Petrological and Geochemical Study of Deposits from the Kalama and the Goat Rocks Eruptive Periods, Mount St

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Petrological and geochemical study of deposits from the Kalama and the Goat Rocks eruptive periods, Mount St. Helens, USA

Baptiste Lemirre1

MSc thesis

ETH Zurich, Institut for petrology and geochemistry

______

Abstract

Understanding the current and past magmatic plumbing system of a volcano is necessary to predict future volcanic activity and potential associated hazards. Mount St. Helens (MSH) is a good candidate to investigate the long-term behavior of a volcano as it is one of the most active volcanoes in the Cascade arc in the United States of America. Its well-established eruptive history reveals an overall composition trend from rhyodacite to andesite superimposed with compositional cycles during the last few thousand years. A pre-1980 cycle of Mount St. Helens is the Kalama eruptive period. It was preceded by a dormant interval which ended in A.D. 1480 and followed by the Goat Rocks eruptive period. The petrology and the geochemistry of the Kalama and the Goat Rocks deposits show this cyclic evolution. The Kalama eruptive period can be divided into three phases. During the early Kalama period, dacitic tephra were erupted. The middle Kalama period consist of the eruption of andesitic lavas, while the late Kalama period is dominated by dacitic lava domes. After a repose interval of 200 years, the Goat Rocks period began with the eruption of a dacitic tepha and ended with the Floating Island andesitic lava flow. The re-examination of the whole rocks geochemistry in combination with the study of the petrology and mineral chemistry of the Kalama and Goat Rocks deposits indicate the implication of different processes to generate the wide variation of magma compositions during the Kalama and the Goat Rocks eruptive period. While magma mixing is the dominant process to produce the first Kalama andesites, middle Kalama andesites and late Kalama dacites were generated by fractional crystallization within a shallow magma reservoir. Moreover, mineral chemistry shows that the plumbing system is composed of several magma reservoirs at different depths beneath Mount St. Helens. ______

Supervisors: Olivier Bachmann2, Maren Wanke2

______1 Ecole Normale superieur de Lyon, France 2 ETHZ, Institut for petrology and geochemistry, Zurich, Switzerland 1. Introduction

Mount St Helens is an active composite stratovolcano in the Cascade Arc in the United States (fig. 2). Since its eruption and the collapse of the north flank in 1980 (fig. 1) which generated massive human and material damages, the eruptive history of Mount St. Helens was well established (Crandell, 1987; Mullineaux and Crandell, 1981; Mullineaux, 1996). The knowledge of the eruptive history of the volcano is useful for the forecast of future eruptions to diminish the hazards caused by a future eruption. The style of the eruption (and so its violence) is broadly related to the Figure 1: North flank of the volcano Mount St. Helens magma composition and its volatile after the 1980 eruption (Clynne, 2005). content. Moreover, Mount St. Helens belongs to a series of volcanic systems that are characterized by prominent geochemical cycles (e.g. Ferriz and Mahood, 1984; Bachmann et al., 2012). Therefore, the geochemical and petrological studies of the Mount St. Helens deposits provide essential elements to understand the cyclic evolution of the volcano and its plumbing system to predict its future activity (and potential risks linked).

Figure 2: Location of volcanoes in the Cascade arc (Wanke, unpub.). 1

During the 40 000 years of its eruptive history, Mount St. Helens has presented a compositional trend from rhyodacite through dacite to andesite (Smith and Leeman, 1987) represented by a progression of magma compositions and mineral assemblages. A cyclic eruption pattern, with cycles of different durations, is superimposed on the overall compositional behavior. This encompasses a three orders cyclicity (Hopson and Melson, 1990). The first order cycles are represented by eruptive stages lasting about 2000-4000 years separated by repose intervals of several thousand years. The second order cycles (hundreds to thousands of years) consist of eruptive periods separated by dormant intervals within each eruptive stage. The second order cycles are only clear within the most recent stage (Spirit Lake), but there are evidences for a similar cyclic behaviour during earlier stages as well (Mullineaux, 1986). The third order cycles (months to tens of years) comprise the patterns of pyroclastic eruptions and lava emission followed by a repose interval. This pattern occurs repeatedly within a single eruptive period like the Kalama one. This study focuses on the two eruptive periods before the 1980 eruption: the Kalama and the Goat Rocks periods. Geochemically, these cycles are both well established (Pallister, 1992). The deposits vary from dacite to andesite during the Early and middle Kalama and then from andesite to dacite during the late Kalama. Therefore, a cycle can be properly defined by the evolution of the magma composition. The first part of this master project is the geochemical study of the Kalama and Goat Rocks deposits as a tool to re-examine the magmatic processes involved throughout a cycle. Magma mixing seems to be essential to produce the wide range of magmatic compositions (Pallister, 1992). However, it might not have been the predominant process, which occurred during each phase of the Kalama eruptive period. For example, fractional crystallization can explain a big part of the geochemical variations observed during the middle Kalama period. The second part of this project focuses on the petrology and the mineral chemistry of the Kalama and Goat Rocks deposits. Mineral textures can provide further evidences of the magmatic processes predominantly, occurring in each phase (third order cyclicity) of the cycle. Moreover, the cyclicity is underlined by the mineral chemistry during the eruptive periods. Thus, different thermobarometers can be used to constrain the P-T conditions of the magma prior to the eruption and provide information about the structure and the evolution of the magma reservoir beneath the volcano.

2. Eruptive history of Mount St. Helens

The eruptive history of Mount St. Helens volcanic center spans at least 40 000 years. Some studies suggest a magmatic activity in the area of the volcano for 275 ka (Clynne et al., 2008; 40Ar/39Ar ages on plagioclase) or even up to 500 ka (Claiborne et al., 2010; U/Pb dating of zircons). The eruptive history of Mount St. Helens is divided into four eruptive stages (Crandell 1987), each separated by long dormant intervals: the Ape Canyon stage [40-35 ka]; the Cougar stage [23-17 ka]; the Swift Creek stage [13-11 ka]; and the Spirit Lake stage [3.9 ka to present]. Based on radiocarbon ages, the Spirit Lake stage has been divided into six eruptive periods: the Smith Creek [3.9-3.3 ka], the Pine Creek [2.9-2.6 ka], the Castle Creek [2-1.7 ka], the Sugar Bowl [1.2-1.15 ka], the Kalama [A.D. 1480 – 1720] and the Goat Rocks [A.D. 1800 – 1857] periods (fig. 3).

