Searching Asteroid Impact Debris for Ancient Continents

A Thesis Presented

by

Jacob Ott

to

The Department of Earth and Planetary Sciences

in partial fulfillment of the requirements

for a degree with honors

of Bachelor of Arts

April, 2020

Harvard College

Abstract

Large asteroid impacts in Earth’s early history excavated enough of the surface

to generate global scale debris clouds. The condensates from some of the earliest

known impacts’ debris clouds, called impact spherules, can be found today in South

Africa’s Kaapvaal craton. Spherules were originally composed of a combination of

the impacting asteroid and the material it excavated, so the chemistry of spherules

provides potential clues to the compositions of the early Earth’s crust and mantle,

which in turn constrain the growth of continental crust and the evolution of plate tec-

tonics. This study builds upon previous bulk chemical analysis of a 3.24 Ga spherule

bed in Kaapvaal craton called S3. Here, we investigate samples of S3 using Laser-

Ablation Inductively-Coupled-Plasma Mass Spectrometry (LA-ICP-MS). This precise

analytical method reveals significant heterogeneity and varying degrees of alteration

throughout S3. Evidence of baritization and silicification is observed; however, the

extent of alteration is limited. Impactor- and target rock-derived chemistries are

distributed unevenly amongst and within spherules and are subject to nugget effects,

with impactor-sourced Ir residing primarily within pockets <100 µm in scale. Based

on rare earth element (REE) trends, target rock material likely consists of oceanic as

well as continental crust. The presence of the latter within the excavation radius of

the S3 asteroid impact supports greater land coverage on the surface of the ancient

Earth than has been previously inferred from the geologic record.

Contents

1 Introduction 4

1.1 When did plate tectonics begin? . . . . . . . . . . . . . . . . . . . . . 4

1.2 Tectonics in igneous geological record . . . . . . . . . . . . . . . . . . 8

1.2.1 Hadean (4.5-4.0 Ga) . . . . . . . . . . . . . . . . . . . . . . . 9

1.2.2 Eoarchean (4.0-3.6 Ga) . . . . . . . . . . . . . . . . . . . . . . 10

1.2.3 Paleoarchean (3.6-3.2 Ga) . . . . . . . . . . . . . . . . . . . . 11

1.2.4 Mesoarchean to Paleoproterozoic (3.2-1.6 Ga) . . . . . . . . . 13

1.3 Modeling tectonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.3.1 Squishy lid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.3.2 Plume lid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.3.3 Plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.4 Spherule beds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

1.4.1 Formation mechanism . . . . . . . . . . . . . . . . . . . . . . 16

1.4.2 Locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

1.4.3 Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

1.4.4 Alteration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

1.4.5 Interbed comparisons . . . . . . . . . . . . . . . . . . . . . . . 20

1.5 S3 spherule bed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

1

1.5.1 Forming Impact . . . . . . . . . . . . . . . . . . . . . . . . . . 21

1.5.2 Mineralogy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

1.5.3 Insights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

1.5.4 Potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

2 Methods 29

2.1 Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

2.2 Data collection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

2.3 Data processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

2.3.1 Background removal . . . . . . . . . . . . . . . . . . . . . . . 33

2.3.2 Instrument drift . . . . . . . . . . . . . . . . . . . . . . . . . . 34

2.3.3 Use of standards . . . . . . . . . . . . . . . . . . . . . . . . . 35

3 Results and Discussion 37

3.1 Stratigraphic variation . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3.1.1 E27B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3.1.2 F5A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.1.3 E22B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

3.2 Alteration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

3.2.1 Proxies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

3.2.2 Silicification . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

3.2.3 Baritization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

3.2.4 Fuchsitization . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

3.2.5 Carbonatization . . . . . . . . . . . . . . . . . . . . . . . . . . 44

3.2.6 Chloritization . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

3.2.7 Sericitization . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

2

3.2.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3.3 Two target crust compositions . . . . . . . . . . . . . . . . . . . . . . 47

3.4 Mantle component . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

3.5 Impactor component . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

4 Conclusions 55

5 Acknowledgments 56

6 References 57

3

Introduction

When did plate tectonics begin?

Earth’s outermost layer is a brittle surface broken into plates (Figure 1.1), together

known as the lithosphere. The plates are dynamic: driven by convection of the

viscous mantle below, continuously created at mid-ocean ridges and destroyed at

subduction zones (Kent C. Condie 1997). This evolving mosaic cycles material

from Earth’s depths through its crust. Plate-bound CO2 and H2O are liberated

when plates sink into the mantle, creating water-saturated magmas that lower the

mantle’s viscosity, enhancing convection and plate motion, before erupting upward

and forming volcanoes. Erupted CO2 and H2O become part of the atmosphere, acting

as greenhouse gases that warm the planet and encourage weathering of its surface,

specifically of silicate rocks. When silicates are weathered, they react with CO2 and

H2O in air and water to produce carbonate rocks. Thus CO2 and H2O become plate-

bound once more (Langmuir and Broecker 2012). The whole process, known as plate

tectonics, cycles biologically important elements and may act as a sort of engine for

life. Figuring out how this engine started running, i.e. what Earth was like before

plate tectonics, is critical to understanding how life began.

Unfortunately, history stands in the way. This is explained well by (TV Gerya

4

2016): “An egg does not look like a chicken. Therefore, even if we look at the chicken

from 1000 different perspectives it will not help us to understand what the egg looked

like. Modern plate tectonic Earth is like the chicken, whereas pre-plate tectonic Earth

is like the egg. It will not be easy to visualize the pre-plate tectonic Earth.”

The proceeding sections assess bodies of research utilizing different modes of

inquiry into the pre-plate tectonic Earth. First is a discussion of traditional methods

such as inspection of the igneous geological record and modeling, followed by an

introduction to a relatively new source of information on crustal evolution: asteroid

impact debris. This line of evidence is the thesis’ ultimate subject of study.

Figure 1.1: Diagram of major tectonic plates. Plate information from (Kent C. Condie1997).

Cratons

Early in its history, Earth was still hot from the violent snowballing of proto-

planets that led to its creation. Earth’s overall heat flow in this time, a period

5

from 2.5 to 4 Ga termed the Archean, was nearly three times higher than that of

the modern Earth and made for a less viscous mantle. This supported the rising

of deep mantle plumes to the surface where they cooled and thickened the crust

(Petit 2010). These thick points of Earth’s surface, called “cratons”, extended up to

hundreds of kilometers into the mantle, rooting future continents. Cratonic regions

are called shields if ancient igneous/metamorphic rocks are exposed on their surface

or platforms if they are covered by relatively young sediments. The continents of

today all have shields or platforms at their centers, because when subduction occurs,

cratons are preserved by their high buoyancy and mechanical strength. It should be

noted that the cratons at the center of today’s continents did not form independently.

They are often associated with old supercontinents which have since broken apart,

dividing their thick roots so that geologic trends of continents often truncate at craton

boundaries (Cawood et al. 2013). Over time, cratons have become the oldest portions

of Earth’s crust.

Modern crust composition

The early to middle 20th century saw a series of studies that resulted in the

comprehensive estimation of the average composition of Earth’s crust (Clarke and

Washington 1924, Goldschmidt 1954, Poldervaart 1955, S. Taylor 1964, Ronov and

Yaroshevsky 1969) and the later half of the century saw those studies integrated with

knowledge of the mantle (S. Hart 1969, Stuart Ross Taylor and Scott M. McLennan

1995, Roberta L Rudnick 1995, Roberta L Rudnick and Fountain 1995, McLennan

and S. Taylor 1996, Rudnick and Gao 2003). The product was a new understanding

that the mantle was melted into mafic magmas that would rise and either crystallize

into mafic crust or undergo fractional crystallization and/or assimilation with existing

6

mafic crust to produce more or less felsic compositions. The net result of this process

has been continental crust with an andesitic average composition (∼60% SiO2).

