Fulgurite Classification, Petrology, and Implications for Planetary Processes

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Link to Item http://hdl.handle.net/10150/144596 FULGURITE CLASSIFICATION, PETROLOGY, AND IMPLICATIONS FOR PLANETARY PROCESSES

Kristin Marie Block

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AThesisSubmittedtotheFacultyofthe

DEPARTMENT OF PLANETARY SCIENCES

In Partial Fulﬁllment of the Requirements For the Degree of

MASTER OF SCIENCE

In the Graduate College

THE UNIVERSITY OF ARIZONA

2011 2

STATEMENT BY AUTHOR

This thesis has been submitted in partial fulﬁllment of requirements for an advanced degree at the University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. This work is licensed under the Creative Commons Attribution-Share Alike 3.0 United States License. To view acopyofthislicense,visithttp://creativecommons.org/licenses/by-sa/3.0/us/ or send a letter to Creative Commons, 171 Second Street, Suite 300, San Francisco, California, 94105, USA.

SIGNED: Kristin Marie Block

APPROVAL BY THESIS DIRECTOR

This thesis has been approved on the date shown below:

15 July 2010 Timothy Swindle Date Professor of Planetary Sciences 3

ACKNOWLEDGEMENTS

IwouldliketothankMatthewPasekforgenerouslygivinghistime,expertise,and guidance over the past several years. I would also like to thank Tim Swindle and Dolores Hill, whose support made this work possible.

Many thanks to my family, both born and chosen, for helping me along the journey. Chris, Cathi, Sashi, Lee, Jade, Dave, Zippy, Rome, Karen, Di, Kat, Mom and Dad– you’ve enriched my life in countless ways.

Thank you to Sarah H¨orst, one of the most giving people I have known, for her unwavering support. Brilliant, strong, and unabashedly geeky, she’s everything IwanttobewhenIgrowup.

Thanks to Nathan Hurst, who continues to inspire despite being on the “wrong side of the grass”, and who is still no doubt comforting the disturbed and disturbing the comfortable.

Finally, I’d like to thank Galen for learning, laughing, skywatching, rock hounding, theorizing, dancing, daydreaming, and making up songs with me.

This research was supported by grant NNX07AU08G from NASA Astrobiol- ogy: Exobiology and Evolutionary Biology program. 4

DEDICATION

To Dad, Thanks for teaching me about satellites using the cookie box lid, taking me to see the giant magnets, and showing me how to ﬁnd the length of a crazy-straw using a graduated cylinder. Let’s put this on the shelf next to your dissertation, okay? 5

TABLE OF CONTENTS

LIST OF FIGURES ...... 7

LIST OF TABLES ...... 8

ABSTRACT ...... 9

CHAPTER 1 INTRODUCTION ...... 10 1.1 Fulgurite Formation ...... 10 1.2 Previous Chemical and Mineralogical Investigations ...... 13

CHAPTER 2 FULGURITE CLASSIFICATION ...... 19 2.1 Introduction ...... 19 2.1.1 Fulgurite morphological features and terminology ...... 20 2.1.2 Sample acquisition ...... 22 2.2 Classiﬁcation Method ...... 22 2.2.1 Data collection ...... 22 2.2.2 Classiﬁcation criteria ...... 23 2.3 Results ...... 24 2.3.1 Type I Fulgurites ...... 24 2.3.2 Type II Fulgurites ...... 25 2.3.3 Type III Fulgurites ...... 26 2.3.4 Type IV Fulgurites ...... 26 2.3.5 Melt Droplet Fulgurites ...... 27 2.4 Summary ...... 27

CHAPTER 3 FULGURITE PETROLOGY ...... 35 3.1 Introduction ...... 35 3.2 Samples ...... 35 3.2.1 Type I Samples ...... 35 3.2.2 Type II Samples and Associated Melt Droplets ...... 36 3.2.3 Type III Samples ...... 40 3.3 Preparation ...... 41 3.4 Analysis ...... 41 3.5 Results ...... 42 3.5.1 Type I Fulgurites ...... 42 3.5.2 Type II Fulgurites ...... 42 6

TABLE OF CONTENTS –Continued

3.5.3 Type III Fulgurites ...... 51 3.6 Discussion ...... 52 3.6.1 Detrital Zircons ...... 53 3.6.2 Metal Grains ...... 53 3.6.3 Melt Droplets ...... 53

CHAPTER 4 FULGURITES IN THE PLANETARY PROCESS CONTEXT 54 4.1 Introduction ...... 54 4.2 Chemical Reduction in Planetary Materials ...... 54 4.3 Lightning on Early Earth ...... 55 4.4 Fulgurite Formation on Planetary Surfaces ...... 56 4.4.1 Venus ...... 57 4.4.2 Mars ...... 58 4.4.3 Titan ...... 59 4.5 Summary ...... 60

CHAPTER 5 SUMMARY AND FUTURE DIRECTIONS ...... 61

REFERENCES ...... 63 7

LIST OF FIGURES

1.1 Global Frequency of Lightning Strikes per Year ...... 12 1.2 Fulgurite Formation Schematic ...... 13 1.3 Temperature Proﬁle of Lightning-Heated Soil ...... 14 1.4 Fe-Ti-Si Metals Observed in a Fulgurite by Sheﬀer ...... 16 1.5 Fe-Si Metals Observed in a Fulgurite by Essene and Fisher ...... 17

2.1 Fulgurite Morphology ...... 21 2.2 Type I Fulgurite ...... 25 2.3 Type II Fulgurite ...... 29 2.4 Type III Fulgurite ...... 30 2.5 Type IV Fulgurite ...... 31 2.6 Melt Droplet Fulgurites ...... 31 2.7 Fulgurite Classiﬁcation Types ...... 34

3.1 Vernal, Utah Fulgurite ...... 38 3.2 Vernal, Utah Melt Droplet Fulgurite ...... 38 3.3 York County, Pennsylvania Fulgurite ...... 39 3.4 La Paz, Arizona Fulgurite ...... 40 3.5 Groundmass of the Chafee County, Colorado Fulgurite ...... 43 3.6 Groundmass Compositional Diﬀerence, York County, Pennsylvania Tube and Melt Droplet Fulgurites ...... 45 3.7 Zircon Grains in the York County, Pennsylvania Fulgurite ...... 46 3.8 Elemental Analyses of Zircon Grains in the York County, Pennsylva- nia Fulgurite ...... 48 3.9 Fe-P Metal in the York County, Pennsylvania Fulgurite ...... 49 3.10 Metal Textures in York County, Pennsylvania Melt Droplet Fulgurites 50 3.11 Groundmass of the La Paz County, Arizona Fulgurite ...... 51 3.12 Partially Melted Calcite Grain, Yuma, Arizona Fulgurite ...... 52 8

LIST OF TABLES

2.1 Fulgurite Densities ...... 32 2.2 Sample Identiﬁcation ...... 33

3.1 Groundmass Composition of Type I Fulgurites ...... 42 3.2 Composition of the Chafee County, Colorado Fulgurite ...... 46 3.3 Composition of the Quartzite, Arizona Fulgurite ...... 47 3.4 Composition of the Vernal, Utah Fulgurite ...... 47 3.5 Composition of the Vernal, Utah Melt Droplet ...... 48 3.6 Metal Bleb Composition in the York County, Pennsylvania Fulgurite . 49 9

ABSTRACT

A variety of fulgurites from diverse locations have been studied. Morphological features were measured and physical properties documented, and a classiﬁcation scheme was developed. Three major types are introduced and described: Type I, Type II, and Type III, along with two minor types: Type IV and Melt Droplets. Fulgurites representative of each major taxonomic type were investigated using electron microprobe point analyses and x-ray mapping. A range of compositions were found, including nearly pure glass, detrital zircons with baddeleyite rims, Fe-metal with P-rich rims, and unusual Fe-Si metals. The fulgurite formation process is considered within the planetary context through a discussion of lightning detection and potential for formation on other terrestrial bodies. Finally, suggestions for future investigations are presented and discussed. 10

CHAPTER 1

INTRODUCTION

The purpose of this work is to expand the body of knowledge of fulgurite morphology, composition, and relationship to planetary processes. To that end, fulgurites from many locations were examined and measured for morphological similarities and classiﬁed based on easily recognizable features. Representative samples from each major fulgurite type were analyzed using electron microprobe point analyses and x-ray maps. Finally, the role of fulgurite formation as a planetary surface process has been considered, and is presented herein.

