Proposal
Executive summary/Abstract
The objective of this project is to understand the partitioning behavior of phosphorus during olivine serpentinization. Serpentinization occurs when ultramafic rocks brought to the earth’s surface interact with natural water and hydrothermal fluids in the form of metasomatism (Lamadrid et al., 2017). During this process, phosphorus is released into the ambient environment as an essential nutrient for planetary habitability (Pasek et al., 2022). By measuring the concentrations of phosphorus and other trace elements in unaltered olivine, partially serpentinized olivine, and fully serpentinized olivine grains on the same mantle xenolith thin section, the project aims to gain insights into the distribution and availability of different phosphorus species on the earth’s surface. The project utilized an Elemental Scientific NWR 193 uc excimer laser coupled with an Agilent 7500ce ICP-MS to measure trace element concentrations of the three aforementioned alteration states of olivine. Each state on the thin section was petrographically identified under the microscope and labeled as laser spots for analysis. 24Mg was chosen to be the internal standard for this research and a stoichiometric Mg abundance was assumed for the measured olivine grains due to its resistance to serpentinization (Schwarzenbach et al., 2016). The elemental concentrations in different olivines were compared to evaluate their distribution during serpentinization.
Research objectives
The chemical composition and concentration of mantle xenoliths reflect the elemental makeup of pristine peridotite before they were fragmented and brought to the surface through magma transport. This information sheds light on the subsolidus elemental exchange between peridotite and its carrier melt, which incorporates different extents of partial melts from overlying layers including but not limited to the lithospheric mantle and crusts. Assessing the interplay between peridotite and the lithospheric mantle is imperative to understanding the evolution and mass exchange of the earth’s interior (Eggins et al., 1997). Subsequent to eruption, the serpentine formed by the hydrothermal fluid and natural water alteration on ultramafic minerals reveals the evolution of the earth’s surface. The release of phosphorus to natural water during this process is critical for the formation of habitable planets, as it is considered a limiting factor in many scenarios for life emergence (Walton et al., 2021). The proposed method tackles these puzzles by analyzing the trace element concentrations of olivine and serpentine crystal grains on a mantle xenolith thin section. The elements of interest include P, Na, Al, Ca, Ti, K, Cr, Mn, Sr, Ni, Sc, V, Co, Zn, Zr, Hf, La, Ce, Nd, and Sm.
Significance
LA-ICP-MS is known for its high sensitivity and wide analytical working range, therefore, it is an optimal strategy for measuring both the major and trace elements in Olivine. Establishing a feasible LA-ICP-MS method is crucial for high-quality data acquisition. Such a method should minimize isobaric, polyatomic, and doubly charged interference on analyte elements to ensure accurate elemental concentrations. Careful control of laser conditions including laser spot size, laser spot selection, laser energy, and laser wavelength is essential. For example, smaller laser spots can minimize the matrix effect caused by major elements, but they are more prone to elemental fractionation arising from elements having different ablation rates. As a result, an intermediate laser spot should be selected to account for both effects.
Olivine was chosen over pyroxenes as the main study of interest. Although clinopyroxene and orthopyroxene also readily form serpentine, phosphorus preferentially partitions into olivine (Mallmann and O’neill, 2009). Phosphorus is a necessary component for the compartmentalization, metabolism, and replication of organisms (Walton et al., 2021). Investigating the distribution of phosphorus on olivine grains helps derive the amount of phosphorus released into natural water and thus makes it available for prebiotic reactions. These olivine grains also make up an essential part of the earth’s surface phosphorus reservoir that can potentially be reduced and released. Besides the major elements including Mg, Fe, and Si, which were measured by EPMA, the aforementioned elements were chosen to be the analyte because elements like Na, Al, Ca, Ti, K, Cr, Mn, Sr, and Ni are common trace elements found in olivine. Rare earth elements like La, Ce, Nd, and Sm are sensitive to metasomatism and fluid-rock interactions. Transition metals like Sc, V, and Co are susceptible to changing redox conditions. Finally, high-field-strength elements like Zr and Hf are resilient to fluid alteration, making them an ideal baseline to track element redistribution. In order to produce useful data, the results have to be highly precise and accurate, which means they possess high reproducibility and are close to the true values of the target of interest. For trace elements, the accepted data accuracy and precision should both be lower than 10% (Limbeck et al., 2015 and Paul et al., 2023).
