APPLICATION OF LA-ICP-MS TO TEPHRA CORRELATION STUDIES IN AFAR, ETHIOPIA

The history of explosive volcanism in the Afar region of Ethiopia is recorded in Neogene to Quaternary tephra deposits in strata exposed in badlands along the Awash River. High extension rates and complex extension patterns in the Afar have resulted in faulted or discontinuous strata that require tephra correlation methods for linking stratigraphic records. Geochemical fingerprinting of tephras has traditionally used major element data collected using electron microprobes. However, recent improvements to the laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) methodology allow minor and trace elements of small (20 μm), discrete, volcanic glass shards to be analyzed that increases the potential number of elements for correlation and magmatic evolution studies.

We used LA-ICP-MS to measure the composition of over 30 tephra deposits from the Ledi-Geraru research project area and other paleontological sites in the Afar. First, we tested the validity of LA-ICP-MS by investigating the 2.8 million year old Gurumaha Tuff – a stratigraphically, geochemically, and temporally constrained deposit. LA-ICP-MS results confirm that Gurumaha Tuff samples are geochemically indistinguishable and establish a protocol for LA-ICP-MS analytical conditions. Second, the Markaytoli and Ogoyta Tuffs, initially mapped as discrete tephras within a structurally complicated area, are geochemically identical providing a firm stratigraphic tie for interpreting depositional environments. Third, we tested the potential correlation of AST-3 and Fialu Tuffs, tephra deposits within the 2.7 – 0.16 Ma Busidima Formation thought to correlate based on major elements. We used trace elements to identify a sample incorrectly attributed to AST-3, and systematic differences in heavier REE between AST-3 and Fialu indicate they may represent different eruptive events. Our continued work is generating new and more robust tephra correlations to improve stratigraphic links and age models for dating fossil and artifact sites in the Afar. These data also serve as a promising first step in building a discrete grain LA-ICP-MS database to study the magmatic evolution of the Afar.