226Ra–230Th–238U disequilibria of historical Kilauea lavas (1790–1982) and the dynamics of mantle melting within the Hawaiian plume

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  • 226Ra^230Th^238U disequilibria of historical Kilauea lavas(1790^1982) and the dynamics of mantle melting within the

    Hawaiian plume

    Aaron J. Pietruszka *, Kenneth H. Rubin, Michael O. GarciaHawaii Center for Volcanology, Department of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822, USA

    Received 13 January 2000; accepted 3 January 2001

    Abstract

    The geochemical variations of Kilaueas historical summit lavas (1790^1982) document a rapid fluctuation in themantle source and melting history of this volcano. These lavas span nearly the entire known range of sourcecomposition for Kilauea in only 200 yr and record a factor ofV2 change in the degree of partial melting. In this study,we use high-precision measurements of the U-series isotope abundances of Kilaueas historical summit lavas and twoingrowth models (dynamic and equilibrium percolation melting) to focus on the process of melt generation at thisvolcano. Our results show that the 226Ra^230Th^238U disequilibria of these lavas have remained relatively small andconstant withV12 4% excess 226Ra andV2.5 1.6% excess 230Th (both are 2c). Model calculations based mostlyon subtle variations in the 230Th^238U disequilibria suggest that lavas from the 19th to early 20th centuries formed atsignificantly higher rates of mantle melting and upwelling (up to a factor ofV10) compared to lavas from 1790 and thelate 20th century. The shift to higher values for these parameters correlates with a short-term decrease in the size of themelting region sampled by the volcano, which is consistent with fluid dynamical models that predict an exponentialincrease in the upwelling rate (and, thus, the melting rate) towards the core of the Hawaiian plume. The Pb, Sr, and Ndisotope ratios of lavas derived from the smallest source volumes correspond to the Kilauea end member of Hawaiianvolcanoes, whereas lavas derived from the largest source volumes overlap isotopically with recent Loihi tholeiiticbasalts. This behavior probably arises from the more effective blending of small-scale source heterogeneities as themelting region sampled by Kilauea increases in size. The source that was preferentially tapped during the early 20thcentury (when the melt fractions were lowest) is more chemically and isotopically depleted than the source of the early19th and late 20th century lavas (which formed by the highest melt fractions). This inverse relationship between themagnitude of source depletion and melt fraction suggests that source fertility (i.e. lithology) controls the degree ofpartial melting at Kilauea. Thus, rapid changes in the size of the melting region sampled by the volcano (in the presenceof these small-scale heterogeneities) may regulate most of the source- and melting-related geochemical variationsobserved at Kilauea over time scales of decades to centuries. 2001 Elsevier Science B.V. All rights reserved.

    Keywords: Kilauea; volcanoes; Hawaii; lavas; geochemistry; uranium; isotopes; mantle plumes; melting; models

    0012-821X / 01 / $ ^ see front matter 2001 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 2 3 0 - 8

    * Corresponding author. Present address: Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 BroadBranch Road, N.W., Washington, DC 20015, USA. Tel. : +1-202-478-8476; E-mail : pietrus@dtm.ciw.edu

    EPSL 5747 22-2-01

    Earth and Planetary Science Letters 186 (2001) 15^31

    www.elsevier.com/locate/epsl

  • 1. Introduction

    Historical lavas from active Hawaiian shieldvolcanoes such as Kilauea and Mauna Loa areideal for investigating mantle melting becausethey display systematic melting-related variationsof incompatible trace element ratios (e.g. La/Ybor Nb/Y) over short periods of time (years todecades [1^4]). Models based on these uctuationsin lava chemistry suggest that the degree of partialmelting has varied by a factor of V2 for Kilaueasince 1790 [3] and V40% relative for Mauna Loasince 1843 [2], which are signicant portions ofthe overall range inferred for Hawaiian shield vol-canoes (a factor of V2.5 [5]). The origin of theseshort-term changes in the degree of partial melt-ing is poorly understood, but may be related tothe interplay between mantle melting and small-scale source heterogeneities [2,3]. A similar pat-tern of rapid, source- and melting-related geo-chemical variations has also been observed instratigraphic sections of prehistorical shield lavasfrom Hawaiian volcanoes (e.g. Mauna Loa [6]and Mauna Kea [7,8]). Thus, a detailed geochem-ical sampling of lavas over time scales of decadesto centuries may provide the best clues to under-standing the dynamics of mantle melting withinthe Hawaiian plume.

