PROCEEDINGS, Twenty-Eighth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 27-29, 2003 SGP-TR-173 VOLCANISM, PLUTONISM AND HYDROTHERMAL ALTERATION AT MEDICINE LAKE VOLCANO, CALIFORNIA Lowenstern, Jacob B.1, Donnelly-Nolan, J.1, Wooden, J.L.1, Charlier, B.L.A2. 1U.S. Geological Survey, Menlo Park, CA, 94025, USA e-mail: [email protected] 2 Dept. of Geological Sciences, University of Durham, UK granitoids obtained as drillcore and xenoliths. The ABSTRACT resulting data provide a framework for understanding the intrusive history of the volcano and the genesis of Ion microprobe dating of zircons provides insight its geothermal system. We provide evidence for the into the history of granitoid generation beneath timing of intrusive episodes beneath the volcano and Medicine Lake volcano (MLV), northern California. their relationship to ongoing volcanism. Part of the MLV geothermal system is hosted by a 319 ± 8 ka granodiorite intrusion that may be over 6 km in diameter. A sample of this intrusion was BACKGROUND GEOLOGY hydrothermally altered by 171 ka, indicating that the geothermal system was active by that time. Post- The current edifice of MLV has grown since about 100-ka magmatism resulted in crustal melting, 0.5 Ma and overlies older lavas of the Modoc evidence for which is found in granitoid xenoliths Plateau. Basement beneath the volcano is thought to erupted with numerous lava flows. Zircons within be Sierran granite located at a depth of 10-20 km many of these xenoliths, analyzed by the ion (Fuis et al., 1987). Eruptive products of the volcano microprobe (230Th techniques) have ages less than cover about 2000 km2 and are estimated at 600 km3 in 20,000 years, and as young as the age of eruption in volume (Donnelly-Nolan, 1988), making MLV the the very latest Holocene. Sustained crustal input of largest volcano by volume in the Cascade Range. basaltic magma generates the heat to melt crust, and acts as a continuing heat source to the shallow MLV consists of a broad shield with a central 7 x 12 geothermal system. km caldera (Anderson, 1941). From the caldera region, the volcano’s only ash-flow, the “andesite INTRODUCTION tuff” was erupted 171±43 ka (Herrero-Bervera et al., 1994). Argon ages of andesitic lavas forming the rim Medicine Lake volcano, located about 50 km ENE of of the caldera are ~100-120 ka (Donnelly-Nolan et Mt. Shasta in northern California, is a large al., 1994). Since eruption of the ~85 ka Lake Basalt, Quaternary shield volcano that last erupted about nearly all volcanism within the Medicine Lake 1000 years ago (Donnelly-Nolan et al., 1990). For caldera has been silicic in composition (Donnelly- nearly 30 years, it has been the site of geothermal Nolan, 1988). exploration and represents one of the few U.S. prospects under consideration for new development Around 11,000 B.P., voluminous basalts erupted to (Hulen and Lutz, 1999). form the Giant Crater lava field, the basalt of Valentine Cave and other mafic lava flows scattered Deep geothermal drillholes reveal that much of the across the flanks of the volcano. This mafic episode exploitable geothermal system is hosted by was followed by a pause of about 6000 years without hydrothermally altered granitoid rocks located ~ 2 an eruption. Beginning about 5 ka, a variety of km below the Medicine Lake caldera (Fig. 1). basaltic, intermediate and silicic lavas erupted across Granitoid xenoliths, mostly unaltered, are found MLV, including several late Holocene silicic within many Pleistocene and Holocene lava flows eruptions in and near the caldera. The Medicine that cover the volcano (Mertzman and Williams, Lake Glass Flow erupted within the caldera about 1981; Grove et al., 1988; Lowenstern et al., 2000). In 5000 B.P. Just to the west and east of the caldera, this paper we provide U-Pb and U-Th disequilibrium rhyolite eruptions took place respectively at Glass ages of zircon crystals (ZrSiO4) extracted from 1 Fig. 1. Location map for samples from Medicine Lake volcano. Labeled samples are discussed in text. Unlabeled dots represent other studied xenoliths. Drill holes are shown by the stars. textures. Mafic minerals are altered to chlorite and Mountain (885 B.P.) and Little Glass Mountain (1065 secondary, liquid-rich fluid inclusions are abundant. B.P.) (Donnelly-Nolan et al., 1990). Whole rock δ18O on three samples revealed similar degrees of oxygen exchange as the sample from In the 1980’s, geothermal operators drilled four deep GMF 31-17 (δ18O = –0.8 to –3.3 ‰). geothermal wells to depths ranging up to 2932 m. Three, including GMF 31-17, tapped a high- A xenolith of hydrothermally altered granodiorite (68 temperature, liquid-dominated geothermal reservoir wt.