Changes in submarine hydrothermal ³He/heat ratios as an indicator of magmatic/tectonic activity

Edward T. Baker

Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, 7600 Sand Point Way NE, Seattle, Washington 98115, USA

John E. Lupton

Marine Science Institute and Department of Geological Sciences, University of California, Santa Barbara, California 93106, USA

Nature, 346, 556-558 (1990)
Copyright ©1990 Macmillan Publishers Ltd. Further electronic distribution not allowed.

When the megaplume at the north end of the Cleft segment of the Juan de Fuca Ridge was discovered in August 1986, an extensive steady-state hydrothermal plume was also found at the same location [Baker et al., 1987; Lupton et al., 1989]. The steady-state plume has been annually mapped by near-bottom conductivity-temperature-depth transmissometer (CTDT) tows and casts, and sampled for He and other constituents using rosette-mounted Niskin bottles. The temperature anomaly ( T) of the plume relative to ambient water of the same potential density is calculated using the formula [Lupton et al., 1985]

where and are the potential temperature and potential density, respectively, in the plume, and m and b are the slope and intercept, respectively, of the linear regression of the - curve of water immediately above the plume where the hydrographic effect of hydrothermal emissions is negligible (Fig. 1). The slope m is constant at -4.865°C m Kg each year, whereas the intercept b is adjusted to compensate for changes in arising from annual variations of ±0.002 in the salinity calibration. The calculated T values may be an underestimate of the true hydrothermal heating by as much as a factor of two owing to cooling of the plume by entrainment of cold bottom water [Baker et al., 1989; Speer and Rona, 1989], and hydrographic aliasing if vent waters have a salinity substantially different from that of ambient sea water [McDuff, 1988]. Despite some uncertainty in the absolute value of T, we assume that the year-to-year values are correct relative to each other because the consistency of the ambient hydrographic structure and the rise height of the plume (Fig. 1) implies a consistency of plume entrainment processes and vent-fluid salinity.

Fig. 1. Potential density plotted against potential temperature for selected axial valley CTDT casts in 1986, 1987 and 1988. Each curve has a slope of -4.865°C m Kg (shown by the straight line) except where hydrothermally affected. 1986 casts in the axial valley sampled only a thin horizon of ambient water (~27.635-27.640 ) between the steady-state plume and the overlying megaplume. Density of the water in the axial valley was significantly lower in 1986 because of enhanced vertical mixing of the lowermost 1,000 m by the megaplume discharge. Such vertical mixing does not affect the ambient - relationship. The maximum temperature anomaly, defined as the temperature deviation from the background trend along an isopycnal, was about the same on each cast. Insets show vertical profiles of the temperature and light-attenuation (proportional to the concentration of fine-grained hydrothermal precipitates) anomalies for each cast. Increased anomalies above 1,950 m in 1986 indicate the megaplume presence. Depth of the steady-state plume maximum was ~2,100 m each year. For each cast the light-attenuation and temperature anomalies go to zero at the same depth, supporting our interpretation of the vertical distribution of temperature anomaly.

Figure 2 shows areal maps of T in the steady-state plume created by plotting and contouring maximum T values along the tow track-lines and at the vertical cast locations. The size and intensity of the T plume indicate that venting extends more than 20 km along the ridge axis with a vigour comparable to the vent field at 48°N on the Endeavour segment of the Juan de Fuca Ridge, where the heat flux is calculated to be 1-5 × 10 W [Rosenberg et al., 1988; Baker and Massoth, 1987]. When the megaplume vent field was discovered in 1986, T values 0.04°C extended from a few kilometres west of the axial valley to the limits of the survey area to the east and north (Fig. 2a). Plume mapping in 1986 was not detailed enough to locate precisely the vent field creating the plume, but the fact that the highest light-attenuation anomalies were found in the axial valley near station SC22 suggests that the source was in the axial valley and the plume distribution was the result of advection to the east.

Fig. 2. Areal maps of the steady-state plume as described by the maximum temperature anomaly between 2,000 m and 2,200 m along CTDT tow tracks (dashed lines) and vertical casts (crosses or solid symbols) in 1986, 1987 and 1988. He samples were collected during tows (open symbols) or on vertical casts (solid symbols); the locations of the Fig. 1 profiles are indicated. Hatched areas are the walls of the axial valley as defined by the 2,200-m bathymetric contour. In each year the plume was ~200 m thick and centred ~150 m above the floor of the axial valley (see Fig. 1). The plume was more extensive in 1986 than in either 1987 or 1988. The size and location of the 1986 megaplume is shown by shading.

In 1987 and 1988 the T plumes were less extensive and more symmetrical about the axial valley (Fig. 2b, c). Temperature anomalies in the axial valley averaged 0.03-0.04°C with localized highs reaching 0.06°C. Although quantitative flux estimates cannot be derived from plume maps alone, it seems likely that the release of the August 1986 megaplume was accompanied by a higher heat and mass flux in the underlying steady-state plume than has since been characteristic of this vent field.

