Alt, J.C., Kinshita, H., Stkking, L.B., and Michael, P.J. (Eds.), 996 Prceedings f the cean Drilling Prgram, Scientific Results, Vl. 48. CMPSITIN AND CRYSTALLIZATIN TEMPERATURES F PRIMARY MELTS FRM HLE 896A BASALTS: EVIDENCE FRM MELT INCLUSIN STUDIES Andrew W. McNeill and Lenid V. Danyushevsky ABSTRACT Glass and mineral gechemical data are presented fr basalts frm cean Drilling Prgram (DP) Hle 896A. Phencryst assemblages are dminated by calcic plagiclase (An 76 _ 945 ) with livine (F 80 _ 9 6 ), Cr-Al spinel, and clinpyrxene. The rigin f the calcic plagiclase phencrysts was investigated by heating-stage experiments n silicate melt inclusins in plagiclase and livine mineral separates. Trapping temperatures f 95-5 C are inferred fr plagiclase (An 8 _ 945 ). Inclusin cmpsitins, interpreted t be in equilibrium with hst plagiclase, are similar t the range f pillw-rim glasses frm this hle. Primary melts, in equilibrium with F 9 6, are estimated t have -5 wt% and crystallizatin temperatures f ~40 C. INTRDUCTIN High anrthite cntents (t An 945 ) are a characteristic feature f plagiclase phencrysts frm basalts drilled near the Csta Rica Rift in cean Drilling Prgram (DP) Hles 504B and 896A. Plagiclases f similar cmpsitin have been reprted frm a variety f midcean ridge basalts (MRB); the Mid-Atlantic Ridge (Deep Sea Drilling Prject Leg 7, Aument, Melsn, et al., 977; Dnaldsn and Brwn, 977; FAMUS, Stakes et al., 984), the Suth East Indian Ridge (Price et al., 986), the East Pacific Rise (Siqueirs Fracture Zne; Natland, 989), and the Galapags Platfrm (Sintn et al., 99), and are cmmnly assciated with megacrysts f Cr-rich magnesian dipside, frsteritic (F 88 _ 9l ) livine, and Cr-Al spinel. Experimental results suggest that melts with Ca/Na >I are required t crystallize plagiclase f >An 90 under anhydrus cnditins at pressures f <5 kbar (Fisk, 984; Panjasawatwng, 99), althugh mre recent data shw that high melt A will stabilize high-an plagiclase at lwer Ca/Na (Haskall et al., 99; Panjasawatwng et al., 995). A survey f primitive MRB glass cmpsitins frm the literature (T.J. Falln, pers. cmm., 994) indicates that nly ne (glass ; Melsn et al., 976) has the necessary Ca/Na t crystallize plagiclase f ~An 90, but nt higher. The paucity f erupted liquids f this cmpsitin may be explained by pre-eruptin prcesses (magma mixing, lw-pressure fractinatin, and magma chamber density filters; Sintn and Detrick, 99). Hwever, samples f such a liquid may be retained in melt inclusins trapped during grwth f early frmed phencrysts, and such inclusins have been reprted in the literature (e.g., Danyushevsky et al., 988; Sblev and Shimizu, 99). Tw appraches have been emplyed in the study f melt inclusins. The first invlves analysis f naturally quenched inclusins (e.g., Andersn, 974; Clcchiatti, 977; Falln and Green, 986; Natland, 989). The cmpsitins f such melt inclusins have been mdified t varying degrees by pst-trapping crystallizatin; hwever, ratis f elements incmpatible with the hst are thught t preserve sme f the chemical signature f the parental melt. In additin, trapping temperatures are difficult t estimate because f uncertainty 'Alt, i.c, Kinshita, H., Stkking, L.B., and Michael, P.J. (Eds.), 996. Prc. DP, Sci. Results, 48: Cllege Statin, TX (cean Drilling Prgram). Gelgy Department, University f Tasmania, Hbart, G.P.. Bx 5c, Tasmania 700, Australia. McNeill: andrew.mcneill@gel.utas.edu.au; Danyushevsky:.dan@gel.utas.edu.au regarding the amunts and cmpsitins f minerals frmed by psttrapping crystallizatin. The secnd apprach, experimental studies f melt inclusins, invlves reheating the inclusins t melt-back phases frmed by psttrapping crystallizatin and thereby "hmgenizing" the inclusin. Hmgenizatin is defined as the mment f disslutin f the last phase inside an inclusin, nrmally a shrinkage gas bubble, prducing a hmgeneus melt (Redder, 984). In an ideal fluid-saturated system the cmpsitin f the inclusin at the mment f hmgenizatin is equal t that f the trapped melt, and the temperature f hmgenizatin matches the temperature f trapping. Tw techniques are emplyed; heating in a -atm furnace, under cntrlled xygen fugacity (Sintn et al., 99; Jhnsn et al., 995), r in a visually cntrlled heating stage with a pure He atmsphere (Sblev et al., 980). In this cntributin we dcument the petrgraphy and majr element gechemistry f glass and phencryst phases frm Hle 896A basalts, and use heating-stage experiments n plagiclase and livine t determine the trapping temperature and chemistry f silicate melt inclusins. Frm these data we estimate melt cmpsitins in equilibrium with plagiclase An 94 5 and mdel pssible primary magmas. ANALYTICAL AND EXPERIMENTAL TECHNIQUES Glass samples were carefully picked frm pillw rims, munted in epxy, and plished. The glasses and any phencryst phases present were then analyzed by electrn micrprbe. Seven samples were selected n the basis f analyzed glass chemistry and phencryst abundance, and mineral separates were prepared by crushing and sieving. Phencrysts were picked frm the 0.