Provenance tracing using mother-of-pearl and the impact of button processing on the material’s trace elemental and isotopic concentrations

Garment buttons and other artefacts incorporating mother-of-pearl (nacre) are significant items of material culture frequently found on archaeological sites and in museum collections. From around 1860 until the end of the Second World War, Australia was the foremost international source of mother-of-pearl for buttons, which were mechanically mass produced during that time. Since nacre formation is impacted by climate, water chemistry and other environmental factors at the time of shell formation, mother-of-pearl is potentially well suited to geochemical investigations of provenance. And yet, this avenue of inquiry, and the impact of button manufacturing on the compositional nature of mother-of- pearl, have received little attention to date. This study comprised stable carbon (δ13C) and oxygen (δ18O) isotope analysis of six mother-of-pearl samples from at least four different species sourced from the major pearling areas of Australia. It found oxygen isotope to be an effective marker in discriminating between most sample source locations, although some regions were not distinguishable. Additional and much higher density sampling and analysis of modern pearl shell might resolve this issue. The study also determined that mechanical processing during button manufacture has no significant impact on the trace element and isotope compositions of the mother-of-pearl. In doing so, it demonstrates the potential for source tracing mother-of-pearl material culture items to shed new light on the history of the pearling industry in Australia, one of the country’s most historically important maritime industries.

Detrital zircon geochronology and provenance of the Middle to Late Jurassic Paradox Basin and Central Colorado trough: Paleogeographic implications for southwestern Laurentia

Middle to Upper Jurassic strata in the Paradox Basin and Central Colorado trough (CCT; south­western United States) record a pronounced change in sediment dispersal from dominantly aeolian deposition with an Appalachian source (Entrada Sandstone) to dominantly fluvial deposi­tion with a source in the Mogollon and/or Sevier orogenic highlands (Salt Wash Member of the Morrison Formation). An enigmatic abundance of Cambrian (ca. 527–519 Ma) grains at this prove­nance transition in the CCT at Escalante Canyon, Colorado, was recently suggested to reflect a local sediment source from the Ancestral Front Range, despite previous interpretations that local base­ment uplifts were largely buried by Middle to Late Jurassic time. This study aims to delineate spatial and tem­poral patterns in provenance of these Jurassic sandstones containing Cambrian grains within the Paradox Basin and CCT using sandstone petrog­raphy, detrital zircon U-Pb geochronology, and detrital zircon trace elemental and rare-earth ele­mental (REE) geochemistry. We report 7887 new U-Pb detrital zircon analyses from 31 sandstone samples collected within seven transects in west­ern Colorado and eastern Utah. Three clusters of zircon ages are consistently present (1.53–1.3 Ga, 1.3–0.9 Ga, and 500–300 Ma) that are interpreted to reflect sources associated with the Appalachian orogen in southeastern Laurentia (mid-continent, Grenville, Appalachian, and peri-Gondwanan terranes). Ca. 540–500 Ma zircon grains are anom­alously abundant locally in the uppermost Entrada Sandstone and Wanakah Formation but are either lacking or present in small fractions in the overlying Salt Wash and Tidwell Members of the Morrison Formation. A comparison of zircon REE geochem­istry between Cambrian detrital zircon and igneous zircon from potential sources shows that these 540–500 Ma detrital zircon are primarily magmatic. Although variability in both detrital and igneous REE concentrations precludes definitive identifica­tion of provenance, several considerations suggest that distal sources from the Cambrian granitic and rhyolitic provinces of the Southern Oklahoma aulacogen is also likely, in addition to a proximal source identified in the McClure Mountain syenite of the Wet Mountains, Colorado. The abundance of Cambrian grains in samples from the central CCT, particularly in the Entrada Sandstone and Wana­kah Formation, suggests northwesterly sediment transport within the CCT, with sediment sourced from Ancestral Rocky Mountains uplifts of the southern Wet Mountains and/or Amarillo-Wichita Mountains in southwestern Oklahoma. The lack of Cambrian grains within the Paradox Basin sug­gests that the Uncompahgre uplift (southwestern Colorado) acted as a barrier to sediment transport from the CCT.