Geochimica et Cosmochimica Acta, Vol. 69, No. 15, pp. 3857–3872, 2005Copyright © 2005 Elsevier Ltd

Printed in the USA. All rights reserved

doi:10.1016/j.gca.2005.03.001

Source and movement of helium in the eastern Morongo groundwater Basin: The influenceof regional tectonics on crustal and mantle helium fluxes

JUSTIN T. KULONGOSKI,1,2,* DAVID R. HILTON,1 and JOHN A. IZBICKI2

1Fluids and Volatiles Laboratory, Scripps Institution of Oceanography, UCSD, La Jolla, CA 92093-0244, USA2U. S. Geological Survey, California Water Science Center, San Diego, CA 92123, USA

(Received October 11, 2004; accepted in revised form March 2, 2005)

Abstract—We assess the role of fracturing and seismicity on fluid-driven mass transport of helium usinggroundwaters from the eastern Morongo Basin (EMB), California, USA. The EMB, located �200 km east ofLos Angeles, lies within a tectonically active region known as the Eastern California Shear Zone that exhibitsboth strike-slip and extensional deformation. Helium concentrations from 27 groundwaters range from 0.97to 253.7 � 10�7 cm3 STP g�1H2O, with corresponding 3He/4He ratios falling between 1.0 and 0.26 RA

(where RA is the 3He/4He ratio of air). All groundwaters had helium isotope ratios significantly higher thanthe crustal production value of �0.02 RA. Dissolved helium concentrations were resolved into componentsassociated with solubility equilibration, air entrainment, in situ production within the aquifer, and extraneousfluxes (both crustal and mantle derived). All samples contained a mantle helium-3 (3Hem) flux in the range of4.5 to 1351 � 10�14 cm3 STP 3He cm�2 yr�1 and a crustal flux (J0) between 0.03 and 300 � 10�7 cm3 STP4He cm�2 yr�1. Groundwaters from the eastern part of the basin contained significantly higher 3Hem and deepcrustal helium-4 (4Hedc) concentrations than other areas, suggesting a localized source for these components.4Hedc and 3Hem are strongly correlated, and are associated with faults in the basin. A shallow thermal anomalyin a �3,000 m deep graben in the eastern basin suggests upflow of fluids through active faults associated withextensional tectonics. Regional tectonics appears to drive large scale crustal fluid transport, whereas episodichydrofracturing provides an effective mechanism for mantle-crust volatile transport identified by variability in

0016-7037/05 $30.00 � .00

the magnitude of degassing fluxes (3Hem and J0) across the basin. Copyright © 2005 Elsevier Ltd

1. INTRODUCTION

Studies of the helium concentrations and isotope ratios ingroundwaters have identified the atmosphere, crust, and mantleas contributors to the total He inventory. These reservoirs havedistinct 3He/4He ratios, enabling the resolution of measuredhelium concentrations into constituent components. Helium isentrained in groundwater by a variety of processes over a broadrange of time scales; these include the rapid addition of excessair during groundwater infiltration or water-table fluctuation,continuous addition of radionuclide decay products, and/ortransport from deep crustal or mantle reservoirs. Each of theseprocesses imparts a diagnostic imprint on groundwater 3He/4Heratios, thereby allowing quantification of its effect (see reviewsby Ballentine et al., 2002; Kipfer et al., 2002). As groundwateris found at shallow levels in the crust, a means to introducemantle-derived He to (some) aquifer systems is required. In thisrespect, tectonic activity has been identified as a process fortransferring mantle volatiles into the crust with the result thatthe presence of mantle-derived helium-3 (3Hem) dissolved ingroundwater can be used to identify magma intrusion, conti-nental underplating, and/or lithospheric fracturing and rifting(e.g., Lupton, 1983; Oxburgh and O’Nions, 1987; Oxburgh etal., 1986; Torgersen, 1993; Watson and Brenan, 1987).

The continental crust is a low-permeability barrier to mantledegassing, and radiogenic helium-4 (4He) production within thecrust dilutes any transcrustal mantle helium-3 flux. This isreflected in low 3He/4He ratios (�0.02 RA) observed in ground-

* Author to whom correspondence should be addressed(kulongos@usgs.gov).

3857

waters from stable platforms and sedimentary basins (Clarkeand Kugler, 1973; Oxburgh and O’Nions, 1987; Torgersen andClarke, 1987). Although 4He produced in the continental crustdominates the He flux to the atmosphere (Torgersen andClarke, 1987), the effective diffusivity of He through crustalrocks is not rapid enough to account for the observed He flux(Torgersen, 1989). Therefore, an additional mechanism thatinduces vertical mass transport is required to achieve substan-tial crustal He degassing. The purpose of this study is toidentify mechanisms of volatile (He) transport both across themantle-crust boundary and through the crust, and to assess therelative roles played by diffusion, faulting, and seismic activity.To this end, we examine the helium isotope schematics of theeastern Morongo groundwater basin in eastern California,which serves as a type example of a region characterized byextensional tectonics with concomitant faulting and seismicactivity.

2. EASTERN MORONGO BASIN, CALIFORNIA

The Morongo Basin lies �200 km east of Los Angeles as asoutheastern part of the Mojave Desert, a semiarid region withlow annual rainfall (100–150 mm yr�1), low humidity, andhigh summer temperatures (Londquist and Martin, 1991). Thisstudy focuses on the eastern section of the Morongo Basin(EMB), an eastward sloping basin that is filled with unconsol-idated alluvial deposits derived from the surrounding moun-tains and that is split into a number of subbasins by northwest-trending faults (see Fig. 1). Based on hydrologic data,groundwater flow is from the northwest to the east-southeast

(Izbicki, 2003). Before the start of groundwater extraction in

d flowp

3858 J. T. Kulongoski, D. R. Hilton, and J. A. Izbicki

1955, there was surface discharge at Surprise Spring. Ground-water also discharged from the EMB as evaporation from thenow-dry Deadman Lake.

Most of the sites sampled in this study lie within the SurpriseSpring groundwater subbasin (SSB) which lies at the center ofthe EMB (Fig. 1). The SSB has an area of 210 km2 and isbounded to the west by the Emerson Fault, to the east by theHidalgo–Surprise Spring Fault, and to the south by the Trans-verse Arch (Akers, 1986). Four sites were sampled outside theSSB: two sites to the west in the Giant Rock subbasin (GRB)and two sites to the east in Deadman subbasin (DMB).

2.1. Geology of the Study Area

Crystalline basement (pTb) forms the mountains that sur-round the EMB (Fig. 1) and underlies the sediments exposed onthe basin floor (Riley and Worts, 1953). The basement consistsof a Precambrian igneous and metamorphic complex that wasintruded by Jurassic granitic rocks. The SSB is filled withunconsolidated continental Tertiary to Quaternary deposits, upto 1000 m thick, covered by a relatively thin veneer of youngerQuaternary alluvial and playa (Qa) deposits (Fig. 2) (Riley andWorts, 1953). The continental deposits, medium to coarse

Fig. 1. (a) Regional map of southwestern USA showMorongo Basin identifying the Surprise Spring subbasingroundwater flow is indicated with black arrows, combineand the local fault system is shown as white lines.

arkosic sands, represent the water-bearing units in the area,

although some water is found in fractures in the largely imper-meable basement complex (Riley and Worts, 1953).

At the surface, younger Quaternary deposits (Qa) derivedfrom the erosion of the igneous and metamorphic basementrocks form alluvial fans and plains, stream-channel deposits inwashes, and playa dry-lake deposits. These deposits range inthickness from 15 to 30 m and consist of coarse poorly sortedfanglomerates along the mountain fronts to fine sands and siltson the alluvial plains and clays in the dry-lake deposits(Londquist and Martin, 1991). The water table lies below theyounger Quaternary deposits, so they are not considered water-bearing units.