- t.0-mm size fractin, munted in epxy and plished. Between 0 and 90 plagiclase grains in each sample and all livine, spinel, and, where present, clinpyrxene were analyzed t establish the range f phencryst cmpsitins fr each sample. Electrn Micrprbe Analysis All analyses were dne n an autmated three-spectrmeter Cameca SX-50 micrprbe at the University f Tasmania, Central Science Labratry. Standards used were San Carls livine (USNM /444), plagiclase LPL (USNM 5900), Augite KA (USNM 4) (Jarsewich et al., 980), spinel UV-6 (Lavrentev et al., 974), and basaltic glass VG- (USNM 49/5). Analytical cnditins and cunting times fr each analytical label used are given
A.W. McNEILL, L.V. DANYUSHEVSKY Table. Electrn micrprbe experimental cnditins. 00 Cunt times (peak/backgrund; s) xide Plagiclase livine Pyrxene Spinel Glass Si, n; Fe AI 0 Mn Ca Na, K 0 P-T, Cr,, Ni Zn Vltage (kv) Current (na) Beam (µm) 5 0 5 0 5 0 Nte: Blank spaces indicate that element was nt analyzed. 5 0 5 0 5x8 in Table. Relevant standards were analyzed at the start and end f each sessin, and at apprximately 4-hr intervals during lnger sessins. Each standard analysis is the average f three analyses, mineral and melt inclusin analyses are averages f tw pints, and glass analyses are averages f fur pints. Furier Transfrm Infrared Analysis The H cntents f glass samples were determined by Furier transfrm infrared spectrscpy (FTIR) using a Bruker IFS 66 spectrmeter with attached ptical micrscpe and Bruker pus/ir data reductin sftware at the Central Science Labratry, University f Tasmania. The same glass chips used fr electrn micrprbe analysis were used fr FTIR analysis. Sample preparatin, standards, and experimental cnditins used have been described elsewhere (Danyushevsky et al., 99). High-temperature Micrthermmetry Grain munts used fr electrn micrprbe analysis were cut and plished t 50-50 µm thickness and grains hsting primary melt inclusins suitable fr micrthermmetry were extracted. The heating stage used is an ptically cntrlled system using a high-purity He atmsphere, the same as that described by Sblev et al. (980). This stage design allws mre rapid quenching than with cmmercially designed equipment. Using visual cntrl and experimental techniques develped at Vernadsky Institute f Gechemistry, Mscw, hmgenizatin temperatures (7,,) may be estimated t within ±0-0 C (Sblev, 98; Sblev et al., 989). Fllwing the technique referenced abve, the sample phencryst was placed n a 00- t 00-µm thick plate f transparent, refractry mantle livine and a small piece f Au was placed n the sample grain as clse as pssible t the inclusins f interest. The melting pint f Au was used t calibrate temperature during each run. The sample was then placed n a Pt ring inside the tubular Pt 90 Rh () fil heater; the heating stage was sealed and then flushed with He fr 5-6 min befre heating began. Bth plagiclase and livine were heated rapidly t 50 C and then at a rate f 5 C per min, as determined frm a series f kinetic experiments n plagiclase, until clse t T h, as assessed by bserving small (~ 0 µm diameter) inclusins in plagiclase and small (0-0 µm) inclusins in livine. Temperature was then held cnstant r increased slightly until the inclusins f interest were hmgenized. Time at high temperature (>l 90 C) was limited t <6 min fr plagiclase, as it was fund that if an inclusin did nt hmgenize within this time then it was unlikely t hmgenize in a reasnable time (i.e., <l hr). After an experiment, each grain was munted in epxy 470 5.5 6.5 7.5 Ca/Na Figure. Dwnhle variatins in glass Ca/Na. Glass grups are based bth n Ca/Na values and variatins in elements shwn n Figure. and plished until the inclusins f interest were expsed and these were then analyzed. RESULTS Glass Chemistry Majr element analyses f 4 glasses, frming a stratigraphic sectin f Hle 896A, are summarized in Figures and. Full analyses are presented in Fisk et al. (this vlume). n the basis f these data, the drilled sectin