Platinum-iron alloy sample containers for melting experiments on iron-bearing rocks, minerals, and related systems

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MINERALOGICAL MAGAZINE, JUNE 1978, VOL. 4 2, PP. 271-5 Platnum-ron alloy sample contaners for meltng experments on ron-bearng rocks, mnerals, and related systems C. E.

1 MINERALOGICAL MAGAZINE, JUNE 1978, VOL. 4 2, PP Platnum-ron alloy sample contaners for meltng experments on ron-bearng rocks, mnerals, and related systems C. E. FORD Grant Insttute of Geology, Unversty of Ednburgh, West Mans Road, Ednburgh EH 9 3JW SUMMARY. A method of producng platnum-ron alloy sample contaners has been developed that allows experments to be carred out on ron- and volatle-bearng systems wth mnmal net exchange of ron between the sample and ts contaner. Prelmnary experments carred out usng these contaners n the temperature range from IIoo ~ to I25o ~ and at pressures up to lo kb over perods of up to 75 hours produced an average ron loss of Io~ to I6% (as FeO) compared wth more than 5o% FeO loss obtaned usng pure platnum contaners. A more judcous choce ofauoy composton would reduce ths loss stll further. MELTING experments on gneous rocks and analogous synthetc systems can provde consderable nformaton on the crystal-lqud equlbra nvolved n magmatc processes. Unfortunately systems contanng ron n both of ts oxdaton states and also contanng small but sgnfcant amounts of volatle components, such as CO2 and H2, have been partcularly dffcult to nvestgate at hgh temperatures, largely due to the lack of a sutable contaner materal. The loss of ron to noble metal sample contaners s a well-known but poorly quantfed phenomenon. Attempts to mnmze ron loss to platnum have nvolved an unsuccessful method of saturatng the contaner wth ron from a slcate melt, addng suffcent excess ron to the sample to compensate for the subsequent ron loss (Ncholls, t974) and reducng the experment duraton to a mnmum. The latter methods rsk falure to acheve equlbrum, especally wth respect to the dstrbuton of the mnor and trace elements between crystals and lquds. It s possble to carry out meltng experments wthout ron loss n contaner materals such as graphte (Thompson, 974), molybdenum (Bggar, ~97o), and ron metal (O'Hara and Humphres, 1977). These contaners are sutable for volatle-free systems and the low oxygen fugactes approprate Copyrght the Mneralogcal Socety for the moon, meteortes, and the terrestral mantle. A recent comparson has demonstrated substantal losses of Fe to Ag4o-Pd6o, Pt, and Ptgs-Au5 (more than 5 ~ FeO loss) wth most ron loss to the latter (Bggar, I977). Ths paper reports a method of producng platnum-ron alloy sample contaners n whch t s possble to carry out experments wth a mnmum of ron exchange between the sample and ts contaner n systems contanng mxed volatles and both ferrous and ferrc oxde and at temperatures above those for whch the more slver-rch slverpalladum alloys could be used. Platnum-ron alloy contaners. The deal contaner materal s chemcally nert by beng saturable at very low concentratons wth all of the chemcal speces present n the system under nvestgaton. Platnum s saturable wth respect to almost all components except ron (and nckel). The actvty of ron n a slcate melt s specfed by the bulk composton, temperature, and pressure. If a platnum-ron alloy can be made wth exactly the same actvty of ron as that n the slcate system then there wll be no net exchange of ron wth the capsule. If the mechancal propertes of such an alloy, or range of alloys, s acceptable then they would consttute near-deal contaner materals. The range of alloys that would be requred to cover the oxygen fugacty range of the magnette stablty feld s o- 5 wt ~ Fe (Taylor and Muan, I962). Snce platnum-ron alloy tubes are not commercally avalable and t would n any case be mpractcal to stock the wde range of alloy compostons that mght be requred, the followng method of producng contaners approachng the requred composton has been developed: startng materals are laboratory reagent grades of hydrated crystallne sodum meta-slcate, whch s melted to produce glass and then ground, and magnette. A physcal mxture of these materals s

