THE ORIGIN OF BASALT AND CAUSE OF MELTING BENEATH EAST ANTARCTICA AS REVEALED BY THE SOUTHERNMOST VOLCANOES ON EARTH

Jenna L. Reindel

A Thesis

Submitted to the Graduate College of Bowling Green State University in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE

December 2018

Committee:

Kurt S. Panter, Advisor

Peter Gorsevski

Daniel F. Kelley

© 2018

Jenna L. Reindel

All Rights Reserved iii ABSTRACT

Kurt S. Panter, Advisor

The southernmost volcanoes on Earth, Mt. Early and Sheridan Bluff, are two basaltic monogenetic volcanoes located 87°S at the head of the Scott Glacier, in the southern

Transantarctic Mountains. The Early Miocene volcanoes lie ~1000 km from any other volcano and ~200 km from the shoulder of the West Antarctic Rift System (WARS), which is the foci of most Cenozoic alkaline volcanism in Antarctica. WARS is part of a larger diffuse alkaline magmatic province (DAMP) including volcanism in New Zealand and Australia. Dating by the

40Ar/39Ar method indicates that Mt. Early is older than previously determined and closer in age to Sheridan Bluff (~19 Ma). Basalts range in composition from alkaline (~6 wt. % Ne- normative) to subalkaline tholeiite (~6 wt. % Hy-normative). Tholeiite have higher ratios of

Zr/Nb (9) than alkaline basalts (4) and have lower ratios of La/Yb (5), La/Lu (<50), and Gd/Yb

(2) than alkaline basalts (20, 100-150, and ~3 respectively). Fractional crystallization, assimilation-fractional crystallization (AFC), and partial melting are tested as mechanisms to explain compositional variations. Crystal fractionation alone cannot explain the difference in composition. Modelling AFC on plots of Rb, Ba, and La/Nd versus TiO2 show that unrealistic bulk partition coefficients are required to explain the array of compositions using contaminates from the upper crust. I conclude that the coexistence of tholeiite and alkaline basalt is likely due to different degrees of partial melting. The basalts mirror partial melting trends for a common garnet lherzolite source on plots of La/Lu versus Nb/Yb. These models also suggest that tholeiite was produced by higher degrees of melting at shallower depths. Mt. Early and Sheridan Bluff iv basalt, especially tholeiite, are distinctive from DAMP basalt, having lower Ce/Pb (≤ 20), Gd/Yb

(≤ 3) and lack K and Pb anomalies on normalized multi-element plots. Negative K anomalies are a prominent feature of DAMP basalt and are used to support amphibole-rich lithospheric sources.

It is likely that the basalts from Mt. Early and Sheridan Bluff were derived from asthenosphere with little or no input from mantle lithosphere. Causes for volcanism in DAMP include passive extension, metasomatism and plumes. A recent viable mechanism for triggering volcanism in this region is lithospheric delamination and has been proposed based on geophysical evidence.

v ACKNOWLEDGMENTS

This research is funded by NSF Polar Programs grant #1443576.

I would like to thank my advisor, Kurt Panter, for making this research possible and enjoyable. The knowledge you’ve instilled in me is priceless, and I’m grateful for your continued support and guidance. Thank you for giving me the opportunity to experience field work in

Antarctica, for mentoring and believing in me as a scholar, and providing me with professional development.

Thank you to my committee members for your valuable input and guidance. Thank you to John Smellie (University of Leicester, UK) for your knowledge and support during our time in

Antarctica. Thanks to Tim Burton (UK), for being the best mountaineering guide and friend, for teaching me valuable mountaineering skills, and for always making me laugh and feel safe.

Thanks to Kenn Borek Air Ltd for the field support to and from our remote field location.

Thanks to Gordon Moore (University of Michigan), for your expertise and guidance during my microprobe sessions at the Electron Microprobe Analysis Laboratory (EMAL).

Thanks to Charlie Onasch and John Farver for assistance and direction in making thin sections.

Finally, thank you to my family, friends, colleagues and teachers for helping me along the way and for making graduate school the best it could be. Talons up!

vi

TABLE OF CONTENTS

Page

CHAPTER 1. INTRODUCTION...... 1

CHAPTER 2. GEOLOGIC SETTING ...... 4

West Antarctic Rift System ...... 4

Geology of Sheridan Bluff and Mt. Early ...... 6

CHAPTER 3. FIELD OBSERVATIONS AND METHODS OF STUDY ...... 8

This Study: Field Logistics & Observations ...... 8

Sample Preparation for Petrography and Mineral Chemistry ...... 9

Sample Preparation for Dating...... 9

Mineral Chemistry ...... 10

Whole Rock Chemistry ...... 11

CHAPTER 4. RESULTS ...... 13

Petrography ...... 13

Age ...... 14

Mineral Chemistry ...... 14

Whole Rock Chemistry ...... 17

Thermobarometry ...... 20

CHAPTER 5. DISCUSSION ...... 23

Evolution by Crystal Fractionation ...... 24

Crustal Contamination ...... 26

Partial Melting ...... 28

Mantle Sources for Basalt ...... 29 vii

Causes for Volcanism ...... 31

CHAPTER 6. CONCLUSION...... 34

REFERENCES… ...... 36

APPENDIX A. TABLES…………………………………………………………… ...... 48

APPENDIX B. FIGURES……………………………………………………… ...... 79

1

CHAPTER 1. INTRODUCTION

A major objective of this study is to explain the petrogenesis of the magmas erupted at Mt.

Early and Sheridan Bluff, upper Scott Glacier, Antarctica, and whether their source and melt generation is associated with Cenozoic volcanism in the West Antarctic Rift System (WARS).

Basaltic magmas have distinct tectonic origins and melting regimes that can be distinguished from each other both mineralogically and geochemically. The difference in compositions for primitive magmas erupted at different plate boundaries has to do with the depth of melting, amount of melting and mantle source composition. Different lithologies and mantle source compositions have been proposed to explain major element contents and enrichments in incompatible trace elements (Pilet, 2015 and references therein). Alkaline volcanism associated with continental rifts like the WARS is generally considered to be the result of small degrees of partial melting of mantle peridotite at great depths. The peridotite is often considered to contain components of recycled oceanic crust and mantle lithosphere. This means the material was brought down by subduction into the mantle and brought back up by large scale upwelling in the convecting mantle and explains the ideas behind mantle plumes. The long period of time it takes for this process can explain the radiogenic isotope signatures as well. This recycled peridotite material is often considered to have been altered (i.e. metasomatized) and CO2 -enriched in the subduction process to produce low SiO2, CaO, and Al2O3 contents found in alkaline magmas

(Pilet et al., 2011). Another proposed model is the melting of metasomatized hydrous cumulates at the base of the lithosphere that occur in the form of amphibole-bearing metasomatic veins.

High degrees of melting of these amphibole-bearing veins along with interaction of the resultant melts with the surrounding mantle as it rises towards the surface could account for the major and trace elements observed in alkali basalts (Pilet, 2015; Pilet et al., 2008; 2011). Metasomatism in 2 the shallow mantle may also help explain radiogenic isotope signatures in shorter periods of time due to the higher amount of isotope fractionation between the parent and daughter that takes place.

The driving force for late Cenozoic magmatism in West Antarctica has been a topic of considerable debate. Geochemical investigations, supported by geological and geophysical studies, have resulted in models attempting to explain the cause of volcanism, which include deep mantle plumes associated with Late Cretaceous Gondwana break-up (Lanyon et al., 1993;

Weaver et al., 1994), melting of a stratified (heterogeneous) fossil plume (Rocholl et al., 1995;

Hart et al., 1997; Panter et al., 2000), reactivation of lithospheric faulting and active rifting

(Rocchi et al., 2002; Nardini et al., 2009), and active mantle plumes (Kyle et al., 1992; Phillips et al., 2018). In the South Island of New Zealand, lithospheric delamination is a driving mechanism proposed for alkaline and lesser tholeiitic magmatism (Timm et al., 2010). Recently, lithospheric delamination has been proposed for East Antarctica based on geophysical evidence, which might explain melting and construction of the Mt. Early and Sheridan Bluff volcanoes

(Shen et al., 2017; Heeszel et al., 2016).

In general, Late Cenozoic rift-related magmatism in West Antarctica have similar trace element and isotopic signatures (e.g., Sr, Nd, Pb) relative to ocean island basalts (OIB). WARS basalts are also similar in composition to continental volcanism in the southwest Pacific region

(i.e. New Zealand and SE Australia) which led Finn et al. (2005) to propose grouping them together to define a diffuse alkaline magmatic province (DAMP). These continental regions were once part of the same continental landmass known as Gondwana prior to ca.100 Ma.

Australia and New Zealand rifted and were separated from Antarctica by about 83 Ma (Veevers,

2012, and references therein). Thus, models for magmatism on these now widely scattered 3 continental fragments of Gondwana often call upon mantle sources that have a pre-breakup origin. A number of recent studies call upon mantle sources for DAMP magmatism as lithospheric mantle that has been metasomatized and enriched in highly incompatible elements

(Ba, Th, Nb, Ce, K, etc.) relative to primitive mantle (Panter et al., 2006; Timm et al., 2010;

Martin et al., 2013). Aviado et al. (2015) propose that magmas were also generated from volatized asthenosphere during decompression in addition to the metasomatized lithosphere that has been thermally eroded during lithospheric extension. A more recent model by Panter et al.

(2018) suggests that carbonate-rich recycled slab material is present in the asthenosphere.

According to their model, this source was melted at low degree to form amphibole-bearing veins frozen within the lithosphere. Then, at a much later time (~20-30 Ma), the veins were melted at a high degree to produce the alkaline magmatism.

Although Mt. Early and Sheridan Bluff are over 1000 km away from the nearest Cenozoic volcanism related to rifting, and are situated about 200 km from the rift shoulder itself, they have been assumed to be related to WARS and have been assigned to the McMurdo Volcanic Group

(LeMasurier and Thomson, 1990). Yet, prior to this study, a detailed comparison with WARS basalts, as well as basalts within the rest of the DAMP, has not been undertaken. This is a primary objective of my thesis and my findings offer an important new understanding of mantle sources and provides additional constraints on the cause of magmatism in East Antarctica. 4

CHAPTER 2. GEOLOGIC SETTING

West Antarctic Rift System

The West Antarctic Rift System (WARS) is an active area of crustal extension in West

Antarctica, including the Ross Embayment (Fig. 1). The WARS extends nearly 3000 km long from the Ross Sea to the base of the Antarctic Peninsula. It is nearly 750 km wide containing many uplifts (i.e. horsts) and basins (i.e. grabens), including many deep subglacial basins within the Marie Byrd Land province.

A long history of rifting is documented across Antarctica. Initially, a Jurassic magmatic event

(Ferrar Large Igneous Province) marked the extension and early breakup of Gondwana from

Africa and South America (Elliot et al., 2008; Granot et al., 2010). Subduction was occurring all along the paleo-Pacific Gondwana coast prior to Cretaceous rifting and progressively ceased along the paleo-Pacific margin in the Late Cretaceous followed by rifting of Zealandia and

Australia but was still occurring at the base of the Antarctic Peninsula at 83 Ma (Veevers et al.,

2012). Subduction continues today but is restricted to the northwestern portion of the Antarctic

Peninsula where the Drake plate is subducting beneath the South Shetland. The final stages of

Gondwana separation occurred in the Late Cretaceous with the separation of New Zealand from

Antarctica (Wobbe et al., 2012). The first phase of WARS extension between Marie Byrd Land

(West Antarctica) and East Antarctica occurred between ~105 and 80 Ma, and formation of the

Victoria Land basin by ~65 Ma (Huerta and Harry, 2007). The second distinct phase of WARS rifting occurred ~43-26 Ma in the Victoria Land, Northern, and Adare basins (Behrendt et al.

1991; Granot et al. 2010), and ~34-29 Ma in the McMurdo Sound region, which includes the southern part of the Victoria Land basin and its sub-basin called the Terror Rift (Fielding et al.,

2008). Huerta and Harry (2007) explain that the difference between these phases is based on 5 extensional style, which is controlled by the changing strength of the lithosphere. The transition between the two phases of rifting is unique from other continental rift systems (i.e. East African

Rift and Basin and Range) in that it starts from a broad area of diffuse extension (~100 Ma), and progressively changes to a focused area of rifting at the western margin of the rift between the extended lithosphere and East Antarctic craton (~43 Ma). Extension in West Antarctica continued into the Late Oligocene and through the Neogene. In Northern Victoria Land and the

Adare Basin (Fig. 1) extensional events occurred at ~24 Ma, ~17 Ma, and recently <5 Ma, including present day activity, marked by post-spreading deformation and normal faulting, eastward dipping fault blocks with generally NE-SW trends, and near vertical fault blocks and volcanic activity (Granot et al., 2010; Panter et al., 2018). Rifting in McMurdo Sound and southern Victoria Land occurred ~29-23 Ma, followed by subsidence, and then faulting and magmatism from ~7 Ma to recent within the Terror Rift in the southern Victoria Land Basin

(Fielding et al., 2008).

East and West Antarctica are separated by the Transantarctic Mountains (TAM) and have different crustal and lithospheric thicknesses. The West Antarctic lithosphere ranges from 70 to

100 km, while the East Antarctic Craton shows lithospheric depths down to 250 km based on geophysical methods (Heeszel et al., 2016). West Antarctica shows a thinning crust between ~16 and 24 km beneath Ross Sea while East Antarctic crust is around 40 km thick (Huerta and Harry,

2007) with seismic thicknesses between 25 and 38 km recorded beneath the southern TAM

(Shen et al., 2017). A possible cause of uplift of the TAM has been recently linked to lithospheric foundering in the southern TAM, which leads upwelling of the asthenospheric mantle to result in the high elevations seen in the TAM (Shen et al., 2017; Brenn et al., 2017).

This geophysical study by Shen et al. (2017) also places Mt. Early and Sheridan Bluff directly 6 above a shallow (50-80 km) low velocity zone and may suggest lithospheric detachment underneath East Antarctica as a viable mechanism for volcanism (Shen et al., 2017).

Volcanism in the WARS is of the intraplate type and alkaline in composition. The age of the volcanics range from currently active (Mt. Erebus) to about 25 Ma, with most of the volcanism occurring in the Late Miocene to Pliocene. The range of alkaline compositions extend from basanite and tephrite, to intermediate hawaiite, mugearite, benmoreite, and phonolite, to high-silica trachyte and rhyolite (LeMasurier et al., 1990). In general, basalts throughout all

WARS provinces are relatively uniform in compositions, showing enrichments in incompatible elements with distinct K and Pb negative anomalies on Primitive Mantle (PM) normalized multi- element plots. Overall, basalts have consistently low concentrations (<10 for PM) in HREE (e.g.

Yb, Lu) and steep LREE/HREE patterns (La/YbN between 8 and 22), and lastly have relatively restricted ranges of radiogenic isotopic values with most Sr < 0.7035, Nd > 0.5128, 206Pb > 19.5,

207Pb >15.5 and 208Pb >39 (Panter et al., 2018). The similar geochemistry and isotopic composition of basalts from WARS indicate similar mantle sources and processes of melting

(Panter et al., 2018b). Prior to this study, basalts collected from Sheridan Bluff and Mt. Early have not received detailed classification or comparison to basalts from WARS. Their classification and comparison will be discussed below.

Geology of Sheridan Bluff and Mt. Early

After their discovery in the 1960’s, the first geological investigation of Sheridan Bluff and

Mt. Early was conducted in the late 1970’s by E. Stump, S.G. Borg, and M.F. Sheridan (Stump et al., 1980; 1990).

Sheridan Bluff is composed of a series of lava and volcanoclastic deposits with a total thickness of ~200 meters and sits on a glacially eroded Paleozoic granodiorite (Fig. 2A). The 7 lava sequence includes hyaloclastite breccia underneath a series of nine compositionally different subaerial lava flows. Stump et al. (1990) report that some lavas are mildly alkaline basalts while the others are olivine tholeiites. They note phenocrysts present in both alkaline and tholeiite types include olivine, plagioclase, and clinopyroxene, as well as crystal aggregates. The groundmass includes plagioclase, clinopyroxene, opaque oxides, and olivine. Three K/Ar whole rock dates were averaged to put Sheridan Bluff at 18.32 ± 0.35 Ma (Stump et al., 1980; 1990).

Mount Early is ~475 meters thick exposed above the surface of the East Antarctic ice sheet.

Stump et al. (1980; 1990) noted the core of the volcano is pillow lavas grading up into pillow breccia, and then capped by lavas just south of the summit. The sequence is cut by dikes (Fig.

3A). Geochemically, they report that Mount Early has mildly alkaline olivine basalt. Phenocrysts include olivine, clinopyroxene, and plagioclase clusters, and is like the alkaline magma at

Sheridan Bluff. One lava flow was sampled for K/Ar whole rock dating which puts Mount Early at 15.86 ± 0.30 Ma (Stump et al., 1980; 1990). Additional geochemical analyses were recently reported by Licht et al., (2018). This includes two samples from Mt. Early and four samples from Sheridan Bluff (Table 2; Fig. 10).

The bedrock that underlies Sheridan Bluff and Mt. Early consists of late Precambrian-early

Paleozoic granodiorite that was uplifted by the Cambrian Ross Orogeny (Stump et al., 1980).

After uplifting and erosion, deposition of non-marine Devonian-Triassic Beacon Supergroup took place, which consists of thick packages of sandstone, shale, coal and carbonate units and is commonly intruded by Jurassic Ferrar dikes and sills (Barrett, 1991; Ballance et al., 2002). 8

CHAPTER 3. FIELD OBSERVATIONS AND METHODS OF STUDY

This Study: Field Logistics & Observations

The 2015 expedition team included myself as a graduate assistant, Dr. Kurt Panter (BGSU) as the Principal Investigator, John Smellie as a collaborator and glaciovolcanologist (University of Leicester), and Tim Burton as our mountaineering and field safety guide. We were flown to the United States base, McMurdo Station, from New Zealand. After a week of field safety courses and logistical preparations, we were flown to a semi-permanent camp on the Shackleton

Glacier on a ski equipped LC130. We then proceeded to our field location on a Twin Otter, a smaller twin turbo propeller ski equipped plane. We set up camp at the base of Mt. Weaver which is located about equal distance between Mt. Early and Sheridan Bluff on the upper Scott

Glacier (Fig. 1). We conducted field work from December 25th to December 29th, 2015. We used snowmobiles to get from our campsite to the volcanoes and then by foot, sometimes using crampons and linking up to each other with ropes and harnesses when necessary. A total of 25 rock samples were collected at the volcanic edifices on the 2015 expedition. Hand samples were collected based on stratigraphy and lithology and positions recorded using handheld GPS as shown in Figures 4 and 5. Samples were collected using rock hammers and small sledge hammers. When collecting samples, we were careful to avoid weathered/altered portions of deposits in order to insure optimal results for igneous geochemistry.

