dolomitization (aapg course notes 24) [lynton stuart land]

Download Dolomitization (AAPG Course Notes 24) [Lynton Stuart Land]

Post on 03-Jun-2018

212 views

Category:

Documents

0 download

Embed Size (px)

TRANSCRIPT

  • 8/11/2019 Dolomitization (AAPG Course Notes 24) [Lynton Stuart Land]

    1/21

    Dolomitization

    Presented at the 1982 AAPG Fall Education Conference

    in Denver, Colorado.

    Education Course Note Series 24

    Lynton S. Land

    University of Texas at Austin

    fflP

  • 8/11/2019 Dolomitization (AAPG Course Notes 24) [Lynton Stuart Land]

    2/21

    Dolomite

    It is probably safe to s tat e t ha t in 1982 no single model of dolomitization unequivocally

    accoun ts for all aspects of any m assively dolomitized ancient limestone. All models have

    significant flaws, and our unde rstan ding of the dolomitization process and its relation to other

    diagenetic processes (silicification, stylolitization, organic maturation, etc.)

    is

    imperfect.

    Ra ther th an advoca te one solution over another, Iwilltry to summarize some of the strengths

    and w eaknesses of several of the models which have been proposed.

    As a s tar tin g point I will review several imp ortan t aspects of dolomite mineralogy and

    chem istry th at place cons train ts on all models and th at are sometimes overlooked.

    Mineralogy

    Dolomite is a rhombohedral carbon ate w ith the ideal formula

    CaMg[C0

    3

    2

    in which calcium

    and m agnesium occupy preferred

    sites.

    In th e ideal

    m ineral,

    planes of

    C0

    3

    anions alterna te

    with planes of cations w ith the c-axisof the crysta l perpendicular to the altern ating stacked

    anion and cation planes . Ordering occurs by the additional alternation of cation planes

    containing only calcium w ith cation planes con taining only magnesium (Fig.1).Itispossible

    to conceive of a mineral having th e same com position as ideal dolomite ((Cao.

    6

    Mg

    06

    )C0

    3

    )in

    which all cation p lanes are alike, containing equal numb ers of calcium and magnesium

    ions.

    Such a mineral is not dolomite. Such a disordered arrangem ent ofionsoccupiesmorevolume

    than tha t of the ideal dolomite structure and is unstable with respect to an ordered phase.

    Perh aps surprisingly, the two com pounds jus t described, ideal dolomite and a disordered

    1-to-lratio Ca-Mg carbo nate, are both rare in sedimen tary rocks. Ideal dolomite rarely

    comprises ancient dolomitic sediments and never modern sedim ents, and the completely

    disordered polymorph does not occur at

    all.

    T he dolomite which does occur in sedim entary

    rocks is commonly Ca-rich, having compositions which rang e from about Ca(Cao.

    16

    Mgo

    g

    4)(C0

    3

    )

    2

    to ideality, and/or exhib its weak, diffuse, X -ray diffraction, suggesting considerably less

    structu ral order than its composition should dictate. With respect to ideal

    dolomite,all

    such

    natu rally occurring dolomite is metas table, and the capacity ex ists for reactions to occur

    toward a m ore stable (more stoichiometric or better ordered) phase.

    The term p rotodolomite was defined by Graf and Goldsmith(1956)as "single-phase

    rhombo hedral carb onate s which deviate from the composition of the dolomite tha t is stable in

    a given environm ent, or are imperfectly ordered, or both, but which would transform to

    dolomite if equilibrium w ere establish ed." Gaines(1977)modified the definition to include only

    ordered phases. I recommended(1980)th at the te rm be dropped altogether, since almost all

    sedim entary dolomite is really protodolomite by G aines' definition. Wh atisimportantisnot

    what

    we

    call these natu ral m aterials, bu t w hat they really are.

    1

  • 8/11/2019 Dolomitization (AAPG Course Notes 24) [Lynton Stuart Land]

    3/21

    J ^

    o

    CARBONATE MAGNESIUM CALCIUM

    Figure

    1

    Schem atic representation of the crystal stru ctu re of dolomite showing the

    alternatio n of cation an d anion (carbonate) planes, and th e altern ation of calcium and

    magnesium planes.

    2

  • 8/11/2019 Dolomitization (AAPG Course Notes 24) [Lynton Stuart Land]

    4/21

    H ydr oth erm al experim ents (Graf and G oldsmith, 1956; Goldsm ith and Heard, 1961),

    extrap olated to low tem pera ture, d em onstr ate t ha t calcite and dolomite are essentially ideal in

    compo sition at 25 C (Fig. 2). In o ther w ords, any double carbon ate crystal of Ca and M g a t

    25 C which is no t essentially pu re dolomite is either m etastab le or unstab le with respect to a

    mix ture of pure calcite plus pu re dolomite. The same thi ng is true with respect to ideal

    dolomite plus mag nesite. The composition of phases which we observe at E ar th 's surface

    define th e range of metastabili ty. U nstable p hase s are only observed as transien t s ta tes in the

    laboratory. In the case of dolomite, few phases containing more than about 8% excess calcium

    (on a molar basis) hav e been reporte d to da te, although the da ta are adm ittedly sparce.