The Tertiary and Tertiary to Quaternary deposits (Ts, QTf1,QTf2) represent the bulk of the basin fill. The QTf units arealluvial fan deposits and are subdivided based on greateramount of silt, clay, and cementation in the lower unit (QTf1)(Londquist and Martin, 1991). The QTf deposits are 100 mthick in places and contain coarse arkosic sand of moderatepermeability, and low-permeability fine sand to silty clay de-rived from the San Bernardino Mountains. The underlyingTertiary unit (Ts) is composed of sedimentary deposits up to460 m thick and is more poorly sorted than the QTf units, hasless defined bedding, and has more igneous and metamorphic

y area and San Andreas Fault. (b) Map of the Easterned area) and the locations of sampled wells. Direction ofaths and line of geological cross section is marked A–A=,

ing stud(lighten

coarse fragments and detritus. The Ts also contains more in-

N/7E-1

3859Source and movement of helium in the Morongo groundwater basin

terstitial clay, resulting in low permeability for this unit(Londquist and Martin, 1991). This unit is derived from theBullion Mountains to the east. Most of the wells in the EMBdraw water from the QTf units (QTf1 and QTf2), althoughwells 3N/6E-2J1, 3N/7E-32D2, 2N/7E-31J1, and 2N/7E-28P1extract water from the Ts unit. Even though these units may beconsidered as two interconnected aquifers, groundwaters in thelower unit (Ts) have higher specific conductances (�1000�S/L) than, and distinct chemistry from, groundwaters in theupper (QTf) units (�300 �S/L) (Londquist and Martin, 1991).

2.2. Tectonic Environment

The San Andreas Fault system (SAF) delineates the bound-ary between the North American and Pacific plates. It beganforming �30 Ma ago following a shift from subduction totransform motion and development of the northward-migratingMendocino Triple Junction (Furlong, 1993). The relative mo-tion between the North American and Pacific plates along the

Fig. 2. Geologic cross-section of the eastern Morongoonto A–A=) and lithology. 2000 water table indicated by daboxes. Not shown are wells 3N/6E-16A1, 3N/6E-27B1, 2ation 13:1 (based on Londquist and Martin, 1991).

transform boundary over the past 3 to 4 Ma has been estimated

at 49 mm yr�1, based on marine magnetic anomalies (DeMetset al., 1987), with a N 35° W direction of dextral slip betweenthe plates. Geologic offset and geodetic data for the SAFsystem, on the other hand, suggest a long-term slip rate ofbetween 7 and 43 mm yr�1 (Stirling et al., 1996), with a meanvalue of 35 mm yr�1 (Hill et al., 1990), with the main faulttrace striking N 65–70° W. Hill et al. (1990) suggest that thisdiscrepancy, in direction and slip rates, is accounted for by theextensional deformation of the western margin of the NorthAmerican plate at 10 mm yr�1 in an N 56° W direction.Features compatible with extensional deformation in the EMBinclude fault swarms sited along northeast-striking lineations,which are nearly perpendicular to the main trace of the north-west-striking SAF system. However, extension in this area maybe a local feature, resulting from gaps opening between rotatingblocks (Nicholson et al., 1986).

The EMB lies in the seismically active region to the east ofthe SAF known as the Eastern California Shear Zone (ECSZ).

tudy area showing sampled wells (from Fig. 1 projecteday line. Perforations in bore hole casing indicated by open0 (2,4), 2N/7E-11R2, and 2N/7E-3E1. Vertical exagger-

Basin sshed gr

It is dominated by northwest-trending, right lateral strike-slip

3860 J. T. Kulongoski, D. R. Hilton, and J. A. Izbicki

faults. Based on geodetic strain measurements, seismicity, andground rupture, the ECSZ is estimated to accommodate from9% to 23% of the total relative strike-slip motion between theNorth American and Pacific plates (Dokka and Travis, 1990).Recent Southern California Integrated GPS Network observa-tions covering the 1999 Hector Mine earthquake along theLavic Lake–Bullion Fault zone show extension as well asstrike-slip deformation in the region (Hurst et al., 2000).

The EMB is transected by numerous faults (see Fig. 2) whichact as barriers to regional groundwater flow as a consequenceof (1) compaction and textural alteration of the water-bearingunits immediately adjacent to the faults, (2) steep tilting andoffsetting of alternating beds, and/or (3) cementation along thefault from the deposition of gouge and calcium carbonate frommineralized groundwater (Riley and Worts, 1953). As a result,the regional water table exhibits large drops across faults; forexample, from west to east, the water table drops 120 m acrossthe Hidalgo–Surprise Springs fault. Groundwater accumulatesalong the faults and, before 1953, discharged naturally asgroundwater outflow across Surprise Spring Fault (Londquistand Martin, 1991). Although groundwater flow may be re-stricted across the fault, enhanced flow of crustal fluids mayoccur within the fault zone (Riley and Worts, 1953).

The northern boundary of the SSB is a northeast-trendingunnamed fault south of Ames Dry Lake (see Fig. 1), to thenorth of which the top of the basement complex deepensconsiderably and groundwater flow is northward (Moyle,1984). An anticlinal structure, known as the Transverse Arch,bounds the basins to the south and acts as an impediment togroundwater flow as a result of the shoaling basement complex(Moyle, 1984).

The western boundary of the SSB is the Emerson Fault(EMF) zone, a right-lateral strike-slip fault across which thewater level drops 20 m from west to east (Fig. 2). The EMFzone stretches �55 km and has a slip rate of �0.5 mm yr�1

with an estimated 9,000 yr major event interval (Petersen andWesnousky, 1994). It last ruptured on June 28, 1992 (MW �7.3), simultaneously with nearby faults during the Landersearthquake (15 km focal depth).

The SSB is bounded to the east by the 40 km long Hidalgo–Surprise Spring Fault (HSSF) zone. Alternate reversals in rel-ative uplift on either side of the HSSF zone suggest the pres-ence of horizontal compressive forces acting along the strike ofthe fault to produce broad transverse folds whose crests ap-proximately coincide across the fault in some places but alter-nate with troughs in others (Riley and Worts, 1953).

Abrupt changes in water table elevation within the SSBcombined with seismic refraction profiles suggest the presenceof two additional unnamed NW-SE-trending faults that furtheract as partial barriers to groundwater flow (Londquist andMartin, 1991) (Figs. 1 and 2). Downward displacement ofnearly 50 m on the Surprise Spring (east) side of both unnamedfaults has been identified by the comparison of electric logsfrom wells on either side of the faults (Londquist and Martin,1991).

To the east of the SSB lies the Deadman subbasin (DMB)which has three major right lateral strike-slip faults (Fig. 1).The faults, from west to east, are (1) the 22-km-long ElkinsFault, (2) the 30-km-long Mesquite Fault, and (3) the 55-km-

long Lavic Lake–Bullion Fault (LLBF) zone, which last rup-

tured on October 16, 1999 (MW � 7.1) during the Hector Mineearthquake (5 to 14 � 3 km focal depth) (Hurst et al., 2000). Ofnote to this study is that two sampling sites lie near these faults:28P (1–3) near the LLBF and Mesquite Faults, and 31J (1–3)near the Elkins Fault (Fig. 1). In light of the right slip observedduring the 1999 Hector Mine earthquake, the origin of thewesternmost range front of the Bullion Mountains, boundingthe DMB to the east, and the flanking deep local transtentionalbasins, may be right stepping of the bounding strike-slip faultsystem.

Bouguer gravity anomaly surveys identify density variationsof the Earth’s upper and middle crust, providing a means toassess the displacement along a fault resulting from the juxta-position of rocks having different densities (Moyle, 1984).Surveys of the EMB identify 3 to 3.5 km of longitudinal offsetalong the EMF and 3.4 km of gravity anomaly offset along theLLBF system (Roberts et al., 2002). Gravity surveys alsoidentify a significant gravity low, �3 km deep to the northwestof the DMB; this is considered to represent an extremely deepbasin (see Section 6.6). Roberts and others (2002) refer to thisfeature as the Deadman Lake Graben; a strike-slip extensionalbasin formed by a combination of normal block faulting of theBullion Mountains and a right step from the Mesquite Fault tothe LLBF.

3. HYDROCHEMISTRY

The stable isotope characteristics of EMB groundwaters aregoverned primarily by orographic effects, which aid in distin-guishing groundwaters recharged at different locations and/orunder different climate conditions. Based on the age and stableisotope composition of the SSB groundwater (Table 1), re-charge to the basin occurs primarily as westward groundwaterinflow from precipitation in the San Bernardino Mountainswhich lie 22 km to the west-southwest of the EMB, rather thanas direct precipitation or surface-water infiltration (Izbicki,2003). Winter precipitation collected at a station near JoshuaTree, California, at an altitude of 1280 m—17 km southwest ofthe EMB and �700 m higher than the lowest part of theEMB—has a mean �D value of �81‰ and a �18O value of�11.3‰ (Friedman et al., 1992). Most groundwaters from theSSB have �D values lighter than this value (�D � �80.8‰ to�86.7‰ with �18O values from �11.17‰ to �11.66 ‰),suggesting recharge at higher altitudes (see Fig. 3). Groundwa-ters from the DMB, 31J (1–3) and 28P (1–3), along withsamples from wells penetrating the Tertiary unit (Ts), 3N/6E-2J1 and 2N/7E-3A2, plot below this group, and have signifi-cantly lighter �D values (�D � �87.5‰ to �94.9‰), suggest-ing that these groundwaters were recharged at a differentlocation and/or under cooler/wetter conditions (Izbicki, 2003).

