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Core OL-92 from Owens Lake, southeast California

Clay Mineralogical Analyses of the Owens Lake Core

Kirsten M. Menking
Hannah M. Musler
Earth Sciences Board, University of California, Santa Cruz
Jeffrey P. Fitts
James L. Bischoff
U.S. Geological Survey, Menlo Park, California
Robert S. Anderson
Earth Sciences Board, University of California, Santa Cruz

Contents:

Abstract

 X-ray diffraction analyses of the <2 mm fraction of channel samples from the Owens Lake core show illite and smectite to be the dominant clay minerals (accounting for up to 90%), with lesser amounts of chlorite and kaolinite present. Quartz, K-feldspar, and plagioclase are also found in the <2 mm fraction. Two end-member X-ray diffractograms have been identified which characterize predominantly illite- or predominantly smectite-bearing sediments. Increases in smectite content with depth correlate directly with increases in mean grain size and carbonate content with depth, and correlate inversely with increases in illite content with depth.

Introduction

 Owens Lake is the first in a chain of lakes lying to the east of the Sierra Nevada. The lake, which lies at the southern end of the topographically closed Owens Valley, receives runoff primarily from the Sierra Nevada via the Owens River and its tributaries. During "pluvial" periods (high ratio of precipitation to evaporation), Owens Lake exceeded the confines of the southern Owens Valley and spilled over a sill into the China and Searles Lake basins (Smith, 1984). During drier periods, the lake shrank and remained at a level nearer to or lower than the spillover sill. In this paper, we report results of clay mineralogical analyses conducted on sediments from a 323-m long core of Owens Lake which reflect the variations in pluvial-interpluvial climate.

Methods

 Part of the clay-sized fraction from the channel sample grain size analysis (see Menking and others, 1993) was collected for mineralogical analysis. Clays were mounted on glass microscope slides using the filter- peel technique (Moore and Reynolds, Jr., 1989) and were scanned with a Norelco Step-Scanning X-Ray Diffractometer at U.C. Santa Cruz. A step size of 0.022-theta and 5 second dwell time were used. Since the crystalline structures of clay minerals differ primarily in their lattice spacing parallel to the c-axis, the c-axis reflections are most useful in ascertaining which clays are present in any given sample. The filter-peel technique, which ensures the greatest degree of preferred orientation of clay platelets, produces the strongest c-axis reflections, and is therefore favored over other methods of sample preparation.

All minerals were identified by diffraction peak locations and/or by various thermal and chemical treatments (see table 1). To test for the presence of smectite minerals (clay minerals with hydrated inner-layer cations), sample slides were placed on a fritted platform in a covered Pyrex dish containing ethylene glycol. The dish was placed for 8 or more hours into an oven heated to 60 C to allow the ethylene glycol to completely replace any interlayer water held by smectite. Given its larger molecular size, ethylene glycol expands the smectite lattice to a larger spacing than when water is held in the interlayer sites. This lattice expansion is easily noted by a shift in the location of the smectite (001) peak on the X-ray diffractogram from 14 (angstroms) in the air-dried state to 17 when glycolated. After glycolation, samples were re-scanned and changes in peak location and intensity were noted. Chlorite and kaolinite display overlapping diffraction patterns. Therefore, to determine which of these minerals was producing 7 and 3.5 reflections, clays were mounted on X-ray-amorphous tile slides and heated to 550C for 1 hour. At this temperature, kaolinite becomes amorphous causing a reduction in peak intensity in those samples containing it. The chlorite peak may also shift to 6.3 to 6.4 due to dehydroxylation of the inner brucite layer (Moore and Reynolds, 1989). Illite, quartz, K-feldspar and plagioclase were identified by inspection.

Results

X-Ray Diffraction determinations of the clay mineralogy of channel samples show illite, smectite, chlorite and kaolinite to be the primary clay minerals, with some samples containing clay-sized quartz, plagioclase, and potassium feldspar. From a visual inspection of the X-ray diffractograms, we have identified two end-member mineral assemblages. Figure 1 shows a scan representative of those samples displaying small smectite but large illite, quartz, and feldspar peaks, while figure 2 displays a typical large smectite but small illite, quartz, and feldspar peak pattern. Because of the presence of these two end-member patterns, we measured the peak areas of several clay and non-clay minerals in each sample to see if any regular pattern or periodicity existed in the deposition of clays and non-clays. Peak areas of smectite (glycolated (001)), illite (001), quartz (100), plagioclase (002) and K-feldspar (002) were measured and ratioed against each other for each channel sample. Chlorite and kaolinite are both present, and, due to overlapping diffraction peaks, are not easily decoupled from one another. Therefore, we measured the combined chlorite-kaolinite reflection at 7.11Ł (12.42-theta).

Next, we compared ratios between samples. Smectite/quartz and plagioclase/K-feldspar ratios behave inversely, with smectite/quartz ratios showing a low value during what we interpret to be the last full glacial maximum in the Sierra Nevada range (ca. 20 ka, and ~20 m depth in the core), while the plagioclase/K-feldspar value is high (figure 3). Unlike the negative correlation to plagioclase/K-feldspar ratios, smectite/quartz values correlate positively with channel sample carbonate content (figure 4). Illite/quartz and chlorite-kaolinite/quartz values do not correlate positively or negatively to carbonate content. However, they do correlate positively to each other, and to the K-feldspar/quartz and plagioclase/quartz ratios. Likewise, the K-feldspar/quartz and plagioclase/quartz ratios behave similarly.