Two field days were spent at Sheridan Bluff and 10 samples were collected from different lava flows. This volcano (Fig. 2B) sits on top of a glacially striated granitoid surface and consists of a series of lava flows (Fig. 6A & B) that appear to have a radial flow pattern away from a lapilli tuff cone (Smellie et al., 2016). Sheridan Bluff lavas are interpreted to have had no 9 interaction with an ice sheet during eruption, but rather the volcano appears to have been a shield volcano that may have possibly erupted within a lake (Smellie et al., 2016).

Three field days were spent at Mt. Early and 15 samples were collected from different stratigraphic levels (Fig. 3B) and include pillow lavas (Fig. 6C) at the base overlain by lapilli tuff

(Fig. 6D) and then by scoria deposit near the top of the section. Large and small dikes (widths vary from one to several meters) were also sampled (Fig. 6E). Mt. Early is interpreted to have erupted underneath an ice sheet due to its overall shape and lithofacies characteristics (Smellie et al., 2016).

Sample Preparation for Petrography and Mineral Chemistry

A total of 23 rock samples were chosen for petrographic analysis, which involved textural description and the identification of mineral phases and their abundance. The sample preparation laboratory facilities at BGSU were used to cut and polish the thin sections at 30 µm for the optical microscope. Fifteen billets left over from making thin sections were then sent to Wagner

Petrographic (http://www.wagnerpetrographic.com/) to be professionally made into polished thick (~50 µm) sections to be analyzed by electron micro-probe for mineral chemistry.

Sample Preparation for Dating

Groundmass from seven crushed samples were sent to the New Mexico Geochronology

Research Lab at New Mexico Tech (https://geoinfo.nmt.edu/labs/argon/home.html) for dating by the 40Ar/39Ar method. The seven samples were chosen based on stratigraphic position to cover the possible range of age and on how crystalline the groundmass is; the more crystalline the groundmass, the more likely the data is to have less error. This is because finer grained minerals

39 and glass are subject to greater effects of irradiation like net loss or redistribution of Ark by 10 recoil. Four of these samples were also chosen for plagioclase separation based on abundance, grain size, and plagioclase K2O content determined by EMPA. Samples containing > 5 vol.% plagioclase, with phenocrysts > 0.5 mm, and having K2O contents upwards of 0.3 wt.% were selected in order to increase the likelihood of meaningful results.

Mineral Chemistry

Polished thick sections were taken to the Electron Microbeam Analysis Laboratory (EMAL) at the University of Michigan

(http://www.mse.engin.umich.edu/people/rsgold/facilities/electron-microbeam-analysis- laboratory-emal) and analyzed by a Cameca SX-100 electron microprobe. Core and rim analyses were performed on phenocrysts of olivine, clinopyroxene, and plagioclase. Only cores of opaque oxides were analyzed due to their small diameter. Core analyses were measured in the middle of the grain while rims were measured within ~20 µm of the edge of the grain. Up to four phenocrysts of each mineral per sample were measured for chemistry. Olivine and clinopyroxene phenocrysts were measured for weight % concentration of SiO2, Al2O3, MgO, CaO, NiO, Cr2O3,

MnO, FeOt, Na2O, and TiO2. Plagioclase phenocrysts were analyzed for SiO2, TiO2, Al2O3,

FeOt, MnO, MgO, CaO, Na2O, K2O, and P2O5. Spinel group oxides were measured for MgO,

Al2O3, SiO2, TiO2, CaO, V2O3, Cr2O3, FeOt, and MnO.

The composition of each mineral has been normalized using Mineral Formulae Recalculation software

(https://serc.carleton.edu/research_education/equilibria/mineralformulaerecalculation.html) to calculate cations and molecular end member compositions. Concentrations of Mg and Fe2+ were used to determine olivine forsterite – fayalite contents. Calcium, Al and Na were used to classify plagioclase (anorthite – albite), and Ca, Mg, and Fe2+ were used to determine clinopyroxene 11 endmember percentages (wollastonite, enstatite, ferrosilite). Spinel group oxides were split into three different recalculation software. Magnetite (ulvospinel) was normalized with the same program listed above, and the elements were calculated on the basis of four oxygen. Ilmenite was normalized using Andy Tindle software (http://www.open.ac.uk/earth- research/tindle/AGTWebPages/AGTSoft.html), where the elements were calculated on the basis of six oxygen. Chrome-rich spinel was normalized using the program by Gabbrosoft.org on the basis of 32 oxygen.

Whole Rock Chemistry

Analyses were conducted on 24 samples for whole rock major and trace element concentrations. Each rock was cut, crushed, and sieved to < 2 cm pieces and carefully picked using a binocular microscope to extract grains that appear weathered or altered. A minimum of

28 grams of each sample was sent to the Peter Hooper Analytical Lab at Washington State

University (https://environment.wsu.edu/facilities/geoanalytical-lab/) for major and trace element analysis by x-ray fluorescence (XRF) and other trace elements, including rare earth elements

(REE), by inductively coupled plasma mass spectrometry (ICP-MS).

Fused beads for XRF analysis were prepared at the Peter Hooper Analytical Lab. After grinding the crushed rock, weighing with di-lithium tetraborate flux with a 2:1 ratio of flux to sample, fusing at 1000°C in a muffle oven and cooling, the bead is then reground, refused and polished on diamond laps for a smooth analysis surface. Analyses were conducted on the

ThermoARL Advant’XP+ sequential X-ray fluorescence spectrometer, which measures 10 major elements and 19 trace elements (Si, Al, Ti, Fet, Mn, Ca, Mg, K, Na, P, Sc, V, Ni, Cr, Ba, Sr, Zr,

Y, Rb, Nb, Ga, Cu, Zn, Pb, La, Ce, Th, Nd, U). The concentrations are measured by comparing 12 the x-ray intensity to nine USGS standard samples (PCC-1, BCR-1, BIR-1, DNC-1, W-2, AGV-

1, GSP-1, G-2, and STM -1).

The ICP-MS measures 27 trace elements (Ba, Th, Nb, Y, Hf, Ta, U, Pb, Rb, Cs, Sr, Sc, Zr and 14 REE). The Peter Hooper Analytical Lab uses a fusion-dissolution method to help rid unwanted matrix elements. The flux used is di-Lithium-tetraborate, and the dissolution is by open-vial mixed acid digestion. The reagents include HNO3 69-70%, HF 48-52%, HClO4 67-

71%, and H2O2. Solutions are analyzed on an Agilent 7700 ICP-MS with a final dilution factor of 1:4800 relative to the amount of sample fused.

For this study, the 10 major elements and 6 trace elements (Cr, Ni, Ga, Cu, V, Zn) were used from the XRF measurements, and all 27 trace elements measured from ICP-MS were used due to a higher precision in the data. In each technique, the lab conducted a duplicate run on the same sample to test for precision. For XRF analyses, major elements with concentrations over 5 wt. % have standard deviations that are < 0.07 wt.%. For trace element concentrations over 10 ppm, standard deviations that are ≤ 5 ppm. For ICP-MS analyses, trace element concentrations over 10 ppm have standard deviations between 0.01 and 3.5. 13

CHAPTER 4. RESULTS

Petrography

A total of 23 rock samples were made into thin sections and examined under the petrographic microscope for descriptions of texture and for mineral identification (Table 1) as well as to assess overall suitability for mineral chemistry and 40Ar/39Ar dating. The basaltic pillow lavas, lavas and dikes are porphyritic with olivine and plagioclase being the most abundant phenocrysts and clinopyroxene phenocrysts are less abundant. Cubic opaque grains are optically identified as magnetite and high aspect ratio opaque grains are identified as ilmenite (Fig. 7). These opaque oxides are sometimes altered to hematite. Overall, the majority of the phenocrysts appear to be in equilibrium with surrounding matrix and range from euhedral to anhedral. Disequilibrium textures are rare but evidence of resorption in olivine, plagioclase, and clinopyroxene phenocrysts is observed (Fig. 8A-B). Clusters of plagioclase phenocrysts are sometimes present, and both Carlsbad to albite twins are observed, with or without oscillatory zoning (Fig. 8H).

The groundmass textures range from hypocrystalline to hypohyaline, and tachylitic (Fig. 8C-

H). Hypocrystalline samples at both edifices have a crystalline groundmass with an intermediate grain-size (100-200 µm) with only a small percentage of glass. Hypohyaline groundmass at both edifices is predominantly glassy, and contains a higher ratio of glass to crystals. Both hypocrystalline and hypohyaline groundmass is dominated by plagioclase, clinopyroxene, opaque oxides, and olivine. One sample (ME15-011) is referred to as a tachylite because the groundmass contains only black glass (Fig. 8G). Vesicularity ranges from non-vesicular to samples with up to 30% vesicles by volume. On average, Sheridan Bluff samples have higher percentage of phenocrysts, and hypocrystalline groundmass is more prevalent at both edifices.

The textures of groundmass range from pilotaxic to sometimes trachytic. 14

Age

Field observations reveal that Mt. Early and Sheridan Bluff are monogenetic edifices that were likely erupted over a relatively short time-period (i.e. months to a few years). Seven samples of whole rock groundmass and plagioclase separated from three samples were analyzed for dating by 40Ar/39Ar method (Table 1). Overall, dating of the groundmass provided poor results that did not yield reliable isochron or plateau age. All groundmass age determinations in

Table 1 are intergraded (aka. total fusion) ages. The poor results are likely the result of recoil of

39 Ar K during irradiation. Groundmass from sample ME15-003 provides the best intergraded age

(i.e. lowest MSWD but very high at 87.07) at 19.32 ± 0.20 Ma, which if usable suggests that Mt.

Early maybe as much as 4 million years older than previously determined by Stump et al., (1980) and closer in age to Sheridan Bluff. The dating of plagioclase proved to be more reliable and likely represent accurate eruption ages. One sample at Mt. Early yields a plagioclase plateau age of 19.31 ± 0.32 Ma, and the two samples at Sheridan Bluff yield a plateau age of 19.80 ± 0.23

Ma and a weighted mean age of 20.39 ± 0.60 (Table 1). Errors are ± 2 sigma, and while uncertainties are high due to the low-K content, they are much better than groundmass results

(MSWD ≤ 9).

Mineral Chemistry

Four samples from Sheridan Bluff and four samples from Mt. Early were analyzed for mineral chemistry (Fig. 9). The four samples from each volcano were chosen to best represent the compositional spectrum of whole rocks (i.e. alkaline to subalkaline types), which are presented in Figure 10 and the details of their whole rock composition are described in the following section. 15

Three to four olivine phenocryst cores and their rims were analyzed on all eight samples

(Table 2; Fig. 9A). At Sheridan Bluff, olivine compositions from alkaline basalts range from Fo

58 to 82, and olivine in tholeiite basalt range from Fo 80 to 85. At Mt. Early, olivine in alkaline samples range from Fo 71 to 78, and for olivine in basalt that is transitional in composition between alkaline and sub-alkaline types (Fig. 10) the range is from Fo 77 to 84. Normal zoning occurs in olivine phenocrysts. The Mg content of rims are lower than cores with complimentary higher Fe and Ca-contents. Normally zoned olivine in alkaline basalt from Sheridan Bluff have the greatest variation in Mg-content between cores and rims, with the highest being a difference of 22.8 mol % Fo. Normal zoning in olivine for all other basalts from Sheridan and Early show only minor variations between cores and rims.

Clinopyroxene phenocryst cores and rims were measured on five samples, at least one from each composition at both volcanoes (Table 3; Fig. 9B). One to four phenocrysts were analyzed per sample. At Sheridan Bluff, clinopyroxene in alkaline basalt range from Wo 45-48,

En 31-45, Fs 9-21, and tholeiitic basalt from Wo 43-44, En 32-36, Fs 21-22. At Mt. Early, clinopyroxene in alkaline basalt range from Wo 46-47, En 41, Fs 12-14, and for transitional basalt Wo 47, En 30-31, Fs 22-23. The overall range of compositions from diopside to augite

(i.e. increasing Fe2+ and decreasing Ca content) are shown in Figure 9B, with only the tholeiites plotting in the augite field. Normal zoning takes place in clinopyroxene phenocrysts where Mg- content is lower and Ca-content is higher at rims versus cores. Similar to what was found in olivine, the whole rock alkaline samples from Sheridan Bluff show the strongest normal zoning in clinopyroxene (7 mol% En). Normal zoning in clinopyroxene for all other basalt types from

Sheridan Bluff is minor, and no variations from Mt. Early are recorded, however this is based on 16 a limited number of measurements. The clinopyroxene in tholeiitic basalt contain less calcium

(Fig. 9B).

Plagioclase phenocryst cores and rims were measured on seven samples and are predominately labradorite (Table 4, Fig. 9C). At Sheridan Bluff, plagioclase in alkaline samples range from An 48 to 67, and tholeiitic basalt range from An 56 to 67. At Mt. Early, alkaline samples range from An 55 to 62 and for transitional basalt range from An 58 to 68. Similar to olivine and clinopyroxene, plagioclase phenocrysts in alkaline basalt from Sheridan Bluff show the strongest normal zoning, with anorthite content lower at rims relative to cores by as much as

16 mol %. However, unlike coexisting olivine and clinopyroxene phenocrysts, the plagioclase in all other basalt types from both volcanoes also show normal zoning. Sheridan Bluff tholeiite show 10 mol% lower anorthite content in cores relative to rims. Plagioclase from basalts at Mt.

Early show depletions in anorthite content of up to 7 and 4 mol% in alkaline and transitional basalts, respectively, between cores and rims. Two phenocryst rims from alkaline basalt at

Sheridan Bluff have the most sodic values measured (An 48-50%) and plot at the boundary between labradorite and andesine (Fig. 9C).

In summary, all olivine, clinopyroxene and plagioclase phenocrysts measured in Mt.

Early and Sheridan Bluff basalts have more evolved outer rim compositions than cores (i.e. not reversely zoned). The thin, normally zoned outer rims on phenocrysts suggests that most of the crystal growth (i.e. cores) occurred in chemical equilibrium with the melt, which is supported by the scarcity of disequilibrium textures observed (see Section 4.1). It is likely that the rims formed at low pressure when the magmas rose and erupted at the surface.

Five samples were analyzed for spinel group oxides and are classified as magnetite, ilmenite, and Cr-spinel (Table 5-7; Fig. 7; Fig. 9D-E). Ilmenite contains around 20 wt% more 17

TiO2 and around 20 wt% less FeO than the titaniferous magnetite and fall just above (i.e. higher

Ti) the magnetite‒ulvospinel join in Figure 9D. Magnetite and ilmenite coexist in the tholeiite and alkaline basalts from each volcano (Fig. 7A-F) and allow for temperature and oxygen fugacity of the magmas to be calculated (see section 4.5). Ilmenite is optically identified in one transitional basalt (ME15-014) but was not analyzed. Four analyses of Cr-rich spinel (21-25 wt%

Cr2O3) were measured in one transitional basalt from Mt. Early (Fig. 7G-H). Some samples of

Cr-spinel border the field of chromite on the classification diagram, but since the majority fall within Cr-spinel (Fig. 9E), they will be collectively referred to as Cr-spinel in this study.

Whole Rock Chemistry

On a total-alkali versus silica (TAS) diagram, basalts from Mt. Early and Sheridan Bluff fall within the alkali basalt, hawaiite, and mugearite classification fields (Fig. 10A). Both volcanoes display a range in composition from alkaline (up to 6 wt. % Ne-normative) to subalkaline (up to 6 wt. % Hy-normative) (Table 8). For this study, the basalts are divided into three types: alkaline, transitional and tholeiite. The transitional basalts at Mt. Early are categorized based on their whole rock compositions that lie in-between the least evolved tholeiite at Sheridan Bluff and the most evolved alkaline basalt at Sheridan Bluff. They fall within the less evolved portion of the hawaiite field, while the alkaline basalt from Mt. Early are the most evolved and straddle the hawaiite/mugearite boundary (Fig. 10A). The classification of basalts as tholeiite is based on their subalkaline composition (Hy-normative), overall higher MgO and FeO contents and lower trace element concentrations relative to alkaline types. The tholeiitic nature of basalts collected from Sheridan Bluff is confirmed and further distinguished from alkaline types using the classification scheme of Floyd & Winchester (1975) shown in Figure 10B.

Basalts previously analyzed for Mt. Early and Sheridan Bluff (Stump et al., 1980; Licht et al., 18

2018; Table 8) are plotted on Figure 10 for comparison. Also provided by Licht et al. (2018) are whole rock data from basaltic erratics found in the moraine of Mt. Howe, which lies ~40 km south of Mt. Early. Licht et al. (2018) suggest that the volcanic source for the erratics lie 60 km upstream (~south) from the Mt. Howe moraine, identified by a circular negative magnetic anomaly that may indicate a subglacial volcano. In addition, fields representing the composition of intraplate basalts (with restricted SiO2 from 43 to 51 wt. %, and total alkalis <7 wt. %) from the WARS, New Zealand and Australia (i.e. DAMP; Finn et al., 2005) are included in Figures

10-14 for comparison.

Whole rock MgO concentrations in Sheridan Bluff and Mt. Early basalts have a broad range from 10 to 4 wt.% but have a very restricted range of SiO2 from 48 to 51 wt.%. The range in SiO2 content is also restricted within each edifice, but overall Sheridan Bluff shows lower

SiO2, between 48 and 49 wt. %, and Mt. Early is slightly higher SiO2 between 49 and 51 wt. %

(Fig. 10A). Overall, the basalts show decreasing concentrations of FeOt, CaO, and MnO, and increasing concentrations of TiO2, Al2O3, Na2O, K2O, and P2O5 with decreasing MgO content

(Fig. 11). Samples from Mt. Early and Sheridan Bluff analyzed previously (Stump et al., 1980;

Licht et al., 2018) fall within the same compositional range as basalts collected in this study.

Basalt erratics from Mt. Howe (Licht et al., 2018) fall mostly within the range of this suite, except they have notably lower Na2O contents and several samples also show higher CaO concentrations. Basalts from Mt. Early and Sheridan Bluff show overall lower TiO2, FeOt, and higher Al2O3 than basalts from DAMP, and are particularly distinctive from the basalts found in

West Antarctica (i.e. WARS). The tholeiites at Sheridan Bluff have the lowest P2O5 contents relative to all other basalts. 19

Trace element concentrations in Sheridan Bluff and Mt. Early basalt mostly fall within the range of basaltic compositions found in DAMP with the exception of tholeiite, which show relatively lower concentrations of Ba, Nb and Sr (Fig. 12). In addition, it is important to note that with the exception of two samples from the north island of New Zealand (Timm et al.,

2010), the tholeiite from Sheridan Bluff are the only basalts found in DAMP that fall within the tholeiitic basalt field as classified by Floyd and Winchester (1975) shown in Figure 10B.

Tantalum concentrations and Sm/Yb ratios in Sheridan Bluff tholeiite have uniquely lower values over the range of their MgO contents in comparison to all other basalts. Overall, Sheridan

Bluff and Mt. Early basalt display a decrease in concentrations of compatible elements Cr, Sc, and Ni and increases in all incompatible elements with decreasing MgO content. It is important to note, however, that over the range of MgO contents in the tholeiite (~10 to 7 wt. %), concentrations of incompatible elements remain relatively constant. All basalts from Sheridan

Bluff and Mt. Early are enriched in incompatible elements relative to Primitive Mantle (Fig.

