Proposal

Executive Summary/Abstract

In order to gain a comprehensive understanding of the Tuolumne Intrusive Suite, it is imperative to delve into its composition. This suite comprises a collection of plutons that intruded the Cretaceous Sierra Nevada Batholith in California. These intrusive formations are prominently visible within Yosemite National Park and exhibit a temporal span ranging from 91 million years ago (Ma) to 82 Ma. It stands to reason that, according to chronological order, the older units are expected to exhibit a lower felsic composition, while the younger units should display a higher degree of felsicity.

A specific focus is directed towards the Cathedral Peak Granodiorite, characterized by the presence of significantly sized potassium feldspar crystals. An analytical assessment of a single potassium feldspar megacryst will commence by obtaining measurements across various minute tracks extending from the central core of the crystal. Single-point measurements will be executed in the intervals between these tracks. The crystal will then be scrutinized and partitioned into distinct zones, namely the core, mantle, and rim, for comprehensive analysis.

To facilitate the analysis of the potassium feldspar, the isotope Si29 will be employed as an internal standard. Calibration and tuning standards will be derived from NIST610 or NIST612, provided by the National Institute of Standards and Technology (NIST). To ensure the integrity of the data, meticulous attention will be dedicated to maintaining consistent calibration throughout the ablation process.

Research Objective

We need to develop a method to determine the growth history of k-feldspar megacrysts in the Cathedral Peak granodiorite. The main hypotheses are continued growth caused by magma mixing and recharge or growth due to late-stage interactions with fluids. To test these we will be using Ba, Sr, and Rb to determine the effects that the magma had on growth and Nb, Ti, Zr, Gd, and Tb to test for fluid interactions. The Ba, Sr, and Rb have a relationship defined by Słaby et al. 2017 that shows how magma has influenced the growth of the crystal. The fluid testing will be done using fluid immobile elements including high field strength elements, Nb, Ti, and Zr, and heavy rare earth elements, Gd and Tb. The relative depletion of these toward the rim can show that fluids stimulated growth because the fluid itself would be depleted in the listed elements. The specific isotopes we will analyze are Nb93, Ti47, Zr90, Gd157, Tb159, Ba137, Sr88, and Rb85.

Discussion of Significance or need (Justification)

The assumption would have to be that the potassium feldspar grew to or during the late-stage fluid-present crystallization may be relatively depleted in fluid immobile elements such as Nb, Ta, Ti, Zr, and HF but also heavy rare earth elements. It is possible that the megacrysts do not show a noticeable variation in these elements, but the inclusions of accessory phase minerals like iron, Apatite, or Titanite, which are more sensitive to variations in rare earth elements in fluids, may show variability.

Review of Relevant Work (Literature Review)

There have been several recent studies into the use of LA-ICP-MS to study feldspar megacrysts as well as studying the potential origin of these large crystals. In studies that have analyzed feldspar through LA-ICP-MS, there are published operating conditions as well as preferred isotope selections. The article published by Słaby et al. has a supplemental operating conditions document that as of now is our planned operating procedure (Słaby et al., 2017). This article is focused on the analysis of feldspar megacrysts from the Santa Angélica composite pluton and will be useful in aiding our analysis of the megacrysts from the Tuolumne Intrusive Complex. Their analysis used multiple other methods outside of LA-ICP-MS and discerned that the crystals in their area of investigation were crystalized and recrystallized through repetitive magma recharge and mixing. The increased concentration of Ba in certain sections shows that they were recrystallized. The second article by Słaby et al. focuses on feldspar megacrysts from a different locality and through LA-ICP-MS and other methods concludes that they were affected by high-temperature fluids that would allow them to grow to their abnormal sizes (Słaby et al., 2011).  The enrichment of trace elements along cracks and the rim shows that there was continued growth caused by fluid interactions. The Astbury et al. article also provides important methodological information on the analysis of feldspar with LA-ICP-MS. This article provides examples of previously mapped K-Feldspars and how these maps were determined (Astbury et al., 2018). The final article from Chew et al. is an overview of LA-ICP-MS imaging that can provide valuable examples of parameters used for mapping various igneous minerals including K-Feldspar (Chew et al. 2020).  

