1 2 Lessons Rising from the Ash: A Geochemical Study of the Ash Layers from Akaroa Volcano, New Zealand 3 4 5 6 7 Sophia Tsang 1,2 1 Department of Geological Sciences, Brown University, Providence, Rhode Island, 02912, USA 2 Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 ABSTRACT Located close to Christchurch, New Zealand, Banks Peninsula offers rare insight into intraplate volcanism. Recent studies have undertaken characterizing the structure of the Lyttelton and Akaroa volcanoes while others have examined the geochemistry of the lava flows. This study analyses the ash layers found between the previously studied lava flows. Fifteen samples were collected during a week of field work in February 2013 from a variety of locations on the northeastern flank of Akaroa volcano. They were then analyzed by the University of Canterbury s Philips PW 2400 Sequential Wavelength Dispersive X-ray Fluorescence Spectrometer for major and trace elemental data. The data exhibits an increasing, linear trend in silica content with areas having similar compositions although not the same. Ash layers frequently are geographically widespread, but the difference in compositions of the layers suggests that each is from a different eruption. The overall low silica content in these ash layers also implies less explosive eruptions that would not be as conducive to blanket cover of the area during an eruption. The elemental content contrasts with the surrounding lava flows though; the trend actually fits the compositions seen from Mount Herbert volcano.
24 25 26 27 28 29 30 31 32 33 34 35 36 37 INTRODUCTION It has been several decades since the last blank area on the globe has been filled, but that does not mean that each spot on Earth has been extensively studied. Banks Peninsula, New Zealand is one place that remains primarily mysterious. Although the Peninsula is easily accessible and a day trip from Christchurch, the geology is still relatively unknown. (See figure 1.) Recent years have seen an upswing in interest though, leading to several theses and studies. The Peninsula is dominated by two volcanoes, Lyttelton and Akaroa, and has formations from smaller volcanoes such as Mount Herbert. In 2009, Hampton revealed that Lyttelton volcano consists of fifteen separate eruptive centers. Similar work has been undertaken by Hobbs (2012) on Akaroa volcano, showing that Akaroa has at least ten eruptive centers. Hartung (2011) and Johnson (2012) have added to our knowledge of Banks by investigating the geochemistry of the lava flows found. Lava flows are only one product of the volcanoes though and other aspects, such as the accompanying ash layers, should also be studied for a fuller picture of the volcanoes histories. 38 39 40 41 42 43 44 45 46 GEOLOGICAL BACKGROUND Over eleven million years ago, the Lyttelton composite volcano began erupting as an intraplate volcano. The volcanics are still undeformed although there are now Tertiary sandstones overlying some areas, making them a perfect study area (Hampton, 2009). Around nine million years ago, the volcanic source began moving southeast, and the Mount Herbert lavas began erupting. At about the same time, the Akaroa composite volcano in the southeast of present-day Banks Peninsula began erupting (Sewell, 1988). The stacking stratigraphy suggests that between eleven million years ago to seven million years ago, Banks Peninsula was almost
47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 continuously erupting (Hampton, 2009; Sewell, 1988). Since then, alluvial gravels have attached what was Banks Island to the South Island of New Zealand, creating Banks Peninsula. The rims of both Lyttelton and Akaroa volcanoes have also been breached so that they are now harbors (Hampton, 2009; Johnson, 2012). It has been shown that both Lyttelton and Akaroa volcanoes have over ten vents each (Hampton, 2009; Hobbs, 2012). Johnson (2012) examined the geochemistry of the lava flows from one of the eruptive centers on Akaroa volcano and found that the lava flows exhibit cyclical fractional crystallization. Cyclical fractional crystallization is when the fractional crystallization compositions are seen to repeat multiple times from the same volcano. These trends have previously been observed in Saudi Arabia (Camp et al, 1992). Both Camp (1992) and Johnson (2012) have suggested a dual magma chamber system. (See figure 2.) A single, large magma chamber sits beneath Akaroa volcano and feeds multiple smaller, shallower magma chambers. Each of the smaller chambers has been injected multiple times, which is why the fractional crystallization trends in the stratigraphy are cyclic (Johnson, 2012). These magma chambers would feed both the effusive lava flows seen on the surface of Banks Peninsula and the ash layers seen between several of the lava flows. It is also possible that that the ash layers seen could have originated from another, more explosive vent also found on Banks (Hobbs, 2012). These relationships could be partially determined by examining the geochemistry of the ash layers. The formation of ash is dependent on several factors including the silica content of the magma, the vertical and horizontal velocities of the magma, temperature, and dissolved gas content (Verhoogen, 1951; Sparks, 1978). Therefore, the geochemistry of the surrounding lava flows and the ash can provide clues as to the relationship between the two and if they are from