13A-B). Alkaline basalt at both Mt. Early and Sheridan Bluff are enriched in incompatible elements (La 33-49 ppm, Ba 270-484 ppm, Sr 712-1009 ppm), have LaN/YbN ratios >10 and show minor Pb negative anomalies and lack K negative anomalies on the Primitive Mantle normalized, multi-element diagrams. Transitional basalt at Mt. Early have slightly lower concentrations in nearly all incompatible elements relative to alkaline basalt (Fig. 13A).

Tholeiitic basalts at Sheridan Bluff have lower concentrations of incompatible elements (La 14-

16 ppm, Ba 110-144 ppm, and Sr 358-380 ppm), LaN/YbN ratios <5, and lack Pb and K negative anomalies but show minor P negative anomalies (Fig. 13B). Chondrite normalized REE patterns for alkaline basalt from Mt. Early and Sheridan Bluff mirror basalt from DAMP (Fig. 13C-D). 20

However, REE patterns for tholeiite from Sheridan Bluff are flatter and overall less enriched in

LREE (Fig. 13D).

A significant distinction between basalts from Mt. Early and Sheridan Bluff, particularly transitional and tholeiite, from basalts associated with the West Antarctica rift and the rest of

DAMP is the absence or subdued nature of negative anomalies for K and Pb on normalized plots

(Fig. 13A-B). To highlight this distinction, I have plotted a measure of each anomaly (K/K* and

Pb/Pb*) against MgO content in Figure 14. Anomalies of K and Pb are normalized based on their neighboring elements on the normalized multi-elemental diagrams (Fig. 13). All samples for Mt. Early and Sheridan Bluff plot outside of the WARS field for K/K* and Pb/Pb* and plot close to the value of 1.0 (a value of 1.0 represents a relatively flat pattern, i.e. lack of anomaly).

Tholeiitic and transitional basalt also plot outside of the K/K* and Pb/Pb* fields for New

Zealand basalt. All basalt at Mt. Early remains unique to all the fields of DAMP on the MgO versus Pb/Pb* plot.

Thermobarometry

Clinopyroxene and olivine core and rim compositions along with their whole rock data were used to estimate temperature and pressure conditions during crystallization (Fig. 15). Cores and rims were used as individual points and were not averaged. Equilibrium between the mineral and

cpx-liq coexisting liquid [KD(Fe-Mg) ] must have values between 0.27-0.33 (Putirka, 2008). Only the estimates in equilibrium with corresponding whole rock compositions were used for calculations (Tables 9 & 10). Multiple estimates using different equations for calculating temperatures and pressures were provided; one or more estimates may be used to compare or to provide an average if they are within error range (Putirka, 2008). 21

The average temperatures estimated for clinopyroxene phenocrysts range from 1135° to

1206°C with an error of ± 69°C (Table 9). The tholeiite sample (SB15-001) has the highest temperature (1206°C) relative to transitional and alkaline basalt (1135-1191°C) (Fig. 15A).

Clinopyroxene phenocrysts pressure estimates range from 2.3 to 8.9 kbar with an error of ± 4.6 kbar (Fig. 15B). The lowest pressure is recorded in an alkaline sample from Sheridan Bluff

(SB15-003). The tholeiitic basalt has a lower estimated pressure (3.2 kbar) than the transitional and two of the three estimates calculated for alkaline basalts (5.1-8.9 kbar). It is important to note that all pressure estimates overlap and are indistinguishable from one another within error.

Four equations were used to calculate an average temperature for olivine phenocrysts (Table

9). This calculation requires an estimate of pressure. The average pressure estimate calculated for clinopyroxene in all five samples (SB15-001, ME15-004, SB15-003, SB15-008 and ME15-

003) is equal to 6 kbar and is used to calculate all olivine temperatures. The average temperatures estimated for olivine phenocrysts range from 1128° to 1282°C with an average error of ± 49°C. Again, the tholeiitic basalt shows higher average temperatures (1202-1332°C) than the transitional and alkaline basalts (Fig. 15A). As with pressure, the error estimates overlap and are indistinguishable from one another within error.

Three estimates of temperature and two estimates of oxygen fugacity (fO2) were determined for coexisting magnetite and ilmenite measured in four samples (Tables 9 & 10). Average temperature estimates range from 872 °C to 1120 °C and log fO2 range from -14 to -9.5. Like the clinopyroxene and olivine temperatures, the average tholeiite temperature calculated from magnetite-ilmenite pairs is higher than those calculated for alkaline basalts. Magnetite-ilmenite pairs were not measured in transitional basalts. Oxygen fugacity is used to represent the redox 22 state of a system. Mt. Early and Sheridan Bluff averages (Table 10) plot below the FMQ buffer

(Iron-Magnetite-Quartz reaction) on a graph of temperature vs. log fO2. 23

CHAPTER 5. DISCUSSION

The basalts at Sheridan Bluff and Mt. Early have been thoroughly described, classified, and compared to other Late Cenozoic intraplate basalts within Antarctica (i.e. WARS) and elsewhere

(i.e. DAMP). I will now focus on an understanding of their petrogenesis, which includes mantle source and modifications to melt composition during ascent to the surface. A particular feature of interest is the unique compositional association at Sheridan Bluff relative to volcanoes in the

WARS, which is the occurrence of alkaline and tholeiite basalt types. The coexistence of tholeiitic and alkaline basalts is not uncommon in continental intraplate settings elsewhere

(Lustrino et al., 2002; Xu et al., 2005). Tholeiitic magmas are most often considered to be produced by higher degrees of partial melting at shallower mantle depths relative to alkaline melts (Jung et al., 1998; Boyce et al., 2015; Khogenkumar et al., 2016; Kocaarlson et al., 2018).

Higher degrees of melting of peridotite results in a higher proportion of olivine melting which results in magmas with higher silica contents and lower concentrations of incompatible elements relative to alkali basalt. Hoernle et al. (2006) suggest that tholeiitic and alkaline volcanism in southern New Zealand relates to the amount of lithosphere removed from its base, where greater amounts allow higher degrees of melting to form tholeiitic shield volcanoes, and lesser amounts allow smaller degrees of melting and form fields of smaller-volume alkaline volcanoes.

Alternatively, the contemporaneous occurrence of tholeiitic and alkaline volcanism has also been explained by fractionation of pyroxene from tholeiitic melts at moderate pressure (~1 GPa) to produce alkaline magmas (Naumann & Geist, 1999; Wanless et al., 2006).

In order to understand the origin of alkaline and tholeiitic basalt an evaluation of melting and post-melting differentiation processes, namely fractional crystallization and crustal contamination, is undertaken. Contamination of primary magmas by continental crust will 24 change the major and trace element composition of a magma, though only significantly if the composition of the assimilant is distinct from the magma. A more sensitive measure of contamination is provided by radiogenic isotopes (i.e. Sr, Nd, Pb), however in this study my assessment of contamination is based strictly on major and trace element data. Future work on these samples will incorporate radiogenic and oxygen isotopic data.

Evolution by Crystal Fractionation

Fractional crystallization (FXL) is a fundamental way in which magmas evolve. Olivine and pyroxene are the high temperature liquidus phases that are fractionated from basaltic magmas.

Specifically, clinopyroxene fractionation at pressures between 0.5 and 1.5 GPa have been attributed to creating alkaline magmas from a tholeiitic parent at Mauna Loa volcano, Hawaii

(Wanless et al., 2006). In order to provide a qualitative evaluation of the role of mineral fractionation in the evolution of magmas from Mt. Early and Sheridan Bluff whole rock major element and mineral data are plotted on variation diagrams and trends are compared in Figure 16.

Overall, the arrays of whole rock compositions shown in Figure 16 indicate that the removal of olivine and clinopyroxene are likely to be the predominant influence on magma evolution.

Plagioclase is an abundant phenocryst phase but has been ruled out as having a major role in magma evolution based on the lack of Eu negative anomaly and slight Sr positive anomaly (Fig.

13). Both Eu and Sr are compatible in plagioclase. The Sr anomaly is expected to be negative if plagioclase was significantly fractionating, and positive if there is an accumulation. Quantifying the Sr anomaly on a normalized plot shows a very slight decrease in Sr/Sr* with decreasing MgO wt. %, with values just over 1.

To test the effect of crystal fractionation on major element compositions, a simple olivine- only addition model was used. Olivine was added incrementally to a tholeiite (SB15-001) while 25 maintaining an Fe2+/Mg Kd value equal to 0.30 until the Mg# of the whole rock reaches 73 and olivine reaches Fo90 (values in equilibrium with peridotite). With these parameters, it takes <

18% olivine addition to tholeiite to reach these equilibrium mantle values. While this is a reasonable and predictable amount, it also requires that the SiO2 content decrease by 1.24 wt. % and MgO content increase by over 6 wt. %. Yet at Sheridan Bluff, to explain tholeiite as being parental to alkaline basalt the SiO2 content has to increase slightly (between 0.2 and 0.8 wt. %) and MgO content decrease (between 1 and 5 wt. %). For simple olivine addition, total alkalis

(Na2O + K2O) would also decrease by 0.6 wt. % (i.e. simple dilution), however, at Sheridan

Bluff alkalis increase by 2.7 wt. % between tholeiite and alkaline basalt.

To further evaluate the case for FXL in producing alkaline compositions from tholeiite or transitional basalt, whole rock trace elements (i.e. REE) are modelled for both clinopyroxene and

(D-1) olivine fractionation using the Rayleigh equation Ci = Co (F ), where Ci is the elemental concentration derived from FXL of olivine and clinopyroxene, Co is the elemental concentration of the parent magma, F is the percent of liquid remaining, and D is the bulk partition coefficient for clinopyroxene and olivine. Tholeiites are used as parental compositions for alkaline basalt from Sheridan Bluff (samples SB15-001, SB15-002, SB15-004, SB15-005, SB15-009; Table 8) and transitional basalt as parental compositions for alkaline basalts from Mt. Early (samples

ME15-001, ME15-004-ME15-008, ME15-014, Table 8). The models are constructed with the total amount of crystallization set to 20% (F = 0.80) and assumes greater fractionation of clinopyroxene relative to olivine [D = (0.7*Kdcpx) + (0.3*Kdolivine); with Kdcpx values from Green et al. (2000) and Kdolivine from Pilet et al. (2011)]. The results of the models are displayed on chondrite normalized REE plots in Figure 17. In both cases, LREE concentrations of the model compositions are too low and the concentration of HREE are slightly too high to match the 26 compositions of the alkaline basalts. In order for the model to match the LREE contents of the alkaline basalts at Sheridan Bluff an additional 20% FXL would be needed, however, this would cause the concentrations of HREE in the model be too high. These trace element variations, as well as major elements discussed above, are the opposite of what is needed to predict for a tholeiite evolving to alkaline basalt. Therefore, fractional crystallization alone cannot explain the compositional array of Sheridan Bluff and Mt. Early basalts (i.e. the negative correlation between MgO content and incompatible elements shown in Figures 12 and 13). Specifically for

Sheridan Bluff, some other differentiation process must be responsible to dramatically increase the LREE while keeping SiO2 and HREE concentrations nearly constant between tholeiite and alkaline basalt (Figs. 10A & 13D).

Crustal Contamination

The contamination of magma by continental crust is evaluated and to assess whether this process can explain the association of alkaline and tholeiite magmas at Sheridan Bluff.

Contamination of tholeiite magmas by granitoids would increase trace element concentrations and concurrent with fractional crystallization would cause a decrease MgO content but also should increase SiO2 content. In general, the silica content of the entire Mt. Early and Sheridan

Bluff suite does increase slightly (~ 2 wt.%) with decreasing MgO, however there is no increase

(< 1 wt.%) between tholeiite and alkaline basalt at Sheridan Bluff over the range in MgO content

(~ 6 wt.%). Furthermore, the SiO2 content between transitional basalt and alkaline basalt at Mt.

Early shows only a slight increase (~ 1 wt.%) over the range of MgO content (Fig. 11).

Trace element ratios can also be used to assess crustal contamination of magmas. Ratios of

Ce/Pb are generally low in crustal material (< 4, Rudnick & Fountain, 1995). Basalts from

Sheridan Bluff and Mt. Early have Ce/Pb ratios that are low (between 11 and 21) relative to most 27

DAMP basalts (between 17 and 80) but show a negative correlation with MgO (increasing Ce/Pb with decreasing MgO wt.%), which suggests that crustal contamination is not a significant process (Furman, 2007; Ayalew et al., 2016). However, to further evaluate crustal contamination quantitatively, I employ a well-established geochemical model using potential contaminants from the Transantarctic Mountains (TAM).

The combined assimilation-fractional crystallization (AFC) process (DePaolo, 1981) is modeled using common basement compositions found in the Transantarctic Mountains (TAM) of Antarctica. These include eclogite, metasediments and granitoids (Granite Harbor Intrusives) from northern and southern Victoria Land (Allibone et al 1993; Di Vincenzo et al., 1997; Dallai et al., 2003; Henjes-Kunst & Schussler, 2003; Di Vincenzo et al., 2014), as well as average values for the lower and upper continental crust (Taylor and McLennan, 1985). The AFC model

푟 퐶 = ((퐶표 × 푓) + ) × (퐶 × (1 − 푓)) 퐶표 equation of DePaolo, (1981) is 퐿 퐿 (푟−1+퐷) 푎 where 퐿 is the concentration of the original magma, Ca is the concentration of the assimilant, r is the ratio of the mass assimilation rate to mass crystallization rate (Ma/Mc), D is the bulk solid/liquid partition coefficient for the elements fractionating, and f is the fraction of liquid remaining. AFC models for possible contaminant compositions are evaluated on a Rb versus TiO2 plot shown in Figure

18. Nearly all of the AFC model curves that were calculated (only two are shown in Fig. 18) do not encompass the data trend from tholeiite to alkaline basalt. However, by contaminating tholeiite with diorite (sample C6 from Dallai et al., 2003) the compositional array of the basalts can be matched over an interval of 10% crystallization with a Ma/Mc ratio of 0.8 (Fig. 18). In

Figure 19A the AFC model for contamination of tholeiite by diorite, also matches well the

Sheridan Bluff and Mt. Early data array. However to explain La/Nd ratios (Fig. 19B), or any other LREE/MREE ratio, unrealistic bulk partition coefficients are required [(i.e. MREE would 28 have to behave compatibly, which is not reasonable for basaltic systems. (Fujimaki et al., 1984;

McKenzie & O’Nions 1991; Green et al., 2000)]. The unacceptable model results also occur when the contaminant is more mafic (e.g., eclogite, gabbro).

Although the above discussion and modeling results does not disprove the possibility that individual magma batches were contaminated by continental crust, it does however indicate that the overall compositional variations in basalts from Mt. Early and Sheridan Buff (Figs. 10-13) is not a result of contamination. Furthermore, I argue below that the coexistence of tholeiitic and alkaline lavas within the same monogenetic volcano (Sheridan Bluff) cannot be explained by

AFC or FXL processes and must now be evaluated as a result of differences in mantle source composition and/or melting processes.

Partial Melting

As mentioned previously, changes in degree of partial melting have been used to explain the coexistence of tholeiite and alkaline magmas (Jung et al., 1998; Wanless et al., 2006; Boyce et al., 2015; Khogenkumar et al., 2016; Kocaarlson et al., 2018). It generally considered that alkaline basalts are formed at greater depth and by smaller degrees of melting relative to tholeiitic melts. With regards to source composition, trace element concentrations can provide clues about what type of mantle is being melted, which in turn, can offer clues about the relative depths and the degrees of melting. For example, HREE and some HFSE are compatible with garnet (e.g., Yb, Y, Lu) during melting and will remain in the source if garnet is residual. This results in low relative concentrations for these elements in the extracted melt as seen on mantle normalized, multi-element plots (Fig. 13). Higher ratios of La/Yb in relatively unfractionated basalts can be related to smaller degrees of partial melting (Wanless et al., 2006) and is enhanced if garnet remains in the source. Alkaline and transitional basalts from Mt. Early and Sheridan 29

Bluff have relatively high La/YbN ratios (between 11 and 16) and are comparable to WARS and other DAMP basalts (Fig. 13C & D). Garnet is considered to be residual in the source for many

WARS and DAMP basalts (Rocholl et al., 1995; Hart et al., 1997; Panter et al., 2000; 2006;

2018; Nardini et al., 2009; Aviado et al., 2016).

To evaluate melting conditions further I have chosen trace element ratios that have been previously used to distinguish relative differences in degree of melting (Wanless et al., 2006;

Boyce et al., 2015) and relative depth of melting (Boyce et al., 2015) plotted in Figure 20. These geochemical proxies suggest that the tholeiites were formed from higher degrees of melting at shallower depths, and the alkaline and transitional were formed from lower degree of melting at deeper depths. In Figure 20A, the difference in La/Yb ratios require different degrees of partial melting (Wanless et al., 2006). Sheridan Bluff tholeiite are unique from WARS volcanism and

Mt. Rouse, Australia volcanics (Boyce et al., 2015), plotting at values that indicate much higher degrees of melting, while both Mt. Early and Sheridan Bluff basalt plot within the field for New

Zealand basalt (Timm et al., 2010). In Figure 20B, Mt. Early and Sheridan Bluff basalt are unique with regards to WARS volcanism and basalt from Mt. Rouse, Australia (Boyce et al.,

2015), and plot within the trend for New Zealand basalt (Timm et al., 2010) at shallower depths and higher degrees of melting, especially the tholeiites. Overall, basalt from Mt. Early and

Sheridan Bluff maybe from shallower depths (i.e. higher Gd/Yb) relative to WARS basalt.

Differences in degree of partial melting is arguably the most viable case for the coexistence of alkaline and tholeiitic basalts at the two volcanic edifices.

Mantle Sources for Basalt

A difference in degree of partial melting is the first order explanation for the association of different basalt compositions at Sheridan Bluff and Mt. Early, however, modal and 30 compositional differences in the source mantle can also control the geochemistry of the melt that is produced. I test for both source differences and changes in partial melting degree by using non-modal batch melting models presented in Figure 21. For models I have selected source concentrations and modes of natural lithospheric xenoliths that include spinel lherzolite from Mt.

Morning volcano, south Victoria Land (Martin et al., 2013) and garnet and spinel lherzolite from the Vitim volcanic field within the Baikal rift, Siberia (Ionov, 2004). Mantle source compositions derived for West Antarctic alkaline basalts (Hobbs Coast, Hart et al., 1997),

Primitive Mantle (PM, Hoffman 1988) and an Enriched Depleted MORB Mantle (EDMM,

Workman & Hart, 2005) are also used to in this modelling exercise. Spinel-bearing mantle sources do not match the trend of alkaline and transitional basalts shown in Figure 21A. Spinel lherzolite from Vitim (Ionov 2004) could match tholeiite compositions but it unrealistic because the model would need to extend to unreasonably high degrees of partial melting (~30%). Basalts from Mt. Early and Sheridan Bluff are matched more closely by the melting model for garnet lherzolite from Vitim (Ionov 2004). Mt. Early basalts lie between 2 and 4% melting, Sheridan alkaline basalt at nearly 4%, and tholeiite at nearly 15%. Other model sources are tested and shown in Figure 21B. The source compositions derived for basalts from Hobbs Coast (Marie

Byrd Land, Hart et al., 1997) are evaluated for both spinel- and garnet-bearing modes. A spinel lherzolite Hobbs source could explain alkaline and transitional basalt with melting between 4 and

10%, however, it does not explain the tholeiitic basalt based on the unreasonably high (>30%) partial melting that is needed. The melting model for garnet lherzolite source, using the composition of Primitive Mantle (PM, Hoffman, 1988), fits reasonably the basalts from Mt.