Figure 3: Stratigraphic section through the last 3.9 ka of eruptive history of Mount St. Helens (left) and zoom in on the Kalama cycle (right) (modified after Wanke, unpub., Hopson and Melson, 1990; Carey et al., 1995. Mullineaux, 1996; Clynne et al., 2005)

The Kalama eruptive period was preceded by a repose interval of 700 years (1200 if the minor eruption of the Sugar Bowl event is neglected), which ended in A.D. 1480 (Yamaguchi, 1983) with a voluminous explosive euption of the dacitic (66-67 wt% SiO2) tephra Wn (2 km3 DRE; Carey et al., 1989). Two additional tephra eruptions of much smaller volume ensued (layer Wa and Wb) and were followed in A.D. 1482 by the eruption of the We tephra with a DRE volume of 0.4 km3 (Carey et al., 1989). These early Kalama Plinian eruptions were followed by the extrusion of several dacitic domes near the summit (Hoblitt et al., 1980; Clynne et al., 2005) along with dome collapse and pyroclastic flows. This early Kalama phase of dacitic volcanism was followed abruptly by the eruptions of andesites. The andesitic eruptions initially produced the four tephra layers of set X (Xb, Xs, Xm and

Xh). Scoria fragments from the X tephra set have 57-59 wt% SiO2. The early andesitic tephra eruptions were followed by the extrusion of andesitic lavas (Middle Kalama, 56-59 wt% SiO2). The middle Kalama lavas were followed by the extrusion of a summit dacite dome (A.D. 1620 – 1720) along with pyroclastic flow (61-64 wt% SiO2), which corresponds to the end of the Kalama period. Thus, the Kalama eruptive period lasted about 300 years.

The Goat Rocks eruptive period followed after about 200 years of repose. It started with a Plinian eruption of dacitic pumice (tephra layer T, 62.5-63.5 wt% SiO2) followed by the growth of a dacite dome. The Goat Rocks period ended with the extrusion of andesitic lavas (61 wt% SiO2) and the resurgence of the dome.

3. Method

3.1. Sample preparation

19 samples collected in the field by M. Wanke and O. Bachmann were used for the analytical part. They were chosen to represent most of the eruptive phases of the Kalama and the Goat Rocks periods. The lithology of the samples varies from dacite (pumice lapilli) to andesite (lava and scoria clasts).

XRF-pills

In order to study the geochemical evolution of the magma erupted during the Kalama and Goat rocks periods, major and trace elements concentrations of whole rocks were analyzed on fused XRF-pills. Pieces of samples were crushed and milled into fine powders using a hydraulic press and an agate mill and mortar. 1.5g of each sample was used to determine the loss on ignition by heating it up at

950°C for two hours. The desiccated sample was mixed with lithium tetraborate LiB4 (1:5 mixture). The mixture was fused in a gold-platinum crucible using a Claisse M4® fluxer and subsequently cooled in a mold.

Thin sections

Thin sections were prepared to study the petrography of the deposits. Because of the porosity of the volcanic rocks, samples were impregnated with epoxy to fill the pores before the preparation of the sections. Thin sections were first observed with optical microscopy, then with a Scanning Electron Microscope (SEM) and finally, minerals were analyzed with an Electron Probe Microanalyses (EPMA). So thin sections were well polished and not cover with a glass.

Epoxy mounts

For the petrographic study of the tephras, epoxy mounts of minerals were prepared, because the crystallinity is relatively low, not enough representative crystals are present in the thin section. The samples were crushed with a hydraulic press and sieved. Three fractions were separated: 500-1 000 μm; 250-500 μm; and <250 μm. Plagioclases, amphiboles and pyroxenes were picked from the two biggest fractions under a binocular microscope. The minerals were mounted in epoxy and well polished for the microprobe analyses.

Carbon coating

After the observation with the optical microscope, the thin sections and mineral mounts were coated. This is necessary before using the SEM and the EPMA (20 nm thick carbon coat) to avoid charging by the electron beam (see SEM & EPMA part) and distribute the heat on the sample during the measurement.

3.2. Whole rocks geochemistry

X-ray fluorescence (XRF)

Major elements compositions of whole rocks were analyzed by XRF-fluorescence spectrometry, using a WD-XRF (Axios, PANalytical) at the ETH Zurich, to constrain the evolution of the magma chemistry throughout the Kalama and the Goat Rocks eruptive periods. Atoms in the pill are excited by an X-ray bombardment; thereby each element of the target sample produces X-ray fluorescent radiation of a characteristic wavelength, which is detected by a spectrometer. The spectrometer is equipped with

4 diffraction crystals to measure the intensity of each radiation (using the Bragg’s law principle). The intensity is a function of the concentration of the element. To process the data, a calibration based on 30 certified international standards is used.

Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS)

Trace element compositions of the fused disks (XRF-pills) were measured by a laser ablation inductively coupled plasma mass spectrometry at ETH Zurich with a “Geolas Laser ablation system” coupled with a Elan 6100 DRC (Perkin Elmer) mass spectrometer. Few μm3 of sample are ablated with the laser and ionized by argon plasma. Then the gas is analyzed by the quadripole mass spectrometer. A spot size of 90 μm was ablated from the samples and the blank and 40 μm from the standard. The background intensity was measured for 40 sec and the peak for 60 sec. An external standard is used (NIST 610) as well as an internal standard (CaO concentration measured with the XRF) to determine the concentration of each element in the sample from the intensity ratio:

Csamp = IR-samp*CR-std*CI-std/IR-std with Csamp the concentration in the sample; IR-samp the intensity ratio (element/internal standard) in the sample; IR-std in the standard; CR-std the concentration ration (element/internal standard) in the standard and CI-std the concentration of the internal standard. The intensity is the average of the signal minus the average of the background. The data were processed using the data reduction software SILLS on Matlab® (Guillong et al., 2008).

3.3. Petrography and mineral chemistry

Optical microscopy

In order to determine the rock textures and the mineral phases presents in these samples, the thin section were examined under an optical microscope using polarized light. This is useful to have a first overview of the textural features in the minerals, such as zonation, inclusions, rims, etc., before using the SEM.

Scanning electron microscopy (SEM)

Scanning electron microscopy is used in order to produce high resolution images of the polished and coated sample surface (grain mounts or thin sections). Back-scattered electron images were performed on a JEOL JSM-6390LA equipped with a LaB6 filament (to produce the electron beam) at the ETHZ. A focused electron microbeam scans the surface. Excited elements of the target material produce different electronic signals (X-rays, low energy electrons, back-scattered electrons …) in response to this excitation. The elastic scattering of the electrons produce back-scattered electron (BSE) images. Their intensity is strongly related to the average atomic number Z of the target material. Light phases appear darker than heavy phases. Thus, BSE images provide information on the relative composition of the sample and can be used to determine mineral textures like the zoning patterns of minerals. Anorthite (An) contents analyses were performed using the SEM equipped with an EDS. The measurements are less accurate than the EPMA analyses (few plagioclase crystals were analyzed with both methods to test the accuracy and the precision of the EDS) but the error of 2-3% is acceptable to have a good idea of the relative variability of the An content.