    The U-series isotope abundances of lavas (e.g.230Th and 226Ra) are excellent tracers of mantlemelting because the half-lives of 230Th and 226Ra(V75 kyr and 1600 yr, respectively) are thoughtto be similar to the time scale of magmatic pro-cesses (e.g. [9]). This unique feature of U-seriesisotopes oers the potential to study both the na-ture and timing of the processes that fractionateincompatible trace elements during magma gene-sis. This possibility derives from two importantproperties of the 238U decay chain. First, priorto melting, the source regions of basaltic lavaswill generally be in a state of radioactive, or sec-ular, equilibrium in which the 238U, 230Th, and226Ra activities (decay rates) are equal. When sec-ular equilibrium prevails, the (230Th/238U) and(226Ra/230Th) ratios (parentheses indicate activ-ities) of the source are known (and equal tounity), in contrast to most other geochemical trac-ers of the melting process (e.g. stable, incompat-

    ible trace elements). Second, any process thatfractionates these elements, such as partial melt-ing of the mantle, induces a state of radioactivedisequilibrium in which the activities of 238U,

    Fig. 1. Summary of the melting, source, and eruptive historyof Kilauea Volcano. Kilaueas historical lavas record a rapiductuation in the degree of partial melting (inferred from in-compatible trace elements) that correlates with the volcanoseruption rate (and, presumably, its magma supply rate). Thecomposition of the mantle source region tapped by Kilauea(or proportions of the source components) has also changedover time (shown as a % of the early 20th century sourcecomposition, which corresponds to the Kilauea isotopic endmember for Hawaiian volcanoes). The vertical shaded linesmark the explosive summit eruptions of 1790 and 1924. See[3] for the methods and/or references used to calculate eachof these parameters. The average melt fraction and sourcecomposition for the Mauna Ulu rift zone lavas are estimatedusing data from [1] and unpublished Pb isotope analyses (A.Hofmann, personal communication, 1999). The sample sym-bols are grouped according to eruption date: up triangles(1790), circles (1820^1921), diamonds (1929^1954), squares(1961^1982), left triangles (Mauna Ulu), and right triangles(Puu Oo).

    EPSL 5747 22-2-01

    A.J. Pietruszka et al. / Earth and Planetary Science Letters 186 (2001) 15^3116

  • Tab

    le1

    Th,

    U,

    Ba,

    Sran

    d22

    6R

    aab

    unda

    nces

    ,U

    -ser

    ies

    isot

    ope

    acti

    vity

    rati

    os,

    and

    Sris

    otop

    era

    tios

    for

    the

    hist

    oric

    alsu

    mm

    itla

    vas

    ofK

    ilaue

    aV

    olca

    no

    Sam

    ple

    Eru

    ptio

    nda

    te[T

    h][U

    ][B

    a][S

    r][2

    26R

    a](2

    30T

    h/23

    2T

    h)(2

    30T

    h/23

    8U

    )(2

    26R

    a/23

    0T

    h)0

    (234

    U/2

    38U

    )87

    Sr/8

    6Sr

    (Wg

    g31)

    (Wg

    g31)

    (Wg

    g31)

    (Wg

    g31)

    (fg

    g31)

    1790

    -117

    900.

    6960

    0.23

    4479

    .67

    278.

    692

    .0

    0.7

    1.04

    9

    0.01

    11.

    022

    1.14

    71.

    003

    0.

    004

    0.70

    3612

    91.4

    0.

    61.

    040

    0.

    011

    1820

    -118

    20^1

    823

    0.78

    120.