1.1 Fulgurite Formation

Glasses are formed by the abrupt cooling of heated materials. Natural glasses are not often encountered on earth, as they weather quickly in wet environments (OKeefe, 1984). Several diverse mechanisms exist to produce glass in nature. Volcanic glasses like tachylite and obsidian are associated with rapid cooling of molten rock and lava. High friction movement along faults during seismic events can also vitrify rocks, forming pseudotachylite (Spray, 1995). Other natural glasses are linked to extremely high energy events, such as meteorite impacts. Tektites, bedaisites, moldavites, and Libyan desert glass have all been linked to large impacts, though the precise location of source material is not always well known. Artificial analogs to these high energy glasses exist; one such material, Trinitite, was formed during the Trinity nuclear bomb test near Alamogordo, New Mexico in 1945. Fulgurites, formed by the rapid heating of rock or soil by a cloud-to-ground lightning strike, take their name appropriately from the Latin term for lightning, fulgur (Arago, 1821). They are the most widespread variety of natural glass, capable of forming wherever lightning strikes. The large compositional variation observed 11 in fulgurites reflects the diverse target materials present on the Earths surface. Lightning is a ubiquitous phenomenon on Earth, striking the surface approximately 65 times each second (Krider et al., 1968). Most often associated with thunderstorms, a lightning discharge occurs when a portion of the atmosphere at- tains an electric charge sufficient to cause breakdown of the air within the electric field. This field is thought to be produced by the charging of ice particles or water droplets as they collide and are separated by internal circulatory patterns within clouds. Stray electrons further ionize the air along a conductive channel, which can reach from cloud-to-cloud, cloud-to-air, or cloud-to-ground (Uman, 1969; Desch et al., 2002). Regional weather patterns serve to deliver thunderstorms and their associated cloud-to-ground lightning strikes repeatedly to some areas; strike rates are as high 2 1 2 1 as 20 km− yr− in central Florida, and over 150 km− yr− in the Democratic Republic of Congo. On average, three cloud-to-ground lightning strikes occur per square kilometer per year (Krider et al., 1968). While these are average rates, Maier and Krider (1984) found that actual cloud-to-ground strike density may vary by as much as an order of magnitude over distances as small as 20-30 km, indicating that localized strike rates could rise higher than the commonly reported averages due to favorable topography and natural or man-made features. This extremely common process is also very energetic— lightning may dissipate up to 109 Jperflash(Borucki and Chameides, 1984), heating surrounding air to instantaneous temperatures in the range of 105 K (Uman et al., 1978). This energy is capable of melting and vaporizing target material. As lightning strikes the ground, the target material undergoes rapid physical, chemical, and morphological change. Current flows through the target material, following areas of high conductivity or moisture content, plant roots, or other subsurface features. As this material is rapidly heated, resultant boiling produces voids and vesicles surrounding the path of the lightning, allowing escape of volatiles. The expanding envelope of heated air and moisture may serve to insulate some grains from extreme temperatures produced in the strike (Pye, 1982), leaving some grains in the fulgurite 12

Figure 1.1: Global frequency of lightning strikes per year. relatively unaltered while others in close proximity show evidence of exposure to extreme temperatures. This rapid heating and release of volatiles can produce a cylindrically shaped glassy core, sometimes hollow, surrounded by a rough outer surface composed of both melted and unmelted grains (see Chapter 2 for a discussion of morphology and terminology.) These fulgurites can range in size from centimeters to meters, though due to their fragility samples on the lower end of the size range are most often recovered. Branching fulgurites are formed when the current divides along its path to the conducting layer. Most side branches fail to reach the conducting layer and rapidly lose their energy, forming oﬀshoots smaller in diameter than the main body. Vapor- ization of the target material during the strike can result in the explosive exhalation of material within the tube, expelling droplet shaped fulgurites from the central channel. See Chapter 2, Figure 2.1 for an explanation of fulgurite features. Fulgurite morphology is linked to the radial and longitudinal temperature gra- dients experienced along the currents pathways. The temperatures reached during fulgurite formation are not well constrained but clearly exceed the vaporization temperature of silicates in the region closest to the path of the strike, as central voids are commonly observed. Farther from the center temperatures drop oﬀaccording to 13

Figure 1.2: Current travels from the atmospheric source to the target material. The charge passes through a relatively non-conductive area to dissipate in the more conductive layers of the subsurface.

the thermal diffusivity of the struck material, but remain high enough centimeters away to melt the target into a glassy tube. A simple thermal diffusion model (Carter et al., 2010a) using the diffusion of heat in cylindrical coordinates:

∂T ∂2T 1 ∂T = k + ∂t ∂r2 r ∂r

predicts vaporization of material within a radius of 0.5 cm, and melt within 1.0- ∼ ∼ 7 2 1 1.5 cm, using a thermal diﬀusivity coeﬃcient of 10− m s− and initial temperature of 30 105 K. According to this model, which results in realistic fulgurite sizes, the × cooling time to reach a background temperature of 300 K is approximately one minute.

1.2 Previous Chemical and Mineralogical Investigations

A majority of fulgurites are formed in siliceous targets, hence most fulgurite research

has focused on silicate mineralogy, particularly SiO2 phases. Lechatelierite is used as a synonym for fulgurite in older literature, though this term speciﬁcally refers to

amorphous SiO2 glass found in the groundmass of the fulgurite. Studies of fulgurite 14

Figure 1.3: Color contour temperature (K) profile of lightning-heated soil from Carter et al., 2010a. silicate mineralogy have revealed rapid heating and cooling time scales. The lack of crystallites in several fulgurites indicates cooling on the order of 108 Cpersecond (Switzer and Melson, 1972). Other work has confirmed these cooling timescales (Rietmeijer et al., 1999). The mineralogy of fulgurites is affected by this quick temperature change— minerals which would ordinarily melt incongruently, like enstatite, may melt congruently under the fulgurite formation timescales (Switzer and Melson, 1972; Sheffer, 2007). Other minerals have been shown to melt incongruently during fulgurite formation, like biotite (Appel et al., 2006) and presumably other OH-containing minerals. The minerals that melt during formation typically form a glass of nearly identical composition to the starting mineral, then mix with other melts to form glasses which reflect the overall bulk composition. Additionally, there are several compositional and structural changes that take place during the fulgurite formation event. Prior work has shown that some targets struck by lightning lose SiO2 and Na2O, and that the fulgurite glass concentrates 15

FeO, MgO, and CaO. Furthermore, some fulgurites show evidence of shock deforma- tion at the boundary of the central void (Frenzel et al., 1989). These features include kink bands in plagioclase and planar fractures in diopside. Shock features similar to these have also been reported in impacts, suggesting a relationship between lightning and impact physics (Stoffler and Hornemann, 1972; Stoffler, 1974). Much of the attention to fulgurite mineralogy has centered on the presence of minerals indicative of extremely reducing conditions. Many of these primarily oxygen-free minerals or metals have previously been reported only in meteorites, or remain completely unique to fulgurites. Phosphides found in fulgurites include schreibersite, Fe3P, which is common in meteorites (Pasek and Lauretta 2005; Pasek and Lauretta, 2008; Pasek et al., 2007), and TiP, which has no terrestrial or me- teoritic counterpart. Other fulgurite minerals or phases indicative of extreme reduction are a variety of silicides, includeing hapkeite, Fe2Si, previously reported in lunar meteorites, and unnamed phases like Fe3Si7,FeTiSi2,andFeSi(Esseneand Fisher 1986; Cardona et al., 2006; Sheffer 2007; Parnell et al., 2008; see Figure 1.4). Other work has also identified metallic aluminum and Fe-Al-Si melts with formulae

Fe2Al3Si3,andFe10Si23Al27 (Sheﬀer, 2007), though the formation of these phases is probably strongly linked to the conditions of formation of one particular sample, in which a downed power line struck asphalt composed of tar and basalt. Native Si and Fe metal droplets are common to all of these studies. Gold has also been reported in at least one fulgurite (Essene and Fisher, 1986), but whether this material is detrital or formed by reduction processes associated with the lightning strike remains unclear. The formation of many of these minerals is likely connected to the combustion of organic material in the target material during the lightning strike. The combustion of organics like roots or detritus may cause extremely reducing conditions locally, acting as a natural smelter. Indeed, minerals observed in some fulgurite samples are common to industrial smelters (Vonken et al., 2006). Reduction of iron from Fe3+ to Fe2+ or even Fe0 is also common in high energy events like impacts (Chao et al., 1964; Sheﬀer, 2007). Iron silicides and graphite have been previously observed in the glassy melt 16

Figure 1.4: Backscattered electron image of a Fe metal droplet in a fulgurite from West Virginia from Sheﬀer, 2007. in fulgurite interiors (Essene and Fisher, 1986), indicating extreme reduction of the target material. Several mechanisms for reduction in these samples have been suggested. Electrolysis may drive the loss of oxygen, and subsequent reduction, in the melted target material (Essene and Fisher, 1986; Wasserman and Melosh, 2001). Atmospheric nitrogen has been suggested as an oxygen scavenger due to the observation of NOx molecule production from lightning strikes (Desch et al.,

2002) however amounts of N2 gas available in soil may not be sufficient to provide asignificantsourceofoxygenacceptors(Sheffer,2007). It has also been suggested that the oxidation of carbon-rich material in the soil drives reduction in fulgurites. In thermodynamic analyses of stability regions of silica phases, Wasserman et al.(2002)showedthatif,duringfulguriteformation, the system reaches a temperature at which the liquid cannot equilibrate with the vapor in a range where silicon metal is present, the Si metal may be retained in the final cooled fulgurite. The range at which Si metal is present is heavily dependent 17

Figure 1.5: Essene and Fisher (1986) analyzed a several metal drops within the glassy melt of a fulgurite. on carbon content— the more carbon available to act as an oxygen scavenger, the greater the temperature range. Despite its role in this process, the presence of carbon is not necessary to reduction in fulgurites; in a rocket-triggered lightning experiment Jones et al.(2005) showed that reduction of pure NiO to metallic Ni did not require a separate reducing agent. Sheﬀer (2006) suggests that reduction is intrinsic to the rapid heating and cooling experienced by the target material in fulgurite formation. The initial supercritical ﬂuid produced by the lightning strike expands and cools, eventually reaching a region where liquid and vapor coexist. Fully oxidized phases, such as