Electron probe micro analysis (EPMA)

Electron probe micro analysis is a non-destructive microbeam technique based on the same principle as the SEM but optimized for quantitative analyses. The electron probe microanalyses are obtained using the JEOL JXA-8200 microprobe equipped with five wavelength-dispersive spectrometers (WDS) at the ETH in order to analyze the composition of pyroxenes and amphibole. The polished and coated

5 sample is bombarded by an electron beam. Atoms of a small volume (1-3 μm3) of the target material are excited by the focused beam and so emit X-rays with a characteristic wavelength for each element. The intensity is proportional to the concentration of the element in the excited volume. The composition of the volume is determined using a standardization for each element. Radiations are detected with energy- (EDS) or wavelength-dispersive spectrometers (WDS). The EDS measures each energy corresponding to a characteristic radiation. The WDS analyzes emitted radiations as a function of their wavelengths by different crystals (using the Bragg’s law principle: nλ = 2d*sinθ). The intensity of each single X-ray is measured separately. This allows a higher sensitivity and less background noise.

The set-up of the instrument is specific for each group of minerals. For amphibole and pyroxene, the analyzed elements were Si, Al, Mg, Na, Cr, Ti, K, Ca, Fe and Mn. Two elements were measured on each spectrometers (5 elements simultaneously) with a proper calibration for each of them (table 1). Indeed, the standard used must be homogenous, with a known composition and a similar matrix to limit matrix effects. A 15 kV accelerating voltage, a 20 nA beam current and a focused beam (spot size 0) were used for the analyses with the differential mode and a base line at 0.7 kV (0.4 kV for Ti and Ca). The elements were analyzed for 20 sec on the peak and 10 sec on the background (left and right). During the measurement, Smithsonian standards (diopside USNM 117733, Kakanui augite USNM 122142 and Kakanui hornblende USNM 1*3965) were regularly measured as unknowns to provide a quality control (Jarosewich et al., 1980).

Element crystal spectro run # standard Si TAP 1 1 wollastonite Al TAP 1 2 anorthite Mg TAPH 2 2 forsterite H083 Na TAPH 2 1 aegirine Cr PET 3 2 chromite D028 Ti PET 3 1 rutile K PETH 4 1 k-feldspar Ca PETH 4 2 wollastonite Table 1: Standards and method used Fe LIFH 5 1 fayalite H116 for the electron probe microanalyses Mn LIFH 5 2 pyrophyllite of pyroxene and amphibole.

4. Results

4.1. Petrography of the Kalama and the Goat Rocks eruptive periods

The samples represent each eruptive phase of the Kalama and the Goat Rocks eruptive periods (annex 1). The first group of samples is from the W tephra set erupted at the beginning of the Kalama period. It comprises dacitic pumices and is divided into five layers: Wn, Wa, Wb, We and We. All the pumice lapilli exhibit mineral disequilibria, while only the bottom of the Wn unit is banded. Those dacitic tephras contain plagioclase, orthopyroxene and amphibole phenocrysts. Moreover, quenched mafic enclaves of basaltic to basaltic andesitic composition are found in some of the early Kalama eruptive products (unit W, Wanke, pers. comm.). Then, the tephra set X was erupted. This banded X tephra sequence is composed of four layers (Xb, Xs, Xm and Xh). The rocks from the set X have a more mafic composition (darker), they are andesites. The sample from the layer Xb comprises scoria clasts while the samples from the layer Xs and Xm consist of coarse ashes. During the middle Kalama period, andesitic lavas were erupted. They formed

6 for example the Worm Complex flow. The mineralogy of these andesitic deposits (tephra X and Worm Complex) exhibits phenocrysts of plagioclase, orthopyroxene and clinopyroxene. Following these explosive andesitic eruptions, lavas erupted near the summit (e.g., the summit dome) of the late Kalama period. The crystals found in the summit dome are plagioclases, clino- and orthopyroxenes and amphibole. Thus, the rocks of the Kalama eruptive period can be separated into three groups: the early dacitic tephra, the middle andesitic deposits, and the late dacitic dome.

Pumice lapilli were erupted at the beginning of the Goat Rocks eruptive period. They have a dacitic composition with plagioclase, clino- and orthopyroxene and amphibole phenocrysts. These pumice lapilli form the tephra layer T. The Goat Rocks period ended with the eruption of the Floating Island lava flow, which is andesitic in composition and contains plagioclase and pyroxene phenocrysts.

4.2. Geochemistry of the Kalama and the Goat Rocks eruptive period

4.2.1. Major element geochemistry

Analytical data for the Kalama eruptive period define single linear trends on SiO2 variation diagrams for major and some trace elements (fig. 5), excepted for K2O and P2O5 (cyclic trend), throughout all the eruptive period. Rocks of the Kalama eruptive period display a wide range of SiO2 concentration: from 55 to 67 wt% for a period of only ca. 300 years. However, as shown with the color code (dark colors indicate an early eruption within the cycle, lighter colors indicate later eruption), the variation is not uniform from the more evolved towards the less evolved magma; first SiO2 decreases and then it increases according to the chronology of the deposits. During the Goat Rocks period, SiO2 decreases again. While some oxides (CaO, MgO, Fe2O3, P2O5, TiO2) increase with decreasing SiO2, others (Na2O, K2O) decrease with the decrease of SiO2. The TAS diagram (fig. 4) indicates that samples vary from dacite to andesite/basaltic andesite during the first part of the Kalama period, and then from andesite to dacite during the second part of the period. During the Goat Rocks period, deposits vary from dacite to andesite. These observations are confirmed by the petrological data.

10 9 8

6 O, wt%O,

2 5 4

O+K basaltic andesite dacite 2 3 andesite Na 2 1 0 40 45 50 55 60 65 70 SiO , wt% 2

Figure 4: TAS diagram for the Kalama and the Goat Rocks deposits (after Lebas et al., 1986). 7

5 8

4.8 7

4.6 6 O / O wt%

2 4.4 5 CaO /CaO wt% Na 4.2 4 4 3 54 59 64 54 59 64

SiO2 / wt% SiO2 / wt%

2 5

1.8 4 1.6 3

0 0 wt%/ 1.4

2 K 1.2 /Mg0 wt% 2 1 1 54 59 64 54 59 64

SiO2 / wt% SiO2 / wt%

8 0.4

7 0.3

/ wt% /

0.2

3 5

0 5

2 P Fe 4 0.1 3 0 54 59 64 54 59 64

SiO2 / wt% SiO2 / wt%

1.4 Figure 5: oxide vs. SiO2 diagrams for 1.2 the Kalama and Goat Rocks deposits. 1

/wt% Colors correspond to the chronology

2 0.8 of the period: green for the Kalama TiO 0.6 period and red for the Goat Rocks one. 0.4 Dark colors are for the beginning of 54 59 64 the eruptive period while bright colors correspond to the late period. SiO2 / wt%

4.2.2. Trace element geochemistry

Rare earth elements (REE) for the samples decrease with increasing SiO2 (fig. 6) throughout the Kalama and Goat rocks eruptive periods. This anticorrelation is observed for each phase (early, middle and late) of the Kalama cycle as well as during the Goat Rocks eruptive period. The REE decrease (by a factor of ½) is relatively uniformed with little change in the pattern slope.