    Reeder (1981) has shown that the structure of various kinds of dolomite revealed by

    tran sm issio n electron micro scopy an d electron diffraction can be classified into at least th ree

    typ es. All str uc tur es are ordered, alth oug h the degree of order is variab le and difficult to

    quantify. Th e first, char acte ristic only of Holocene dolom ite, con sists of irregular " m osa ics" on

    a scale of tens or hund reds of An gstro m s. The crystals are characterized by extremely high

    densities of crystallograph ic faults and dislocations, and can be tho ugh t of as an aggre gate of

    "m icro-c rystals" who se compositions m ay vary, forming a very discontinuous latt ice. This

    leads to man y unsatisfied or s trained chemical bon ds and to X-ray diffraction pat tern s w ith

    bro ad, gene rally weak reflections. This kind of dolom ite is also characterized b y large trac e

    element s ub stitutio ns, especially s tr ont ium (Behrens and Land, 1972), and sodium (Land and

    Ho ops, 1973). Qu alitative da ta su ggest th at th is material is extremely soluble compared to

    be tter ordered forms of dolomite. M y a tte m pt s to beneficiate samples composed of mixtures of

    this kind of dolomite and arago nite (for example, supratidal cru sts from Florida and th e

    Ba ham as) by slow leaching in acetic acid resulte d in only slight concentratio n of the dolomite

    by selective solution of arago nite. C0

    2

    for isotopic ana lyses of H olocene dolomite is evolved

    much faster tha n from finely groun d ancient dolomite. All evidence suggests tha t Holocene

    dolom ite is a uniq ue, highly soluble materia l. It is clearly a m etas tab le phase, unkno wn (in an

    unmodified form) in ancient rocks.

    The second and m ost comm on kind of sedimen tary dolomite exhibits a lamellar or "tw eed"

    stru ctu re w hen examined b y transm ission electron microscopy and electron diffraction, which

    Reeder (1981) ha s interp rete d as a stru ctu ral and /or compositional mod ulation on a scale of

    several hundred A ngstro ms (Fig.3).At p resen t this kind of dolomite is thou ght to consist of

    two intim ately intergro wn lamellar dom ains parallel to the rhom b face with slightly different

    stru ctu res and/or compo sitions. The te xtu re resembles spinoidal decomposition, or solid s tat e

    unm ixing on a scale of a few h undre d a ng strom s from a single homogeneous precursor. The

    exact s tr uct ure and comp osition of the two doma ins or lamellae is not known, althoug h one

    m ust be more stable (and presum ably more mag nesium rich) than the other. This type of

    dolomite is clearly me tastab le, bu t continued stabilization can not proceed spontaneo usly

    becau se it is limited b y solid sta te diffusion. C ontinue d stabilization can occur as a result of

    solution-reprecipitation p rocesses however, and it has been de m onstrated th at bulk Ca-rich

    dolomites dissolve more rapidly than ideal dolomite (Busenberg and Plummer, 1982).

    Continued stabilization tow ard a more stoichiometric dolomite would presum ably be

    pro m ote d if pore fluids in th e rock chan ged t o enable dissolving out of th e less stable, Ca-rich

    dom ain. Poro sity could easily increase under the se conditions.

    3

  • 8/11/2019 Dolomitization (AAPG Course Notes 24) [Lynton Stuart Land]

    5/21

    1000 -

    800

    TEMP.

    (C)

    600

    400

    Ordered Dolomite

    Dolomite + Magnesite

    Lower limit

    of

    experimental data

    Ranges of metastable p hases

    observed in natu re

    10

    20 30 40 50

    60

    MOLE Mg C0

    3

    Figure

    2

    Stability relations in the system C aC0

    3

    MgC0

    3

  • 8/11/2019 Dolomitization (AAPG Course Notes 24) [Lynton Stuart Land]

    6/21

    Figure 3Dark field transmission electron micrograph of a calcian dolomite

    (Caj

    i

    2

    Mg

    088

    (C0

    2

    )

    2

    )of Eocene age. The prominent mod ulated struc ture is typical of sedimen tary

    dolomite, and such cry stals are meta stable with respect to ideal stoichiometric dolomite.

    Photograph by Richard Reeder.

    A th ird k ind of dolomite is nearly ideal in composition, and when examined by transmission

    electron microscopy and e lectron diffraction is observed to

    be

    homogeneous, consisting of

    large single domains . This kind of dolomite

    is

    presently known mostly from ancient, deeply

    buried sequences and from metam orphic rocks.