UNIVERSITY OF GOTHENBURG Department of Earth Sciences Geovetarcentrum/Earth Science Centre

ISSN 1400-3821 B609 Bachelor of Science thesis Göteborg 2011

Mailing address Address Telephone Telefax Geovetarcentrum Geovetarcentrum Geovetarcentrum 031-786 19 56 031-786 19 86 Göteborg University S 405 30 Göteborg Guldhedsgatan 5A S-405 30 Göteborg SWEDEN

Fractionation of trace elements in the

Rävsön Ulvö gabbro, central Sweden

Mattias Ek

Abstracts Abstract The concentrations of strontium, vanadium, cerium and barium are analysed in whole

rock samples and plagioclase crystals in the exposed ~90m thick middle and upper

zone of the Rävsön Ulvö gabbro. The plagioclase composition was used to calculate

a supernatant liquid composition through the stratigraphy. Strontium shows a ~50%

decrease in the supernatant liquid concentration towards the sandwich horizon and,

vanadium decreases ~80% towards the sandwich horizon. The supernatant liquid

concentration of cerium remained relatively stable throughout crystallisation while

barium was enriched by ~80%. The calculated liquid composition is used to evaluate

several models for fractionation crystallisation. A modified version of the diffusion-

driven fractionation model could potentially explain the observed fractionation. A

thinner crystallisation zone increases the gradient of incompatible elements in the

crystallisation zone. This allows for diffusive exchange between the magma chamber

and the crystallisation zone for barium, which has higher tracer diffusion rate than

cerium due to smaller ionic charge. However, further studies are needed to assess this

modification.

Sammanfattning Koncentrationen av strontium, vanadin, cerium och barium bestäms i hela

bergartsprover och i plagioklaskristaller i en ~90 meter tjock stratigrafi genom Rävsön

Ulvö gabbron. Sammansättningen av plagioklas används för att räkna ut en primitiv

magmasammansättning genom stratigrafin. Strontiumhalten i magman minskade

med ~50% mot sandwich horisonten, och vanadin visar en ~80% minskning under

samma sträcka. Ceriumhalten är stabil igenom hela stratigrafin medan halten av

barium ökar med ~80% mot sandwich horizonten. Den uträknade

magmasammansättningen används för att evaluera olika modeller för

fraktioneringskristallisation och en modifierad version av den diffusionsdrivna

fraktionerings modellen kan potentiellt resultera i den observerade fraktioneringen.

Modifikationen utgörs av att man förkortar kristallisationszonen och därmed ökar

gradienten för de inkompatibla ämnerna. Denna modifkation tillåter en ökning av

barium i magmakammaren genom diffusion, men inte av cerium. Ytterligare studier

behövs för att fastställa om denna modifikation ensamt kan skapa den observerade

fraktioneringen.

Keywords Ulvö Gabbro Complex, partitioning coefficients, laser ablation, plagioclase, layered

intrusions, mafic sills, diffusion-driven fractionation.

Index Abstracts ................................................................................................................................ 1

Abstract ................................................................................................................................... 1

Sammanfattning .................................................................................................................... 1

Keywords ................................................................................................................................. 1

Index ...................................................................................................................................... 2

Introduction ........................................................................................................................... 1

Geological Setting ................................................................................................................. 1

Previous Work .......................................................................................................................... 1

Partition Coefficients .............................................................................................................. 2

Fractionation Models ............................................................................................................. 2

Redistribution of Phenocrysts ............................................................................................ 2

Compositional Convection ............................................................................................... 2

Compaction ........................................................................................................................ 2

Diffusion Driven Fractionation ........................................................................................... 3

Methods ................................................................................................................................. 4

Results .................................................................................................................................... 5

Whole Rock Composition...................................................................................................... 5

Plagioclase Composition ...................................................................................................... 7

Supernatant Liquid Composition ......................................................................................... 8

Discussion .............................................................................................................................. 8

Selection of Primitive Plagioclase ........................................................................................ 9

Evaluation of Fractionation Models ................................................................................... 10

Partition Coefficients ............................................................................................................ 11

Conclusion ........................................................................................................................... 11

Acknowledgements ........................................................................................................... 12

References ........................................................................................................................... 12

Appendix ............................................................................................................................. 13

~ 1 ~

Introduction The aim of this study is twofold; firstly the

fractionation of compatible and

incompatible elements in whole rock

samples and plagioclase crystals in the

Rävsön Ulvö gabbro will be described.