2 272 C.E. made wth compostons n the range from ~o to 25 wt ~ of magnette. Cut lengths of o. I mm wall thckness platnum tube, ether 2 mm or 3 mltl n dameter and cm or 2 cm long wth both ends open, are mmersed vertcally n a crucble contanng about 2o grams of the mxture, whch s then held at 12 ~ for 24 hours or more n an atmospherc pressure furnace n whch the oxygen fugacty s controlled by mxng H 2 and CO2. At the end of ths perod the crucble s removed and the tubes recovered by dssolvng the glass n hot agtated water. Ths may take several days. Typcal dstrbutons of ron n the platnum tube wall after 24 and 72 hours are llustrated n fg.. 9,%o Fe ~ \ I hours I \ "-- 72 hours \ \ I p FIG. t. Dstrbuton of ron n platnum tube wall as a functon of tme. The ron content of the platnum quckly reaches a maxmum value at the tube surface. There s vrtually no ron n the centre ofa o.i mm thck wall after 24 hours, about 66 ~o of the maxmum amount after 72 hours, and about 7o ~ of the maxmum amount after 2 hours. In subsequent dscusson the ron content referred to s the maxmum ron content at the nternal surface of the tube wall. The 24-hour ron dstrbuton, although bound to lead to ron exchange wth the charge n subsequent experments, produces a capsule wth suffcent ductlty to wthstand the deformaton nvolved n hgh-pressure experments, except perhaps those n sold meda apparatus. Capsules wth the 72-hour ron dstrbuton tend to dsntegrate when crmped, especally at ron contents n excess of 15 ~. The capsules are arc welded wth an ron electrode n an argon stream. The ron content of the tubes s dependent on oxygen fugacty and the composton of the startng FORD mxture. At I2 ~ the startng mxtures used contan only lqud, although magnette crystallzes readly on coolng. Fg. 2 shows the ron contents of alloys produced as a functon of oxygen fugacty and melt composton at I2 ~ The scatter n the data probably represents falure to fully equlbrate the large mass of melt wth the gas phase and consequent varaton of the alloy composton at the tube wall of + I ~o Fe. In addton, at low ron contents and tow oxygen fugactes, the melt may become sgnfcantly depleted n ron. No ron s absorbed from a melt n equlbrum wth pure CO2. '...".. I I I ",r I, I B,'.,-y 9 : " I..,,...;,C~, " I " I I e :..~ I I -, A -; k -" -" -',o -, -',2 Io to 2 FIG. 2. Iron content of platnum-ron alloys as a functon of oxygen fugacty and composton of the Na2SO3- FeaO, melt at 12 ~ Symbols: 9 preparaton n 25 ~o magnette mxture; [] preparaton n I7~ magnette mxture; preparaton n I ~ magnette mxture. Dash lnes show the maxmum ron contents to be expected. The dash-dot lne represents alloy compostons n equlbrum wth magnette at 3 ~ (Taylor and Muan, I962). Other methods of producng the alloys were tred and proved unsatsfactory. Iron or magnette sank n molten sodum chlorde or sodum carbonate and produced an uneven ron dstrbuton. Ferrous chlorde n sealed tubes heated at pressure lberated suffcent chlorne to rupture the capsules on depressursaton. Meltng experments on ZP422, an alkal basalt from the Mdland Valley of Scotland, llustrate the advantages of platnum-ron alloys over pure platnum. The results of these experments wll be descrbed n detal elsewhere. The composton of ZP422 s as follows: SO 2 46"5, TO2 2-2, A12 3 4'87, Fe2Oa 4"4, FeO 8.64, MnO o't9, MgO 7-I6,

