Helium isotope studies in the Mojave Desert, California: implications for groundwater chronology and regional seismicity Justin T. Kulongoski a, * ,1 , David R. Hilton a,2 , John A. Izbicki b,3 a Fluids and Volatiles Laboratory, Scripps Institution of Oceanography, Geoscience Research Division, University of California, San Diego, La Jolla, CA 92093-0244, USA b U.S. Geological Survey, Water Resources Division, San Diego, CA 92123, USA Received 23 December 2002; accepted 30 July 2003 Abstract We report helium isotope and concentration results for groundwaters from the western Mojave River Basin (MRB), 130 km east of Los Angeles, CA. The basin lies adjacent to the NW – SE trending San Andreas Fault (SAF) system. Samples were collected along two groundwater flowpaths that originate in the San Gabriel Mountains and discharge to the Mojave River located f 32 km to the northeast. Additional groundwater samples were collected from Mojave River Deposits underlying the Mojave River. The primary objective of this study is to identify and quantify crustal and mantle helium contributions to the regional groundwater system. A total of 27 groundwaters, sampled previously for chemistry and isotope systematics (including 14 C activity) have measured helium concentrations that increase along flowpaths from 9.9 10 8 to 1.0 10 4 cm 3 STP g 1 H 2 O. Concomitantly, 3 He/ 4 He ratios decrease from 0.84R A to 0.11R A (R A equals the 3 He/ 4 He ratio in air = 1.4 10 6 ). We did not record 3 He/ 4 He ratios equivalent to crustal-production values (f 0.02R A ) in any sample. Dissolved helium concentrations were resolved into components associated with solubility equilibration, air entrainment, mantle-derivation, in-situ production within the aquifer, and extraneous crustal fluxes. All samples contained the first four components, but only older samples had the superimposed effects of helium derived from a crustal flux. The radiogenic He component has chronological significance, and good concordance between 4 He and 14 C ages for younger groundwaters ( < 25,000 year) demonstrates the integrity of the 4 He-chronometer in this setting. Helium-rich waters could also be dated with the 4 He technique, but only by first isolating the whole crustal flux (3 – 10 10 6 cm 3 STP cm 2 year 1 ). Mantle-derived 3 He ( 3 He m ) is present in all MRB samples irrespective of distance from the SAF. However, regional-aquifer groundwaters near the terminus of the flowpath have a significantly greater content of mantle-derived 3 He in comparison with more modern samples. We propose that faults in the basin other than the SAF may be an additional source of mantle-derived helium. The large range in 3 He m concentrations may be related to fault activity; however, groundwaters with lower and more constant 3 He m contents may indicate that seismic activity along the SAF has been 0009-2541/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2003.07.002 * Corresponding author. Fax: +1-858-822-3310. E-mail addresses: [email protected], [email protected] (J.T. Kulongoski), [email protected] (D.R. Hilton), [email protected](J.A. Izbicki). 1 Now at U.S. Geological Survey, San Diego, CA 92123, USA. 2 Fax: +1-858-822-3310. 3 Fax: +1-858-637-9201. www.elsevier.com/locate/chemgeo Chemical Geology 202 (2003) 95 – 113
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www.elsevier.com/locate/chemgeo
Chemical Geology 202 (2003) 95–113
Helium isotope studies in the Mojave Desert, California:
implications for groundwater chronology and regional seismicity
Justin T. Kulongoskia,*,1, David R. Hiltona,2, John A. Izbickib,3
aFluids and Volatiles Laboratory, Scripps Institution of Oceanography, Geoscience Research Division,
University of California, San Diego, La Jolla, CA 92093-0244, USAbU.S. Geological Survey, Water Resources Division, San Diego, CA 92123, USA
Received 23 December 2002; accepted 30 July 2003
Abstract
We report helium isotope and concentration results for groundwaters from the western Mojave River Basin (MRB), 130 km
east of Los Angeles, CA. The basin lies adjacent to the NW–SE trending San Andreas Fault (SAF) system. Samples were
collected along two groundwater flowpaths that originate in the San Gabriel Mountains and discharge to the Mojave River
located f 32 km to the northeast. Additional groundwater samples were collected from Mojave River Deposits underlying the
Mojave River. The primary objective of this study is to identify and quantify crustal and mantle helium contributions to the
regional groundwater system.