Secondly the study will evaluate several

fractionation models that could

potentially explain the fractionation seen

in the Rävsön Ulvö Gabbro.

Focus will be on the stratigraphic

variations of compatible and

incompatible elements in whole rock,

plagioclase crystals and a calculated

liquid composition. The liquid composition

is calculated from the most primitive

plagioclase crystals using partition

coefficients to give an estimate of the

supernatant liquid composition from

which the plagioclase crystallised.

Strontium and vanadium will represent the

compatible elements, and cerium and

barium represent the incompatible

elements in this study.

Redistribution of phenocrysts model,

compositional convection model and the

compaction model are the most popular

models currently used to describe the

fractionation seen in many mafic sills, and

together with the diffusion-driven

fractionation model they are briefly

described and evaluated for their

potential in explaining the fractionation

observed in the Rävsön Ulvö Gabbro.

Geological Setting The Rävsön Ulvö gabbro (RUG) is a part of

the Ulvö gabbro complex (Fig. 1), a series

of saucer shaped alkali gabbroic sills that

intruded the Nordingrå formation at ~1.26

Ga (Gorbatschev et al., 1979; Larson,

1980, Hogmalm et al. 2006). The UGC is

included in the larger Central

Scandinavian Dolerite Group (CSDG)

believed to be related to the break-up of

Baltica and Laurentia (Gorbatschev et al.

1979; Hogmalm et al. 2006). Emplacement

depth is estimated to have been ~3 km,

and the sills are typically ~300 m thick

(Larson, 1980; Lundqvist et al. 1990).

The RUG is exposed on the eastern

shoreline of the Rävsön peninsula in a ~90

m thick sequence of layered igneous

rocks. The exposed stratigraphy is thought

to represent the upper two thirds of the

total thickness (Larson, 1980). The

stratigraphy dips beneath the Nordingrå

anorthosite at the upper contact.

Previous Work The UGC has been the subject to

numerous petrological studies during the

last 160 years, however only a few of

these studies have focused on the

stratigraphic variations within the Ulvö

gabbro complex (Larson, 1973; Lundqvist

& Samuelsson, 1973; Larson, 1980; Larson

et al., 2008; Hogmalm et al., Submitted a;

Hogmalm et al., Submitted b;). The

diffusion-driven fractionation model used

in this article, was developed on the Norra

Ulvön gabbro located in the Ulvö gabbro

complex (Hogmalm et al., Submitted a).

The Ulvö gabbro complex is also the type

locality of the mineral ulvöspinel, a

Figure 1. Location of the Rävsön Ulvö Gabbro.

Modified from Claeson et al. (2007).

~ 2 ~

titanium-iron oxide (Morgensen, 1946). The

sampled stratigraphy used in this study

was originally collected as part of a

different study (Larson, 1980).

Partition Coefficients Partition coefficients are often used to

calculate the liquid from which a certain

mineral crystallised, and are defined as

the equilibrium concentration ratio of an

element in a mineral to that in a liquid (Eq.

1). Plagioclase is the most common

mineral used in petrology as it occurs in all

mafic sills and is usually the first mineral to

crystallise.

Equation 1.

The partition coefficients for the

plagioclase-melt system have been

determined to be variable depending of

the anorthite concentration (Bindeman et

al., 1998), as seen in Eq. 2 for strontium,

cerium and barium whereas vanadium

uses the simplified formula seen in Eq. 3. R

is the gas constant, T is the temperature

(Kelvin) and An is the anorthite

concentration.

Equation 2. Equation 3.

The partition coefficients for

plagioclase are empirically determined

and are thus subject to a large

uncertainty. To better appreciate the

uncertainty, partition coefficients from

two different articles are used in this study

(Table 1).