Crust history

Ancient igneous activity produced minerals called zircons (ZrSiO4) that, for crys-

tallographic reasons, make exceptional samples of study for crustal evolution. First,

zircons tend to include portions of apatite and biotite that can be used to infer

the composition of the zircons’ original host magmas, providing hints regarding the

tectonic context of their origin (Jennings et al. 2011). Zircons can also be precisely

U-Pb dated. If a zircon is found in sedimentary detritus, dated, and its host magma

determined to have formed under a plate-tectonics regime (if their host magmas are

determined to have been formed by subduction zone style partial melting), then the

zircon’s crystallization age provides a lower bound on the timing of plate-tectonics

onset (Cawood et al. 2013).

Zircons can also be studied for Hf and O isotopes. Newly generated crust, as

found in island arcs, has depleted εHf (176Hf/177Hf normalized to chondrite values)

relative to old crust generated from the depleted mantle (DePaolo 1981, Vervoort and

Blichert-Toft 1999, Dhuime, C. Hawkesworth, et al. 2011), and so zircons’ Hf isotopic

ratios can be used to model ages (Dhuime, C. Hawkesworth, et al. 2011). Hf-derived

ages can then be compared to U-Pb derived crystallization ages, with discrepancies

indicating crystallization from reworked crustal magmas. Model ages of zircons can

next be combined with O isotope data. Rocks that have experienced a sedimentary

cycle exhibit high δ18O because their 16O is reduced by surficial processes such as

evaporation, i.e. high δ18O in igneous minerals indicates their parent magmas formed

by reworking of pre-existing crustal material rather than mantle extraction (Eiler

7

et al. 2000). In applying all these angles of study, one must be careful to consider

sedimentary processes capable of resetting Hf and O isotopes after crystallization.

O and Hf isotope studies were performed on a suite of zircons by Dhuime, Chris

J Hawkesworth, et al. 2012 so they could effectively be classified as originating in

reworked or new crust. The suite of zircon ages was then used to chart changes in

proportions of reworked and new crust over Earth’s history, where a high proportion of

new crust indicated a higher rate of crust formation over a given time period (Cawood

et al. 2013). A steep drop off in continental crust formation around 3 Ga is inferred

along with the possibility that by this time 60-70% of the modern crust volume had

been extracted. However, the potential for resetting of O and Hf by sedimentary

processes makes this figure ambiguous and leaves hidden this highly sought secret of

Earth’s surface history.

Tectonics in igneous geological record

Igneous rocks carry a fingerprint of the tectonic regime under which they were

formed. But because the ancient geological record has been severely altered by plate

recycling, associations within it are difficult to establish. Studies have considered a

variety of local regimes responsible for crust formation on early Earth, from plume-

dominated volcanic systems to an enigmatic process known as “drip tectonics”. Al-

ternatives to plate tectonics consistently appear to be possible but not to the point

of certainty, and as a result few constraints on the onset of plate tectonics have

been established. Relevant results of the igneous geological record’s inspection are

summarized below, broken down by geologic epoch.

8

Hadean (4.5-4.0 Ga)

The oldest terrestrial rocks on record are of a class known as tonalite-trondhjemite-

granodiorite (TTG), formed by partial melting of hydrated mafic rocks such as those

in oceanic crust. Gneisses of >4 Ga are known to cover nearly 40 km2 of Earth’s

surface, and there is data to suggest gneisses as old as 4.06 Ga exist. The oldest

TTGs are 4.03 Ga gneisses in Canada’s Slave craton (Bowring and Williams 1999).

The magmas responsible for these TTGs have been determined to be tonalitic (felsic)

in composition, produced by partial melting of pre-existing, less-differentiated crust.

But while partial melting is a trademark of plate tectonics, the pressures required

to produce the observed tonalitic compositions are less than that seen in subduction

zones, and so the Canadian gneisses could predate crust formation through plate

tectonics.

A mantle plume-dominated volcanic system is proposed by (Oliver Nebel et al.

2014) as an alternative to early crust formation by subduction. They analyze Archean

komatiites from Pilbara craton (Australia) using Hf and Fe isotopes. The komatiites

are found to have high εHf and light Fe isotope enrichment indicative of an old, melt-

depleted mantle source. The Hf isotope signature of Pilbara komatiites are also found

to be in line with that of Hadean detrital zircons, suggesting that the komatiites are

linked to a Hadean process. The proposed way in which Archean crust becomes linked

to the Hadean is through mantle plumes: hot mantle plumes accumulate at the base

of the Hadean crust, which in parts becomes destabilized over time and sinks into

the mantle, only to return again in a plume. This recycling through plume activity

may also be responsible for evidence of local, plate-tectonic like crust recycling in a

pre-plate tectonic Hadean.

9

Eoarchean (4.0-3.6 Ga)

In 2018, slightly younger TTGs (∼3.7 Ga) than those of Slave craton (4.03 Ga) were

discovered in Tarim craton in Northwestern China (Ge et al. 2018). Thermodynamic

modeling and trace element concentrations indicate these rocks formed under pres-

sures of 1.8 to 1.9 GPa and at temperatures of 800-830°C. This range of pressures and

temperatures represents a series of formation depths from which a geothermal gradient

of 400-450°C GPa−1 was calculated. This geothermal gradient is too cold to represent

normal surface conditions and is interpreted to represent conditions of a subduction

zone, where seawater acts as a coolant and depresses temperature variations with

depth.

However, evidence of subduction within one locale does not necessarily support

a global plate tectonic regime (Chris J. Hawkesworth and Brown 2018). If folding

occurs within a region, it is possible for that region’s lithospheric root to thicken and

become unstable. When this happens, the root’s end can be cut off from the rest

of the lithosphere, effectively subducting and turning into what is called a “sinker”

(Figure 1.2). The thickening of the lithospheric root in general resembles the process

of subduction, causing partial melting of hydrated basalt which in turn produces felsic

melts that intrude the overlying crust to form TTG (O. Nebel et al. 2018). O. Nebel

et al. 2018 report that this process occurs in all cratons and is evidenced by dome-like

structures. Such a dome is indeed observed in Tarim craton (Yang and Chen 2018),

suggesting that the region’s TTG is unrelated to the global crust regime at time of

formation.

10

Figure 1.2: Visualization of drip tectonics. Folding of lithosphere (colored region)results in lengthening of a lithospheric root until the root’s end breaks off and sinksinto the mantle.

Paleoarchean (3.6-3.2 Ga)

South Africa’s Kaapvaal craton is home to the Barberton Greenstone Belt (Fig-

ure 1.3), a well-preserved Archean terrane that can be divided into two rock suites:

TTGs aged 3.5-3.2 Ga and granites-monzogranites-sysenites (GMS) aged 3.2-3.1 Ga

(Yearron 2003). Yearron 2003 examine major- and trace- element trends against SiO2

within TTGs, concluding TTGs to be sourced from a heterogeneous set of magmas.

The magmas range in composition from juvenile to more evolved crust, with the

youngest TTGs being sourced from the most evolved. GMSs were generated through

11

upwelling of alkali basaltic magma and subsequent partial melting of the TTG suite.

Van Kranendonk 2011 examine metamorphic assemblages present in the Barberton

Greenstone Belt, finding two assemblages of interest: one corresponding to high-

pressure (P) and moderate-temperature (T) metamorphism found near the edges of

large dome structures (possibly formed by orogenic processes (Yearron 2003)) and

another corresponding to moderate P-T metamorphism found near the cores of the

same domes. This juxtaposition of metamorphic conditions is said to reflect the

sinking of the Barberton Greenstone Belt’s base rock into the mantle. Ultimately,

the picture painted by Yearron 2003 and Van Kranendonk 2011 is one independent

of plate tectonics.

Figure 1.3: Map of Africa. Kaapvaal craton highlighted in yellow. BarbertonGreenstone Belt highlighted in green.

