We report [Li] and d 7 Li values for a well-characterized suite of 52 geographically (165-184°W), compositionally (SiO 2 = 46-70 wt.%), and temporally (0-38 Ma) diverse lavas and intrusive samples. The d 7 Li in these rocks range from À0.7‰ to +14.2‰, with 32 of the 35 lavas and 12 of the 17 intrusive samples falling within the depleted mantle range (d 7 Li +1.6 to +5.6‰), as sampled by mid-ocean ridge basalts (MORB). The d 7 Li values of Aleutian lavas do not exhibit the spatial trends observed in other slab component tracers, nor do d 7 Li values correlate with any slab component indicators, such as radiogenic isotopes, oxygen isotopes, or trace element ratios such as Cs/La and Th/La. The d 7 Li values in Aleutian intrusions also do not exhibit temporal trends, however, an overall positive relationship exists between d 7 Li and Th/Nd. Mixing models for d 7 Li and 143 Nd/ 144 Nd values suggest that Aleutian samples within or above the MORB d 7 Li range can be explained by addition of <1-2% sediment-derived aqueous fluid and 3% sediment melt to depleted mantle; both are required to explain the range in d 7 Li that is observed. Sediment-derived fluid exerts a stronger control on Aleutian samples having higher d 7 Li values than the MORB range, while sediment melt skews the Li isotopic compositions of MORB-range samples to slightly lower values than if sediment fluid was the only slab influence. Our study demonstrates that a slab signature may be deciphered via modeling even in arcs where spatial trends in d 7 Li values and correlations with slab component indicators are lacking.
Electron probe micro-analysis (EPMA) is a standard analytical tool for geologists, metallurgists and material scientists. EPMA can provide very high spatial resolution, quantitative chemical analyses of a wide variety of materials. Used predominantly for rock and mineral analyses by geologists, the electron probe is gaining traction as an invaluable tool to measure metal compositions, degradation of electrical components and in some special cases, biological samples. In this work, we present a case where EPMA analysis was used in the forensic analysis of soil.Soil is formally defined as a mixture of rock and sediment that has been modified by physical and chemical interaction with organic material and rainwater over a period of time [1]. Soil formation is strongly related to local geology such that a mass of soil will consist of varying proportions of minerals that are usually derived from the underlying geologic unit, mixed with organic matter. Due to geological variations, the individual components themselves will also vary in composition across several different chemical systems such as major elements, minor elements, trace elements, and isotopes. The combinations of all these variations mean that the mineral components of a soil sample from any given location may have a distinctive composition that can be used to differentiate it from soil in other locations. By establishing the compositional differences between soil from various locations, it then becomes possible to determine if soil in trace evidence, such as on a suspect's shoe, is consistent with the crime scene and not from other alibi locations. In addition to natural variations in soil composition, materials may be brought into an area for the purposes of construction or erosion abatement and will have different mineral compositions from those of the native soil. These natural and induced variations should provide ample differences that can be exploited to identify a location through careful analysis of the soil mineral composition. Key to this study was the ability to perform precise, quantitative chemical analyses on very small sample quantities.The case, NC v. Jordan Peterson, examined forensic soil evidence collected from a pair of shoes and sweat pants worn by the suspect and from two dental stone casts of foot prints from the crime scene (referred to in the Forensic report and here after as samples 6A and 7A; [2,3]). All samples contained white mica. The suspect declined to provide potential locations for exclusion samples. To demonstrate the variability of mica compositions in a larger context, material containing mica representative of local Piedmont rock types was collected for inclusion in the chemical analysis. Three "Index Sample" sites were selected and sampled based on rock type and ease of access.Two different procedures were used to mount samples for EPMA; polished acrylic mounts and double sticky-sided carbon tape adhered to a microscope slide. Polished acrylic mounts were prepared for the index samples due to the abundance of mica. ...
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