Statement_of_Work.docx

Statement of Work I. General information A. Introduction The USGS Geology, Energy, and Minerals Science Center is conducting garnet geochronology on a suite of newly collected metamorphic rock samples from Michigan’s Upper Peninsula. Determining Lutetium–hafnium (Lu–Hf) and Samarium–neodymium (Sm–Nd) garnet ages will provide critical insights into the timing of metamorphic and deformational events recorded in these rocks. These geochronologic constraints are essential for enhancing our understanding of the bedrock geology of the Upper Midwestern United States and refining regional tectonic models.

B. Background This new work integrates with several longstanding USGS research projects focused on the bedrock geology of the Upper Midwest which aim to refine mapping in the area and understanding of the tectonic history. These efforts are motivated by the importance of these data for prospectivity models for critical mineral resources. Age constraints on metamorphism and deformation are some of the most important information which these models require. These samples submitted for garnet geochronology will provide the first garnet age-dates in the area, and thus, are fundamentally important to several scientific and agency goals.

C. Scope The Lu-Hf and Sm-Nd isotope analyses will be done by isotope dilution. This process includes acid dissolution of garnet and whole rock aliquots in bomb pressure vessels at high temperature. Solutions will be spiked with enriched isotope tracers of 149Sm-150Nd and 176Lu-180Hf and then purified using cation exchange column chromatography. The isotope compositions of the purified solutions will be analyzed on a multi-collector ICPMS (inductively coupled plasma mass spectrometer). Additionally, two aliquots of garnet from each sample will be subject to treatment using Sulfuric acid (H2SO4) to remove phosphate inclusions which can complicate Sm-Nd isotope measurements of garnet.

II. Work Requirements A. Technical Requirements The contractor shall have experience in Lu-Hf and Sm-Nd isotope analyses of geologic materials, isotope dilution, garnet geochronology, and in multi-collector mass spectrometry. For each sample, two whole rock aliquots and two garnet aliquots must be analyzed for Lu-Hf and Sm-Nd isotopes. Additionally for each sample, two garnet aliquots will be analyzed for Sm-Nd isotopes after being subject to H2SO4 treatment, and two aliquots will be measured for Lu-Hf isotopes. The isotope measurements shall be made using a multi-collector ICPMS with demonstrable consistent prior measurement of standard solutions JNdi (For Nd) and JMC (for Hf).

Deliverables The contractor shall provide the deliverables in a digital format. Raw data will be provided in Excel Spreadsheet files. The corrected and final data for all samples shall be of publication quality and will be provided in a single Excel Spreadsheet. The contractor will also provide the measured values of the standard solutions analyzed with the unknowns during the isotope analyses.