was divided int fur intervals with samples frm each interval frming subsequent grups: Grup cmprised samples frm Cres 48-896A-R t 0R, Grup frm Cres 48-896A-5R and 6R, Grup frm Cre 48-896A-7R, and Grup 4 frm Cre 48-896A-0R. The transitin frm Grup t ccurs acrss the massive flw f Cres 48-896A-0R t 4R, and transitins between the lwer grups are in pillw lava sequences and appear ver m f cre. These transitins d nt appear t crrelate with changes in either phencryst assemblage r abundance, as defined by shipbard lgging. Mst samples are frm Grups and 4. These tw grups frm a single trend n element- plts, cnsistent with livine-plagiclase c-crystallizatin. Glasses frm Grup have higher Na, Ti, and A cmpared with the main trend (Fig. ) whereas thse frm Grup have higher, Ca, and Fe, and lwer A and Na than the main trend. The H cntents f pillw-rim glasses are all <0.% and there is n variatin f H cntent between the glass grups, within the precisin f the technique used. These lw abundances are cnsistent with analyses f glasses f similar Mg# (= 00 Mg/[Mg + Fe]) and K frm the East Pacific Rise and Galapags Ridge (Byers et al., 986; Michael, 988). Seven glasses, all frm the high part f the suite, frm Grups,, and were selected fr micrthermmetry (Table ). The fllwing discussin f mineralgy and melt inclusin cmpsitins refers nly t these samples. Petrgraphy and Phencryst Mineral Chemistry The petrgraphy f Hle 896A basalts has been described in detail by the Shipbard Scientific Party (99) and nly the majr characteristics f selected samples are summarized here. The samples used are all frm glassy pillw rims and inner varilitic znes, and they 8.5
PRIMARY MELT CMPSITIN AND CRYSTALLIZATIN frm Cres 48-896A-9R, 5R and 7R, suggesting that it is mre widespread but in lw abundances. Plagiclase α Grup -Grup A-Grup *-Grup 4.4 8.8 9. (wt*j Figure. Selected majr xides vs. fr Hle 896A pillw-rim glass samples. Glass grups have been defined n the basis f these variatins and Ca/Na as shwn n Figure. Table. Analyses f representative pillw-rim glass samples frm Hle 896A. Cre, sectin: Interval (cm): Piece n.: Glass grup: Depth (mbsf): Si, Ti A Fe Mn Ca Na K, P5 Cr H Mg# Ca/Na R- 6-8 4 0. 48.94 0.70 6.45 8.7 0. 9.6.0.70 99. 65.7 7.7 4R- 0-9.0 49. 0.7 6.7 8.80 0.5 9.5.00.74 5 99. 65.6 7.5 9R- 5-0 4 67.95 49.68 0.8 5.79 9.0 0.4 8.96.8.7 5 99.7 64.0 7.5 5R- 89-9 4. 48.96 0.87 6.5 8.99 0.6 9.09.89.4 99.8 64.4 6.0 5R- 6-64 4.4 49.4 0.85 6.55 9.7 0.5 9.09.8.6 5 5 00. 64.0 5.9 7R- 05-09 4.6 49.8 0.67 6.7 9.6 0. 9.45.64.6 5 5 00.7 64.7 8.4 7R- 4-0 5 4.8 49.0 0.64 6.07 9. 0. 9.4.66.6 5 99.86 65.0 8.4 Ntes: See text fr details n glass grups. All Fe as Fe. Mg# = 00 [Mg/(Mg + Fe)l. range frm sparsely prphyritic (l%-% phencrysts) t highly prphyritic (>0% phencrysts). Thrughut mst f the drilled sectin, and in all samples used in this study, plagiclase is the dminant phencryst phase, with lesser livine (fresh nly in glassy rims), clinpyrxene, and spinel. Shipbard descriptins indicate that clinpyrxene phencrysts are restricted t the interval 5.-87.5 m belw seaflr (mbsf) (Cres 48-896A-8R t R); hwever, we recvered grains f phencrystal pyrxene (high Mg# and Cr ) Plagiclase ccurs as anhedral t euhedral phencrysts that range frm 0. t 5 mm, with thse >0.9 mm interpreted t be megacrysts n the basis f crystal size distributin patterns (Shipbard Scientific Party, 99). Plagiclase ccurs as single crystals, in glmercrysts with livine and/r clinpyrxene, and may be included in livine and in the rims f sme clinpyrxenes. Red-brwn spinel and glass inclusins are cmmn. Plagiclase phencrysts cmmnly have striking ptical zning, and are bth nrmally and reversely zned. These zning patterns are described in detail by Shipbard Scientific Party (99). Plagiclase "cre" cmpsitins range frm An 78 t An 94 5 (Table ; Fig. ) and are similar t thse frm the extrusive sectin f nearby Hle 504B (Natland et al., 98). Hwever, mst samples hst plagiclase grains f up t An 9 _ 9 nly, and plagiclases f >An 9 were fund nly in Sample 48-896A-9R- (Piece 4). Despite the wide range f plagiclase cmpsitins in each sample, histgrams f An cntent (Fig. ) are characterized by well-defined maxima that are statistically indistinguishable frm the arithmetic means f An cntent fr each sample. T check if these maxima reflect cmpsitins f plagiclases in equilibrium with the hst glasses, the arithmetic mean f An cntent is pltted vs. the Ca/Na f the hst glass n Figure 4. Tw bservatins can be made: () samples frm each pillw-rim glass grup, with similar