CHEMICAL AND ISOTOPIC CHARACTERISTICS OF ... - NGDS

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1. Temperature profile of the CSDP Shady. Rest corehole measured on July 7, 1987. The temperature log was run by Ron Jacobson and Bob. Myers of Sandia ...
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Geolhermal Resources Council . TRANSACTIONS. Vol. 11. Oclober 1987

CHEMICAL AND ISOTOPIC CHARACTERISTICS OF THERMAL FLUIDS IN THE LONG VALLEY CALDERA LATERAL FLOW SYSTEM, CALIFORNIA

Lisa Shevenell,l Fraser Goff,l C. O. Grigsby,l Cathy J. Janik,2 P. E. Trujillo Jr.,l and Dale Counce l lEarth and Space Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545 2Branch of Igneous and Geothermal Processes, U.S. Geological Survey, Menlo Park, CA 94025

ABSTRACT

of the Long Valley caldera has been described by Bailey et al. (1976) and Bailey and Koeppen (1977). Characteristics of the geothermal system associated with the caldera have been studiej by several investigators (Mariner and Willey, 1976; Sorey et aI., 1978; Blackwell, 1985; Farrar et aI., 1985; Sorey, 1985; Sorey et a1. 1986; and Shevenell et al., 1986). According to Sorey (1985), recharge to the hydrothermal system occurs on the western edge of the caldera along the Sierra Nevada front. Meteoric wa te I' then percolates downward and is heated at depth by a relatively young intrusion, which is probably associated with the Inyo dike chain. Thermal fluids flow la terally toward the east, a round the southern edge of the resurgent dome, and become increasingly mixed with dilute mete o r i c groundwaters to form the more diluted fluids found within the caldera.

Chemical and isotopic data of thermal waters in Long Valley caldera have been used to identify both the origins and characteristics of the fluids and to eval ua te mixing and boi ling processes occurring within the lateral flow system of the caldera. Recharge to the Long Valley geothermal system occurs in the western part of the caldera with the water being heated at depth and flowing laterally eastward in the subsurface. The la teral flow sys tem was recently intersected by the Shady Rest Continental Scientific Drilling Program (CSDP) corehole at 335 m (1100 ft) with fluids in this 202'C zone being more concentrated than non-boiled fluids to the east. As the Na-K-HC03-CI thermal fluids flow eastward, they are increasingly mixed with isotopically depleted, dilute groundwa tel's similar to cold waters east of Lake Crowley. Near surface boiling of Casa Diablo well fluids at 100'C forms waters with the compositions of Colton and Casa Diablo hot springs. Waters to the east of the Casa Diablo area are mixtures of meteoric water and boiled thermal fluids with a composition close to that of Col ton Hot Spring. There is no correlation between 3H and 36Cl in thermal fluids or between these components and conservative species, and i t appears that cold fluids involved in mixing must be relatively old waters, low in both meteoric 3H and 36Cl.

Several shallow holes have been drilled in the Long Valley caldera that show evidence of intersecting the lateral flow system, with temperature reversals occurring in the upper portions of the drill holes (Sorey et aI., 1978; Blackwell, 1985). A recent CSDP hole was cored near the Shady Rest campground in Mammoth Lakes in the western part of the caldera. The Shady Rest corehole exhibits strong evidence of intersecting the lateral flow system in the western caldera region (Table 1). The samples listed in Table 1 were collected and analyzed in 1984, 1985, and 1986 according to the procedures of Trujillo et al., 1987. This report combines chemical and isotopic data from the springs within the caldera, from fluids in the Casa Diablo wells, and from fluids in the Shady Rest corehole to evaluate mixing and boiling processes in the Long Valley lateral flow system. This characterization of the thermal fluids associated with Long Valley is part of a background study for future CSDP projects and is designed to help identify future drilling targets.

