Proposal
- Executive summary/Abstract
In our study of sedimentary provenance and crustal thickness variations, we try to develop a method for U-Pb dating and multi-trace element determination on ablated zircons using Laser Ablation Quadrupole ICP-MS. The target element concentration includes U-Pb and Rare Earth Elements series, Hf and Sr-Y. The developing methods are expected to test the possibility to double date the zircons after initial ablation from High Resolution ICP-MS for its accuracy and precision. We expect to utilize rapid multielement analyses from Quadrupole LA-ICP-MS to reproduce concentrations and counts measurements with the widely accepted zircons standards for quality controls. The expected outcome includes the consistent U-Pb age measurement from the initial dating, as well as accurate measurement of trace elements and REEs with distinguishable elemental signals from matrix interferences and low background noise. We plan to test for the optimal analytical conditions of the sample introduction systems and ICP operating conditions. The limitation of the method and possible interferences will be discussed after analyses.
- Research objective or hypothesis and specific objectives of the method
The laser ablation system will be utilized based on the conditions from a previous study by Yuan et al. (2004) and Yuan et al. (2008). The analytical sequences of zircons analyses are based on the external standard calibration method and template used in Arizona LaserChron (Pullen et al., 2018). The expected internal standard calibration is Si elemental signals to correct for the systematic shift of elemental and isotopic signals. The primary external standard, FC-1, of known U-Pb age will be used for U-Pb fractionation measurement. We also plan to use Dynamic Reaction Cell with ICP-MS to increase resolution and signals of both U-Pb and REEs accurate. The operating conditions of the laser ablation and mass spectrometer will be based on the set up used in Yuan et al. (2004)
Ultimately, we are interested in examining the sedimentary provenance through the use of detrital zircons from sedimentary rocks, so we can constrain the preferential sedimentary source of the Middle Magdalena Valley, Colombia. In other words, which of the two cordilleras is feeding the valley? Is it the Eastern Cordillera or the Central Cordillera, or a mix of both? Is one cordillera depositing more sediments than the other, or in equal amounts? Adding on, these sedimentary provenances can also allude to temporal variabilities such as when were these two cordilleras depositing sediments into the valley. We will be examining the sedimentary provenance of the Middle Magdalena Valley using the U-Th-Pb isotopic system in detrital zircons, in which both U-238 and Th-232 decay to Pb-206 and Pb-208, respectively (Gehrels, 2010).
The first trace elements system we are interested in is the geochemical signature Sr/Y to examine the crustal thickness variations below the Eastern and Central Cordillera and compare their differences (Chapman et al., 2015). Perhaps one of the cordilleras has a thicker crustal root or is more exhumed than the other due to the subduction of the Caribbean plate. For future work, these crustal thickness variations can be tied to thermochronology, either U-Th/He or fission tracks where we can examine the exhumation rates of these magmatic arcs as one arc might preferentially exhume at a faster rate due its interaction, or lack thereof, with the subducting Caribbean plate.
Similarly, the Rare Earth Elements (REEs) series are unique trace elements to the geological setting with unique initial conditions for crystallization and partition in heavy minerals such as zircon and apatite. REEs have the ability to determine continental crust magma generation phases. Thus, we should be able to see an enrichment in incompatible elements in our samples due to their theorized origin (magmatic arc). A key REE in the lanthanide series that can verify the generation of our samples in upper continental crust is the Eu2+ anomaly. Eu2+ preferentially fractionates in plagioclase above 10-12 kbar, and becomes unstable below this pressure range, thus constraining the melt to upper continental crust in origin if the anomaly is present (Taylor and McClennan, 1981). Adding on, another trace element that provide critical information to the origin of the magmatic arcs in the Central and Eastern Coridlleras of Colombia is Hf. Epsilon Hf (εHf) can be used to determine the source of melt that is supplying the magmatism in the magmatic arc (Chapman & Ducea, 2019).
- Discussion of significant or need
There are high concentrations of trace and rare earth elements in (detrital) zircon grains. However, the mainstream current research efforts focus on a specific set of elemental and isotopic analysis e.g. U-Th-Pb isotopic system. Usually, these isotopic systems are analyzed with a magnetic sector or high resolution ICP-MS or Multi-Collector ICP-MS for its high resolution of isotopic components. However, the major limitation of both ICP-MS instruments are the short range of isotopic analysis and relatively longer period of analysis per grain, which is unpreferable for multi-elemental analysis such as a series of REEs. This gives way to the possibility of using Quadrupole ICP-MS for its rapid and acceptable precision and accuracy. ICP-Q-MS is proven to be a viable method for trace elements measurement with acceptable accuracy and precision with optimal setup (e.g. Yuan et al., 2004; Yuan et al., 2008, Pullen et al., 2018). Additionally, the REEs do not require as high precision as the individual isotopic analysis. Previous work have done the laser ablation mode of the standard geological sample and proved that the ICP-MS are suitable for providing the REEs and trace element data that are useful for the geological interpretation.