Review of relevant work
Numerous researches have been done to assess the trace element concentrations of mantle xenoliths by utilizing LA-ICP-MS, mostly as an attempt to investigate the interactions between upper mantle rocks and carrier melts, to quantify the elemental distribution in the mantle, or to study the partitioning habit of trace elements within coexisting mineral phases (Eggins et al., 1998, Mason et al., 2007, Norman et al., 1998, Su et al., 2012). Unsurprisingly, they all decided to measure ultramafic minerals including olivine, clinopyroxene, orthopyroxene, spinel, and garnet, which are representative mantle crystals. For olivine, the most commonly used calibration standards are NIST 610 and NIST 612 due to their broad range of well-characterized trace element concentrations that match typical peridotite compositions (Liu et al., 2013). 29Si (and sometimes 25Mg) is the most popular internal standard of choice for olivine due to its abundance and consistency across a broad range of ultramafic minerals (Bussweiler et al., 2019, Stead et al., 2017). A laser wavelength of 193 nm and 266 nm, a repetition rate of 4 to 30 Hz, a laser spot size of 30 to 160 μm, and a laser energy of 0.5 to 2 mj/pulse were the adopted laser conditions (Bussweiler et al., 2019, Eggins et al., 1998, Liu et al., 2013, Mason et al., 2007, Norman et al., 1998, Stead et al., 2017, Su et al., 2012). A frequently encountered analytical challenge of this technique is elemental fractionation due to the ununiform ablation rates of different elements (Liu et al., 2013). Although this effect can be addressed by increasing the laser spot size to ≥100 μm, another issue coming with it is the mass-load-induced matrix effect in which the increased aerosol production affects the accurate measurements of low-concentration elements (Liu et al., 2013). An alternative is to use a matrix-match calibration that permits the use of a small spot size while minimizing downhole fractionation effects (Bussweiler et al., 2019). Common polyatomic interference include CrAr (92Zr), TiO (64Zn), Cr (40Ar12C+), Nb (40Ar16O+), and Cu (40Ar23Na+) and were mostly mitigated through mass filtering and by using a collision/reaction cell (Bussweiler et al., 2019, Eggins et al., 1998, Liu et al., 2013, Mason et al., 2007, Norman et al., 1998, Stead et al., 2017, Su et al., 2012). Data precision and accuracy were kept below 10 % for all studies.
Materials and methods
Objectives
This method aims to understand the partitioning behavior of phosphorus and other trace elements during olivine serpentinization by measuring the trace element concentrations of unaltered and serpentinized olivine grains on thin sections of the mantle xenolith. Besides phosphorus, Na, Al, Ca, Ti, K, Cr, Mn, Sr, Ni, Sc, V, Co, Zn, Zr, Hf, La, Ce, Nd, and Sm are also measured because elements like Na, Al, Ca, Ti, K, Cr, Mn, Sr, and Ni are common trace elements found in olivine. Rare earth elements like La, Ce, Nd, and Sm are sensitive to metasomatism and fluid-rock interactions. Transition metals like Sc, V, and Co are susceptible to changing redox conditions. Finally, high-field-strength elements like Zr and Hf are resilient to fluid alteration, making them an ideal baseline to track element redistribution.
Sample Preparation
Two 30 μm thin sections of mantle xenoliths (detailed origins need further confirmation) that have undergone serpentinization were prepared and provided by Dr. Douglas Smith, an emeritus professor at the Jackson School of Geoscience at the University of Texas at Austin. The thin sections were observed for olivine crystal grains under a binocular microscope. The unaltered and serpentinized olivine grains were labeled on a computer as laser spot locations. A total of 48 laser spots (24 for unaltered olivine and 24 for serpentine) were measured for trace element concentrations including P, Na, Al, Ca, Ti, K, Cr, Mn, Sr, Ni, Sc, V, Co, Zn, Zr, Hf, La, Ce, Nd, and Sm. The major elements of the thin sections including Mg, Fe, and Si were measured previously by EPMA.
Instrument Calibrations
Numerous researches have been done to assess the trace element concentrations of mantle xenoliths by utilizing LA-ICP-MS, and olivine is one of the most frequently measured mineral phases due to its high abundance in peridotites. In this study, NIST 610 and/or NIST 612 were used as calibration standards due to their homogeneity and their broad range of well-characterized trace element concentrations that match typical peridotite compositions (Liu et al., 2013). 24Mg was chosen to be the internal standard for this research and a stoichiometric Mg abundance was assumed for the measured olivine grains due to its resistance to serpentinization (Schwarzenbach et al., 2016). A laser wavelength of 193 nm, a repetition rate of 5 Hz, a laser spot size of 50 μm, and a laser energy of 1 mj/pulse were the adopted laser conditions based on a review of relevant literature (Bussweiler et al., 2019, Eggins et al., 1998, Liu et al., 2013, Mason et al., 2007, Norman et al., 1998, Stead et al., 2017, Su et al., 2012). Due to the low thickness of the thin sections, line scan and raster ablations were adopted on olivine grains larger than 300 μm to enhance signals. Instead of the conventional argon gas, helium was used as the carrier gas, as it was found to reduce fractionation effects in the silicate matrices and produce a 2- to 4-fold signal enhancement when combined with a 193 nm laser (Eggins et al., 1998 and Horn et al., 2003).