100

normalized abundancenormalized

- Chondrite

1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Rare Earth Elements

Figure 6: Chondrite-normalized rare earth element abundances (after Sun and McDonough,

1989). Colors: brown, >65 wt% SiO2; orange, 60-65 wt% SiO2; pink, <60 wt% SiO2. Symbols: x for

set W; + for set X and Middle Kalama lavas; o for summit dome; triangle for Goat Rocks.

The spider-diagram (fig. 7) of the bulk rock composition for the Kalama and the Goat Rocks eruptive periods has the typical pattern of a subduction zone with a depletion of Nb & Ta and an enrichment in Pb and large-ion lithophile elements (K, Rb, Ba, etc.). Moreover, the light (L) RRE are enriched compared to the heavy (H) REE (La/Yb=5.9 for the W tephra set). This enrichment of LREE relative to HREE (which are more fluid-mobile) is another typical feature of subduction related rocks.

Goat Rocks deposits present a positive Eu anomaly for the REE abundances (fig. 8) superimposed to the anticorrelation already observed throughout the two Kalama and Goat Rocks cycles (fig. 6). For the Goat Rocks dacite T tephra, the ratio Eu/Eu* (with Eu*=(Sm+Gd)/2) is equal to 1.6 while Eu/Eu* is equal to 1.2 in the Goat Rocks andesites (Floating Island lava flow).

1000.0

100.0

10.0

1.0

0.1 Cs Rb Ba Th U Nb Ta K La Ce Pb Sr P Nd Sm Zr Hf Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu

Figure 7: Spider-diagram with N-MORB normalization after Sun and McDonough, 1989.

100

normalized abundancenormalized

- Chondrite

1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Rare Earth Elements

Figure 8: REE abundances for the Goat Rocks deposits. Dark red: T tephra unit (square for the bottom layer, diamond for the medium layer and triangle for the top layer); bright red (circle): floating island lava flow.

4.2.3. Cyclic evolution during the Kalama and the Goat Rocks eruptive periods

The analyses of the whole rock geochemistry of the Kalama and the Goat Rocks deposits produce multiple trends on SiO2 variation diagrams for some elements (fig. 9). The trends form ‘counter- clockwise loops’ with respect to the stratigraphic position. Thus, the Kalama eruptive period can be divided into three different phases according to the geochemistry. Those phases correspond to the three phases defined according to the petrology of the deposits: the early, the middle and the late Kalama periods. The phases are underlined by the variations of major element compositions throughout the period as SiO2 first decreases and then increases. Cr is used as an example to describe the phases as it expresses well the trends observed. During the early Kalama cycle (A.D. 1480 –

1505), Cr increases (from 10 ppm to more than 100 ppm) as SiO2 decreases from 67 wt% to 56 wt% on a linear trend. Then, Cr decreases abruptly (by a factor of 10) at near-constant SiO2 (2 wt% of variation) in middle Kalama andesitic lavas. Finally, Cr decreases slightly (about 10 ppm of decrease) with increasing SiO2 from 58 wt% to 64 wt% in the late Kalama summit dome. During the Goat Rocks eruptive cycle, a single linear trend is observed as if the cycle was not complete. SiO2 decreases from 63.5 wt% to 61 wt% while Cr increases from 13 ppm to 25 ppm. Those three phases for the Kalama period and the single trend for the Goat Rocks eruptive period are distinguishable for others elements as well (e.g. Ta, Ce, Zr). Although for elements like Ba or Rb, the variations of the trace element are a little bit different (negative slope), the three trends can still be defined.

Figure 9: SiO2 diagrams for samples from the Kalama and the Goat Rocks eruptive periods. On the left: Cr vs. SiO2 (positive slope). On the right: Rb vs. SiO2 (negative slope). Arrows represent the trends. Analyses of SiO2; Cr and Rb were by XRF and LA- ICPMS respectively.

4.3. Petrography and mineral chemistry of the Kalama and Goat Rocks eruptive periods

4.3.1. Mineral textures

Plagioclases

The plagioclase (plg) crystals from the Kalama and the Goat Rocks eruptive periods present a huge diversity in textures. The largest range of textural features is observed for the early Kalama deposits (tephra set W, fig. 10). Both grains with bright (high-anorthite) and dark (low-anorthite) cores can be observed within each single sample. These are overgrown by rims often showing reverse or inverse (or both) oscillatory zonation. The grains exhibit several resorption zones superimposed on the

11 oscillatory zoning. Melt inclusions within the cores or the rims are frequent. A typical feature is inner zones with wavy resorption horizons overgrown by a bright An-rich zone that slightly decreases into darker zones (fig. 10b & c).

Plagioclase grains from the early Kalama display the highest variability of the An content within a single grain (fig. 10a). A variation of the An content from An62 to An28 was measured within a single grain from the Wa tephra. The An62 corresponds to the core which might have been inherited; this derived from an andesitic magma while the An29 zone corresponds to the crystallization from a very silica magma (e.g., rhyolite).

The middle Kalama samples exhibit less diversity. Although andesitic lavas erupted during the middle Kalama contain characteristic resorption features as the ones shown by the W tephra set (with sometimes wide bright zones with a lot of melt inclusions; annex 2), lots of grains presents a bright core with a darker rim (normal zonation). During the late Kalama eruptive period, the majority of the plagioclase crystals have normal zoning patterns (bright core with dark rim, fig. 10c). Few crystals show resorption horizons or an inherited core with melt inclusions and an oscillatory zoned rim in the dacitic summit dome samples.

(a) (b)

(d)

(c)

Figure 10: BSE images of selected plagioclases (plg) crystals. Textural features include a)

patchy zoned crystal with a normal zonation of the high anorthite (An) content core from

the tephra W, b) an An-poor core truncated by a wavy resorption horizon of high An and

being overgrown by a normally zoned rim from the tephra W, c) same feature as b) observed

in an andesitic lava (middle Kalama) and small normally zoned crystals, d) a resorbed core

surrounded by oscillatory zoning in the summit dome. See annex 2 for more images.