Fractionation Models The following section will briefly describe

the most popular fractionation models in

use today, in addition to the newly

developed diffusion-driven fractionation

model.

Redistribution of Phenocrysts

The redistribution of phenocrysts model

(Marsh, 1980; Marsh, 1989; Marsh 1996)

states that all changes in composition

within a sill are the result of the

introduction of a phenocryst-rich magma

from a different mush column, that then

settles onto the floor of the intrusion

through crystal settling (Fig. 2a). The

magma chamber is stagnant and goes

through no convection meaning that the

intrusion is incapable of fractionation

without the introduction of phenocrysts.

This results in a stratigraphy that has a step

like change in mineral composition rather

than a continuous change (Fig. 2a).

Compositional Convection

In the compositional convection model

(Jaupart & Tait, 1995; Tait & Jaupart, 1996;

Latypov, 2003) the magma chamber is

constantly convecting, due to thermal

and chemical gradients that occur in the

magma chamber. This results in a

crystallisation front that is continuously

replenished by more primitive magma,

resulting in a depletion of compatible

elements, while the incompatible

elements will become enriched in the

magma as crystallisation progresses (Fig.

2b).

Compaction

The compaction model (Meurer &

Boudreau, 1998a; Meurer & Boudreau,

Table 1. The X and Y values used with Eq.1 and Eq.2 to calculate partition coefficients for

strontium (Sr), vanadium (V), cerium (Ce) and Ba (Ba) for Bédard (2006) and Bindeman et

al. (1998). Examples of calculated partition coefficients are included for An60 and An40 for

each element.

Bédard (2006)

Bindeman et al. (1998)

X Y An60 An40

X Y An60 An40

Sr -20,71 23,85 2,69 4,79

-30,4 28,5 2,42 5,16

V 0,09 0,08 0,034 0,052

Ce -33,82 -3,68 0,123 0,175

-17,5 -12,4 0,135 0,141

Ba -35,20 10,14 0,380 0,668

-55 19,1 0,292 0,738

~ 3 ~

1998b) predicts that the magma chamber

crystallises as a large mush column, with

cumulus mineral crystals containing the

same composition throughout the

stratigraphy. The cryptic layering is formed

as compaction with related extension

occurs within the mush column. The

extended portion of the mush column is

infiltrated by more evolved magma which

recrystallises the existing crystals in

addition to growing new ones, resulting in

compatible elements becoming depleted

and incompatible elements becoming

enriched (Fig. 3a). However the

fractionation is not as extreme as seen in

the Compositional convection model due

to the fact that the more evolved liquid is

diluted into a less evolved partially

crystallized mush, thus complete

fractionation is very difficult to achieve.

Diffusion Driven Fractionation

The diffusion-driven fractionation model

(Hogmalm et al. Submitted a), like the

compositional convection model, invokes

a convecting magma chamber.

However, there is no direct interaction

between the convecting magma

chamber and the crystallisation zone as

seen in the compositional convection

model. The movement of elements

between the magma chamber and the

crystallisation zone is instead controlled by

diffusion. The chemical gradient

developed by the depletion of

compatible elements in the crystallisation

zone results in a diffusive flux from the

overlying magma chamber. There is not a

significant increase of incompatible

elements within the crystallisation zone

until the distance between the enriched

Figure 2. The expected evolution of incompatible (dash-dash line) and compatible (dash-dot line)

in the supernatant liquid, with increasing concentration to the right of the figures. The grey scale

represents the chemical variation within the mush column. a) The redistribution of phenocrysts

model. The hollow arrow represents input of phenocryst rich magma from a different mush column.

The thin solid arrows represent the settling of phenocrysts onto the floor of the chamber. b) The

compositional convection model. The large arrows demonstrate the convection in the magma

chamber, which continuously replenishes the crystallisation front.

~ 4 ~

liquid and the convecting magma

chamber is too large for effective

diffusion. This results in a depletion of

compatible elements while the

incompatible elements remain

unaffected as crystallisation progresses

(Fig. 3b).

