Flow of Mantle Fluids Through the Ductile Lower Crust: Helium Isotope Trends

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Science  30 Nov 2007:
Vol. 318, Issue 5855, pp. 1433-1436
DOI: 10.1126/science.1147537


Heat and mass are injected into the shallow crust when mantle fluids are able to flow through the ductile lower crust. Minimum 3He/4He ratios in surface fluids from the northern Basin and Range Province, western North America, increase systematically from low crustal values in the east to high mantle values in the west, a regional trend that correlates with the rates of active crustal deformation. The highest ratios occur where the extension and shear strain rates are greatest. The correspondence of helium isotope ratios and active transtensional deformation indicates a deformation-enhanced permeability and that mantle fluids can penetrate the ductile lithosphere, even in regions where there is no substantial magmatism. Superimposed on the regional trend are local, high 3He/4He anomalies indicating hidden magmatic activity and/or deep fluid production with locally enhanced permeability, identifying zones with high resource potential, particularly for geothermal energy development.

Mantle volatiles, principally water and CO2, play an important role in lithospheric rheology and the production of buoyant fluids that can be injected into the shallow crust. Regional and local trends in the crustal occurrence of mantle volatiles provide insight into the coupling between mantle-crust tectonics (1, 2), heat and mass exchange between the mantle and crust (35), and the occurrence and distribution of economic resources such as ore minerals and oil, gas, and geothermal fluids (6). Mantle-derived volatiles in the crust are traceable through He isotopic compositions of hydrologic fluids (7). Once injected into a crustal-fluid system, mantle He will be continuously diluted with radiogenic helium-4 (4He) acquired from the U,Th–rich crust, and therefore surface-fluid He isotopic compositions also provide a measure of the mantle He flux and the integrated permeability-fluid pressure gradient (flow rate) through the crust (1). To enter the hydrologic system, mantle He must pass through a ductile lower crust, which is believed to be an impermeable boundary because of an inability to maintain open fractures on long time scales (810). A general assumption is that the passage of fluids through this boundary must occur either by direct intrusion and degassing of mantle-derived magmas (6) or by diffusion through the ductile boundary layer (11). However, two recent studies in areas void of recent volcanism (1, 12) have found evidence for fault-controlled advective flow of mantle fluids through the ductile boundary. How and why this occurs is not well understood.

We conducted a regional study of He isotopic compositions of thermal fluids collected from surface features and wells throughout the northern Basin and Range Province (B&R), western North America (Fig. 1 and table S1) (13). As a result of the tectonic influence exerted by the relative motion of the Pacific and North American Plates (14, 15), the B&R is a vast extended region of anomalous thermal gradients, large heat flux, high regional elevation, thin (ned) crust, and lithospheric- and asthenospheric-mantle melting. Presently, extension is accommodated by high-angle normal faults, and the locus of major extension and its associated magmatism occurs at the margins of the province (16), primarily within the Walker Lane, a narrow 100-to-200–km–wide transtensional volcanic zone along the eastern side of the Sierra Nevada that extends north into a transitional zone between the Sierra Nevada and the subduction-related volcanic arc of the Cascades. East of the Walker Lane, B&R extension occurs at a slowing rate, over a much wider area and in the absence of active or recently active volcanism. The highest extension rates in this area occur west of the central Nevada seismic belt (CNSB), a north-northeast–trending belt of Holocene seismic activity.

Fig. 1.

Sample location map of the B&R and surrounding areas. The symbol colors delineate 3He/4He ratios, expressed as Rc/Ra (the air-corrected sample ratio normalized to the ratio in air), and the symbol shapes identify the type of thermal area. Tectonic zones are outlined: red, northern B&R; yellow, the Walker Lane transtensional zone (WL) and the CNSB; green, the Sierra Nevada batholith (SN); and light blue, the Cascades volcanic zone. TZ, transition zone between the Cascades, WL, and B&R.

