished new and detailed data sets. Any skepticism that a close correspondence could exist between geological and hydrothermal patterns was dispelled by comprehensive and detailed studies within individual segments [e.g., Embley et al., 1991; Haymon et al., 1991]. These and similar data now enable an examination of the geological indexes of hydrothermal venting on intermediate to superfast spreading ridges at the scale of individual tectonic segments. Scheirer and Macdonald [1993] have quantified ridge crest morphology with measurements of cross-axial inflation along nearly the entire EPR from 18°N to 23°S. Multichannel seismic (MCS) investigations of the EPR from 9° to 13°N [Detrick et al., 1987] and 14° to 20°42S [Detrick et al., 1993] have provided unprecedentedly detailed views of the seismic structure of the ridge crest. Fine-scale petrographic sampling of recent lavas in these same areas [e.g., Thompson et al., 1985; Langmuir et al., 1986; Sinton et al., 1991; Batiza and Niu, 1992; Perfit et al., 1994] has yielded information on the temperature and composition of magma beneath the ridge crest. Finally, multisegment patterns of hydrothermal discharge have been deduced from continuous mapping of temperature, optical, and chemical anomalies in waters above the Juan de Fuca Ridge (JDFR, 44°30 to 48°30N) [Baker and Hammond, 1992], northern EPR (8°40 to 13°N) [Bougault et al., 1990; Baker et al., 1994] and southern EPR (13°50 to 18°40S) [Urabe et al., 1995; Baker and Urabe, 1996].

Using these and other data, I examine here the correlation between hydrothermal venting patterns and key morphological, structural, and petrological parameters on intermediate, fast, and superfast spreading segments of the mid-ocean ridge. The correlations are examined first at a fine scale of continuous along-axis variations, then on a scale of second- to fourth-order [Macdonald et al., 1991] tectonic segments, and finally at a multisegment, regional scale.

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