Ca/Na, are characterized by similar mean plagiclase cmpsitins; and () an increase in melt Ca/Na with increasing An is cnsistent with lw-pressure experimental data n plagiclase-melt equilibria. This indicates that mst plagiclases frm each sample are in equilibrium with their hst glasses. livine livine phencrysts were separated frm all samples except Sample 48-896A-4R- (Piece ), and representative analyses are given in Table 4. They generally cmpse <4% f the sample and range in size frm 0. t 5 mm. The livines are euhedral, cmmnly with skeletal vergrwths, and may include spinel, plagiclase, and glass. The phencrysts are unzned, with variatins f <l F unit frm cre t rim. As with the plagiclase phencrysts, livines als have a significant cmpsitinal range, frm F 80 t F 9, 6, and a well-defined distributin maximum. Hwever, unlike the plagiclases all samples used have this maximum at the same F f 87-89 (86% f analyses). The Mg# values f all seven glasses are very similar (64-65.7). Assuming an livine-melt Mg-Fe + K d f 0., liquids in equilibrium with livines ~F g8 (the mst abundant F f studied samples) shuld have an Mg' (= 00 Mg/[Mg + Fe + ]) f 68.8. T btain this result, the Fe-^/EFe f the studied glasses shuld be -0.7. This value is significantly higher than expected fr nrmal MRB (Christie et al., 986; Michael and Chase, 987), suggesting that the dminant livine cmpsitins (F 87 _ 89 ) are nt in equilibrium with the hst glasses and, mst likely, are crystallized frm mre primitive melts. Assuming a mre realistic Fe^/IFe f -9, the calculated livine in equilibrium with the studied glasses is F 8f 87. We nte als that plagiclase inclusins analyzed frm five livine phencrysts (F 88 _ 88 8 ) have a wide cmpsitinal range (An 86 _ 9. ); significantly, this range f cmpsitins ccurs in a single livine (see Table ). Spinel Spinels are a minr cmpnent f the phencryst assemblage in all samples examined, ccurring as phencrysts and inclusins in plagiclase and livine. Grain size ranges frm 0 t 50 µm. Representative analyses are included in Table 5. Spinels shw a well-defined
A.W. McNEILL, L.V. DANYUSHEVSKY Table. Representative plagiclase analyses, Hle 896A. Analysis n.: 4 5 6 7 8 9 0 Si AI Fe " Ca Na, K 0 44.58 4.6 0.7 0.0 8.9 0.75 0 99.6 46.8.77 0.8 0. 7.7.4 0 99.69 44.67 5.08 0.6 0.0 8.79 0.79 0 99.89 46.6 4.46 0.7 0.7 8.8.0 00.75 47.40.7 0.5 0.4 7.6.6 0 9 4.9 5.07 0.6 0.6 9.07 0.6 99.0 44.0 5.06 0.7 0.4 9.0 0.6 0 99.5 45.9 4.9 0.4 0. 8.0.7 99.4 47..56 0.5 0.40 6.4.80 99.07 46.6.7 0.4 0. 7.90.4 99.94 45.7 4.46 0.4 0.8 8.. 00.6 4 6.7.5 0.55 0.5 7.5.54 0 99.75 44.66 4.70 0.57 0.9 8.65 0.86 0 99.6 Catins calculated n the basis f 8 xygens Si Al Fe Mg Ca Na K An Ab r Mg#.075.899 0.944 68 00 5.00 9. 6.7 56.9.4.87 0.876 0. 00 4.996 88.8. 66.4.067.94 0.9 00 5.0 9.9 7. 49.8.4.860 0.89 0.5 5.04 88.5.5 56.5.59.80 9 6 0.859 0.4 00 5.007 85.8 4. 45.6.050.99 0.954 55 5.04 94.5 5.5 44..049.94 0 0.96 57 00 5.07 94.4 5.6 40..099.877 6 0.906 5 5.06 89.5 0.4 0. 54.7.9.779 8 0.86 0.6 0 5.000 8. 6.5 0. 57.4.5.84 0.885 0.7 5.0 87.4.5 0. 46.5.06.87 9 0.900 0 5.009 89.9 0 0. 59.5.4.86 0.868 0.8 00 5.0 86..7 44.8.074.900 0.98 7 00 5.05 9. 7.7 7. Ntes: Alphanumeric cdes fllwing sample numbers are identifiers f grain munt and grain number. = Sample 48-896A-9R- (Piece 4), A9-54 (hsts melt inclusin, Table 7). = Sample 48-896A-9R- (Piece 4), A9- (hsts melt inclusin, Table 7). = Sample 48-896A-7R- (Piece 5), A-6 (hsts melt inclusin, Table 7). 4 = Sample 48-896A-9R- (Piece 4), -5 (hsts inclusin 4, Table 7). 5 = Sample 48-896A-9R- (Piece 4), -8 (hsts inclusin 5, Table 7). 6 = Sample 48-896A-9R- (Piece 4), A9-58. 7 = Sample 48-896A-9R- (Piece 4), A9-. 8 = Sample 48-896A-7R- (Piece 5), A- (hsts spinel, Table 5). 9 = Sample 48-896A-7R- (Piece 5), A-9 (hsts spinel, Table 5). 0 = Sample 48-896A-4R- (Piece ), T- (hsts spinel, Table 5). = Sample 48-896A-4R-I (Piece ), T-5 (hsts spinel 4, Table 5). and = Sample 48-896A-7R- (Piece 5), A-88 (inclusin in livine I, Table 4). trend f decreasing Mg' with Cr# (= 00 Cr/[Cr + Al]) (Fig. 5A), similar t ther MRB suites (e.g., Sigurdssn and Schilling, 976), indicating that spinel crystallizatin accmpanied livine and plagiclase fractinatin (Allan et al., 988). Calculated Fe + /Fe + values f all spinels have a narrw range, with an average f. (Fig. 5B). Althugh there is a pssible trend f increasing Fe + /Fe + with increasing Cr#, we think that the average Fe + /Fe + is representative f this suite. Clinpyrxene Clinpyrxene is present as bth phencrysts and