Carbon-14 (14C) and �13C data are available for 14 of theEMB groundwater samples included in this work. In Table 1,we tabulate calculated 14C residence times (time since re-charge, in years) (Izbicki and Michel, 2003). Downdipchanges in the groundwater 14C activity and �13C wereevaluated using the program NETPATH (Plummer et al.,1994), which incorporates mass-transfer reactions betweendissolved constituents and aquifer material. This techniquedraws upon mineralogic analysis of the aquifer as well as

chemical (major and minor ions) and isotopic analysis of the

Table 1.

Eastern Morongotownship/range, well ID#

14C-ageb

(res. time), yr�18O,permil

�D,permil 4Hes Rs/RA

c Nes Nea4Hea

4Heex4Hedc (Rex/RA

c) 3Hem3Hem flux

He-age(res. time), yr

Giant Rock sub-basin (�20%) (�1%) (�2%) (�10%) (�7%) (�7%) (�7%)3N6E-16A1 700 �11.17 �80.8 1.04 � 0.02 1.0 � 0.06 3.13 1.07 0.31 0.23 0.20 1.03 3.1 9.4 2100e

3N/6E-27B1 nd �11.37 �84.1 0.97 � 0.01 1.0 � 0.06 3.70 1.65 0.47 0.03 0.02 1.18 0.4 0.0 0e

Surprise Spring sub-basin3N/6E-2J1a nd �11.44 �93.7 12.4 � 0.2 0.71 � 0.01 3.54 1.48 0.43 10.5 10.26 0.68 106 9.2 6300e

3N/7E-19N1 1700 �11.24 �81.5 1.09 � 0.02 0.94 � 0.06 2.76 0.70 0.20 0.38 0.31 0.86 4.4 8.9 2400e

3N/7E-20C1 nd �11.19 �81.6 1.60 � 0.02 0.75 � 0.04 3.73 1.67 0.48 0.63 0.48 0.40 2.9 3.4 3500e

3N/7E-32D2a nd �11.42 �83.3 11.4 � 0.2 0.26 � 0.01 3.64 1.58 0.46 10.22 7.64 0.20 26 2.4 60000e

3N/7E-32D3 14200 �11.88 �87.5 4.56 � 0.07 0.40 � 0.03 4.58 2.53 0.73 3.29 2.68 0.19 7.7 2.1 14900e

3N/7E-32D4 7000 �11.25 �82.6 2.35 � 0.03 0.73 � 0.03 4.31 2.26 0.65 1.16 0.86 0.49 7.6 5.6 5800e

3N/7E-32D5 6600 �11.29 �82.7 3.08 � 0.04 0.78 � 0.03 5.87 3.81 1.10 1.41 1.13 0.56 11 6.8 7700e

3N/7E-32D6 6300 �11.4 �84.3 2.12 � 0.03 0.67 � 0.03 3.06 1.00 0.29 1.28 1.01 0.49 8.4 5.7 7700e

3N/7E-29R1 3500 �11.24 �81.6 1.47 � 0.02 0.82 � 0.03 3.07 1.01 0.29 0.66 0.51 0.64 5.6 7.0 3500e

3N/7E-32J1 nd �11.47 �85.0 2.08 � 0.03 0.59 � 0.03 3.98 1.93 0.56 1.03 0.83 0.19 2.1 1.3 4800e

3N/7E-28R1 nd �11.32 �83.7 2.54 � 0.05 0.68 � 0.04 4.02 1.96 0.57 1.42 1.14 0.47 8.9 5.5 6600e

2N/7E-3B1 6500 �11.36 �83.5 1.62 � 0.03 0.78 � 0.05 3.63 1.57 0.45 0.66 0.38 0.51 4.2 5.1 2000e

2N/7E-3A2 4700 �11.27 �84.5 1.72 � 0.03 0.85 � 0.06 4.88 2.82 0.81 0.42 0.22 0.43 2.0 3.0 1200e

2N/7E-2D1 6550 �11.45 �84.2 1.47 � 0.02 0.77 � 0.04 3.33 1.28 0.37 0.91 0.63 0.36 4.0 3.5 2900e

2D1 (replicate) nd �11.45 �84.2 1.88 � 0.03 0.67 � 0.05 3.67 1.61 0.46 0.60 0.32 0.48 3.6 4.5 1900e

2N/7E-3E1 nd �11.38 �84.3 2.00 � 0.04 0.59 � 0.03 3.56 1.50 0.43 1.07 0.95 0.27 3.3 2.4 2900e

3N/7E-35P2 4900 �11.24 �82.5 1.50 � 0.02 0.74 � 0.05 3.60 1.54 0.44 0.57 0.36 0.34 2.2 2.5 2600e

2N/7E-10D2 nd �11.52 �85.0 3.27 � 0.05 0.39 � 0.02 2.75 0.70 0.20 2.54 2.35 0.24 7.7 2.7 4300e

2N/7E-10D4 nd �11.66 �86.7 4.02 � 0.06 0.87 � 0.03 11.4 9.37 2.70 0.81 0.72 0.41 4.2 4.1 2200e

2N/7E-11R2 12000 �12.57 �93.2 1.88 � 0.03 0.64 � 0.03 3.52 1.47 0.42 0.95 0.43 0.33 3.7 0.0 12800d

Deadman sub-basin3N/8E-31J1a 7700 �11.57 �88.9 135.3 � 2 0.29 � 0.004 3.56 1.50 0.43 130 130 0.28 496 369 11200f

31J1a (replicate) nd �11.57 �88.9 160.8 � 2 0.29 � 0.006 2.84 0.78 0.22 155 154 0.28 596 373 13300f

3N/8E-31J2 nd �11.46 �84.7 37.3 � 0.5 0.31 � 0.005 2.98 0.92 0.27 35.3 35.2 0.30 142 387 3200f

3N/8E-31J3 nd �11.26 �84.0 15.6 � 0.2 0.35 � 0.008 6.54 4.48 1.29 13.4 13.3 0.27 48 342 1300f

3N/8E-28P1a 8400 �11.66 �94.6 253.7 � 4 0.35 � 0.006 3.14 1.09 0.31 243 242 0.34 1150 1366 11100g

3N/8E-28P2 nd �11.75 �94.2 92.0 � 1 0.36 � 0.006 12.1 10.1 2.90 85.0 84.8 0.34 395 1338 4300g

3N/8E-28P3 nd �11.59 �94.9 86.4 � 1 0.35 � 0.004 3.33 1.27 0.37 82.1 81.9 0.34 385 1350 4300g

nd: No data available. He and Ne concentrations (�10�7) in cm3 STP g�1 H2O, 3Hem concentrations in (10�14) in cm3 STP g�1 H2O, and 3Hem flux in (10�14) in cm3 STP cm�2 y�1. Rechargetemperature 8°C, Heeq � 4.69 � 10�8 in cm3 STP g�1.

a Tertiary unit (Ts).b Interpreted 14C residence times from Izbicki and Michel (2003).c Helium ratio in air, RA � 1.384 � 10�6.d Helium ages calculated with � � 0.2 and Jo � 3 � 10�9 (cm3 STP 4He cm�2 y�1).e Helium ages calculated with � � 0.2 and Jo � 1 � 10�7 (cm3 STP 4He cm�2 y�1).f Helium ages calculated with � � 0.2 and Jo � 1 � 10�5 (cm3 STP 4He cm�2 y�1).g Helium ages calculated with � � 0.2 and Jo � 3 � 10�5 (cm3 STP 4He cm�2 y�1).Subscript letters defined as follows: a � excess air entrainment; dc � deep crustal source; ex � excess helium (terrigenic) source; m � mantle source; s � concentration measured in groundwater sample.

3861Source

andm

ovement

ofhelium

inthe

Morongo

groundwater

basin

m.

3862 J. T. Kulongoski, D. R. Hilton, and J. A. Izbicki

groundwaters. Calculated errors on the 14C residence times,based on knowledge of chemical interactions within thegroundwater system, are of the order of �20% (Davis andBentley, 1982). The residence times of groundwaters fromthe Tertiary and Quaternary units (QTf) in the SSB are�8,000 yrs, whereas groundwaters extracted from the Ter-tiary unit (Ts) in the SSB and from the DMB are older, with14C residence times between 10,000 and 14,000 yrs (Izbickiand Michel, 2003) (Table 1).