12

Mesoarchean to Paleoproterozoic (3.2-1.6 Ga)

The onset of plate tectonics could be evidenced in a collection of global geochemical

transitions between 3 and 2 Ga according to a recent series of studies (Kent C Condie

2003, Kent C Condie 2005, K. Condie 2015, Kent C Condie et al. 2016). Using

ratios of incompatible elements such as Nb/Th and Zr/Nb, young oceanic basalts

are analyzed in order to characterize their mantle sources’ chemistries. Three mantle

domains are identified: depleted mantle (DM), enriched mantle (EM), and hydrated

mantle (HM), the last of which is seen to increase in prevalence in the basalts studied

over 3.5 to 2 Ga. A continuously increasing fraction of hydrated mantle-sourced

basalt could be caused by the continuous propagation of plate tectonics and the

increase in subduction processes that would result. Kent C Condie et al. 2016 notes

additionally the increased appearance of eclogite in diamonds over the 3-2 Ga period.

Eclogites are reported to form from capture of subcontinental fluids by subduction and

continental collision (Shirey and Richardson 2011), a.k.a. the Wilson cycle of plate

tectonics. Shirey and Richardson 2011 report that all diamonds older than 3.2 Ga

are peridotitc in composition (formed from mantle plumes), while younger diamonds

become increasingly eclogitic. Other geological shifts noted by Kent C Condie et al.

2016 are increases in the frequency of collisional orogenies, appearances of ophiolites

(subaerial oceanic crust that was uplifted by plate tectonic-related thrusting), and

arrival of large igneous provinces. These shifts are each consistent with the cooling of

Earth’s mantle and the resulted strengthening of the lithosphere, but it is reported

that the associations between the geologic records upon which these shifts are based

must be understood in greater detail before concluding if and to what degree these

shifts were caused by a global transition to plate tectonics.

13

Modeling tectonics

With the evolution of computers, many models for early Earth geophysical pro-

cesses have been developed. However, without access to a comprehensive geologic

record–specifically, without a history of deep Earth processes–these models are limited

and often contradict each other. Several categories of models are considered here in

the context of their models preferred tectonic regimes, including models for rock

formation conditions, mantle convection, crust-mantle interaction, and global heat

flow.

Squishy lid

Modeling of TTG formation P-T conditions has been proven feasible by Rozel

et al. 2017. It is inferred by Rozel et al. that the volcanism-, plume-dominated

tectonic processes reported by Fischer and Taras Gerya 2016 for the formation of

TTGs result in too cold of geothermal gradients to create original continental crust

compositions. A “squishy lid” regime like that of Venus (Van Kranendonk 2011)

is preferred, in which cold, strong plates are separated by regions of warmth and

weakness due to intrusive magmatism (Louro Lourenco et al. 2017). In this regime,

plate boundaries are very mobile and dependent on the location of specific magma

intrusions. A squishy lid is supportive of the eclogite formation described by Shirey

and Richardson 2011 and is supportive of the surface velocities necessary for the

non-subduction zone related TTG formation mechanisms inferred by Yearron 2003,

O. Nebel et al. 2018, and Van Kranendonk 2011.

Jain et al. 2018 expand upon the work done by Rozel et al. 2017 using thermo-

chemical mantle convection models. In the models, ratios of intrusive and eruptive

14

magmatism are varied along with temperature and friction coefficients. The result

is model preference of a 1 Gyr period of plume lid tectonics and linear crustal

growth followed by a cubic-root time dependency of crustal growth unaccompanied

by tectonic regime shift. It should be noted that the dome structures characteristic of

cratons are not produced in the model. Regardless, the observation of a sudden shift

in crustal formation rate without any tectonic transition stands in stark contrast to

the studies described above relating halting of crustal growth to the onset of plate

tectonics.

Plume lid

Magmatic-thermomechanical modeling of crust-mantle evolution performed by

Fischer and Taras Gerya 2016 suggests two stages of evolution in the early Earth

(>3.2 Ga). The first is a period of “quiet growth”, in which crust and lithosphere

volumes are enhanced by plume activity. This is followed by a period of “catastrophic

removal” by eclogitic delamination (in which a dense lower lithosphere is replaced by

a buoyant underlying asthenosphere) and subsequent dripping. Models show this

regime as operating on a periodicity of ∼100 Myr and capable of supporting TTG

formation as well as the crust formation rates >3.2 Ga presented by Cawood et al.

2013.

Plates

Hopkins et al. 2008 perform thermobarametric analysis of 4.02–4.19 Ga zircons

from Jack Hills, Australia, finding formation conditions implicative of a heat flow

much lower than the global Hadean average. Regions of similarly reduced heat flow

relative to the global average on the modern Earth are found only near subduction

15

zones, so the study infers the presence of plate boundary interactions. As has been

discussed in the previous section, subduction zone style P-T conditions can be created

locally through crust-mantle interaction without the formation of plate boundaries.

Spherule beds

As TV Gerya 2016 puts it, “An egg does not look like a chicken”. The paucity

of the geologic record when it comes to the old, deep rocks in which past tectonic

regimes would have been recorded is a constant source of frustration for researchers

and the root of persistent ambiguities in the field of crustal evolution. In order to

expand the geologic record, signatures of the Earth’s surface composition must be

sought in creative places. One such place so far examined is the debris of ancient

asteroid impacts known as “spherule beds”.

Formation mechanism

Asteroids that impact Earth excavate material from depth equivalent to several

times the impactor diameter. A world-enveloping vapor plume is generated which

then condenses into spherical droplets and is deposited in a global layer. The remnants

of these debris layers are now referred to as “spherule beds” and are composed of

“spherules”, roughly sand-sized particles bearing correlated levels of extraterrestrial

Cr and Ir from their meteoritic components (G. R. Byerly 2002, Donald R. Lowe

et al. 2003). The chemistry of a spherule bed generally represents a mixture of the

impactor and target material involved in an individual impact event, in addition to

locally sourced detrital input and products of any post-deposition alteration processes.

Because spherules can be found only for impact events large enough to deposit debris

16

in the few Archean terranes that would persist to the modern day, they usually contain

excavated mantle in addition to crust. They provide a geochemical signature covering

a scale as broad and deep as asteroids can excavate.

17

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18

Locations

The world’s oldest spherule beds can be found in the Pilbara (Australia) and

Kaapvaal (South Africa) cratons (Figure 1.3). The Pilbara and Kaapvaal cratons are

among Earth’s best-preserved Archean terranes, and since Archean impact craters

have long since been lost to plate tectonics, spherule beds are the only evidence

of Archean impacts and thus the best place to look for records of Earth’s ancient

geochemistry with the breadth and depth required for charting crustal evolution.

Components

Crust

Presence of a normal mid-ocean ridge basalt (N-MORB) component within spherules

would imply shallow melting of the upper mantle prior to the impact (Krull-Davatzes,

G. R. Byerly, et al. 2014), potentially indicating some form of mid-ocean ridge spread-

ing and plate tectonics. On the other hand, presence of primitive crust would provide

no indication of plate tectonics processes. Other processes, such as plume activity

and slower spreading centers could result in an enriched MORB or alkaline basalt

crust component, respectively (S. Ross Taylor and Scott McLennan 2008). Trace

elements are often used to distinguish these compositions, and so the determination

of the specific crust composition contained within spherules is highly dependent on

the trace element sensitivity of their chemical analysis. It is also dependent on spatial

resolution, as spherules possess a variety of mineralogies (Krull-Davatzes, D. R. Lowe,

et al. 2012), each with different trace element compatibilities, such that crustal signal

strength is potentially highest in individual spherules, the resolution of which requires

sub-mm precision.

19

Mantle

If spherules contain mantle material depleted in highly incompatible elements (a

composition referred to as depleted MORB mantle, or DMM), those elements must

have been extracted through crust formation. As a result, DMM presence in spherules

indicates continental crust extraction prior to an impact even if the spherules do not

directly contain continental crust (Krull-Davatzes, G. R. Byerly, et al. 2014).

Alteration

Several processes control a spherule bed composition’s deviation from original

impactor and target concentrations through its formation and long after. Siderophile,

chalcophile, and volatile lithophile elements are mobilized at the moment of impact,

and the concentrations of these elements can be higher or lower in the final spherule

deposit than in the starting plume material, but starting levels of refractory lithophile

elements and Cr are retained (Norman and Mittlefehldt 2002). Starting Ir values are

also retained, although Ir is non-uniformly distributed and is subject to “nugget

effects” (Goderis et al. 2013), complicating the process of extracting a meteoritic

component from any non-bulk analysis of a spherule bed. Following vapor condensa-

tion and deposition, carbonatization and authigenic phosphate formation are known

to affect spherules (Krull-Davatzes, D. R. Lowe, et al. 2012), as is input of local

detritus.