Methods The whole rock major element analyses

were determined using X-ray fluorescence

by Larson (1980).

The trace elements compositions of

whole rock samples were determined

using inductively coupled plasma mass

spectrometry (ICP-MS) on an Agilent 7500,

at the University of Gothenburg. The

samples were prepared using the

following procedures: approximately 0.1 g

of rock powder was dissolved in one part

HF and four parts HNO3, and left to dry on

a hotplate. The sample was then re-

dissolved in HNO3, and 15 ppb indium and

15 ppb rhenium was added as an internal

standard. This resulted in a dilution factor

of approximately 3800. Four multi-element

standards (Merck and Agilent) each at

10, 0.1 and 0.01 ppb concentration levels

were used for calibration, while drift was

monitored by analysing the JB-1 rock

standard every fifth sample. JB-1 was

analysed as an unknown sample to

determine accuracy, which is 10% for all

elements used in this study.

The major element composition of

plagioclase was determined using an

Oxford Instruments energy dispersive

spectrometer detector connected to a

Hitatchi S-3400N scanning electron

Figure 3. The expected evolution of incompatible (dash-dash line) and compatible (dash-dot line)

in the supernatant liquid, with increasing concentration to the right of the figures. The grey scale

represents the chemical variation within the mush column. a) The compaction model. Showing the

migration of magma (thin black arrows) due to compaction (thick white arrows), to areas

affected by of extension (thick black arrows) in the mush column. b) The diffusion-driven

fractionation model where compatible elements (thin black arrows) diffuse from the convecting

magma chamber (big arrows) into the crystallisation zone.

~ 5 ~

microscope, using the Inca platform. The

analyses were carried out at the University

of Gothenburg.

The trace element composition of

plagioclase was determined by laser

ablation (LA) ICP-MS on thin sections

(~100 µm in thickness) of each sample,

using a Cetac ASX-200 Nd-YAG UV laser

for sampling and an Agilent 7500 ICP-MS

for analysis. A spot size of 200µm, energy

at 7,5mJ, and a repetition rate of 4Hz

were used for the analyses. NIST 612 was

used as a standard and the raw data was

reduced using an in-house program. The

precision is estimated to be better than

10% for all elements analysed in this study.

Results The Rävsön stratigraphy is divided into two

units, the upper zone and the middle zone

(Fig. 4a). A lower zone is most likely

present underneath the middle zone

however it is not exposed on the Rävsön

peninsula.

The upper zone consists of plagioclase,

oxides and olivine as cumulus phases and

clinopyroxene occurs as a poikilitic post

cumulus phase (Fig. 4b). The middle zone

is divided into the following groups from

the floor upwards; the rhythmically

layered zone a and the rhythmically

layered zone b. The contact between the

upper zone and middle zone is called the

sandwich horizon, and is defined as the

zone where the upper and lower

solidification fronts meet, and contains the

lowest anorthite concentrations in

plagioclase and highest concentration of

incompatible elements in whole rock. The

rhythmically layered zone a contains rocks

with cumulus plagioclase, oxides and

olivine, and interstitial poikilitic

clinopyroxene, while the rhythmically

layered zone b contains rocks where

plagioclase, oxides, clinopyroxene and

olivine occur as cumulus phases.

Whole Rock Composition The concentration of phosphorus (Fig. 5a)

in the upper zone remains stable at ~900

ppm. In the middle zone the

concentration remains stable at ~1000

ppm between the lower sample and 50

m, increasing to ~4000 ppm at the

sandwich horizon. The concentration of

phosphorus in whole rock corresponds

well with the modal abundance of

apatite in the stratigraphy.

The strontium concentrations (Fig. 4b)

decrease from ~500 ppm at the lower

sample to ~375 ppm at 20 m, then

increases to ~410 ppm at the sample

below the sandwich horizon. The

sandwich horizon sample has a

concentration of ~310 ppm. In the upper

zone the concentration decreases from

~600 ppm at the upper sample to ~450

ppm above the sandwich horizon.