A total of 27 groundwaters, sampled previously for chemistry and isotope systematics (including 14C activity) have
measured helium concentrations that increase along flowpaths from 9.9� 10� 8 to 1.0� 10� 4 cm3 STP g� 1 H2O.
Concomitantly, 3He/4He ratios decrease from 0.84RA to 0.11RA (RA equals the 3He/4He ratio in air = 1.4� 10� 6). We did not
record 3He/4He ratios equivalent to crustal-production values (f 0.02RA) in any sample.
Dissolved helium concentrations were resolved into components associated with solubility equilibration, air
entrainment, mantle-derivation, in-situ production within the aquifer, and extraneous crustal fluxes. All samples contained
the first four components, but only older samples had the superimposed effects of helium derived from a crustal flux. The
radiogenic He component has chronological significance, and good concordance between 4He and 14C ages for younger
groundwaters ( < 25,000 year) demonstrates the integrity of the 4He-chronometer in this setting. Helium-rich waters could
also be dated with the 4He technique, but only by first isolating the whole crustal flux (3–10� 10� 6 cm3 STP cm� 2
year� 1). Mantle-derived 3He (3Hem) is present in all MRB samples irrespective of distance from the SAF. However,
regional-aquifer groundwaters near the terminus of the flowpath have a significantly greater content of mantle-derived 3He
in comparison with more modern samples. We propose that faults in the basin other than the SAF may be an additional
source of mantle-derived helium. The large range in 3Hem concentrations may be related to fault activity; however,
groundwaters with lower and more constant 3Hem contents may indicate that seismic activity along the SAF has been
0009-2541/$ - see front matter D 2003 Elsevier B.V. All rights reserved.
He and Ne concentrations (� 10� 7) in cm3 STP g� 1.
Regional aquifer Heeq = 4.61�10� 8 in cm3 STP g� 1; Mojave River Deposits Heeq = 4.50� 10� 8 in cm3 STP g� 1.
4N/7W-33J1 is the origin of both flowpaths.a Interpreted 14C ages from (Izbicki and Michel, in preparation).b 3H values from (Izbicki and Michel, in preparation).c Helium ratio in air, RA= 1.4� 10� 6.d Helium ages calculated with /= 0.2 and J0 = 3� 10� 8.e Terminal site (>22 km along flowpath).f Helium ages calculated with /= 0.1 and J0 = 1�10� 5.g Helium ages calculated with /= 0.1 and J0 = 3� 10� 8.h Helium ages calculated with /= 0.1 and J0 = 3� 10� 6.
J.T. Kulongoski et al. / Chemical Geology 202 (2003) 95–113 101
regional aquifer (e.g. 4N/4W-3A2) because surface
flows along the entire length of the Mojave River
infiltrate the shallow aquifer system. As a result, the
age of the groundwater in the MRD does not increase
along the flowpaths as it does in the regional aquifer
system.
J.T. Kulongoski et al. / Chemical Geology 202 (2003) 95–113102
4. Analytical methods
Of the 25 boreholes sampled for this study (21 in
May 2000 and 4 in June 2001), 15 are 24-cm ID
production wells equipped with electric turbine pumps,
and 10 are 6-cm ID multi-level observation wells (4N/
ð9ÞSubstituting (9) into Eq. (7) gives the linear equa-
tion (Y=mX + b) (Castro et al., 2000; Stute et al.,
1992; Weise and Moser, 1987)
3Hes � 3Hea4Hes � 4Hea
¼ Req � Rex þ3Het4Heeq
� �
�4Heeq
4Hes � 4Heaþ Rex ð10Þ
in which Y is the measured 3He/4He ratio corrected for
air bubble entrainment, X is the fraction of 4He in
water resulting from air-equilibration with respect to
the total 4He corrected for air bubble entrainment, and
Rex is the 3He/4He ratio of the excess (crustal and
mantle) helium.