Fe2O3 decompose into more stable oxides, such as FeO and oxygen. Rapid cooling blocks the equilibration of the vapor and liquid phases. Oxygen remains in the vapor phase, leaving the liquid reduced compared to its initial composition. This liquid is then quenched, preserving the reduced chemistry. Fulgurites are among the ﬁrst natural, terrestrial materials yet identiﬁed to carry reduced phosphorus species. Phosphorus in most geologic materials exists in the +5 oxidation state as phosphates, however, a series of extracts of fulgurites of various types have demonstrated the presence of reduced phosphorus species of oxidation 18 state +1 and +3 (Pasek et al., 2008; Pasek and Block, 2009). Raman spectroscopy mapping of a fulgurite cross-section (Carter et al., 2010b) revealed additional indications of the range of conditions induced in the target material at the time of formation. The preservation of polyaromatic hydrocarbons and presence of anatase, a low-temperature polymorph of TiO2,showthatinsomere- gions of the fulgurite, temperatures did not exceed 2000 K. Other areas of the same fulgurite show shocked quartz, traditionally associated with impacts due to the extreme pressures required for generation. Carter et al.calculateapressureof27GPa within the central part of the fulgurite, within the accepted range for formation of shocked quartz. Investigations of fulgurites are becoming more common, however most research focuses on single samples. A more comprehensive look at fulgurites from a variety of sources is required to determine the range of formation conditions experienced in the target material. 19

CHAPTER 2

FULGURITE CLASSIFICATION

2.1 Introduction

Due to the ubiquity of lightning strikes to the Earth’s surface, fulgurites are not rare finds. However, because the relatively thin body of fulgurite research has focused on individual samples rather than broader populations, no thorough approach at aconsistentfulguritetaxonomyhasbeenpreviouslysuggested.Classificationis an important and necessary step in the ongoing development of fulgurite studies. A coherent, standardized classification scheme provides common terminology for communication. It creates a basis for understanding the scope of fulgurite research, placing studies of single specimens into the appropriate larger context. Much like the meteorite terms stony, stony-irons, and irons, the groups introduced below are based on recognizable common attributes, and can be further refined by chemical and petrologic studies. Upon examination of fulgurites formed in a variety of locations, it is apparent that a reasonable classification scheme sorts individual samples into four broad groups based on macroscale features. The major groupings, termed Type I, Type II, Type III, and Type IV, are most appropriate for fulgurites formed from electrical discharge into natural materials. Artificial fulgurites have been produced for the purposes of studying lightning strike characteristics and effects (Jones et al., 2005). In those cases, where the target material is similar to those discussed below, the classification scheme is appropriate to apply. While artificial fulgurites do not necessarily reflect the range of conditions under which fulgurites can form, because the induced voltage can be monitored and to a certain extent controlled, they may be valuable in the efforts to constrain conditions during natural formation and should be classified with their naturally produced analogs. For fulgurites formed by ar- 20 tificially produced electrical discharge into target material which is not analogous to natural surface material, as in the case of the research presented by Jones et al. (2005), the classifications presented in this work are not appropriate. In addition to these classifications, a minor group, Melt Droplet Fulgurites, is proposed. For most fulgurites, provisional placement into a specific classification can be achieved through quick visual evaluation. Several distinguishing features, discussed below, are macroscale in size and are generally readily visible to the naked eye or under hand lens. This may prove particularly useful when acquiring fulgurites for study whose formation location is not well documented. Further simple physical measurements can be made to support or modify the initial classification. Other microscale and chemical features common to each type are discussed in Chapter 3. Because evaluation of that type requires extensive, sometimes difficult sample preparation and more advanced analysis, microscale and chemical features are not as valuable as physical characteristics for the purposes of quick taxonomic evaluation.

2.1.1 Fulgurite morphological features and terminology

The fulgurite features referred to in the classiﬁcation scheme below are illustrated in Figure 2.1.Thefulguriteitselfisdeﬁnedasthesolidmaterialthathasundergone physical or chemical change by way of lightning strike to a target material— in most cases soil or rock. It is further distinguished from the surrounding material by its structure; in the case of lightning strikes to loose ground material such as sand or clay, the fulgurite is the solid material that can be readily separated from the target. In the case of lightning strikes to rock, the fulgurite is the entire assemblage of melted material along with the original rock, only some of which may have been exposed to the extreme conditions required for thermal alteration. Most fulgurites are cylindrical or elongated conical in shape. Many have a either asingleormultipleclosely-spacedvoidsatthecenterfromwhichvaporizedmaterial or volatiles escaped during formation. This void space is aligned along the direction of current travel, assigned here as the z-axis. Small voids or ancillary vents may extend radially from the central void. If the fulgurite is retrieved with little loss 21

Figure 2.1: Common fulgurite morphology and features. Right: Deschutes, Oregon fulgurite and 1 cm marker, image courtesy Flandrau Science Center. of material, this central void is usually surrounded by the interior fulgurite wall, extending the length of the sample. This wall typically consists of melt glass, though unaltered grains may be embedded within. The exterior fulgurite wall consists of apartiallymeltedorbakedmaterial.Lightlyalteredorunalteredtargetmaterial may be loosely adhered to the outer fulgurite wall. Fulgurites are generally roughly symmetric in the x-y plane, however some exhibit branched structures extending from the main body. The main body is considered to be the portion of the fulgurite with the largest outer wall-to-outer wall diameter. In cases where the sample is not intact, the main body is distinguished from ancillary branching structures by the location and size of the central void and interior wall melt. 22

2.1.2 Sample acquisition

Although fulgurites are popular among some collectors for their attractive, unusual colors or dramatic origin, their scientiﬁc value is not commonly recognized, and there is as yet no readily available science-based source for purchasing or borrowing samples for study. Most samples were obtained from rock and mineral collectors rather than researchers. Even small museum collections are more frequently maintained for their appearances rather than for the purposes of scientiﬁc study. Often times potentially useful data are not collected; conditions of the surrounding material, positioning in the target soil, and collection method are rarely recorded. As samples change hands multiple times, potentially valuable anecdotal information from the collector is lost, sometimes leaving the general collection location as the only associated data. With that in mind, care has been taken to maintain any data obtained from the collector or seller. Most samples studied in this work are from the private collection of Matthew and Virginia Pasek. Fulgurites were given names based on their reported discovery location.

2.2 Classiﬁcation Method

2.2.1 Data collection

Individual fulgurite pieces, even when collected from the same location, were weighed separately. Calipers were used to measure length (parallel to the z-axis) and diameter (in the x-y plane) for each sample. Where the diameter varied across the length of the fulgurite or along diﬀerent orientations, at least three measurements were recorded. Where appropriate, the diameter of the central void was measured. Similarly, thicknesses of the glassy and unaltered portions of the cross-section were measured. Although not strictly necessary for the purposes of classiﬁcation, additional information such as glass color, number of major voids, branching, and other data were collected in order to build the body of knowledge concerning fulgurite morphology. 23

Images were taken to document the large range of physical characteristics exhibited by the fulgurites studied.

2.2.2 Classiﬁcation criteria

Provisional classiﬁcation was done by visual inspection on the basis of morphological attributes described in the following sections. Measurements of the members of each class were then examined to determine whether this visual assessment reﬂected other, quantitative characteristics. Because fulgurites often are not recovered in a single, unbroken piece, length and mass alone are inappropriate to use for cross-class comparisons. Rather, density was calculated as if the samples were solid cylinders; this allows the size of the void space to be incorporated as an identifying character- istic. The standard density formula, with average length and outer wall diameter measurements for asymmetrical samples, was used.

mass ρ = fulgurite π ¯ 2 ¯ 4 DouterL Additionally, the glass thickness, as measured from the innermost point of glassy material (often the center of the fulgurite) to the radial extent where distinct grain edges become visible, was compared to the total wall thickness as a simple percentage. Target material was then noted for the members of each type. These ex- aminations revealed common density, melt percentage, and target materials within each type for fulgurites classified based on gross morphological characteristics. No samples required re-classification from the provisional status upon examination of density and target material; glass percentage served to further differentiate types. These attributes provide classification results consistent with considering morphology alone. 24

2.3 Results

2.3.1 Type I Fulgurites

Sixteen individual specimens from four different source locations were classified as Type I fulgurites. These samples have very thin inner and outer walls, making them extremely fragile. Due to their fragility, most samples are clearly broken, the recovered portion shorter than the original fulgurite produced. Most in this sample group have lengths less than 4cm,howevercarefulexcavationhasrevealedsimilar ∼ fulgurites that are much longer, and unbroken. Some have a large internal void space, however it is apparent that in others, the void spaces have collapsed prior to cooling, leaving a small central void. In the case of one sample, Morocco (MOR), no central void remains, however the external morphology is consistent with other collapsed Type I fulgurites. The external color varies between gray and tan. The walls of these fulgurites are very thin, and nearly all of their thickness is comprised of the melted material that can most clearly be seen lining the inner walls. A very thin veneer of unaltered grains interspersed with melted grains can be seen on the exterior of the outer wall. This coating is often so thin– on the order of several grains– it is difficult to measure without the aid of a microscope. The glass lining the interior walls tends to be slightly darker in color than the external surface, and very smooth. Some of these fulgurites exhibit small, feathering-like patterns extending from the central tube. These structures are most prominent on collapsed specimens. Little branching is found among fulgurites of this type. An example of a Type I fulgurite can be seen in Figure 2.3. These fulgurites formed from lightning strike into beach or dune sand target material. Because of their very thin inner and outer walls, Type I fulgurites have a 3 low density if calculated as a solid cylinder (0.7 0.2 g cm− , see Figure 2.7)and ± have a glass percentage of nearly 100%, to the accuracy of the calipers used. 25

Figure 2.2: A Type I fulgurite from New South Wales, Australia.