Figure 4. a) The stratigraphic position of the upper

zone (UZ), sandwich horizon (SH), middle zone (MZ),

rhythmically layered zone a (RZa) and b (RZb). b)

Shows where plagioclase (Pl), clinopyroxene (Cpx),

olivine (Ol) and oxides (Ox) occur in cumulus (solid

line) or as an interstitial (dash-dash line) phase. c)

The variation of anorthite (An%) in the RUG

stratigraphy.

~ 6 ~

Figure 5. The Whole rock concentrations plotted

against height in the RUG for a) phosphorus (P),

b) strontium (Sr), c) vanadium (V), d) cerium

(Ce), and e) barium (Ba).

The concentration of vanadium (Fig 5c)

increases from ~350 ppm at the lower

sample to ~550 ppm at 20 m, then

decreases to ~100 ppm at the sandwich

horizon. In the upper zone an increase

from ~250 ppm at the upper sample to

~400 ppm above the sandwich horizon is

observed.

The cerium concentration (Fig. 5d)

in the middle zone increase from ~35 ppm

at the lower sample to ~80 ppm just

below the sandwich horizon, the

sandwich horizon has a cerium

concentration of ~70 ppm. In the upper

~ 7 ~

zone an increase from ~30 ppm at the

upper sample to ~50 ppm above the

sandwich horizon is observed.

The barium concentrations (Fig. 5e) in

the upper zone increase from ~500 ppm

at the upper sample to ~700 ppm above

to sandwich horizon. In the middle zone

an increase from ~500 ppm at the lower

sample to ~850 ppm at the sample below

the sandwich horizon. The sandwich

horizon has a concentration of ~700 ppm.

Plagioclase Composition The most ‘primitive’ plagioclase

analyses were selected (see appendix) for

each sample using the following criteria:

1) the analysed spot is located in a

plagioclase core, defined by the crystal

displaying twinning. 2) The core shows no

signs of recrystallisation in either the

polarising microscope or in backscatter

images from the SEM. 3) Has the highest

concentration of calcium and strontium

and the lowest concentration of barium in

the data acquired from the laser ablation

ICP-MS. Analyses with abnormally high

concentrations of rare earth elements is

usually associated with inclusions of trace

phases in the analysed area. Reasons for

selecting the most primitive plagioclase

analyses will be discussed below.

Figure 6. The concentration plotted against stratigraphic height in the RUG for all plagioclase

analyses (diamonds) and the earliest crystallised plagioclase (circles) for a) strontium (Sr), b)

vanadium (V), c) cerium (Ce), and d) barium (Ba).

~ 8 ~

The anorthite concentration (An%) (Fig.

4c) of plagioclase in the middle zone

decreases from ~58 An% at the lower

sample to ~39 An% at the sandwich

horizon. In the upper zone the An%

decreases from ~58% to ~48% above the

sandwich horizon.

The strontium concentration (Fig. 6a) of

the middle zone initially decreases from

~1000 ppm at the lower sample to ~800

ppm at 10 m, increasing to ~1000 ppm at

the sandwich horizon. In the upper zone

the concentration decreases from ~900

ppm at the upper sample to ~800 ppm

above the sandwich horizon.

The vanadium concentration (Fig. 6b)

in the middle zone remains stable at ~3.25

ppm between the lower sample and 10

m, then decreasing to ~0.6 ppm at the

sandwich horizon. In the upper sample

the concentration decreases from ~2.5

ppm at the upper sample to ~2 ppm

above the sandwich horizon.

The cerium concentration (Fig. 6c)

in the middle zone decreases from ~2.2

ppm at the lower sample to ~1.9 ppm at

50 m where it increases to ~2.75 ppm at

the sandwich horizon. In the upper zone

the concentration remains stable at ~2.4

ppm.

The barium concentration (Fig. 6d) in

the upper zone increases from ~200 ppm

at The upper sample to ~500 ppm above

the sandwich horizon. In the middle zone

the concentration increases from ~400

ppm at the lower sample to ~1100 ppm at

the sandwich horizon.