quenched crystals. Phencrysts range frm 0.5 t 7.0 mm lng, are subhedral, ften with a runded r crrded frm, and may partially enclse plagiclase phencrysts (Shipbard Scientific Party, 99). Representative analyses are given in Table 6. Melt Inclusins Melt inclusins that wuld be classified as primary, secndary, and pseud-secndary, using the criteria f Redder (984), ccur in all phencryst types. Inclusins in plagiclase and livine were used in heating experiments, and are described in detail belw. Primary inclusins ccur in -0% f plagiclase grains. They range frm t 50 µm acrss and vary in shape frm rund t runded negative crystal frms. The inclusins ccur in three main patterns: randmly distributed in the cres f crystals having inclusinfree rims; scattered thrughut the grain (ften arranged parallel t, and elngate alng, 00); and mre rarely, cncentrated in cncentric znes. Inclusins that lack fluid bubbles are rare, as are daughter crystals, althugh sulfide blebs ccur in sme larger inclusins. Rarely, inclusins are assciated with arcuate fractures in the hst plagiclase. There are sme dwnhle variatins in inclusin ccurrence; inclusins frm samples higher in the hle (Cres 48-896A-R, 4R, and 9R) are typically larger (mst large primary inclusins are 40-50 µm), mre runded, and ccur mre cmmnly randmly distributed thrughut the grains. Thse frm Cres 48-896A-5R and 7R are much smaller (generally <40 µm diameter), are assciated with mre fracturing than thse frm higher in the drilled sectin, ccur cncentrated in the cres f phencrysts, and are mre numerus. Planes f - t 5-µm runded inclusins, f larger irregularly shaped inclusins, sme f which are linked by healed fractures, and f large (>50 µm) irregularly shaped inclusins typically with crystallgraphic cntrl, are the three main types f secndary and pseud-secndary inclusin seen. The last type f inclusin cmmnly has a cmplex three-dimensinal frm and may cmprise a large cmpnent, by vlume, f the hst crystal. These inclusins may result frm rapid crystal grwth, represent glass jining tw crystals, r may result frm resrptin fllwed by rapid vergrwth; whatever the case, they are nt useful fr heating experiments and will nt be discussed further. Primary inclusins in livine range frm 0 t 500 µm in diameter, but are generally 0-00 µm, and were recrded in 70%-80% f grains, althugh mst phencrysts hst nly ne r tw inclusins. Mst inclusins lack vapr bubbles, have a rund t runded-tubular frm, and are clear green t brwn with nly rare bvius daughter crystals. Cmpsite inclusins (cnsisting f simultaneusly trapped glass and plagiclase) were fund in a single phencryst frm Sample 48-896A-5R-, Piece. There appears t be n dminant crystallgraphic cntrl n inclusin rientatin. Cmpsitins f primary inclusins in plagiclase, livine, and spinel, which were quenched during eruptin as glass r glass with a fluid bubble (naturally quenched melt inclusins), are given in Table 7. The bulk cmpsitins f these inclusins are variably mdified by pst-entrapment crystallizatin f vergrwths n the hst mineral. As can be seen n Figure 6A, variatins in A and cntent are much larger in naturally quenched inclusins than in pillw-rim glasses, and als define a trend perpendicular t that prduced by ctectic livine-plagiclase crystallizatin. Many inclusins in plagiclase have higher and lwer A cntents than the glasses, whereas inclusins in livine shw the ppsite effect (i.e., increasing A, decreasing ). These variatins are interpreted t result frm pst-trapping vergrwths n the walls f inclusins. The field f pillw-rim glasses verlaps a small segment f the trend defined by the inclusins. This implies that inclusins which have cmpsitins similar t the glasses are the least mdified by pst-trapping 4
PRIMARY MELT CMPSITIN AND CRYSTALLIZATIN R-, Pc.4 5 Mean = 87. 4R-, Pc. 0 5-0 - 5-0 - 9R-, I, Pc.4 5 I I p v. i :' vli 'Xvl.v.M 9 9 75 77 79 8 β 85 87 8θ An (%) 5R-, *n Pc. 5R-, Pc. 7R-. Pc. 7R-, Pc.5 I I f T VJ':':':'i T^T 75 77 79 8 8 86 87 An (%) 9 9 96 75 77 79 8 8 85 87 An (%) 9 9 96 Figure. Histgrams f plagiclase phencryst cmpsitin frm glass samples used in this study. Pc. = piece. crystallizatin, and deviatin f (r A ) frm the glass field is a measure f the extent f pst-trapping mdificatin. A simple calculatin demnstrates that the least magnesian inclusin ( 4 wt%) and the mst magnesian inclusin ( -6 wt%) are prduced by pst-trapping crystallizatin f -5 wt% livine and -45 wt% plagiclase, respectively. Whereas pst-trapping mdificatin des nt significantly affect Na cntents f