In addition to looking at how peak-area ratios vary with depth, we employ another technique, described by Hallberg and others (1978) and modified by Hay and others (1991), to determine the relative abundances of clay minerals in the clay-sized fraction of the core. This technique is based upon the fact that certain clay mineral diffraction peaks are inherently larger or smaller than others. For instance, a sample containing equal proportions of smectite and illite will typically display a glycolated smectite (001) peak that is about three times larger than the corresponding illite (001) peak. Therefore, conversion of diffraction peak areas to relative abundances of the minerals from which those peaks arise requires only multiplication of the illite peak area by three, summation of this product with the smectite peak, and the assumption that the two minerals constitute 100% of the clay minerals present. The 7 combined chlorite-kaolinite peak is typically half as intense as the 17 glycolated smectite peak such that a multiplication factor of 2 can be used to raise chlorite-kaolinite to parity with smectite. Figure 5 shows the results of this analysis for the Owens Lake core. Smectite and illite are the dominant phases, with chlorite-kaolinite accounting for only about 10% of the clay minerals (chlorite-kaolinite is not shown in figure 5 but represents the offset between the smectite and illite curves). In addition, smectite and illite are inversely abundant, with smectite concentrations very low during the "last glacial maximum" (ca. 20 m depth), while illite concentrations are high. Comparison of smectite and illite abundances, versus depth, to both the carbonate curve from the channel sample analyses and the mean grain size curve from the point sample analyses shows striking similarities (figure 6).

Discussion

Like Newton (1991), we interpret the presence of non-clay minerals (quartz, plagioclase, and K-feldspar) in the clay-sized fraction to be the result of glacial abrasion in the Sierra Nevada which produced large volumes of glacial flour. Of the true clay minerals, illite and chlorite may be recycled from metasedimentary roof pendants, but smectite is more likely produced from the weathering of feldspars during soil formation, and it may, therefore, be more indicative of climates conducive to chemical weathering (Chamley, 1989). Smectite is also a common weathering product of volcanic ash, so care must be taken to distinguish between true climatic variations and volcanic events (Chamley, 1989). It is interesting to note that the illite/quartz and chlorite-kaolinite/quartz peak area ratios correlate positively with the K-feldspar/quartz and plagioclase/quartz ratios. Since feldspars typically indicate immature, mechanically produced sediments, their correlation with illite and chlorite (though probably not kaolinite) may indicate a mechanical origin for these clays. Conversely, smectite/quartz correlates negatively with plagioclase/K-feldspar. Since plagioclase is less resistant to weathering than is K-feldspar, plagioclase/K-feldspar ratios should be lower in chemically weathered sediments than in those sediments unaltered by chemical processes. Because smectite is a chemical weathering product, ratios of smectite/quartz should be high in those sediments that were chemically altered and low in those sediments that were unaltered. Therefore, smectite/quartz and plagioclase/K-feldspar ratios should, and do, behave inversely (figure 3).

An alternative explanation for the variation between predominantly smectite and illite sedimentation is that smectite may have formed authigenically in Owens Lake during periods of high salinity. The fact that the percent smectite versus depth curve (derived using the multiplicative factors of Hallberg and others, 1978) so closely resembles the percent carbonate versus depth curve (figure 6) may indicate an authigenic origin for much of the smectite content of the core. In addition, authigenic K-feldspar is quite common in saline lakes (Hay and others (1991) found it in Searles Lake sediments). If smectite and K- feldspar are largely authigenic, the smectite/quartz and plagioclase/K- feldspar ratio curves would still vary inversely. In this instance, the varying smectite/quartz and plagioclase/K-feldspar values would result not from the differential chemical weathering of plagioclase and K-feldspar but from the periodic precipitation of K-feldspar and smectite in an authigenic setting. Times characterized by active authigenic mineral formation would be relatively enriched in smectite over quartz and in K- feldspar over plagioclase. Periods with less authigenic activity would show lower values of smectite relative to quartz and of K-feldspar relative to plagioclase. Presently, we can not rule out either a detrital or authigenic origin for clay minerals and feldspars in the core. Future work will attempt to answer whether these minerals are detrital, authigenic, or both.

References

Table 1: Diffraction peaks used to identify minerals in the clay fraction of the Owens Lake core channel samples 
Mineral     (hkl)    2-theta
illite      (001)     8.7      10.1
chlorite    (001)     6.2      14.2
            (002)    12.4       7.11
            (003)    18.8       4.72
            (004)    25.1       3.54
smectite    (001)     5.2      17.0
kaolinite   (001)    12.4       7.11
            (002)    25.1       3.54
quartz      (100)    20.8       4.26
K-feldspar  (002)    27.5       3.24
plagioclase (002)    27.9       3.18

Figures

  1. Typical small smectite-large quartz plus feldspar peak XRD scan. Note the small smectite peak and the large and abundant quartz and feldspar peaks.

  2. Typical large smectite-small quartz plus feldspar peak XRD scan. Note the huge smectite peak and the relative lack of significant quartz and feldspar peaks.

  3. Smectite/quartz and plagioclase/K-feldspar with depth. The two curves are inversely correlated.

  4. Smectite/quartz and carbonate content plotted against depth. The two curves are positively correlated.

  5. Plot showing percentages of smectite and illite as calculated by the method of Hallberg and others (1978), and Hay and others (1991). Note the inverse correlation of smectite and illite.

  6. Comparison of smectite, illite, and carbonate contents, and mean grain size with depth. Smectite, illite, and carbonate contents from channel sample analyses; mean grain size from point sample analyses.

Appendix

Peak areas of selected clay and non-clay mineral reflections, in counts per second.

U.S. Department of Interior, U.S. Geological Survey
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