70 71 72 73 74 the same vent and magma injection. If they are related, then there is a strong possibility that they are from the same volcanic vent. Additionally, learning about the ash layers can put constraints on the eruption conditions. The eruption conditions are also related to the size of the ash deposit. By correlating layers across Akaroa volcano, the stratigraphies from different vents can be correlated, which will provide further insight into the history of the Peninsula. 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 METHODS In February 2013, over one hundred samples were collected from the remnants of Akaroa volcano in the process of mapping the area around Okains Bay in greater depth. Of those samples, fifteen of them were taken from red, baked layers between lava flows. Due to the welding, grain size, location in the stratigraphy, and widespread nature of these samples, they were assumed to be ash layers. Soil horizons would also be between lava flows although they would likely contain evidence of life in the area since they would take several years to form to the thicknesses seen (Schaetzl & Anderson, 2005; Buol et al., 1973). The samples collected were from different layers in the stratigraphy with samples also collected from the lava flows on either side of the ash bed layers. Whenever possible, the samples were always over 10 cm in diameter without any weathered edges. This was to ensure that at least 12 grams of unweathered sample could be generated after washing, sawing, crushing, and powdering the samples. The University of Canterbury then used a Philips PW 2400 Sequential Wavelength Dispersive X-ray Fluorescence Spectrometer to collect major and trace elemental data. Beads were created from the field samples, flux, and oxidant by being pressed at temperatures in excess of 1300 K for more than 15 minutes. The beads were then run in the
93 94 95 spectrometer in a rhodium tube set. The major elements were analysed at 50kV/55mA while trace elements were at 60kV/46mA. See Johnson (2012) and Hartung (2011) for more details about the process used at the University of Canterbury. 96 97 DATA 98 99 100 101 102 103 104 105 106 107 108 109 Looking at the compositions of the ash layers, several trends quickly appear. Each ash layer has a different silica oxide composition. With the exception of EO 24, the silica oxide content of the ash layers exhibits a very linear trend with an R 2 value of.82. Just as the silica increases, the iron (III) oxide decreases. The correlation is almost as strong with an R 2 value of.77. When the two elements are compared against each other, a very strong trend emerges without any outliers. The same cannot be said for a comparison of iron (III) oxide and magnesium oxide which have almost no correlation (R 2 value of.13). This could be due in part to the lack of a change in the magnesium oxide data across the different samples. Aluminum oxide did not change significantly across samples either. Together, this data creates the last diagram and reveals that all of the samples except for EO 24 are in the basalt range of compositions. See the attached table in the appendix for more information about specific percentages of cations in the samples. 110 111 112 113 114 115 RESULTS The strong linear trend in the silica content of the samples implies fractional crystallization occurred in the magma chamber that fed the vents that created the sampled ash layers. Since these ash layers could be from different vents, this implies that the lower magma chamber proposed by Johnson (2012) connects all of them. The iron (III) oxide trends support
116 117 118 119 120 121 122 123 124 125 126 127 128 that idea since it decreases as the silica increases. This makes sense since iron (III) partitions out of the melt faster due to its size. Surprisingly, the magnesium oxide does not exhibit the same trend. Commonly, iron (III) will replace magnesium in the mineral structure as the fractional crystallization takes place. This could imply that the melt was just magnesium poor to begin with. The basaltic composition of the ash is not particularly surprising since the surrounding lava flows also tend to be relatively low in silica (Johnson, 2012). This also has implications on the ash fall area. More silica rich lava compositions tend to erupt more explosively due to the polymerization of the chains. A more explosive eruption is likely to have material reach higher altitudes. Since material is falling from a greater height, it has more time to be picked up and pushed far away by wind. None of the samples seem to have similar enough compositions to be from the same ash fall. Therefore, the low polymerization suggests that these eruptions were not particularly explosive. 129 130 131 132 133 134 135 136 137 138 DISCUSSION The ash samples collected on Banks Peninsula suggest very small, effusive eruptions. This supports that the ash layers found are in very limited geographical regions. Volumes this small are practically unheard of though (Carey & Sparks, 1986). This could be partially due to the very primitive magma that was being erupted. Due to the relatively large grain size, the density of the ash, and the low explosivity implied by the composition, it is possible that these ash layers are extremely localized. Most of the ash layers have compositions that are even more primitive than the surrounding lava flows that Johnson (2012) studied. The compositions of the ash layers in each area do seem to be related to each other, though. Although the ash layers are