% SiO2) was found within the 171 ka “andesite and are capable of geothermal production (Hulen and tuff”. This unit is thought to have vented ~ 6km Lutz, 1999). Another well, 17A-6, encountered distant from the GMF 31-17 drilling site (Donnelly- commercial temperatures but was not flow tested Nolan and Nolan, 1986). The xenolith, sample because of mechanical difficulties (Hulen and Lutz, 997M-f, is petrographically similar to the granitoids 1999). from the geothermal drillholes and has a whole-rock δ18O of 0.5 ‰. SAMPLES We also studied unaltered granitoid xenoliths found Granodiorite (64 wt.% SiO ) from geothermal 2 within younger volcanic units at MLV. The xenoliths drillhole GMF 31-17 was made available by California Energy Corporation. The sample came range in composition from diorite to granite and from a depth of ~2570 m. This granodiorite body is include granophyric, seriate and equigranular textures. The fine- to medium-grain size, granophyric located within the present-day geothermal system and quench textures and vapor-rich fluid inclusions imply shows clear evidence for hydrothermal alteration. In that these granitoids crystallized in the upper crust (< thin-section, mafic minerals have been converted to 8 km) where pressures are low and temperature chlorite and Fe-oxides. Abundant, secondary, liquid- gradients high. The xenoliths range from unmelted to rich fluid inclusions give the feldspar phenocrysts a highly melted (up to 50%). Presumably, the melting grainy appearance. Whole-rock analysis for oxygen isotope composition yielded a δ 18O of –1.5 ‰, occurred during final transport to the surface within a hot magma column. Phenocrysts show no signs of revealing that abundant meteoric-derived water has hydrothermal alteration and δ18O of whole-rocks and interacted with the rocks, replacing the original phenocryst separates display magmatic compositions magmatic oxygen. from (+6 to +10‰). Sr isotopic compositions (87Sr/86Sr) of many of the xenoliths range up to Hydrothermally altered granitoid is also found 0.7050, far more radiogenic than basalts and basaltic continuously from 2348 m to 2933 m within andesites from MLV (Grove et al., 1988). That, and geothermal drillhole 17A-6. Chemical analyses of δ 18 drill cuttings of this material range in composition the elevated O imply a significant crustal component to the granitoid xenoliths (Grove et al., from 68 to 74 wt.% SiO2 and thin sections display a range of micrographic to seriate to equigranular 1988; Lowenstern et al., in prep.) About 35 xenoliths have been collected and analyzed for their chemical 2 and isotopic compositions, but only six have been further prepared for geochronology (Table 1). They come mostly from the following Holocene lavas: Medicine Lake Glass Flow (MD1 and MD2), Crater Glass Flow (CG1 and CG2) and Little Glass Mountain (LGM1). In addition, we studied a single xenolith from the ~85 ka Lake Basalt (LB1). Sample Field Wt.% δ18O Host Name Name SiO2 ‰ Eruption GMF GMF 31-17 31-17 63.9 -1.5 – 997M-f 997M-f 67.7 0.5 ~171 ka LB1 561M 71.0 9.3(q) ~85 ka CG1 2050M 64.9 6.0 ~ 1 ka CG2 2049M 73.0 8.3 ~1 ka MD1 86-3 72.6 7.8 ~ 5 ka MD2 680M 74.2 8.8(q) ~ 5 ka Fig. 2. Cathodoluminescence image of zircon crystal LGM1 2029M 73.7 8.8 ~ 1 ka from LGM1. Dark bands correlate with areas high in U. The white circles Table 1. Granitoid samples from MLV. Oxygen represent areas analyzed with the isotopes for whole rocks, except where SHRIMP-RG and yielded 230Th/238U ages indicated (q = quartz separate). Host of 50±10 and 55±10 ka (1 σ analytical eruption = time of eruption that brought errors). This crystal is among the oldest xenolith to surface. from this sample and may have been remobilized from an earlier intrusion. SAMPLE PREPARATION AND ANALYTICAL TECHNIQUES For samples younger than 200,000 years, low 230 238 Zircons were separated with standard magnetic radiogenic Pb and potential Th– U disequilibrium and heavy-liquid techniques and individual zircons makes U-Pb dating rather imprecise. Such young samples were thus dated by taking advantage of were mounted in epoxy, ground nearly to half- 230 238 thickness using 1500 grit wet-dry sandpaper and Th– U disequilibrium systematics (Reid et al., polished with 6- and 1-µm diamond abrasive. Each 1997; Lowenstern et al., 2000). Because zircon incorporates U preferentially with respect to Th, a grain was photographed in reflected light and imaged 230 with an SEM-based cathodoluminescence detector finite time (about 5 half-lives of Th; i.e., ~375,000 years) must elapse before 230Th is in isotopic (Fig. 2). Isotopic data were acquired on the 238 Stanford/USGS SHRIMP-RG (Sensitive, high- equilibrium with its parent isotope, U. Prior to that time, and especially for samples < 150,000 years old, resolution ion microprobe – reverse geometry) at 230 238 Stanford University. Grains older than 300,000 years the Th– U system is effective as a geochronometer.