ROOTS OF THE HALEMA`UMA`U CRATER ACID SULFATE SYSTEM ON KILAUEA VOLCANO

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1 ROOTS OF THE HALEMA`UMA`U CRATER ACID SULFATE SYSTEM ON KILAUEA VOLCANO Meghann F.I. Decker Department of Geology University of Hawai i at Hilo Hilo, HI ABSTRACT Blocks of acid sulfate altered rock ejected from Halema`uma`u crater in the March 2008 eruption provide evidence of deeper processes that are driving the shallow hydrothermal alteration in this area. Previous work shows that the surficial alteration is dominated by remnant opal (SiO 2 ) and anatase (TiO 2 ), which are compositionally similar to materials found at Home Plate on Mars. The opal deposits are indicative of water-related alteration on Mars, but two different theories have been proposed for their origin. The initial theory suggested the Home Plate deposits are similar to the acid sulfate leaching deposits at Halema`uma`u (Squyres et al., 2008), however Ruff et al. (2011) suggested that the Home Plate deposits represent hot spring sinter deposits similar to those at Yellowstone. Analysis of the samples ejected in the 2008 Halema`uma`u eruption demonstrate that materials leached from the surface are at least partially being redeposited at depth as anhydrite and natroalunite, which differ greatly from alteration related to hot springs sinter deposits. INTRODUCTION Hydrothermal systems are a potential habitat for living organisms. Therefore identifying these on Mars is important in the search for life beyond the Earth. Opaline rich soils and hydrated silica outcrops have been found adjacent to the Home Plate region. The presence of opal can indicate a rather large percentage of water. Typically on earth opal can contain up to 21% water bound within its structure, which suggests these soils were derived by hydrothermal alteration (Ruff et al., 2011). Two different theories have been proposed as possible terrestrial scenarios that can result in deposits similar to those that have been found on Mars. Hydrated silica is a key mineral to compare Martian deposits to terrestrial deposits by observing their aqueous alteration (Smith, 2013). The initial theory suggests the Home Plate deposits are similar to the acid sulfate leaching deposits at Halema`uma`u crater on Kilauea (Squyres et al., 2008). More recently, it has been suggested that the Home Plate deposits represent hot spring sinter, which would require the involvement of much more water (Ruff et al., 2011) in a liquid state as opposed to steam and condensate. Further studying of the unresolved issue of whether the silica found around the Home Plate region was produced as a result of acid sulfate leaching or precipitation from a hot spring environment would aid in determining what the aqueous environment is or was like on Mars (Squyres et al., 2008). 42

2 KILAUEA ANALOG Kilauea s summit magma chamber has been degassing through fractures within and around Halema`uma`u crater since the 1924 collapse. Due to degassing and the presence of boiling groundwater, sublimates and precipitates formed inside fractures and around the rim of the crater (Figure 1). In March 2008 an explosive event occurred for the first time since 1924 within the summit caldera. A new vent opened up within Halema`uma`u crater. As a result of this explosion altered blocks from within fractures were ejected. Of the volcanic gasses emitted from the summit magma chamber below, sulfur dioxide (SO 2 ) plays a significant role in the alteration of basaltic rock. Presently the new vent emits a number of toxic gasses, of which the sulfur dioxide emissions can reach up to 2000 tons per day (Figure 1). Prior to 2008, sulfur, escaping through the water table from the underlying magma chamber, mixed with meteoric water and oxygen resulting in the formation sulfuric acid (H 2 SO 4 ) near the surface. The sulfuric acid then attacks and leaches the surrounding basaltic rock. Figure 1. Aerial view of Halema`uma`u crater outlined distribution of 2008 ejecta. Prior to the March 2008 eruption little was known about the acid sulfate system on Kilauea, however during the eruption a series of blocks from within the crater were ejected. Dr. Ken Hon and Lopaka Lee collected 15 samples of the ejecta from the area outlined in Figure 1 along with 60 surficial deposits from the surrounding area. After two former space grant fellows Liliana Desmither and Ryan Bishop both analyzed surficial deposits they gained insight on the how volcanic gases interact with surficial basaltic rocks to produce a variety of sublimates and acidic alteration products. The surficial samples they analyzed were opal (SiO 2 ) and anatase (TiO 2 ) rich deposits. Mars exploration rover (MER) Spirit found amorphous (lacking crystalline form) silica (SiO 2 ) within Gusev crater (Rice et al. 2010). Spirit determined that the presence of opaline silica forming was associated with adjacent volcanic material. The deposits found are widespread and occur both with and without sulfur (Bishop 2011). The Martian deposits are very similar to the 43

