Abstract. Prehistory of Sumatra well characterized by its abundant numbers of obsidian industry, one of which is in Jambi Province. However, determination on the geochemical characteristic of obsidian from Jambi is still lacking. Portable X-Ray Fluorescence analysis (pXRF) on obsidian samples from Merangin and Sarolangun proved the existence of three different obsidian sources in Jambi. It is obtained through the determination of pXRF on the particular trace elements: Rb, Sr, Zr, Y, Nb, Ti and Mn. The results are then analyzed by the Principal Component Analysis (PCA) to arrange the same obsidian sources. This result is then corroborated with the Independent Sample T-Test. This analysis reveals the similarity in trace-element concentration amongst the same source, as well as their differences within different sources. This study contributes to the identification of two new obsidian sources from Sarolangun that have never been reported before. As a result, there are five known-sources of obsidian in Southern Sumatra, in which three other sources were previously identified by Ambrose et al. (2009) and Reepmeyer et al. (2011).Abstrak. Prasejarah Sumatra terkenal dengan industri obsidiannya yang melimpah, salah satunya yaitu di wilayah Jambi. Namun demikian, determinasi karakteristik geokimia obsidian dari wilayah Jambi hingga saat ini masih sangat terbatas jumlahnya. Analisis Portable X-Ray Fluorescence (pXRF) pada sampel obsidian dari Merangin dan Sarolangun membuktikan adanya tiga sumber obsidian yang berbeda di wilayah Jambi. Perbedaan tersebut diperoleh melalui determinasi pXRF pada unsur-jejak Rb, Sr, Zr, Y, Nb, Ti dan Mn. Hasil determinasi kemudian dianalisis dengan metode Principal Component Analysis (PCA) untuk mengelompokkan sumber-sumber obsidian yang sama. Hasil analisis tersebut kemudian diperkuat oleh analisis Independent Sample T-Test yang menunjukkan kemiripan proporsi unsur-jejak pada sumber yang sama, sekaligus perbedaannya pada sumber yang berlainan. Studi memberikan kontribusi berupa identifikasi dua sumber obsidian baru dari Sarolangun (Batang Asai 1 dan 2) yang belum pernah dilaporkan sebelumnya. Dengan demikian terdapat lima sumber obsidian di Sumatra Bagian Selatan, dimana tiga sumber lainnya (i.e. Kerinci, OKU dan Tapus) telah berhasil diidentifikasi oleh Ambrose dkk. (2009) dan Reepmeyer dkk. (2011).
Gamma ray log is a logging tool to capture the radioactive level of a rock or formation measured in API units. This logging tool generally has a capability to differentiate between permeable and impermeable layers. Usually the impermeable layer tends to have higher radioactivity compared to the permeable one except for the feldspar bearing formation. In addition, another capability of this logging tool is to determine the kind of clay mineral, by using ratio data between Thorium and Potassium. This laboratory experiment used Spectral Gamma Ray (SGR) equipment at LEMIGAS Routine Core Laboratory. The quality of gamma ray log measurement is significantly affected by the speed of the conveyor belt. During the experiment, the measurement speed of 30 m/hour is the optimum speed to achieve good quality data and time efficiency with the data amount of 169 points/meter. The result of SGR measurement gives the reading on the content of Uranium, Thorium and Potassium. The Thorium and Potassium content are compared and plotted in Quirein graphic which was modified by Schlumberger in 1985. Using this crossplot, we can identify the presence of Chlorite, Montmorillonite, Kaolinite, Illite, mixed layer Feldspar, Mica, and Glauconite minerals. A case study conducted on Wells A1, A2, A3 and A4, indicated that the result of this crossplot were similar to the measurement using XRD.
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