Across the northern B&R, three general He isotope trends are apparent. First, ratios >3.0 Ra (7) occur only at the western margin of the B&R, reflecting active or recently active shallow-crustal volcanism within the Walker Lane (Coso, Long Valley, and Steamboat) and the Cascade volcanic chain (Lassen, Mount Shasta, Medicine Lake, Crater Lake, and Newberry Crater). Second, the preponderance of ratios >0.6 Ra occur in the northwest B&R in an area ranging from the CNSB west to the transition zone with the Cascade volcanics. Third, east of the CNSB, the He ratios decline, ranging from 0.1 to 0.3 Ra.

Collectively, the minimum 3He/4He ratios define a regional baseline trend of decreasing ratios from west to east, as illustrated by the shaded band in Fig. 2. East of the Walker Lane and Cascades, the occurrence of mantle He—as indicated by baseline 3He/4He ratios (∼0.2 to 2.0 Ra) that are much greater than those of average crustal He (∼ 0.02 Ra)—is not supported by magma intrusion, as this region has no evidence for current or recent volcanic activity. Instead, the baseline trend is strongly correlated with a change in the direction and magnitude of strain detected by present-day Global Positioning System (GPS) velocities (Fig. 3) (17, 18). West of the CNSB, a nearly pure east-west extension rate of ∼3 mm/year shifts to N40°W and increases to 12 to 13 mm/year. The accelerating dextral shear component is driven by a drag force due to the relative movement of the Pacific and North American Plates (19). We hypothesize that the increase in total strain and, specifically, the northwest-orientated dextral shear component greatly enhance average fluid-flow rates, allowing for a more rapid flow of mantle fluids through the crust and preserving the high 3He/4He ratios observed at the surface. The enhanced flow rate must persist through the brittle-ductile transition, through the ductile lower crust, and into the mantle lithosphere. If, as expected, fluids passing through the ductile crust enter the base of the brittle zone at, or near, lithostatic pressure, then the east-to-west increase in the flow rate is primarily governed by an east-to-west increase in average permeability.

Fig. 2.

Air-corrected He isotopic composition of geothermal fluids above 38°N latitude in the B&R, TZ, and Cascades, plotted as a function of longitude. The shaded curve depicts an east-to-west baseline trend defined by minima in the local 3He/4He ratios.

Fig. 3.

Compilation of present-day GPS strain rates across the northern B&R, relative to the North America reference frame. The data are from GPS networks located in a band from 38°N to 41° N latitude (15, 16). West of 242°E to 242.5° E longitude, the data show a combined increase in total magnitude of strain and an increase in lateral dextral shear strain superimposed on the east-west extension. This is most evident in the Walker Lane, but it impacts most of the B&R in and to the northwest of the CNSB, as illustrated by the Fig. 3 insert taken from (19). The broad colored band is the regional baseline He isotope trend from Fig. 2, shown superimposed on the trends in strain rate. CP, Colorado Plateau; GV, Great Valley.

The high-angle normal faults that presently accommodate B&R extension and that are probably fluid-flow pathways are not expected to penetrate the ductile lower crust. In extensional regimes, because of gravitational loading, the maximum principle stress is perpendicular to the geoid. With increasing depth, the brittle-to-ductile shift in rheology refracts the maximum stress acting on the fault, resulting in nearly horizontal shear zones or detachment faults at the boundary between the high-viscosity upper crusts and low-viscosity lower crusts (20, 21). Strain localization induced by an increasing dextral shear component superimposed on the extensional stress field must mechanically couple the brittle and ductile crustal zones, generating vertically oriented downward fault splays that extend through the ductile crust and into the mantle. These splays would act as conduits for fluid flow. The close correspondence between He isotope ratios and the rate of dextral shear strain suggests that fluid-flow rate (and hence, permeability) through the ductile zone is a function of the rate of dextral shear strain. Once the flow has initiated, the high-pressure pore and fracture fluids may help to maintain permeability, enhancing flow through the ductile zone (22).