    2641

    98.3

    431

    0.6

    98.6

    0.

    61.

    041

    0.

    015

    1.01

    41.

    103

    1.00

    3

    0.00

    30.

    7036

    3899

    .4

    0.8

    1.03

    9

    0.01

    118

    6618

    661.

    156

    0.39

    6914

    1.7

    382.

    40.

    7035

    4818

    94-2

    (gro

    undm

    ass)

    1894

    1.13

    30.

    3931

    144.

    238

    5.1

    162.

    7

    1.0

    1.06

    7

    0.01

    61.

    020

    1.21

    21.

    004

    0.

    004

    0.70

    3547

    1.08

    2

    0.00

    918

    94-2

    (gla

    ss)

    1894

    1.20

    10.

    4135

    146.

    038

    2.6

    0.70

    3548

    95-T

    AJ-

    319

    121.

    254

    0.43

    9914

    4.3

    398.

    20.

    7035

    3619

    1719

    171.

    206

    0.42

    4313

    2.5

    397.

    716

    2.8

    1.

    81.

    093

    0.

    012

    1.02

    41.

    114

    1.01

    3

    0.00

    30.

    7034

    91K

    il191

    919

    191.

    225

    0.42

    7913

    5.0

    401.

    516

    7.4

    1.

    31.

    085

    0.

    011

    1.02

    31.

    129

    1.00

    3

    0.00

    20.

    7034

    7116

    5.0

    0.

    819

    2919

    291.

    306

    0.42

    8914

    6.5

    392.

    517

    1.3

    2.

    51.

    035

    0.

    010

    1.04

    21.

    131

    1.00

    5

    0.00

    30.

    7035

    5116

    8.7

    3.

    31.

    042

    0.

    013

    1931

    HM

    1931

    ^193

    21.

    336

    0.43

    9315

    0.1

    400.

    00.

    7035

    4619

    54-2

    1954

    1.28

    70.

    4284

    150.

    440

    0.9

    0.70

    3607

    K61

    -22

    1961

    1.23

    20.

    4141

    145.

    838

    8.7

    155.

    3

    1.1

    1.04

    8

    0.00

    91.

    029

    1.08

    11.

    006

    0.

    003

    0.70

    3584

    1.05

    1

    0.01

    519

    71S

    Sep

    1971

    1.08

    70.

    3633

    132.

    038

    6.4

    0.70

    3580

    1982

    A-2

    0A

    pr19

    820.

    9832

    0.32

    9712

    1.3

    360.

    112

    6.2

    0.

    71.

    052

    0.

    014

    1.02

    91.

    103

    0.70

    3566

    1.04

    2

    0.00

    9

    All

    unce

    rtai

    ntie

    sar

    e

    2c.

    The

    repr

    oduc

    ibili

    tyof

    the

    Th,

    U,

    Ba,

    and

    Srco

    ncen

    trat

    ions

    (and

    rati

    osth

    ereo

    f)is

    0.

    3%or

    bett

    er.

    The

    unce

    rtai

    ntie

    sfo

    rth

    e22

    6R

    aco

    ncen

    trat

    ions

    and

    the

    (230

    Th/

    232T

    h)an

    d(2

    34U

    /238

    U)

    rati

    oslis

    ted

    inth

    eta

    ble

    are

    the

    wit

    hin-

    run

    mea

    sure

    men

    ter

    rors

    .T

    he(2

    30T

    h/23

    8U

    )an

    dth

    eag

    e-co

    rrec

    ted

    (226

    Ra/

    230T

    h)ra

    tios

    are

    calc

    ulat

    edus

    ing

    the

    aver

    age

    226R

    aab

    unda

    nces

    and

    (230

    Th/

    232T

    h)ra

    tios

    for

    each

    sam

    ple.

