WHOLE ROCK GEOCHEMISTRY

A manual comparison of major and trace elements and variation diagrams reveals distinct eruptive units that correlate with lithofacies boundaries that were established by textural, stratigraphic, and geomorphological evidence. The narrow suite of basaltic samples neatly falls within the range of tholeiitic lava (Figure 3.2). The alkali content of the samples is low and displays no definitive trend. Total alkalis versus silica therefore does not aide in the initial discrimination of lava groups. Instead, incompatible elements reveal strongly linear trends of enrichment and reveal discrete chemical clusters (Figure 3.3).

The linear trend is particularly strong for CaO, TiO2, Al2O3, and Sr (Figure 3.4).

Figure 3.2 AFM diagram show the distribution of Austurfjöll samples. All samples plot neatly as tholeiites.

Figure 3.3 The enrichment of the basaltic melt is highlighted by an incompatible element diagram and total alkali diagram. Symbols represent individual eruptive units of Austurfjöll. Gabbro nodules, Holocene spatter,

and lava clasts from diamictons are included for comparison. Symbols will be used consistently throughout this document unless otherwise noted.

Figure 3.4 Variation diagrams of major and trace elements against MgO. Plots along the left represent components of plagioclase and components on the right are incompatible elements. All oxides are in wt%.

The Austurfjöll sample suite was divided into seven eruptive units associated with the construction of Austurfjöll massif, two additional sedimentary units, one post-Austurfjöll eruptive unit, and one unit of gabbro nodules. Tables of the complete results appear in Appendix B (Table 1 and B-3). The units identified include gabbro nodules, Unit 1 (A, B), Unit 2, Unit 3, Unit 4, Unit 5, Unit 6, Unit 7, Dm1, Dm2 and Holocene spatter / lava. Units 1-7 are stratigraphically and chronologically defined units containing samples of similar geochemistry. Unit 1 i s divided into A and B as they are stratigraphically equivalent, but have unique geochemical signatures. Units Dm1 and Dm2 are define from samples of lava contained within diamicton units, the geochemistry of which helps establish the relationship between the eruptive units and the diamictons. Variation diagrams comparing elements relevant to the phenocryst species identified in petrographic investigations (plagioclase and clinopyroxene) provided the most useful visual distribution of the chemical units. Of particular interest is CaO/TiO2, CaO, Al2O3 (Figure 3.5) and trace element diagrams comparing V/Y or two incompatible elements Zr/Ti (Figure 3.6). All four diagrams display a strong linearity in the distribution of the data, with well defined consistent clusters of data points representing each eruptive unit.

Microprobe analysis of ten samples produced highly similar results to the XRF whole rock geochemistry, and provided additional information about the volatile species present in the samples to support FTIR results (See Chapter 8). The samples analyzed by microprobe confidently fell within the Unit 2 and Unit 5 eruptive units both stratigraphically and chemically, supporting the divisions defined above (See Table A-4).

Normalized trace element trends of samples using primitive mantle (Lyubetskaya and Korenaga, 2007) reveal that the samples are enriched in incompatible elements relative to Mid-Ocean Ridge Basalt (MORB) lavas (Figure 3.7). This trend in enrichment spans from Unit 3 as the most enriched and the gabbro as the least. The trends are fairly similar for most of the eruptive units, with some of trends

accentuated between the least enriched group, Unit 6, and most enriched Unit 3. The signature of the trace elements is dominated by the behavior of Sr, which correlates with enrichment of CaO in major element plots.

Rare earth element analyses were conducted on only 15 samples at McGill University. The REE trends produced through the normalization with chondrites (Anders and Grevesse, 1989) reveal the same relative enrichment of eruptive units that was observed in the major and trace elements. One sample of rhyolite dome that pre-dates the basaltic subaqueous sequence shows similar trends in REE, but has notably higher values and a st rong negative Eu anomaly (Figure 3.8). All samples are enriched in incompatible elements relative to compatibles of the REE series. Unit 5 is notably less enriched than Units 1, 2, and 3. Analyses of Units 1, 4, 6, and 7 were not conducted.

Figure 3.5 Variation diagram of CaO vs. TiO₂ and CaO vs. Al₂O₃. These comparisons highlight the linear trend of the Austurfjöll dataset. CaO/TiO₂ likely reflects the relative abundance of feldspars and oxides,

where high CaO reflects high plagioclase content and high TiO₂ reflects high oxide content.

Figure 3.6 Trace element variation diagrams contrasting incompatible and compatible elements. The linear trend of the data set is more pronounced than in the major elements. V/Y likely shows the influence of Cpx

and oxide minerals and the overall enrichment of the samples.

Figure 3.7 Multi-element diagrams highlighting the relative enrichment of trace elements in the Austurfjöll samples. Samples are normalized to the primitive mantle (Lyubetskaya and Korenaga, 2007).

Figure 3.8 REE plot of 15 samples including a sample of the rhyolite dome with a distinctive Eu anomaly, indicating the role of plagioclase in the evolution of the melt. Units 2 and 3 show very limited variability

between samples. Unit 5 shows the lowest REE concentrations of all of the Austurfjöll lavas analyzed.

Unit 1 is the lowest exposed unit of the sequence and precedes the glaciovolcanic eruptive activity. The unit is divided into two sub-units that differentiate two chemical suites of subaerial lavas that are stratigraphically equivalent. Overlying the lavas are diamictite (facies Dia1) deposits that contain clasts of basaltic lava with a chemical composition unique to the Austurfjöll suit that are assigned to unit Dm1. The lavas display subaerial textures and glacial scour (facies L2). The subaqueous eruptive sequence includes units 2 through7 and Dm2. Within units that contain both porphyritic and microcrystalline lithofacies, there is no significant trend in the results of porphyritic samples relative to those of micro-porphyritic samples. However, the linear trend of eruptive units 2, 3, 5 and 6 of increasing concentration of incompatible elements and plagioclase cations correlates to the abundance of porphyritic samples within the unit, rather than chronology. Of the subaqueous eruptive sequences, Unit 3 is the most evolved (Figure. 3.3, 3.6) and displays no porphyritic samples. Unit 6 is the least evolved unit and is the only exclusively porphyritic unit. Units 4 and 7 are both defined by small outcrops and a single geochemical sample. As such it is hard to evaluate the consistency of this trend through these units, which plot reasonably well along this trend. The Holocene samples are subaerial and occur on the margins of the massif, locally overlying glaciovolcanic units and plot toward the less evolved end of the spectrum of Austurfjöll compositions.