2.3.2 Type II Fulgurites

Twenty-two individual specimens from eight different source locations were classified as Type II fulgurites. The exterior of Type II fulgurites is often friable and pieces dislodge easily when the sample is handled, but because of their overall size they are less fragile than Type I fulgurites (i.e.,theyreadilycrumblewhenrubbed,butare unlikely to break under their own weight). These samples can be extremely large, some in this sample group have masses in excess of 100 g. Many Type II fulgurites have a large void, some surpassing 10 cm in diameter. The walls surrounding the void are variable in thickness, though generally 1 cm or larger. Ancillary tubes branching from the central void are not rare, with 20% of Type II fulgurites having some sort of side vent from the main void. The external color of Type II fulgurites is usually a medium gray. While the external color of unaltered target material is relatively uniform between samples, the glass color of these fulgurites exhibits a surprising variation not seen among the other types, including bottle green, dark blue, white, orange, and brown. Glass lining the interior wall can be smooth, however a frothy texture can be exhibited in the glass outward from the central void, and within some branches. Type II fulgurites may have plant matter, small pebbles, or other detritus from the target area embedded in the outer wall. The samples in this class of fulgurites were formed from lightning strike into fine- 3 grained material like clay and silt. The average density is 1.2 0.2 g cm− .This ± 26 type shows the greatest variety in glass percentage– samples ranged from 19-90% glass.

2.3.3 Type III Fulgurites

Seventeen individual specimens from four diﬀerent source locations were classiﬁed as Type III fulgurites. These are not fragile and only moderately friable, and are consequently often recovered in sections longer than 4 cm. Some have a distinct, symmetrical central void, while others show several small voids which follow the length of the cylinder. All central voids are generally quite small in diameter, on the order of 3-4 mm. The outer wall-to-outer wall diameter is typically small, ∼ rarely surpassing 3 cm, however the wall thickness is consistently and noticeably greater than that of Type I fulgurites. The external color of Type III fulgurites is dependent on grain composition and may match the color of the target material, though is usually tan. A small amount of interior glass is visible in cross section and is generally gray and slightly darker than the target material and exterior surface of the fulgurite. They rarely have ancillary features, such as branching or feathering structures, beyond the central tube. Partially melted and embedded grains are common surface features of these fulgurites. Type III fulgurites were formed from lightning strike into dry desert soils, such as those found in the American Southwest. Type III fulgurites are the most dense 3 of the three classes at 2.1 0.5 g cm− .Theyhavethelowestglasspercentage, ± ranging from 0-10%. ∼

2.3.4 Type IV Fulgurites

Although not studied in this body of samples, a fourth class is proposed, encompass- ing those fulgurites produced from lightning strikes directly onto larger rocks. Type IV fulgurites have been long recognized as a geologic phenomena, and are frequently encountered on mountain tops (Switzer and Melson 1972, Frenzel and Stahle 1984, 27

Clocchiatti 1990, among others). Inconsistently referred to in previous literature as rock fulgurites, these specimens typically exhibit a glassy material veneer on the surface of the aﬀected rock (see Figure 2.5). Some have additional fused material; depending on the location of the strike, this fused material may take the classical fulgurite shape.

2.3.5 Melt Droplet Fulgurites

Melt droplets are often more easily recognized as fulgurites than their cylindrical counterparts. Because they do not share the morphological characteristics common to tube fulgurites, they are classiﬁed separately, however they are often found in concert with Type II samples. As the name implies, these fulgurites are generally round in shape and very smooth, made of glassy material presumably ejected from the target. Fulgurite melt droplets appear to have cooled very quickly— no grains or remnant detritus are adhered to the exterior. Melt droplets shown in Figure 2.6 were found with two Type II samples studied in this work: the fulgurites from Vernal, Utah (VUT) and Western New York (WNY).

2.4 Summary

Based on a quick visual inspection, fulgurites can be placed into one of four major groups, Type I, Type II, Type III, or Type IV, or the minor group Melt Droplet Ful- gurites. Although these broad categories are based on general macroscale features, they reflect common source materials, glass fractions, and sample densities. Type I fulgurites are the most delicate samples, exhibiting thin inner and outer walls, comprised nearly entirely of glass. The members of this group were formed from lightning strikes into beach or dune sand and are the least dense of the major types. Type II fulgurites are friable yet have thick interior and exterior walls, often with frothy glass texture. Their relatively uniform exterior gray color lies in contrast with the range of glass color seen in their interiors. The target material for Type II fulgurites is fine-grained clay or silt. Both sample density and glass 28 fraction lie between those of Type I and Type III fulgurites. Type III fulgurites are neither fragile nor friable, and were formed from lightning strikes into desert soil. Their central voids and glassy portions tend to be very small in comparison to their wall thickness, and they have the highest density of the loose-soil target material samples. Type IV fulgurites are readily recognizable as “rock fulgurites”, with a small area of glassy veneer over an existing larger rock. Also easily identifiable by their morphology are Melt Droplet fulgurites. While they differ in structure from the major groups, they are commonly found in conjunction with Type II fulgurites. Because fulgurite morphology is related to the temperature profile produced during formation, readily identifiable physical characteristics correspond to differences in the target material from which the fulgurite was formed. An examination of sample density can place fulgurites into one of the major groups; calculation of the glass fraction compared to the total thickness can further differentiate samples or clarify classification of boundary cases. Although classification into a single type does not imply identical formation location, the provenance of fulgurites can be interpreted based on morphology. As fulgurite research continues and advances, a wide variety of physical and chemical characteristics should be used to refine and extend the classification scheme presented above. This work serves as a starting point from which to conduct further investigations. Like any taxonomy, the classification in this work can and should evolve with the collection of new data. 29

Figure 2.3: A Type II fulgurite from Western New York. Although broken and incomplete, the outer wall (above)showsthestandardTypeIIgreycolor,whilethe cross-section (below)showsgreen,white,andamberglassyareaswithintheinner wall. 30

Figure 2.4: A Type III fulgurite from Yuma, Arizona. The outer wall reﬂects the appearance of the target material (left). Small voids which run the length of the fulgurite are visible in cross-section (right). 31

Figure 2.5: A Type IV fulgurite from Mohave County, Arizona. The branching portion was created around the pre-existing rock, which shows a glassy veneer near the joining point.

Figure 2.6: Two melt droplet fulgurites, from York County, Pennsylvania (left)and Vernal, Utah (right). 32 ) Glass 3 − 0.50 0.03 σ Sample Density (g cm GSL-01GSL-02 1.12 1.38 0.03 0.00 YAZ-01YAZ-02YAZ-03YAZ-04 2.18YAZ-05 2.45YAZ-06 2.27 2.02 0.05 1.85 0.06 2.77 0.00 0.07 0.04 0.03 Average 2.06 0.05 SWY-01SWY-02SWY-03 1.62 1.76 2.11 0.08 0.06 0.06 LPAZ-01LPAZ-02LPAZ-03LPAZ-04 2.01LPAZ-05 1.73LPAZ-06 3.23 1.96 0.10 2.36 0.05 2.14 0.09 0.05 0.09 0.04 Type III ) Glass 3 − 0.22 0.23 σ Sample Density (g cm YPA-01YPA-02YPA-03YPA-04 1.47YPA-05 1.35YPA-06 1.07 0.96 0.88 1.28 0.72 1.15 0.84 0.84 0.90 0.89 QAZ-01 1.23 n/a ENV-01 1.14 0.56 Average 1.24 0.66 VUT-01VUT-02VUT-03VUT-04 0.83 1.34 1.38 1.69 0.89 0.91 0.67 0.75 CCO-01CCO-02 1.51 1.21 n/a n/a GNC-01GNC-02GNC-03GNC-04 1.02GNC-05 1.69 1.14 0.95 0.33 1.28 0.43 0.30 0.31 0.41 . Type II WNY-01WNY-02WNY-03WNY-04 1.02 1.43 1.23 1.18 0.33 0.84 0.80 n/a 2.2 ) Glass 3 − 0.20 0.01 σ Sample Density (g cm Type I ALG-01ALG-02ALG-03ALG-04 0.70ALG-05 0.76ALG-06 0.44ALG-07 0.68 1.00 ALG-08 0.84 1.00 ALG-09 0.90 1.00 ALG-10 0.62 0.96 ALG-11 0.44 1.00 ALG-12 0.79 1.00 0.99 1.00 0.59 1.00 0.71 1.00 1.00 1.00 1.00 Average 0.70 1.00 NSW-01NSW-02NSW-03 0.41 0.62 1.15 1.00 1.00 1.00 MOR-01 0.63 1.00 Table 2.1: Measuredidentification abbreviations, densities see and Table glass fractions of fulgurite samples, grouped by broad classification types. For 33 YAZ Yuma, Arizona, United States YPA York County, Pennsylvania, United States QAZ Quartzite, Arizona United States VUT Vernal, Utah, United States NSWSWY New South Wales, Australia Sheridan, Wyoming, United States WNY Western New York, United States Designation Location Table 2.2: Abbreviations used for sample identification purposes GSL GreatSaltLake,Utah,UnitedStates ALG Algeria(exactlocationunknown) ENV Eli, Nevada, United States CCO Cha ff ee, Colorado, United States GNC Greensboro, North Carolina, United States MOR Morocco (exact location unknown) LPAZ La Paz, Arizona, United States Designation Location 34 : Upper left :Proposedtypedivisions. Bottom :Boxinset. Upper right Figure 2.7: MeasuredAll densities measured and densities. glass fractions of fulgurite samples, showing broad classification types. 35

CHAPTER 3

FULGURITE PETROLOGY

3.1 Introduction

Though the exterior of fulgurites often retains the appearance of the target material, it is clear the areas closer to the path of the electrical discharge have been thermally altered. The eﬀect of the dramatic temperature spike on the chemistry and morphology of fulgurite grains and groundmass has been explored for several samples (see also Pasek and Block, 2009).

