American Mineralogist, Volume 78, pages 968-979, 1993 Crystal chemistry of Fe3+and H+ in mantle kaersutite: Implications for mantle metasomatism M. D.qnnv Dv.q.n Department of Geology and Astronomy, West ChesterUniversity, West Chester,Pennsylvania, 19383, U.S.A. STBpHBNJ. Ml.crwn'r-r Department of Geosciences,The PennsylvaniaState University, University Park, Pennsylvania 16802, U.S.A. AuNn V. McGurnn Department of Geosciences,University of Houston, Houston, Texas 77204, U.S.A. Launa R. Cnoss, J. Dlvro RonenrsoN Department of Chemistry and Center for Applied Energy Research,University of Kentucky, kxington, Kentucky 40506, U.S.A. Ansrru.cr Chemical and crystal chemical analyseshave been performed on a suite of subconti- nental, mantle-derived hornblende (kaersutite)samples. Mdssbauer techniques have been utilized to investigateFe valenceand site occupancies,U extraction techniqueshave been used to determine bulk H contents, proton-induced y-ray emission (PIGE) analysis was employed to measure F, and electron microprobe techniques coupled with the above measurementshave been utilized to determine major-element contents of hornblende. Similar analyseswere performed on a suite of metamorphic amphibole samples from Coscaet al. (1991).Comparison with their wet chemicalresults on Fe3+/Fe2+permitted determination of C : 1.22, the correction for differential recoil-free fraction effects,which was used to correct the mantle sample M0ssbauerdata. The results of the analysesfor the kaersutite samplesshow a nearly l:l inverse relationship between the Fe3+and H+ con- tents. Although the range of Fe3+/H+ in the less oxidized kaersutite samples may be explained by partial H loss during entrainment and ascent,the nearly total dehydrogena- tion of the Fe3*-rich megacrystswould require time scalessignificantly longer than what is expectedfor transport. Thus, it seemslikely that these oxykaersutite samplesgrew in a more oxidized metasomatic fluid, where incorporation of H was not required for charge compensation. As megacrystsfrom the same location show wide variation in Fe3* and H+, it appearslikely that significant variations in the oxidation stateof the mantle metaso- matic fluid occurred over relatively small temporal or spatial scales. INrnonucrroN while others lack hydrous phases.Improved understand- volatile in metasomatism and A 1964paper by Oxburgh was among the first to sug- ing of the role of species phasesis needed. gestthat the Earth's upper mantle must contain td;;- the crystal chemistry of the hydrous studies bv Dvar et al' (1989' eral amphibolein ordei to provide sufficienrK - ^l:ttlt^Sineralogical wood' virgo' and alkalis to serve as a source region for ururi ";e;il;r 1992a' 199.2b)among others including on the presenceof Fe3* Becausethe presenceof amphibole implies "or"u"lt.".,ig"ifl"*t coworkers have focusedattention " in tvpical mantle phases(McGuire et al', 1989;Wood volatile component in the mantle, this rugg"rtToi- *i !o1! relaredworkbyvarne(1970)ledronumero"*;li;r;" ung virgo, 1989;Dyar et al., 1989;Canil et al.' 1990) in metasomatizedperidotites (McGuire et al'' 1991; the influenceof volatiles on the melting uetra.,^ororpe- 1na 1992a). The studies on peridotite indicate ridotite (e.g., Bailey, lg7}, lg72) and on tfre efects ana Dyar et al., metasomatismare associated characteristicsofmetasomatism(e.g.,Menzies,1983; Dt"k that contrastingstyles of with sets of Fe3*/Fe,",ratios in component et al., 1984).Field observations(Wilshire and Sh;;;i;, distinctive phases; of an interrelationship 1975;Harteetal., l975;StoschandSeck, 1980;R"J;; they are also suggestive Fer+ and H* in individual minerals' et al., 1984; Wilshire, 1987; Nielson et al., f Sglj h""" between led to a variety of models for mantle metasomatic pro- The goal of our presentresearch is to examine the crys- cesses.There is current debate over the mechanismsihat tal chemistry of mantle-derived amphibole by studying produce the different styles of metasomatism. For ex- its major-element contents, Fe-site occupanciesand va- ample, it is not understood why some styles of metaso- lences,and H* content and partitioning behavior. Data matism are characterizedby the presenceof amphibole, on the isotopic behavior of H are presentedin brief in 0003-004x/93l0910-0968$02.00 968 DYAR ET AL.: Fe3+AND H+ IN MANTLE KAERSUTITE 969 250 200 1.75 o. 1.50 $ r.zs B t.oo 0.50 1.5 3.0 4.5 6 0 7.5 9.0 10.5 12.0 13.5 15.0 Wt% Fe2O3 Fig. 1. FerO. and HrO contentsof oxykaersutitesand mantle amphiboles. Symbols representdata from Barnes (1930), solid circles;Aoki (1963),solid squares;Best (1970),solid triangles; Frg.2. Schematicdrawing ofthe octahedralsites adjacent to Wilshire and Trask (1971),solid diamonds;Aoki (1971),open theH* atomsin thehornblende structure. O sitesare