Interbed comparisons

A single spherule bed can on its own give quite a lot of information about the

geochemical state of the Earth at the time of impact, but the study of multiple

20

spherule beds–representing independent impact events of the same geologic epoch–has

even greater potential. Consider a scenario in which ten spherule beds are discovered

relating to impacts from the period 3.3-3.2 Ga. In all these fictional spherule beds

there is observed to be DMM; in three reside some portion of continental crust; and in

the other seven reside only oceanic crust. This fictional result would imply a slightly

reduced land fraction in the mid-Archean, perhaps consistent with the figure of 60-

70% extraction of current crust volume by 3 Ga reported by Cawood et al. 2013.

Study of multiple independent spherule beds offers statistical snapshots of crustal

evolution that can be used to supplement the effort to understand the Archean epoch

through models and proxies.

S3 spherule bed

There exists a spherule bed in Kaapvaal craton, South Africa termed “S3” with

multiple accessible outcrops that has been the basis of most spherule research to date.

S3 is composed nearly entirely of spherules’ whose primary textures have been well-

preserved, providing for those spherules a pseudomorphic limit on the effects of post-

depositional alteration (G. R. Byerly 2002, Donald R. Lowe et al. 2003) and enabling

more precise determination of original impactor and target rock compositions.

Forming Impact

The thickness of the S3 bed at the time of its deposition is conservatively estimated

at 30 cm (Krull-Davatzes 2006), and considering the packing geometry within spherule

deposits (Jaeger and Nagel 1992) this must have been produced by an impactor about

30 km in diameter. Such an impactor would produce a crater of diameter 556 km

21

(Melosh 1996), comparable to the size of Texas (Figure 1.5), and vaporize rock down

to 16 km depth, well into the mantle, even considering a thick Archean crust. Kyte

et al. 2003 used raw Cr isotope data to determine that the meteoritic material of S3

was most like that of a CV group carbonaceous chondrite.

Figure 1.5: Diagram comparing a 556 km impact radius to the size of Texas.

22

Mineralogy

Studies on S3 have charted the mineralogical evolution of spherules from start-

ing assemblages of olivine [(Mg,Fe)2SiO4], chromite [FeCr2O4], pyroxene, and glass

to pyrite [FeS2], quartz [SiO2], K-spar [KAlSi3O8], chromite, and sericite follow-

ing dissolution-replacement reactions and devitrification during diagenesis (Krull-

Davatzes, D. R. Lowe, et al. 2012).

Quartz has been found to compose 40-50% of spherule material in one locality

(Krull-Davatzes, D. R. Lowe, et al. 2012). 50 µm nuggets of sericite of compositions

varying from muscovite to Ba- and Cr-rich mica are observed to take up 30-40%

of the remaining mineralogy. Quartz and phyllosilicate then, neither of which are

primary minerals, take up upwards of 75% of spherule mineralogy just by them-

selves. Beyond these dominant phases, fine grained Fe- and Ti-oxides are found

in spherules’ scalloped, pockmarked, and corroded rims. These metal oxides are

associated with carbonaceous material determined to be graphite by laser Raman

spectroscopy. This is unsurprising, considering the previous observation of graphite

in the Barberton Greenstone Belt by Tice et al. 2004. Intergrown with Ti-oxides are

apatite grains, sometimes occupying up to 80% of individual spherules. Other trace

minerals found at this locality by Krull-Davatzes, D. R. Lowe, et al. 2012 include

monazites [(Ce,La)PO4], baddeleyite [ZrO2], zircon [ZrSiO4], and barite [BaSO4], all

for the most part restricted to the matrix surrounding spherules.

Ni-, Cr-rich spinel [(Fe2+, Ni) (Cr, Fe3+, Al)2O4] is found by Krull-Davatzes,

G. R. Byerly, et al. 2010 and Gary R Byerly and Donald R Lowe 1994 to be the only

primary mineral remaining in S3, residing within a relatively small class of spherules

usually exhibiting plagioclase [(Na,Ca)(Al,Si)4O8] and pyroxene pseudomorphs. If

23

like other impact-derived Cr-spinels, the spinel’s Ni is likely sourced by the impactor

responsible for S3’s formation (Robin and Molina 2006).

Insights

Impact plume evolution

Krull-Davatzes 2006 document stratigraphic trends in S3 morphology consistent

across outcrops separated by a distance of 40 km. Analysis of REE elements revealed

refractory elements to be concentrated most highly at lower stratigraphic heights,

consistent with hypothesized temporal sequences of condensation from chondritic-

basaltic vapor plumes.

Archean atmosphere

Further study of S3 by Krull-Davatzes, G. R. Byerly, et al. 2010 examines

Fe3+/FeT ratios in spherules’ spinel mineralogy. The fraction of ferric Fe is indicative

of the oxygen fugacity of S3’s associated impact vapor plume and that of the 3.24 Ga

atmosphere. Comparison of results to similar studies of younger impact events’ debris

indicate that O2 levels were significantly lower 3.24 Ga than during the Cretaceous-

Paleogene boundary (65 Myr), the Eocene (56-34 Myr), and the Late Pliocene (5.3-2.6

Myr). It is also inferred from the morphological/chemical variability of spinels that

the vapor plume was rather heterogeneous in terms of cooling rate and temperature.

Element mobility under Archean alteration processes

In addition to characterizing S3’s mineralogy, Krull-Davatzes, D. R. Lowe, et al.

2012 used X-ray Fluorescence (XRF) Inductively Coupled Plasma Mass Spectrometry

24

(ICP-MS), and Laser Raman Spectroscopy to observe small-scale element mobiliza-

tion. REEs such as Ce and Eu vary significantly, indicating mobilization, inferred

to be due to diagenesis and mild metamorphism. Consistent Cr/Ir ratios are also

reported, suggesting these elements’ limited mobility and the absence of artificial

Ir enrichment due to sulfide mineralization (a process proposed by Martın-Peinado

and Rodrıguez-Tovar 2010). Platinum group elements are seen to be segregated

into spherules bearing primary phases of Ni-, Cr-rich spinel, suggesting their limited

mobility as well.

Archean DMM

Krull-Davatzes, G. R. Byerly, et al. 2014 perform bulk chemical analysis of S3

samples, followed by principal component analysis (PCA) and application of a mixing

model. Spherule principal components were determined by first identifying elements

that were relatively unchanged with respect to original bolide plus target values. Such

elements include generally incompatible elements Nb, Ta, Zr, and Hf, as well as immo-

bile elements Ti, Al, Sc, Cr, and Ir, the last of which defines the meteoritic material.

Concentrations of these elements in potential principal components (CI chondrite, CV

chondrite, various basalt types, komatiite, continental crust, MORB, DMM, primitive

mantle, etc.) were then used to determine relative proportions of components that

could optimally reproduce S3 bulk chemistry. Spherules can be reproduced by a

mixture of 15% CV group carbonaceous chondrite, 65% normal MORB, and 20%

DMM and cannot be reproduced by any mixture involving continental crust. The

presence of N-MORB in S3 points either to significant mantle melting and seafloor

spreading prior to 3.24 Ga or to the existence of a bizarre, uniquely Archean tectonic

regime (Krull-Davatzes, G. R. Byerly, et al. 2014). The coexistence of oceanic crust

25

and depleted mantle within spherules suggests that some amount of continental crust

had been extracted prior to 3.24 Ga, but since no continental crust is directly inferred,

no continental crust was concluded to have been excavated during the forming asteroid

impact. It was inferred that this could be either because (1) Archean continental crust

covered such a small area that even an impactor 30 km in diameter was unlikely to

strike land even nearshore environments, or (2) continental crust extracted from the

mantle was recycled through subduction or lamination and dripping prior to 3.24 Ga.