3.2 Samples

3.2.1 Type I Samples

Type I fulgurites (see Chapter 2 for a complete discussion of fulgurite classiﬁcation) are formed from lightning strikes into dry beach or dune sand. These fulgurites often have a large central void and thin, glass-lined walls, though some specimens appear to have collapsed inward during formation, resulting in a clumpy exterior texture with no main central void. Due to the thin walls, members of this group tend to be very fragile, however they are not friable; glassy melt lining the interior adheres grains ﬁrmly to the main mass. They are the least dense of the fulgurites 3 at an average of 0.7 0.2 g cm− (see Table 2.1 and Figure 2.7). ±

Morocco

The Morocco fulgurite has no obvious central void, however a z-axis can be inferred from the elongated shape and radially extending glassy channels. The exterior has an irregular, clumpy shape, and is primarily medium grey in color, with grains of lighter sand. Several protrusions are broken oﬀ, revealing what appears to be 36 the interior texture– highly vesicular dark grey material with darker, amorphous glassy areas on the millimeter scale. Grain size is varied, ranging from 0.1-1.5 mm, corresponding with very ﬁne sand to coarse sand on the Wentworth Scale. Prior to cutting for analysis, MOR-01 was 5 cm in length along the z-axis, with an irregular width of 0.5 to 1 cm in the x-y plane. Prior to cutting, MOR-01 was approximately 7.4 g in mass.

New South Wales

The New South Wales, Australia (NSW-01) fulgurite is thin-walled and cylindrical, with a large central void extending the length of the z-axis (4.5 cm). The outer diameter of the x and y axes are 1 cm and 2 cm, respectively. It is primarily white in color, with dark striations. Exterior grains are ﬁne to medium sand sized (0.2- 0.4 mm), while the interior is completely lined in melt glass, slightly darker in color than the exterior grains. This fulgurite is extremely fragile due to its small size and thin, millimeter thick walls. Prior to preparation for analysis, NSW-01 was 2.5 g in mass.

3.2.2 Type II Samples and Associated Melt Droplets

Type II fulgurites form from lightning strikes into ﬁne-grained material like clay and silt or loess (see 2). These fulgurites are larger than Type I specimens, and can have branches comparable in diameter to the main structure. Many Type II fulgurites have a large central void with thick walls. The external color of Type II fulgurites is dependent on the target material and is usually gray. Type II fulgurites exhibit substantial variation in glass color, even within samples; bottle green, milky light blue, dark blue, orange, and brown have been observed. Fulgurites of this type are often recovered with plant matter or other debris embedded in the outer surface. 3 The average density is 1.2 0.2 g cm− . ± 37

Chafee County, Colorado

The Chafee County, Colorado fulgurite (CCO-01) is, like most fulgurites of this type, a medium gray in exterior color and very friable. It is roughly 4 cm in length, with an external diameter of 1.5 cm at one end. The inner walls of the other end of the sample are branched, however the branch morphology isn’t completely reflected externally; rather than distinct branches, the fulgurite has a widened end with two separate vents leading to the interior. Porous, light green glassy material fills the interior of the fulgurite along the z-axis and continues into the vents as the interior walls branch. The exterior surface of this fulgurite reflects the target material and is primarily clay-sized grains, with a few adhered angular medium to coarse sand-sized grains and small bits of plant matter. This fulgurite is approximately 1.5 cm in length. Prior to preparation, CCO-01 was 15.5 g in mass.

Quartzite, Arizona

This Quartzite, Arizona (QAZ-01) fulgurite, formed from clay-rich target material, is covered in light gray clay-sized grains, with several larger sub-angular inclusions. The glass visible in the central portion of the sample is dark green, while the more distal glass progressed from light green to white. Several very small, sub-mm sized glass-lined vents are visible on the exterior wall. This is a small, friable fulgurite, and an original mass of 2.4 g and length of about 2.5 cm.

Vernal, Utah

The fulgurite sample from Vernal, Utah (VUT-04) is brown-gray in color. Although the exterior grains are largely clay to silt-sized, 1 - 3 mm sub-rounded pebbles are also loosely adhered to the fulgurite. The sample is roughly cylindrical, about 5.5 cm in length and 1 cm in diameter. Examination of the ends and a broken area of the sample reveals a dark green interior glass with small, elongated voids on the order of 1 mm in width. Nearly the entire thickness of the fulgurite walls is comprised of 38 this glass. Prior to preparation, VUT-04 was 12.1 g in mass.

Figure 3.1: The Vernal, Utah Type III fulgurite, VUT-04.

Ameltdropletwasrecoveredinthevicinityofthisfulgurite.Thisglassdroplet consists of two small green elongated spherules, approximately 1 cm in largest diameter, fused together. Several smaller spherical drops < 4mmindiameterareadhered to the outside. The texture is very smooth, though a few sand size particles can be seen on the surface. See Figure 3.2.

Figure 3.2: The Vernal, Utah melt droplet fulgurite. 39

York County, Pennsylvania

The York County, Pennsylvania fulgurite (YPA-03) was also found in proximity to ameltdropletfulgurite,alsoanalyzedanddescribedbelow.Theouterwallsofthe sample are dark gray, mostly clay-sized particles, with infrequent larger, sub-cm sized angular inclusions. The fulgurite has an outer diameter of 1.5 - 2.0 cm, and a small, asymmetrical central void, 0.75 cm in maximum width. This central void ∼ is lined with brown glass, which extends into vents between the interior void and exterior walls. Within the central void the glass is very smooth, however a vesicular glassy texture extends into the fulgurite wall. This sample is extremely friable, and lost nearly 0.5 g of its original 7.1 g mass during handling and preparation for analysis. See Figure 3.3.

Figure 3.3: The York County, Pennsylvania Type II fulgurite, YPA-03.

The melt droplet associated with the York County, Pennsylvania fulgurite is a dark green glass drop approximately 2 cm in length. It is an elongated oval shaped, with two sections having been broken oﬀ. The interior is almost solid glass, with sub- mm sized round trapped gas bubbles. Prior to preparation for microprobe analysis, the York County melt droplet was 2.1 g in mass. 40

3.2.3 Type III Samples

Type III fulgurites include fulgurites formed from desert sands and caliche (see Chapter 2). These fulgurites are generally long, though diameters rarely surpass 3cm.TypeIIIfulguritesdonotalwayshaveadistinctcentralvoid;insomecases several small, millimeter-scale voids will extend parallel to the z-axis. Glass is visible in cross section and is generally gray in color and darker than the target material. Fulgurites of this type rarely have ancillary features beyond the central tube. Partially melted and embedded grains are common surface features of these fulgurites. Type III fulgurites are the most dense of the three classiﬁcations at 3 2.1 0.5 g cm− . ±

La Paz County, Arizona

La Paz County, Arizona fulgurite (LPAZ-04) has a light, sandy-colored exterior, with moderately well sorted medium to coarse sand grains. Several larger granules up to 3mminsizeareobserved.Thisfulguriteiscylindricalinshapewithadiameter of approximately 1 cm. Because LPAZ-04 is a sturdy sample, a 9 cm section was recovered from the strike site. A distinct cylindrical central cavity of 2 mm is visible in the cross-section and extends the length of the z-axis. This central cavity is surrounded by a column of lighter material with smaller, 0.1-0.3 mm vent holes. Prior to preparation for analysis, LPAZ-04 was 30.0 g in mass.

Figure 3.4: The La Paz, Arizona Type III fulgurite, LPAZ-04. 41

Yuma, Arizona

The Yuma, AZ fulgurite (YAZ-04) is a short cylinder, roughly 2.5 cm in length and 1 cm in diameter. Its exterior is well sorted, ﬁne grained sand. Small holes, 0.5 ≤ mm are visible within the cylinder’s cross-section. None of these vessicles appear to penetrate from the center of the fulgurite radially outward– all run parallel to the z-axis. The YAZ-04 fulgurite is 6.3 g in mass.

3.3 Preparation

Fulgurite samples were cut perpendicular to the z-axis to allow for examination of interior materials at varying radial distances from the path of the lightning strike. For samples larger than 1 inch in diameter, a portion of the cross-section was used. In all cases, either ethanol or isopropyl alcohol were used as lubricants to minimize water introduction to the center of the sample. The samples were mounted in epoxy and polished, again using ethanol or isopropyl alcohol as lubricants. It should be noted for future work that fulgurite samples take much longer to polish to the smoothness required for microprobe analysis than other geological samples. Metal grains, hard fused glass, and friable, fragile material are in very close proximity. Grains towards the outer edges are often easily dislodged, creating pits and providing material which can scratch the harder, more easily polished glass and metal. For this work, as is standard, samples were polished with gradually ﬁner material, only proceeding to the next stage when the surface viewed under microscope showed an even polish. This often necessitated polishing for several hours or more at a single stage of coarseness before proceeding. Prior to analysis, samples were carbon coated according to standard sample preparation procedures.

