Proposal

Abstract

Zircon is highly resistant to weathering, both chemical and physical, making it a great “storage” device for elements that may be lost over the years in other minerals. Zircon is a relatively common accessory mineral in igneous and metamorphic rocks, and often survives erosion and deposition processes in sedimentary rocks. Zircon is a nesosilicate (ZrSiO4) with a tetragonal crystal system. It has the potential to incorporate radioactive elements into its crystal structure, such as uranium & thorium, further making it a renowned geochronometer. Zircon often exhibits zoning, creating an uneven distribution of these elements within a single crystal.

This project will involve developing a method to to conduct trace element Rare Earth Element analysis of zircons from the Cerro Tenerife Mountains. We will use LA-ICP-MS to analyze a series of igneous ash and detrital zircon samples for trace element and REE concentrations as a proxy for crustal thickening. La/Yb and Eu/Eu* are used to determine crustal thickness (Chiaradia, 2015, Tang et al., 2020). We will also measure the REEs, in addition to U, Th, and Pb, as these elements are important geochronometers. Understanding the distribution of these elements within a zircon grain is very important for laser targeting, as hitting a depleted or enriched zone of these elements will impact data.

Objective

Our objective is to develop a method that will measure concentrations of significant trace elements and REEs to see if zoning plays a significant role in element distribution. This would be done by performing line scans and spot analysis on a suite of igneous ash and detrital zircons from rim to core. The data collected from this method will be used to calculate the ratios of La/Yb and Eu/Eu*, which are commonly used as a proxy for crustal thickening (Tang et al., 2020; Profeta et al., 2015; Kay and Mpodozis, 2002).

Significance

Zircons are able to store trace and rare earth elements in their crystal structure, however a majority of the literature focuses on the U-Pb system. This is due to the importance of age dating rocks in order to understand Earth’s history and how the history can apply to the future. This is typically done using HR-ICP-MS for its higher resolution, however this use of ICP-MS typically takes a longer time to analyze the desired sample, which isn’t preferable for an analysis of multiple elements, such as the lanthanide series. By developing a method to analyze trace and rare earth elements using a quadrupole, elemental analysis could be both more accessible and more efficient to collect trace and rare earth element data.

The Cerro Tenerife mountains consist of the Tobifera, Zapata, Punta Barrose, and the Cerro Torro formations. These formations are from the upper Jurassic to upper Cretaceous, containing various volcanic rocks, rhyolitic flows, shales, sandstones and conglomerates sitting on top of a metamorphic basement (Fildani & Hessler, 2005). Igneous samples from the Zapata and detrital samples from the Punta Barrosa will be analyzed to determine if there is a thickening upward sequence.

The relative contributions of tectonic shortening versus magmatic additions to crustal thickening remain difficult to quantify, making it challenging to fully understand the processes behind crustal evolution in continental arcs. Crustal thickness estimates for the Andes can be derived using the geochemistry of subduction-related volcanic rocks or the minerals crystallized within them. The ratio of light to heavy rare earth elements (LREEs and HREEs, respectively, e.g., La/Yb) has been shown to correlate with crustal thickness on a global scale (Chiaradia, 2015). LREEs are more abundant in the residues of thinner arcs (Kay and Mpodozis, 2002; Profeta et al., 2015). Higher La/Yb ratios in whole rock samples are associated with magmas that form at greater pressures and depths, which correlate to thicker crust (Profeta et al., 2015). In intermediate to felsic rocks, the europium (Eu) anomaly in zircons has been shown to correlate with the whole-rock La/Yb ratio (Tang et al., 2020; Profeta et al., 2015; Kay and Mpodozis, 2002).

Literature Review

Carappa et al. (2022) analyzes detrital syntectonic deposits of the Santa Maria Conglomerate, to reconstruct paleo-crustal thickness. Zircon standards, including Fish Canyon (28.5 Ma) and R33 (419.3 Ma), were used to establish age calibration, while Sri Lanka zircon served as the trace element standard (Gehrels et al., 2009; Balica et al., 2020). The europium (Eu) anomaly (Eu/Eu* = EuN / √[SmN * GdN]) in zircons was used as a proxy for crustal thickness, following the methodology of Tang et al. 2020). Detrital zircon trace-element data from volcanic ash layers within the conglomerate, combined with existing paleo-crustal thickness data, suggest that crustal thickness was approximately 35 km around 38 million years ago (Ma) and about 44 km around 12 Ma.