inclusins we analyzed in livine, it has a dramatic effect n the Na cntent f inclusins in plagiclase (Fig. 6B) and thus Ca/Na values (Fig. 6C). When nly the least mdified inclusins (i.e., thse with similar t the pillw-rim glasses) are cnsidered, the range f Ca/Na values in inclusins in plagiclase is apprximately twice that shwn by the pillw glasses (Fig. 6D). Inclusins in plagiclase frm Grup samples have Ca/Na values that cver the entire range. Hwever, inclusins in plagiclase frm Grup have lw Ca/Na values, and inclusins frm Grup have relatively high Ca/Na values when cmpared with Grup glasses. Als, inclusins in livine (all frm Grup ) have Ca/ Na values that are similar t inclusins in plagiclase frm the same grup. There is a remarkable difference in the range f Ti cntents between inclusins in livine and plagiclase (Fig. 6E). The range in livine is the same as that in the glasses, whereas inclusins in plagiclase shw large variatins in Ti cntent that are independent bth f the degree f pst-trapping mdificatin and f the sample grup. Inclusins in bth livine and plagiclase shw a wide range f K cntents, several times that f the pillw-rim glasses (Fig. 6F), that are als independent bth f the degree f pst-trapping mdificatin and f the glass grup. Inclusins with least mdified cmpsitins (i.e., thse with similar t the glasses) are shwn
A.W. McNEILL, L.V. DANYUSHEVSKY 78 80 8 84 86 88 90 9 94 An (%) Figure 4. Crrelatin f plagiclase phencryst An cntents and Ca/Na values f equilibrium melts fr melt inclusins and pillw-rim glasses. t = Grup t glasses, respectively (symbls are the same as n Fig. ), 4 t 6 = experimentally reheated inclusins in plagiclase frm Grups t, respectively. Fr cmparisn the field f -atm experiments n MRB cmpsitins is als shwn (data frm Bender et al., 978; Duncan and Green, 987; Gaetani et al., 994; Grve and Bryan, 98; Grve et al., 98; and Walker et al., 979). separately n Figure 7. Variatins in K, Ti and, Na cntent f these inclusins are nt related t each ther r t the glass grup. Hwever, we nte that all ur analyzed high K inclusins have high Na cntents, althugh lw K inclusins with high Na cntent als exist. Inclusins in spinel d nt shw any unusual cmpsitinal features and are nt shwn n these plts as they verlap with the pillw-rim glasses. In summary, the cmpsitins f naturally quenched inclusins have a much wider range than the pillw-rim glasses, and inclusins in plagiclase shw much mre cmpsitinal scatter than thse in livine and spinel. We emphasize the large variatin in Ti cntents f inclusins in plagiclase, which is nt seen in inclusins frm livine and spinel and the pillw-rim glasses. In additin, sme inclusins in bth livine and plagiclase are characterized by unusually high K cntents. Inclusins in plagiclase interpreted t have undergne significant pst-trapping mdificatin are characterized by severe Na depletin and subsequent increase in Ca/Na values. The lw Na and high Ca/Na that are characteristic f many naturally quenched inclusins in plagiclase are nt matched by cmpsitins f inclusins in livine and spinel r cmpsitins f the pillw-rim glasses. A thrugh analysis f these features is beynd the scpe f this paper, hwever we nte that the results presented abve highlight the danger f using even incmpatible element ratis frm naturally quenched inclusins in plagiclase as petrgenetic indicatrs. Fluid Inclusins Fluid inclusins were fund in plagiclase phencrysts and indicate that the melt was fluid saturated during plagiclase crystallizatin. Hwever, n fluid inclusins were fund in livine phencrysts. Plagiclase Experimental Results Inclusins f 0-0 µm diameter were fund t be the mst useful fr experiments, as mst inclusins >0 µm diameter leaked during reheating (indicated by increased bubble size and T h ), whereas thse f <I µm diameter were t small t easily bserve the hmgenizatin prcess. All inclusins used fr experiments were glassy, with n bvius daughter crystals at rm temperature. n heating t 600-800 C, the inclusins darkened as daughter crystals frmed and bubbles appeared (if nne were present initially). In sme grains, cnsiderable fracturing was bserved at temperatures f >400 t 600 C. With increasing temperature, generally at >50 C, the clr f the inclusins became lighter again, as the daughter crystals started melting. Clser t T h, at >l 90 C, mst silicate daughter crystals within the vlume f the inclusin had melted, leaving a fluid bubble, and in many experiments a small, black, ~- t -µm diameter, Fe-Cu-Ni sulfide glbule (identified