Project Proposal

Three extrusive volcanic rock samples from different places along the slope of the inner crater of Mt. Vesuvius all contain large euhedral amphiboles despite having different chemical compositions. There is no age data for these samples, but sample Types A and B were obtained from the crater with Type A right on the rim and Type B from 100 meters below. Type C was taken from the lava field produced in the last major eruption in 1944. The presence of amphiboles in all three of these samples could indicate that all deposits came from the same magma source. An analysis of the trace element concentration within these amphiboles via LA-ICP-MS could prove this hypothesis. The focus of the analysis is on light rare earth elements (LREEs) as these have been shown to preferentially partition into the crystal structure rather than the melt at certain temperatures. Twelve amphiboles, four from each sample type will be separated from the rock, mounted on slides, and subjected to laser ablation ICP-MS. This will provide high accuracy data about the amounts of these trace elements building a distinct signature for these amphiboles. The data could be used for future projects to determine the temperature, depth, and tectonic setting of the magma chamber at amphibole crystallization.

Research Objective

Hand samples of three separate pyroclastic deposits were collected from three locations on the slope of the inner crater of Mt. Vesuvius, located near Naples, Italy. The three types are classified based on the color indicative of their chemical composition. Type A is light in color indicating a high level of felsic material, a matrix dominated by silica and aluminum oxide. Type B is similar to Type A however this category is more enriched in iron giving it a deep red color. Both Type A and Type B are estimated at 20-30% vesicles, and are classified as pyroclastic deposits of rhyolitic composition. Based on their position in the strata building up the inner cone of Mt. Vesuvius, it is likely these two types were produced in different explosive eruptions, however without age data this cannot be confirmed. Type C is more compact (<10% vesicles) and gray in color suggesting higher mafic content with more Mg and Ca present. Type C differs from the other two types in how it was produced as well with it being the product of an extrusive lava flow rather than tephra from an explosion. It is possible that Type A, the sample closest to the crater could have been deposited at the same time as Type C due to the combination of an explosion and lava flow in the 1944 eruption. Despite these differences, all three types contain amphibole phenocrysts large enough to be seen with the naked eye. Some measure at >5 mm, but most fall at around 2-4 mm in size. The amphiboles are also very euhedral in shape. This combination of size and shape suggests a deep magma source in which partial crystallization of amphiboles occurred at a slow enough rate to grow larger crystals and these must have had sufficient space to form well-defined shapes. Assessing the trace element compositions of the amphiboles acts as a fingerprint for this specific magma body where they formed. Assuming all these crystals in all three sample types grew in the same chamber, their trace signature should be the same. The signature can be discovered by analyzing the following elements via LA-ICP-MS and comparing their compositions across the three sample types: Sc, Y, La, Nd, Sm, Eu, Gd, Rb, Ba. With this data, the question of whether Vesuvius amphiboles originate from the same chamber could be answered, and a hypothesis about the temperature and pressure conditions corresponding to the magma chamber depth.

Justification

As stated above, in order to determine where the amphiboles in the samples formed, their trace element signature must be analyzed. Laser ablation ICP-MS is the most efficient method for acquiring in-situ high accuracy data given its ability to assess small concentrations of elements in the system. The analytes chosen (Sc, Y, La, Nd, Sm, Eu, Gd, Rb, Ba) were selected due to being incompatible meaning these elements will preferentially enrich the mineral phase (in this case amphibole) compared to the melt during the process of partial crystallization. The solid phase partition coefficient for amphibole is not as high as that of pyroxene, garnet, or olivine which is why it is often not chosen if there are better phenocrysts present. Due to this, the LREEs in addition to Rb and Ba are chosen as elements that are most likely to provide higher counts that will be well above the limit of detection for the LA-ICP-MS. However, here amphibole is the only phenocryst, and despite the challenge, others have begun using this mineral phase for analysis. Previous studies on amphibole show that light rare earth elements (LREEs) in addition to Rb and Ba favor partition into amphibole rather than the melt (Torres Garcia et al 2020, Keller et al 2024). This should result in concentrations small enough to be properly resolved, but large enough to be detected and relevant for research. The incompatibility of certain elements are affected by temperature. Further research could involve using the data collected here to estimate the temperature of the magma chamber at time of amphibole crystallization which connects with depth and tectonic setting. However, it should be noted that there can be variation in the amount of incompatible elements present depending on the Ca versus Na content in the mineral (Marks et al 2004). There are also numerous other factors that can cause variety in trace element concentration such as oxygen fugacity of the magma chamber (Ye et al 2021). For simplicity’s sake, this analysis will only seek to determine the similarities and potential differences between the three populations of amphiboles. Discrepancies between the populations from the three types could result from a number of factors, and this will prompt further analysis and study. 