4. ANALYTICAL METHODS

During July and October 2000, groundwater samples were col-lected in annealed copper tubes using standard procedures (Weiss,1968). A total of 27 wells were sampled: 19 in the SSB, 2 in theGRB, and 6 in DMB to the east (Fig. 1). Seven of the wells are 36cm inside-diameter (ID) production wells equipped with electric

Fig. 3. �D vs. �18O for EMB groundwater samples plotThe QTf-SSB samples are distinguished from the Ts andthe SSB samples (solid black line), and regression for thevalues for precipitation at Joshua Tree, CA, altitude 1280

turbine pumps. The remaining 20 wells were pumped with a posi-tive-displacement double-reciprocating submersible pump. They in-

clude five 36 cm ID steel cased wells and fifteen 5 cm ID observa-tion multidepth wells. The multidepth wells—3N/7E-32D (2– 4)(SSB), 3N/8E-31J (1–3) (DMB), and 3N/8E-28P (1–3) (DMB)—have separate wells within the same borehole that penetrate todifferent depths in the aquifer. The wells are separated by low-permeability bentonite grout.

Dissolved He and Ne were extracted from groundwater samples andanalyzed at the Fluids and Volatiles Laboratory of the Scripps Institu-tion of Oceanography. Measurement protocols (Kulongoski and Hilton,2002) called for releasing dissolved gases under high vacuum, remov-ing the active gases with a hot (700°C) titanium sponge, and purifyingthe He�Ne fraction with a charcoal finger frozen to �196°C beforeisolating a known fraction (�0.4%) of the gas in a 1720-glass break-seal. The He�Ne fraction was then transferred to a magnetic-sectormass-spectrometer-based system where the gas was further purifiedwith a titanium sponge (700°C) and frozen charcoal (�196°C) beforebeing exposed to a cryogenic trap to separate He from Ne, which werethen sequentially inlet to the mass spectrometer (MAP 215E). Mea-sured peak intensities were compared to prepared atmospheric-air

respect to the global meteoric water line (dash-dot line).amples by their stable isotope variations. Regression for

dotted line). Open circle is the mean winter stable isotope

ted withDMB sDMB (

standards, and measurement errors were determined for He and Ne

3863Source and movement of helium in the Morongo groundwater basin

concentrations from the reproducibility of duplicate analyses (usuallygood to �2%). Errors on 3He/4He ratios principally reflect countingstatistics on the 3He ion beam (��4%).

5. RESULTS

Table 1 presents helium isotope and abundance results for 29samples from the EMB (including two replicates), along withneon abundance results. Results are reported as sample heliumand neon concentrations (3Hes,

4Hes, Nes) in cm3 g�1 H2O atstandard temperature and pressure (STP), and helium isotoperatios (Rs/RA), where sample 3He/4He ratios (Rs) are normal-ized to the 3He/4He ratio of air (RA � 1.4 � 10�6). Alsotabulated are the calculated parameters including groundwater14C and 4He residence times, helium and neon concentrationsdue to excess air entrainment, excess 4He, mantle derivedhelium-3, and helium-3 fluxes (see section 6.1 and table foot-notes for more details).

5.1. Helium Concentrations

Helium concentrations measured in the EMB groundwatersamples (Hes) range from 1.0 � 10�7 to 254 � 10�7 cm3 STPg�1 H2O. SSB groundwaters from the QTf units along flowpath(A–A=), contain helium concentrations with a limited rangefrom 1.0 to 4.6 � 10�7 cm3 STP g�1 H2O. Groundwaters fromthe Ts unit have higher helium concentrations ranging from11.4 to 12.4 � 10�7 cm3 STP g�1 H2O, whereas heliumconcentrations in samples drawn from the DMB are consider-ably higher, ranging from 15.6 to 254 � 10�7 cm3 STP g�1

H2O. All EMB groundwaters have measured helium concen-trations greater than that expected for air-saturated water at theestimated recharge temperature of �8°C (4.69 � 10�8 (Weiss,1971)), indicating addition of He from extraneous sources.

5.2. Helium Isotope Ratios

The 3He/4He ratios for all EMB groundwater samples (Rs)range from 1.0 to 0.26 RA (RA � air 3He/4He). SSB 3He/4Heratios range between 0.94 and 0.40 RA, while samples from theTertiary unit (Ts) have ratios ranging from 0.71 to 0.26 RA.3He/4He ratios from the DMB are low and exhibit a muchsmaller range, between 0.35 and 0.29 RA. The lowest 3He/4Heratio for the EMB, 0.26 RA, is considerably higher than theproduction or the deep-crustal value of 0.02 RA expected ingroundwaters from radiogenic production in the aquifer matrix(Andrews, 1985). 3He/4He ratios greater than 0.1 RA, aftercorrection for excess air entrainment (see section 6.1.1), indi-cate the presence of mantle-derived helium. Other explanationsfor high 3He/4He ratios, such as radioelement heterogeneity,high lithium and/or tritium contents, or fractionation processessuch as the preferential release of 3He from the aquifer matrix(Ballentine and Burnard, 2002; Martel et al., 1990), have notbeen identified in the EMB and may be dismissed in this case(see section 6.1.2 and Kulongoski et al., 2003, for more detaileddiscussion).

6. DISCUSSION

6.1. Resolution of Helium Components

Measured 3He and 4He concentrations represent the sum of

several helium components which, upon resolution, provide

insight into the provenance of helium fluxes into the ground-water system. The sources of helium (3He and 4He) include insitu production (Heis) from the radiogenic decay of Li, U, andTh in the aquifer matrix, fluxes from the mantle (Hem) and thedeep crust (Hedc), air-equilibrated helium (Heeq), dissolved airbubbles (Hea), and tritiogenic helium-3 (3Het). The measuredhelium concentrations (3Hes and 4Hes) in water samples may beinterpreted as the sum of these components (e.g., Torgersen,1980; Weise and Moser, 1987). Each component has a distinc-tive 3He/4He ratio, so measured values can be resolved into thevarious components. Diagnostic 3He/4He ratios of the differentcomponents or sources are: air-equilibrated water Req � 1.36� 10�6 � 0.98 RA (Clarke et al., 1976), mantle Rm � 1.1 �10�5 � 8 RA (Craig and Lupton, 1981), and in situ and deepcrustal production Ris Rdc 2 � 10�8 � 0.02RA (Mamyrinand Tolstikhin, 1984).

6.1.1. Atmospheric components

The total (measured) helium concentration in a groundwatersample includes atmospheric helium introduced from twosources: solubility-equilibration (Heeq) and excess-air entrain-ment (Hea). The contribution of these atmospheric heliumcomponents to the total helium in the groundwater samples iscalculated using measured concentrations of helium, neon, andthe He/Ne ratio along with an adopted air-equilibration tem-perature (of recharge) (Torgersen, 1980). Given the relativeinsensitivity of the solubility of He and Ne to temperature(Ozima and Podosek, 1983), the recharge temperature has asmall effect on the concentrations of helium and neon, and cantherefore be estimated from the mean annual winter groundtemperature at the EMB recharge zone, when most precipita-tion occurs. The present-day value, 8°C (Engstrom, 1981), isused in this study as the recharge temperature for groundwater.

Calculating the amounts of air-entrained helium (Hea) relieson the assumption that the dissolved neon measured in a sampleis the sum of the neon from air equilibration (Neeq) and fromair-bubble entrainment (Nea) and that there is no fractionationof the entrained-air He/Ne ratio. Subtracting the neon due to airequilibration from the measured neon concentration gives theamount of neon from air-bubble entrainment, which is thenused to calculate the amount of air-entrained helium (Hea)because the helium-neon ratio in air (Hea/Nea � 0.2882) isknown (Weiss, 1971).

The amounts of He and Ne in each sample from air-bubbleentrainment are listed in Table 1. Helium due to air-bubbleentrainment ranges from 0.20 to 2.90 � 10�7 cm3 STP g�1

H2O with a mean value of 0.61 � 10�7 cm3 STP g�1 H2O(n � 29). The presence of air-derived He and Ne may reflectoverpressure effects during groundwater infiltration and/orrapid fluctuations in the water table (Heaton et al., 1986).Previous studies of groundwater in the regional aquifer of theadjacent Mojave River Basin found mean air-bubble He con-tents of 0.65 � 10�7 cm3 STP g�1 H2O (n � 16) (Kulongoskiet al., 2003).