Distinguishing pristine spherules

A unique approach to the study of spherules is taken by Fritz et al. 2016 and Schulz

et al. 2017, who analyze a drill core from the center of the Barberton Greenstone Belt

that includes numerous distinct spherule layers. Fritz et al. 2016 use spectroscopic

imaging and µm-X-ray fluoresence (µXRF) to characterize the whole drill core, in-

cluding the marine sediments separating spherule layers. It is found that spherule

layers can be distinguished based on their high Al2O3 content and that alteration

phases in spherules tend to be associated with enrichment of K. These results aid

future spherule studies by providing a means to isolate pristine spherule material.

Another way to evaluate pristinity of spherules is to compare their compositions

to the surrounding matrix (distinct compositions suggest homogenizing alteration

processes have not occurred). Still another way could simply be to determine the

number density of spherules. If alteration damages spherule morphology, then one

would expect to observe fewer spherules in more altered samples and less Cr and

Ir in those samples’ matrix (Figure 1.6). It may of course not be so simple, as

different alteration processes have different physio-chemical effects. For instance,

silicification and formation of quartz within spherules could prevent their removal by

26

future alteration processes. Regardless, testing the use of these two alteration proxies

[(1) spherule/matrix bimodality, (2) spherule density] together is of particular interest

to LA-ICP-MS analyses, which afford the precise spatial resolution and elemental

sensitivity required.

Figure 1.6: Schematic of spherule layer exposures. (a) Exposure exhibiting highspherule density. Matrix material (red box) contains relatively high Cr and Ir due tomixing with spherules. (b) Exposure exhibiting low spherule density. Matrix materialcontains relatively low Cr and Ir due to low degree of mixing with spherules.

Potential

Previous chemical analyses of spherule components have left room for analyses

with greater precision, such as that offered by LA-ICP-MS. LA-ICP-MS study enables

several insights that are simply impossible on a bulk scale: intra-spherule as well

as inter-spherule chemical trends can be observed, the strongest signals of spherule

components of interest such as oceanic/continental crust can be isolated, and, once

isolated, any chemical signature of crust can be analyzed through its rare earth

element (REE) profile. Ratios of REEs have been well-documented to distinguish

oceanic and continental crust (Moyen and Martin 2012).

27

To provide a constraint on continental crust’s extent and rate of formation during

the Archean, PCA studies like Krull-Davatzes, G. R. Byerly, et al. 2014 should

be done for other spherule beds representing distinct impact events. This way a

picture of the crust’s evolution through the Archean can be pieced together. These

future studies would benefit greatly from an understanding of how spherules’ various

chemical components are distributed, as reported in the following sections, as with

that knowledge one could target chemical analyses to where signals of target rock

chemistry are strongest and there is greatest potential to glean insights into Earth’s

ancient surface.

28

Methods

Samples

Bulk rock samples of S3 were collected in 2019 by Roger Fu, Alec Brenner, and

Emily Stoll (Stanford) from the S3-206 outcrop in Barite Valley, Kaapvaal Craton

(South Africa) (Figure 2.1). This is the same stratigraphic horizon and locality

sampled by Krull-Davatzes, G. R. Byerly, et al. 2014. Samples were extracted from

the full 30 cm thickness of the spherule bed. At the Paleomagnetics Lab in the

Department of Earth and Planetary Sciences at Harvard University, we drilled a suite

of 1-inch rounds forming a continuous transect along the 30 cm thick exposure. These

drill cores were then polished down to 1 µm grit for LA-ICP-MS analysis (Figure 2.2).

Three members of this suite were analyzed as part of this study, representing S3 at

three different stratigraphic heights (5, 22, and 27 cm). The packing density of

spherules varies across stratigraphic heights from as low as 20-25 spherules/cm2 at

5 cm to as high as 45-50 spherules/cm2 at 22 cm (Table 2.1, Figure 2.3. Samples

of varied spherule density where chosen to reveal the extent to which S3 has been

homogenized by post-deposition alteration (if regions of low and high spherule density

are chemically similar, homogenizing alteration processes must have taken place).

Also analyzed are two barite samples, one of pure barite provided by the Harvard

29

Museum of Natural History and one of barite characteristic of the Barite Valley

sampling locale.

Figure 2.1: Map of South Africa. Barite Valley coordinates (25°54’49” S, 31°3’17” E)indicated.

Sample Spherule density (cm−2) Spherule diameter (mm)

E27B 35-40 ∼2E22B 45-50 ∼1F5A 20-25 ∼2

Table 2.1: Spherule density and size comparison.

Data collection

Spherule sections were analyzed on LA-IC-PMS in the Langmuir Lab at the

Department of Earth and Planetary Sciences at Harvard University using 100 µm

30

Figure 2.2: Visible light images of polished 1-inch drill cores from S3. (a) E27B.Analyzed spherules circled in black. (b) E22B. (c) F5A. (d) E25A (not analyzed).

spot size. For each section, several distinct spherules were identified as typical and

analyzed using a grid of 25 evenly-spaced 100 µm spots. Then, a grid of 50 evenly-

spaced 100 µm spots was measured across the full area of each section. Finally, at

least 20 spots were measured on matrix material.

31

Figure 2.3: Comparison of spherule densities and appearance across S3. Red dotsindicate identified spherules. (a) E27B. (b) E22B. (c) F5A.

Figure 2.4: Visualization of LA-ICP-MS spot analyses. (a) 20-pt grid analysis of anE27B spherule. (b) 50-pt grid analysis of entire E22B section.

Data processing

LA-ICP-MS is very effective for precisely determining the distribution of a sam-

ple’s chemistry. To utilize the precision offered by LA-ICP-MS, however, several

obstacles had to be overcome.

32

Figure 2.5: Flowchart demonstrating relationships between the chemistries of S3’sconstituent components. Shown in red are ions uniquely enriched/depleted in impact-plume derived compositions. Shown in blue are ions uniquely enriched/depleted dueto alteration processes. Generally immobile elements are displayed in bold.

Background removal

Not all ions captured by the spectrometer during analysis actually originated in

the sample. Some were “background” ions. To provide information on the quantity

and species of background ions present, the spectrometer takes measurements for

some time before the laser begins ablating the sample. To be properly accounted

for, background ions were subtracted from data. This was done by splitting a

measurement’s data into “background” (laser = off) and “peak” (laser = on) currents

and finding the difference of the averages of the two subsets. That is, for each

ion the counts were averaged for the background measurements and that average

was subtracted from the average for the peak measurements. This process can be

33

expressed as:

Vi =1

n

n∑j=1

Pij −1

m

m∑j=1

Bij; (2.1)

where Vi is the net ion current measured for a particular ion, Pi a vector containing

all peak ion currents (laser = on) reported for a particular ion i in the time of one

measurement, and Bi a vector containing all background ion currents (laser = off)

reported for a particular ion i in the time of one measurement.

Instrument drift

Non-linear drift in spectrometer readings is known to occur between LA-ICP-MS

measurements. To account for this, measured ion currents are typically normalized

to some reference parameter. This reference parameter can be chosen in several

ways. Samples can be spiked with an internal standard–a known amount of a specific

ion–such that the measured intensities of that ion can be compared to the known con-

centration. The factors relating the measured ion current to the known concentration

can then be used to correct instrument variation. One may also use as reference an

ion or set of ions already present in the sample that are assumed to be of uniform

distribution. This is known as normalizing to Extracted Ion Current (EIC). One

may simply use all ions measured as reference ions and assume the total ion intensity

(also known as total ion current, TIC) to be constant across all measurements. The

normalization of LA-ICP-MS data is discussed at length by Uerlings et al. 2016,

December 21, but most generally, the procedure is given by:

Xki =

IkiV kr

(1

n

n∑j=1

V jr ) (2.2)

34

where Xki is the normalized current (intensity) of ion i in measurement k, V j

r is

the measured current of the reference ion r in measurement j (j is iterated over all

n measurements in data set), V kr is the measured current of the reference ion r in

measurement k, and Iki is the unnormalized current of ion i in measurement k.

In this study, measurements were taken in a relatively short time interval. Because

of this, and because procedural difficulties preclude spiking S3 samples with a known

standard, and because no elements are known to be uniformly distributed in S3,

instrument drift is ignored.