139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 interspersed between the lava flows, their compositions do not relate to the lava flows. Instead, they relate to each other, and their compositions evolve together over time. This suggests that they are not originating from the same upper magma chamber as the lava flows seen. The ash layers in each area do seem to be from the same magma chamber though. Therefore, the ash layers are originating from different vents than the lava flows they are found with. The bulk composition of each area s ash layers also implies an overall evolution of the magma chamber they are being fed from. It is possibly they are all from one vent, which is how the fractionation continues across areas. The evolution also allows for the timing of the ash layers relative to each other to be determined. This has implications on the timing of the vents identified by Hobbs (2012) and the resulting lava flows. Until recently, the geological map of Banks Peninsula has grouped all of Akaroa volcano as one formation. More recent work done by Frontiers Abroad students including Johnson (2012) and Hobbs (2012) have shown how complex Akaroa volcano truly is. Akaroa volcano is now known to have ten separate vents (Hobbs, 2012). The ash layers seen across the volcano do not seem to match with any of the compositions erupting from these vents. They do plot well together though. This means that they can be used to establish the relative eruption times of the lava flow events. For example, the ash samples taken in this study reveal that Laverick s Bay s lava flows are older than those in found in East Okains Bay. This would also have implications as to why the ash layers seem localized. To determine the volume of the ash fall, multiple samples of the same layer would need to be found and their thicknesses measured. Since the vents did not erupt simultaneously, and thus build the topography evenly, finding the same ash layer would require taking cores in the newer areas like East Okains Bay. The ash layers are more easily eroded than the lava flows, so they would not still be on the top of the stratigraphy in older areas. In this study, not all of the ash layers in every area were sampled.
162 163 164 165 166 167 Additionally, the geochemistry of the ash layers fits in better with the geochemistry seen from other volcanoes on Banks Peninsula that were known to be erupting at the same time (Sewell, 1988). Therefore, it would seem that the ash layers should be very widespread. These ash layers likely originated from a vent on Mount Herbert. This also provides a chance to correlate eruptions between Akaroa volcano and Mount Herbert although that project requires more data than this study has access to. 168 169 170 171 172 173 174 175 176 177 178 CONCLUSION Previous studies on Banks Peninsula have focused on the lava flows on the different volcanoes. By examining the ash layers, the entire Peninsula can be connected since the ash layers found on Akaroa volcano actually have bulk compositions closer to the lava flows found on Mount Herbert volcano which is known to have been erupting at the same time. The ash layers are then interspersed in the stratigraphy and not all exposed. Therefore, the timing of the eruption of the different Akaroa vents can be determined. The compositions also reveal a striking fractional crystallization trend in the magma chamber from which the ash layers originated. Overall, this study ties the history of Banks Peninsula volcanoes much more closely together and shows how more field data would greatly enhance our understanding of the area. 179 180 181 182 183 184 FUTURE WORK There is significant left to be done to determine the number of ash falls from Akaroa volcano. First, more field work should be conducted to collect more samples. These should be taken from a variety of sites across the northeastern quadrant of the peninsula. Care should be taken to take samples of the ash layers from the same vents. The geochemistry data from these
185 186 187 188 189 samples should match at least some of the samples already taken, which will provide a fuller history of Akaroa volcano. The volume of ash fall from Akaroa volcano still needs to be determined. Similar ash compositions need to be found first, though. Second, samples of lava flows from Mount Herbert volcano should be collected. The geochemistry of the lava flows and the ash layers seen on Akaroa volcano should be compared more in depth. 190 191 192 193 194 195 196 ACKNOWLEDGEMENTS I would like to thank the 2013 Frontiers Abroad- Geology students for all of the work they did in the field. Also, thank you to Adrienne Emmerich for her eiditing of the draft of this paper. Additionally, Drs. Darren Gravely and Sam Hampton have been providing much needed guidance on this project. Finally, Drs. Catherine Brown and Rob Spiers have been indispensable in the lab and on the spectrometer. 197 198 199 200 201 202 203 204 205 REFERENCES Buol, S. W., F. D. Hole, & R. J. McCracken, (1973). Soil genesis and classification. Ames, Iowa: The Iowa State University Press. Camp, V. E., M. J. Roobol, & P. R. Hooper. (1992). The Arabian continental alkali basalt province: Part III. Evolution of Harrat Kishb, Kingdom of Saudi Arabia. Geological Society of America Bulletin, (104)4, 379-396. Doi: 10.1130/0016-7606. Carey, S. & R. S. J. Sparks, (1986). Quantitative models of the fallout and dispersal of tephra from volcanic eruption columns. Bulletin of Volcanology, (1986)48, 109-125.