3.4 Analysis

Samples were analyzed using a Cameca SX50 electron microprobe. Point analyses, X-ray maps, and backscattered electron images were obtained. A current of 20nA 42 and an accelerating voltage of 15kV with 20 second peak count times were used.

3.5 Results

3.5.1 Type I Fulgurites

The Type I fulgurites analyzed, MOR-01 and NSW-01, showed very similar composition. Both had a groundmass largely consisting of nearly pure SiO2 glass, with infrequent, small areas of increased Al2O3 or TiO2 abundance, due to isolated il- menite or other melted grains. No signiﬁcant visual or compositional diﬀerence was observed across their thin walls; elemental abundances were nearly uniform regard- less of radial distance from the central void.

Na2O K2O SiO2 MgO Al2O3 FeO TiO2 Total MOR-01 average 0.20 0.58 93.28 0.28 2.91 1.04 0.34 98.90 N=19 σ 0.66 0.26 3.44 0.23 1.84 0.76 0.28 NSW-01 average 0.03 0.04 93.08 0.47 0.01 0.10 4.22 98.00 N=16 σ 0.08 0.08 8.65 0.02 1.16 0.18 8.19 Table 3.1: Microbrobe analysis results for the groundmass of two Type I fulgurites, MOR-01 and NSW-01. Groundmass composition for MOR-01 is averaged from 19 points, that of NSW-01 is averaged from 16 points.

3.5.2 Type II Fulgurites

Chafee County, Colorado

Microprobe examination of the Chafee County, Colorado fulgurite revealed a cracked, pitted, fractured sample. Figure 3.5 shows the voids and brecciation typical to CCO-01. Where point analyses could be completed, two main compositions emerged: a darker, nearly pure SiO2 glass, and a lighter silicate with varying amounts of Al and K (see Table 3.5.2). Glass was found throughout the sample, but was more dominant towards the center. 43

Figure 3.5: Backscatter electron image showing the fractured, brecciated texture of CCO-01. Dark areas are void spaces.

Quartzite, Arizona

The Quartzite, Arizona (QAZ-01) fulgurite showed similar SiO2 glassy areas, as well as medium toned and bright areas with varying amounts of Al and Fe. Unaltered zircon grains were also observed. See Table 3.5.2 for compositions.

Vernal, Utah

Much of the groundmass VUT-04 was fractured and porous in texture. This is particularly true towards the outer rim, where fragments are only very loosely associated with the main body. While the condition of the fulgurite diﬀers with radial distance from the center, the composition does not. Like QAZ-01, backscattered electron images of this fulgurite show darker areas of SiO2 melt glass, and medium- toned and light areas with varying amounts of Al, Fe, and Ca. See Table 3.5.2 for details. Although the melt droplet associated with VUT-04 was not analyzed as a part of this work, the results of microprobe analysis were made available by Matthew Pasek. The glass composition within the melt droplet was nearly identical to that of VUT- 04, however the bulk (non-glass) composition varied slightly, showing marginally less 44

Al2O4,andmoreSiO2 (Table 3.5.2). The measurements also show a larger range of SiO2 and Al2O3 values in the bulk ground mass of melt droplet compared to the bulk groundmass of VUT-04.

York County, Pennsylvania

The YPA-03 fulgurite is primarily comprised of silicate glass with varying amounts of Mg, Fe, and Al. Over 40 point analyses in various areas show almost a continuum of SiO2 and Al2O3 weight percentages in the ground mass (see Figure 3.6). Areas of nearly pure melt glass ( 98.1% SiO )wereobserved,asweredetritalzirconium ∼ 2 silicate grains (ZrSiO4). Towards the distal edges, these zircons were unaltered, however those found near the central void showed an signs of thermal alteration.

Baddeleyite rims (ZrO2) were observed in backscatter electron images (Figure 3.7) and point analyses (Figure 3.8). Baddeleyite is a refractory mineral found areas which have experience high temperatures, such as kimberlite intrusions. Many small metal blebs, from 1-20µ m in diameter, were observed. These blebs are primarily Fe, though some were enriched up to 10% P or more by weight, and ∼ the larger exhibited a P-rich rim shown in Figure 3.9.Someweretoosmalltoobtain reliable point analyses, however there is no evidence to suggest the composition of those differ significantly from their larger counterparts. See Table 3.6 for point analyses of these metal melt drops. In contrast to YPA-03, the associated melt droplet showed a narrow range of Si and Al values within the groundmass. Figure 3.6 illustrates the difference between the groundmass composition of the tube and melt droplet fulgurite, taken from ten measurements in various areas. In addition to the glassy groundmass, this fulgurite has two main components: nearly pure ( 97.5% SiO )meltglassofsimilar ∼ 2 composition to that in YPA-03, and large metal blebs, some exceeding 500 µmin size. They generally consisted of two Fe and Si bearing metal types in a melt-like texture (see Figure 3.10). Microprobe point analyses showed the composition to be consistent with FeSi (fersilicite), Fe5Si3 (xifengite), and Fe3Si (gupeiite). Large metal blebs, on the order of 102µmweresurroundedbysmallermetal 45

Figure 3.6: YPA-03 shows a continuum of SiO2 and Al2O3 values, while the composition in the associated melt droplet is consistent across the groundmass. Oxides comprising < 5% are not reﬂected above. drops, many sub-µminsize.Thesesmallerdropsdidnotshowthevariationin composition seen in the larger blebs, however many were too small to obtain point analyses. 46

Figure 3.7: Backscatter electron image of typical unaltered detrital zircon grain from the exterior portion of YPA-03 (left)andzirconwithbaddeleyiterimfoundcloser to the center of the sample (right).

Na2O K2O SiO2 MgO Al2O3 FeO TiO2 Total Glass avg 0.02 0.03 99.80 0.01 0.08 0.03 0.01 100.02 N=22 σ 0.04 0.07 0.47 0.01 0.10 0.05 0.02 Al-rich areas avg 2.02 6.45 55.15 2.63 20.91 11.00 0.97 100.09 N=50 σ 0.54 1.78 12.70 0.76 4.34 19.12 0.55

Table 3.2: Microbrobe analysis results for CCO-01. Glass composition is averaged from 22 points, Al-rich area composition is averaged from 50 points. 47

Na2O K2O SiO2 MgO Al2O3 FeO TiO2 Total light avg 2.21 2.77 47.60 1.78 12.87 8.15 3.48 98.00 N=10 σ 0.28 1.08 3.34 2.20 1.81 2.66 3.17 medium avg 2.29 4.41 59.95 2.71 17.29 4.73 0.85 98.58 N=13 σ 1.04 1.81 2.32 1.04 2.51 1.64 0.36 dark avg 0.03 0.04 97.99 0.00 0.06 0.09 0.02 98.26 N=12 σ 0.02 0.04 0.56 0.01 0.01 0.04 0.03 Table 3.3: Microbrobe analysis results for QAZ-01, showing three compositionally diﬀerent areas based on backscattered electron image brightness. Glass (dark) composition is averaged from 12 points, medium-toned area from 13 points, and light area from 10 points. Note that oxides comprising < 0.2% in each area are not included in this table.

Na2O K2O SiO2 MgO Al2O3 FeO TiO2 CaO Total light avg 4.02 2.46 57.62 4.45 15.52 6.01 1.16 7.57 98.85 N=12 σ 0.68 0.51 3.96 1.37 2.05 2.06 1.01 2.36 medium avg 4.54 3.26 62.62 3.00 16.43 4.00 0.57 5.45 99.90 N=8 σ 0.61 0.26 3.18 0.78 1.14 5.45 0.07 1.34 dark avg 0.03 0.01 99.39 BDL 0.04 0.10 0.02 0.02 99.61 N=12 σ 0.04 0.01 0.59 BDL 0.05 0.02 0.03 0.01

Table 3.4: Microbrobe analysis results for VUT-04, showing three compositionally diﬀerent areas based on backscattered electron image brightness. Glass (dark) composition is averaged from 12 points, medium-toned area from 8 points, and light area from 12 points. Note that oxides comprising < 0.2% in each area are not included in this table. 48

Na2O K2O SiO2 MgO Al2O3 FeO TiO2 CaO Total bulk avg 3.74 2.92 69.58 2.58 13.91 3.69 0.51 5.20 102.13 N=43 σ 0.34 2.92 1.46 0.19 0.48 0.26 0.05 0.36 glass avg 0.08 0.12 99.39 BDL 0.19 0.05 0.03 0.01 99.87 N=8 σ 0.06 0.07 0.36 BDL 0.32 0.04 0.02 0.01 Table 3.5: Microprobe analysis results for the melt droplet associated with VUT-04 shows nearly identical glass composition, but varying amounts of Al2O4 and SiO2 in the bulk groundmass. Bulk composition is averaged from 43 points and glass composition is averaged from 8 points.

Figure 3.8: Point analyses taken across four detrital zircon grains show the lower O and Si concentrations in baddeleyite rims seen in Figure 3.7. 49

Figure 3.9: Low resolution fast scan (left)andbackscatteredelectronimage(right) of larger metal blebs in YPA-03. Fe is red, P is blue.

