Geomorphology 53 (2003) 199–208 www.elsevier.com/locate/geomorph Cosmogenic 36Cl ages of Quaternary basalt flows in the Mojave Desert, California, USA Fred M. Phillips* Department of Earth and Environmental Science, New Mexico Tech, Socorro, NM 87801, USA Received 24 September 2001; received in revised form 30 April 2002; accepted 1 May 2002 Abstract Basalt flows provide excellent opportunities for calibration and intercomparison of Quaternary dating methods, remote sensing methods, and rates of geomorphic processes. The immediate motivation for this study was to provide chronology for a blind test of the utility of rock varnish microstratigraphy as an indicator of the age of flow emplacement. Five basaltic eruptive centers in the Mojave Desert of California were sampled for cosmogenic 36Cl analysis. Multiple samples were taken from most centers and, with one exception, produced good agreement. Assuming a surficial erosion rate of 1 mm/kyrÀ1, the flows yielded the following ages: Amboy Crater, 79 F 5 ka; Pisgah Crater, 22.5 F 1.3 ka; Cima field, I-Cone, 27 F 1.3 ka; Cima field, A- Cone, 21 F 1.6 ka and 11.5 F 1.5 ka; Cima field, flow of unidentified origin, 46 F 2 ka. The ages from the Cima I and A cones are in good agreement with previous cosmogenic 3He dating. Ages from the three previously undated flows are significantly older than previous estimates based on flow appearance. Tanzhou Liu performed varnish microstratigraphic analysis on samples collected from the same sites. His results were submitted for publication without knowledge of the 36Cl ages. His age estimates agree well with the 36Cl ages for the three previously undated flows, strongly supporting the validity of varnish microstratigraphy as a chronological correlation tool. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Mojave Desert; Cl-36; Volcanic rocks; Basalts; Geochronology; Exposure age; Quaternary 1. Introduction on three flows not previously quantitatively dated: Amboy Crater, Pisgah Crater, and an unnamed flow in A number of relatively fresh, youthful-appearing the Cima field (Fig. 1). basaltic cones and flows in the Mojave Desert have There were several motivations for dating these attracted geological attention since the time of the flows. The primary one was as a test of the rock earliest investigators (Darton et al., 1916).Inthis varnish microstratigraphy method for surface dating study, 36Cl surface-exposure dating has been used to and paleoenvironmental reconstruction (Liu, 1994; obtain ages on two previously dated flows, those from Liu et al., 2000). Rock varnish from arid regions is the ‘‘A’’ and ‘‘I’’ cones in the Cima volcanic field, and typically deposited as alternating dark, manganese- rich layers and reddish, iron-rich layers. The manga- nese-rich layers are thought to be deposited during * Tel.: +1-505-835-5540; fax: +1-505-835-6436. intervals of relatively humid climate and the iron-rich E-mail address: [email protected] (F.M. Phillips). ones during more arid intervals. If the varnish chem- 0169-555X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-555X(02)00328-8 200 F.M. Phillips / Geomorphology 53 (2003) 199–208 117 W 116 W 115 W 114 W 11552'30" W 11545'W (A) AVF 'N NEV Amboy CALIFOR ADA N • MD99-5, 6, 8 • Las 3433'45' N NI Vegas A Amboy Crater ARIZONA MOJAVE 15 Kingman Baker × CVF • DESERT N 35 N 36 Barstow 'N • Needles × 40 3430 PVF • × Amboy Undifferentiated AVF lava flows 012km • Site and sample MD99-5 numbers 11622'30'' W 3445' N (B) PVF Railroad N MD99-1, 2, 3 W • Pisgah Crater 11630'' I-40 Flow Units and Types Phase III (pahoehoe) Phase II (pahoehoe and aa) 012km • Sites and sample MD99-1 numbers Phase I (pahoehoe) 3445' N 115 50' W 115 45' W (C) CVF vent I 700 • MD99-9, 10, 11 35 12' 30" N vent H vent G 1100 MD99-12 • Kel vent F vent E baker Road • MD99-16, 17, 18 vent A vent B Contour interval 50 m 0 0.5 1.0 1000 N km Quaternary lava flows 35 10' N 800 and scoria cones 35 10' N 35 12' 30" N • 900 MD99-12 Sites and sample numbers 115 50' W 115 45' W Fig. 1. Location maps of 36Cl samples from late Quaternary lava flows in the Mojave Desert of California, western United States. (A) Amboy volcanic field; (B) Pisgah volcanic field; (C) southern portion of the Cima volcanic field. Geologic map of flows in B by Wise (1966), presented in Greely et al. (1988). Figure modified from Liu (2003-this issue). F.M. Phillips / Geomorphology 53 (2003) 199–208 201 istry varies in a regionally coherent fashion and surface erosion and where the likelihood of cover by relatively complete microstratigraphic sections are scoria or sand was small, inasmuch as removal or common, then the sequence of layering can poten- addition of material can affect cosmogenic nuclide tially provide a record of regional