Early and Sheridan Bluff between 1% (alkaline) and 7% (tholeiite) partial melting. Partial 31 melting of an EDMM results in model compositions that are too depleted to match any of the basalts at Mt. Early or Sheridan Bluff.

Based on modeling of partial melting of natural and theoretical mantle source compositions

(Fig. 21) it can be concluded that all of the basalts at Mt. Early and Sheridan Bluff are most likely from a common garnet-bearing mantle source, with tholeiite formed at higher degrees of partial melting and alkaline basalt at lower degrees of partial melting. This conclusion is also based on the observation of trace element patterns, particularly the fact that HREEs (Lu and Yb) are similar in concentration amongst all basalts while LREE vary significantly (Fig. 13).

Causes for Volcanism

If the basaltic magmas were generated by different degrees of partial melting of a common garnet-bearing source, then what could cause this change in melting within these small volume, monogenetic edifices over a relatively short period of time? Recent geophysical evidence places

Mt. Early and Sheridan Bluff directly above a shallow (50-80 km) low velocity zone in the upper-most mantle. The lower seismic velocities likely reflect elevated temperatures and therefore could promote melting to generate volcanism (Shen et al., 2017; Heezsel et al., 2016).

The cause of the shallow low seismic velocity is interpreted by Shen et al. (2017) to be a result of lithospheric delamination. This mechanism has also been previously proposed for DAMP volcanism in New Zealand (Hoernle et al., 2006). Shen et al. (2017) indicate that the removal of mantle lithosphere causes warm asthenosphere to intrude to shallow levels. The absence of lithospheric mantle beneath Mt. Early and Sheridan Bluff may explain some of their unique

(relative to WARS and DAMP) geochemical characteristics such as the lack of negative K anomalies on mantle normalized multi-element plots. Negative K anomalies are a prominent feature in WARS basalts (Fig. 13 & 14) and are considered to be a result of residual amphibole 32 in a metasomatized mantle source. Amphibole (kaersutite or pargasite) is stable at temperatures

~< 1150°C (Wallace & Green, 1991) and therefore can exist within the lithosphere mantle but would not be present within the asthenospheric mantle (>1350°C). Amphibole in the continental lithosphere of West Antarctica has been documented in xenoliths (Kyle, 1986; Rocchi et al.,

2002; Coltorti et al., 2004; Perinelli et al., 2006; 2011; Armienti & Perinelli, 2010; Perinelli et al., 2017) and is commonly modeled to explain the source of melts for WARS basalt (Rocchi et al., 2002; Nardini et al., 2009; Aviado et al., 2016; Panter et al., 2018). Therefore, the lack of negative K anomalies on mantle normalized multi-element plots of Sheridan Bluff and Mt. Early basalts supports the geophysical evidence that the lithospheric mantle beneath this portion of

East Antarctica has been removed and that melt sources are likely to be exclusively asthenospheric.

If lithospheric delamination is the driving mechanism for volcanism in this region, then two scenarios that seem most likely in explaining the coexistence of alkaline and tholeiite basalts at

Sheridan Bluff are; 1) that lithospheric delamination provided a high-relief sub-plate topography that facilitated contemporaneous melting over a range of depths, or 2) that melting was generated at approximately the same depth but from a modally heterogeneous source where higher degree melts were produced initially from the melting of more fusible materials followed shortly by lower degrees of melting as those materials became exhausted. A possible source for the second scenario would be one that contains both garnet-pyroxenite and garnet-peridotite available for melting (Hole, in review; Lambart, 2017). In this scenario, tholeiitic magmas would be generated initially by melting of the pyroxenite (higher degree) and alkaline basalt from peridotite (lower degree) at the same temperature (Hole, in review). Both lithologies can contain garnet to explain REE patterns. Garnet is a high pressure phase stable at pressures of ~2.5 GPa 33

(~92.5 km) but is stable at lower pressures in pyroxenite (Hole, in review; Lambart et al., 2013).

There are many complexities to address with the second scenario (e.g., types of pyroxenite, mantle potential temperatures, melt migration mechanisms/melt-rock interactions, anhydrous vs hydrous melting, etc.), however, further evaluation is beyond the scope of this paper and they will need to be carefully considered in future studies of Mt. Early and Sheridan Bluff basalts. 34

CHAPTER 6. CONCLUSION

Overall, basalts at Mt. Early and Sheridan Bluff contain compositions of alkaline and tholeiitic basalt. A transitional basalt composition has been defined in this study based on being compositionally in between the alkaline and tholeiite basalt presented here. The basalts display typical mineralogy for their compositions, including phenocrysts of olivine, plagioclase, and clinopyroxene. Olivine, clinopyroxene, and magnetite and ilmenite pairs were measured for thermobarometry calculations. Although these results come with high errors, the thermobarometry could suggest the tholeiites have a higher average temperature and lower average pressure. New 40Ar/39Ar age determinations on groundmass and plagioclase separates reveal that Mt. Early is ~19 Ma, which is significantly older than previously determined by K/Ar dating (Stump, 1980). Sheridan Bluff is dated at ~20 Ma. The results of this study show that basalt from Mt. Early and Sheridan Bluff, especially tholeiitic samples, have distinct geochemical differences from the WARS and the larger SW Pacific region that includes DAMP.

Fractional crystallization of olivine and clinopyroxene and assimilation-fractional crystallization involving compositions of Antarctic crust were tested as mechanisms to understand the affiliation of alkaline and tholeiite basalt coexisting at the same monogenetic center. However, the geochemistry suggest that alkaline and tholeiitic basalt may be related by different degrees of partial melting; where tholeiitic basalt is produced by higher degree melts at shallower depths.

However, it is difficult to explain what caused the change since it would have to occur suddenly to account for the coexistence of both compositional types at Sheridan Bluff. Models for the source and cause of DAMP volcanism include mantle plume, metasomatized lithospheric mantle, and asthenospheric mantle decompressed as a result of lithospheric delamination. Melting triggered by extension is not likely to cause such a rapid change in degree of fusion. A mantle 35 plume is also unlikely since it should produce much greater volumes. Lithospheric delamination may be the most viable mechanism for triggering volcanism in this region and would provide a high-relief sub-lithospheric topography from which melting can occur over a range of pressures.

This is supported by geophysical evidence that shows Mt. Early and Sheridan Bluff above a shallow low velocity zone in the upper-most mantle which could account for high enough temperatures to be a source for volcanism (Shen et al., 2017). The volcanism may also constrain the timing of this event at ~19-20 Ma. 36

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APPENDIX A. TABLES

Table 1. Summary of deposits, petrography, composition and age of samples from Mt. Early and Sheridan Bluff. Sample location(1) lithofacies(2) rock type(3) age(4) age(5) phenocrysts(6) phenos % texture(7) vesicle % ME15-001 ME pillow interior trans - - Ol > Pl 11 Hh 1 ME15-002 ME lapilli tuff/hyaloclastite breccia ------ME15-003 ME dike in the lower orange lapilli tuff alk 19.32 ± 0.20 Ma - Pl > Ol > CPX 10 Hc 0 ME15-004 ME pillow interior trans - - Ol > Pl 3 Hh 3 ME15-005a ME white material ------ME15-005b ME black “columnar intrusions” trans 19.42 ± 0.06 Ma - Ol > Pl 5 Hh 30 ME15-006 ME in the tuff behind "the Nubbin" trans - - Ol > Pl 10 Hc 30 (large) ME15-007 ME in the tuff behind "the Nubbin" trans - - Ol > Pl 10 Hc <1 ME15-008 ME pillow interior behind "the Nubbin" trans 19.31 ± 0.03 Ma - Ol > Pl 8 Hc <1 ME15-009 ME summit dike alk 18.88 ± 0.05 Ma - Pl > Ol > CPX 12 Hc 1 ME15-010 ME dike alk - - Pl > Ol > CPX 7 Hh 30 ME15-011 ME grey lapilli tuff above "the Nubbin" alk 19.53 ± 0.06 Ma 19.31 ± 0.32 Ma Pl > Ol 4 tachylite 30 ME15-012 ME dike alk - - Pl > Ol > CPX 6 Hc 10 ME15-013 ME float on the face near summit alk - - Pl > Ol 6 Hc 0 ME15-014 ME pillow outcrop trans - - Ol > Pl 4 Hh <1 ME15-015 ME float ------SB15-001 SB lowest lava above lapilli tuff thol 20.46 ± 0.08 Ma - Ol > Pl 7-10 Hc 10 SB15-002 SB float (next to 001) thol - - Ol > Pl 5-7 Hh 5 SB15-003 SB lava flow-mid top alk - 20.39 ± 0.60 Ma Pl > Ol 10 Hc 3 SB15-004 SB lava flow-lower thol - - Pl > CPX > Ol 25 Hc 5 SB15-005 SB lava flow-mid lower thol - - Pl > Ol > CPX 10 Hc 5 SB15-006 SB lava flow-mid alk - - Ol > CPX > Pl 10 Hc 5 SB15-007 SB float alk - - Pl > CPX > Ol 15 Hh 10 (large) SB15-008 SB lava flow - top northern flow alk 20.26 ± 0.04 Ma 19.80 ± 0.23 Ma Pl > CPX > Ol 10 Hh 20 SB15-009 SB lava flow - top north-east flow thol - - Ol > CPX 10 Hh 5 SB15-010 SB lava flow - top southern flow alk - - Pl > Ol 10 Hc 5 (1) ME: Mount Early. SB: Sheridan Bluff. (2) lithofacies refer to Figures 2 & 3. (3) Alk: alkaline basalt. Thol: tholeiitic (subalkaline) basalt. Trans: transitional alkaline basalt. (4) 40Ar/39Ar dating on groundmass reflect intergraded age ± 2 sigma. These samples were subject to recoil of 39Ar during irradiation. The bold date yielded the best result. (5) 40Ar/39Ar dating on plagioclase reflect intergraded age ± 2 sigma. Bold dates represent better accuracy in the results. (6) Ol: olivine. Pl: plagioclase. CPX: clinopyroxene. (7) Hh: hypohyaline: higher glass to crystal ratio. Hc: hypocrystalline: higher crystal to glass ratio. Tachylite: groundmass is completely glass. Phenocrysts and vesicles represent volume %. Large vesicles range from 1 to 7 mm. 49

Table 2. Olivine chemistry from electron microbeam analysis. SB01 SB01 SB01 SB01 SB01 SB01 SB01 Sample ol rim 1 ol core 1 ol rim 2 ol core 2 ol rim 3 ol core 3 ol rim 4 Location SB SB SB SB SB SB SB Rock type thol thol thol thol thol thol thol Analysis rim core rim core rim core rim

SiO2 39.38 39.97 40.05 40.25 40.53 40.61 39.28

TiO2 0.01 0.01 0.01 0.00 0.00 0.01 0.02

Al2O3 0.04 0.04 0.05 0.06 0.06 0.05 0.03

Cr2O3 0.04 0.06 0.03 0.03 0.03 0.03 0.02 FeO 17.54 15.93 15.83 14.46 15.58 14.22 18.10 MnO 0.24 0.24 0.19 0.16 0.17 0.22 0.30 MgO 42.20 43.49 43.86 44.62 43.55 45.17 41.87 CaO 0.28 0.23 0.23 0.22 0.23 0.23 0.25

Na2O 0.00 -0.01 -0.01 0.01 0.02 0.00 -0.03 NiO 0.16 0.18 0.19 0.23 0.18 0.25 0.14 Total 99.90 100.12 100.41 100.07 100.36 100.81 99.96 (Structural formula on the basis of 4 oxygen)* Si 1.01 1.01 1.01 1.01 1.02 1.01 1.00 Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Al 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Fe3+ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Fe2+ 0.37 0.34 0.33 0.30 0.33 0.30 0.39 Mn 0.01 0.01 0.00 0.00 0.00 0.00 0.01 Mg 1.61 1.64 1.65 1.67 1.64 1.68 1.59 Ca 0.01 0.01 0.01 0.01 0.01 0.01 0.01 (End member) Fo 80.57 82.49 82.74 84.21 82.87 84.52 79.95 Sample numbers are abbreviated to only include the location - SB: Sheridan Bluff. ME: Mount Early - and the number in sequence (01, 02, 03, etc.). The number for the year collected (“15” for 2015) has been omitted. Alk: alkaline basalt. Thol: tholeiitic (subalkaline) basalt. Trans: transitional alkaline basalt. Fo: forsterite. *Structural formula of elements and end member calculations from Mineral Formulae Recalculation. John Brady, Smith College and Dexter Perkins, University of North Dakota https://serc.carleton.edu/research_education/equilibria/mineralformulaerecalculation.html 50

Table 2 (continued). Olivine chemistry from electron microbeam analysis. SB01 SB03 SB03 SB03 SB03 SB03 Sample ol core 4 ol rim 1 ol core 1 ol rim 2 ol core 2 ol rim 3 Location SB SB SB SB SB SB Rock type thol alk alk alk alk alk Analysis core rim core rim core rim

SiO2 39.63 37.18 38.90 38.63 39.63 37.10

TiO2 0.02 0.07 0.01 0.02 0.01 0.06

Al2O3 0.06 0.02 0.04 0.03 0.02 0.02

Cr2O3 0.05 -0.02 0.01 0.02 0.01 -0.01 FeO 15.00 27.16 18.15 21.81 18.02 31.99 MnO 0.19 0.46 0.29 0.31 0.27 0.50 MgO 44.49 34.28 41.50 38.33 41.58 30.76 CaO 0.22 0.41 0.29 0.38 0.28 0.45

Na2O -0.02 0.03 0.01 0.02 0.01 0.01 NiO 0.23 0.02 0.07 0.07 0.10 0.03 Total 99.87 99.61 99.28 99.62 99.93 100.91 (Structural formula on the basis of 4 oxygen)* Si 1.00 1.00 1.00 1.01 1.01 1.01 Ti 0.00 0.00 0.00 0.00 0.00 0.00 Al 0.00 0.00 0.00 0.00 0.00 0.00 Cr 0.00 0.00 0.00 0.00 0.00 0.00 Fe3+ 0.00 0.00 0.00 0.00 0.00 0.00 Fe2+ 0.32 0.61 0.39 0.48 0.39 0.73 Mn 0.00 0.01 0.01 0.01 0.01 0.01 Mg 1.67 1.37 1.59 1.49 1.59 1.24 Ca 0.01 0.01 0.01 0.01 0.01 0.01 (End member) Fo 83.67 68.46 79.73 75.13 79.90 62.38 51

Table 2 (continued). Olivine chemistry from electron microbeam analysis. SB03 SB03 SB03 SB08 SB08 SB08 Sample ol core 3 ol rim 4 ol core 4 ol rim 1 ol core 1 ol rim 2 Location SB SB SB SB SB SB Rock type alk alk alk alk alk alk Analysis core rim core rim core rim

SiO2 39.81 36.13 39.36 39.70 39.83 37.35

TiO2 0.01 0.05 0.01 0.01 0.01 0.03

Al2O3 0.03 0.02 0.02 0.02 0.04 0.01

Cr2O3 0.01 -0.01 0.02 0.03 -0.01 0.01 FeO 17.47 34.84 17.21 17.71 17.31 31.27 MnO 0.30 0.57 0.25 0.26 0.27 0.59 MgO 42.40 28.01 42.28 42.13 42.92 31.13 CaO 0.26 0.46 0.25 0.29 0.25 0.36

Na2O 0.02 0.02 -0.02 -0.01 -0.02 0.01 NiO 0.12 0.01 0.08 0.07 0.08 0.03 Total 100.43 100.09 99.46 100.21 100.68 100.78 (Structural formula on the basis of 4 oxygen)* Si 1.01 1.00 1.01 1.01 1.01 1.01 Ti 0.00 0.00 0.00 0.00 0.00 0.00 Al 0.00 0.00 0.00 0.00 0.00 0.00 Cr 0.00 0.00 0.00 0.00 0.00 0.00 Fe3+ 0.00 0.00 0.00 0.00 0.00 0.00 Fe2+ 0.37 0.81 0.37 0.38 0.37 0.71 Mn 0.01 0.01 0.01 0.01 0.01 0.01 Mg 1.60 1.16 1.61 1.60 1.62 1.26 Ca 0.01 0.01 0.01 0.01 0.01 0.01 (End member) Fo 80.68 58.10 80.91 80.37 81.03 63.20 52

Table 2 (continued). Olivine chemistry from electron microbeam analysis. SB08 SB08 SB08 SB08 SB08 SB10 Sample ol core 2 ol rim 3 ol core 3 ol rim 4 ol core 4 ol rim 1 Location SB SB SB SB SB SB Rock type alk alk alk alk alk alk Analysis core rim core rim core rim

SiO2 40.34 39.04 39.58 39.05 39.89 38.82

TiO2 0.01 0.01 0.01 0.02 0.00 0.01

Al2O3 0.03 0.02 0.01 0.01 0.04 0.03

Cr2O3 0.03 0.01 0.02 0.04 0.04 0.01 FeO 17.29 20.23 16.83 20.54 17.04 20.71 MnO 0.32 0.35 0.28 0.30 0.26 0.34 MgO 42.82 39.80 42.55 40.03 42.98 39.59 CaO 0.27 0.30 0.27 0.32 0.25 0.37

Na2O 0.01 0.02 -0.01 -0.01 0.01 0.03 NiO 0.07 0.07 0.09 0.07 0.12 0.06 Total 101.19 99.85 99.63 100.38 100.64 99.98 (Structural formula on the basis of 4 oxygen)* Si 1.01 1.01 1.01 1.01 1.01 1.00 Ti 0.00 0.00 0.00 0.00 0.00 0.00 Al 0.00 0.00 0.00 0.00 0.00 0.00 Cr 0.00 0.00 0.00 0.00 0.00 0.00 Fe3+ 0.00 0.00 0.00 0.00 0.00 0.00 Fe2+ 0.36 0.44 0.36 0.44 0.36 0.45 Mn 0.01 0.01 0.01 0.01 0.01 0.01 Mg 1.61 1.54 1.62 1.54 1.62 1.53 Ca 0.01 0.01 0.01 0.01 0.01 0.01 (End member) Fo 80.95 77.18 81.29 77.05 81.29 76.62 53

Table 2 (continued). Olivine chemistry from electron microbeam analysis. SB10 SB10 SB10 SB10 SB10 ME03 Sample ol core 1 ol rim 2 ol core 2 ol rim 3 ol core 3 ol core 1 Location SB SB SB SB SB ME Rock type alk alk alk alk alk alk Analysis core rim core rim core core