Use of standards

The final piece of data processing performed involved using measured ion currents

to acquire an absolute composition (in terms of parts per million, weight percent, or

weight percent oxide). Measuring a standard tells one what concentrations of every

element produce what ion currents in the spectrometer at a given time. Knowing

this, one can convert ion currents to concentrations. This is done by:

xki = x0iXk

i

X0i

(2.3)

where xki is the concentration of ion i in measurement k, x0i is the concentration of

ion i in the known standard, Xki is the normalized current of ion i in measurement k,

and X0i is the normalized current of ion i measured in the standard. A complication

could arise in the conversion process if the standard(s) used has a known composition

but that composition lacks isotope information. Consider, for instance, when one’s

spectrometry dataset contains data for both 137Ba and 138Ba but it is only known

that the standard used has 1 ppm Ba (no isotope is specified). Such lack of isotope

35

data is handled in this study by combining data for all isotopes of an element prior

to unit conversion (e.g., 1 count 137Ba and 10 counts 138Ba becomes 11 counts Ba).

Here, the United States Geological Survey (USGS) reference glass GSE-1G is used.

36

Results and Discussion

S3’s chemistry can be broadly categorized into two compositions: the first corre-

sponding to spherules at the time of their deposition and the second corresponding to

the assemblage of alteration minerals formed post-deposition. Because spherules form

from the assemblage of crust, mantle, and asteroid material ejected during an impact

event, they are particularly enriched in elements of high concentration across the

board of crust, mantle, and asteroid compositions. S3 has been previously associated

with the impact of a CV chondrite into oceanic crust (Krull-Davatzes, G. R. Byerly,

et al. 2014), which is convenient, as chondrites, crust, and mantle are all Cr-enriched

relative to most alteration minerals, with the exception of fuchsite [K(Al,Cr)2(OH)2].

Discussed in the proceeding sections are stratigraphic variation, alteration effects,

and potential component compositions (continental crust, oceanic crust, chondrite,

etc.) observed in S3. Major element trends are used to evaluate stratigraphic

variations. Two alteration proxies [(1) spherule/matrix bimodality, (2) spherule

density] are used to evaluate pervasiveness of alteration. Finally, potential component

compositions are interpreted using trends of rare Earth elements (REE) and transition

metals.

37

Stratigraphic variation

Figure 3.1: (a) Si (ppm) vs. Mg (ppm). E27B shown in blue; F5A in red; E22B ingreen. (b) E27B. Si vs. Mg. Measurements of spherules plotted in purple; matrix inorange. (c) F5A. Si vs. Mg. (d) E22B. Si vs Mg.

E27B

Figure 3.1(a) compares concentrations of Mg and Si the three stratigraphic heights

of S3 at which samples were taken and analyzed. E27B exhibits Si concentrations

from 5-90 wt.% and Mg concentrations of 0-0.8 wt.%. These concentrations are broken

down and classified by their corresponding morphology (spherule or matrix) for each

38

Figure 3.2: Comparison of E27B spherule compositions. (a) Si (ppm) vs. Mg (ppm).(b) Cr vs. Ti. (c) Nd vs. Yb. (d) Ir (counts) vs. Cr (counts). Counts are usedinstead of concentrations due to issues standardizing Ir.

sample in Figures 3.1(b-d). It can immediately be seen that sample E27B possesses

a chemical trend within spherule morphologies distinct from that of its matrix. This

trend is characterized by higher concentrations of Si and Mg. Figure 3.2(a), which

plots Mg and Si concentrations in E27B broken down into measurements of different

spherules, shows that within individual spherules Mg and Si are anticorrelated (∼-100

ppm Si/ppm Mg).

Figure 3.3 compares concentrations of several major and minor elements with

concentrations of Cr across S3. E27B exhibits significant Cr content (up to 0.7 wt.%),

39

Figure 3.3: (a) Cr (ppm) vs. Al (ppm). E27B shown in blue; F5A in red; E22B ingreen. (b) Cr vs. Mg. (c) Cr vs. Si. (d) Cr vs. Ba. Barite Valley barite (BVAL2)and pure barite samples are added.

and analysis points of high Cr are typically associated with high Al (5-30 wt.%) and

low Ba (<5 wt.%). Figure 3.3(d) compares the Ba content of spherule samples with

that of naturally occurring barite crystals found near S3 in Barite Valley (BVAL2)

and with that of pure barite. It can be seen that despite E27B’s regions of high Cr

content, there are pockets of nearly pure Ba (up to 80 wt.% Ba).

Figure 3.4(a) compares concentrations of Rb and Cr in S3. K is not included in

LA-ICP-MS analyses, but Rb is included, and it is known that Rb often substitutes

for the K+ cation in K-bearing minerals (Higuchi and Nagasawa 1969), and so the

40

Figure 3.4: (a) Rb (ppm) vs. Cr (ppm). E27B shown in blue; F5A in red; E22B ingreen. (b) E27B. Rb vs. Cr. Measurements of spherules plotted in purple; matrix inorange.

general correlation of Rb and Cr in E27B, along with the restriction of relatively

high Cr and Rb concentrations to spherule morphology (Figure 3.4(b)) suggests that

spherule material is often found with K.

F5A

F5A exhibits three distinct trends in Si v. Mg space: one vertical trend (∼0 ppm

Mg) and two trends with nearly equal negative slopes (Mg increases as Si decreases).

The negative trends are similar in profile to those of E27B and E22B, although

like E22B, F5A is also depleted relative to the E27B profile (0-50 wt.% Si; 0-0.25

wt.% Mg). Figure 3.1(c) reveals that the two anticorrelation trends seen for F5A in

Figure 3.1(a) correspond to generally different morphologies, with more Mg-, Si-rich

points in F5A corresponding exclusively to spherule morphology and Mg-, Si-depleted

regions displaying either matrix or spherule morphology.

Figure 3.3(c) indicates that F5A possesses relatively low and homogeneous Ba

(<5 wt.%) as well as nearly zero Cr.

41

Correlation of Rb and Cr (Figure 3.4(a)) suggests that spherule material is often

found with K, but at a higher ratio of Rb to Cr than in either E27B or E22B.

E22B

A chemical trend can be seen in E22B in Figure 3.1(a) that is similar to that of

E27B in profile but depleted relative to the E27B profile in both Si and Mg (0-55 wt.%

and 0-0.6 wt.%, respectively). Looking at Figure 3.1(d), it can be seen that for E22B

morphology is a poor indicator of chemical composition, implying homogenization of

spherules at this stratigraphic height with their surrounding matrix.

Like F5A, E22B possesses relatively low and homogeneous Ba (<5 wt.%), although

there is evidence of nuggets of up to 50 wt.% Ba. Unlike F5A there are points of

significant Cr-enrichment up to nearly 0.2 wt.%.

Rb and Cr are also correlated in E22B at a ratio between Rb/Cr values of F5A

and E27B, indicating that spherule material at lower stratigraphic heights could be

generally enriched in K (Figure 3.4(a)) relative to spherule material.

Alteration

As described in a preceding section, Krull-Davatzes, D. R. Lowe, et al. 2012 doc-

umented multiple alteration processes that have affected S3 following its deposition.

Here, these processes are discussed in the context of LA-ICP-MS measurements.

Proxies

According to Figure 3.1(b-d), only in E22B do spherule and matrix morphologies

possess similar concentrations of Si and Mg, suggesting it is the only sample to

42

have undergone near-complete homogenization of Mg and Si through post-deposition

alteration. Fritz et al. 2016 found that high Al content in spherules indicates better

preservation, and Figure 3.3(a) shows that Al is generally higher in samples with

higher Cr (higher in E27B than E22B and F5A). Considering this alongside ob-

servations of spherule/matrix bimodality, E27B appears to bear the most original

spherule material of the samples analyzed, consistent with its relatively pristine visual

appearance (Figure 2.2(a)), followed in degree of alteration by E22B (Figure 2.2(b))

and F5A section (Figure 2.2(c)).