Pyroxenes

The diversity of features exhibited by the orthopyroxenes (opx) from the Kalama and Goat Rocks periods is less significant than in plagioclase crystals. The large majority of the pyroxenes has a bright core (Fe-rich) with a thin darker rim (more Mg-rich) and lots of mineral (e.g. apatite, magnetite and ilmenite) and melts inclusions up to 200 μm. Others, less common, textural features comprise oscillatory and patchy zoning patterns. The late Kalama lavas exhibit a characteristic feature (fig. 11): a relatively round clinopyroxene core is surrounded by small orthopyroxenes.

(a) (b)

apt

ox (mgt/ilm)

amp

opx

cpx

Figure 11: BSE images of a) a typical orthopyroxene (opx) with a thin darker rim and numerous inclusions of magnetite (mgt), ilmenite (ilm) and apatite (apt), b) opx with patchy zones, c) opx featuring slight oscillatory zonation, d) a clinopyroxene (left bottom) surrounded by small opx and an amphibole (top right) surrounded by a decompression rim (px, plg, oxide). (a), (b) and (c) are from the tephra W while (d) is from the summit dome.

Amphiboles

Amphibole (amp) is an abundant Fe-Mg bearing crystal phase in the dacitic tephra units. The textural variations in the amphiboles are broader than those in the pyroxenes, and almost comparable to those in plagioclases. The majority of the amphibole cores in the early Kalama period are brighter than the rim. Cores are often overgrown by a dark rim or numerous oscillatory zones (both normal and reverse). (fig. 12a & c). Some of the grains contain lots of melt inclusions, few oxide inclusions and/or more or less strongly resorbed zones (annex 2). While breakdown rims are not observed in the first tephra layers (Wn, Wa & Wb), grains from the We and Wd tephras layers are more often surrounding by a thin (10 to 30 μm) breakdown rim (fig. 12c). Those breakdown rims are composed of melt, plagioclases, pyroxenes and Fe-Ti oxides.

The only amphibole crystals found within the middle Kalama andesitic lavas are small (few μm) grains with pyroxenes, oxides and melts forming aggregate as a destabilization rim (fig. 12d). All the amphibole phenocrysts from the late Kalama eruptive period (summit dome) exhibit a breakdown rims while small amphibole crystals are almost all decomposed.

(a) (b)

Figure 12: BSE images of selected amphibole (amp) crustals. Textural features include a) an amp with an oscillatory zoned pattern, b) a strong resorbed core of amp, c) an amp with a bright core and a thin darker rim surrounded by a decompression rim. The decompression rim is composed of px, plg & Fe-Ti oxides (ox). d) The result of the destabilization of an amp. a), b) and c) are from the tephra W and d) from the andesite lavas.

4.3.2. Mineral chemistry

Electron probe microanalyses were performed to analyze the chemistry of the pyroxenes and the amphiboles throughout the Kalama and the Goat Rocks eruptive periods.

Pyroxenes

The middle Kalama period corresponds to the phase with an important decrease in Cr. During this period of the cycle, the Cr content decreases in the pyroxenes (maximum value measured and average decrease). Moreover, the cores are enriched in Cr compared to the rim (fig. 13). That means the chromium has a compatible behavior in the pyroxenes, most of it is incorporated into the crystallizing pyroxenes. Thus the residual melt is depleted in Cr.

Worm Complex lavas 600 Then, SiO2 versus magnesium number (Mg# = MgO/(MgO+FeOt)*100) diagram define a linear 500 trend throughout the Kalama and the Goat Rocks eruptive periods. Mg# increases from 55

400 to 75 while SiO2 increases from 50 to 54 wt%.

With respect to the chronology of the deposits,

300 first SiO2 and Mg# increase both during the

Cr / ppm/Cr early Kalama period and then decrease during 200 the middle and late Kalama period. Finally, they increase again during the Goat Rocks period. 100 Thus, each phase of the Kalama and Goat Rocks eruptive periods can also be distinguished using 0 the mineral chemistry of the orthopyroxene phenocrysts. Moreover, the increase of SiO Figure 13: Chromium content of the opx (core & 2 and Mg# is already observed within the W rim) from the three middle Kalama andesites. tephra layers during the early Kalama while Symbols: X for the core, + for the rim. Colors: based on the whole rock chemistry, the W set darker for the older lava and brighter for the does not show any trend. The pyroxene younger lava. chemistry highlights the variations within a single tephra set (fig.14). However, the whole rock chemistry defines a real cyclic evolution, while the pyroxene chemistry does not. Indeed, the SiO2 and Mg# of the early Kalama pyroxenes are the lowest and the Goat Rocks pyroxenes have the highest SiO2 and Mg#. The chemistry of the pyroxene phenocrysts expresses a linear trend towards more silicic compositions with higher Mg# superimposed to the cyclic behavior during the Kalama and the Goat Rocks eruptive period.

54 4

53 3

2 52 1 SiO 2 51

48 50% 55% 60% 65% 70% 75% 80%

Mg#

Figure 14: SiO2 vs. Mg# diagram. Colors code: from dark green to bright green in respect to the chronology of the Kalama deposit. (1) early Kalama period: from Wn to Xb. x represents the bottom of the W tephra while + indicates the top; (2) middle Kalama period (Worm Complex Andesite); (3) late Kalama (Summit Dome); (4) Goat Rocks period in red (floating island lava flow).

Amphiboles

The dacitic tephra units contain different types of clinoamphibole (about 10-11 wt% CaO) as a major crystal phase. Using the classification scheme of Leake et al. (1997), the clinoamphiboles can be divided into tschermakite/pargasite (Tsch-Prg), magnesiohornblende/edenite (Mg-Hbl) and to lesser extent magnesiohastigite (Mg-Hst). All types of amphiboles exhibit broad compositional ranges consistent with their large textural variations: Tsch-Prg (SiO2=39.6-44.9 wt%, Al2O3=8.9-15.2 wt%), and Mg-Hbl (SiO2=44.2-45.8 wt%, Al2O3=8.3-9.4 wt%). Both types of clinoamphiboles occur in all dacitic units. While the high-Al Tsch-Prg is hardly abundant in the T tephra of the Goat Rocks period, the Kalama dacitic eruptions show bimodal distributions of high-Al Tsch-Prg and the low-Al Mg-Hbl populations (fig. 15). Tsch-Prg do not show any Eu anomaly (or only a small negative one for few analyses), while Mg-Hbl exhibit a negative Eu anomaly (Wanke, unpub.). Tsch-Prg are more depleted in REE than Mg-Hbl (fig. 16). Mg-Hbl might have crystallized from a melt, which was more evolved and had already fractionated plagioclase. Thus Mg-Hbl crystallized in a later stage of magma evolution than crystallization of Tsch-Prg.