Balica at al. (2020) describe a method used to analyze trace elements in detrital zircons that span from 4.4 Ga to the present day. While not necessarily done using a quadrupole, the method had produced data that outlined crustal thickening during the Archean. Balica et al (2020). also outline their analytical method, using a dwell time of 0.001 – 0.300 sec, and varied settle times based on the analyte (0.001 – 0.034 sec). 

Trace element analysis in zircons has been widely accepted as a medium for the analysis of continental crust thickening, as REEs are resistant to weathering and diagenesis (Taylor et al., 1981). Trace elements are also concentrated differently depending on when the zircon crystallized. Most notably, zircons that crystallized during the Archean have no Eu anomaly present, along with lower REE concentrations and varied abundance patterns (Taylor et al., 1981). 

One question that is left to be answered is how trace element analysis can be applied to zoned zircons, as zoned zircons reflect a continuous history of crystallization, potentially depicting major events that could potentially affect a zone’s elemental composition (Hanchar et al., 1993). Our method hopes to propose a way to conduct a trace element analysis of zircon zonation in order to describe the crustal history that occurred during crystallization.

Materials/Methods

The zircons and standards were pre-mounted in epoxy prior to this study, so sample prep was minimal. Each mount was scanned for BSE images to determine which spots will be analyzed. 4 mounts in total will be analyzed. 1 mount containing 3 samples of zircons in ash containing approximately 30-50 grains. The other 3 mounts are detrital zircon samples containing approximately 1000 grains. 15 grains from each mount will be scanned for trace and rare Earth elements. Line scans of 30 µm will be used on 15 grains from each sample. 4 grains were additionally scanned using lines of 30 µm spots. Sri Lanka Zircon is often used as a trace element standard for LA-ICP-MS. It has a well-characterized trace element composition, making it suitable for calibrating and validating the accuracy of trace element measurements. MAG-1 is another zircon standard that is also used for trace element analysis, especially in studies involving magma crystallization processes. NIST SRM 610/612 Glass Standards are commonly used for trace element calibration in LA-ICP-MS analysis, including in studies of zircon trace element abundances (Yuan et al., 2004). Si (Silicon) will be used as an internal standard in zircon analysis because of it’s abundant and relatively unaffected by fractionation during ablation.

The trace elements that we aim to measure include the lanthanide series of REEs and trace elements U-238, Th-232, Pb-208, Lu-175, Yb-172, Tm-169, Er-166, Ho-165, Dy-163, Tb-159, Gd-157, Eu-153, Sm-147, Nd-146, Pr-141, Ce-140, La-139, Y-89, Ti-49, Ti-50, and Sc-45. The La/Yb* ratios and Eu/Eu* anomaly derived from whole-rock geochemical estimates to produce paleocrustal thickness after Profeta et al., 2015 and Tang et al., 2020.

Outcomes

Higher energy or longer ablation times may be required to achieve measurable signal intensities for the HREEs (e.g., Lu, Yb, Dy). This is because the ionization efficiency of HREEs in the ICP tends to be lower compared to LREEs. We expect to see higher amounts of La/Yb ratios and HREEs as you go go up the section. We expect to observe the Eu/Eu* anomaly as you go higher in the section. We expect to find comparable trace element and REE measurements that will support existing trace element data. We expect REE distribution to vary across zone boundaries, though to what degree is uncertain.

Timeframe/Budget

The budget for this project is dependent on the cost of sample preparation and the amount of time spent in the ICP-Q-MS lab, running the laser analysis. Sample preparation is estimated to be 10 hours for mineral separation with a rate of $18/hour. Mounting and polishing is estimated to be 1.5 hours for each mount, with epoxy and polishing materials costing $50. Laser ablation analysis is estimated to take 8 hours including initial tuning of the ICP-MS, picking spots for ablation, optimization of the laser systems, and actual measurements of the whole series of zircons grains. The service rate for LA-ICP-MS is 65$/hour. The total budget for this project is $750.