Silicification

Mg and Si are anticorrelated in all spherule morphologies of all samples, suggesting

silicification has possibly affected S3 across its entire 30 cm thickness. Figure 3.1(b)

demonstrates that in spite of silicification there is a distinct chemical difference be-

tween spherule and matrix morphologies in E27B. It could be that greater silicification

in E27B, evidenced by higher Si concentrations, has hardened spherules, protecting

them from future alteration. Anticorrelation of Mg and Si in F5A (Figure 3.1(c))

could indicate post-deposition dilution of spherules by a Si-rich fluid (silicification)

at this stratigraphic height as well.

Baritization

Despite any preservation of spherules by silicification in E27B, it can be seen

that there are pockets of nearly pure Ba, indicating hydrothermal alteration by Ba-

rich fluid (baritization). In F5A, low concentrations of Ba (Figure 3.3(d)) suggest

little baritization affected S3 at this stratigraphic height, but at the same time the

complete lack of Cr indicates that nearly all original original spherule material has

43

been lost. An alteration mechanism other than baritization, such as chloritization or

carbonatization, must be responsible for removing spherule material from F5A with-

out homogenizing spherule and matrix compositions. As with F5A, low Ba content

suggests that an alteration mechanism other than baritization must be responsible

for the homogenizing of E22B.

Fuchsitization

The correlation of Rb and Cr seen in all samples may be explained by the presence

of fuchsite [K(Al,Cr)2(OH)2]. Cr-enriched fuchsite could be produced where spherule

material is present as a result of fluid alteration. If fuchsite is present across S3

stratigraphy, then the higher Rb/Cr ratios of F5A could reflect lower amounts of

starting spherule material. The presence of fuchsite or another K-bearing alteration

phase is supported by the work of Fritz et al. 2016, who found that K-enrichment

of impact spherules from Barberton Mountain Land, South Africa is associated with

alteration (although it is unclear if these spherules are associated with S3 or another

spherule bed).

Carbonatization

Figures 3.5 show Ca trends in S3, in which any signature of carbonatization would

be visible. Carbonatization is associated with an increase in Ca (Figure 2.5), so if

carbonatization were to have affected S3, one would expect to see anticorrelation

between Ca and Cr (carbonatization would dilute Cr-rich spherule chemistry). How-

ever, Figure 3.5(a) shows very low Ca concentrations (generally less than 0.1 wt.%)

and no clear trend between Ca and Cr in all samples. Furthermore, Figure 3.5(c)

suggests that in E27B, where Ca exists in highest concentration, Ca resides as much

44

Figure 3.5: (a) Cr (ppm) vs. Ca (ppm). E27B shown in blue; F5A in red; E22B ingreen. (b) Fe vs. Ca. (c) E27B. Cr vs. Ca. Measurements of spherules plotted inpurple; matrix in orange.

in spherules as in matrix material, whereas one would expect to see greater Ca in the

more porous and unsilicified matrix material were significant carbonatization to have

occurred.

Chloritization

Chloritization enhances Mg and Fe concentrations while decreasing Ca (Figure 2.5).

Figure 3.5(b) shows that Fe-rich pockets in E27B occur only where Ca is present in

concentrations less than 0.1 wt.%, suggesting the possibility of localized chloritization.

45

Figure 3.6: (a) Cr (ppm) vs. Fe (ppm). E27B shown in blue; F5A in red; E22B ingreen. F5A measurements plot near origin, beneath E22B measurements. (b) Fe vs.Mg.

However, there is no obvious anticorrelation between Cr and Fe (Figure 3.6(a)), and

the Fe-rich pockets of E27B are not clearly enriched in Mg (Figure 3.6(b)) as one may

expect.

Sericitization

A third possible mechanism of alteration is sericitization, associated with a de-

crease in Ca (Figure 2.5). This means sericitization would be evidenced by a correla-

tion between Cr and Ca, as any Ca contained within spherules would be lost to the

altering fluids. While Ca concentrations are quite low in all samples, it does indeed

appear that there is a correlation 3.5(a), at least in E27B. However, this trend is

largely restricted to spherule morphology and it is unclear if it is reflected in matrix

material, where one would expect to see more evidence of alteration.

46

Summary

From these analyses, we cannot quantify the extent to which S3 has been affected

by carbonatization, sericitization, or chloritization; however low Cr, Si, and Ba

contents at lower stratigraphic heights indicate that some alteration other than sili-

cification must have occurred. The likely solution is that a combination of alteration

reactions involving Ca occurred, to the effect that Ca trends are now impossible to

deconvolve.

Two target crust compositions

Ti and Cr in S3 are shown in Figure 3.7. Ti is chosen because it is characteristically

enriched in crust compositions relative to mantle, chondrites, and most alteration

minerals. All samples exhibit heterogeneous distribution of Ti (Figure 3.7(a)), al-

though only in E27B and E22B is Ti found along with much Cr (>0.1 wt.%). And

only in E27B are Ti and Cr both restricted to spherule morphology (Figure 3.7(b)),

suggesting that only in E27B has crust material not been wholly mobilized away from

original spherule material. Still, the most Cr-rich points analyzed on E27B that have

been interpreted as having the most spherule material are not those with the greatest

concentration of Ti. It is unclear if this is a result of post-deposition unmixing of

spherules’ constituent components or original spherule heterogeneity.

Figures 3.8-3.9 plot trends of various rare earth elements (REEs). Ratios of REEs

have been well-documented to distinguish oceanic and continental crust (Moyen and

Martin 2012). Oceanic crust is relatively rich in Lu and Yb and depleted in Ce and

Nd (Godard et al. 2006); vice versa for continental crust (R.L. Rudnick and S. Gao

2014). The shallower of the two distinct slopes visible in Figures 3.8-3.9, richer in

47

Figure 3.7: (a) Cr (ppm) vs. Ti (ppm). E27B shown in blue; F5A in red; E22B ingreen. (b) E27B. Cr vs. Ti. Measurements of spherules plotted in purple; matrix inorange. (c) F5A. Cr vs. Ti. (d) E22B. Cr vs Ti.

Lu and Yb, could then reasonably represent an oceanic crust component within S3,

while the steeper slope could represent a continental crust counterpart.

The observed REE concentrations and trends are compared to those characteristic

of continental crust (R.L. Rudnick and S. Gao 2014) and mid-ocean ridge basalt

(MORB) (Godard et al. 2006) in Figure 3.10. Table 3.1 lists approximate REE ratios

for S3 as well as the range of measured REE concentrations, comparing them with

those of continental crust, MORB, DMM, and carbonaceous chondrites. Looking

at Figure 3.10 and Table 3.1 it is clear that Nd/Yb and Ce/Lu differ between S3

48

Figure 3.8: (a) Nd (ppm) vs. Yb (ppm). E27B shown in blue; F5A in red; E22B ingreen. (b) E27B. Nd vs. Yb. Measurements of spherules plotted in purple; matrix inorange. (c) F5A. Nd vs. Yb. (d) E22B. Nd vs Yb.

and known crust compositions. Considering S3 is composed of an assemblage of

components, most of which have different REE concentrations than continental crust

or MORB, this is unsurprsing. What is most interesting is that the ratio of Nd/Yb

to Ce/Lu (that is, Nd/Y bCe/Lu

) for the steep and shallow REE trends in S3 are very similar

to those of continental crust and MORB, respectively. Additionally, concentrations

of Yb and Lu along these trends are similar to those one would expect for continental

crust and MORB. In the shallow trend, values of Nd/Yb, Ce/Lu, and Nd/Y bCe/Lu

agree

with those of DMM, however the enrichment of each Ce, Nd, Yb, and Lu in the

49

Figure 3.9: (a) Ce (ppm) vs. Lu (ppm). E27B shown in blue; F5A in red; E22B ingreen. (b) E27B. Ce vs. Lu. Measurements of spherules plotted in purple; matrix inorange. (c) F5A. Ce vs. Lu. (d) E22B. Ce vs Lu.

shallow trend relative to DMM (Workman and Stanley R. Hart 2005) discourage

a mantle source. In the steep trend, concentrations of Ce, Nd, Yb, and Lu and

associated ratios (particularly Ce/Lu) are all much higher than those of both DMM

and carbonaceous chondrite (Workman and Stanley R. Hart 2005, Friedrich et al.