I NT RODUC TI ON Long Valley is an elliptically-shaped caldera located at the boundary between the Sierra Nevada and Basin and Range tec tonic provinces in east-central California. The caldera formed 0.7 Ma with the eruption of 600 km3 of the Bishop Tuff. Between 0.6 and 0.1 Ma, rhyolites, rhyodacites and basalts were extruded around the resurgent dome which is located in the west-central portion of the caldera. The Inyo domes, located in the western third of the caldera, are recent rhyolitic eruptive domes with ages as young as 500 to 600 years and posai bly were fed by the Inyo dike chain. Mammoth Mountain, which is at the southe rn terminus of the Inyo dike chain, ranges in age from 250,000 yr at ita base to 50,000 yr at the summit. The geology and geolog i c history

SHADY REST COREHOLE In June 1986, the Shady Rest hole was continuously cored in the southwestern moat a r ea of Long Valley caldera to a completed depth of 426 m (1397 ft) and a maximum temperature of

195

:IF · f

Shevenell et al. Chemical

Table 1.

a

and Isotopic

Sample No.

b

Data of Thenmal and Nonthenmal Waters Temp. Date Collected ('C)

NCKUe

Shady Rest Core Hole at 1100 ft Shady Rest Core Hole at 1100 ft Casa Diablo Well '1 Casa Diablo Well '1 Calla Diablo Well 13 Casa Diablo Well 13 Casa Diablo Hot Spring Calla Diable Hot Spring Casa Diablo Hot Spring Colton Hot Spring Colton Hot Spring Casa Diablo Pool Hot Creek Hot Spring Hot Creek Hot Spring Hot Creek Hot Spr ing Hot Creek Hot Spring Little Hot Creek Hot Spr ing Little Hot Creek Hot Spring Hot Creek Gorge Well Meadow Hot Spring Hot Spring North of Whitmore Big Alkali Hot Spring Whitmore Warm Spring East Crowley Warm Spring Big Spr ing Fish Hatchery Spring Laurel Spring Red's Meadow Hot Spring Soda nat Spring

LV86-9 LV86-11 d LV85-12 d LV85-23d LV85-24 LV86-13 Ml.V-S MLV-25 LV86-12 MLV-ll MLV-26 MLV-12 MLV-7 MLV-27 LV86-2 LV86-6 MLV-13 LV86-7 HC-l MLV-20 HLV-17 MLV-18 MLV-16 t4LV -21 MLV-19 ~LV-15d

LV85-7 MLV-5 MLV-4

11/86 11/86 5/85 5/85 5/85 11/86 11/8li 2/85 11/86 11/8li 2/85 11/84 11/811 2185 5/86 11/86 11/84 11/86 11/84 11/84 11/84 11/84 11/84 11/84 11/84 11/84 5/85 11/84 11/84

202 202 168 168 171 170 95.4 95.4 911.0 95.0 93.5 69.0 85.9 84.8 94.0 93.0 82.7 85.0 93.2 67.2 57.1 58.2 33.4 30.2 10.2 111.5 11.8 13.3 13.3

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in Long Valley Caldera.

Field pH

510 2

5.94 5.9li 8.75 5.60 6.10 6.07 8.50 8.50 8.5 8.75 8.35 7.35 8.19 8.02 8.4 8.04 7.15 7.0 7.20 6.81 7.92 7.17 7.71 8.57 7.40 7.18 8.55 7.02 6.49

290 37li 290 382 275 li18 210 392 245 378 255 336 280 420 307 436 290 450 249 395 280 395 212 370 138 375 160 386 135 379 150 380 86 1100 90 390 201 357 200 23li 119 303 190 376 69 112 71 83 20 51 21 53 21 6.1 148 132 78 485

Na

K

li2 li5 40.0 36.0 38.0 32 311

34

43 2li 23 20 20 20 19 23 22 26 18.7 36 23 29 7.6 5.3 3.5 11.11 1.2 5.8 19

Li

2.78 2.83 3.40 3.20 3.00 2.66 3.27 3.511 3.111 3.05 3.13 2.85 2.55 2.64 2.54 2.113 2.76 2.66 2.64 1. 70 2.2 1.70 0.78 0.29 0.03 0.11 (0.01 0.97 2.06

Ca

9.9 7.3 1.8 1.6 2.0 5.5 1.6 1.7 1.7 2.9 2.0 9.9 5.3 6.8 9.5 5.2 23.9 22.8 16.1 4.1 14.8 24.9 19.9 5.4 4.9 12.3 15.7 60.7 43.1

Mg

0.17 0.12 0.15 0.14 0.18 0.20 0.12 0.02

Z 0.2

0

~ ~