Clay Diagenesis in Tutuila, American Samoa

Chris Bartek2010/11/30

Clay Diagenesis: A Factor in Geochemical Modeling?

• Geochemical characterization allows stone and ceramic material to be sourced to a geological setting

• Assumes that there will be relationships between cultural material and broad geological source locations

• Also assumes compositional group variation is not measuring variation in diagenesis

Hypothesis• Test the assumption and explore the impact of

diagenesis on clay• Diagenesis IS a factor in compositional group variation• Clay samples (n=21) were collected across Tutuila,

American Samoa

Method

• Using LA-ICP-MS, James and Eckert (2009) found that these clays differentiated by volcanic series Olomoana, Pago, and Leone

• LA-ICP-MS clay data, this study uses PHREEQC to model changes in saturation indices and chemical composition

• Saturation indices indicate whether a phase precipitates or dissolves, and analyzing chemical composition of the clay samples confirms any changes that may have happened to the clay

Saturation Indices

• Element concentrations given by LA-ICP-MS were used in creating an input file.

• Saturation indices were given with the following 18 phases: Al(OH)3(a), albite (NaAlSi3O8), chalcedony (SiO2), chlorite (14A) (Mg5Al2Si3O10(OH)8), chrysotile (Mg3Si2O5(OH)4), Fe(OH)3(a), gibbsite (Al(OH)3), goethite (FeO(OH)), H2(g), H2O(g), hematite (Fe2O3), kaolinite (Al2Si2O5(OH)4), O2(g), quartz (SiO2), sepiolite (Mg2Si3O7*5OH:3H2O), sepiolite (d) (Mg2Si3O7*5OH:3H2O), SiO2(a), and talc (Mg3Si4O10(OH)2)

Group 1

• 3 groups of clay can be established on the basis of what species will precipitate and what species will dissolve

• Group 1 consists of clay series 175, 177, 619, 1200 and subsamples 621B, 621C, 1197A, and 1197C

• These clays are characterized by major precipitates: hematite, kaolinite, gibbsite, and goethite. Minor precipitates include Al(OH)3(a), albite, Fe(OH)3(a), and chalcedony. While these phases precipitate, chlorite (14A), chrysotile, H2(g), H2O(g), O2(g), sepiolite, sepiolite (d), SiO2(a), and talc dissolve

Group 1

Group 2• Group 2 contains subsamples 621A and 1197B• Similar to group 1, except chlorite and talc

precipitate in both subsamples 621A and 1197B instead of dissolving

• Differences between subsamples 621A and 1197B

• Subsample 1197B is similar to subsample 621A where in both subsamples, chlorite and talc precipitate, but chrysotile, sepiolite, and sepilote (d) also precipitate, and Al(OH)3(a) dissolves in 1197B

Group 2

Group 3

• Group 3 consists of clay series 617• More similar to group 2 than group 1 due to

the precipitates characterizing group 3• Al(OH)3(a) dissolves

Element Concentration

• Changes in clay chemistry can also be detected with element concentration

• All of the clays analyzed in this study are generally high in Al, Na, and Si, while remain low in Fe and Mg

• Group 1 is characterized by high Al and low Mg

Group 1

Group 2

• Group 2 is characterized by low Al and high Mg

Group 3

• Group 3 is differentiated by high Na concentration

• All clays in series 617 are high in Na

Conclusions

• These data are indicative of chemical change altering Group 3 in the post-depositional process

• Group 3 contains clays with high concentrations of Na and supersaturated chlorite

• Clay series 617 has a low Al concentration, along with the highest Na concentrations among the clay samples

Conclusions

• Possible that the clay of group 3 was initially deposited near a coast and was influenced by low temperature and low pressure.

• Diagenesis differentiated the compositional groups of this study

References

•Cochrane, Ethan E., and Hector Neff (2006) Investigating compositional diversity among Fijian ceramics with laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS): implications for interaction studies on geologically similar Islands. Journal of Archaeological Science 33, 378-390. •James, William D. and Suzanne L. Eckert (2009) Clay Procurement Practices on Tutuila Island, American Samoa during the Ceramic Period. Poster presented at the 74th Meeting of the Society for American Archaeology. Atlanta, Georgia.•Johnson, Philip R., Frederick B. Pearl, Suzanne L. Eckert, and William D. James (2007) INAA of Pre-Contact Basalt Quarries on the Samoan Island of Tutuila: a Preliminary Baseline. Journal of Archaeological Science 34, 1078-1086.