    The

    accu

    racy

    and

    repr

    oduc

    ibili

    tyof

    our

    Th

    isot

    ope

    rati

    om

    easu

    rem

    ents

    wer

    eve

    rie

    dw

    ith

    the

    Uni

    vers

    ity

    ofC

    alif

    orni

    aSa

    nta

    Cru

    z(U

    CSC

    )T

    hst

    anda

    rdA

    .O

    urav

    erag

    efo

    rU

    CSC

    Th

    A

    was

    (230

    Th/

    232T

    h)=

    1.08

    6

    0.01

    2(n

    =39

    ),w

    hich

    is0.

    3%lo

    wer

    than

    the

    pred

    icte

    dva

    lue

    (1.0

    89).

    Rep

    licat

    ean

    alys

    esof

    the

    Nat

    iona

    lIn

    stit

    ute

    ofSc

    ienc

    ean

    dT

    echn

    olog

    y(N

    IST

    )U

    stan

    dard

    SRM

    U01

    0ga

    ve(2

    34U

    /238

    U)=

    0.99

    93

    0.00

    54(n

    =10

    ),w

    hich

    is0.

    04%

    high

    erth

    anth

    ece

    rti

    cate

    valu

    e(0

    .998

    9).

    Rep

    licat

    ean

    alys

    esof

    the

    NIS

    TSr

    stan

    -da

    rdSR

    M98

    7du

    ring

    two

    peri

    ods

    ofti

    me

    sepa

    rate

    dby

    am

    ajor

    mac

    hine

    upgr

    ade

    gave

    87Sr

    /86Sr

    =0.

    7102

    59

    0.00

    0012

    (n=

    10)

    and

    87Sr

    /86Sr

    =0.

    7102

    68

    0.00

    0014

    (n=

    14).

    All

    data

    are

    expr

    esse

    dre

    lati

    veto

    87Sr

    /86Sr

    =0.

    7102

    59fo

    rSR

    M98

    7.T

    hew

    ithi

    n-ru

    nun

    cert

    aint

    ies

    onin

    divi

    dual

    87Sr

    /86Sr

    mea

    sure

    men

    tsw

    ere

    alw

    ays

    equa

    lto

    orle

    ssth

    anth

    eex

    tern

    alre

    prod

    ucib

    ility

    ofSR

    M98

    7.T

    heto

    tal

    proc

    edur

    albl

    anks

    for

    abun

    danc

    em

    easu

    rem

    ents

    wer

    e:T

    h(1

    ^6pg

    ),U

    (2^7

    pg),

    Ba

    (0.2

    ^1ng

    ),Sr

    (20^

    60pg

    ),an

    d22

    6R

    a(6

    0.01

    fg).

    Bla

    nkco

    rrec

    tion

    sof

    0.00

    3^0.

    04%

    for

    Th,

    0.02

    ^0.1

    %fo

    rU

    ,an

    d0.

    005^

    0.07

    %fo

    rB

    aw

    ere

    mad

    eto

    deri

    veth

    en

    alco

    ncen

    tra-

    tion

    s(S

    ran

    d22

    6R

    abl

    anks

    wer

    ene

    glig

    ible

    ).T

    heto

    tal

    proc

    edur

    albl

    anks

    for

    isot

    ope

    rati

    om

    easu

    rem

    ents

    wer

    ene

    glig

    ible

    :T

    h(1

    0^20

    pg)

    and

    U(3

    ^4pg

    ).T

    hede

    cay

    cons

    tant

    sus

    edar

    e:V2

    38U

    (1.5

    51U

    103

    10yr3

    1),V2

    34U

    (2.8

    35U

    103

    6yr3

    1),V2

    32T

    h(4

    .948U

    103

    11yr3

    1),V2

    30T

    h(9

    .195U

    103

    6yr3

    1),

    andV2

    26R

    a(4

    .332U

    103

    4yr3

    1).

    EPSL 5747 22-2-01

    A.J. Pietruszka et al. / Earth and Planetary Science Letters 186 (2001) 15^31 17

  • 230Th, and 226Ra are unequal. The magnitude ofthe 226Ra^230Th^238U disequilibria in basalticlavas is believed to depend, in part, upon the ratesof mantle melting and upwelling [10^12].