using EDS n the micrprbe). These sulfides ften calesced with the fluid bubble at temperatures clse t T h, making accurate determinatin f the exact temperature f bubble disappearance difficult. In large inclusins, the daughter crystals ften recrystallized t a single livine f ~ µm diameter that culd nt be remelted in the time available. n quenching a successful run, mst small (<5 µm diameter) inclusins remained glassy, with n bubble; hwever larger inclusins ften had a vapr bubble indicating that they were nt hmgenized. As the melt has been demnstrated t be fluid-saturated during plagiclase crystallizatin, then T h, which crrespnded t the disappearance f the fluid bubble, shuld equal the trapping temperature (Sblev and Danyushevsky, 994). Hmgenizatin was achieved in 40 runs; 8 inclusins were analyzed frm these runs. Hmgenizatin temperatures ranged frm 95 t 5 C. Based n these results, n crrelatin exists between T h and the hst plagiclase An cntent. Hwever, as the range f T h btained is within the errr f the technique used (+5 C), the existence r therwise f a crrelatin between T h and hst anrthite cannt be determined. Cmpsitins f hmgenized inclusins are shwn n Figure 8. The cmpsitinal range f majr elements verlaps that f the pillw-rim glasses but shws mre scatter. The cmpsitinal ranges f minr elements (Ti, K, and Na ) shw significantly mre variatin, with a similar range t the cmpsitins f least mdified naturally quenched inclusins (Fig. 7). As can be seen n Figures and 8 and as discussed in the mineralgical sectin, the cmpsitinal variatins f this suite are cnsistent with ctectic livine-plagiclase crystallizatin. As plagiclase phencrysts are in general in equilibrium with pillw-rim glasses, melt trapped by these plagiclases shuld be c-saturated with livine and plagiclase. Hwever, as is bvius frm Figure 8, the majr element cmpsitins f many inclusins lie ff the ctectic as defined by the glasses. Assuming that n chemical exchange ccurred between hst and inclusin after trapping, then three majr prcesses can affect the cmpsitin f an analyzed inclusin. verheating during the run (hmgenizatin btained at higher temperature than the temperature f trapping, a functin f the kinetics f melting) and/r analytical verlap with the hst plagiclase, caused by the uncntrlled thickness (depth) f inclusins being analyzed, will bth result in enrichment f the melt in plagiclase cmpnents. Pr quenching, particularly cmmn in small inclusins such as thse used in this study, will result in depletin f the melt in plagiclase cmpnents. livine and plagiclase mineral-melt gethermmeters are a useful tl that can reveal inclusin cmpsitins affected by these prcesses. In this paper, we use a calibratin by Danyushevsky et al. (in press), develped t estimate the livine-plagiclase ctectic, that uses the livine-melt gethermmeter f Frd et al. (98) and the plagiclase-melt gethermmeter f Weaver and Langmuir (990). A crrectin has been intrduced t the latter t btain a best fit f the livine-plagiclase ctectic ver the MRB cmpsitinal spectrum. Cmpsitins affected by trapping f the hst mineral during
PRIMARY MELT CMPSITIN AND CRYSTALLIZATIN Table 4. Representative livine analyses, Hle 896A. Analysis n.: 4 5 6 7 8 9 0 Si? Fe~ Mn Ca Cr Ni ' 40..5 0. 47.55 0. 6 0.6 99.70 4.46 0.5 47.58 0.5 0.6 99.89 40.5 0.6 0.0 47.80 0.0 0.8 99.77 40.45 0.96 47.68 0. 6 0.6 99.8 4. 0.5 46.84 0.55 0.6 0.0 00.6 40.8 0.59 0. 48.60 0. 0.7 00.0 40.40 0. 0.6 48.58 0.4 0. 99.86 40.7 8.5 0.5 50.6 0. 6 0.6 99.9 8.77 7.97 0.0 4.94 0. 0.7 99.4 9.6 4.04 0. 45.64 0.0 0. 00. Catins calculated n the basis f 4 xygens Si Fe Mµ Ca Mn Cr Ni F 0.997 0..757 09 0.00 88. 0.99 0.8.758 09 0.007 88..00 0.0.760 08 04 06.999 88.9.000 0.7.757 08 0.000 88.6 0.99 0.5.7 5 0 07 04.004 87. 0.99 0.8.78 08 04.009 89. 0.995 0.09.784 09 0 04.005 89.5 0.994 0.68.86 08 0.006 9.6 0.996 0.86.606 06 07 04.004 80.6 0.99 0.94.70 08.008 85. Ntes: Alphanumeric cdes fllwing sample numbers are identifiers f grain munt and grain number. I = Sample 48-896A-7R- (Piece 5), A-88 (hsts plagiclase and, Table ). = Sample 48-896A-7R- (Piece 5),A-4I (hsts melt inclusin 7, Table 7). = Sample 48-896A-R- (Piece 4), A4-0 (hsts melt inclusin 6, Table 7). 4 = Sample 48-896A-7R-I (Piece ), A0-40 (hsts melt inclusin 8, Table 7). 5 = Sample 48-896A-5R- (Piece ), A5-8 (hsts spinel 5, Table 5). 6 = Sample 48-896A- 9R- (Piece 4), A9-6 (hsts spinel 6, Table 5). 7 = Sample 48-896A-5R- (Piece ), A8-5. 8 = Sample 48-896A-5R- (Piece II), A8-8. 9 = Sample 48-896A-5R- (Piece ), A8-. 