indicated circles;Boettcher and O'Neil (1980), open squares;and Graham by largeoutlined circles and identified by theirnumbers; cation et al. (1984),open triangles.The lack ofa closelyaligned linear sitesare small, solid circles. The H atomsits close to the03 site trend among these variables arises from three main factors: (1) with an O-H distanceof 0.934A lPecharet al., 1989),and so group part polyhedra HrO was, in most cases,determined by weight loss with no dis- theOH takes in thecoordination of both crimination of possibleCO, contribution; (2) FerO, was, in most Ml andM3. cases,detennined by calorimetry, which could not detect con- tributions from Ti and Mn oxidation; and (3) since the true (1984) examined the major-elementand volatile com- substitution here involves oxidation ofFer+ to Fe3+due to loss ponents of amphibole in xenoliths from the Grand Can- of H* ions, the plot of oxide data obscuresany evidence of a yon and found a strong positive correlation between Ti crystal chemical trend. contents and H+ deficiency.An intriguing study (Brynd- zia et al., 1990) investigated the relationship among fo,, Dyar et al. (1992b) and will be pursued, togerher wirh O OH- content, DD, and Fe3+/Fe,.,in samplesfrom Shep- isotope data, in a later work (Dyar et al., in preparation). pard and Epstein (1970) and Kuroda et al. (1978) and We present here the results of a detailed study of 20 kaer- found only small ranges of dD and Fe3+/Fe,o,;however, sutite samples, in order to assessthe relationships among no primary data were given. Recent work by Deloule and H+, Fe3+, and other major elements within the amphi- coworkers(e.g., Deloule et al., 1990, l99l) has concen- bole structure. trated solely on the isotopic variation displayed by horn- blende. Unfortunately, none of these more recent studies B,c,cxcnouNo included complete (published) Fe3* data on all samples, Beginningwith Barnes(1930) and then Winchell (1945), and so other workers (e.g.,Boettcher and O'Neil, 1980) Aoki (1963, 1970; l97l), Best(1970, 1974),and Ernsr could do little more than note a generalinverse correla- and Wai (1970),geochemists have used atomic absorp- tion betweenFerOr/FeO and HrO content(Fig. l). tion and wet chemical analysesto study the dehydration Many crystallographic studies of the kaersutite struc- of amphibole. Incomplete characteization of minor con- ture using X-ray diffraction have been published, includ- stituents such as Cr3* and Tia+ and suspicionsthat Ti3+ ing Papike and Clark (1968),Papike et al. (1969),Kita- might contribute to errors in Fe3+determinations ham- mura and Tokonami (1971), and Hawthorne and Grundy pered these workers in evaluating the detailed crystal (1973).Powder (Jirdk et al., 1986)and single-crystal(Pe- chemistry of their samples. Furthermore, HrO analyses char et al., 1989) neutron diffraction studies of this min- were generally determined by weight loss, a technique eral have further improved our understandingofthe cat- that yielded resultsgreatly biasedby the contents ofother ion distributions and H positions in kaersutite (Table l, volatile phases, especially COr, which are now known Fig. 2). from fluid inclusion studies to be abundant in minerals Additional relevant X-ray data complemented by ex- derived from the upper mantle (seee.g., Roedder, 1965; perimental work has been performed recently by Popp et Bergman,198 1; Andersenet al., 1984). al. (1990)at TexasA&M University (Phillipset a1.,1988, More recent geochemicaland petrologic studies of am- 1989;Clowe et al., 1988).Results showed that dehydro- phibole have focused more closely on major-element genation accompanying heat treatment causespreferen- analysescoupled with studies ofH isotope behavior. Ku- tial ordering oftrivalent cations at the cis-Ml and trans- roda et al. (1978)examined 6D ofhornblendefrom intru- M3 sites. Some trivalent cations in M2 may relocate to sions in alkali basalts;Boettcher and O'Neil (1980) fol- Ml or M3 as oxidation occurs.Fe3+-H+ oxysubstitution lowed with a thorough examination of DD,HrO content, and Al substitution were identified as the primary mech- and D'8Oin amphibole;and Graham et al. (1984)derived anisms accompanying oxidation and dehydrogenation. activation energiesfor H isotope exchange.Matson et al. Popp et al. (1990) surveyedthe literature on Fe3+-rich 970 DYAR ET AL.: FC3*AND H* IN MANTLE KAERSUTITE Tmle 1. Cationdistributions in kaersutitebased on structurerefinements Site Hawthorneand Grundy(1973) Pechar et al. (1989) Phillipset al. (1989) A Na, + Kyand Nao543, and Ko4s y' 0.265Na+ 0.205K 0.43Na T1 0.53Si + 0.47A1 0.57Si + 0.43A1 0.35Ar+ 0.65Si r2 0.94Si + 0.06A1 0.90si + 0.10A1 0.02Ar+ 0.98si M1 0.343Fe3*+ 0.657M9 0-26Ti+ 0.03ca + 0.32FeF*+ 0.39M9 0.41sFe+ 0.585M9 M2 0.056Fe3*+ 0.502M9 + 0.185At+ O.257Ti 0.17A1+