Clark et al. PNAS - SI Appendix Regional Geology and Sample ...

Clark et al. PNAS - SI Appendix Regional Geology and Sample Allocation Regional geology: The Fiji-West Polynesia region includes the island groups of Fiji, Tonga, Samoa, Uvea (Wallis), Futuna and Rotuma. The area is conventionally subdivided by the 'Andesite Line' (1) separating island arc-deriving igneous rocks of Fiji, Rotuma and Tonga from the intra-plate volcanic islands of Samoa and Uvea further north. The Tongan Arc is subdivided by the Tofua trough which separates the Tofua chain consisting of a series of north-south aligned basaltic andesite to dacite islands (2) from low-lying limestone capped fore-arc islands on the Tongan Platform (3). Like all subduction-related lavas, those of the Tongan Arc are characterized by an enrichment of large-ion lithophile elements (LILE) in contrast to high field strength elements (HFSE) and heavy rare earth elements (HREE) (4-6). The geology of the Fijian islands is complex, involving both subduction related and intraplate components, and a detailed description is beyond the scope of this paper. Arc-deriving igneous rocks are the oldest Late Eocene formations on the main island of Viti Levu. Exposed lavas include tholeiitic basalts and boninites similar to Eua (7). There have been several episodes of uplift that have exposed plutonic intrusions and sedimentary rocks that are otherwise uncommon in Western Polynesia (8). The islands of the Lau Group are situated on the Lau Ridge, a remnant arc, separated from the active Tonga-Kermadec Arc system by the opening of the Lau Basin spreading centre. The volcanic rocks of the Lau Group are subdivided into three discrete groups: Lau, Korobasaga and Mago (9). The oldest lavas of the Lau group are basalt to rhyolite flows, the Korobasaga group has a basalt-to-basaltic andesite spread with dacites exposed on Lakeba and Vanau Balavu. The youngest volcanics are the Mago group consisting of alkalic basalts with strongly enriched LILE and HFSE contents similar to ocean island basalts (10).The lavas of Rotuma appear to have a primitive island arc geochemistry (11). They consist of an older basalt formation, associated with dolerite dykes and olivine gabbros, and a later formation of basaltic andesites. Both formations have a tholeiitic composition with low potassium values, but slightly enriched TiO2 and FeO contents indicative of back-arc basin basalts (12). The Samoa islands are products of the Samoan hotspot (13) with the island of Uvea located on the westernmost end of the archipelago (14). The islands contain a series of shield volcanoes and post-erosional lava flows. Shield volcanic deposits consist of tholeiitic to alkalic basalt sequences with a distinctively enriched TiO2 content and slightly enriched alkali content, even for tholeiites. These older sequences are overlain by highly undersaturated post-erosional volcanics (15) giving rise to the complex chemistry of exposed surface rocks of the Samoan islands. Igneous rocks exposed on Uvea are significantly younger than the islands of Samoa (14). Quaternary volcanism comprises two sequences of older alkaline basalts and hawaiites, and younger tholeiites. The young age of Uvea