SiO2 39.77 38.24 39.51 37.82 39.72 38.26

TiO2 0.01 0.03 0.00 0.03 0.02 0.03

Al2O3 0.01 0.03 0.04 0.03 0.02 0.03

Cr2O3 0.03 0.02 0.03 0.01 0.01 0.02 FeO 16.32 23.57 17.32 26.38 17.76 23.57 MnO 0.19 0.36 0.27 0.42 0.23 0.40 MgO 43.49 37.48 42.77 34.81 42.30 37.44 CaO 0.26 0.39 0.25 0.41 0.26 0.28

Na2O 0.01 0.01 0.00 0.02 0.00 0.00 NiO 0.15 0.04 0.09 0.03 0.09 0.04 Total 100.24 100.15 100.27 99.96 100.40 100.06 (Structural formula on the basis of 4 oxygen)* Si 1.00 1.00 1.00 1.01 1.01 1.00 Ti 0.00 0.00 0.00 0.00 0.00 0.00 Al 0.00 0.00 0.00 0.00 0.00 0.00 Cr 0.00 0.00 0.00 0.00 0.00 0.00 Fe3+ 0.00 0.00 0.00 0.00 0.00 0.00 Fe2+ 0.34 0.52 0.37 0.59 0.38 0.52 Mn 0.00 0.01 0.01 0.01 0.00 0.01 Mg 1.64 1.46 1.62 1.38 1.60 1.46 Ca 0.01 0.01 0.01 0.01 0.01 0.01 (End member) Fo 82.15 73.23 80.97 69.42 80.45 73.28 54

Table 2 (continued). Olivine chemistry from electron microbeam analysis. ME03 ME03 ME03 ME03 ME03 Sample ol rim 2 ol core 2 ol rim 3 ol core 3 ol rim 4 Location ME ME ME ME ME Rock type alk alk alk alk alk Analysis rim core rim core rim

SiO2 37.73 38.18 38.53 38.48 38.09

TiO2 0.02 0.01 0.04 0.03 0.01

Al2O3 0.01 0.03 0.00 0.03 0.02

Cr2O3 -0.01 -0.01 -0.01 0.01 0.00 FeO 25.07 22.95 23.11 22.39 24.32 MnO 0.37 0.35 0.42 0.37 0.32 MgO 36.09 37.87 37.95 38.38 37.07 CaO 0.28 0.27 0.28 0.26 0.29

Na2O -0.01 0.00 0.00 0.00 0.00 NiO 0.04 0.09 0.08 0.05 0.07 Total 99.61 99.74 100.40 100.01 100.19 (Structural formula on the basis of 4 oxygen)* Si 1.00 1.00 1.00 1.00 1.00 Ti 0.00 0.00 0.00 0.00 0.00 Al 0.00 0.00 0.00 0.00 0.00 Cr 0.00 0.00 0.00 0.00 0.00 Fe3+ 0.00 0.00 0.00 0.00 0.00 Fe2+ 0.56 0.50 0.50 0.49 0.53 Mn 0.01 0.01 0.01 0.01 0.01 Mg 1.43 1.48 1.47 1.49 1.45 Ca 0.01 0.01 0.01 0.01 0.01 (End member) Fo 71.37 74.05 73.89 74.76 72.54 55

Table 2 (continued). Olivine chemistry from electron microbeam analysis. Sample ME03 ol core 4 ME04 ol rim 1 ME04 ol core 1 ME04 ol rim 2 ME04 ol core 2 Location ME ME ME ME ME Rock type alk trans trans trans trans Analysis core rim core rim core

SiO2 38.82 40.07 40.65 40.25 40.60

TiO2 0.03 0.02 0.01 0.02 0.00

Al2O3 0.01 0.02 0.05 0.04 0.04

Cr2O3 0.00 0.04 0.05 0.04 0.02 FeO 22.27 19.99 15.61 17.39 14.94 MnO 0.33 0.30 0.31 0.22 0.22 MgO 38.63 40.79 44.28 43.05 44.63 CaO 0.28 0.27 0.24 0.25 0.21

Na2O 0.02 0.00 0.01 0.01 0.01 NiO 0.05 0.07 0.20 0.19 0.21 Total 100.45 101.59 101.40 101.46 100.89 (Structural formula on the basis of 4 oxygen)* Si 1.01 1.02 1.01 1.01 1.01 Ti 0.00 0.00 0.00 0.00 0.00 Al 0.00 0.00 0.00 0.00 0.00 Cr 0.00 0.00 0.00 0.00 0.00 Fe3+ 0.00 0.00 0.00 0.00 0.00 Fe2+ 0.48 0.42 0.33 0.36 0.31 Mn 0.01 0.01 0.01 0.00 0.00 Mg 1.49 1.54 1.65 1.61 1.66 Ca 0.01 0.01 0.01 0.01 0.01 (End member) Fo 74.99 77.88 82.95 81.06 83.76 56

Table 2 (continued). Olivine chemistry from electron microbeam analysis. Sample ME04 ol rim 3 ME04 ol core 3 ME05 ol rim 1 ME05 ol core 1 ME05 ol rim 2 Location ME ME ME ME ME Rock type trans trans trans trans trans Analysis rim core rim core rim

SiO2 39.46 39.98 39.46 39.52 39.28

TiO2 0.03 0.01 0.03 0.01 0.02

Al2O3 0.01 0.02 0.02 -0.01 0.02

Cr2O3 0.01 0.03 0.00 0.02 0.03 FeO 20.47 16.25 19.55 18.42 19.06 MnO 0.32 0.27 0.29 0.31 0.26 MgO 40.00 43.41 40.94 42.02 40.97 CaO 0.30 0.25 0.33 0.25 0.30

Na2O 0.01 0.02 0.03 0.00 0.03 NiO 0.09 0.19 0.12 0.12 0.12 Total 100.71 100.44 100.77 100.66 100.10 (Structural formula on the basis of 4 oxygen)* Si 1.01 1.01 1.01 1.00 1.01 Ti 0.00 0.00 0.00 0.00 0.00 Al 0.00 0.00 0.00 0.00 0.00 Cr 0.00 0.00 0.00 0.00 0.00 Fe3+ 0.00 0.00 0.00 0.00 0.00 Fe2+ 0.44 0.34 0.42 0.39 0.41 Mn 0.01 0.01 0.01 0.01 0.01 Mg 1.53 1.63 1.56 1.59 1.57 Ca 0.01 0.01 0.01 0.01 0.01 (End member) Fo 77.10 82.13 78.27 79.72 78.75 57

Table 2 (continued). Olivine chemistry from electron microbeam analysis. Sample ME05 ol core 2 ME05 ol rim 3 ME05 ol core 3 ME05 ol rim 4 ME05 ol core 4 Location ME ME ME ME ME Rock type trans trans trans trans trans Analysis core rim core rim core

SiO2 39.87 39.46 39.83 39.29 40.11

TiO2 0.02 0.02 0.02 0.05 0.01

Al2O3 0.01 0.04 0.02 0.02 0.03

Cr2O3 0.02 0.02 0.04 0.01 0.02 FeO 18.70 19.07 17.71 20.76 16.23 MnO 0.29 0.27 0.19 0.26 0.19 MgO 41.62 40.51 42.43 39.50 43.66 CaO 0.28 0.29 0.26 0.28 0.25

Na2O 0.01 -0.01 -0.02 0.00 0.00 NiO 0.13 0.12 0.16 0.11 0.20 Total 100.95 99.78 100.63 100.27 100.69 (Structural formula on the basis of 4 oxygen)* Si 1.01 1.02 1.01 1.01 1.01 Ti 0.00 0.00 0.00 0.00 0.00 Al 0.00 0.00 0.00 0.00 0.00 Cr 0.00 0.00 0.00 0.00 0.00 Fe3+ 0.00 0.00 0.00 0.00 0.00 Fe2+ 0.40 0.41 0.38 0.45 0.34 Mn 0.01 0.01 0.00 0.01 0.00 Mg 1.58 1.56 1.60 1.52 1.64 Ca 0.01 0.01 0.01 0.01 0.01 (End member) Fo 79.31 78.55 80.58 76.71 82.31 58

Table 2 (continued). Olivine chemistry from electron microbeam analysis. ME11 ME11 ME11 ME11 ME11 ME11 Sample ol rim 1 ol core 1 ol rim 2 ol core 2 ol rim 3 ol core 3 Location ME ME ME ME ME ME Rock type alk alk alk alk alk alk Analysis rim core rim core rim core

SiO2 35.64 36.88 38.16 38.25 38.66 38.49

TiO2 0.02 0.02 0.01 0.02 0.04 0.02

Al2O3 0.01 0.02 0.01 0.01 0.02 0.04

Cr2O3 0.00 -0.01 -0.02 0.00 0.02 0.01 FeO 17.76 18.71 22.26 22.03 23.25 22.58 MnO 0.27 0.30 0.33 0.36 0.32 0.37 MgO 36.12 37.27 36.98 38.22 37.26 38.23 CaO 0.25 0.24 0.28 0.26 0.29 0.27

Na2O 0.00 0.01 -0.01 0.02 0.00 0.02 NiO 0.04 0.06 0.05 0.06 0.07 0.05 Total 90.11 93.51 98.04 99.22 99.94 100.07 (Structural formula on the basis of 4 oxygen)* Si 1.02 1.02 1.02 1.00 1.01 1.00 Ti 0.00 0.00 0.00 0.00 0.00 0.00 Al 0.00 0.00 0.00 0.00 0.00 0.00 Cr 0.00 0.00 0.00 0.00 0.00 0.00 Fe3+ 0.00 0.00 0.00 0.00 0.00 0.00 Fe2+ 0.42 0.43 0.50 0.48 0.51 0.49 Mn 0.01 0.01 0.01 0.01 0.01 0.01 Mg 1.54 1.53 1.47 1.50 1.46 1.49 Ca 0.01 0.01 0.01 0.01 0.01 0.01 (End member) Fo 77.82 77.47 74.17 74.98 73.50 74.52 59

Table 3: Clinopyroxene chemistry from electron microbeam analysis. SB01 SB01 SB01 SB01 SB01 SB01 Sample px rim 1 px core 1 px rim 2 px core 2 px rim 3 px core 3 Location SB SB SB SB SB SB Rock type thol thol thol thol thol thol Analysis rim core rim core rim core

SiO2 48.76 48.99 48.92 48.87 48.69 48.46

TiO2 2.40 2.13 2.24 2.24 2.13 2.44

Al2O3 2.80 2.79 2.79 3.00 2.63 2.81

Cr2O3 0.01 0.02 0.00 0.12 0.01 0.03 FeO 12.49 12.86 13.16 12.19 13.79 12.07 MnO 0.20 0.27 0.28 0.27 0.24 0.26 MgO 11.35 11.08 10.95 11.80 10.43 11.54 CaO 20.26 20.26 20.15 19.89 20.00 20.16

Na2O 0.49 0.56 0.57 0.45 0.54 0.52 NiO 0.02 0.01 0.01 0.01 0.00 0.03 Total 98.79 98.97 99.05 98.84 98.46 98.30 (Structural formula on the basis of 6 oxygen)* Si 1.87 1.88 1.88 1.87 1.89 1.87 Ti 0.07 0.06 0.06 0.06 0.06 0.07 Al 0.13 0.13 0.13 0.14 0.12 0.13 Cr 0.00 0.00 0.00 0.00 0.00 0.00 Fe3 0.02 0.03 0.03 0.02 0.02 0.03 Fe2 0.38 0.38 0.39 0.37 0.43 0.36 Mn 0.01 0.01 0.01 0.01 0.01 0.01 Mg 0.65 0.63 0.63 0.67 0.60 0.66 Ca 0.83 0.83 0.83 0.82 0.83 0.83 Na 0.04 0.04 0.04 0.03 0.04 0.04 (End members) Wo 44.24 44.31 44.13 43.41 44.18 44.18 En 34.48 33.73 33.38 35.82 32.04 35.18 Fs 21.28 21.96 22.49 20.77 23.78 20.64 Sample numbers are abbreviated to only include the location - SB: Sheridan Bluff. ME: Mount Early - and the number in sequence (01, 02, 03, etc.). The number for the year collected (“15” for 2015) has been omitted. Alk: alkaline basalt. Thol: tholeiitic (subalkaline) basalt. Trans: transitional alkaline basalt. Wo: wollastonite. En: enstatite. Fs: ferrosilite. *Structural formula of elements and end member calculations from Mineral Formulae Recalculation. John Brady, Smith College and Dexter Perkins, University of North Dakota https://serc.carleton.edu/research_education/equilibria/mineralformulaerecalculation.html 60

Table 3 (continued). Clinopyroxene chemistry from electron microbeam analysis. SB01 SB01 SB03 SB03 SB03 SB03 Sample px rim 4 px core 4 px rim 1 px core 1 px rim 2 px core 2 Location SB SB SB SB SB SB Rock type thol thol alk alk alk alk Analysis rim core rim core rim core

SiO2 48.82 48.03 44.68 50.28 44.30 50.36

TiO2 2.09 2.42 4.69 2.09 5.54 1.96

Al2O3 2.47 2.95 6.68 2.67 6.66 2.52

Cr2O3 0.04 0.06 0.02 0.03 0.01 0.00 FeO 12.19 12.13 10.01 9.43 11.89 9.39 MnO 0.30 0.32 0.22 0.20 0.27 0.20 MgO 11.62 11.53 10.46 13.07 9.52 12.83 CaO 20.22 19.90 21.25 21.19 20.83 21.31

Na2O 0.49 0.52 0.74 0.49 0.97 0.49 NiO 0.00 -0.02 -0.02 0.01 0.01 0.02 Total 98.25 97.83 98.73 99.45 100.00 99.09 (Structural formula on the basis of 6 oxygen)* Si 1.88 1.86 1.71 1.89 1.69 1.90 Ti 0.06 0.07 0.14 0.06 0.16 0.06 Al 0.11 0.13 0.30 0.12 0.30 0.11 Cr 0.00 0.00 0.00 0.00 0.00 0.00 Fe3 0.04 0.04 0.06 0.01 0.08 0.00 Fe2 0.36 0.35 0.26 0.29 0.30 0.30 Mn 0.01 0.01 0.01 0.01 0.01 0.01 Mg 0.67 0.67 0.60 0.73 0.54 0.72 Ca 0.84 0.83 0.87 0.85 0.85 0.86 Na 0.04 0.04 0.05 0.04 0.07 0.04 (End members) Wo 44.05 43.82 48.72 45.33 48.05 45.83 En 35.23 35.33 33.37 38.91 30.55 38.40 Fs 20.72 20.85 17.90 15.75 21.40 15.77 61

Table 3 (continued). Clinopyroxene chemistry from electron microbeam analysis. SB08 SB08 SB08 SB08 SB08 SB08 Sample px rim 1 px core 1 px rim 2 px core 2 px rim 3 px core 3 Location SB SB SB SB SB SB Rock type alk alk alk alk alk alk Analysis rim core rim core rim core

SiO2 48.05 51.26 50.92 50.95 51.08 51.06

TiO2 2.36 1.36 1.76 1.67 1.72 1.02

Al2O3 5.09 2.96 2.66 2.63 3.08 4.56

Cr2O3 0.49 0.39 0.06 0.02 0.06 0.62 FeO 7.11 6.38 7.87 8.03 7.84 5.57 MnO 0.14 0.10 0.17 0.16 0.15 0.13 MgO 13.16 14.54 13.55 13.71 13.85 15.09 CaO 21.90 22.10 21.96 21.83 21.96 21.30

Na2O 0.48 0.43 0.51 0.46 0.45 0.57 NiO -0.02 0.00 -0.01 0.00 0.01 0.01 Total 98.76 99.51 99.46 99.44 100.19 99.92 (Structural formula on the basis of 6 oxygen)* Si 1.81 1.90 1.91 1.91 1.90 1.88 Ti 0.07 0.04 0.05 0.05 0.05 0.03 Al 0.23 0.13 0.12 0.12 0.13 0.20 Cr 0.01 0.01 0.00 0.00 0.00 0.02 Fe3 0.04 0.01 0.01 0.01 0.01 0.02 Fe2 0.18 0.19 0.24 0.24 0.24 0.15 Mn 0.00 0.00 0.01 0.01 0.00 0.00 Mg 0.74 0.81 0.76 0.76 0.77 0.83 Ca 0.88 0.88 0.88 0.88 0.87 0.84 Na 0.04 0.03 0.04 0.03 0.03 0.04 (End members) Wo 47.87 46.72 46.76 46.27 46.38 45.66 En 40.01 42.76 40.15 40.43 40.69 45.02 Fs 12.12 10.52 13.09 13.29 12.93 9.32 62

Table 3 (continued). Clinopyroxene chemistry from electron microbeam analysis. Sample ME03 px rim 1 ME03 px core 1 ME04 px rim 1 ME04 px core 1 Location ME ME ME ME Rock type alk alk trans trans Analysis rim core rim core

SiO2 49.07 49.36 43.62 45.01

TiO2 1.65 1.73 4.76 4.00

Al2O3 4.95 4.25 7.01 4.84

Cr2O3 0.03 0.10 -0.01 0.02 FeO 7.98 7.24 12.06 12.86 MnO 0.12 0.16 0.20 0.26 MgO 13.46 13.62 9.69 9.39 CaO 21.07 21.81 20.38 20.70

Na2O 0.71 0.55 0.72 0.81 NiO 0.02 0.02 -0.01 -0.01 Total 99.06 98.87 98.41 97.88 (Structural formula on the basis of 6 oxygen)* Si 1.83 1.85 1.69 1.76 Ti 0.05 0.05 0.14 0.12 Al 0.22 0.19 0.32 0.22 Cr 0.00 0.00 0.00 0.00 Fe3 0.07 0.05 0.08 0.09 Fe2 0.18 0.18 0.31 0.33 Mn 0.00 0.01 0.01 0.01 Mg 0.75 0.76 0.56 0.55 Ca 0.84 0.88 0.84 0.87 Na 0.05 0.04 0.05 0.06 (End members) Wo 45.78 46.99 47.09 47.25 En 40.70 40.83 31.15 29.83 Fs 13.52 12.18 21.75 22.92 63

Table 4: Plagioclase chemistry from electron microbeam analysis. SB01 SB01 SB01 SB01 SB01 SB01 SB01 Sample pl rim 1 pl core 1 pl rim 2 pl core 2 pl rim 3 pl core 3 pl rim 4 Location SB SB SB SB SB SB SB Rock type thol thol thol thol thol thol thol Analysis rim core rim core rim core rim

SiO2 52.27 50.85 51.77 50.91 52.49 50.29 52.17

TiO2 0.07 0.08 0.06 0.07 0.09 0.08 0.09

Al2O3 30.14 30.63 30.40 30.80 28.78 30.46 29.21 FeO 0.48 0.44 0.62 0.40 0.70 0.37 0.53 MnO -0.01 0.02 -0.01 0.03 0.12 -0.09 0.01 MgO 0.15 0.18 0.14 0.19 0.12 0.11 0.14 CaO 12.72 13.49 12.70 13.42 11.64 13.54 12.22