3 amorphous silica found by both Bishop and DeSmither on Kilauea. Through their research they found that the surficial deposits had been heavily altered by the sulfuric acid. The deposits they examined were originally basaltic in composition (Figure 2) but the alteration was so severe the opal has replaced the basalt by a vapor process due to the strong acid leaching process (Bishop, 2011). Only opal and anatase remained in the deposits while everything else had been leached out of the basalt, but still preserving the original vesicle structure (Figure 3). Figure 2 (left). Unaltered basalt Figure 3 (right). Completely altered and leached basalt dominantly composed of residual opal In continuation with their research, my mentor, Dr. Ken Hon and I have selected 10 of the 15 blocks ejected during the 2008 eruption for analysis. Through this research we hope to gain a better understanding of the alteration process that is occurring at depth as a result from the leached surficial material that is carried down the system. METHODS The samples were initially photographed under the microscope as references and for visual identification. Then all of the samples were analyzed using a Bruker D- 8 -ray Diffractometer (RD). The outer crystalline material was finely ground, homogenized and analyzed to determine the average bulk composition. A scanning electron microscope (SEM) was also used to examine crystalline structure within 5 of the 10 samples. The SEM provides an alternate way of analyzing samples by examining the intact crystal structure, rather than ground bulk composition. This is key in determining the relationship between different mineral assemblages forming and can provide insight on the process of alteration. Specifically the relationship between two of the most abundant minerals present in the samples, gypsum (CaSO 4 2H 2 O) and anhydrite (CaSO 4 ) was of concern. The relationship between those two minerals is reversible with small changes in temperature or water content as seen in figure 4 below. 44

4 Figure 4. Relationship between reversible gypsum and anhydrite Available from: RESULTS The ten samples I analyzed were HMB-1, HMB-2, HMB-3, HMB-5, HMB-6, HMB-7, HMB-9, HMB-10, HMB-12, and HMB-13. After analyzing all the samples with the RD it became clear that the outer rind of each of the blocks (Figure 5) was being completely altered and alteration was continuing into the interior of the rocks within the vesicles. Figure 5. HMB-2, basaltic block with outer rind alteration and interior vesicle alteration 45

5 To better understand the system driving the alteration, examine the diagram below (Figure 6). The area surrounding the Halema`uma`u crater is dominated by fractures extending deep into the caldera. These fractures allow sulfur dioxide (SO 2 ) escaping from the magma deep within the caldera to escape. As this gas escapes it mixes with the meteoric water percolating through the fractures to form sulfuric acid (H 2 S0 4 ). The sulfuric acid attacks the surface rock leaching everything except SiO 2 (opal) and TiO 2 (anatase) from the originally basaltic rock. The leached material contains elements such as Ca, Mg, Fe, and Al in solution. As this solution continues percolating through the fractures it continues to attack the surrounding rock altering the basalt. The leached material then precipitates out as secondary minerals such as anhydrite (CaS0 4 ) and gypsum (CaS0 4 2H 2 0) coating the outer layer of rock. Depending upon the vesicularity of the rock, the fluid begins to alter the interior of the basalt within the vesicles. This process occurs at depth where the ejecta samples were believed to have originated from as seen by the red box in figure 6. Figure 6. Diagram of the acid sulfate leaching system on Kilauea. Yellow arrows show upwelling H2S and SO2-rich vapor. Green arrows show downwelling acidic condensates. Through the use of both the SEM and RD it became apparent that the alteration of the ejecta was complex. After RD analysis of the outer rinds of each of the ejecta samples my research showed they were almost completely altered to opal (SiO 2 ), gypsum (CaS0 4 2H 2 0) and anhydrite (CaS0 4 ). With trace amounts of sulfates such as natroalunite (NaAl 3 (SO 4 ) 2 (OH) 6 ), alunite (KAl 3 (SO 4 ) 2 (OH) 6 ), melanterite (FeSO 4 7H 2 O), pickeringite MgAl 2 (SO 4 ) 4 22(H 2 O), and natrojarosite (NaFe 3+ 3(OH) 6 (SO 4 ) 2 ) also present. These sulfates formed as a result of solfataric or hydrothermal sulfate-bearing solutions reacting with the basalt. As seen in figures 7 and 8, HMB-5 was completely altered to gypsum and natrojarosite. 46

