Proposal

Trace Elements in the Monterey Formation: Evaluating Critical Mineral Enrichment and Oil & Gas Indicators with ICP-MS

Research problem:

The Monterey Formation contains a complex shale (organic mudstone) matrix with high silica and organic content. While such material has historically been difficult to fully digest, this study employs HF microwave-assisted acid digestion, which effectively dissolves both siliceous and organic components. The main remaining challenges now involve achieving accurate measurement of trace metals due to ICP-MS interferences (argon, oxides, and isobars) and matrix effects, which must be carefully addressed to ensure high-quality data relevant to both critical mineral resource assessment and oil and gas evaluation.

Objective:

The purpose of this study is to develop a solution-based ICP-MS workflow that provides accurate, precise trace metal data following complete HF microwave digestion of this complex shale. This method will apply helium collision–reaction cell mode, selection of cleaner isotopes, and optimized plasma and equipment tuning. These results will help assess whether the Monterey Formation contains critical minerals and hydrocarbons that justify additional investigation and/or extraction.

Importance:

The development of a reliable solution-based ICP-MS workflow for the Monterey Formation is significant because it addresses both critical mineral potential and hydrocarbon evaluation in a geochemically complex shale unit. Accurate determination of trace and redox-sensitive elements is essential for reconstructing depositional conditions, assessing source rock quality, and identifying potential zones of economic enrichment. Because many of these analytes occur at trace to ultra-trace concentrations (ppb–ppm), method development must achieve high accuracy and precision for the data to be useful for evaluating economically significant enrichments. Previous studies have analyzed and discussed oil enrichments in the Monterey, but analytical complications such as matrix suppression and spectral interferences have limited the accuracy, precision, and reproducibility of trace metal analyses. A robust solution ICP-MS method is therefore necessary to overcome these challenges to provide dependable data to support any findings of the previously mentioned resources.

Relevant Work:

Research on organic-rich, siliceous sediments demonstrates multiple factors that can lead to inaccurate trace metal analysis. Incomplete digestion, matrix suppression, volatility of redox-sensitive elements, and the limited availability of certified reference materials (CRMs). These all create challenges especially in complex shale systems. Together, these challenges make careful method development and validation essential.

Monterey shales are both organic-rich and silica-rich, which makes them notoriously difficult to fully digest. High organic content can lead to element loss or isotope fractionation if oxidation is incomplete, while the abundant silicates require aggressive dissolution conditions—both of which complicate accurate trace metal analysis (Dong et al., 2025). Multi-acid approaches are often necessary, yet incomplete digestion remains a risk, especially for redox-sensitive elements such as U, Mo, Re, and V (Kolotov et al., 2025). To address these challenges, this study utilizes HF microwave digestion, which is competent enough to completely break down both organic matter and silicate minerals. This allows for the majority of the focus of this study to reduce interferences and matrix effects rather than dissolution limitations.

Trace metals including V, Ni, Mo, U, Re, Co, and Cr are particularly sensitive to matrix effects because of their low concentrations in shale. Matrix suppression can bias results even when internal standards are used. This requires careful matrix matching or targeted chemical separation for achieving accurate results. (Yang et al., 2025; Ibad and Padmanabhan, 2022). Redox-sensitive elements such as U, Mo, Re, and V may be lost or fractionated if the sample is not fully oxidized during digestion. For example, Dong et al. (2025) displayed thallium fractionation during ashing, illustrating how volatile trace metals can behave unpredictably under oxidative conditions. Previous work on coal and shale ash highlights the potential for trace metal “mobility” during heating and oxidation (Novković et al., 2024; Chajduk et al., 2013), emphasizing the need for controlled, complete oxidation during HF microwave digestion to preserve accurate redox-sensitive element concentrations.

Phosphate-rich sediments offer useful context for understanding trace element behavior. Graul et al. (2023) reported that Tremadocian shelly phosphorites from shallow marine settings show multi-stage REE + Y enrichment during early diagenesis. The Heavy REE enrichment and Ce and Eu anomalies indicate that phosphate phases can effectively retain trace metals during early diagenetic processes. For the Monterey Formation, these phosphate associated patterns imply that organic matter, silica, and phosphate together may influence trace metal distribution and redox-sensitive element behavior. Although the Monterey Formation is dominated by organic-rich shale, phosphate grains are present in some intervals, providing localized sites where trace metals could be sequestered or modified.

Another challenge is the lack of suitable CRMs. No single reference material contains all elements of interest, requiring multiple overlapping CRMs to validate methods (Tyutyunnik et al., 2023). High precision is critical for detecting economically meaningful trace metal enrichments. LA-ICP-MS studies show coefficients of variation ≤5% (Yang et al., 2025), but solution-based ICP-MS often struggles to achieve this in shale or ash matrices. Achieving relative standard deviations better than ±5% remains a key consideration given the low (ppb–ppm) concentrations and complex matrix.

Materials and Methods:

We will analyze 16 pulverized shale samples from the Monterey Formation core (from Platform Holly offshore Santa Barbara) using solution mode ICP-MS. Samples will be digested using HF microwave-assisted acid digestion, ensuring full breakdown of both organic matter and silicate minerals. Blanks, duplicates, and a reference shale will be included for quality control. Helium collision mode will be used with carefully chosen isotopes to reduce interference, and results will be reported in ppm to compare enrichment and redox trends in shale.

Analytes: V, Ni, P, Mo, U, Re, Co, Cr, Fe, Mn, Ba, Al, Ti & REE

Potential Results:

By developing a reliable solution-based ICP-MS workflow for the Monterey Formation, this study has the potential to produce data that is original and useful to the scientific community. By measuring trace metals accurately, we can learn about the conditions when the shale was deposited and how the metals are linked to any organic matter and silica found in the rock. The method developed here provides data relevant to both petroleum and critical mineral evaluation. Trace metal distribution can indicate hydrocarbon generation viability, while concentrations of elements such as V, Ni, Mo, and U can contribute to critical resource evaluation. Establishing a robust analytical method therefore creates a foundation for future resource evaluation in Monterey and provides a method template for other complex shale systems.

Budget and Timeline:

The budget for this project is fairly straightforward, with each ICP-MS analysis costing about $20 per sample. However, it costs $60 per unknown to microwave digest. Since we plan to analyze 16 shale samples from the Monterey Formation, the total budget will fall around $1700 accounting for blanks and standards. All other costs, such as reference standards (USGS shale CRMs), calibration solutions, acids, digestion vessels, and consumables, will be covered by the lab. The project timeline is organized around course deadlines: a rough draft of the proposal website is due September 25, followed by the final proposal on October 23. By November 6, all sample preparation must be complete, which means having our 16 samples digested and diluted, along with blanks, spikes, duplicates, and reference materials, ready for ICP-MS analysis. Data collection will take place in mid-to-late November, with time allocated for troubleshooting and reruns as needed. In early December, results will be processed into concentrations, enrichment factors, and figures such as depth profiles and REE patterns. The project will conclude with a published final website and a short presentation, both of which are due on December 11.