igneous formations forbids association with the Samoan hot spot trail and is more likely associated with the melting of subducted oceanic crust, although Uvea lavas appear to have a similar isotopic composition to some Samoan shield lavas (14:283). Figure S1 shows the results of two discriminant function analyses using geological sample major and trace element values to successfully distinguish samples from Fiji-Lau islands, Tonga-Kermadec Arc and Samoa islands. Sample allocation: A five step process was used to provenance lithic artifacts to a source area. First, a geological dataset of samples (n=193) from Fiji islands-Lau Arc, Tonga-Kermadec Arc and Samoa islands was examined with discriminant function analysis (DFA) to separate island arc igneous rocks from ocean island volcanics (Figure S1). Second, a DFA of geological reference data (n=138) from the Tongan Arc and intra-plate volcanics north and east of the Andesite Line (Samoa, Uvea, Rotuma) was made to classify archaeological samples using the pXRF results (Figure S2 and Figure S3). As expected the DFA (Figure S3) showed clear separation between rocks deriving from island arc and ocean island contexts (see also XRF, SEM-EDXA results in Figure S4). Third, source allocation of 26 archaeological samples identified as from the Tongan Arc with pXRF were examined by DFA of 247 reference samples from central Tonga, northern Tonga and Eua and classifying archaeological samples using 17 trace and rare earth elements obtained with LA-ICPMS. Fourth, samples with a composition indicating an origin in the Fiji islands (or archipelagos west of Fiji), were examined with a DFA (Figure S1) using XRF, SEM-EDXA and LA-ICP-MS archaeological sample data (Figure S5). Fifth, radiogenic results for two samples (203, 204) identified with LA-ICP-MS as probable imports from the Society Islands and Samoa respectively were compared with isotopic results from the potential source areas (Figure S6). pXRF (Archaeology and Natural History, ANU) A Bruker Tracer III-V PXRF equipped with a rhodium tube, peltier-cooled Si-PIN detector at a resolution of approximately 170eV FWHM at the Mn Kα peak (5.9keV at 1000 counts per second) and a 1024 channel configuration multichannel analyzer was used. Instrument parameters were 40 keV, 15 µA, using a 0.1524 mm Cu, 0.0254 mm Ti and 0.3048 mm Al filter in the x-ray path and a 100 second live-time count at 185 FWHM. Interferences from air were minimized by placing the instrument as close as possible to the flat surface of a sample. Net values of the samples were calculated with the Bruker ARTAX Spectra 7.1 package. Nine correction cycles were run for background stripping and peak deconvolution. Net values were calibrated by linear regression against 14 international standards (AGV-1, BCR-1, BCR-2, BHVO-1, BHVO-2, BIR, CRPG-BR, DNC-1, JB-1, NIST1633a, NIST1646, NIST2704, NIST278, NIST27D, RGM-1, WSE) and six in-

house standards (GC-006, GC-11, GC-188, GC-200, KILAUEA 93-1489, TAFAHI) in Microsoft Excel 2010 (see Figure S2 for calibration algorithms). Discriminant function analysis on the data set using log10-transformed values and absolute counts of Ti, Rb, Sr, Y, Zr, Nb was successful in providing initial artifact groups (Figure S3). There is clear separation between arc deriving material from Tonga and intra-plate hotspots such as the Samoan hotspot trail (39). Rotuma and Uvea plot discrete from both arc and hotspot deriving lavas in the 95% confidence interval ellipses, but with a strong overlap in the 50% normal distribution ellipses. The overlap in the 50% mean ellipses between Samoa, Rotuma and Uvea source materials indicates that the unambiguous separation of these source areas is, as expected, difficult with pXRF alone. Samoan samples tend to have higher Ti and Fe, Rotuma samples display lower concentrations of Ti around 10,000 ppm and have higher Rb/Sr ratios than lavas from Uvea (Data S2). Samples identified as unlikely to derive from the Samoan islands or the Tonga-Kermadec Arc were distinguished by Ti values of 1,100 ppm and Nb >110 ppm have not been reported from Samoa, even in the high-Sr volcanic suits on Upolu. Shoshonite (sample 694), Hawaiite (sample 695) and Mugearite (sample 696) with low TiO2 content but higher K2O are known from Fiji (see below), but have not been recorded in Tongan Arc lavas. LA-ICP-MS (Research School of Earth Sciences) LA-ICP-MS analysis of 87 samples was made to investigate artifact sources using trace and rare earth elements (S3) due to the overlap of igneous rock geochemistry in the study area. Only one adze quarry in the Tongan Arc has been geochemically analyzed (46) and we employ published data from geological studies to assist identification of artifact provenance. Ewart et al. (5) recorded important latitudinal geochemical variation along the Tongan Arc. There is a marked changed in Zr/Ba and Nb/Yb ratios, element abundances of HFSE while element ratios of Zr/Sm decreasing in the northern island of Niuafoou suggesting increasing magma source depletion and element ratios of Sc/Y increasing in the north of the Tongan Arc. Trace elements from 247 subaerial geological samples from islands in the Tonga Arc listed in the online GEOROC database (http://georoc.mpchmainz.gwdg.de/georoc/) were analyzed with discriminant function analysis (Rb, Sr, Y, Zr, Nb, Ba, Ce, Nd, Sm) to determine if northern volcanic islands (Niuafoou, Tafahi, Niuatoputapu, Fonualei) could be differentiated from central Tongan volcanics (Hunga Haapai-Hunga Tonga, Tofua, Kao,