6.1.2. Terrigenic components

The excess helium (He ) in a sample is the sum of in

ex

situ–produced, deep crustal, tritiogenic, and mantle helium

3864 J. T. Kulongoski, D. R. Hilton, and J. A. Izbicki

components. The contribution of each of these components tothe total helium-3 is calculated using 4He concentrations mul-tiplied by the 3He/4He ratios characteristic of the differentsources (see section 6.1). Excess helium-3 (3Heex) and heli-um-4 (4Heex) are calculated by subtracting the concentration ofhelium due to air equilibration (Heeq) and air-bubble entrain-ment (Hea) from the measured helium concentration of thesample (Hes). The ratio of excess helium-3 to excess helium-4(3Heex/4Heex) is quoted as the air-normalized excess heliumisotope ratio (Rex/RA) in Table 1.

The total dissolved helium in a sample may be separated intoits various components using the following linear equation (seeKulongoski et al., 2003; Stute et al., 1992; Weise and Moser,1987 for the detailed derivation of the following formulas):

3Hes�3Hea

4Hes�4Hea

ÇY

� �Req � Rex

3Het

4Heeq�

Çm

4Heeq

4Hes�4Hea

ÇX

� RexÇ

b

(1)

in which Y is the measured 3He/4He ratio corrected for air-bubble entrainment, X is the fraction of 4He in water resultingfrom air equilibration with respect to the total 4He in thesample, corrected for air-bubble entrainment, and b or Rex isthe isotopic ratio of nonatmospheric excess helium.

In Figure 4, we plot Y vs. X (as defined above) for all EMBsamples. This plot presents the evolution of the EMB ground-water system from recharge conditions (right side of plot), inwhich all of the dissolved helium is from air equilibration[4Heq/(4Hes � 4Hea) 1], to X values dominated by crustaland/or mantle contributions in which air-equilibrated He is asmall fraction [4Heq/(4Hes � 4Hea) � 0.05]. As a reference,included in Figure 4 is the evolution trajectory (labeled “a”)representing the addition of radiogenic helium-4 (0.02 RA) toair-saturated water (ASW). In this plot, each sample uniquelydefines its own terrigenic 3He/4He component from its Y value.Figure 4 is used to estimate (1) Rex (the Y-axis intercept),which is the isotopic composition of the excess helium contri-bution to this groundwater system (including deep crustal andmantle fluxes), and (2) the possible contribution of tritiogenic3He (related to the gradient “m”), which is diagnostic of recent(young) water.

The principal point to note from Figure 4 is that all EMBsamples plot above the radiogenic helium–ASW trajectory (linea). A linear regression of the total EMB database gives theequation Y � 11.8 (X) � 4.4 (r2 � 0.76) and is labeled line“b.” Significantly, the intercept (of line b) on the Y-axis is 0.32RA—this value is the 3He/4He excess and is considerablygreater than anticipated for in situ radiogenic production ofhelium. The gradient (11.8) is consistent with complete decayof 4.8 tritium units (TU) (1 TU yields 2.5 � 10�15 cm3 STP3He g�1 H2O); this represents the background tritium value inthe EMB. There are three possibilities to explain the observa-tion of the apparently high value of the 3He/4He excess: (1)mixing of older groundwater with younger nuclear bomb–tritiated water, (2) a contribution of 3He from lithium decay incrustal materials, and/or (3) a contribution from a mantle-derived helium flux.

For groundwater residence times �50 yr, no excess nuclearbomb tritium is expected. The groundwaters sampled in this

study have helium- and/or 14C-derived residence times signif-

icantly greater than 50 yr (Izbicki and Michel, 2003) (Table 1).If the background tritium value is taken as 4.8 TU, this wouldproduce 1.2 � 10�14 cm3 STP 3He g�1 H2O. For EMBgroundwater samples older than 50 yr, 3He concentrations aresignificantly higher, ranging from 2 to 1150 � 10�14 cm3 STP3He g�1 H2O (Table 1); therefore the decay of nuclear bomb–produced tritium should not represent a significant contributorto the total 3He.

The production of 3He from the reaction involving thermalneutron capture by lithium (6Li(n,�)3H (��)3He) may be dis-counted due to the unrealistically large concentrations of lith-ium (�850 ppm Li) in aquifer rock necessary to produce theobserved 3He/4He ratio (0.32 RA). Assuming a lithium contentof �50 ppm, an upper limit for a sedimentary lithology (An-drews, 1985; Ballentine and Burnard, 2002), the calculated3He/4He production ratio in the rock is 2.59 � 10�8 or 0.02 RA

(Kulongoski et al., 2003).By default, we conclude that the high Rex in the EMB

groundwaters results from an influx of mantle-derived helium-3(3Hem). Using the regression value of 0.32RA we can estimatethat the average contribution of mantle He is 4.6% of the totalhelium, assuming a simple binary mixture of mantle (Rm �8RA) and crustal (Rdc �0.02 RA) components. Although amantle endmember composition of 6 RA has been proposed forsubcontinental lithospheric mantle (Dunai and Porcelli, 2002),this would make only a small difference to the calculatedpercent of mantle-derived He in any given sample.

6.1.3. Intrabasin differences in Rex

In Figure 4 we show an expanded view of the lower lefthandportion of the main plot, focusing on the oldest and mostevolved groundwaters (i.e., 4Heq/(4Hes � 4Hea) � 0.16) in theSSB and DMB. There are three groups (of three points each)near the Y-axis, representing the values for the multidepthwells 3N/7E-32D (2–4) in the SSB and 3N/8E-28P (1–3) and3N/8E-31J (1–3) in the DMB. Each group has a unique Rex

value, and lines d, e, and f represent the evolution of ground-water from the air equilibrium value (Req) to the average Rex

values for each group. Taking the average of the Y-interceptsfor each group gives the following: Rex � 0.20 RA � 0.02 RA

for wells 3N/7E-32D (2–4) (SSB), Rex � 0.28 RA � 0.01 RA

for wells 3N/8E-31J (1–3) (DMB), and Rex � 0.34 RA � 0.003RA for wells 3N/8E-28P (1–3) (DMB).

It is significant to note that for these three groups of mul-tidepth wells, the Rex values increase across the basin fromwest to east. None of the groundwaters in the EMB reflect thedeep crustal excess ratio (Rdc � 0.02RA) nor do they evolve toa basin-wide mean Rex value; rather, the subbasins, althoughhaving similar lithologies and lying in close proximity, havedifferent Rex values. As we discuss in section 6.3, this mayindicate the presence of a source of 3He located beneath theDeadman subbasin.

6.2. Excess 4He—Geochronologic Implications

Excess helium-4 is introduced to the groundwater reservoirby in situ production or from a deep crustal flux. The accumu-lation of radiogenic 4He from in situ production may be used to

estimate groundwater age, and deep crustal fluxes (4Hedc) from

3865Source and movement of helium in the Morongo groundwater basin

Fig. 4. Measured 3He/4He ratios corrected for air bubble entrainment vs. the relative amount of 4He derived fromsolubility with respect to total 4He, corrected for air bubble entrainment. Lines (a) and (b) represent the evolution of the3He/4He ratio in water samples from atmospheric equilibrium values (Req) to excess ratios (Rex) of 0.02 RA and 0.32 RA

respectively (see Eqn. 1 in the text). Line (c) represents an addition of 10% of He of mantle origin with 3He/4He � 1.1 �10�5 and taking into account an average crustal production 3He/4He ratio of 0.2 � 10�7 as well as a background 3H contentof 4.8 TU. Lines (e), (f), and (g) on the inset plot represent the evolution of the 3He/4He ratio in water samples from

atmospheric equilibrium values (Req) to excess ratios (Rex) of 0.20 RA, 0.28 RA, and 0.34 RA, respectively.

3866 J. T. Kulongoski, D. R. Hilton, and J. A. Izbicki

continental degassing may be used to model constraints on thechemical and thermal evolution of the Earth (O’Nions andOxburgh, 1988).

The amount of in situ–produced helium that accumulates ingroundwater depends upon the radioelement content and po-rosity of the aquifer, and it can be quantified using the equation(Andrews and Lee, 1979)

4Hesol � � �1.19 10�13�U�

� 2.88 10�14�Th�� (1 � �)

�(2)

where 4Hesol is the 4He solution rate (cm3 STP g�1 H2O yr�1);[U] and [Th] are the uranium and thorium concentrations in theaquifer material (ppm); 1.19 � 10�13 and 2.88 � 10�14 (cm3

STP yr�1 �g�1 natural U and Th) are the 4He production ratesfor U and Th series in equilibrium with their decay products; is the bulk density of the aquifer material (g cm�3); is thefraction of helium produced in the rock that is released into thewater, assumed to be unity; and � is the fractional effectiveporosity of the aquifer material. If the presence of excesshelium (4Heex) in groundwater is attributed solely to in situproduction, then the helium age of the groundwater may beestimated by dividing the excess helium-4 (4Heex) by the so-lution rate (4Hesol).