206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 Hampton, S. J. & J. W. Cole, (2009). Lyttelton volcano, Banks Peninsula, New Zealand: Primary volcanic landforms and eruptive centre identification. Journal of Geomorphology, 104, 284-298. doi: 10.1016/j.geomorph.2008.09.005 Hartung, E. (2011). Early magmatism and the formation of a Daly Gap at Akaroa shield volcano, New Zealand. University of Canterbury. Hobbs, D. (2012). Remote sensing geomorphology of Akaroa volcano and detailed mapping of Okains Bay, Banks Peninsula, New Zealand. University of Canterbury. Johnson, J. (2012). Insights into the magmatic evolution of Akaroa volcano from the geochemistry of volcanic deposits in Okains Bay, New Zealand. University of Canterbury. Schaetzl, R. J. & S. Anderson, (2005). Soils: Genesis and geomorphology, Cambridge, UK: Cambridge University Press. Sparks, R. S. J. (1978). The dynamics of bubble formation and growth in magmas: A review and analysis. Journal of Volcanology and Geothermal Research, 3(1978), 1-37. Sewell, R. J. (1988). Late Miocene volcanic stratigraphy of central Banks Peninsula, Canterbury, New Zealand. New Zealand Journal of Geology and Geophysics, 31(1), 41-64. Verhoogen, J. (1951). Mechanics of Ash Formation. American Journal of Science, 249, 729-739. 223 224 225 226 227 228
229 FIGURES 230
231
60.00 EO24 232 Percent Silica Oxide (SiO2) 55.00 50.00 45.00 40.00 35.00 LB2L1 LB2L3 LBS32 LBS33 LB2L4 LAVIIP LB2L5 EO25 LB2L2 LAV3PS LAVH2P SBAI EO50GA EO35A EO28A 21.00 19.00 LB2L5 LAVIIP LB2L4 LB2L1 LB2L3 Percent Iron (III) Oxide (Fe2O3) 17.00 15.00 13.00 11.00 LBS32 LBS33 LB2L2 SBAI EO25 LAV3PS EO50GA LAVH2P EO28A 9.00 EO35A 233 7.00 EO24
234 235
30.00 Percentage of Oxide 25.00 20.00 15.00 10.00 LB2L5 LAVIIP LB2L4 LBS32 LBS33 LB2L1 LB2L3 LAV3PS EO25 LB2L2 LAVH2P SBAI EO28A EO50GA EO35A EO24 Al2O3 MgO 236 5.00 0.00 EO50GA LB2L4 LBS33 LB2L3 LB2L5 LAVIIP LB2L1 EO25 LB2L2 LAV3PS LAVH2P EO28A EO35A LBS32 SBAI EO24 237
50 45 Laverick's Bay Ash Samples Percentage of Cation Oxide 40 35 30 25 20 15 SiO[2] TiO[2] Al[2]O[3] Fe[2]O[3] MgO CaO Na[2]O P[2]O[5] 10 5 238 0 70 East Okains Bay Ash Samples 60 50 Percentage of Cation Oxides 40 30 20 SiO[2] TiO[2] Al[2]O[3] Fe[2]O[3] MnO MgO CaO Na[2]O K[2]O P[2]O[5] 10 239 0
240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 FIGURE CAPTIONS Figure 1: Geological map of Banks Peninsula. The geological formations noted are very broad although they can be subdivided very easily. The map is also incomplete since there are patches of other formations on Akaroa volcano. The bottom right map indicates where Banks Peninsula is; the larger map shows where Okains Bay is. All of the field areas in this study were located in or near Okains Bay. Formation dates from Timm et al (2009). Figure courtesy of Johnson (2012). Figure 2: This is a diagram from Johnson (2012) to illustrate the proposed magma system. Mantle peridotite melts at depth and rises to a chamber near the Moho. The slightly evolved magma from this chamber eventually rises to another magma chamber near the upper crust-lower crust boundary. The magma in this chamber could rise straight to the surface or feed another smaller magma chamber. Due to the compositions seen, it seems that the magma resulting in the ash layers rose and erupted almost immediately. As the eruptive sequence of these ash layers progressed, a magma chamber formed where the magma could evolve. Therefore, the compositions of these layers became more evolved as time went on. Figure 3: A plot with all of the ash layers displaying how silica oxide content of the samples changes. None of the ash layers have the same amount of silica although the geographical areas are related to each other. Overall, all of the ash layers are related. None of the other cation data for samples matched either. Figure 4: This plot displays how the iron (III) oxide content of the samples changes. As the silica increases, the iron (III) oxide decreases. Figure 5: There is a very strong correlation between the silica content increasing as the iron (III) oxide content decreases. This implies that the iron (III) is being partitioned out of the melt quickly.