In Fig. 5, we plot Y versus X (as defined above) for
MRB data to estimate (i) Rex (the Y-axis intercept)
which is the effective contribution of terrigenic helium
to this system (including deep crustal and mantle
fluxes), and (ii) the possible contribution of tritiogenic3He (from the gradient) which is diagnostic of recent
(young) water. In effect, this plot represents the evo-
lution of the MRB groundwater system from recharge
conditions (Req) in which all of the dissolved helium is
from air-equilibration 4Heq/(4Hes–
4Hea)f 1, to X-
axis values dominated by crustal and/or mantle con-
tributions, 4Heq/(4Hes–
4Hea) < 0.005. Also included in
Fig. 5 are trajectories representing the evolution of
groundwater helium ratios from (i) the addition of
radiogenic He with a 3He/4He ratio typical of crustal
lithologies (0.02RA, Ozima and Podosek, 1983)—line
(a); (ii) addition of radiogenic He with a slightly higher3He/4He ratio (0.11RA—equal to the lowest measured
value)—line (b); (iii) addition of 0.5� 10� 14 cm3STP3He (representing complete decay of 2 TU) to evolu-
tionary trajectory (b)—line (c); (iv) addition of
5.5� 10� 14 cm3STP3He (representing complete de-
cay of 22 TU) to evolutionary trajectory (b)—line (d);
and (v) for reference, the integrated effects of complete
decay of 2 TU and a 10% addition of mantle helium
(8RA) to radiogenic He (0.02RA)—line (e).
In Fig. 5, 19 of the 25 MRB samples plot (within
error) between lines (b) and (c) which represent the
respective evolution of the 3He/4He ratios from Req,
and Req plus 2 TU, toward the helium excess ratio Rex
( = 1.59� 10� 7 or 0.11 RA). Also shown in Fig. 5 is a
close-up plot of the Y-axis, focusing on the oldest, and
therefore most evolved, groundwater samples (4Heq/
(4Hes–4Hea) < 0.005). Included are lines (a, b, c, and
d) as described above. This figure shows the five most
evolved regional aquifer samples (closest the Y-axis)—
the mean value of which (1.59� 10� 7F 6% at the 2rlevel) is taken as the MRB Rex value. We interpret this
Rex value as representative of the regional flux to the
MRB groundwater system. It is noteworthy that this
value is f 4 to 5 times higher than the expected deep
crustal excess ratio Rdc ( = 0.02RA).
There are three possibilities that can explain the high
value of Rex: (i) mixing of older groundwater with
younger post-bomb tritiated water, (ii) a contribution of
Fig. 5. Measured 3He/4He ratios corrected for air bubble entrainment versus the relative amount of 4He owing to solubility with respect to total4He, corrected for air bubble entrainment. Lines (a) and (b) represent the evolution of the ratios with 3He/4He crustal ratios of 0.2� 10� 7and
1.59� 10� 7, respectively (see Eq. (10) in the text). No He of tritiogenic origin is present in these cases. Lines (c) and (d) show the evolution of
line (b) when 2 TU and 22 TU are added and totally decayed to produce 0.5 and 5.5� 10� 14 cm3 STP g� 1 TU� 1of 3Het. Line (e) represents an
addition of 10% of He of mantle origin with 3He/4He = 1.2� 10� 5 and taking into account an average crustal production 3He/4He ratio of
0.2� 10� 7 as well as a background 3H content of 2 TU.
J.T. Kulongoski et al. / Chemical Geology 202 (2003) 95–113106
nucleogenic 3He from lithium decay in crustal materi-
als and/or preferential release of nucleogenic 3He, and/
or (iii) a contribution of a mantle-derived helium flux.
In the case for tritiogenic 3He, it is significant that
the sample (well 4N/4W-1C4) with the highest mea-
sured 3He/4He ratio (1.24RA) plots along line (d)—
consistent with a starting composition equivalent to the
complete decay of 22 TU superimposed on air-equil-
ibrated water (Req) followed by addition of radiogenic
helium with Rex = 1.59� 10� 7. This site has a signif-
icant tritium signal (3H1C4 = 12.2 TU, see Table 1),
suggesting mixing with a large component of recent
water. Precipitation in Santa Maria, CA (f 300 km
west of the MRB), reached 700 TU during the peak of
the nuclear weapons testing in 1963 (Michel, 1989);
therefore, it is not surprising that some modern sam-
ples have measurable tritium contents. Other samples
from the MRD have low TU and therefore a smaller
contribution from modern recharge: this can explain
their only slightly elevated helium isotope ratios (i.e.
their position on trajectory (c) in Fig. 5). However,
owing to the antiquity of most groundwater samples
J.T. Kulongoski et al. / Chemical Geology 202 (2003) 95–113 107
(Table 1)—particularly for the regional aquifer sys-
tem—and their distribution along the meteoric water
line (Fig. 3), mixing with young tritium-rich water can
be discounted in nearly all cases.