Na2O 4.21 3.82 4.27 3.81 4.87 3.65 4.60

K2O 0.19 0.12 0.17 0.14 0.24 0.14 0.21

P2O5 0.03 -0.01 -0.01 0.03 0.01 0.01 -0.02 Total 100.24 99.61 100.11 99.80 99.07 98.58 99.16 (Structural formula on the basis of 8 oxygen)* Si 2.37 2.32 2.34 2.32 2.40 2.32 2.38 Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Al 1.61 1.65 1.62 1.65 1.55 1.66 1.57 Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Fe3 0.02 0.02 0.02 0.02 0.03 0.01 0.02 Fe2 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Mg 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Ca 0.62 0.66 0.62 0.65 0.57 0.67 0.60 Ba 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Na 0.37 0.34 0.37 0.34 0.43 0.33 0.41 K 0.01 0.01 0.01 0.01 0.01 0.01 0.01 (End members) An 61.85 65.69 61.59 65.50 56.13 66.64 58.79 Ab 37.03 33.63 37.44 33.69 42.48 32.52 40.02 Or 1.12 0.68 0.97 0.81 1.39 0.83 1.19 Sample numbers are abbreviated to only include the location - SB: Sheridan Bluff. ME: Mount Early - and the number in sequence (01, 02, 03, etc.). The number for the year collected (“15” for 2015) has been omitted. Alk: alkaline basalt. Thol: tholeiitic (subalkaline) basalt. Trans: transitional alkaline basalt. An: anorthite. Ab: albite. Or: orthoclase. *Structural formula of elements and end member calculations from Mineral Formulae Recalculation. John Brady, Smith College and Dexter Perkins, University of North Dakota https://serc.carleton.edu/research_education/equilibria/mineralformulaerecalculation.html 64

Table 4 (continued). Plagioclase chemistry from electron microbeam analysis. SB01 SB03 SB03 SB03 SB03 SB03 SB08 Sample pl core 4 pl rim 1 pl mid 1 pl core 1 pl rim 2 pl core 2 pl rim 1 Location SB SB SB SB SB SB SB Rock type thol alk alk alk alk alk alk Analysis core rim mid core rim core rim

SiO2 50.84 52.87 53.15 51.02 54.77 52.63 54.61

TiO2 0.08 0.12 0.11 0.06 0.16 0.11 0.13

Al2O3 30.34 28.64 29.00 30.37 27.78 29.04 28.07 FeO 0.56 0.52 0.43 0.45 0.53 0.47 0.42 MnO 0.04 -0.04 0.00 -0.05 -0.02 0.05 0.04 MgO 0.16 0.09 0.10 0.12 0.11 0.09 0.07 CaO 13.39 11.50 11.62 13.21 10.07 11.65 10.28

Na2O 4.09 4.87 4.72 3.82 5.67 4.77 5.47

K2O 0.13 0.39 0.35 0.22 0.49 0.35 0.50

P2O5 -0.01 0.00 -0.02 0.02 0.01 0.06 -0.01 Total 99.62 98.94 99.46 99.24 99.56 99.22 99.59 (Structural formula on the basis of 8 oxygen)* Si 2.31 2.41 2.42 2.34 2.48 2.40 2.47 Ti 0.00 0.00 0.00 0.00 0.01 0.00 0.00 Al 1.63 1.54 1.55 1.64 1.48 1.56 1.50 Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Fe3 0.02 0.02 0.02 0.02 0.02 0.02 0.02 Fe2 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Mg 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Ca 0.65 0.56 0.57 0.65 0.49 0.57 0.50 Ba 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Na 0.36 0.43 0.42 0.34 0.50 0.42 0.48 K 0.01 0.02 0.02 0.01 0.03 0.02 0.03 (End members) An 63.94 55.35 56.47 64.83 48.14 56.27 49.50 Ab 35.33 42.43 41.50 33.90 49.08 41.71 47.63 Or 0.73 2.22 2.03 1.27 2.78 2.02 2.87 65

Table 4 (continued). Plagioclase chemistry from electron microbeam analysis. SB08 SB08 SB08 SB08 SB08 SB08 SB08 Sample pl core 1 pl rim 2 pl mid 2 pl core 2 pl rim 3 pl mid 3 pl core 3 Location SB SB SB SB SB SB SB Rock type alk alk alk alk alk alk alk Analysis core rim mid core rim mid core

SiO2 50.76 52.35 51.67 51.97 52.04 50.13 50.81

TiO2 0.08 0.09 0.08 0.06 0.10 0.04 0.08

Al2O3 30.47 29.54 30.46 29.84 29.87 30.68 30.05 FeO 0.29 0.52 0.34 0.42 0.53 0.42 0.33 MnO 0.08 0.04 0.03 0.05 0.03 0.04 0.06 MgO 0.13 0.11 0.10 0.13 0.10 0.12 0.06 CaO 13.26 12.00 12.86 12.37 12.13 13.40 13.04

Na2O 3.77 4.36 3.93 4.14 4.42 3.59 3.74

K2O 0.23 0.29 0.21 0.26 0.31 0.21 0.24

P2O5 -0.01 0.00 0.00 0.02 0.02 0.03 0.01 Total 99.05 99.29 99.66 99.27 99.53 98.65 98.42 (Structural formula on the basis of 8 oxygen)* Si 2.33 2.39 2.35 2.38 2.37 2.31 2.35 Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Al 1.65 1.59 1.64 1.61 1.60 1.67 1.64 Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Fe3 0.01 0.02 0.00 0.02 0.02 0.02 0.01 Fe2 0.00 0.00 0.01 0.00 0.00 0.00 0.01 Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Mg 0.01 0.01 0.01 0.01 0.01 0.01 0.00 Ca 0.65 0.59 0.63 0.61 0.59 0.66 0.65 Ba 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Na 0.34 0.39 0.35 0.37 0.39 0.32 0.33 K 0.01 0.02 0.01 0.02 0.02 0.01 0.01 (End members) An 65.15 59.33 63.61 61.32 59.19 66.52 64.93 Ab 33.51 38.98 35.16 37.14 39.02 32.24 33.66 Or 1.34 1.69 1.24 1.54 1.79 1.24 1.41 66

Table 4 (continued). Plagioclase chemistry from electron microbeam analysis. SB10 SB10 SB10 SB10 SB10 ME04 ME04 Sample pl rim 1 pl mid 1 pl core 1 pl rim 2 pl core 2 pl rim 1 pl core 1 Location SB SB SB SB SB ME ME Rock type alk alk alk alk alk trans trans Analysis rim mid core rim core rim core

SiO2 51.11 51.19 50.59 52.91 50.29 50.12 50.05

TiO2 0.05 0.07 0.09 0.11 0.09 0.07 0.08

Al2O3 31.03 30.10 30.41 29.49 31.06 30.76 30.66 FeO 0.70 0.53 0.45 0.45 0.50 0.47 0.48 MnO 0.01 0.04 -0.12 0.03 0.01 0.11 -0.03 MgO 0.12 0.10 0.13 0.11 0.10 0.14 0.17 CaO 13.28 12.90 13.31 11.75 13.61 13.21 13.28

Na2O 3.72 3.88 3.83 4.55 3.63 3.59 3.75

K2O 0.24 0.23 0.22 0.30 0.21 0.19 0.21

P2O5 0.04 0.00 0.00 0.03 -0.03 0.04 0.03 Total 100.28 99.04 98.92 99.73 99.48 98.71 98.68 (Structural formula on the basis of 8 oxygen)* Si 2.32 2.35 2.32 2.40 2.30 2.31 2.30 Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Al 1.66 1.63 1.65 1.58 1.67 1.67 1.66 Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Fe3 0.03 0.02 0.02 0.02 0.02 0.02 0.02 Fe2 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Mg 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Ca 0.65 0.63 0.65 0.57 0.67 0.65 0.65 Ba 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Na 0.33 0.35 0.34 0.40 0.32 0.32 0.33 K 0.01 0.01 0.01 0.02 0.01 0.01 0.01 (End members) An 65.45 63.84 64.93 57.76 66.61 66.25 65.37 Ab 33.17 34.78 33.81 40.47 32.16 32.60 33.41 Or 1.38 1.38 1.26 1.77 1.24 1.15 1.22 67

Table 4 (continued). Plagioclase chemistry from electron microbeam analysis. ME04 ME04 ME04 ME04 ME04 ME04 ME04 Sample pl rim 2 pl mid 2 pl core 2 pl rim 3 pl mid 3 pl core 3 pl rim 4 Location ME ME ME ME ME ME ME Rock type trans trans trans trans trans trans trans Analysis rim mid core rim mid core rim

SiO2 50.49 50.16 50.85 50.40 50.61 49.70 49.49

TiO2 0.06 0.07 0.07 0.06 0.06 0.07 0.08

Al2O3 30.49 30.92 30.28 30.47 30.12 30.60 30.40 FeO 0.56 0.61 0.32 0.53 0.58 0.56 0.53 MnO 0.08 0.02 0.06 -0.04 -0.01 0.03 -0.02 MgO 0.11 0.11 0.16 0.14 0.15 0.12 0.12 CaO 13.19 13.51 12.89 12.98 12.82 13.59 13.57

Na2O 3.71 3.44 3.91 3.78 4.07 3.63 3.45

K2O 0.18 0.18 0.18 0.20 0.22 0.19 0.19

P2O5 0.00 0.00 0.01 -0.03 0.00 0.04 0.02 Total 98.89 99.01 98.73 98.51 98.61 98.53 97.82 (Structural formula on the basis of 8 oxygen)* Si 2.32 2.31 2.34 2.32 2.33 2.29 2.30 Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Al 1.65 1.68 1.64 1.66 1.63 1.66 1.67 Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Fe3 0.02 0.01 0.01 0.02 0.02 0.02 0.02 Fe2 0.00 0.01 0.00 0.00 0.00 0.00 0.00 Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Mg 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Ca 0.65 0.67 0.63 0.64 0.63 0.67 0.68 Ba 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Na 0.33 0.31 0.35 0.34 0.36 0.33 0.31 K 0.01 0.01 0.01 0.01 0.01 0.01 0.01 (End members) An 65.55 67.71 63.87 64.72 62.72 66.64 67.76 Ab 33.35 31.22 35.08 34.12 36.02 32.26 31.13 Or 1.09 1.08 1.05 1.16 1.26 1.11 1.12 68

Table 4 (continued). Plagioclase chemistry from electron microbeam analysis. ME04 ME05 ME05 ME05 ME05 ME11 ME11 Sample pl core 4 pl rim 1 pl core 1 pl rim 2 pl core 2 pl rim 1 pl core 1 Location ME ME ME ME ME ME ME Rock type trans trans trans trans trans alk alk Analysis core rim core rim core rim core

SiO2 50.68 52.13 51.65 51.05 51.31 52.70 50.61

TiO2 0.06 0.10 0.05 0.07 0.09 0.10 0.07

Al2O3 30.27 29.34 29.49 29.71 29.63 28.96 30.01 FeO 0.47 0.54 0.43 0.72 0.47 0.68 0.58 MnO 0.05 -0.02 0.00 -0.04 0.03 -0.03 -0.01 MgO 0.16 0.14 0.18 0.17 0.14 0.09 0.09 CaO 12.88 11.75 12.31 12.83 12.16 11.30 12.63

Na2O 3.77 4.54 4.18 4.05 4.25 4.85 4.07

K2O 0.18 0.33 0.24 0.24 0.24 0.32 0.24

P2O5 -0.02 0.03 0.02 0.03 0.03 0.01 0.00 Total 98.50 98.88 98.54 98.84 98.35 98.96 98.28 (Structural formula on the basis of 8 oxygen)* Si 2.34 2.39 2.38 2.35 2.36 2.41 2.34 Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Al 1.65 1.58 1.60 1.61 1.61 1.56 1.63 Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Fe3 0.02 0.02 0.02 0.03 0.02 0.03 0.02 Fe2 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Mg 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Ca 0.64 0.58 0.61 0.63 0.60 0.55 0.62 Ba 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Na 0.34 0.40 0.37 0.36 0.38 0.43 0.36 K 0.01 0.02 0.01 0.01 0.01 0.02 0.01 (End members) An 64.69 57.76 61.08 62.78 60.38 55.23 62.29 Ab 34.26 40.34 37.49 35.84 38.20 42.92 36.33 Or 1.05 1.90 1.42 1.38 1.42 1.85 1.38 69

Table 4 (continued). Plagioclase chemistry from electron microbeam analysis. Sample ME11 pl rim 2 ME11 pl core 2 ME11 pl rim 3 ME11 pl core 3 Location ME ME ME ME Rock type alk alk alk alk Analysis rim core rim core

SiO2 51.05 52.08 51.89 51.14

TiO2 0.07 0.08 0.13 0.09

Al2O3 30.03 28.32 29.15 29.47 FeO 0.61 0.62 0.68 0.70 MnO -0.02 0.00 0.02 0.01 MgO 0.09 0.11 0.07 0.10 CaO 12.29 11.24 11.56 11.92

Na2O 4.18 4.69 4.63 4.39

K2O 0.27 0.34 0.34 0.29

P2O5 0.03 0.00 0.02 0.02 Total 98.59 97.49 98.49 98.12 (Structural formula on the basis of 8 oxygen)* Si 2.35 2.42 2.38 2.36 Ti 0.00 0.00 0.00 0.00 Al 1.63 1.55 1.58 1.60 Cr 0.00 0.00 0.00 0.00 Fe3 0.02 0.02 0.03 0.03 Fe2 0.00 0.00 0.00 0.00 Mn 0.00 0.00 0.00 0.00 Mg 0.01 0.01 0.00 0.01 Ca 0.61 0.56 0.57 0.59 Ba 0.00 0.00 0.00 0.00 Na 0.37 0.42 0.41 0.39 K 0.02 0.02 0.02 0.02 (End members) An 60.92 55.84 56.80 58.97 Ab 37.49 42.17 41.22 39.31 Or 1.58 1.99 1.98 1.72 70

Table 5. Magnetite chemistry from electron microbeam analysis. SB01 SB01 SB01 SB03 SB03 SB03 SB03 Sample Mt 1 Mt 2 Mt 3 Mt 1 Mt 2 Mt 3 inc 2 Mt 4 inc 2 Location SB SB SB SB SB SB SB Rock type thol thol thol alk alk alk alk

SiO2 0.09 0.16 0.08 0.08 0.19 0.07 0.27

TiO2 26.18 27.27 24.50 26.89 27.22 27.56 27.96

Al2O3 1.19 0.84 1.32 2.19 1.20 2.28 0.97

Cr2O3 0.25 0.35 0.15 0.02 -0.01 0.00 0.02 FeO 67.83 66.50 69.72 66.92 67.06 66.65 67.36 MnO 0.59 0.64 0.55 0.75 0.75 0.73 0.69 MgO 1.76 0.91 1.20 1.68 1.12 1.53 0.79 CaO 0.13 0.13 0.11 0.05 0.20 0.03 0.04

V2O3 0.69 0.68 0.65 0.51 0.20 0.40 0.34 Total 98.71 97.47 98.28 99.07 97.94 99.25 98.44 (Structural formula on the basis of 4 oxygen)* Si 0.00 0.01 0.00 0.00 0.01 0.00 0.01 Ti 0.73 0.77 0.68 0.74 0.76 0.76 0.78 Al 0.05 0.04 0.06 0.09 0.05 0.10 0.04 Cr 0.01 0.01 0.00 0.00 0.00 0.00 0.00 Fe3 0.48 0.39 0.56 0.42 0.41 0.38 0.37 Fe2 1.62 1.71 1.60 1.63 1.68 1.65 1.73 Mn 0.02 0.02 0.02 0.02 0.02 0.02 0.02 Mg 0.10 0.05 0.07 0.09 0.06 0.08 0.04 Sample numbers are abbreviated to only include the location - SB: Sheridan Bluff. ME: Mount Early - and the number in sequence (01, 02, 03, etc.). The number for the year collected (“15” for 2015) has been omitted. Alk: alkaline basalt. Thol: tholeiitic (subalkaline) basalt. Trans: transitional alkaline basalt. *Structural formula of element calculations from Mineral Formulae Recalculation. John Brady, Smith College and Dexter Perkins, University of North Dakota https://serc.carleton.edu/research_education/equilibria/mineralformulaerecalculation.html 71

Table 5 (continued). Magnetite chemistry from electron microbeam analysis. SB03 SB03 SB08 SB08 SB10 SB10 ME03 ME03 Sample Mt 4 Mt 5 Mt 1 Mt 2 Mt 1 Mt 2 Mt 1 Mt 2 Location SB SB SB SB SB SB ME ME Rock type alk alk alk alk alk alk alk alk

SiO2 0.14 0.05 0.06 0.10 0.15 0.01 0.08 0.08

TiO2 27.98 28.17 25.72 26.22 28.35 27.61 23.94 24.47

Al2O3 1.15 1.79 1.40 1.51 1.13 1.24 1.74 1.11

Cr2O3 0.00 0.00 0.02 0.01 0.09 0.12 -0.01 0.02 FeO 66.23 66.00 68.53 67.94 65.84 66.33 70.00 70.66 MnO 0.79 0.81 0.68 0.71 0.80 0.80 0.67 0.69 MgO 1.37 1.44 1.46 1.42 2.29 1.92 1.61 0.89 CaO 0.07 0.05 0.03 0.05 0.03 0.06 0.09 0.09

V2O3 0.34 0.42 0.43 0.36 0.41 0.45 0.61 0.52 Total 98.06 98.73 98.33 98.32 99.10 98.55 98.72 98.54 (Structural formula on the basis of 4 oxygen)* Si 0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.00 Ti 0.78 0.78 0.72 0.73 0.78 0.77 0.66 0.68 Al 0.05 0.08 0.06 0.07 0.05 0.05 0.08 0.05 Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Fe3 0.37 0.36 0.50 0.47 0.38 0.41 0.60 0.58 Fe2 1.69 1.68 1.62 1.63 1.64 1.64 1.56 1.61 Mn 0.02 0.03 0.02 0.02 0.02 0.03 0.02 0.02 Mg 0.08 0.08 0.08 0.08 0.13 0.11 0.09 0.05 72

Table 6. Ilmenite chemistry from electron microbeam analysis. SB01 SB01 SB01 SB03 SB03 SB03 SB08 SB08 ME03 Sample Il 1 Il 2 Il 3 Il inc 1 Il 2 inc 1 Il 3 inc 2 Il 1 Il 2 Il 1 Location SB SB SB SB SB SB SB SB ME Rock type thol thol thol alk alk alk alk alk alk

SiO2 0.06 0.04 0.06 0.10 0.10 0.08 0.18 0.03 0.22

TiO2 48.93 48.45 48.80 26.38 27.12 51.27 50.27 50.66 49.42

Al2O3 0.04 0.06 0.09 0.85 0.79 0.04 0.06 0.05 0.15 FeO 47.89 49.04 47.99 68.35 67.35 45.25 46.89 46.16 46.72 MnO 0.67 0.68 0.64 0.78 0.80 0.95 0.73 0.78 0.79 MgO 0.80 0.54 0.82 0.83 0.96 1.54 1.10 1.10 0.68 CaO 0.16 0.06 0.15 0.04 0.06 0.09 0.19 0.09 0.09