Late) and volcanics on Eua in the south. As expected, individual islands in the central Tonga group have an overlapping geochemistry, but there was excellent separation between central Tongan volcanics (n=163, 99%) and northern islands (Fonualei n=27, 93%, Tafahi n=13, 92%, Niuatoputapu n=11, 91%, Niuafoou n=21, 100%) and Eua (n=12, 100%). The discriminant function correctly grouped 97.6% of cases with the first two eigenvalues contributing 93% of between group variance. We tested the efficacy of the discriminant function by entering three reference samples collected on Niuafoou (samples 234, 237, 238) as ungrouped cases and all were correctly placed with Niuafoou geological samples. A further 26 archaeological samples were identified as deriving from the Tonga Arc based on trace element values and ratios were entered as ungrouped cases with the majority (n=22) placed in the central Tonga volcanic group (samples 26, 27, 28, 29, 33, 34, 35, 39, 43, 44, 46, 48, 49, 206, 213, 217, 228, 233, 235, 242, 244, 705) and four with Eua (samples 193, 195, 685, 690). Volcanic pebbles used to decorate graves at Lapaha today are often collected on Niuafoou, but in the past grave pebbles were collected exclusively from volcanics in central Tonga and no adze-flakes nor manuports indicate the importation of lithics from northern Tonga. Archaeological literature on basaltic adze sources in Samoa has focused on the small island of Tutuila (52 square km) where 17 large and small adze quarries have been recorded (34, 35, 47). Trace elements can distinguish lithics from Samoan island sources as incompatible elementenrichment increases with decreasing age along the Samoan hot spot trail (33:27). Using the trace element data published by Collerson and Weisler (34) several adzes-flakes found in Tongan sites likely derive from Tutuila (samples 36, 51, 53, 54, 208, 241, 243, 678, 683). Slightly lower incompatible element concentrations (Rb, Th, Ba) and more depleted REEs (La-Lu) indicate samples 31, 41, 42, 210, 211, 212, 679 and 681 are not from Tutuila and are from the older islands of Savaii or Upolu (33), with older shield volcanic settings rather than post-erosional volcanics from Nb/U and Ba/Sm ratios. XRF sourcing of artifact sample 688 with high Sr-values to the posterosional lavas on Upolu (15), is supported by its trace element values (Rb, Th, Ba). Sample 204 was identified as Samoan, but it had depleted incompatible element values (Rb, Th, Ba) and very low REEs, which suggest one of the western islands of the Samoan chain as the source. The low TiO2 value of sample 204 indicates Uvea as a potential source (14), and the sample was further analyzed with MC-ICP-MS (see below). Sample 697 has a trace element composition similar to reference samples from Rotuma (samples 130, 134, 135, 137, 140, 142), although a source in Fiji is also possible. Due to the geochemical variability in Fijian volcanic rocks we distinguish between samples of probable Fijian origin ('Fiji') and those which are less securely sourced labeled as '?Fiji', as they might originate from the Tongan Arc. An unusual combination of depleted HFSE and enriched

REEs was noted for adze samples 694, 695 and 696, and the element signatures indicate possible mantle-mixing of post-subduction ocean island basalts with arc-deriving magmas (9). High Ce/Yb ratios and enriched LILEs have been found in the Mago volcanic group in the Lau Islands (9, 42). The geochemistry of samples 695 and 696 supports an origin in the Lau Group while sample 694 is also likely to derive from east Fiji, although a source in the large islands of Fiji cannot be excluded. Samples are identified as deriving from 'Fiji' (e.g. samples 10, 21, 198, 245, 694, 695, 696) if they have a composition that rules out an origin in the Tongan Arc (e.g. K2O >2wt%, Rb >30 ppm) and Rotuma-Uvea-Samoa (e.g. TiO2