6 Figure 7 (left). RD analysis of HMB-5 altered to gypsum and natrojarosite Figure 8 (right). SEM analysis of HMB-5 showing natrojarosite Anatase (TiO 2 ) a secondary oxide and two high temperature silica (SiO 2 ) polymorphs tridymite and cristobalite were also present within the samples. The anatase is believed to precipitate out of solution at great depth within the system. After analyzing HMB-1 with the SEM anatase was found precipitating on the surface of a large Ti-rich crystal with some SiO 2 (Figure 9) while the surrounding area was altered to anhydrite. Figure 9. SEM image of HMB-1 showing both anatase (TiO 2 ) on left, and anhydrite (CaSO 4 ) on right 47

7 Trace amounts of cristobalite and tridymite present in the samples represents a change from opal A to opal CT at depth due to the increased temperature. Most of the alteration occurring is opal plus anhydrite or gypsum with minor anatase or other sulfates locally. The anhydrite gypsum reaction shows that temperatures are higher at depth and the amount of water increases in the condensate. While Anhydrite was found in a majority of the samples as seen in the table below (Figure 10), anhydrite was not found on the surficial deposits, just gypsum. Sample Opal Anhydrite Gypsum Cristobalite Jarosite Anatase Natroalunite Tridymite Hump HMB-1 HMB-2 Interior HMB-3 HMB-5 HMB-6 HMB-7 HMB-9 HMB-10 HMB-12 Interior HMB-13 Figure 10. Table of 10 exterior and 2 interior samples analyzed and results of the most common minerals found present CONCLUSION With such a diverse range of secondary minerals forming at depth it is clear that the relationship between them is complex. The alteration occurring at Halema`uma`u occurs without the presence of standing water. Instead fluctuations within magmatic gasses mixing with meteoric water and changes in temperature produce a complex assemblage of secondary minerals and alteration products. At the surface, opal and anatase have replaced the basalt by leaching all other elements out of the rock. At depth the acid leaching alteration continues and is still dominated by opal and anatase, however an array of other sulfates and silica polymorphs are forming as well from the precipitation of material leached from the surface rock. After analyzing the ejecta samples from Halema`uma`u it is evident the deposits found on Kilauea are dominated by sulfur and require no standing water. This differs however from the Yellowstone analog where sinter deposits are dominated by chlorine and require a large body of super heated standing water. The deposits found by both MER Spirit and Opportunity are higher in sulfur rather than chlorine. This type of deposit is produced on earth by acid, or water with a low ph. Which suggests that the opaline deposits found at Home Plate are similar in composition and also formed from water-related alteration. While there is a great variation in geochemistry of the samples collected by MER Spirit and Opportunity, there is also a complex variation in the geochemistry of the samples collected from Halema`uma`u. In conclusion the evidence for water on Mars also consists of deposits formed from water with an extremely low ph, which indicates the acid sulfate-leaching environment on Kilauea is similar to acid environment on Mars. 48

8 ACKNOWLEDGEMENTS I would like to personally thank the NASA Hawai`i Space Grant Consortium for giving me the opportunity to gain valuable experience doing hands-on scientific research. The valuable research skills I have obtained will assist me in my future scientific endeavors. I would also like to thank Dr. Ken Morris for providing me with a lab equipped with an RD machine, and most of all, my mentor, Dr. Ken Hon, for all his guidance and assistance throughout the course of my project. REFERENCES Bishop R. (2011) Mineralogical study of volcanic sublimates from Halema uma u Crater, Kilauea Volcano. Hawaii Space Grant Consortium. DeSmither L. (2011). Distribution of opaline alteration in fumaroles from Halema uma u Crater, Kilauea Volcano. Hawaii Space Grant Consortium. Rice M., Bell III J., Cloutis E., Wang A., Ruff S., Craig M., Bailey D., Johnson J., de Souza Jr. P., Farrand W. (2010) Silica-rich deposits and hydrated minerals at Gusev crater, Mars: Vis-NIR spectral characterization and regional mapping. Icarus 205(2), Ruff S., Farmer J., Calvin W., Herkenhoff K., Johnson J., Morris R., Rice M., Arvidson R., Bell III J., Christensen P., and Squyres S. (2011) Characteristics, distribution, origin, and significance of opaline silica observed by the Spirit rover in Gusev crater, Mars. J. Geophys Res. Planets 116, Smith M., Bandfield J., Cloutis E., Rice M. (2013) Hydrated silica on mars: Combined analysis with near-infrared and thermal-infrared spectroscopy. Icarus 223(2), Squyres W., Arvidson E., Ruff S., Gellert R., Morris R., Ming D., Crumpler L., Farmer J., Des Marais D., Yen A., et al. (2008) Detection of silica-rich deposits on Mars. Science 320,