Cr2O3 0.01 0.00 0.00 0.03 -0.02 0.00 0.02 0.01 0.00

V2O3 0.46 0.45 0.39 0.22 0.20 0.25 0.36 0.42 0.41 Total 99.02 99.30 98.94 97.58 97.37 99.46 99.79 99.30 98.49 (Structural formula on the basis of 6 oxygen)* Si 0.00 0.00 0.00 0.00 0.01 0.00 0.01 0.00 0.01 Ti 1.87 1.85 1.86 0.99 1.02 1.93 1.90 1.92 1.90 Nb 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Al 0.00 0.00 0.01 0.05 0.05 0.00 0.00 0.00 0.01 Fe3 0.26 0.30 0.27 1.96 1.90 0.12 0.18 0.14 0.18 Fe2 1.77 1.77 1.77 0.90 0.92 1.78 1.78 1.81 1.82 Mn 0.03 0.03 0.03 0.03 0.03 0.04 0.03 0.03 0.03 Mg 0.06 0.04 0.06 0.06 0.07 0.12 0.08 0.08 0.05 Ca 0.01 0.00 0.01 0.00 0.00 0.00 0.01 0.00 0.00 Sample numbers are abbreviated to only include the location - SB: Sheridan Bluff. ME: Mount Early - and the number in sequence (01, 02, 03, etc.). The number for the year collected (“15” for 2015) has been omitted. Alk: alkaline basalt. Thol: tholeiitic (subalkaline) basalt. Trans: transitional alkaline basalt. *Structural formula of element calculations from Gabbrosoft.org 73

Table 7. Cr-Spinel chemistry from electron microbeam analysis. Sample ME04 Cr 1 ME04 Cr 2 ME04 Cr 3 ME04 Cr 4 Location ME ME ME ME Rock type trans trans trans trans

Cr2O3 21.01 24.05 24.49 24.69

Al2O3 33.93 15.27 16.16 18.52

TiO2 1.37 6.14 5.46 5.07 FeO 29.66 44.15 43.79 41.83 MgO 13.61 7.51 8.36 8.63 MnO 0.26 0.39 0.35 0.34

SiO2 0.09 0.07 0.06 0.13 CaO 0.00 0.17 0.05 0.04

V2O3 0.13 0.36 0.33 0.32 Total 100.06 98.10 99.06 99.58 (Structural formula on the basis of 32 oxygen)* Cr 3.87 5.07 5.06 5.02 Al 9.33 4.80 4.98 5.62 Ti 0.24 1.23 1.07 0.98 Fe3 2.23 3.45 3.59 3.21 Fe2 3.64 6.59 6.20 5.97 Mg 4.73 2.98 3.26 3.31 Mn 0.04 0.07 0.06 0.05 Ni 0.00 0.00 0.00 0.00 Sample numbers are abbreviated to only include the location - SB: Sheridan Bluff. ME: Mount Early - and the number in sequence (01, 02, 03, etc.). The number for the year collected (“15” for 2015) has been omitted. Alk: alkaline basalt. Thol: tholeiitic (subalkaline) basalt. Trans: transitional alkaline basalt. *Structural formula of element calculations from Andy Tindle software. http://www.open.ac.uk/earth-research/tindle/AGTWebPages/AGTSoft.html 74

Table 8. Whole rock major element (XRF) and trace element (ICPMS and XRF) compositions* Sample ME15-001 ME15-003 ME15-004 ME15-005a ME15-005b ME15-006 ME15-007 ME15-008 Location ME ME ME ME ME ME ME ME Rock type trans alk trans inc trans trans trans trans (Major element compositions in wt. %) SiO2 48.75 49.39 49.76 79.00 49.83 49.39 50.07 49.92 TiO2 1.95 2.26 2.00 0.14 1.96 1.97 2.00 2.00 Al2O3 15.92 17.48 16.10 8.55 16.17 16.01 16.10 16.27 FeO 9.38 9.38 9.51 0.61 9.18 9.35 9.32 9.22 MnO 0.15 0.14 0.15 0.01 0.14 0.14 0.14 0.15 MgO 7.01 4.86 6.65 0.19 6.50 6.59 6.40 6.46 CaO 8.54 7.94 8.80 0.96 8.83 8.74 8.77 8.82 Na2O 3.61 4.69 3.80 1.40 3.75 3.59 3.75 3.70 K2O 1.67 2.27 1.75 4.19 1.71 1.75 1.78 1.77 P2O5 0.53 0.79 0.54 0.02 0.53 0.54 0.53 0.54 Total 97.50 99.20 99.06 95.06 98.60 98.06 98.86 98.85 LOI 1.87 0.32 0.30 4.19 0.60 0.95 0.57 0.62 Mg# 57.13 48.02 55.50 35.71 55.81 55.69 55.05 55.55 (CIPW normative minerals) Ne 0.2 5.3 0.8 0.0 0.1 0.0 0.0 0.0 Hy 0.0 0.0 0.0 0.5 0.0 1.1 0.4 0.6 (Trace element compositions in ppm) La 33.29 49.81 34.06 15.89 33.43 33.68 33.78 34.07 Ce 66.75 97.56 68.87 34.77 67.50 68.07 68.76 68.60 Pr 7.91 10.94 8.17 4.28 7.96 8.06 8.09 8.05 Nd 30.70 40.83 31.69 16.63 31.13 31.60 31.16 31.46 Sm 6.43 7.90 6.76 3.87 6.55 6.54 6.66 6.55 Eu 1.97 2.43 2.06 0.44 1.99 2.04 2.02 2.03 Gd 5.88 6.86 5.99 3.63 5.94 5.97 5.96 6.06 Tb 0.90 1.05 0.94 0.63 0.93 0.93 0.94 0.94 Dy 5.14 5.95 5.38 3.83 5.33 5.36 5.35 5.43 Ho 1.00 1.12 1.03 0.79 1.02 1.01 1.02 1.03 Er 2.52 2.86 2.62 2.20 2.57 2.56 2.59 2.60 Tm 0.35 0.40 0.35 0.32 0.35 0.35 0.37 0.35 Yb 2.07 2.32 2.15 2.00 2.09 2.11 2.11 2.12 Lu 0.31 0.36 0.33 0.31 0.32 0.31 0.32 0.32 Ba 326.42 483.71 341.86 600.44 333.84 344.72 346.11 341.06 Th 3.78 5.17 3.91 10.24 3.87 3.89 3.91 3.89 Nb 38.50 65.79 38.85 6.79 37.65 37.70 38.64 38.67 Y 24.89 28.11 25.72 19.94 25.35 25.43 25.52 25.46 Hf 4.55 5.53 4.66 2.62 4.54 4.54 4.59 4.64 Ta 2.15 3.57 2.20 0.72 2.14 2.18 2.17 2.23 U 1.06 1.47 1.08 2.73 1.13 1.06 1.12 1.12 Pb 5.21 6.32 5.46 14.87 5.10 5.48 3.78 5.49 Rb 21.91 39.03 31.43 151.33 31.58 31.60 24.48 31.97 Cs 0.55 1.07 0.82 8.39 0.89 0.92 0.16 0.81 Sr 774.42 986.77 726.65 63.16 727.88 729.65 762.38 722.95 Sc 23.17 17.21 23.76 2.76 23.22 23.77 23.55 23.49 Zr 197.90 263.30 202.92 76.60 196.26 199.76 197.86 202.48 Cr 168.95 39.50 161.00 4.82 175.50 169.60 181.00 157.50 Ni 100.10 39.60 89.20 4.32 93.30 97.20 95.80 84.70 Ga 20.20 22.09 20.90 8.24 19.20 19.40 19.90 19.00 Cu 44.48 33.93 43.20 48.94 44.40 44.40 44.20 43.40 V 185.87 187.66 194.10 5.73 189.20 193.00 193.50 195.60 Zn 85.87 87.26 86.60 10.25 89.60 90.80 85.70 87.10 SB: Sheridan Bluff. ME: Mount Early Alk: alkaline basalt. Thol: tholeiitic (subalkaline) basalt. Trans: transitional alkaline basalt. Inc: calcareous inclusion. *Major elements and Cr-Zn from XRF, La-Zr from ICPMS 75

Table 8 (continued). Whole rock major element (XRF) and trace element (ICPMS and XRF) compositions* Sample ME15-009 ME15-010 ME15-011 ME15-012 ME15-013 ME15-014 SB15-001 SB15-002 Location ME ME ME ME ME ME SB SB Rock type alk alk alk alk alk trans thol thol (Major element compositions in wt. %) SiO2 49.77 49.66 49.26 49.61 49.18 49.49 48.58 48.68 TiO2 2.25 2.10 2.24 2.19 2.21 1.99 1.46 1.53 Al2O3 17.39 17.17 17.59 17.53 17.06 16.03 15.72 15.95 FeO 8.99 8.73 8.62 8.92 8.63 9.43 10.67 10.47 MnO 0.14 0.13 0.14 0.14 0.13 0.15 0.17 0.17 MgO 4.71 4.20 4.27 4.64 4.41 6.95 10.19 8.78 CaO 8.08 8.38 7.92 8.03 8.06 8.79 8.87 9.22 Na2O 4.64 4.29 4.65 4.40 4.59 3.77 3.08 3.19 K2O 2.27 1.94 2.23 2.19 1.68 1.74 0.72 0.76 P2O5 0.77 0.60 0.78 0.75 0.77 0.54 0.24 0.26 Total 99.03 97.22 97.71 98.38 96.72 98.87 99.69 99.00 LOI 0.24 2.26 1.67 1.06 2.33 0.12 0.00 0.84 Mg# 48.30 46.18 46.90 48.12 47.68 56.79 63.01 59.93 (CIPW normative minerals) Ne 4.5 1.6 4.2 2.8 1.9 1.0 0.0 0.0 Hy 0.0 0.0 0.0 0.0 0.0 0.0 4.8 4.8 (Trace element compositions in ppm) La 48.52 37.85 49.55 48.52 49.09 33.97 14.84 15.32 Ce 94.90 75.79 96.67 94.72 96.29 68.14 31.67 32.72 Pr 10.80 8.92 10.98 10.78 10.82 8.06 4.05 4.15 Nd 40.00 34.29 40.31 39.83 40.30 31.08 16.85 17.32 Sm 7.71 6.97 7.88 7.78 7.73 6.67 4.16 4.24 Eu 2.37 2.14 2.40 2.36 2.35 2.01 1.46 1.50 Gd 6.79 6.27 6.82 6.75 6.71 5.94 4.42 4.59 Tb 1.05 0.97 1.08 1.05 1.06 0.94 0.78 0.79 Dy 5.82 5.49 5.91 5.80 5.94 5.25 4.68 4.84 Ho 1.10 1.07 1.12 1.10 1.11 1.00 0.96 0.99 Er 2.83 2.72 2.86 2.80 2.81 2.53 2.58 2.64 Tm 0.38 0.37 0.39 0.38 0.38 0.35 0.36 0.37 Yb 2.33 2.22 2.37 2.36 2.35 2.10 2.24 2.35 Lu 0.35 0.34 0.38 0.35 0.35 0.31 0.34 0.37 Ba 472.01 389.21 476.41 476.15 481.26 327.99 134.67 138.16 Th 5.09 4.23 5.32 5.25 5.20 3.84 1.65 1.71 Nb 62.72 43.23 64.95 62.15 64.35 38.75 13.33 13.74 Y 28.19 26.53 28.46 28.09 28.13 25.06 23.76 24.70 Hf 5.44 4.90 5.49 5.43 5.61 4.54 2.88 3.05 Ta 3.34 2.38 3.45 3.37 3.43 2.17 0.82 0.82 U 1.51 1.23 1.43 1.33 1.52 1.07 0.46 0.89 Pb 6.80 6.61 6.83 6.84 6.45 5.34 2.66 2.72 Rb 39.12 33.40 38.92 34.45 13.14 31.28 15.09 21.59 Cs 1.20 0.88 1.12 0.75 1.14 0.81 0.37 0.79 Sr 966.12 842.48 966.11 1008.64 998.35 711.98 359.28 359.00 Sc 18.41 18.33 17.01 17.52 17.90 23.40 27.12 28.12 Zr 257.58 217.17 261.33 254.78 261.51 202.21 124.39 127.10 Cr 51.10 54.40 34.50 43.20 42.10 177.70 326.46 291.14 Ni 38.90 34.80 32.10 36.20 39.70 100.30 193.23 149.75 Ga 21.50 21.10 21.70 21.60 20.30 20.10 17.91 19.10 Cu 34.40 28.00 34.40 31.30 32.80 45.10 56.22 55.42 V 192.00 189.10 188.60 184.30 186.70 192.60 178.70 184.87 Zn 91.70 93.00 87.90 87.10 91.80 87.40 87.86 87.06 76

Table 8 (continued). Whole rock major element (XRF) and trace element (ICPMS and XRF) compositions* Sample SB15-003 SB15-004 SB15-005 SB15-006 SB15-007 SB15-008 SB15-009 SB15-010 Location SB SB SB SB SB SB SB SB Rock type alk thol thol alk alk alk thol alk (Major element compositions in wt. %) SiO2 48.55 49.37 48.60 48.68 48.64 48.82 48.60 48.63 TiO2 2.25 1.85 1.54 2.07 2.13 2.07 1.54 2.13 Al2O3 17.13 15.99 17.01 16.85 17.38 17.17 16.15 17.26 FeO 9.10 10.93 10.00 9.27 8.84 8.99 10.63 9.05 MnO 0.16 0.18 0.16 0.15 0.15 0.15 0.17 0.15 MgO 5.09 7.49 7.86 6.49 5.21 6.01 8.72 5.69 CaO 8.97 9.66 9.78 9.14 9.28 9.32 9.33 9.25 Na2O 4.44 3.33 3.28 4.15 4.26 4.22 3.25 4.40 K2O 2.02 0.76 0.62 1.66 1.78 1.77 0.74 1.80 P2O5 0.78 0.27 0.24 0.65 0.72 0.67 0.26 0.73 Total 98.47 99.82 99.08 99.11 98.38 99.21 99.39 99.08 LOI 0.00 0.00 0.42 0.47 0.82 0.00 0.16 0.13 Mg# 49.94 55.00 58.37 55.53 51.24 54.39 59.40 52.86 (CIPW normative minerals) Ne 5.7 0.0 0.0 4.2 4.3 4.8 0.0 5.7 Hy 0.0 5.9 3.2 0.0 0.0 0.0 2.7 0.0 (Trace element compositions in ppm) La 47.80 16.29 14.29 38.42 44.65 43.31 16.65 44.28 Ce 90.36 35.37 30.16 73.44 83.99 80.88 34.06 84.04 Pr 10.25 4.53 3.80 8.46 9.59 9.28 4.67 9.50 Nd 38.53 19.18 16.26 32.41 35.97 34.82 19.65 35.71 Sm 7.77 4.78 4.19 6.70 7.36 7.22 4.91 7.19 Eu 2.44 1.63 1.52 2.23 2.33 2.32 1.60 2.35 Gd 7.28 5.18 4.61 6.38 6.79 6.62 5.13 6.77 Tb 1.15 0.91 0.80 1.03 1.09 1.07 0.87 1.08 Dy 6.71 5.49 4.97 5.92 6.26 6.10 5.41 6.17 Ho 1.30 1.13 1.01 1.14 1.24 1.20 1.09 1.19 Er 3.34 3.01 2.80 2.89 3.14 3.07 2.95 3.10 Tm 0.46 0.42 0.40 0.41 0.44 0.42 0.42 0.42 Yb 2.81 2.70 2.42 2.50 2.68 2.60 2.55 2.60 Lu 0.42 0.42 0.38 0.38 0.40 0.38 0.39 0.39 Ba 336.88 141.19 109.55 269.63 319.01 305.07 144.12 312.85 Th 5.13 1.90 1.53 4.13 4.85 4.61 1.77 4.74 Nb 57.57 15.69 14.05 47.03 53.59 51.72 14.42 53.53 Y 32.56 28.12 25.17 28.47 30.91 30.01 27.47 29.91 Hf 5.47 3.49 3.06 5.11 5.47 5.30 3.10 5.43 Ta 3.13 0.95 0.86 2.65 2.94 2.87 0.88 2.98 U 0.96 0.52 0.46 1.27 1.43 1.37 0.48 1.47 Pb 5.00 2.66 2.08 3.88 4.59 4.27 2.55 3.94 Rb 28.61 14.16 9.85 23.14 25.06 24.37 12.89 24.67 Cs 0.75 0.26 0.21 0.39 0.64 0.51 0.23 0.70 Sr 839.26 358.19 360.11 782.09 877.88 868.54 380.33 890.27 Sc 24.92 33.75 30.06 25.97 25.65 26.76 29.52 26.42 Zr 257.95 146.03 130.18 251.24 266.43 258.82 129.74 267.29 Cr 67.66 236.70 229.94 146.46 71.30 112.00 275.60 98.70 Ni 29.35 97.90 86.86 59.80 30.60 42.70 147.40 38.61 Ga 19.60 17.90 18.01 18.61 19.70 19.00 19.00 19.00 Cu 40.00 57.10 49.95 46.47 37.20 43.80 54.20 42.19 V 196.91 218.10 191.54 192.63 197.70 193.80 186.30 192.83 Zn 78.90 92.40 80.20 77.11 77.10 79.50 94.70 73.83 77

Table 9. Temperature and pressure estimates from olivine and clinopyroxene. Clinopyroxene Putirka et al. 1996 Putirka et al. 1996 sample n type Eqn T1, Temp ± 69 °C Eqn P1, P ± 4.6 kbar SB15-001 1 thol 1206 3.2 ME15-004 1 trans 1191 5.1 SB15-003 3 alk 1135 2.3 SB15-008 6 alk 1183 7.0 ME15-003 2 alk 1181 8.9 Olivine Beattie 1993 Putirka et al. 2008 sample n type Temp ± 53 °C Eqn 22, Temp ± 43 °C SB15-001 3 thol 1301 1293 ME15-004 1 trans 1223 1212 ME15-005 5 trans 1217 1212 SB15-003 1 alk 1179 1177 SB15-008 4 alk 1211 1200 ME15-003 5 alk 1188 1181 ME15-011 4 alk 1159 1151 Olivine continued Putirka et al. 2008 Sisson & Grove 1992 All Equations sample n type Eqn 21, Temp ± 52 °C Eqn 2, Temp °C Avg T °C SB15-001 3 thol 1332 1202 1282 ME15-004 1 trans 1228 1126 1197 ME15-005 5 trans 1227 1127 1196 SB15-003 1 alk 1183 1088 1157 SB15-008 4 alk 1201 1112 1181 ME15-003 5 alk 1177 1087 1158 ME15-011 4 alk 1142 1061 1128 Temperature in degrees celsius. Pressure in kilobars (kbar). n: number of points (probe data) used for the average of that sample Thol: tholeiitic. Trans: transitional basalt. Alk: alkaline. Calculation spreadsheet from Putirka et al., (2008). 78

Table 10. Temperature and oxygen fugacity estimates from magnetite-ilmenite pairs.