In order for radiogenic processes (in situ) to pro-
duce 3He/4He ratios f 0.11RA from the 6Li(n,
a)3H! 3He reaction, the lithium content in the aquifer
material would have to be greater than 300 ppm (with
the observed [U] and [Th]). Typical Li concentrations
in sandstone lithologies, such as the MRB, are f 15
ppm (Andrews, 1985), and thus the contribution of3He from lithium decay may be discounted owing to
the requirement of unrealistically large concentrations
of lithium. However, preferential release of nucleo-
genic 3He over in-situ produced 4He may lead to3He/4He ratios higher in the fluid phase in comparison
with the production site. In the case of the MRB
(Rex = 0.11RA vs. Ris = 0.02RA), there is a factor of
f 6 between the fluid and the production 3He/4He
ratios. In theory, therefore, complete release of 3He and
partial release (f 16%) of 4He could account for the
‘excess’ helium isotope ratios observed in the fluids.
Although different 3He/4He ratios have been observed
between fluids and rocks previously (e.g. the Carnme-
nellis granite–groundwater system; Martel et al.,
1990) if large fractionations of 3He and 4He have
occurred in the MRB, then groundwater ages based
upon 4He would be significantly younger compared to
other chronometers. As shown in the next section,
there is good agreement between 4He and 14C ground-
water ages, particularly for younger waters in the
regional aquifer system.
We conclude that a simple two-endmember mixing
model between mantle (Rmf 8RA) and crustal
(Rdcf 0.02RA) components can account for the high
Rex in these samples. Assuming the above endmember
ratios, there is an average 1.2% mantle contribution to
the total He inventory for a majority of the ground-
waters in this study. A slightly higher mantle contri-
bution (3–5%) is required to account for the four
regional aquifer sites which plot between line (c) and
line (d) in Fig. 5. These results are discussed in detail
in Section 6.3.
6.2. Terrigenic 4He—geochronological implications
The terrigenic contribution to the 4He inventory is
principally controlled by two components: in-situ pro-
duction within the aquifer and a deep crustal flux.
Assuming that the deep crustal flux may be quantified
(or neglected), the former component has chronologi-
cal significance for the groundwater. Stute et al. (1992)
propose an approach by which the different helium
components may be separated, enabling either ground-
water ages or crustal helium fluxes to be estimated.
The relationship between apparent (corrected)
groundwater age (scorr, in years) and the deep crustal
flux entering the aquifer ( J0, in cm3 He STP cm� 2
year� 1) is given by the following equation (Stute et
al., 1992):
scorr ¼4Heex
J0
/z0qþ 4Hesol
� � ð11Þ
where Heex is the excess 4He (4Heis +4Hedc) in cm3
STP g� 1 H2O, / is the effective porosity of the
aquifer, z0 is the depth (m) at which this flux enters the
aquifer, and q is the density of water (f 1 g cm� 3).
Note that the 4He solution or accumulation rate (4Hesolin cm3 STP g� 1 H2O year� 1) is given by the
following equation which combines the radioelement
content of the aquifer {in brackets} with its physical
properties (e.g. Andrews and Lee, 1979):
4Hesol ¼ q � K � f1:19� 10�13½U þ 2:88
� 10�14½Thg � ð1� /Þ/
ð12Þ
In this equation, [U] and [Th] are the uranium and
thorium concentrations in the aquifer rock (ppm); q is
the bulk density of the aquifer rock (g cm� 3), K is the
fraction of helium produced in the rock that is released
into the water, assumed to be unity, and / is the
fractional effective porosity of the aquifer rock.
In circumstances in which there is no extraneous
flux of helium into the aquifer ( J0 = 0), Eq. (11)