If an external flux contributes helium to the system, it may bequantified (if the groundwater age is known) or the relativegroundwater age determined (if the flux is known). With thistechnique, the corrected groundwater age (�corr) is calculatedfrom (Weise and Moser, 1987; Stute et al., 1992)

�corr �

4Heex

� J0

�z0w

� 4Hesol� (3)

where 4Heex has the units cm3 STP g�1 H2O; 4Hesol, thehelium solution rate � 4.31 � 10�12 cm3 STP g�1 H2O yr�1

(Eqn. 2, � � 0.2); J0 (cm3 STP He cm�2 yr�1) is the 4Hedc fluxentering the base of the aquifer; � is the effective porosity ofthe aquifer; z0 is the depth (m) at which this flux enters theaquifer; and w is the density of water (�1 g cm�3). In theEMB aquifer, z0 is taken as the distance from the middle of theperforations in the well casing to the contact with the basement(see Fig. 2).

Employing Eqns. 2 and 3 with SSB data from the QTf unitsresults in helium residence times that range from 8.9 to 76.4 ka(assuming � � 20%, � 2.15 g cm�3 (Moyle, 1984), mean[U] � 2.3 ppm, [Th] � 7.9 ppm (Bushnell and Morton, 1987),and J0 � 0). Significantly older 4He residence times (237.4 to5,627 ka) are calculated for SSB samples drawn from theTertiary unit (Ts) and samples from the DMB. These values areseveral orders of magnitude greater than residence times deter-mined by 14C dating. Clearly, for the DMB and Ts, additionalsources of helium have contributed to the measured He con-centrations resulting in the anomalously high helium ground-water residence times.

To constrain the relationship between the deep crustal fluxand the in situ–produced 4He, we plot the 4He residence times

(Eqn. 3) vs. the 14C residence times of the groundwaters in

Figure 5. For this calculation, we set the crustal He flux (J0) tothree values—zero (i.e., no crustal flux), 1 � 10�7, and 300 �10�7 (units of cm3 STP He cm�2 yr�1)—and the porosity (�)to two values, 0.1 and 0.2.

Two trends are apparent when employing the above range inJ0 values in Eqn. 3 to establish concordance between the heliumand 14C residence times for EMB groundwaters: (1) Ground-waters from the SSB require a much smaller J0 value (0 to 1 �10�7 cm3 STP He cm�2 yr�1) than the more helium-richgroundwaters from the DMB (100 to 300 � 10�7 cm3 STP Hecm�2 yr�1); and (2) site 3N/8E-28P1 requires the largestcrustal flux of 300 � 10�7 cm3 STP He cm�2 yr�1, more than3 times the flux for site 3N/8E-31J1 (3 km distant) and 300times the flux of deep SSB site 3N/7E-32D2 (12 km distant).

The above observations may be explained thus: 14C andhelium residence times of the SSB samples from the Tertiaryand Quaternary units (QTf) plot along the reference line whenlittle or no crustal flux is adopted (Fig. 5a,d). This observationagrees with observations that He in shallow (young) ground-waters (�50 ka) is dominated by in situ production (Bethke etal., 1999; Kulongoski et al., 2003; Torgersen and Ivey, 1985).In Figure 5, older (and deeper) helium-rich samples from theTertiary unit (Ts) and DMB cluster in a separate group whichrequire J0 values between 100 to 300 � 10�7 cm3 STP Hecm�2 yr�1 for agreement between helium and 14C residencetimes. This range of J0 is significantly higher than the averagecrustal degassing value of 33 � 10�7 cm3 STP He cm�2 yr�1,calculated by O’Nions and Oxburgh (1983), and degassingfluxes observed in Australia, J0 � 36.2 � 10�7 cm3 STP Hecm�2 yr�1(Torgersen and Clarke, 1985), and Namibia, J0 �31.4 � 10�7 cm3 STP He cm�2 yr�1 (Torgersen and Clarke,1985).

It is important to note that the large range in J0 is notexpected over short distances, and it can not be explained byheterogeneous lithologies, because of the fairly uniform com-position of the EMB alluvial deposits. An alternative explana-tion for variability in J0 could be focused flow of helium-richgroundwater as a consequence of subregional flow cells in theaquifer (Bethke et al., 2000; Bethke et al., 1999; Zhao et al.,1998). However, this explanation may not be appropriate forthe EMB because shallow basement features which directgroundwater upwelling are not observed on the plot of gravityanomalies in the areas of high helium concentrations; thebasement deepens rather than shoals in the area of high J0, andthere is no evidence for subregional flow cells resulting frombasement irregularities.

A third, and more probable, explanation for the observedvariability in the crustal degassing flux (J0), is the release of Hefrom the solid (crustal) lithology to the proximal pore fluid asa result of dilatant fracturing (Torgersen and O’Donnell, 1991),followed by time-dependent hydraulic transport of the He-richfluids through the crust (Nur and Walder, 1990). Radiogenicproduction of helium (4Hedc) is a continuous process; however,the transfer of accumulated 4Hedc from the solid phase toproximal fluids may be discontinuous and enhanced by frac-turing and/or thermal alteration (Ballentine and Burnard, 2002;Torgersen, 1980; Torgersen, 1993; Torgersen and O’Donnell,1991). Calculations by Torgersen and O’Donnell (1991) showthat fracturing (10 m spacing of fractures) could release accu-

mulated He on geologically short time scales (1.5 ka). Episodic

�7 cm3

3867Source and movement of helium in the Morongo groundwater basin

faulting and fluid flow could then explain variations in themagnitude of crustal degassing fluxes (J0). This possibilitywould explain the marked variability in J0 throughout theheavily fractured EMB. It is significant that the sites requiringthe greatest crustal fluxes for 4He-14C residence time concor-

Fig. 5. Plots of corrected 4He groundwater residence tifor three crustal fluxes (Jo � 0, 1 � 10�7, and 300 � 10

dance, 3N/8E-28P (1–3) and 3N/8E-31J (1–3), lie close to

faults in the DMB. It has been noted previously that the releaseand transport of helium through the upper brittle crust inseismically active basins appears to be enhanced relative to adiffusion-dominated basin-wide crustal helium flux which hasbeen postulated for stable aseismic platforms (Stute et al.,

llowing Stute et al., 1992) vs. 14C-based residence timesSTP He cm�2 yr�1) and porosities (�) � 0.1 and 0.2.

mes (fo

1992; Torgersen, 1989; Torgersen and Clarke, 1987).

3868 J. T. Kulongoski, D. R. Hilton, and J. A. Izbicki

The general conclusion that emerges from these observationsis that application of 4He chronology in an extensively faultedgroundwater basin requires close scrutiny of groundwater flowto assess flow systematics and the contribution of a crustal Heflux. In the case of the EMB, the residence time of both youngand older waters may be calculated given reasonable estimatesof the magnitude of the crustal He flux; however, the crustalflux may exhibit local variability depending upon groundwater-fault interactions. On the other hand, knowledge of the 14Cgroundwater residence times enables variability in the magni-tude of crustal degassing fluxes to be identified, and the eval-uation of tectonic controls on the enhanced transport of 4He and3He through the crust.

6.3. Mantle-Derived Helium: Quantification and Fluxes

The contribution of mantle-derived He to the total He in anEMB groundwater sample may be quantified by resolving themeasured He concentration into its constituents, and assuminga binary mantle-crust mixture with end-member compositions(mantle � 8 RA; crust � 0.02 RA; see section 6.1). In this way,we can compute the absolute concentrations of He (3He and4He) contributed by each endmember. Table 1 gives the calcu-lated mantle 3He concentrations (3He ) for each EMB ground-

Fig. 6. Absolute concentration of excess 3He measured in ground-water samples (�10�14 cm3 STP g�1 H2O) in the EMB. Also shownare the depth to basement in gray-shading, regional faults (white lines),and region of thermal anomaly (dotted contour).

m

water. The 3Hem values range over 4 orders of magnitude from

0.2 � 10�14 cm3 STP g�1 H2O at well 3N6E-27B1 in theGRB, to 1150 � 10�14 cm3 STP g�1 H2O at well 3N/8E-28P1in the DMB. For comparison, the concentrations of 3Hem ingeothermal fluid samples from the Long Valley Caldera (LVC),a seismically active region to the north, range from 1787 to4542 � 10�14 cm3 STP g�1 H2O (Hilton, 1996), and in theseismically active Mojave River Basin (MRB) to the west ofthe EMB 3Hem concentrations in groundwaters range from 0.02to 1398 � 10�14 cm3 STP g�1 H2O (Kulongoski et al., 2003).