30 80

25 Wn 60 T 20 15 40 10

20 # ofanalyses # # ofanalyses # 5 0 0 7 8 9 10 11 12 13 14 15 16 7 8 9 10 11 12 13 14 15 16 Al O Al O 2 3 2 3 Figure 15: Histograms showing the frequency distribution of Al2O3 in calcic amphiboles form the tephras W (Wn layer) and T. Blue colors indicate Tsch-Prg, red colors Mg-Hbl, green colors Mg-Hst compositions after Leake et al. (1997).

1000 1000 magnesiohorblende/edenite

100 100

10 10 tschermakite/pargasite We T

1 1

sample /chondritesample sample /chondritesample La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Figure 16: Variability of rare earth element concentrations in clinoamphiboles for tephras W and T (Wanke, unpub.; normalization to chondrite after Sun and McDonough, 1989).

4.4. Thermobarometry

4.4.1. Two-pyroxene thermobarometer

The two-pyroxene thermobarometer (Putirka, 2008) is used to constrain the variations in P-T conditions in the magma reservoir during the Kalama and the Goat Rocks eruptive periods. Opx and cpx must be in equilibrium. The Fe-Mg exchange coefficient is used to test the equilibrium. For this,

SiO2 vs. MgO diagrams were used. Opx and cpx from a same sample display two linear trends.

Couples were chosen using those trends: opx with low SiO2 and MgO contents were considered to be in equilibrium with cpx with low SiO2 and MgO content and so on. For some samples, it is difficult to find couples of opx and cpx in equilibrium (e.g. Xb) to have a good approximation of the P-T conditions. This thermobarometer yielded a wide variability of pressure and temperature for each sample (more than 100°C of variation within a single sample, fig. 17).

1250 Worm Complex 1200

Xb C

° 1150 Floating Island 1100 Figure 17: Variations of the temperature in the 1050 magma reservoir using the two-pyroxene

Temperature /Temperature 1000 thermometer for the 950 andesitic lavas of the middle Kalama and the 900 Relative chronology Goat Rocks period (Putirka, 2008).

4.4.2. Amphibole thermobarometer

Amphiboles are stable under a wide range of conditions that are mainly influenced by pressure, temperature, oxygen fugacity and magma composition. The Al concentration is controlled by some important substitution reactions. Thus, an amphibole thermobarometer (Ridolfi, 2010) can be used to determine the P-T conditions in the magma reservoir during the crystallization of amphiboles. The Tsch-Prg population crystallized over an interval of 847±22 to 1011±22°C and 169±19 to 801±88 MPa, while the Mg-Hbl population crystallized at cooler and shallower levels of 817±22 to 881±22°C and 122±15 to 226±31 MPa (fig. 18). The variations between the deposits from each phase of the Kalama and Goat Rocks periods are almost as important as the error.

1050 Wn Summit Wa dome 1000 Wb We Wd T

950

° T [ T 900

850

800 Relative chronology

Figure 17: Variations of the temperature from the dacitic deposits of the Kalama and the Goat Rocks eruptive period using the amphibole thermobarometer (Ridolfi et al., 2010). (See annex 3 for the pressure). Blue colors indicate Tsch-Prg, red colors Mg-Hbl, green colors Mg-Hst compositions after Leake et al. (1997). 17

5. Discussion

5.1. Magma mixing and fractional crystallization, two magmatic processes involved during the Kalama and Goat Rocks eruptive periods

Petrographic and geochemical features suggest that different magmatic processes must have occurred to explain the compositional variations observed during the Kalama and the Goat Rocks eruptive period. Indeed, to produce such a wide range of composition in a short period of time, magma mixing and fractional crystallization must be both involved during the two periods. While fractional crystallization (and assimilation) is a common process to produce a wide range of magma compositions, magma mixing also seems to play an important role in the plumbing system beneath the Mount St. Helens.

Figure 19: SiO2 variation diagram for rocks of the Kalama cycle (this study) and the north-flank basalts (Wanke, unpub.)

The major element compositions of the Kalama rocks define single linear trends on SiO2 variation diagrams for major and some trace elements (fig. 19) throughout the eruptive period. These relations suggest a two-component magma mixing to produce the andesitic magmas of the middle Kalama eruptive period. Two potential endmembers could be magmas similar to the north-flank basalt erupted during the Castle Creek period (Smith, 1984; Pallister, 1992, Wanke, unpub.) and the W tephra, which was erupted at the beginning of the period and represents more than 2/3 of the volume of magma erupted during this period (Carey et al., 1995). However, as shown by the color code (getting brighter with time) of the figure 19, if the only process is magma mixing, the mixing is not progressive throughout the period with a linear trend towards the north flank basalt (decrease of

SiO2). This indicates that others magmatic processes during the Kalama cycle must have occurred and not only a linear mixing between two magmas in one magmatic chamber.

The trace element data show that REE decrease with increasing SiO2 (fig. 6) throughout the Kalama and Goat rocks eruptive periods. This anticorrelation is contrary to the effect expected from removal major phenocryst phases during fractionation. Although accessory phase fractionation can produce decreasing REE with increasing SiO2, the relatively uniform decrease in the REE with little change in the pattern slope combined with the variation of other major and trace elements are more readily explained by mixing of a similar Kalama dacite with REE-enriched basalt similar to that erupted during the preceding Castle Creek period.

The most characteristic feature of the pyroxenes (thin darker rims) can be explained by the crystallization during the magma ascent, and thus places constraints on the pre-eruptive composition of the magma. However, few pyroxenes throughout the cycle exhibit patchy zones and oscillatory zoning, which indicate repeated injections of new magma into the main reservoir during the Kalama and the Goat rocks eruptive periods.

In conclusion, magma mixing and fractional crystallization are both present throughout the Kalama and the Goat Rocks eruptive periods. However, geochemical and petrological data of Kalama and Goat Rocks deposits allows us to distinguish several phases during those cycles with dominant process for each phase.

5.1.1. Magmatic mixing as the dominant process involved during the early Kalama eruptive period

Geochemical data

During the early Kalama cycle, Cr increases as SiO2 decreases on a linear trend towards the north- flank basalts (Wanke, unpub.) erupted during the preceding Castle Creek period (fig. 20). This evolution suggests a magma mixing as the dominant process during the early period to generate the andesitic magma. As seen before, two potential endmembers can be a dacitic melt (similar in composition to the tephra W) and a basalt like the north-flank basalts erupted during the previous period. In fact, the quenched mafic enclaves found in some of the early Kalama eruptive products indicate that mafic magma was present in the system during that time. However, the quenched structure (mingling) of these enclaves might point to a slightly more evolved magma as an endmember to enable true mixing. This could be formed by an andesite closer in composition to those erupted during the middle Kalama period. Thus, the magma mixing might have been more complex and might have involved more than two endmembers.