Unequivocal evidence for temporal variability in hydrothermal processes at this site comes from the progressive decrease in the He/T ratio of the steady-state plume (Fig. 3). In August 1986, the He/T ratio in the plume was (4.4 ± 0.64) × 10 cm STP g °C (1 uncertainty), the highest ratio yet reported for hydrothermal emissions on a mid-ocean ridge and a factor of 14 higher than the He/ T ratio in the overlying megaplume [Lupton et al., 1989]. Concentrations of He in the plume were as high as 0.30 × 10 cm STP g (He = 238 ; He = [(He/ He)/(He/He)]-1). In September 1987 the ratio in the steady-state plume decreased to (2.4 ± 0.14) × 10 cm STP g°C (1), a factor of 1.8 below the 1986 value. By September 1988 the ratio had decreased further, to (1.3 ± 0.1) × 10 cm STP g°C (1), a factor of 3.4 below the 1986 value. The yearly decreases apparently resulted from a decreasing concentration of He in the plume, as the range of T values is about the same each year. In the following discussion we use the conservative assumption that hydrographic cooling and vent-fluid salinity changes had a negligible influence on the observed T values, so that the He/T ratios in the plumes are equivalent to the He/heat ratios in the original fluids. We cannot verify this because no high-temperature vents have yet been located at this site.

Fig. 3. He concentration plotted against temperature anomaly for samples in steady-state plume from 1986, 1987 and 1988. Sample locations shown in Fig. 2. Least-squares trend of 1986 megaplume samples (which had a maximum temperature anomaly of ~0.26°C) shown for comparison. Least-squares regression shows that the He/T ratio decreased from 4.4 to 2.4 to 1.3 × 10 cm STP g °C from 1986 to 1988. A t-test analysis indicates that these ratios are all significantly different from each other at the 95% confidence level.

We propose that the observed He/heat trend represents accelerated degassing from a magma body whose solidification rate was abruptly increased either by vertical intrusion into cooler crust or by a local deepening of the boundary between brittle and ductile deformation. Sub-surface releases of He have been reported at magmatically active terrestrial sites in Japan [Wakita et al., 1978; Sano et al., 1986] and Long Valley Caldera in California [Welhan et al., 1988] in association with earthquakes and local faulting. Two degassing processes can enhance the expulsion of He from a magma body and thereby temporarily increase the He/heat ratio of the surrounding hydrothermal fluids: pressure release or crystal-melt fractionation.

Because of the low and pressure-dependent solubility of C0 in silicate liquids [Stolper and Holloway, 1988], CO exsolution forms vesicles in a rising magma. Basalts from the Juan de Fuca Ridge have a mean vesicularity, of ~1% [Dixon et al., 1988]. Determinations of the solubility constant of He in a tholeiitic basalt melt [Jambon et al., 1986] show that for 1% vesicularity, 60% of the total He is contained in vesicles and is thus more readily available for loss from the melt. The discovery of basalts with [He] > 50 × 10 cm STP g, ten times larger than that of typical mid-ocean ridge basalts [Sarda and Graham, 1990], suggests that a large proportion of magmatic He is commonly lost by pre-eruptive degassing.

As the intrusion solidifies, He is progressively concentrated in the remaining melt by crystal-melt fractionation because the distribution coefficient for He between crystals and a basaltic melt is 0.1-0.01 [Kirsten, 1968; Kurz et al, 1982; Hiyagon and Ozima, 1986]. Freezing of a basaltic melt at a liquidus temperature of 1,150°C excludes 90-99% of the He from individual crystals but liberates only ~100 cal g, ~25% of the total heat lost on cooling to seawater temperature. A thick and slowly cooling intrusion might thus supply an elevated, but decreasing, He/heat flux for several years.

An alternative to magma intrusion is the incremental deepening of the cracking front [Lister, 1983] above a magma chamber (J. R. Delaney, personal communication). A sudden increase in the permeability of the crust, perhaps caused by tectonic stretching, might allow deeper penetration of cold sea water and an accelerating solidification of the magma.

Lupton et al. have suggested that He/heat variations in mid-ocean-ridge vent systems do not arise from geographic inhomogeneities in parent magmas but are symptomatic of dynamic conditions such as the age and permeability of the circulation system feeding the vent field. The changing He/heat ratios reported here support that prediction and may thus describe a changing hydrothermal circulation system. A sudden permeability increase caused by crustal fracturing associated with magma solidification could accelerate the release of preequilibrated fluids having a "normal" He/heat ratio, causing a megaplume event. Fluids remaining in the now rejuvenated, highly permeable, circulation system are enriched with degassing He faster than they are heated, causing He/heat ratios to exceed the theoretical ratio of ~2 × 10 cm STP cal expected for upper mantle magmas. Indeed, the He/heat ratio in the steady-state plume immediately after the megaplume disruption was more than twice this theoretical ratio.

As the system ages, circulation approaches equilibrium and the fluid pathways become more tortuous as a result of mineral precipitation. He/heat ratios decline as the residence time of fluids in the circulation system increases and the extraction rates for heat and He converge. The compositionally unchanging fluids at the 21°N vent field on the East Pacific Rise [Campbell et al., 1988] have a low He/heat ratio (0.4 × 10 cm STP cal) [Welhan and Craig, 1983; Lupton et al., 1980] that may reflect equilibrium conditions. Recent studies of Icelandic geothermal systems [Poreda et al., in press] and the degassing behaviour of mid-ocean-ridge magma [Sarda and Graham, 1990] also suggest that He/heat ratios can decline as a magmatic system ages. Based on our observations, the timescale for a return to equilibrium conditions can be of the order of a few years, in agreement with radiochemical estimates of the residence time of hydrothermal fluids in the crust [Kadko and Moore, 1988].

Acknowledgements. We thank G. Massoth for collecting the He samples, S. Walker for processing the CTDT data, K. Thornberry, D. Dion and A. Faizullabhoy for the He isotope measurements, and D. Graham for comments on the manuscript. This work was supported by the NOAA and the NSF.

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