3 PT-FE SAMPLE CONTAINERS FOR MELTS 273 T A B LE I. Results of experments on ZP422 n platnum and sealed and unsealed platnum-ron alloy capsules A B C Sealed Pt tube ~ FeO Sealed PtFe tube ~ FeO Open PtFe tube ~ FeO ~5 ~-~ ~ ~ loss Tube loss Tube loss Z ~",~ Assemblage a b % Fe Assemblage a b ~ Fe Assemblage a b 16 12oo I 2 G GI 11 o 17 I2oo I 55 G GI 1I 5 13 Gl* o 1 25 Gl Gl+O Gl+Ol+Plag o I 48 Gl Gl+Ol+Plag I Gl+Ol+Plag II25 I 48 Gl+Ol+Ore Gl+Ol+Ppx+Plag 1I Gl+Ol+Cpx+Plag IIO I 26 Gl+Ol+Cpx+Ore Gl+Ol+Cpx+PlagI Gl+Ol+Cpx+Plag 9-I + Ore + Ore 21 12oo 5 66 G Gl Gl 14 IO o 5 24 Gl+Cpx 4 ~ 28 IO Gl+Ol+Cpx 5 I2 IO Gl+Ol+Cpx+Plag x6o 5"5 49 Gl+Cpx Gl+Ol+Cpx Gl+Ol+Cpx+Plag33 lo "5 18"5 GI I GI+Cpx 17 2 II Gl+Ol+Cpx+Plag4o I5O Gl+Cpx+Ore II Gl+Cpx+Ore II Gl+Ol+Cpx+Plag3o o Gl+Cpx Gl+Cpx+Plag+ 36 I6 1I Gl+Ol+Cpx+Plag45 22 Ore + Ore o 7" G Gl+Cpx o 1 9 Gl+Ol+Cpx+Plag o 8 28 Gl+Cpx Gl+Cpx Gl+Ol+Cpx+Plag I G GI GI IO I39 G I GI+Cpx II Gl+Cpx+Plag oo IO I9. 5 G It Gl+Cpx I8 15 1I Gl+Cpx+Plag 3 ~ II5 o lo 73 Gl+Cpx Gl+Cpx+Plag Gl+Ol+Cpx+Plag Ore + Ore a: value derved from mcroprobe analyss. b: value derved from the FeOMgO rato of the mcroprobe analyss. * Glass from outsde a leaked capsule. GI = Glass, O1 = Olvne, Cpx = Clnopyroxene, Plag = Plagoclase. CaO Io-t 5, Na2 2"77, K2 "93, H2+ I-O8, H2- "24, P25.37, total 99.I4 (analyst: S. Rzzelo). Fnely ground rock powder, dred at I IO ~ was loaded and sealed nto platnum tubes, and both sealed and unsealed platnum-ron alloy tubes contanng about o ~ ron. The open-tube experments lose combned votatles, ncludng oxygen, durng the experment. Experments were carred out n nternally heated gas meda pressure vessels (Ford, I976) and sold meda pressure vessels (O'Hara et al., I971). The oxygen fugactes of the experments are unknown n exact numercal terms. However, f there s no ron exchange between charge and capsule the oxygen fugacty of the charge may be consdered as beng defned by the platnum-ron alloy composton used. If there s substantal ron loss from the charge then the oxygen fugacty s lower than that approprate for the platnum-ron alloy used. Conversely, substantal ron gan by the charge ndcates that the oxygen fugacty s hgher than that approprate for the alloy composton used. The results of experments are gven n Table I. Iron losses to pure platnum, and to sealed and unsealed platnum-ron alloys are shown n fgs. 3, 4, and 5. The ron contents of the charges were determned by electron-mcroprobe analyss. Ths method has been adopted for routne use n preference to wet chemcal analyss snce there s frequently nsuffcent materal (< Io mg) for both, mcroprobe analyss of the ndvdual phases s n any case normally desrable and n those cases, nvolvng all glass charges, where both mcroprobe analyss and wet-chemcal analyss for FeO have been carred out the results were dentcal (A. Graham, pers. comm.). For wholly melted charges analyses of the glasses, whch were homogenous, were obtaned from a defocused beam about IO~m n dameter. For partally melted charges analyss was of an area of about 35o #m square. Whole-sample mcroprobe analyss s not entrely satsfactory for partally melted charges snce the occurrence of cracks and holes tends to reduce analyss totals and non-unform dstrbuton of phases may produce samplng bas n the area chosen for analyss. For the purposes of estmatng ron loss or gan the mcroprobe analyss totals have been normalzed to the startng composton total mnus H2 and one oxygen from Fe23.