- Review of relevant work
Yuan et al. (2004) and Yuan et al. (2008) have evaluated the method for determination of U-Pb and trace elements series in geologically important zircons standards using Quadrupole-ICP-MS and Multicollector-ICP-MS. These authors proved that ICP-Q-MS is suitable for rapid determination and acceptable accuracy (2STDV error). Yuan et al. (2004) also demonstrate that two stage determination of the same spot laser ablation on zircons grains can be accomplished with accuracy and precision using 30 microns to 60 microns spot laser.
Trace element and REEs analyses has been recommended to couple with the U-Pb age dating method to test for the accuracy and credibility of the U-Pb age result (Belousova et al.,2002, Hidaka et al., 2002, Rubatto, 2002). However, the development of the new method of U-Pb zircons dating has improved so much that there is no requirement for trace and REEs measurements to test for the significance of the age result. Nonetheless, we plan to measure the REEs to interpret for its significance in geological settings for its fingerprint signals of the known geological setting based on the coupled U-Pb measurements to see if REEs results are accurate.
However, the question remains if there is maximum limitation required for acceptable accuracy of trace element and REEs measurements and U-Pb measurement in one peak hopping cycle (Yuan et al., 2004). In our study, we recognize the need to optimize the plasma condition for its robustness as well as the laser ablation conditions such as pit sizes and ablation durations for the most consistent and highest amount of zircons aerosols.
- Materials and methods
In this study, we plan to follow the method presented by Yuan et al. (2004) using the quadrupole ICP-MS from Perkin Elmer/SCIEX. The sample preparation has already been achieved from the Arizona LaserChron center (ALC) with the resin mount of a series of zircons standards and initial unknowns. The objective is to date and measure elements from the same spots on grains of zircons from initial measurement from ALC. We plan to ablate the same spot of the zircon grains with bigger laser diameter spot size (>15 microns diameter), but with laser pulse conditions as used by Yuan et al. (2004).
The primary and secondary standard used is FC-1, R33, and Sri Lanka F, which are highly standardized by U-Pb age. The sequence of analysis is standard bracketing starting from the mixture of primary and secondary standard zircons, followed by the initial unknown grains of the same age population determined from the initial analysis. The measurement series are bracketed by a couple primary or secondary standard to evaluate the consistency of the ICP-MS measurement from fractionation of the U and Pb isotopes (Pullen et al., 2018)
The instrumental parameters are adjusted based on the parameters used in Yuan et al. (2004). The additional usage of CRC is not required.
The trace elements that we aim to measure include the lanthanide series of REEs and trace elements U, Pb, Sr, Sc, Y, Hf. These elements will be used to determine the Sr/Y ratio, U-Th-Pb isotopic system and determine REE concentrations. The interference equations will be calculated for all trace elements and possible interferences from isobaric, oxides ions, and polyatomic ions such as Hg-204 and Pb-204.
- Discussion of possible outcome
Instead of using only one method and instruments to measure multiple trace element and isotopic determination such as HR-ICP-MS or Q-ICP-MS, the benefit for this study is the possibility to double date zircons for U-Pb Age and trace elements using both type of Mass spectrometry. This method and instrument set up for secondary measurement of trace elements and REEs can be an alternative for rapid and relatively inexpensive determination of trace and REEs series. The accuracy will be evaluated based on the age consistency of U-Pb measured from two instruments. It is important to note that we do not have any REEs measurement from the initial analysis from HR-ICP-MS. We expect to find comparable U-Pb ages from two measurements that will validate our new trace elements measurements.
- Timeframe and budget
The budget needed to develop our method depend on the amount of time spent in ICP-Q-MS lab, which is estimated to take at least three 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. Considering the rate of service at the ICP-Q-MS lab of $73/hour for LA-ICP-MS, it expects to cost at least $219. The approximate number of grains to analyze is twenty-one grains including fifteen grains of three known ages population of zircons and six grains of primary and secondary standards.