Petrological Sourcing of Polynesian Lithic Artifacts: Criteria, Methods, and Uncertainties Society of Hawaiian Archeology Earth Day, 2015 John Sinton Department of Geology and Geophysics University of Hawai‘i at Mānoa

Petrology* of Volcanic Rocks *Petrology – the study of rocks, Πέτρος Some things that can be measured in volcanic rocks: Texture grainsize, fabric Mineralogy Chemical Composition Major elements; trace elements; isotopic ratios Age Rock classification schemes combine chemical composition and texture Basalt: a fine-grained, mafic volcanic rock, typically with <52.5 wt % SiO 2, blah, blah ……

Magmatic Processes 1.Melting -The genetic imprint when the magma is “born” 2.Differentiation -How that magma changes composition as it rises, and resides prior to eruption 3. Eruption (or not) -The cooling history of a magma affects the mineralogy and texture of the rock Melting and differentiation combine to control chemical composition (like genetics and environment combine to affect human personality) A devil A born devil On whose nature Nurture can never strike -Prospero’s description of Caliban

Alkalic differentiation trend Tholeiitic differentiation trend Chemical effects of melting and magmatic differentiation melting

Names of Hawaiian volcanic rocks Names of Hawaiian volcanic rocks (based on composition) Alkalic series Tholeiitic series

Because so many variables combine to control the composition and texture of a volcanic rock, there is a supposition that each particular lava flow or dike is likely to be (nearly) unique. Two flows from the same volcano might bear similarities in some characteristics, but probably not all. extent of partial melting mantle source composition crustal storage, fractionation, and assimilation intrusion environment eruption rate eruption conditions and duration

What to measure? Elemental abundance

Atomic number Neutron number What to measure? Isotopic ratios

Silicon and oxygen comprise ~45% (by weight) of the entire planet Earth … and almost 75% of Earth’s crust The most important rock forming minerals are silicates (containing silicon and oxygen)

O Si Ti Al Fe +2, Mn Mg Ca Na K+K P Sum99.97 All Fe as FeO SiO TiO Al 2 O FeO*13.99 MnO0.20 MgO5.17 CaO9.78 Na 2 O2.87 K2OK2O1.10 P2O5P2O Sum99.97 All Fe as Fe 2 O 3 SiO TiO Al 2 O Fe 2 O 3 *15.31 MnO0.20 MgO5.09 CaO9.63 Na 2 O2.83 K2OK2O1.08 P2O5P2O Sum99.97 Normalized SiO TiO Al 2 O Fe 2 O 3 *15.32 MnO0.20 MgO5.09 CaO9.64 Na 2 O2.83 K2OK2O1.08 P2O5P2O Sum Major element* chemical analyses (all values in weight per cent) *Elements that typically occur in abundances greater than 0.1 weight % and which together make up >99 % by weight of any given igneous rock The same data expressed as oxide wt % Note that analytical total is conserved

Absolute Precision Detection Limit Sc23 V33 Cr33 Co33 Ni33 Cu33 Zn33 Rb10.6 Sr11 Y11 Zr21 Nb10.5 Ba108 Pb12 Th12 U12 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Trace elements by WD-XRF (ppm) Additional trace elements analyzed well by ICP-MS Typical absolute precision ~0.5 ppm Typical LLD ~0.1 ppm

The confidence with which an artifact can be “sourced” to a specific location depends on two principal uncertainties 1.Number of elements (observations) and reliability of data obtained on the artifact 2.Composition and variability of the source region, as determined by number and quality of analytical data for that location Each of these uncertainties can be quantified

Uncertainties associated with geochemical data 1.Precision : How reproducible are the data? Will you get the same number if you re-analyze a particular sample by the same method in the same lab? 2.Accuracy : How close are the analytical data to the absolute value in the sample?

Not shown: Pu‘u Wa‘awa‘a glass source Most of the known quarries can be distinguished Not many data for most Hawaiian quarries Source Variability : What is the variation of the source? How well characterized is it?