Individual drop analyses Point Fe P Ni Si Total 1 84.21 13.45 0.63 0.051 98.33 2 86.89 10.81 0.47 0.06 98.23 3 96.20 2.23 0.03 0.08 98.54 4 93.46 3.67 0.19 0.05 97.38 5 86.51 9.39 0.15 1.45 97.50 6 95.08 2.06 0.14 0.12 97.40

Line scan across a single metal drop Point Fe P Ni Si Total 7a 86.16 9.65 0.19 0.05 96.05 7b 91.87 3.02 0.18 0.05 95.11 7c 94.54 2.76 0.18 0.05 97.53 7d 86.01 11.15 0.27 0.09 97.52

Table 3.6: Results of point analyses of metal blebs in YPA-03. 50 ); compositions in the bottom center inset correspond medium ). dark ,xifengite( 3 Si 5 ) and Fe light )andFeSi,fersilicite( medium Si, gupeiite ( 3 ,xifengite( 3 Si 5 Figure 3.10: These large metalcorrespond blebs to show Fe metals of varyingto Fe Fe and Si composition. Compositions in the top center inset 51

3.5.3 Type III Fulgurites

La Paz County, Arizona

The composition of the LPAZ-04 fulgurite reﬂects its target material; it is primarily calcite, though it has areas of nearly pure quartz melt. Twelve point analyses from diﬀerent parts of the fulgurite show this quartz melt to be an average of 99.39% SiO2. Measurements of the calcite groundmass returned very low totals due to the inability of the electron microprobe to measure C. Little, if any compositional variation is seen across the diameter of the sample. See Figure 3.11 shows a backscattered electron image of a portion of LPAZ-04 representative of the entire fulgurite.

Figure 3.11: Backscattered electron image shows the calcite and silicate glass composition of LPAZ-04.

Yuma, Arizona

The YAZ-04 fulgurite is largely calcite, with areas of nearly pure quartz melt up to 100 µminwidth.Barite(BaSO4), hematite (Fe2O3), and zircon (ZrSiO4)grains were observed within the calcite matrix. X-ray maps indicate P is dispersed in low concentrations throughout the fulgurite, however one area was identiﬁed containing higher abundances. This region, seen in Figure 3.12,appearstobeapartiallymelted 52 calcite grain with a mottled, lighter material roughly 5-10 µminwidthconsistingof P ( 10-15 wt%), Si ( 2-4 wt%), and Ca ( 31-36 wt%) interspersed. As color variations in the P-rich mottled area were on the micron scale, point analyses could not be used to determine the composition of individual areas, however energy dispersive x-ray spectroscopy (EDS) measurements indicate the brighter areas are associated with higher P.

Figure 3.12: Backscattered electron image shows compositional diﬀerences across a melted calcite grain in YAZ-04.

3.6 Discussion

Nine fulgurites were prepared and analyzed according to standard electron microprobe procedures. These fulgurites represented the range of target material compositions; two were Type I, four were Type II with one associated melt droplet, and two were Type III. Upon microprobe analysis, Type I fulgurites showed very little variation in composition. As can be inferred from their appearance, they were 53 largely pure glass. Isolated small grains of other compositions were infrequent. No radial compositional variation was observed.

3.6.1 Detrital Zircons

While detrital zircons were found in many of the fulgurites examined, those in YPA- 03 showed an interesting feature. Towards the center of the fulgurite, detrital zircons were surrounded by baddeleyite (ZrO2) rims. These rims are a result of the extreme temperatures experienced by grain embedded in the target material, in this case clay, during fulgurite formation. Furthermore, the temperatures attained in the center of this fulgurite, or in the localized areas examined, must have been greater than those attained in other fulgurites not showing baddeleyite-rimmed zircon grains.

3.6.2 Metal Grains

Type II fulgurites showed the greatest compositional variation within each sample. One in particular, YPA-03, showed a variety of interesting features. Small reduced Fe-metal melt blebs were observed, some exhibiting a P-rich rim. Additionally, the melt droplet associated with YPA-03 showed much larger metal grains, of reduced Fe-Si metal. Weight percentages obtained from point analyses correspond to Fe3Si, Fe5Si3,andFe5Si3.Previously,Fe-SiandFe-Ti-Simetalshave been observed by Sheﬀer (2007) and Essene and Fischer (1986).

3.6.3 Melt Droplets

From the analysis of two melt droplets associated with Type II fulgurites, it is clear that the melt droplets represent a limited portion of the original target material. Their composition is restricted compared to the associated fulgurite, with narrow ranges of SiO2 and Al2O3 values. The composition of glass grains are nearly identical, however. Additionally, one melt droplet, that associated with YPA-03, showed higher metal content, and much larger metal blebs, than seen in the fulgurite itself. 54

CHAPTER 4

FULGURITES IN THE PLANETARY PROCESS CONTEXT

4.1 Introduction

Although fulgurites themselves are small in scale, they reﬂect a highly energetic formation process stemming from a phenomenon common on early Earth, and possible on other terrestrial planets. Furthermore, because lightning strikes can be concentrated in small areas due to atmospheric circulatory patterns or topography, fulgurite production may be a highly inﬂuential process in localized areas. Due to their potential for high energies and chemical alteration, lightning strikes and the materials they produce must be considered in the context of other planetary environments.

4.2 Chemical Reduction in Planetary Materials

Reduced materials, including Fe, Fe-Si and Fe-Si-Ti metals have been observed within fulgurites. Other planetary materials, originating both on Earth in the case of tektites, and in the solar nebula, in the case of enstatite chondrites, likewise exhibit characteristics stemming from formation under reducing conditions. Tektites are thought to be formed from terrestrial surface material heated by meteorite or asteroid impact and quickly cooled. They are found dispersed across large areas, such as the Ivory Coast and Australian strewn fields. Because of their low water content, 0.002-0.02 wt%, tektites were believed by some to have formed ∼ on the Moon, it has been shown that the thermodynamic history of tektites is violent enough to drive water out of the target material (Melosh, 1998; Sheffer 2007; and many others). Mössbauer measurements of Fe3+/Fe2+ measurements show extreme reduction in a variety of tektites, with ratios of 0.00-0.12 (Fudali et al., 1987; Rossano 55 et al., 1999). Modeling of isentropic cooling shows most, if not all Fe3+ in the starting material is reduced to Fe2+ at a variety of modeled entropies (Sheffer, 2007). Lunar regolith agglutinates are millimeter-sized aggregates of minerals and glasses formed from micrometeorite impacts to the lunar surface. These materials similarly show reduction, however because most Fe on the lunar surface is in the 2+ oxidation state, Fe within these materials is reduced to Fe metal. Keller and McKay (1997) found rims around agglutinate grains containing Fe metal and Fe- sulfides. Physical separation of impact melt and vapor is thought to be responsible for the iron reduction seen in these materials (Sheffer, 2007). Other notable planetary materials exhibiting reduction are enstatite chondrites. Enstatite chondrite meteorites are divided into two groups, EH and EL, the former more reduced. The EH group also exhibits higher Si within Fe,Ni metal (2-3 wt% versus <1 wt% in EL). As the name implies, enstatite MgSiO3 is the dominant silicate mineral found in these meteorites. Additionally, less common phases such as oldhamite (CaS), osbornite (TiN), and perryite [(Ni,Fe)x(Si,P)] are observed. It is clear that enstatite chondrites formed under highly reducing nebular conditions (Keil, 1968, and others). Because non-anthropogenic reduced iron metal is rarely seen on the Earth’s surface, processes which produce or deliver this material can be important to localized geochemistry. The presence of iron metal can likewise be considered an indicator of one of several processes. Fulgurite formation should be considered along with tektite, lunar regolith agglutinate, and enstatite chondrite formation as a planetary process relevant to chemical reduction.

4.3 Lightning on Early Earth

Lightning has long been invoked in discussions of the formation of organic compounds, and consequently life, on early Earth (most notably Miller, 1953). Glycine, alanine, and other amino acids were produced from inorganic constituents subject to a simulated lightning discharge within a reducing environment. More recently, 56 the role of lightning in nitrogen ﬁxation has been investigated. Lightning is one of the few abiotic phenomena capable of breaking the nitrogen-nitrogen triple bond, allowing for recombination with O2 or other atmospheric gases (Desch et al., 2002), usually to form NOx compounds. It has been proposed to be the primary source of activated nitrogen compounds on the surface of the early earth (Navarro-Gonzales et al., 2001). Although materials investigated through microprobe analyses in this work do not follow nitrogen in atmophilic tendencies or bonding structure, the highly energetic nature of lightning can cause distinct chemical changes to occur in the target soil. Speciﬁcally, reduced P has been previously attributed to have played a role in the origin of life due to its increased solubility over phosphate (Gulick 1955, Schwartz 2006), and increased reactivity (de Graaf and Schwartz 2005, Pasek et al., 2008). Lightning activity on the early Earth could produce biologically favorable materials both in the atmosphere and in the soil.

4.4 Fulgurite Formation on Planetary Surfaces

Although no fulgurites have been identiﬁed on the surfaces of other planets, their formation is not necessarily limited to the Earth environment. Gibbard (1996) describes the chain of events necessary for lightning production:

1. Convection of a condensable material to an altitude suitable for condensation

2. Appropriate temperature and composition of particles to allow for exchange of charge

3. Particles of suﬃcient size to exchange the necessary amount of charge, and to avoid electrostatic levitation

4. Number of particles suﬃcient to reach electric ﬁeld breakdown before dissipation

5. Conductivity low enough within the surrounding gas to allow for signiﬁcant charge build-up before discharge 57

Non-terrestrial lightning has been most notably observed within the atmospheres of Jupiter, by Voyager plasma wave and radio emission detection (Gurnett et al., 1979, and others), and Galileo Solid State Imager (SSI) (Little et al., 1999, and others), and Saturn, through Saturn Electrostatic Discharges (SEDs) detected via Cassini/RPWS (Radio and Plasma Wave Science Instrument) (Fischer et al., 2006, and others). Because these gas giants lack an actual “surface” to strike, their relevance to fulgurite formation is limited to the demonstration of lightning detection methods.