Literature Review

The most relevant study for the trace analysis of amphiboles in relation to silica enriched volcanic deposits similar to those at Mt. Vesuvius is the work by Keller et al. (2024). The group analyzed trace elements in amphiboles found in dacites from the Mt. St. Helens complex using LA-ICP-MS. A wide range of elements were analyzed including La, Nd, Eu, Sc, Ba, Rb, Zr, etc. The group found that the amphiboles found in different rock suites exhibited differing trace element patterns. In their method, mineral separates were handpicked from the crushed tephra samples, placed in epoxy, imaged with SEM, and subjected to electron microprobe analysis. For the laser ablation setting, Keller et al. (2024) ablated for 40 s with a pulse rate of 10 Hz and a spot diameter of 30 microns. The standards used were NIST-612 and a USGS GSD-1D synthetic glass. The same or similar standards will be used for this project. The results from the standards were used for drift correction and data reduction. The group placed the analytical error at about 5% for elements above the detection limit due to the reliability of the glass standards. Hampel et al. (2011) also did trace element characterization on amphiboles which included analytes such as Li, Co, Cu, Zn, Sr, Y, Mo, Ag, Sn, Sb, Te, Ce, W, and Pb. The majority of these elements will not be analyzed because many are not as incompatible and/or more difficult to analyze due to the possible isobaric or polyatomic interferences. Torres García et al. (2020) conducted trace element analysis via LA-ICP-MS on amphiboles in hornblendites found in the Cretaceous magmatic arcs of the Fuegian Andes. Here, the analytes were alkalis, LREEs, and incompatible elements with strong crustal affinity (Th, Rb, Ba). Based on the concentrations, the group concluded that trace element patterns deviate from typical magmas formed at subduction settings. Laser excitation was carried out at 50% power on a 50 micron spot diameter with a pulse frequency of 10 Hz. Another study (Ye et al 2021) used similar standards to Keller et al (2024) including the NIST-612 glass along with another standard, the NIST-610, to assess compositional variations in amphiboles from granite in East Junggar, China.  

Sc has only one isotope at 45 amu and there are no argon related polyatomic interferences. The only possible problem could come from 89Y2+, a doubly charged ion. Y is also monoisotopic with the only polyatomic interference an argon hydroxide molecule. Since this is laser ablation, there should be no excess hydrogen or oxygen from water or solutions introduced into the system. The abundance of 139La is so high (99.9%) that La is essentially monoisotopic, and there are no relevant interferences. The most abundant isotope of Nd is 142 amu, but there could be an issue due to the abundance of 142Ce (11%). 144Nd is the second most abundant and has two interferences, one 144Sm with a tiny abundance (3.1%) and the other a Ru oxide molecule. Similar issues occur with Sm, Eu, and Gd. Fisher et al. (2011) measured 149Sm and used published Sm isotopic abundances to correct for the 144Sm interference with 144Nd via interference equations, but 142Nd would still be a better choice if 142Ce could be corrected for since 142Nd is more abundant and has no polyatomic interferences. 151Eu could be the better choice since there are less polyatomics involved, and the isotope combinations in these oxides are harder to create. 160Gd seems to be the best choice for this element as the polyatomic interferences involve rare earth isotopes and oxygen isotopes that are overall low in natural abundance, meaning they are not as likely to form and maintain stability. 85Rb is the most abundant isotope of this species, but it has several polyatomic interferences most of which are irrelevant given the use of laser ablation instead of solution mode ICP-MS. The only possible problems come from Zn oxides, but the oxygen in these molecules would have to be 17O or 18O which are extremely low in natural abundance. 138Ba is most abundant and has interferences that are rendered redundant through miniscule abundance. Overall, the majority of the elements proposed for analysis in this project should be relatively simple to measure with little to no significant interference. All information about interference and isotopic abundances was taken from data provided by Wee and Keating (2004).

Materials and Methods

The samples have already been acquired, so there is no need for collection. The amphiboles will need to be extracted from the rock and mounted on slides as thin sections for laser ablation analysis. The amphiboles will be mounted using epoxy and/or crystal bond and polished until they are 100-200 µm so that there is enough material for the laser to penetrate. Four amphiboles will be taken from each sample type totaling 12 total amphiboles tested. Depending on the sizes of the minerals chosen, four to six amphiboles will be placed on a given slide and those best for analysis will chosen once the sample drawer is loaded. For this project, the Agilent 7500 ICP-MS hardware with attached laser ablation chamber will be used. The slides will be set up on the laser ablation stage in addition to standards which will act as quality control of the data. Since this is a silicate mineral, silicate standards would be most useful here. The standards will include the NIST 612 glass and a USGS basaltic glass.The most beneficial type of laser analysis to do is a line scan that begins at the center of each amphibole and moves to the edge. The line would progress briefly along the edge before terminating. If these amphiboles show any sign of internal zoning in the analyte compositions, a line scan will expose that in the data. While it is unlikely for these particular analytes to be distributed in this manner, it does not hurt to investigate the possibility. After the data is acquired, it will be filtered, reduced, and tested in order to insure its accuracy.

Possible Outcomes

Ideally, this method will result in an accurate and precise dataset of the concentrations of the analytes (Sc, Y, La, Nd, Sm, Eu, Gd, Rb, Ba) which can be used to determine 1) whether or not the amphiboles formed in the same magma chamber, 2) the approximate temperature this magma chamber was at when these amphiboles formed, and 3) the tectonic setting where this magma chamber was created. The latter two are beyond the scope of this current project, but the data from this method could yield all of these outcomes. This method could also be used on amphiboles from other volcanic systems to determine where they come from as it is not uncommon for amphiboles to be found in pyroclastic and pumice deposits from volcanic eruptions.

Timeframe and Budget

A basic outline of the schedule is provided below:

• • Sample Preparation (includes separating amphiboles from the rock and mounting on slides) → completed by November 7
  • Running LA-ICP-MS Analysis (includes setting up line scan paths on samples and standards in addition to getting machine ready for the run) → completed by November 22, estimated time on machine is approximately 4 hours total
  • Data Processing → completed by December 10

• Project will take approximately two months to complete

The rate for the Jackson School of Geosciences Quadrupole LA-ICP-MS lab is $65/hr. Since this project is expected to take at least 4 hours of runtime, the proposed budget is $260. Samples, standards, and sample prep equipment are expected to be provided in house.