2002). While lighter lanthanides Ce and Nd appear to be depleted in S3 to values

less than a fourth of crust values, this is consistent with greater mobility of lanthanides

under hydrothermal conditions (Kikawada et al. 2001).

E27B exhibits both continental crust and MORB trends, exclusive to spherule

50

Sample Nd/Yb Ce/Lu Nd/Y bCe/Lu

Ce Nd Yb Lu

S3 (shallow REE trend) ∼1 ∼6.25 0.16 0-5 0-5 0-5.25 0-0.75S3 (steep REE trend) ∼20 ∼320 0.06 0-15 0-7 0-2 0-0.25

MORB 3.2 22.7 0.14 13.17 11.48 3.59 0.58Continental crust 13.5 203.2 0.07 63 27 2 0.31

DMM 1.59 9.48 0.17 0.55 0.58 0.37 0.06Carbonaceous chondrite 2.79 2.11 1.32 0.93 0.67 0.24 0.44

Table 3.1: Rare earth element comparison of S3, continental crust, MORB, DMM, andcarbonaceous chondrite. All concentrations are in units of ppm. “S3 (shallow REEtrend)” refers to the shallower of the two REE profiles present in Figure 3.8 and 3.9.Accordingly, “S3 (steep REE trend)” refers to the steeper profile. Continental crustcomposition comes from R.L. Rudnick and S. Gao 2014; MORB composition fromGodard et al. 2006; DMM from Workman and Stanley R. Hart 2005; carbonaceouschondrite from Friedrich et al. 2002.

Figure 3.10: (a) Nd (ppm) vs. Yb (ppm). E27B shown in blue; F5A in red;E22B in green; MORB composition (Godard et al. 2006) in cyan; Continental crust(R.L. Rudnick and S. Gao 2014) in magenta. Black dotted lines are projections ofthe two Nd/Yb ratios prevalent in dataset. Magenta dotted line represents Nd/Ybratio characteristic of continental crust. Cyan dotted line represents Nd/Yb ratiocharacteristic of MORB. Black arrow indicates light lanthanide depletion due todiagenesis. (b) Ce vs. Lu.

morphologies, indicating coexistence of oceanic and continental crust within its spherules.

Meanwhile, F5A exhibits only the MORB trend, implying any continental crust

originally within F5A has been diluted beyond the point of detection. E22B appears

to exhibit both continental crust and MORB trends but does so primarily within its

51

matrix material and at lower concentrations and lower spatial frequency than E27B,

hinting once more at significant alteration of E22B.

The presence of continental crust in S3 is a surprising result, because previous

studies of the spherule bed (Krull-Davatzes, G. R. Byerly, et al. 2014) that modeled

its component phases based on bulk chemistry found N-MORB and depleted MORB

mantle (DMM) as the only target rock phases. While the presence of DMM in S3

indicates continental crust had formed prior to the 3.24 Ga impact, the absence of

continental crust itself suggested that there was no continental crust within the impact

radius. The REE trends reported on here suggest that continental crust was indeed

present within the S3 forming impact’s excavation radius.

Mantle component

Figure 3.11 compares amounts of Ni, Ti, Hf, Ir, and Cr in S3. Ti and Hf generally

indicate crust material presence, so the lack of Ni at points of high Ti or Hf in

Figure 3.11(a-b) suggests Ni is not associated with crust material. Additionally, Ni

is not correlated with Ir or Cr (Figure 3.11(c-d)). This suggests that the Ni is either

post-depositional or has been transported away from its original host material.

Rare earth element ratios in general agree with those of DMM 3.1, however they

appear generally too enriched to have been sourced from the mantle (Workman and

Stanley R. Hart 2005). Ultimately, mantle material is not distinguishable in S3.

This is perhaps unsurprising given the relatively small amount of mantle material

that would be excavated by even a very large asteroid impact and the even smaller

amount that would persist through post-deposition alteration.

52

Figure 3.11: (a) Ni (ppm) vs. Hf (ppm). E27B shown in blue; F5A in red; E22B ingreen. (b) Ni vs. Ti. (c) Ni vs. Ir. (d) Ni vs. Cr.

Impactor component

Figure 3.12 compares Ir, Hf, and Cr counts in S3. Spectrometer counts are plotted

instead of concentrations here because Ir was absent in the standards measured

during LA-ICP-MS analysis. Hf is thought to be characteristic of crust material

while Ir is thought to be entirely asteroidal. F5A exhibits slight positive correlation

of Ir with Hf (Figure 3.12(a)), although there is very little Ir present at any given

point (consistent with the smaller spherule material present (Figure 2.2(c))). For the

more Ir-rich E27B, there is no clear relationship between Ir and Hf nor Ir and Cr

53

Figure 3.12: (a) Ir (counts) vs. Hf (counts). E27B shown in blue; F5A in red; E22Bin green. (b) E27B. Ir vs. Hf. Measurements of spherules plotted in purple; matrixin orange. (c) Ir (counts) vs. Cr (counts). E27B shown in blue; F5A in red; E22B ingreen. (d) E27B. Ir vs. Cr. Counts are used instead of concentrations due to issuesstandardizing Ir.

(Figure 3.12(a,c)), although points of high Ir, Hf, and Cr are restricted to spherule

morphology (Figure 3.12(b,d)), supporting a case for separation of crust and impactor

material within spherules.

54

Conclusions

LA-ICP-MS analysis of S3 reveals localized preservation of the impactor and

target rock compositions involved in its formation. Baritization and silicification

are documented to have affected these compositions but not so much they cannot be

studied. Based on rare earth element trends within S3, it appears that the spherule

bed was formed by the impact of an asteroid into a nearshore environment, such that

continental crust and mid-ocean ridge basalt were excavated and incorporated into the

global debris cloud. The presence of continental crust within the impact excavation

radius indicates extensive continental crust extraction prior to 3.24 Ga and supports

a rather large land fraction during the Archean. Just how large the land fraction

actually was may remain unclear until additional spherule beds are studied using

precise analysis techniques such as those reported here.

55

Acknowledgements

I would like to first thank my advisor, Dr. Roger Fu, both for advising me in thisproject and for mentorship over the past two years. I have been educated in the wayyou communicate complexities scientific and otherwise, inspired by the inquisitive,creative, and egalitarian lab you have created, and heartened by your friendship. Youand your Quantum Diamond Microscopes–but mostly you–have a helluva thing going.

Thank you to Dr. Zhongxing Chen and Dr. Charles Langmuir for your assistance inthis project as well. Zhongxing, hours on the spectrometer felt like minutes thanks toyour easy-going conversation. Charlie, through our discussions and through readingyour writing, it has been impressed upon me that rocks are not just verbs but entirevolumes of stories.

Thank you to everyone I have worked with in the Paleomag lab: Alec Brenner,Danielle da Cruz, Kimberly Hess, Hairou Fu, Michael Volk, Oren Ben Dor, RaisaTrubko, and Zoe Levitt. Through you I have come to appreciate the process ofscientific inquiry, terrible food, and starry nights.

Thank you to every member of the EPS 174 expedition to Brazil: Abby, Angelique,Caleb, Candice, Caro, Ibyata, Janine, Julio, Jason, Leo, Lia, Mary, Nicolas, Rachel,Riccardo, Robert, Roger (again), Theo, Sadie, and Victoria. I hope each of yourfutures is well supplied with rice and beans, friendly caramel colored dogs, and fastfriends.

Anisha Mittal, Ashwin Krishna, Chloe Lemmel-Hay, Eli Langley, Ian Saum, MaggieBeazer, Michelle Walsh, and Miro Furtado–just call me.

I write sitting in my parents’ house in Abbott, Texas, abundantly comfortable andsafe during a time impoverished of those things. Mom and Dad, your love speaksloud in actions and in words, and you can’t realize how grateful I am. I will try totell you tomorrow though.

long live mass hall. long live pub night. semper cor.

56

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