Supernatant Liquid Composition The supernatant liquid compositions are

calculated for the most primitive

plagioclase analyses using partition

coefficients from Bédard (2006) and

Bindeman et al. (1998), except for

vanadium where only the data from

Bédard (2006) is available. The

concentration of strontium (Fig. 7a) in the

upper zone decreases from ~490 ppm

(Bindeman) and ~410 ppm (Bédard) at

the upper sample to ~280 ppm

(Bindeman & Bédard) above the

sandwich horizon. In the middle zone

there is a decrease from ~390 ppm

(Bindeman) and ~350 ppm (Bédard) to

~210 ppm (Bindeman & Bédard) at the

sandwich horizon.

The vanadium concentrations (Fig. 7b)

in the middle zone remain stable at ~90

ppm (Bédard) between the lower sample

and 10m, and then decrease to ~10 ppm

(Bédard) at the sandwich horizon. In the

upper zone the concentration decreases

from ~90 ppm (Bédard) at the upper

sample to ~50 ppm (Bédard) above the

sandwich horizon.

The cerium concentrations (Fig. 7c)

decrease from ~19 ppm (Bédard) and

~17 ppm (Bindeman) at the lower sample

to ~13 ppm (Bédard) and ~15 ppm

(Bindeman) at 50 m. The cerium

concentrations then increase to ~17 ppm

(Bédard) and ~20 ppm (Bindeman) at the

sandwich horizon. In the upper zone the

concentrations decrease from ~22 ppm

(Bédard) and 18 ppm (Bindeman) at the

upper sample to ~16 ppm (Bédard) and

~16 ppm (Bindeman) above the

sandwich horizon.

The barium concentrations (Fig. 7d) in

the upper zone increase from ~600 ppm

(Bédard) and ~800 ppm (Bindeman) at

the upper sample to ~1050 ppm (Bédard)

and ~1150 ppm (Bindeman) above the

sandwich horizon. In the middle zone the

concentrations increase from ~900 ppm

(Bédard) and ~1100 ppm (Bindeman) at

the lower sample to ~1700 ppm (Bédard)

and ~1600 ppm (Bindeman) at the

sandwich horizon.

Discussion The discussion is divided into three parts;

the first part discusses the reasoning

behind selecting the most primitive

plagioclase analyses for calculation of the

liquid composition. The second part will

evaluate the different models presented

earlier based on the liquid composition

calculated from the plagioclase

composition. The final part discusses the

~ 9 ~

Figure 7. The calculated supernatant liquid concentration plotted against the height in the RUG.

Black Cricles represents concentrations calculated using partition coefficients by Bédard (2006)

and hollow diamonds use partition coefficients by Bindeman et al. (1998) for a) strontium (Sr), b)

vanadium (V), c) cerium (Ce), and d) barium (Ba).

use of partition coefficients to calculate

liquid composition.

Selection of Primitive Plagioclase An average of nine plagioclase crystals

were analysed for each sample within the

stratigraphy and the analyses show

considerable scatter (Fig. 3). Reasons for

this scatter can be attributed to several

factors: a few of the analyses were

analysed on plagioclase rims (Fig. 8),

which would have crystallised from a

more evolved liquid thus having higher or

lower concentration of certain elements.

Small inclusions of other minerals in the

plagioclase crystals will generate

anomalous peaks in concentration during

LA ICP-MS analysis (Fig. 9), inclusion of e.g.

apatite increases the concentration of

the rare earth elements as their partition

coefficients for apatite is high (Watson &

Green, 1981). Several crystals show signs

of post cumulus re-crystallisation as can

be seen in SEM backscatter images and

polarising microscope analysis, which

would alter the original composition.

~ 10 ~

Evaluation of Fractionation Models The models will be evaluated mainly

based on their capacity to predict the

changes seen in the supernatant liquid

composition calculated from plagioclase

analyses. Whole rock analysis will also be

used for evaluation when applicable as

whole rock compositions can be affected

by post cumulus processes (Sparks et al.,

1985).