Hawaiian Quarries: elements analyzed well by XRF -Many quarries indistinguishable with these elements -WD-XRF uncertainties smaller than symbols -ED-XRF fields from Mintmeier et al. (2012)

Case Studies 1. Vitaria Quarry, Rurutu, Austral Islands [Rolett et al., JAS 2014] Marquesas Society Is. Tuamotus Austral Is. Rurutu Raivavae Tubuai Tahiti Gambier Geologic map (Guille et al., 2000)

Adze quarry associated with major ceremonial complex Viliamu Teuruarii [Rolett et al., JAS 2014]

The Vitaria source composition is well defined by geological samples and artifacts most differentiated (highest SiO 2, lowest MgO) of the Recent Volcanics on Rurutu [Rolett et al., JAS 2014]

Mean ± 1 standard deviation from the mean for each element or oxide The Vitaria source is relatively uniform in composition Mean composition and variability of Vitaria source (mean + 1  )/mean (mean - 1  )/mean [Rolett et al., JAS 2014]

Unknowns (with uncertainty) compared to source Unknowns compared to mean source composition. Error bars show analytical uncertainty for each element or oxide Some adzes from Raivavae match the Vitaria (Rurutu) source for all elements within uncertainty Major element uncertainties smaller than symbol size. Raivavae adzes from Peabody Museum [Rolett et al., JAS 2014]

Raivavae adze E36447 is not from the Vitaria source, (likely from Tubuai) [Rolett et al., JAS 2014] Unknowns (with uncertainty) compared to source Some elements (Al 2 O 3, FeO, Zn, Nb) are consistent with Vitaria source, other elements are not

Case Studies 2. Tuamotu adze (C7727) Collerson & Weisler, Science, 2007 Tuamotu adze C7727 assigned a source on Kaho‘olawe, based on isotope ratios

C7727 A source in Samoa, Society Islands, Marquesas, and Austral Islands is not permitted by isotopic data A source in the Hawaiian Islands is consistent with isotopic data

C7727 analytical uncertainty Kaho‘olawe Mō‘iwi Adze C7727 is unlike any analyzed rock from Kaho‘olawe

2. Tuamotu adze C7727 Isotopic data consistent with a source in the Hawaiian Islands Major and trace element data inconsistent with a source on Kaho‘olawe Isotopic data alone can indicate or eliminate island chains; rarely can such data identify specific volcanoes

Case Studies 3. Unusual adze from ocean near Sand Island, O‘ahu Measurements of 870 Hawaiian adzes in Bishop Museum collection (T. Dye & J. Kahn) Sinton & Sinoto, in prep.

Unusual apatite-bearing alkalic lava, with flow layering Indistinguishable from sample C-159 (Macdonald, 1968) Distinctive -Grainsize -Flow layering -Brown apatite + magnetite

Pu‘u Pāpa‘i Keonekū‘ino, East Moloka‘i Geology modified from Stearns and Macdonald, 1947)

Adze and Pu‘u Pāpa‘i share unusual chemical composition Ne-hawaiite with low SiO 2, and extreme P 2 O 5 (highest in Hawaiian Islands)

… and similar, equally unusual apatite compositions Ca-phosphate: Ca 5 (PO 4 ) 3 (OH, F, Cl)

Working area at Pu‘u Pāpa‘i

3. Adze BP Adze rock indistinguishable from Pu‘u Pāpa‘i -texture -mineralogy -whole-rock composition -apatite compositions This composition is extreme in P 2 O 5 (and Sr) for the Hawaiian Islands and elsewhere in Polynesia Likelyhood that Earth exactly repeated itself close to zero Evidence for working of cores present in the field

Element Y Element X Case Studies 4. Hypothetical Artifact analyzed in lab A Source rock analyzed in Lab B It’s a (statistical) match! (but not a very good one) What now? 1.Analyze artifact in Lab B 2.Analyze elements M, N, O, P, Q, R……. 3.Make a thin section 4.Consider other sources

“Sourcing” - matching analytical or observational data to that of known quarries, volcanic units, volcanoes, island groups …. The confidence with which an artifact can be “sourced” to a specific location depends on two principal uncertainties Composition and variability of the source region Few quarries are presently adequately characterized 1.Quarry sampling depends on geology -dikes, lava flows, colluvial scatters and alluvial deposits will require different sampling strategies 2.Analyze with the most comprehensive and best data available -petrography, mineralogy, texture -major element oxides -range of trace elements

1.No free lunch More and better data are preferable to fewer and less precise data 2.One size does not fit all Nested approach Emphasize diagnostic characteristics The confidence with which an artifact can be “sourced” to a specific location depends on two principal uncertainties How well analyzed is the artifact Collaborators: Paul Cleghorn, Tom Dye, Steve Eminger, Jenny Kahn, Pat Kirch, Peter Mills, Barry Rolett, Yosi Sinoto, Marshall Weisler MAHALO