Reports: G2
47811-G2 Element Mobility During Zeolitic Alteration of Volcanic Ash: Implications for Tephra Correlation in Sedimentary Basins
Using funds granted by the American Chemical Society Petroleum Research Fund (and other sources), I have conducted two field expeditions to Olduvai Gorge, Tanzania (2008, 2009). In both years, I sampled Pliocene-Pleistocene tephra altered under saline-alkaline lacustrine and groundwater conditions.
The purpose of the sampling was to determine the compositional changes associated with different degrees and types of tephra alteration in a closed-basin saline-alkaline lacustrine environment. I focused my sampling on one specific ash layer (Tuff IF) to limit the influence of differences in starting composition. I conducted detailed sampling at three outcrops near the center of the paleo-lake deposit (total of 15 samples), and visited three new sites within the lake beds (Localities 77, 78, and 82) to determine the mineralogical and geochemical variability. These samples supplement the altered tephra samples from the lake margin area collected in 2002 and 2006. I also visited the Lake Natron basin (modern saline-alkaline lake) to sample tephra altered in a different lake basin.
In 2009 I also visited three salt springs located within the paleolake basin to sample the modern saline groundwater and examine the geochemistry and mineralogy of spring precipitates and tephra altered in contact with modern saline-alkaline groundwater.
Samples were returned to the lab and analyzed by X-ray Fluorescence (XRF, for major and minor element composition) and X-ray Diffraction (XRD, for authigenic mineral assemblage). Select samples were also analyzed by Scanning Electron Microscopy (SEM) and Electron Microprobe (EPMA) to examine the textural relationships of coexisting zeolites and clay minerals and the major and minor element partitioning between them.
The mobility of major and minor elements during closed-basin zeolitic and argillic alteration of tephra was determined using the results of these analyses. All samples were compared to the freshest sample- one that still contained fresh glass and revealed no authigenic minerals in its XRD pattern. An interesting pattern resulted: even the freshest samples showed incipient leaching of mobile elements (Na, K, etc.). Argillic samples showed an increase in Mg (related to the formation of Mg-rich smectite). Zeolitized samples dominated by phillipsite and authigenic K-feldspar showed an increase in K and Rb.
Most importantly, the zeolitic samples showed marked decreases in traditionally “immobile” elements Ti and Zr compared to the fresh and argillic samples. This loss of Ti and Zr may be related to some process of complexation, possibly involving F. It also indicates that it is not safe to use Ti and Zr as “immobile” elements under these conditions. Al and Nb appear to be less affected. Results of this aspect of the project were reported in a 2009 publication.
During the XRD component of this project, the mineral jarosite was observed in some of the most altered samples. This was surprising, as jarosite generally indicates acidic conditions, while the coexisting phillipsite forms only under alkaline conditions. This apparent contradiction could result from pyrite oxidation. If conditions were reducing at the time of deposition, pyrite could have formed in the lake sediments along with clays and zeolites. Much later, perhaps in modern groundwater, the pyrite could have been exposed to more oxidizing conditions and oxidized. The oxidation of pyrite produces sulfuric acid, which can lead to a local reduction in pH. Jarosite could have formed in this microenvironment. The fact that phillipsite is the dominant mineral shows that these acidic conditions did not last long. I presented results of this aspect of the project at GSA and the LPSC, and am currently preparing a manuscript for submission to EPSL detailing this unusual geochemical system.
There is still a lot of work to be done on this project. I have recruited a Tanzanian student to work on this project, likely starting in January 2010. As part of his MS thesis he will process the tephra, water, and lake clay samples I collected in 2009, and (provided I am granted a no-cost extension) will conduct additional fieldwork in 2010. We will be looking at the partitioning of elements between co-existing authigenic phases to help explain the patterns of element mobility observed in the bulk samples, will be examining how resistant various phenocrysts are to diagenesis, and will be exploring the behavior of trace elements in this system.