The redistribution of phenocrysts

(Marsh, 1988; Marsh 1989;Marsh 1996)

model would need an infinite amount of

mush columns to produce the chemical

variation seen in the RUG. The phenocryst-

rich magma would also have to be

introduced in the right sequence to

produce e.g. the continuous decrease of

anorthite concentration. Although it is

theoretically possible for the model to

create the stratigraphic variations seen in

the RUG, it is in reality virtually impossible.

The model has come under recent

criticism and the reader is referred to

Latypov (2009) for a thorough evaluation.

The compositional convection (Jaupart

& Tait, 1995; Tait & Jaupart, 1996; Latypov

2003) model can to some extent explain

the fractionation of strontium, vanadium

and barium in the supernatant liquid.

However the fractionation is not as well

developed as is generally predicted for

the model. The near constant

concentration of cerium in the

supernatant liquid during crystallisation is

in contradiction with what is predicted by

the model. The fractionation of strontium

and barium in whole rock is not as well

developed as might be predicted by the

compositional convection model. The

model is therefore not able to satisfactory

describe the fractionation seen in the

RUG.

The compaction model (Meurer &

Boudreau, 1998a; Meurer & Boudreau

1998b) predicts less fractionation than

what is predicted in the compositional

convection model, and can thus

satisfactory explain the strontium,

vanadium and barium variations in whole

rock compositions. However, like the

compositional convection model it fails to

explain the nearly constant concentration

of cerium in the supernatant liquid. The

model further requires re-crystallisation of

plagioclase as all plagioclase initially

would have had the same composition

and this is unlikely to have happened in

the RUG. Evidence for this is that although

some plagioclase grains show signs of re-

crystallisation, strong compositional zoning

of plagioclase is preserved. According to

the compaction model the plagioclase

cores should all be of similar composition.

It is therefore unlikely that the compaction

model can explain the observed

fractionation.

The diffusion-driven fractionation

model (Hogmalm et al., Submitted a)

deviates from the previous two models in

that it predicts no enrichment of

Figure 8. The circle represents the area analysed

using laser ablation. a) A plagioclase core showing

twinning. b) A plagioclase rim showing no twinning.

a) b)

Figure 9. Counts per second for lanthanum (La)

measured using LA ICP-MS in plagioclase,

showing an anomalous increase in counts at 17s.

~ 11 ~

incompatible elements, while it has similar

depletion of compatible elements in the

supernatant liquid. It can therefore

explain the strontium and vanadium

fractionation and the near stability of

cerium; however the enrichment of

barium contradicts the model. According

to the model incompatible elements

should show similar stratigraphic

variations, regardless of ionic charge or

size and differences in partition

coefficients. The model as stated in

Hogmalm et al. (Submitted a) is therefore

incapable of producing the fractionation

seen in the RUG.

There is a difference in the diffusion

rate of barium and cerium, with barium

having a higher tracer diffusion rate

(Mungall, 2002). However, in the diffusion-

driven fractionation model the necessary

chemical gradient does not develop until

the distance to the convecting magma is

too large for effective diffusion. If the

distance was to be shortened, thus

creating a stronger chemical gradient it is

possible to get diffusion of incompatible

elements. If the crystallisation zone was to

be modified so that a strong chemical

gradient is achieved in a distance that

could facilitate diffusion of the faster

barium but not cerium, it would be

possible to get enrichment of barium but

not cerium (Fig. 10). However, further

studies are needed to determine if this

modification alone is capable of

producing the fractionation seen in the

RUG.

Partition Coefficients The partition coefficients used in this study

contain large uncertainties as is evident

from the calculated supernatant liquid

compositions for cerium as seen in Fig. 7c.

The cerium liquid composition decreases

at the lower sections of the stratigraphy,

more so for the partition coefficient from

Bédard (2006) than Bindeman et al.

(1998). There is no logical reason why

there should be a depletion of an

incompatible element and this decrease

might represent uncertainties in the

partition coefficients, it is thus likely that

some factor affecting partition

coefficients in sills has been overlooked.