Materials and Methods

We plan to analyze a single k-feldspar megacryst to learn more about the history of k-feldspar growth in the cathedral peak granodiorite. While a single megacryst won’t explain the growth of all megacrysts in the Tuolumne intrusive suite, it will provide important insights into the area that could potentially be expanded upon in future research. The scope of this method project limits us from exploring other megacrysts and we should be able to obtain quality data from a single crystal. The method will be capable of expansion to other megacrysts in the future. To ensure quality results we plan to take measurements over multiple small tracks extending from the core of the crystal as well as some single point measurements between. Since the crystal is very large a full rim-core-rim transect may not be feasible within our allotted time. Multiple small tracks will still supply data about the elemental distribution throughout the crystal that can be used to conclude. The small tracks will be placed within the zones of the crystal defined by mineral inclusions. The particular crystal we have chosen to analyze has distinct zones that can be split into the core, mantle, and rim, this division is supported by the Chambers et al. paper (Chambers et al., 2019). This is done to show any distinct changes in elemental concentrations across boundaries. Potentially there could be more placed within these zones to show any concentration changes within that would be important to analyze if our time budget permits. The single-point measurements will be used to determine if the concentrations are relatively consistent in areas not being measured by the tracks mentioned previously. The publications often use Si29 as an internal standard for k-feldspar analysis, so that would be our choice (Astbury et al., 2018)(Słaby et al.,2016 and 2011). The publications often use NIST610 or NIST612 as the main calibration and tuning standards (Astbury et al., 2018)(Ubide et al., 2015)(Chew et al., 2021)(Oppenheim et al., 2021). These standards provide the necessary consistency and concentrations to properly tune the machine and keep it calibrated. Calibration can be done consistently throughout ablation to maintain the quality of the data. 

Discussion of Possible Outcomes

The possible outcomes of our analysis are that the crystals grew during magma mixing and chamber recharge which introduced additional component elements that further expanded the crystals, or that late-stage fluid interactions caused further expansion of the crystal. If the crystal’s growth was caused by magma recharge the analyses will show spikes in trace elements such as Ba, Sr, and Rb that are rapidly taken up by the feldspar, or a consistent trend across the rim of the crystal (Słaby et al., 2016). If the crystals grow due to fluid interactions the rim of the crystal will be relatively depleted in fluid immobile elements such as Nb, Ti, Zr, Gd, and Tb (Słaby et al., 2011). This is because these elements can not be as easily carried by fluids as they would be in magma. The distribution would remain largely consistent or increase if the crystals grew completely in magma while a notable decrease toward the rim would likely indicate fluid interactions. These elemental proxies should provide an important insight into the growth history of the megacryst and we can conclude the single crystal based on them. To determine the interactions that took place within the full batholith many more samples would need to be analyzed, and this method could be used to do so.

Timeframe and Budget

The timeframe for the project largely depends on the speed at which we operate the LA-ICP-MS machine. A faster scan speed leads to a shorter time spent on the machine total. The scan speed used in reference materials is between 5 and 20 μm/s this also depends on the size of the ablation beam. To maintain data integrity but reduce the time required to complete the experiment it is best to use a scan speed higher than half the beam size with a high repetition rate. Because we plan to analyze 8 elements the scan speed must be slower to allow for quality data to be collected. Astbury et al. analyzed similar elements and used a scan speed of 12 μm/s with a beam width of 30 μm and a repetition rate of 8hz (Astbury et al., 2018). This brings the overall speed to 1cm/11.11min and thus we could run a full rim-core-rim transect in about an hour and 20 minutes. If this is outside our time budget the smaller tracks can be run and bring our total time to much less. This leaves our budget at $146 maximum assuming 2 hours of run time at $73 an hour.