We note also that 3Hem contents in groundwaters from themultidepth wells in the SSB and DMB increase with depth (seeTable 1; explanation in Table footnote). The multidepth wellsprovide a means of estimating the regional 3Hem accumulationrates because the calculated He residence times of the ground-waters also increase with depth. Dividing the concentration of3Hem by the He residence times of the groundwater gives the3He accumulation, per year, at each depth. Interestingly, thethree multidepth wells, 3N/7E-32D (2–6), 3N/8E-31J (1–3),and 3N/8E-28P (1–3), have unique average 3Hem accumulationrates: 9.53 � 10�18, 4.27 � 10�16, and 9.51 � 10�16 (cm3

STP g�1 H2O yr�1), respectively. The observed fluxes ofmantle-derived 3He can also be estimated for these sites bycombining the percentage mantle contribution and the crustalfluxes (J0) (section 6.2) needed to establish 4He and 14C con-cordance. The 3Hem flux at site 3N/7E-32D (2–6) equals 4.5 �10�14 cm3 STP cm�2 yr�1, at site 3N/8E-31J (1–3) equals367.7 � 10�14 cm3 STP cm�2 yr�1, and at site 3N/8E-28P(1–3) equals 1351 � 10�14 cm3 STP cm�2 yr�1.

When the absolute 3Hem concentrations (symbol size as afunction of concentration) of groundwaters are plotted on amap of the EMB (see Fig. 6), along with the surface expressionof faults in the region, a thermal anomaly zone (Trexler et al.,1984), and the depth to basement based on gravity anomalysurveys (Roberts et al., 2002), two additional observationsbecome apparent. First, sites with the highest concentrations of3Hem appear to lie in close proximity to the recently active(1999) Lavic Lake–Bullion fault system. Second, the two siteswith the highest concentrations of 3Hem lie within or close tothe observed thermal anomaly. Possible explanations for thisdistribution of 3Hem concentrations in EMB groundwaters in-clude (1) an association between high 3Hem and seismic activ-ity, as in the nearby Mojave River Basin (Kennedy et al., 1997;Kulongoski et al., 2003); and (2) the presence of a shallowmagma body, such as at Long Valley Caldera. These possibil-ities are evaluated in the next two sections.

6.4. Transfer of Helium Into and Through the Crust

Two processes can be considered for the transfer of heliumfrom the mantle into and through the crust: a basin-widediffusive flux and a fault-controlled advective flux. In deter-mining which mechanism is more appropriate for the EMB, ourconstraints are the need to explain the spatial (and vertical)distribution of 3He and 4He in the basin, including their appar-ent association with faults, as well as variations in magnitude ofthe 3He and 4He degassing fluxes. In this respect, a key obser-vation is the relation between the mantle 3He concentration andthe calculated flux of 4He required for concordance between the4He and 14C chronometers (Fig. 5).

We dismiss a simple basin-wide diffusive flux of 3He as the

3869Source and movement of helium in the Morongo groundwater basin

dominant process to transfer mantle volatiles to the shallowcrust in the EMB (Marty et al., 1993; Stute et al., 1992). In sucha scenario, one would anticipate a gradual increase in 3Hem

concentrations with increasing groundwater residence time(Marty et al., 1993; Stute et al., 1992). However, the 14Cresidence times for SSB groundwaters (1.7 to 14.2 ka) are notsignificantly different from the 14C residence times of DMBgroundwaters (7.7 to 8.4 ka), yet there is nearly 100 times more3Hem in the average DMB sample (4.59 � 10�12 cm3 STP g�1

H2O, n � 7). It seems implausible that the DMB waters couldhave accumulated such a large amount of 3Hem from a simplebasin-wide flux over such a short (�10 ka) time period andgroundwaters of similar age in the SSB, less than 5 km distant,did not. In addition, the nonuniform 3Hem accumulation ratesand fluxes observed in the multidepth wells in the EMB argueagainst the ubiquitous diffusion and accumulation of 3Hem

throughout the basin.A more plausible explanation for the transfer of mantle-

derived 3He involves the presence of deep faults which may actas conduits for material transfer (including volatiles) into andthrough the crust. Kennedy et al. (1997) showed that elevated3He/4He ratios (i.e., greater than crustal production values) indeep pore fluids from the San Andreas Fault (SAF) could beused to estimate the flow rate of mantle fluids through the SAFzone. By taking the vertical gradient of 3He/4He values of faultzone pore fluids to reflect dilution of mantle helium (Rm � 8RA) with crustal (radiogenic) helium (Rdc � 0.02 RA), theycalculated an upward flow rate of 1 to 10 mm yr�1, which isconsidered a lower limit because hydrodynamic dispersion, theeffects of mixing between fault fluids and radiogenic crustalfluids, or episodic flow events are not taken into account. For acrustal thickness of 30 km, and a flow rate of 10 mm yr�1, itwould require nearly 3 Ma for mantle fluids to reach the surfaceunder these conditions. In the same time period, He would beexpected to move only �1 m by diffusion through bulk granite(DHe � 5 � 10�7 m2 a�1; Ballentine and Burnard, 2002) or�300 m by diffusion through groundwater (DHe � 0.03 m2

a�1; Ballentine and Burnard, 2002), further emphasizing therole of advection in the transport of helium.

Advective flow of (deep) crustal fluids containing mantle-derived He throughout the EMB, particularly in the vicinity offaults, implies that the Eastern California Shear Zone (ECSZ)extends to considerable depths, possibility into the lowermostcrust. In this way, episodic fracturing and fluid flow would playthe key role in enhancing the transfer of mantle volatilesthrough the brittle-ductile boundary—estimated at 10–15 kmdepth for typical crustal thermal gradients—and towards thesurface (Kennedy et al., 1997; Nur and Walder, 1990; Rice,1992). On the other hand, if faults in the ECSZ are limited inextent to relatively shallow depths, e.g., by a midcrustal de-collement (e.g., Jones et al., 1994; Webb and Kanamori, 1985),then their role would be to transport volatiles through theshallow crust if the fractures connect with the decollement. Inthis case, faults may act as conduits, and fault activity may actas the release valve for accumulated 3Hem (and 4Hedc) into theshallow crust. Either mechanism would explain the correlationbetween high 3Hem and 4Hedc observed in the groundwaters ofthe EMB (next section) and for the Mojave River Basin (Ku-

longoski et al., 2003).

6.5. Influence of Faults and Seismic Activity in the EMB

In this section, we assess the role of faults—their surfacedistribution and activity—in controlling the distribution of3Hem concentrations in groundwaters of the EMB.

The presence of faults, particularly if they lead to greaterpermeability in the fault zone, should act to channel He fluxesto the surface. If this were the case then wells in close prox-imity to a fault should have associated high 3Hem (and 4Hedc)fluxes as a result of relatively rapid transport of helium via thefault zone. However, this is not the case for the EMB, with sitessampled near the Hidalgo–Surprise Springs Fault zone (Fig. 6)showing no evidence of high concentrations of 3Hem. Further-more, the multidepth wells of the EMB, which are all locatednear major fault systems, have distinctly different fluxes of3Hem (section 6.3). This leads to the conclusion that the pres-ence of faults alone does not lead to enhanced transport of deepcrustal and mantle fluids.

However, fault activity or episodic fault rupture (Nur andWalder, 1990) may be the reason why 3Hem fluxes could varybetween sites which are geographically close. In this scenario,pore fluid pressure increases until it reaches the level of theleast compressive stress in the crust, when it induces hydraulicfracturing and fluid release, followed by pore pressure drop,and resealing of the system (Nur and Walder, 1990). Localhydrofracture, induced by fluid pressure increases (i.e., fromporosity reduction, dehydration, vertical fluxes), would createlocal interconnected networks allowing fluid flow (Miller andNur, 2000). Such localized networks may explain why theobserved 3Hem accumulation and mantle flux rates differthroughout the EMB. It should be noted that significant time(Ma) is necessary for advective transport of 3Hem from themantle to the shallow crust, and therefore it is unlikely that anindividual rupture event would immediately result in high 3Hem

groundwater concentrations unless these faults directly tappeda shallow 3He reservoir (Gulec et al., 2002; Sano et al., 1986).However, the cumulative activity (slip) along faults may facil-itate transfer and transport of volatiles from the mantle to thecrust over geologic time scales, resulting in the heterogeneous3Hem distribution observed in the EMB. In this respect, it isinformative to consider the recent seismic record for the EMBregion.