Accuracy and Precision Assessments
Accuracy and precision were enhanced by minimizing matrix effects, elemental fractionations, and interferences of polyatomic, isobaric, and doubly-charged species. A frequently encountered analytical challenge of LA-ICP-MS is elemental fractionation due to the ununiform ablation rates of different elements (Liu et al., 2013). Although this effect can be addressed by increasing the laser spot size to ≥100 μm, another issue coming with it is the mass-load-induced matrix effect in which the increased aerosol production affects the accurate measurements of low-concentration elements (Liu et al., 2013). An alternative is to use a matrix-match calibration (if it’s available) that permits the use of a small spot size while minimizing downhole fractionation effects (Bussweiler et al., 2019). Some common polyatomic interferences include 27Al40Ar (overlaps with 67Zn), 48Ti16O (overlaps with 64Zn), and 52Cr40Ar (overlaps with 92Zr). Common isobaric interferences include K40 and Ar40 with Ca40, 48Ca with 48Ti, and 50V with 50Cr. Common doubly charged interferences include 80Sr2+ with 40Ca, 96Zr2+ with 48Ti, and 180Hf2+ with 90Zr. These interferences can be minimized by alternative isotope selection, optimization of plasma and torch conditions to minimize oxides and doubly charged species, interference equations, cooling the plasma, and using the collision/reaction cell. With all these factors accounted for, the data should have an accuracy and a precision of around 5-10%.
Precision can be calculated as follows:
Percent accuracies can be assessed by evaluating the recoveries obtained from quality control standard replicates. More specifically, a known concentration of an element called a “spike”, is added to a quality control standard with the same element, which its concentration is also known. The increase in counts for the quality control standard after spike addition is converted to concentration to see how well it matches the known concentration of the spike. In practice the calculation should be as below:
Discussion of possible outcomes
The final method is expected to provide trace element concentrations within olivine grains on a mantle xenolith thin section that has undergone serpentinization. The measured laser spots should cover pristine olivine grains and the unaltered and altered regions on a serpentine grain. The analyzed elements should include P, Na, Al, Ca, Ti, K, Cr, Mn, Sr, Ni, Sc, V, Co, Zn, Zr, Hf, La, Ce, Nd, and Sm with reasonable accuracy and precision. With this information, the final method will provide insights into the redox conditions during the crystallization and serpentinization of olivine, the element exchange, redistribution, and release before, during, and after being transported to the surface. With the measured P concentrations in unaltered and serpentinized olivine, an estimation of the amount of release P can be done by performing a mass balance equation. Not only does this information quantify the amount of P as an available nutrient, but it also determines the amount of P trapped in surface olivine and serpentine. Since the thin section samples are relatively thin (30 μm), careful calibration on laser conditions is required to minimize the burning of the underlying glass slides. Line scan and raster ablations might be used to maximize sample counts. The tradeoffs of these methods are the requirement for longer analytical time and larger grain size (hundreds of μm), resulting in fewer measurements.
Timeframe and budget
Although thin sections were provided and the olivine major elements were measured, I still needed to observe them under a binocular microscope to discover olivine and serpentine grains and label them as laser spots. This will take around one to two days. I also need to do extensive tests and refinements on the proposed method since it’s heavily based on related studies, and their methods might not directly apply to mine. This should take around two days. Instrument calibration and setup of internal and calibration standards should take around two hours. For sample analysis, if we assume an analytical time of 50 seconds per laser spot and we have 100 spots and account for the time taken by sample washouts, standard measurements, and instrument settling, the analysis should take around four hours. Combined with the calibration and setup time, the entire process should take one full day. Finally, the subsequent data interpretation and visualization should take another three days. The total time required for this project should take around seven to eight days.
Considering an LA-ICP-MS session costs 65$/hr and a total analytical time of around 6 hours, the total cost should be 65 x 6 = 390$.