263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 Figure 6: Unlike figure 5, the magnesium oxide is not being partitioned out of the melt quickly. There is almost no correlation between the magnesium oxide content of the samples and the iron (III) oxide content of the samples. This suggests that the magnesium content is not changing much over time. This could be due to there not being significant magnesium oxide being available originally or it is not crystallizing out of the melt. This could be due to the elemental availability of other cations and which minerals are being formed. Figure 7: This plot shows that the aluminum oxide and magnesium oxide percentages did not change significantly over time as the silica content did. These are generally fairly common elements, so this trend is not frequent. Normally, both cation contents would vary over the fractional crystallization. Figure 8: This figure, modeled after LeBas et al. (1986) is a total alkali-silica (TAS) classification scheme for categorizing samples based on their rock types. The samples almost all follow a picrite to trachyte trend of evolution. Figure 9: This plot shows how the samples from Laverick s Bay have related elemental compositions. All of the elemental compositions vary rather consistently and have similar compositions. Figure 10: This plot shows how the samples from East Okains Bay have related elemental compositions. All of the elemental compositions vary rather consistently and have similar compositions. 282 283 284 285
286 287 288 289 290 TABLE Sample SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Number (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) LB2L5 35.58 4.65 22.41 20.85 0.26 2.68 2.15 0.58 0.12 0.15 LAVIIP 36.61 4.69 22.66 20.64 0.27 2.75 2.23 0.64 0.14 0.14 LB2L4 37.56 4.32 20.07 19.74 0.26 3.93 2.37 0.33 0.06 0.26 LBS32 38.30 3.10 25.83 15.61 0.10 0.90 1.20 1.10 0.17 0.87 LBS33 38.66 3.84 20.19 17.25 0.23 3.70 2.83 1.83 0.08 0.73 LB2L1 39.38 4.52 22.87 18.25 0.20 2.21 1.27 0.55 0.15 0.31 LB2L3 39.66 4.16 18.67 19.32 0.23 3.93 2.17 0.28 0.06 0.23 EO25 41.59 3.28 21.58 15.62 0.28 2.23 1.89 2.54 0.99 0.84 LB2L2 42.14 4.01 21.39 17.23 0.51 2.49 1.54 0.77 0.24 0.27 LAV3PS 42.52 2.69 23.38 14.98 0.23 1.92 1.91 1.99 0.36 0.21 LAVH2P 44.04 2.46 21.72 12.90 0.22 2.26 2.55 3.74 1.19 0.68 SBAI 45.76 3.18 21.95 16.15 0.17 1.29 0.72 1.32 0.50 0.22 EO50GA 46.59 3.78 16.54 14.32 0.19 4.85 8.44 3.57 1.22 0.59 EO28A 48.36 2.78 17.94 12.85 0.19 2.62 6.28 4.34 1.87 1.07 EO35A 48.91 2.19 21.73 8.81 0.13 2.56 8.51 3.84 1.11 0.56 EO24 60.50 1.16 15.76 7.13 0.12 1.11 1.64 3.58 2.20 0.33 TABLE CAPTION Table 1: Major elemental analysis of the samples collected from Banks Peninsula in February 2013.