Recent seismic activity in the EMB includes the rupture ofthe Emerson Fault during the 1992 Landers earthquake, andrupture of the Lavic Lake–Bullion Fault, which extended intothe DMB within 4 km of site 3N/8E-28P (1–3) during the 1999Hector Mine earthquake (see section 2.2). In Figure 6, thehighest concentrations of 3Hem are observed at sites near theterminus of the 1994 rupture of the LLBF, and high concen-trations were also observed in wells near the Emerson Fault(3N/7E-32D2 and 3N/6E-2J1). These observations support themodel of episodic hydrofracture-induced flow enhancing thetransfer of mantle volatiles to the shallow crust.

Groundwaters in the EMB which contain relatively highconcentrations of 3Hem are also observed to have the highestconcentrations of crustal (radiogenic) helium (4Hedc) (see Fig.7). The relation between high 4Hedc and 3Hem concentrations(Fig. 7) suggests a similar transport mechanism (Ballentine etal., 1991 and may be attributed to an increased permeability

through the active fault zone. Transmission of mantle and deep

3870 J. T. Kulongoski, D. R. Hilton, and J. A. Izbicki

crustal helium through fault zones represents a plausible mech-anism for enhanced fluid/gas transport through the crust. Thismechanism then justifies the model of a continental degassingflux that provides a steady state He atmosphere.

6.6. Thermal and Gravity Anomalies in the EMB:Relationships with 3Hem

Finally, we consider the influence of the shallow thermal(65°C at a depth of 275 m; see Fig. 6) and gravity anomalieswhich occur in the eastern part of the basin (Trexler et al.,1984). In the case of the thermal anomaly, two models havebeen proposed that could supply heat (and 3Hem) in extensionalbasins. One is magma emplacement, including shallow magmaintrusion (Oxburgh et al., 1986; Torgersen and Clarke, 1992),which supplies heat and 3Hem by either hydrofracturing(Watson and Brenan, 1987), or crustal underplating (Lachen-bruch and Morgan, 1990). Alternatively, extension of the litho-sphere could supply both heat and 3Hem from the resultingthinning of the crust (e.g., Lupton, 1979; McKenzie, 1978;Oxburgh et al., 1986). The observed thermal maximum, at adepth of 275 m, argues against a large underlying heat source.Magma intrusion or crustal underplating would generate aspatially extensive thermal profile that increases with depth.The thermal anomaly in the EMB covers only about a third ofthe basin, and temperatures decrease below the 275 m thermalmaximum. Given that the diffusion coefficient of heat in crustalrock is �10�6 m2 a�1 (Carslaw and Jaeger, 1957), and thediffusion coefficient of He through the crust is estimated to be10�8 m2 a�1, the 3Hem flux should lag the heat flux by severalhundred million years (Torgersen, 1993). This is not observedfor the EMB and offers further support for the notion thatvolatile and heat transport is dominated by advective flow.

The gravity anomaly survey shown in Figure 6 does notindicate a shallow magma intrusion coinciding with the heatanomaly region. However, the survey identifies two gravitylows: one in the north of the DMB and one in the south. The

Fig. 7. Plot of mantle-derived helium-3 (3Hem) vs. deep-crustalhelium-4 (4Hedc) in the eastern Morongo Basin groundwater samples.

gravity lows represent deep basins or grabens (�3000 m

deep)—a result of extensional tectonics, block rotation, andright stepping in the LLBF (Nicholson et al., 1986).

We conclude that stretching, erosion, and the lateral flow ofcrustal materials would explain higher heat flow in extensionalbasins (see Torgersen; 1993), but that high 3Hem fluxes do notresult from a shallow magma intrusion. The heat anomaly in theEMB may indeed be a result of crustal thinning, but the sourceof the 3Hem flux into the basin is not magma intrusion; rather,we argue that active faults penetrate the thinned crust to con-siderable depth to transfer heat and volatiles to the shallowgroundwater systems. Much like 3Hem in hydrothermal sys-tems, which is localized by hot water advection, helium-3 inextensional basins may be associated with localized flowthrough higher permeability fault zones (Welhan et al., 1988).The shallow thermal maximum (275 m) in the eastern EMBsupports the hypothesis of upflow and lateral transfer of fluids,possibly through the fault system (Trexler et al., 1984). Asdiscussed in section 2.2, transform faulting in the ECSZ isaccompanied by extensional deformation in the EMB. Exten-sional thinning and crustal-block rotation could therefore ex-plain the observed gravity lows, while providing a source ofheat and 3Hem delivered to the groundwater basin by activefaulting.

7. CONCLUDING REMARKS

In the eastern Morongo Basin, groundwaters entrain heliumfrom several distinct sources, including the atmosphere, mantle,and crust (from within both the aquifer system and the deepercrust). The helium components of groundwaters are resolvedusing standard modeling techniques and reasonable assump-tions of endmember compositions to identify characteristicfluxes.

In conclusion, we summarize our observations regarding thesources of helium and the impact of faults on the heliumdistributions in EMB groundwaters:

(1) Measured 3He/4He ratios in all samples from the easternMorongo Basin lie between 0.3 and 0.4 RA, significantly higherthan the ratio for a continental groundwater basin (0.02RA).Crustal (radiogenic) and mantle-derived helium may be quan-tified after eliminating the atmospheric (equilibration and air-entrainment) helium components.

(2) Concentrations of mantle-derived helium-3 (3Hem) anddeep-crustal helium-4 (4Hedc) are significantly higher in theDeadman subbasin, in particular at sites in close proximity toactive faults. Heterogeneous 3He and 4He fluxes occur overshort distances, suggesting that an advective flow regime drivesHe transport in and through the crust. We suggest that episodicfracturing in the ECSZ regulates 3Hem and 4Hedc fluxes into theeastern Morongo Basin. This mechanism is important world-wide for determining whole crust degassing rates (Oxburgh andO’Nions, 1987; Torgersen, 1989).

(3) In the eastern Morongo Basin, large crustal fluxes (J0)accompany enhanced 3Hem fluxes, and both have significantvariations in magnitude over small spatial scales. In otherextensional basins, enhanced 3Hem fluxes have been attributedto the shallow emplacement of magma, but in the EMB nomagma source has yet been identified. We conclude that faultspenetrate the thinned crust to supply 3He to the system. This

m

implies either that faults extend to the Moho and act as conduits

3871Source and movement of helium in the Morongo groundwater basin

for mantle 3Hem and scavenge 4Hedc from the deep crust(released during fracturing) or that 4Hedc and 3Hem are trappedin a common shallow reservoir, such as by a decollement zone,and are delivered to the shallow crust by fault activity. As aresult, the whole crustal flux (J0) appears to vary spatially,much like mantle 3He fluxes.

(4) Regardless of the depth and type of reservoirs tapped bythe fault system, tectonic forcing, i.e., episodic seismic activity,is the dominant mechanism that regulates fluid transport andthe transfer of volatiles through fault zones into the uppermostcrust.

(5) The radiogenic Heis in all samples of the eastern Mo-rongo Basin allows for estimates of the groundwater residencetime in the basin. For a majority of the SSB groundwaters, thereis concordance between 4He and 14C residence times, therebydemonstrating the integrity of the 4He-dating method. Ground-waters from the Tertiary unit (Ts) and DMB give significantlyhigher 4He residence times compared to estimates based on 14Cdating. This observation is consistent with the presence of anenhanced crustal flux of helium (J0 � 0.03 to 300 � 10�7 cm3

STP cm�2 yr�1) which is comparable to estimates derived forthe nearby Mojave River Basin (Kulongoski et al., 2003).Reasonable estimates of groundwater residence times are stillpossible for the DMB groundwaters once the crustal flux com-ponent is resolved from He produced in situ in the aquifersystem.

Acknowledgments—Special thanks go to Dennis Clark for help withsampling. Comments by K. Howard, C. Ballentine, and an anonymousreviewer improved this work. Funding for this project was provided bythe United States Geological Survey and the Hydrological SciencesProgram of the National Science Foundation (award EAR-0001133).

Associate editor: J. Matsuda

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