4.4.1 Venus

Lightning detection at Venus has been a controversial; many studies present results supporting its existence, while other searches have failed to produce evidence.

Clouds are thought to be composed of H2SO4 droplets, which like water are polar and can be charged to produce the electric potential required for lightning discharge. Optical detection, though problematic because of the ease with which the light can be obscured in the atmosphere, has been reported from Venera 9 (Krasnopolsky, 1980) and Earth-based telescopes (Hansell, Wells, and Hunten, 1995). Radio signals can be used to detect the signatures of lightning more reliably. Pioneer Venus detected whistler-mode signals of the expected polarization when the magnetic ﬁeld was favorably positioned; where the magnetic ﬁeld dipped into the atmosphere, these signals were allowed to reach the spacecraft. Recent analysis of magnetometer data from Venus Express also shows whistler-mode electromagnetic signals consistent with atmospheric lightning (Russell et al., 2007). Because clouds are nearly 50 km above the surface of Venus, cloud-to-ground lightning is not predicted (Russell, 2010). The greatest possibility for lightning of this type is over high altitude areas, where the cloud deck is closest to the surface. Maxwell Montes, at 11 km high, is the most likely area for cloud-to-ground strikes based on altitude. It it unclear what the energy of such lightning discharges would be, however fulgurites formed from basaltic target material on Earth can give clues to the geochemistry that could result. 58

AfulguriteanditsrelatedunalteredtargetmaterialfromClineButte,Oregon were analyzed using M¨ossbauer spectroscopy by Sheﬀer (2007). This fulgurite was the only sample from basaltic target material, and the only sample to show oxidation compared to its unaltered target— the fulgurite glass contained 30% Fe3+ while the target material contained only 18%. It is not clear, however, that this composition is representative of basaltic fulgurites or even the sample in question, as large Fe-oxide grains may have been mistakenly included in the glass analysis.

4.4.2 Mars

Lightning has been theorized to exist in the martian atmosphere. Unlike the terrestrial and Venusian atmospheres, the polar molecules H2OandH2SO4 are not available to serve as charge separating particles, rather suspended dust is the most likely participant. Collisions between dust and sand particles, and those particles and the surface have been shown to transfer charge (Renno and Kok, 2008; Kok and Renno, 2008). Additionally, modeling shows that the resulting electrical discharges could occur of on a variety of scales. Discharges between grains (Renno et al., 2003), on the scale of the saltation layer (Kok and Renno, 2009), and on the scale of a large dust storm (Renno et al., 2004) have been theorized. Additionally, large-scale electrostatic fields have been invoked as oxidant producers near the surface (Atreya et al., 2006). Non-thermal radiation peaks detected consistent with large-scale electric discharge have been reported in a martian dust storm (Ruf et al., 2009). This is consistent with electrical field measurements produced by triboelectric interactions of particles in terrestrial dust devils and dust storms. Due to the low density of the atmosphere (> 10 mbar), it is unclear whether this “lightning” discharge would follow similar forking paths to those observed on Earth. It is also unclear whether lightning would strike the surface, rather than discharging in the atmosphere. Some constraints on potential fulgurite formation on Mars can be inferred from in situ soil condition measurements. The Thermal and Electrical Conductivity Probe (TECP) on the Phoenix Mars Lander measured conductivity across two small probes 59 inserted into the soil. During the early part of the mission, in the beginning of martian summer, conductivity measurements were consistent with a fully open circuit; the soil demonstrated no charge carriers on the scale of the probe. Interestingly, during late summer, nighttime increases in dielectric permittivity were observed, consistent with accumulation of water molecules (Smith et al., 2009). Because larger fulgurites are formed when the thickness of the non-conducting layer of the target material is greater, there is potential for fulgurite formation in areas with deeper ice tables, or in polar regions during summer when ice is not accumulating. Because southern spring and summer are particularly active times for regional and global dust events, if martian lightning could strike the ground, fulgurite formation may be most likely in the southern mid-latitudes. Because of the activity of the martian atmosphere and the variety of composition of potential target material, the possibility for martian fulgurites is intriguing. However, until a more thorough study of generated electric potential in the atmosphere, electrical discharge area, and induced regolith temperatures is conducted, the possibility for fulgurite formation and its effect on the target material remains highly speculative.

4.4.3 Titan

Due to its thick atmosphere and rich hydrocarbon composition, Titan has been a popular subject for the possibility of lightning. However, the optically thick cloud and haze layers prevent lightning detection at optical wavelengths. The planetary radioastronomy instrument aboard Voyager I was used to search for evidence of lightning, however no signals were detected, indicating an upper energy limit of 106 J ∼ per ﬂash (Desch and Kaiser, 1990). Further investigations, using Cassini/RPWS also failed to detect lightning ﬂash-induced radio emissions (Fischer et al., 2007). The Huygens Probe also carried instruments capable of detecting the signatures of lightning. The Permittivity, Waves, and Altimetry (PWA) analyzer, a part of the Huygens Atmospheric Structure Instrument (HASI) measured extremely-low and very-low (ELF-VLF) frequencies (Grard et al., 1995; Fulchignoni et al., 2002). 60

Apeakinelectronconcentrationandconductivitywasobservedatanaltitudeof 63 km, which has been linked to electron attachment in aerosol layers in that portion of the atmosphere (Hamelin et al., 2007). Fulchignoni et al.(2005)showthat narrow-band emissions might be the result of lightning activity between this boundary and the surface, however they caution that such an interpretation is simply allowed by, rather than suggested by, these data. Although lightning has not been detected in Titan’s atmosphere, lightning generated from movement of material following an impact is a possibility. Additionally, the similarity of Titan’s composition to that of early Earth presents a potential model for lightning’s role in the organization of pre-biotic molecules. Cloud-to- ground lightning strikes on Titan could provide energy for recombination of hydrocarbons, though it is unclear what sort of path current would take once in contact with the surface material. If the struck surface is ice, as it would be in Titan’s current environment, electrical charge would dissipate, with heat serving to melt, and possibly vaporize the target.

4.5 Summary

Although fulgurites are small, with diameters on the order of centimeters and lengths ranging from centimeters to meters, their formation process is highly energetic. Lightning itself has been shown to break the N-N triple bond and provide reduced Ptothesoil.Sincelightingactioncanbespatiallyconcentratedbyweatherpat- terns and surface topography, the chemical changes derived from strikes may be particularly important to certain localized areas. Although lightning is ubiquitous on Earth, its existence on other terrestrial planets remains, at the moment, controversial. Venus, Mars, and Titan are the most likely terrestrial candidates for lightning strikes, however it is unknown what conditions lightning would produce on these bodies. 61

CHAPTER 5

SUMMARY AND FUTURE DIRECTIONS

An effort was made with this work to expand the body of knowledge of fulgurites, their physical and mineralogical attributes, and advance their consideration as a planetary process with potential importance to the early Earth, and localized areas of the surface of this planet and others. Awiderangeoffulguritesfromavarietyoftargetareaswereexaminedfor morphological similarities that could lead to a coherent taxonomy. Extensive measurements were taken, the most relevant of which are presented in Chapter 2,along with the resultant classification system. Like any broad classifications, the ones discussed in this work can and should be expanded upon and refined as new data are collected. Representative samples from each type were examined using electron microprobe point analyses and x-ray mapping. Type I fulgurites showed little diversity in composition and were nearly pure SiO2 glass. Type II fulgurites exhibited the largest range of composition. While unaltered detrital zircons were found towards the distal edges of samples, one sample showed alteration to baddeleyite in those towards the center of the fulgurite. Metal blebs largely composed of Fe, with P-rich rims were documented. Larger metals in an associated melt droplet were found to have the formulae FeSi (fersilicite), Fe5Si3 (xifengite), and Fe3Si (gupeiite). A comparison of a fulgurite sample and its associated melt droplet showed that the tube fulgurite displays a much wider range of compositions with respect to Si and Al content than the droplet. The composition of Type III fulgurites reflected their calcite soil origins, and also included SiO2 melt. Although fulgurites are small in scale, their relevance to planetary processes should be considered. Fulgurite formation is an energetic processes with an energy density within an order of magnitude of that of impacts. They have only been found 62 on Earth, however their formation is not necessarily limited to the terrestrial environment. The potential for lighting on Venus, Mars, and Titan presents interesting possibilities for alteration of surface material on those bodies. Future work should continue to place individual fulgurite samples within the broader context of the range of fulgurite types. Type II fulgurites should continue to be investigated for unusual metal compositions and reduced chemistry. An effort to analyze fulgurites along with their associated melt droplets can give additional clues to the formation and rapid cooling processes. Although physical investigation is a powerful tool for elucidating the fulgurite formation process, it should be coupled with further chemical and thermodynamic modeling. Additionally, because lightning conditions are poorly constrained and vary from strike to strike, experimental analyses should be done to provide some constraints for chemical modeling. The effect of the unusual fulgurite chemistry on the biota of the surrounding target material is an interesting avenue for investigation of microbial response to the influx of the more biologically available reduced elements, like P. Samples taken in areas of high fulgurite concentration could show what effect, if any, this nutrient influx has on typical soil microbiology. Characterizing this response may give clues to the importance of lightning in oligotrophic soils, on the early Earth, and in non- terrestrial micro-environments. 63

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