Figure 20: Cr vs. SiO2 diagram for samples from the Kalama and the Goat Rocks

eruptive periods. Dotted arrows represent potential endmembers. Analyses of SiO2 and Cr were by XRF and LA-ICPMS respectively. Data for the basalts are from Wanke (unpub.).

Mineral textures and chemistry

The typical zoning and resorbed features exhibited by the plagioclases in the W tephra have been interpreted by Blundy et al. (2006) as a result of the interplay between decompression crystallization and resorption due to heating by adjacent batches of crystallizing magma. Moreover, the variability of the An content within a single grain indicates that almost all crystals have experienced numerous compositional environments and do not record the development of a single magmatic batch. In addition of the textures, the geochemistry of the plagioclases suggests a high variability of the magmatic composition from rhyolitic to andesitic within a short period of time. Those variations are more readily explained by complex magma mixing between several melt batches of different compositions. After the first big eruption, the magma reservoir is recharged by the injection of magmas from different reservoirs located in the deeper crust during the early Kalama eruptive period.

5.1.2. The importance of the fractional crystallization compare to the magma mixing during the middle Kalama cycle

Pyroxenes and whole rocks geochemistry

During the middle Kalama time (abrupt decreasing of Cr), the anticorrelation between REE and SiO2 is still observed (fig. 21). However it can be produced by fractionation of pyroxenes in combination with accessory minerals such as apatite or Fe-Ti oxides. They are typically found as inclusions in the pyroxene crystals. Indeed, the crystallization and fractionation of pyroxenes in combination with apatite, ilmenite, and magnetite produces an important decrease in Cr in the melt, since Cr is a highly compatible element in pyroxene and apatite (KdCr-Px > 10; Bacon & Druitt, 1988; KdCr-apatite > 9; Luhr et al., 1984 for andesite; fig. 13). The fractionation of pyroxenes that contain about 53 wt% of SiO2 might be responsible for the small increase of 2 wt% of SiO2 observed (from 56 to 58 wt%; fig. 20).

100

normalized abundancenormalized

Chondrite Figure 21: Chondrite- normalized rare earth 1 element abundances. La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Symbols: x for set X; + for Rare Earth Elements Middle Kalama lavas; o for summit dome.

Mineral textures

Although few resorption horizons are observed, most of the plagioclase grains exhibit a bright core with a darker rim. It is typically interpreted as a result of cooling and fractional crystallization within a

20 magmatic reservoir. This reservoir is sometimes disturbed by the injection of new hotter magma causing magma mixing, which is still a significant process. Anyway, fractional crystallization potentially combined with minor amounts of assimilation is the predominant process here as highlighted by the geochemistry.

5.1.3. Fractional crystallization during the late Kalama cycle

Geochemistry of the late Kalama deposits

During the late Kalama period, Cr decreases slightly with increasing SiO2. Moreover, the anticorrelation between SiO2 and REE is still observed. These results can be explained by the fractional crystallization of amphibole in combination with accessory mineral phases. The low SiO2 content of amphibole (40-45 wt%) shifts the magma composition towards higher SiO2 contents, while the relatively compatible behavior of the middle rare earth element decreases their abundance. This is supported by the occurrence of amphibole in the dacites of the summit dome, while they are lacking in the andesites. Thus, fractional crystallization seems to be the dominant process generating the magma evolution during the late Kalama eruptive period.

Mineral textures

Although the recharge of the main shallow reservoir might have continued (rare resorption horizons indicate mixing), the darker rims of the late Kalama plagioclases are interpreted as a result of cooling of the magma in the reservoir. The amount of injected magma became negligible compare to the volume of magma in the reservoir. Thus, the fractional crystallization was the dominant process to produce the dacitic lavas of the summit dome.

5.1.4. Dominant process during the Goat Rocks period

Geochemistry

Finally, the Goat Rocks trend (fig. 20) can not be explained by mixing with a magma similar to the Castle Creek basalts. If magma mixing was the dominant process in generating the andesitic Floating Island lava flow, one endmember could have been a magma similar to the dacite erupted at the end of the Kalama eruptive period (summit dome) and the second one similar to the middle Kalama andesite. But also fractional crystallization and extraction of the melt from an andesitic magma could have produced this trend. To explain the positive anomaly of Eu observed in the REE patterns of the Goat Rocks deposits, an assimilation of plagioclases must be involved. This means that during the late Kalama cycle and the following repose interval (or right before), fractional crystallization of at least plagioclase occurred and they are assimilated during the followed eruptive period.

5.1.5. Summary of the dominant process during each phase of the Kalama and the Goat Rocks eruptive periods

To conclude, the geochemical data suggest a complex interplay between different processes in the generation of the diverse magma composition observed during the Kalama and the Goat Rocks eruptive periods. During each phase of the cycle, the dominant mechanism responsible for the magma variations seems to have been different. Magma mixing is a good candidate to explain the trend from dacitic to andesitic magmas during the early Kalama period, while fractional crystallization seems to be more important in the evolution during late Kalama period. The middle Kalama period has been an intermediate phase between the early and the late Kalama period, when the injection and mixing of new magma has still been involved to explain the variability of the

21 textural features. But magma mixing became more and more negligible compared to the volume of magma present in the shallow reservoir. And so that fractional crystallization became the predominant process from the middle Kalama period on.

5.2. Magma ascent rate

The breakdown rims of amphiboles are interpreted as the result of decompression and loss of volatiles during magma ascent. The kinetic (the width of the rim) of melt and amphiboles (volatile-bearing phenocrysts) reactions are controlled by the ascent rates of dacitic and andesitic magmas (Rutherford and Hill, 1993; Rutherford, 2008). During the early Kalama period, the ascent of the magma forming the We and Wd tephras was slower (about 5-7 days) than the first fast magma ascent (<3 days for the Wn, Wa & Wb tephras) of the Kalama Figure 22: Plot of experimentally determined amphibole reaction- eruptive period (fig. 22). This might rim widths as a function of a constant rate ascent duration at have been triggered by an important temperatures of 830 to 900°C (after Rutherford, 2008). Green injection of new magma into the colors: dark corresponds to the range of width for the early W set, shallow reservoir at the beginning of medium for the late W and bright for the summit dome. As the rim the cycle. width varies within a single sample, the duration is approximate.

During the late Kalama period, amphiboles are also surrounding by decompression rims, typical of a slow magma ascent during the dome formation (about 10 days), which occurred at the end of the Kalama cycle. The typical feature found in the late Kalama dacite (cpx surrounded by small opx) might also be a reaction between the clinopyroxene and the dacitic melt during the formation of the summit dome.