$274 C. E. FORD ~ Fe FeO as t 12.5 ~- TubeCharge ml~o = 75I 14) +\\ x ~ -~-+-11 J AIII - - -... 1175 5. " 2.5 I I 1 2 3 lo 5 6 Tme (hours) 12 Fe loss as FeO ;,'o 8 1 2 3O 4 5 6 Fe m ~FeO 12.5 1.$

4 274 C. E. FORD ~ Fe FeO as t 12.5 ~- TubeCharge ml~o = 75I 14) +\\ x ~ -~-+-11 J AIII " 2.5 I I lo 5 6 Tme (hours) 12 Fe loss as FeO ;,'o O Fe m ~FeO O 2.5 Fe ~ss ~: sfeo t * 2 3 ' 7 3o,~ do ~o 7 so Tme (hours) 8O 9 FIGS. 3-5: FIG. 3 (top left). Iron contents of ZP422 charges lz5, after experments n sealed platnum tubes, data from Table I. Symbols: 9 FeO content determned by mcro- 1. probe analyss; + FeO loss determned from FeOMgO rato of mcroprobe analyss. Curves correlate experment z5 temperatures wth amount of ron loss. FIG. 4 (top rght). Iron contents of ZP422 charges after experments n ~ sealed platnum-ron alloy tubes. Symbols are n fg. 3. FIG. 5 (bottom). Iron contents of ZP422 charges after experments n open platnum-ron tubes, * glass col- 2.5 lected from outsde a leaked capsule, other symbols as n fg- 3. o + el...,~ T- --o... : ~-- -,": * 1~ ~o 3'o 2o ~ ~ ; g Fe loss as FeO The effect of samplng bas or analytcal error on ron loss can be assessed from consderaton of the bulk FeOMgO rato wth the assumpton that these oxdes are usually assocated n the ferromagnesan mnerals. The + symbols n fgs. 3, 4, and 5 represent ron loss estmated from the FeOMgO rato of the mcroprobe analyss (Table I, % FeO loss column b) and should be read n conjuncton wth the % FeO loss scale. The small consstent dfferences of about 5 % FeO loss probably represent analytcal bas, ether n the accepted bulk composton quoted or n the mcroprobe technque, whle the larger dfferences probably represent samplng bas. The loss of ron to pure platnum s substantal. It exceeds 2 % of the amount present after 2 hours and 5o % of the amount present after 5o hours. The loss of ron to sealed platnum-ron alloy tubes s very much lower, averagng zo % or t6 % FeO loss (by FeOMgO rato and analyss respectvely) for the seres of experments n sealed capsules over all tmes up to 73 hours. The use of capsules wth a hgher ron content would have reduced ths ron loss stll further. The loss of ron to open platnumron capsules appears to be somewhat more erratc than to sealed platnum-ron capsules. Ths s ascrbed to varous amounts of oxygen loss to the argon pressure medum, whch has an oxygen fugacty below the ron-wstte buffer assemblage; tarnshed ron capsules are recovered untarnshed. The analyss wth 68 % FeO loss n fg. 5 s of a glass that had leaked out of the capsule and was attached to ts outsde where t would have lost oxygen at a faster rate. A number of experments were carred out n sealed platnum-ron capsules n sold meda apparatus. All of these experments lost substantal amounts of ron. Ths s probably due to shearng of the capsule walls, makng them effectvely open capsule experments, and equlbraton of the charges wth the graphte furnace at a consderably lower oxygen fugacty than that ntended. The results of these experments requre further verfcaton and are not reported here. It may be possble to enclose the farly brttle platnum-ron alloy capsules n a second more ductle membrane to ensure a hermetc seal for sold meda experments. The platnum-ron alloy composton approprate for an experment on a gven startng composton s dependent on the ron actvty of that composton under the gven expermental

5 PT-FE SAMPLE CONTAINERS FOR MELTS 275 condtons. Intally ths ron actvty, and hence the requred alloy composton, wll be unknown. However, a few experments n alloys of dfferent compostons should quckly establsh the one most sutable. In any case, there s an ncreased margn for error where the experments are of short duraton. REFERENCES Bggar (G. M.), I97o. Bull. Am. Ceram. Soc. 49, Mneral. Mag. 41, Ford (C. E.), x976. Natural Envronmental Research Councl Publcaton, Seres D, No. 6, t43-6. Ncholls (I. A.), t974. Contrb. Mneral. Petrol. 45, O'Hara (M. J.) and Humphres (D. J.), Phl. Trans. R. Soc. Lond. A 286, 313-3o. --Rchardson (S. W.), and Wlson (G.), 197I. Contrb. Mneral. Petrol. 32, Taylor (R. W.) and Muan (A.), Trans. MetaU. Soc. A.I.M.E. 224, 5oo-2. Thompson (R. N.), I974. Contrb. Mneral. Petrol. 45, 317-4I. [Manuscrpt receved 16 November 1977]

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