Obsidian elemental composition

Table 5.4 Olivine major element compositions (in weight %). Samples occur in different types of rocks (1) = forsterite from a metamorphosed limestone (2) = hortonolite from an olivine gabbro (3) = fayalite from a pantelleritic obsidian (4) = fayalite from an Fe-gabbro (5) = forsterite from a cumulitic peridotite (6) = forsterite from a tectonitic peridotite. Samples (1) to (4) from Deer et al. (1983) sample (5) from Ottonello et al. (1979) sample (6) from Piccardo and Ottonello (1978). ...

Most obsidian sources are chemically homogeneous, with variations in composition on the order of a few percent or less. However, the individual sources have different trace-element compositions as a reflection of the compositions of parent rocks and changes taking place in the magma chamber prior to eruption. The major elements are restricted to a relatively narrow range of composition but the abundances of trace elements can differ by orders of magnitude between obsidian sources. If the variations within sources are smaller than the differences between sources, then the provenance of obsidian artifacts can be successfully established. 

Jampatilla, Peru, obsidian source, 536 Jen Tsung, coins, elemental composition, 233,237/-239/ Jiskairumoko, Peru, ochre artifacts instrumental neutron activation analysis, 480-505 mathematical and statistical data treatment, 492-501... 

Obsidian is a volcanic rock that forms when lava cools very quickly. The chemical elements in the melt freeze in random order. The result is obsidian or volcanic glass, which has no orderly internal structure and can be of any elemental composition. 

The use of trace element analysis to determine the provenance of archaeological materials has expanded rapidly in the last decade. It is now a well-established technique for the identification of obsidian source deposits (J), and is nearly as established for turquoise (2), steatite (3), and some ceramic materials (4). Native copper has received much less attention. Friedman et al. (5), Fields et al. (6), and Bowman et al. (7) used trace element analyses to determine the type of geological ore from which copper was extracted. However, only our efforts (8) and the work of Goad and Noakes (9) have focused on collecting and analyzing native copper from all potential deposits of a given region to provide a data base for statistical comparison with artifact trace element compositions. 

Minerals are inorganic compounds that are found in nature and have both a well-defined composition and crystalline arrangement of atoms. Coal and petroleum hydrocarbons are organic and thus not minerals. Obsidian is not a mineral because it has neither crystalline structure nor a specific composition. Stones such as chert and flint, which are mainly silica, SiO, have a relatively precise composition but lack crystalline structure, so are not minerals. While copper is a mineral, brass and bronze do not occur in nature and do not have a fixed elemental composition, so they are not minerals. A synthetic material can be a mineral, however, as long as it is also found in nature. Hematite can be produced artificially by firing ceramics in an oxidizing environment, but it is still considered mineral because hematite can be found in nature. A synthetic ruby is likewise a mineral because rubies do occur in nature, but modem cubic zirconia is not. 

Much study has been made on obsidian, whose production can often be linked to a specific volcanic eruption on the basis of the trace and minor element composition [101,102]. The results of such a study are important both for understanding trade patterns in obsidian artifacts, and for identifying the occurrence of tephra in stratigraphic layers, thus providing a basis for chronological information. 

In most obsidian provenance studies, the ability to employ compositional differences to discriminate between sources depends, to a certain extent, on the number of elements measured. Because instrumental neutron activation analysis (INAA) is capable of measuring 25-30 elements in obsidian with excellent precision, numerous combinations of trace and major elements are available for comparing differences between sources. The main requirements for success are that all sources have been located and analyzed, and that the internal variation measured within the sources be smaller than the compositional differences measured between the sources (23). 

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