SB15-001, thol, n=3 Avg T °C Avg log fO2 Range T °C Range log fO2 Powell & Powell 1977 1070 1012 to 1150 Spencer & Lindsley 1981 1180 -9 1057 to 1361 -10 to -6 Anderson & Lindsley 1985 1110 -10 1006 to 1258 -11 to -8 All methods average 1120 -9.5 1006 to 1361 -11 to -6

SB15-003, alk, n=6 Avg T °C Avg log fO2 Range T °C Range log fO2 Powell & Powell 1977 948 924 to 965 Spencer & Lindsley 1981 820 -15 788 to 843 -16 to -15 Anderson & Lindsley 1985 847 -16 813 to 871 -16 to -15 All methods average 872 -15.5 788 to 965 -16 to -15

SB15-008, alk, n=2 Avg T °C Avg log fO2 Range T °C Range log fO2 Powell & Powell 1977 952 936 to 967 Spencer & Lindsley 1981 881 -14 836 to 926 -15 to -13 Anderson & Lindsley 1985 887 -14 852 to 922 -15 to -13 All methods average 907 -14 836 to 967 -15 to -13

ME15-003, alk, n=2 Avg T °C Avg log fO2 Range T °C Range log fO2 Powell & Powell 1977 925 917 to 932 Spencer & Lindsley 1981 858 -14 848 to 868 -14 to -14 Anderson & Lindsley 1985 860 -15 851 to 869 -15 to -14 All methods average 881 -14.5 848 to 932 -15 to -14 Thol: tholeiitic. Alk: alkaline. n: number of pairs. ILMAT: A Magnetite-Ilmenite Geothermobarometry Program (version 1.20) By: Luc D Lepage ([email protected]) 79

APPENDIX B. FIGURES

Figure 1. BEDMAP2 of Antarctica showing the bed height elevation in meters. The green triangle represents the location of Mt. Early and Sheridan Bluff. The red triangles represent the location of other Late Cenozoic volcanoes (created using ArcMap). WARS: West Antarctic Rift System. SP: South Pole. MBL: Marie Byrd Land. SVL: Southern Victoria Land. NVL: Northern Victoria Land. AP: Antarctic Peninsula; the volcanism at AP is not part of WARS. The Ross Embayment area includes the Ross Sea and Ross Ice Shelf. The inset is high resolution aerial photography of the volcanic edifices near 87° S and 153° W (Polar Geospatial Center). The inset includes the position of the field camp of this study, at the base of Mt. Weaver. 80

A

B

Figure 2. A: Sketch of the east face of Sheridan Bluff (made from a photograph). PB, pillow breccia; B, breccia; gr, granodiorite. Numbers indicate sample sites (Fig. A from Stump et al, 1980). B: Geologic maps of Sheridan Bluff, cross sectional and aerial views, by John Smellie, 2016. 81

A

B

Figure 3. A: Sketch cross section of Mount Early (made from a photograph). PL, pillow lava; PB, pillow breccia; B, breccia. Numbers indicate sample sites (Fig. A from Stump et al., 1980). B: Photograph and geologic map of Mt. Early annotated by John Smellie, 2016. 82

MgO wt. % 4.20 - 4.86 4.87 - 7.00

Figure 4. Sample locations on Mt. Early. Samples are symbolized based on their wt. % MgO composition. The blue squares are alkaline and the orange pentagons are transitional basalt. Sample numbers are abbreviated to only include the location ME (Mt. Early) and the number in sequence (01, 02, 03, etc.). The number for the year collected (“15” for 2015) has been omitted. Figure made using GPS locations in ArcMap. (Aerial photography from Polar Geospatial Center). 83

SB01 SB02 SB04 SB07 SB05 SB06 SB03

SB09

SB08 SB10

MgO wt. % 5.09 - 6.48 6.49 - 10.19

Figure 5. Sample locations on Sheridan Bluff. Samples are symbolized based on their wt. % MgO composition. The green triangles are alkaline and the purple diamonds are tholeiitic basalt. Sample numbers are abbreviated to only include the location SB (Sheridan Bluff) and the number in sequence (01, 02, 03, etc.). The number for the year collected (“15” for 2015) has been omitted. Figure made using GPS locations in ArcMap. (Aerial photography from Polar Geospatial Center). 84

A D

B

C E

dike Pillow basalt

Lapilli tuff

Figure 6. Field images showing samples collected from different units at Mt. Early and Sheridan Bluff. A: Sheridan Bluff lava flows samples SB15-005 & SB15-006. B: Sheridan Bluff upper lava flow unit samples SB15-009 & SB15-010. C: Mt. Early pillow basalts samples outlined in red. Sample location of ME15-001 & ME15-002. D: Detail image of lapilli tuff above “the Nubbin”. E: Mt. Early dike outlined in red, behind “the Nubbin” sample ME15-010. (Photos A-C & E taken by Kurt Panter, photo D taken by Tim Burton). 85

A B

C D

E F

G H

Figure 7. Magnetite and ilmenite pairs are found in alkaline rocks at Mt. Early and in the alkaline and tholeiite basalts at Sheridan Bluff. Cr-spinel is found in the transitional basalt at Mt. Early. A: Sample ME15-003 (alkaline) showing magnetite and ilmenite pairs in plain-polarized light (PPL). B: Same as A but in cross- polarized light (XPL). C: Sample SB15-008 (alkaline) showing magnetite and ilmenite pairs between phenocrysts of olivine and plagioclase in PPL. D: Same as C but in XPL. E: Sample SB15-003 (alkaline) showing magnetite and ilmenite pairs in PPL. F: Sample SB15-001 (tholeiite) showing magnetite and ilmenite pairs surrounded by plagioclase and clinopyroxene in PPL. G: Sample ME15-004 (transitional) showing a grain of cr-spinel alongside of an olivine phenocryst in PPL. H: Same as G but in XPL.

86

A B p l cp x

C D pl

E F

ol ol

G p H l

Figure 8. Disequilibrium textures and groundmass textures. A: Sample ME15-003 (alkaline) showing resorption of anhedral clinopyroxene next to a euhedral olivine phenocryst in plain-polarized light (PPL). B: Sample ME15- 014 (transitional) showing resorption of euhedral olivine and plagioclase phenocrysts in cross-polarized light (XPL). C: Sample SB15-006 (alkaline) showing a subhedral olivine phenocryst surrounded by predominantly crystalline groundmass (hypocrystalline) consisting of plagioclase, clinopyroxene, and optically identified magnetite and ilmenite (opaque oxides) in PPL. D: Same as C but in XPL. E: Sample SB15-002 (tholeiite) showing euhedral to subhedral olivine phenocryst surrounded by predominantly glassy groundmass (hypohyaline) containing plagioclase, clinopyroxene, and opaque oxides in PPL. F: Same as E but in XPL. G: Sample ME15- 011 (alkaline) showing a cluster of plagioclase phenocrysts in a tachylitic groundmass consisting entirely of glass in PPL. H: Same as G but in XPL; Carlsbad to albite twinning can be seen through the plagioclase phenocrysts, with or without oscillatory zoning which is subtle in the largest phenocryst in this image. 87

Figure 9. Mineral classification diagrams. A: Olivine compositions range from Fo84 to Fo58. B: Clinopyroxene compositions of diopside for alkaline and transitional basalt and augite for tholeiitic basalt. C: Plagioclase compositions plot within labradorite field. Two rim compositions from alkaline basalt at Sheridan border andesine-labradorite. D: Magnetite (ulvospinel) and ilmenite found in alkaline and tholeiite basalt. E: Chrome- rich spinel group oxides (cr-spinel) found in the transitional basalt. 88

A

B

Figure 10. A: Total alkali vs. silica (TAS) diagram, normalized to 100% volatile free, for classification of basalts (LeBas et al., 1986). Cross-cutting line represents the boundary between alkaline and sub-alkaline basalts with respect to the TAS plot (Irvine & Baragar 1971). B: Classification of tholeiite vs. alkaline basalt with respect to trace elements (Floyd & Winchester 1975). 89

Figure 11. Major element oxides in weight %. Comparisons are between this study, WARS, New Zealand, and Australia basalts (i.e. DAMP), including data from Stump et al., (1980) and Licht et al., (2018). Symbols and fields are the same as in Figure 10 (see key). All major element oxides are normalized to 100% volatile free; FeO is expressed as FeO total. 90

Figure 12. Select trace elements in ppm against MgO in weight %. Comparisons are between this study, WARS, New Zealand, and Australia basalts (i.e. DAMP), including Mt. Howe erratics from Licht et al., (2018). Symbols and fields are the same as in Figure 10 (see key). 91

A B PM PM Mt. Early Sheridan Bluff

C D Chondrite Chondrite Mt. Early Sheridan Bluff

Figure 13. Multi-elemental diagrams showing Mt. Early and Sheridan Bluff against fields for DAMP; Symbols and fields are the same as in Figure 10. A: Primitive mantle (PM) normalized (McDonough & Sun, 1995) trace elemental diagram for Mt. Early against basalts from DAMP, including data from Licht et al., (2018). Mt. Early basalt are enriched in incompatible elements and show minor Pb negative anomalies and lack K negative anomalies. Transitional basalt at Mt. Early have slightly lower concentrations in nearly all incompatible elements relative to alkaline basalt. B: The same primitive mantle (PM) normalized trace elemental diagram for Sheridan Bluff against basalts from DAMP, including data from Licht et al., (2018). Alkaline basalt are enriched in incompatible elements and show minor Pb negative anomalies and lack K negative anomalies. Tholeiitic basalts at Sheridan Bluff have lower concentrations of incompatible elements and lack Pb and K negative anomalies but show minor P negative anomalies. C: Chondrite normalized (Sun & McDonough, 1989) Rare Earth Element (REE) diagram for Mt. Early against basalts from DAMP, and Mt. Howe erratics from Licht et al., (2018). Incompatible and REE patterns for Mt. Early mirror basalt from DAMP. D: The same chondrite normalized Rare Earth Element (REE) diagram for Sheridan Bluff against basalts from DAMP, and Mt. Howe erratics from Licht et al., (2018). Incompatible and REE patterns for tholeiite are flatter and overall less enriched than alkaline basalt. 92

Figure 14. Normalized K/K* and Pb/Pb* anomaly plots against MgO in weight %. These two anomalies of Mt. Early and Sheridan Bluff are shown in comparison to DAMP; symbols and fields are the same as in Figure 10 (see key).

K and Pb are normalized based on their neighboring elements on the primitive mantle (PM) normalized multi- elemental diagrams, where K/K* = ((K20*8301.5)/(SQRT((Ta/0.037)*(La/0.648))*240)), and Pb/Pb* = ((Pb)/(SQRT((Ce/1.675)*(Nd/1.25))*0.15). Normalization values used in calculations come from McDonough & Sun, (1995).

A value of 1.0 represents a relatively flat pattern (i.e. lack of anomaly). All samples for Mt. Early and Sheridan Bluff plot outside of the WARS field for K/K* and Pb/Pb*, and border the lines for a value of 1.0. Tholeiitic and transitional basalt also plot outside of the K/K* and Pb/Pb* fields for New Zealand. The alkaline and transitional basalt at Mt. Early remains unique to all the fields of DAMP on the Pb/Pb* plot. 93

A Equations from Table 9: Eq. T1; cpx Eq. 21; olivine Eq. 22; olivine Beattie 93; olivine

SB-001 SB-003 SB-008 ME-003 ME-004 ME-005 ME-011 (n = 1; 3) (n = 3; 1) (n = 6; 4) (n = 2; 5) (n = 1; 1) (n = 5) (n = 4)

B

SB-001 SB-003 SB-008 ME-003 ME-004 (n = 1) (n = 3) (n = 6) (n = 2) (n = 1)

Figure 15. Sample numbers are abbreviated to include only the location SB or ME (Sheridan Bluff or Mt. Early) and the number in sequence (-001, -002, -003, etc.). The number for the year collected (“15” for 2015) has been omitted. The colors indicate whether that sample is tholeiitic, alkaline, or transitional, and are coordinated with the symbols represented for whole rock data in Figure 10 (see key). A: Temperature estimates from clinopyroxene (CPX) and olivine mineral chemistry (Table 9; Putirka et al., 2008). Different symbols represent different equations used for that average (see Table 9). N = number of points (from electron-microprobe) used in the average of that sample. The colon separates n for clinopyroxene and n for olivine, respectively. CPX and olivine show the tholeiite to have an average higher temperature than the transitional and alkaline basalt. B: Pressure estimates calculated from clinopyroxene chemistry (Table 9; Putirka et al., 2008). N = number of points (from electron-microprobe) used in the average of that sample. The alkaline samples have a higher average pressure than tholeiitic or transitional basalt. 94

Figure 16. Clinopyroxene (CPX) and olivine mineral controls on evolution of whole rock compositions. CPX and olivine compositions (open and closed red circles) are from this study (Tables 2 & 3). The symbols for whole rock data of this study is the same as in Figure 10 (see key). All major elements are in weight %. CaO is highly compatible in CPX, so the ratio CaO/Al2O3 would decrease with decreasing MgO (wt. %) if CPX is fractionating out; the ratio would stay constant with decreasing MgO (wt. %) if the fractionation was dominated by olivine. 95

Figure 17. Chondrite normalized Rare Earth Element (REE) diagrams (Sun & McDonough, 1989). Sheridan Bluff and Mt. Early symbols are the same as in Figure 10 (see key). The black diamonds represent the fractional crystallization model at Sheridan Bluff, where each tholeiite sample was tested with 20% crystallization. The black pentagons represent the fractional crystallization model at Mt. Early, where each transitional sample was (D-1) tested with 20% crystallization. The equation used is Ci = Co(F ) where the total partition coefficient (D) in the model takes into account 70% CPX (Kd values from Green et al., 2000) and 30% olivine (Kd values from Pilet et al., 2011). In both cases the Light Rare Earth Elements (LREE; La-Eu) of the model compositions are too low in concentration and Heavy Rare Earth Elements (HREE; Gd-Lu) are slightly too high in concentration to match alkaline basalts. 96

Figure 18. A plot of Rb (ppm) versus TiO2 (wt. %) showing Mt. Early and Sheridan Bluff basalts (same symbols defined in Fig. 10) and possible crustal contaminants.

The best fit AFC curve is produced using a Granite Harbour Intrusives contaminant (Northern Victoria Land diorite, sample C6), where the parent is SB-005 from Sheridan Bluff, the D value (partition coefficient) for Rb is 0.1 and the D value for TiO2 is 0.5. F starts at 1 (100% liquid) and increases in segments of 0.05; for example, a value of 0.9 means 90% liquid remains and 10% crystallization has taken place.

The second AFC curve is for a Southern Victoria Land granitoid (Pearse Pluton granite, sample P49909). The same parent, same D values, and same F values are used with this contaminant. This is an example of a contaminant that doesn’t fit the data for Mt. Early and Sheridan Bluff, and therefore will not be considered as a contaminant. Other contaminants tested include eclogite, metasediments, other SVL granitoids, and the upper and lower crust compositions. These contaminants produce similar AFC curves as the Pearse Pluton granite from SVL granitoids, and therefore will not be considered for contamination. 97

A

B

Figure 19. A plot of Ba versus Rb, and La/Nd ratio versus Rb (all trace elements in ppm) showing Mt. Early and Sheridan Bluff basalts (same symbols defined in Fig. 10) and possible crustal contaminants (same symbols defined in Fig. 18). To verify diorite (sample C6; Dallai et al., 2003) as an assimilant, trace elements are modeled using the same parent (SB-005; tholeiite from Sheridan Bluff) and D values (Rb = 0.1). F starts at 1 (100% liquid) and increases in segments of 0.05; for example, a value of 0.9 means 90% liquid remains and 10% crystallization has taken place. A: In a plot of Ba versus Rb, the compositional array of the basalts can be matched over an interval of 10% crystallization with a Ma/Mc ratio of 0.8, and appropriate D values (i.e. incompatible). B: In a plot of La/Nd versus Rb, inappropriate partition coefficients are needed to fit the compositional array of basalts; Nd would have to behave compatibly with a D value of 1.5 (shown above in red). Another AFC curve is shown as an example of what the trend would look like with the appropriate D values (i.e. incompatible), which does not fit the compositional array of basalts. 98 A

B

Depth

Figure 20. Symbols and fields are the same as in Figure 10. A: A plot of trace element ratios La/Yb versus Zr/Nb that has been previously used to distinguish relative differences in degree of melting (Wanless et al., 2006). Sheridan Bluff tholeiite are unique from WARS volcanism and Mt. Rouse, Australia volcanics (Boyce et al., 2015), plotting at much higher degrees of melting. Early/Sheridan basalt follow more closely the trend for New Zealand basalt (Timm et al., 2010). B: A plot of trace element ratios Gd/Yb versus La/Lu that has been previously used to distinguish relative differences in degree of melting and relative depth of melting (Boyce et al., 2015). Early/Sheridan basalt are unique with regards to WARS volcanism and Mt. Rouse, Australia volcanics (Boyce et al., 2015), and plot within the trend for New Zealand basalt (Timm et al., 2010) at shallower depths and higher degrees of melting, especially the tholeiites. 99

A

B

Figure 21. Symbols for Mt. Early and Sheridan Bluff are the same as in Figure 10. A: Plot of La/Lu versus Nb/Yb displaying Early/Sheridan and natural lithospheric xenolith compositions of garnet and spinel lherzolites from the Vitim volcanic field within the Baikal rift, Siberia (Ionov 2004) and spinel lherzolite from Mt. Morning volcano within the West Antarctic rift (Martin et al., 2013) at different degrees of melting. B: Logarithmic plot of La/Lu versus Nb/Yb displaying Early/Sheridan and both garnet and spinel lherzolite source compositions derived for West Antarctic alkaline basalts (Hobbs Coast, Hart et al., 1997), Primitive Mantle (Hoffman 1988) and an Enriched Depleted MORB Mantle (EDMM, Workman & Hart, 2005) modelled at different degrees of melting.

The non-modal batch melting equation comes from Shaw (1970), equation 15: Cl/Co = 1/[Do + F(1-P)], where Cl is the fractional melt to solve for, Co is the source abundance (xenolith or model trace element composition), Do is the starting mode and P is the melting mode (Salters, 1996; Kd values for trace elements from Pilet et al., 2011, and references therein), and F is the amount fractionating (%). The starting mode for a garnet lherzolite uses olivine (0.53), orthopyroxene (0.04), clinopyroxene (0.38), and garnet (0.05), and melt modes of olivine (0.05), orthopyroxene (-0.49), and clinopyroxene (1.31); the melt mode for garnet is 0. The starting mode for a spinel lherzolite uses olivine (0.53), orthopyroxene (0.15), clinopyroxene (0.3), and spinel (0.02), and melt modes of olivine (0.375), orthopyroxene (-0.5), and clinopyroxene (1.125); the melt mode for spinel is 0.