    Early U-series studies of Hawaiian shield lavasused radioactive decay counting techniques toidentify small, but signicant, 230Th and 226Raexcesses relative to their respective parents, 238Uand 230Th, and proposed a magmatic (mantlemelting) origin for these disequilibria [13^15].More recent, high-precision decay counting andmass spectrometric U-series investigations of his-torical Kilauea and Mauna Loa lavas observedsmall 230Th excesses and moderate 226Ra excessesup to 28% [16^20]. Modeling of the small 226Ra^230Th^238U disequilibria of these lavas (comparedto mid-ocean ridge basalts [21,22] and lavas fromother ocean island volcanoes [23]) has led to theidea that the rates of mantle melting and upwell-ing for tholeiitic basalt production within the Ha-waiian plume are relatively high [16^19,23].

    This paper presents new U-series isotope, Srisotope, and trace element abundance data forthe historical summit lavas of Kilauea Volcano(1790^1982). These samples span nearly the entireknown range of source composition for this vol-cano and record a factor of V2 change in thedegree of partial melting [3]. In this study, wefocus on mantle melting at Kilauea using the U-series isotope systematics of its historical lavasand two ingrowth models for producing 226Ra^230Th^238U disequilibria (dynamic melting [10]and equilibrium percolation melting [12]). Ourgoal is to nd an internally consistent modelthat accounts for the 226Ra^230Th^238U disequilib-ria of Kilauea lavas in the context of other geo-chemical evidence (from incompatible trace ele-ments and Pb, Sr, and Nd isotope ratios [3]) fora short-term change in the mantle source andmelting history of this volcano (Fig. 1).

    2. Results

    High-precision measurements of the Ba, Th, U,Sr, and 226Ra concentrations and (230Th/232Th),(230Th/238U), (226Ra/230Th), (234U/238U), and 87Sr/86Sr ratios of Kilaueas historical summit lavas by

    thermal ionization mass spectrometry (TIMS) arereported in Table 1 (for additional sample infor-mation and analytical methods see the EPSL On-line Background Dataset1). In this section, we de-scribe the temporal geochemical variations of thelavas (Fig. 2). All uncertainties are 2c.

    2.1. 230Th^238U disequilibria

    The 230Th^238U disequilibria of Kilauea lavashave remained nearly constant since 1790(V2.5 1.6%, 2c) with a narrow 1.4^4.2% rangein the amount of excess 230Th (Fig. 2). The 230Th^238U disequilibria from other recent studies of Ki-lauea lavas (except [19,20]) are signicantly morevariable than our results and include some sam-ples with (230Th/238U)6 1 [16,17]. Despite thesmall (230Th/238U) range observed in this study,the samples display a systematic temporal varia-tion. The (230Th/238U) ratios increase subtly fromthe 19th to early 20th centuries (V1%), increaseabruptly after the explosive summit eruption of1924 (V2%), decrease slightly thereafter (V1%),and remain essentially constant during the late20th century. These short-term changes in the230Th^238U disequilibria (V1^2%) are close toour average analytical uncertainty ( 1.1%) basedon repeated Th isotope standard measurements( 1.1%) and our 0.3% reproducibility of Th/U ratios. However, we believe these subtle dier-ences are signicant because (1) they were veriedby duplicate Th isotope ratio measurements formost samples, (2) lavas that erupted during re-stricted periods of time have extremely constantTh/U, (230Th/232Th), and (230Th/238U) ratios (the1917 vs. 1919 lavas or the late 20th century lavas;Fig. 2), and (3) duplicate (230Th/232Th) analyses(and recalculated (230Th/238U) ratios) of severalsamples (1820-1, Kil1919, and six Mauna Uluand Puu Oo lavas; A. Pietruszka, unpublisheddata) using new, high-precision plasma ionizationmulti-collector mass spectrometric (PIMMS) tech-niques [24] agree withinV0.3% of our TIMS data(Fig. 2). A systematic uctuation in exc

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