paleoenvironmental ages of surfaces (Gosse and Phillips, 2001).All conditions (Liu et al., 2000; Broecker and Liu, 2001). samples were taken from pahoehoe lava flows, with The ‘‘age’’ (by correlation) of the basal varnish layer the exception of those from the Cima ‘‘A’’ cone, should then yield an estimate of the timing of initial which was an aa flow. The presence of glassy, frothy, exposure of the rock surface. flow-top pahoehoe basalt was used as a primary In order to provide a rigorous test of the varnish criterion for minimal erosion on all flows except the microstratigraphy technique, Tanzhou Liu (Lamont- ‘‘A’’ cone. Sample thickness and estimated bulk Doherty Earth Observatory, Columbia University, density (affected by vesicularity) were noted for all New York) and I sampled the same outcrops on the samples, although Gosse and Phillips (2001) have basalt flows at the same time. The samples were shown that error in estimating sample thickness or processed completely independent and have been vesicularity have only a small influence on calculated submitted for publication without knowledge of the cosmogenic in situ nuclide ages. Sample locations other’s results. The varnish microstratigraphy results were determined by global positioning system. can be found in Liu (2003-this issue). The samples were prepared by digestion in mixed 35 A second motivation for the study was the inter- HF and HNO3. A spike of isotopically enriched Cl comparison of Quaternary dating methods. The ‘‘A’’ was added during the digestion so that the Cl content and ‘‘I’’ cones at the Cima volcanic field have of the samples could be determined during the accel- previously been chronologically studied using K– erator mass spectrometry (AMS) analysis by means of Ar, 40Ar/39Ar, 3He, and thermoluminescence. The the isotope dilution mass spectrometry method. The 36Cl results for the flows from this study provide a sample preparation methodology is described in valuable intercomparison. The 36Cl ages for the Pis- Ayarbe (2000). The 36Cl/Cl ratio was measured by gah and Amboy flows will provide opportunity for AMS at PRIME Laboratory (Purdue University) additional future intercomparisons. (Elmore et al., 1979). Major oxides and U and Th Finally, these Mojave Desert flows have been were measured by X-ray fluorescence at New Mexico extensively used for a variety of applied studies. They Tech (Bureau of Geology) and B and Gd by prompt have been important ‘‘calibration’’ sites for interpre- gamma emission spectrometry. tation of remote sensing data (Farr, 1992; Arvidson et The 36Cl age calculations were performed using the al., 1993; Guinness et al., 1997; Greely et al., 1988). data-reduction program CHLOE (Phillips and Plum- Their geochemistry has yielded important information mer, 1996). Elevation-latitude scaling was according on evolution of basic magma bodies (Smith and to Lal (1991). The recently revised production for- Carmichael, 1969; Reid and Ramos, 1994; Farmer et mulation and production rates of Phillips et al. (2001) al., 1995). The study of post-eruptive modifications of were employed. This formulation explicitly accounts the flow surfaces have played an important role in the for muon production. Other aspects of the production development of theories for geomorphic processes in calculations were as described in Gosse and Phillips arid environments (Dohrenwend et al., 1984; McFad- (2001). Calculated ages and sample location data are den et al., 1987; Wells et al., 1995). All of these, but given in Table 1 and the 36Cl ages for the various especially the last, stand to benefit by improved volcanoes are compared in Fig. 2. Chemical data are chronology for the flows. given in Table 2. Apparent cosmogenic ages are affected by erosion, therefore ages in Table 1 are given for three hypothesized erosion rates: zero, 1, 2. Methods and 5 mm/kyr. Based on field observations, the fact that the samples were carefully selected for sites with Samples were collected from the flow surfaces minimal erosion, and general observations of bare- using hammer and chisel. Care was taken during rock erosion rates in arid environments, the ages sample collection to locate sample sites with minimal calculated based on the assumed 1 mm/kyr erosion 202 F.M. Phillips / Geomorphology 53 (2003) 199–208 Table 1 36Cl ages and sample data for Mojave Desert basalt flows a b Sample no. Flow Zero erosion age 1 mm/kyr age 5 mm/kyr age Longitude Latitude Elevation ST Kf,e (ka) (ka) (ka) (jN) (jW) (m) (g/cm2) MD99-1PC Pisgah 24.2 F 2.7 23.3 F 2.5 21.3 F 2.2 34.7558 116.3769 685 0.998 166 MD99-2PC Pisgah 23.0 F 2.7 23.1 F 2.5 21.1 F 2.1 34.7558 116.3769 685 0.998 166 MD99-3PC Pisgah 21.0 F 2.1 21.0 F 1.9 18.9 F 1.6 34.7558 116.3769 685 0.970 166 MD99-5AC