The effect of oxidation on cerium has

been determined to negligible. There are

very few studies using plagioclase

analyses to calculate liquid compositions

in mafic intrusions. More studies are

needed of to evaluate the accuracy of

the partitioning of trace elements

between plagioclase and melt, in systems

with strong changes of anorthite

concentration in plagioclase.

Conclusion In the Rävsön Ulvö gabbro (RUG) there is

a progressive depletion of the compatible

elements strontium and vanadium in the

liquid due to fractional crystallisation of a

supernatant liquid. The fractionation of

the incompatible elements barium and

cerium differ from each other in that

barium becomes enriched while the

Figure 10. The dash-dot arrow represent the diffusion distance of compatible elements, the dash-dash

arrow is the maximum reach of cerium by diffusion and the solid arrow is the maximum distance that

barium can diffuse. The grey scale represents the chemical gradient in the crystallisation zone. a) The

crystallisation zone from the diffusion-driven fractionation. b) The modified version of the crystallisation

zone from the diffusion-driven fractionation where the size of the crystallisation zone has been

decreased.

~ 12 ~

cerium concentration of the liquid

remains the same throughout the

stratigraphy. The redistribution of

phenocrysts, compositional convection

and compaction model all fail to

satisfactorily explain the observed

fractionation. A modified version of the

diffusion-driven fractionation model could

potentially be able to reproduce the

fractionation seen in the RUG, although

further studies are needed.

Acknowledgements This study was carried out as a Bachelor of

Science thesis at the University of

Gothenburg under the supervision of

Johan Hogmalm (PhD), and Prof. David

Cornell served as the thesis examiner. The

assistance by A Firozaan and J Andersson

during thin section preparation was

greatly appreciated. The manuscript was

improved by comments from J Andersson,

DH Cornell, A Robijn and an anonymous

reviewer. The work was supported by a

grant awarded to S.Å. Larson by the

Swedish Research Council (60-1159/2002).

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Appendix

Sample Height An% Sr D B1 D B2 V D B1 Ce D B1 D B2 Ba D B1 D B2

235 Pl2 - 1 88 67,2 982 2,32 1,98 2,75 0,0281 2,48 0,107 0,129 173 0,318 0,220

236 Pl - 6 77 59,4 1005 2,75 2,49 3,29 0,0351 2,53 0,125 0,136 268 0,390 0,305

237 Pl2 - 1b 74 49,9 907 3,53 3,47 2,19 0,0437 2,47 0,147 0,139 530 0,500 0,459

131 Pl2 - 2b 66 41,4 1000 4,65 4,96 0,61 0,0514 2,85 0,169 0,139 1111 0,642 0,693

238 Pl2 - 3 63 43,0 836 4,47 4,69 N/A 0,0499 2,49 0,161 0,136 956 0,607 0,634

240 Pl2 - 4b 49 46,1 976 4,15 4,23 N/A 0,0471 1,91 0,147 0,130 815 0,545 0,536

121 Pl - 10 23 48,2 850 3,73 3,72 2,63 0,0452 2,12 0,150 0,138 541 0,522 0,494

122 Pl2 - 1 19 54,3 816 3,12 2,95 2,45 0,0398 1,95 0,137 0,139 409 0,446 0,380

124 Pl - 2 11 56,4 789 2,95 2,74 3,31 0,0378 2,06 0,132 0,138 379 0,422 0,346

125 Pl2 - 2 9 57,6 845 2,87 2,64 3,33 0,0368 2,19 0,129 0,137 340 0,409 0,330

126 Pl - 12 0 59,7 962 2,73 2,47 3,26 0,0349 2,34 0,124 0,136 352 0,387 0,302

Table 1: The concentration of strontium (Sr), vanadium (V), cerium (Ce) and barium (Ba) in ppm

for the most primitive plagioclase crystals in each sample, used to calculate a supernatant

liquid composition. Sample is the name of the analysed plagioclase crystal, Height is the

stratigraphic height in metres, An% is the anorthite concentration (Ca/(Ca+Na+K)), D B1 is the

partition coefficient calculated from Bedard (2006) and D B2 is the partition coefficients

calculated from Bindeman et al. (1998).