There are many sampled features that have elevated 3He/4He ratios with respect to the regional baseline trend. Although a few, such as Roosevelt Hot Spring and Covefort, are associated with crustal magma systems (23), the majority are not. An example of the latter is the Dixie Valley thermal system (Fig. 2), where a detailed study (24) found a range in He isotope ratios (∼ 0.3 to 0.8 Ra), with the highest ratios restricted to fluids emerging directly from the Stillwater range-front fault system, a high-angle normal fault that defines the western margin of the valley. A recent east-west high-resolution deep magnetotelluric study that crossed through Dixie Valley revealed a zone of low resistance, a possible indication of fluids at a depth of ∼25 km within the mantle lithosphere (25), suggesting that the Dixie Valley He anomaly reflects a combination of local, deep production of mantle fluids (zone of low resistance) and a range-front fault system with enhanced permeability (high fluid-flow rates and, hence, high 3He/4He ratios). The range-front fault is the primary conduit supplying ∼20 × 109 kg of 250°C water in support of a 62–megawatt-electric geothermal power plant (26). It follows that other “local He anomalies” may be indicative of similar systems and represent geothermal targets with high potential.

Our regional He isotope study of fluids across the northern B&R clearly demonstrates a strong correlation between an east-to-west increase in the magnitude of dextral shear strain and an east-to-west increase in baseline He isotope ratios. In the absence of active or recently active magmatism, the elevated He isotope ratios require amagmatic flow of mantle fluids through the ductile lower crust, suggesting that the increase in dextral shear strain rates creates and maintains permeable pathways through the ductile zone. Elevated He isotope ratios in surface fluids along amagmatic sections of the San Andreas fault (1) and a recently observed series of nonvolcanic tremors deep (20 to 40 km) beneath the same section of the fault provide additional support for the existence of deep-mantle fluids, their potential importance in fault mechanisms (27), and nonmagmatic fluid flow through the ductile zone.

The high 3He/4He anomalies superimposed on the regional trend indicate enhanced crustal permeability coupled with local zones of deep fluid production and/or hidden magmatic activity. These local anomalies may be indicative of a heterogeneous distribution of mantle volatiles that promote melt production. Assuming a CO2/3He ratio of ∼2 × 109 M (characteristic of mid-ocean ridge basalts) (6) as a proxy for mantle-derived volatiles, the calculated fluid-flow rate through the Dixie Valley geothermal system (24) translates into a mantle CO2 flux of ∼2 × 10–6 to 20 × 10–6 mol cm–2 year–1. As of yet, no definitive geochemical or isotopic evidence for the presence of mantle CO2 has been found, other than carbon isotopic compositions (δ13C= –6.5 ± 2.5 per mil) (22) not dissimilar from those of mantle CO2 (6). An estimated CO2/3He ratio in Dixie Valley fluids of ∼40 × 109 (13) is ∼20 times the mid-ocean ridge value, suggesting that most (∼ 95%) of the CO2 is not mantle-derived or, alternatively, that the subcontinental mantle CO2/3He ratio is much greater than that observed at mid-ocean ridges.

Earth's crust stores an enormous resource of thermal energy produced primarily from the radioactive decay of U, Th, and K that is dispersed throughout Earth. It has been estimated that, within the United States (excluding Hawaii and Alaska), there are ∼9 × 1016 kilowatt-hours (kWh) of accessible geothermal energy. This is a sizable resource compared to the total energy consumption in the United States of 3 × 1013 kWh annually. In order for geothermal systems to develop and mine the heat source naturally, adequate fluid sources and deep permeable pathways are a necessity. The deep pathways provide access to high temperatures that can drive fluid convection cells. He isotopes provide a quantitative or, at least, a qualitative estimate of deep permeability from surface measurements, and anomalies superimposed on regional trends can identify potential resources.

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Materials and Methods

Table S1


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