5.3. P-T conditions variations throughout the Kalama and the Goat Rocks periods

First, as phenocrysts of amphibole were not stable in the magma reservoir during the middle Kalama period, the magma reservoir was relatively hot and dry. The two-pyroxene thermobarometer displays a wide range of pressure and temperature during the Kalama and the Goat Rocks periods. Moreover, as noticed in the result part, the test of equilibrium between a couple of cpx and opx used by Putirka (2008) is not always verified. This means that pyroxene phenocrysts exhibit numerous compositional environments and do not record the development of a single magmatic batch with well-constrained P-T conditions. The same interpretation is deduced with the amphibole thermobarometer as it also displays a wide range of P- T conditions. However, the Mg-Hbl might have crystallized before the Tsch-Prg, and the summit dome dacites do not contain Mg-Hbl. An interpretation might be the earlier crystallization of Tsch- Prg in a deeper reservoir (during the repose interval) whereas the Mg-Hbl crystallized just before the eruption at lower temperature in a shallower reservoir (fig. 16, annex 3). During the late Kalama period, the temperature was higher. New hotter magma was injected into the shallow reservoir after the big eruption at the beginning of the cycle. Subsequently, there was not enough time for Mg-Hbl to crystallize before the eruption of the summit dome lavas.

6. Conclusion

Mount St. Helens shows a cyclic eruption patterns. During the Spirit Lake stage, the two last pre-1980 periods (Kalama and Goat Rocks) consisted of a repose interval followed by an eruption interval. While geophysical data show the presence of several magma reservoirs in the shallow crust beneath the volcano (Waite and Moran, 2009, fig. 23), geochemical data suggest the presence of at least one shallow reservoir and one deeper reservoir.

Figure 23: Example of the dimensions of the upper magma plumbing system beneath Mount St. Helens as inferred from seismic tomography using local earthquake data since the 18th May 1980 eruption (Waite and Moran, 2009)

Cyclic evolution of the geochemical and petrological data allows defining several phases that might have composed the Kalama and the Goat Rocks eruptive periods. Before the first eruption of the Kalama period, the main shallow reservoir was filled by a dacitic and relatively cooled magma. Injection of new hotter magma might have triggered the first important eruption of the W tephra set. Then, the recharge produced the middle Kalama andesites by the mixing with a less evolved and hotter magma coming from a deeper reservoir. As the reservoir was filled, the volume of the injections was more and more negligible compared to the volume of the magma in the main reservoir during the middle and the late Kalama. Thus, the fractional crystallization became the dominant process to generate the dacitic lavas at the end of the Kalama period. The Kalama eruptive period was followed by a dormant interval and then a new cycle began with the eruption of the T tephra set. The mixing between a dacitic and an andesitic magma in the main shallow reservoir produced the Floating Island lava flow at the end of the Goat Rocks period.

Acknowledge

I would like first to thank Olivier Bachmann for supervising me during this master project as well as Maren Wanke, the co-supervisor for her advises and her help during this semester. Thanks both for their implication and their enlightening discussions. I also thank all the staff of the institute for the help during the lab work and the volcanology group for their welcome. Thanks to the ERASMUS program and the coordinators at the ETHZ and the ENS for simplifying the administration registration.

References

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Annex 1: Description of the samples

Eruptive Period Unit Sample Petrology Comment Age Fe-Mg minerals Minerals analyzed on H78-44-1 andesite Floating Island lava flow AD 1800- Opx, Cpx thin-section Goat Rocks (upper flow) 1838 T MW13- dacite pumice lapilli AD 1800 Opx, Amp mount 19/20/21 H78-88-2 dacite summit dome Opx, Cpx, Amp thin-section

H78-70-16 dacite summit dome Opx, Cpx, Amp thin-section

H78-90-2 dacite summit dome Opx, Cpx, Amp thin-section

H78-37-2 andesite Highest worm complex AD 1482- Opx, Cpx thin-section Kalama andesite flow, lava 1668 H78-37-22 andesite Worm complex andesite AD 1482- Opx, Cpx thin-section flow, lava 1668 MW13-41 andesite Middle Kalama andesite, Opx, Cpx thin-section lava Xb MW13-17 andesite scoria clasts Opx, Cpx thin-section

Wd MW13-3 dacite pumice lapilli Opx, Amp mount

We MW13-2 dacite pumice lapilli AD 1482 Opx, Amp mount

Wb 9-4-85-2 dacite pumice lapilli Opx, Amp mount

Wa MW13-52 dacite pumice lapilli Opx, Amp mount

Wn MW13-18 dacite pumice lapilli + bombs AD 1479 Opx, Amp mount (main) Wn MW13-16 dacite pumice lapilli AD 1479 Opx, Amp mount (lower)

Annex 2A: BSE images of plagioclases and amphibole

(a) (b)

(e)

a) Plagioclase from the Wn tephra showing an oscillatory zoning pattern and a resorption horion. b) Plagioclase crystal from the Worm complex lava flow. It exhibits an inherited core resorbed surrounded by a wide brighter rim which contains lots of melt and oxide inclusions. c) Plagioclase phenocryst from the Summit Dome dacite. The An-poor core is truncated by a wavy resorption horizon of high An. This horizon is overgrown by a normally zoned rim. d) Clinoamphibole from the Wn tephra layer which contains melt and Fe-Ti oxides inclusions. e) Amphibole from the Summit Dome. The core is destabilized and the crystal is surrounded by a wide decompression rim.

Annex 2B: BSE images of orthopyroxenes and oxides

(a) (b)

me ap

(e)

mgt

ilm

a) Orthopyroxene (opx) from the Wa tephra layer. The grain contains melt (me), apatite (ap), and Fe- Ti oxide (ox) inclusions. Size of the oxide inclusions: up to 200μm of diameter. b) Opx from the Wn tephra. It exhibits a thin darker rim but no decompression rim; it is directly in contact with the melt. c) Opx crystal from the We tephra. It is surrounded by a decompression rim (melt, oxide, px). d) Opx grain from the Wd tephra with a resorbed bright core and a decompression rim. e) Ilmenite (ilm) and magnetite (mgt) phenocrysts in the melt of the Wn tephra layer.

Annex 3: Amphibole thermobarometer (Ridolfi, 2010)

900 Wd 800 Summit dome 700 Wb We 600 Wn

500 Tsch-Prg T Mg-Hbl

P [MPa] P 400 Mg-Hst 300

200

100

0 Relative chronology

Annex 3: Variations of the pressure from the dacitic deposits of the Kalama and the Goat Rocks eruptive period using the amphibole thermobarometer (Ridolfi et al., 2010). Blue colors indicate Tsch-Prg, red colors Mg-Hbl, green colors Mg-Hst compositions after Leake et al. (1997).