0 = Sample 48-896A-9R-I (Piece 4), A9-4. Table 5. Representative spinel analyses, Hle 896A. Analysis n.: 4 5 6 7 8 9 Si> Ti"? A, Fe^ Fe Mn Cr,, Ni ' Zn 0.45 5.6 6.77 4.9 0.6.9 8. 0.4 9 00.55 0. 0.9 7. 5.99 0.5 5 8.00 8. 0.4 9 00.74 0.0 4.85 5.64.0 0.4 7.08 9 0.9 6 99.5 9 0. 5.64 5.7 9.96 0. 7.98 0.7 0.9 0.4 99.96 0.6 0.57 4.96 6.57.4 0.5 5.49 8.80 99. 0.84 4.5 4.08 8.99 0.9 9.99 40.55 0. 0. 99.5 0. 0. 9.7 5.4 0. 0.4 8.74 7.8 0. 0.9 0.0 7.58 5.59 0.57 0. 8.0 8.0 0.0 00.87 0.8.0 5.87.80 0.0 5.07 4. 0.5 00.8 Catins calculated n the basis f xygens Si Ti Al Fe + Fe + Mn Ms Cr^ Xi Zn Mg' Cr# 9 7.70.7.009 4.997 7.0 7 6 6.4 50. 9 9.84.0.97 0 6.0 5.0 5 75..8 6 5 9.449 0.977. 7 5.858 5.47 5 0 7.4 6.7 0 6 9.57 0.97.89 6.086 5.44 5 76. 6. 9 7.09.94.50 5.580 7.4 6 69.0 5.0 08 0.66 4.444.785 4.7.9 8.44 48.4 65.5 4 5 0.7 0.894.856 6 6.9 4.75 06 76.8.5 50 9.9 0.94.978 5 6.0 5.008 6 75..6 9 5 6.54.064.580 5.44 8.55 9 67.7 55.S Ntes: Alphanumeric cdes fllwing sample numbers are identifiers f grain munt and grain number. Fe^^ calculated assuming stichimetry. = Sample 48-896A-4R- (Piece ), T- (inclusin in plagiclase 0, Table ). = Sample 48-896A-4R- (Piece ), T-5 (inclusin in plagiclase, Table ). = Sample 48-896A-7R- (Piece 5), A-9 (inclusin in plagiclase 9, Table ). 4 = Sample 48-896A-7R- (Piece 5), A- (inclusin in plagiclase 8, Table ). 5 = Sample 48-896A-5R- (Piece ), A5-8 (inclusin in livine 5, Table 4). 6 = Sample 48-896A-9R- (Piece 4), A9-6 (inclusin in livine 6, Table 4). 7 = Sample 48-896A-5R- (Piece ). A-57 (hsts glass inclusin 9, Table 7). 8 = Sample 48-896A-5R- (Piece ), A-. 9 = Sample 48-896A-7R- (Piece 5), A-. Mg'= 00 [Mg/(Mg + Fe + )]. analysis are identified by calculated plagiclase temperatures higher than the run temperature, whereas pr quenching will yield lwer calculated temperatures (Fig. 9A). Cmparisn f livine- and plagiclase-calculated temperatures can reveal inclusin cmpsitins that are the result f verheating r pr quenching (Fig. 9B). Sixty-three inclusins frm 9 runs were fund t satisfy the criteria discussed abve. Their 7), and cmpsitins are given in Table 8 and are cmpared with pillw-rim glasses n Figure 0. The remaining inclusins have a mre restricted majr element cmpsitinal range, cnsistent with livine-plagiclase ctectic crystallizatin, but they still shw wide variatins in minr element cntents. Gd crrelatin exists between Ca/Na values f inclusins and hst An cntents (Fig. 4), cnsistent with lw-pressure experimental results n plagiclase-melt equilibria and with plagiclase-hst glass relatinships. This indicates that the cmpsitins f hmgenized inclusins are in equilibrium with hst plagiclases. As with the leastmdified naturally quenched inclusins (Fig. 6D), hmgenized inclusins frm different grups vary cnsistently in Ca/Na ; inclusins frm Grup span the range f bserved Ca/Na values. The ranges in Ca/Na f hmgenized inclusins appear t be slightly smaller than thse f naturally quenched inclusins, but this may be related t sampling. In bth cases, Ca7Na values higher than thse in the pillw-rim glasses were fund. The trend f A and Ca vs. defined by inclusins is the same as thse f glasses; hwever, it appears that inclusins have higher Si and lwer Fe cntents at the same than the glasses. Higher Si cntents 7
A.W. McNEILL. L.V. DANYUSHEVSKY 70 65 60 55 45 40 5 0 Φ Δ F87 8 CD Inclusins in plagiclasβ Inclusins in livine Grundmass F88.5 F89 45 50 55 60 65 Mg 70 75 80 Table 6. Representative clinpyrxene analyses, Hle 896A. Analysis n.: Si n; AI Fe Mn Ca Na-,0 Cr 5.97 0.7.5.9 7.77.89 0. 0.4 00. T 50.60 0.56 4.48 6.0 0.5 5..7 0. 0.44 00.8 46.04.4 8.64 X.98 0.6..68 0.7 99.55 Catins calculated n the basis f 6 xygens Si Ti Al Fe Mn Mg Ca Na Cr En Is W Mg#.960 65 97 0 0.96 0.89 08 4.00 49. 5.0 45.7 90.9.866 6 0.95 0.86 0.84 0.884 6 4.0 44. 9.7 46. 8.9.77 0 0.84 0.8 0.688 0.877 7 4.04 7. 5. 47.4 70.8 4 50.49 0.60 4.74 6.40 0.8 5.8.98 0.8 0. 99.96.866 0.06 0.98 06 0.86 0.870 06 4.08 4.9 0.4 45.7 80.9 5 5.6 0.0.6 4.56 6.9.45 04 7 0.86 99.60.90 08 0.7 0.4 0 0.900 0.886 5 4.04 46.7 7. 46.0 86.5 B.5 " Ntes: Alphanumeric cdes fllwing sample numbers are identifiers f grain munt and grain number. = Sample 48-896A-9R- (Piece 4), A9-0. = Sample 48-896A-7R-I (Piece 5), A-8. = Sample I48-896A-R- (Piece 4), A4-4. 4 = Sample 48-896A-9R- (Piece 4), A9-6. 5 = Sample 48-896A-7R- (Piece 5), A-67. All Fe as Fe. Mg# = 00 [Mg/(Mg + Fe)]..5.5-0 5 40 45 50 Cr# 55 60 65 70 Figure 5. Cmpsitinal variatin f spinels. A. Cr# (00 Cr/[Cr + Al]) vs. Mg' (00 Mg/[Mg + Fe + ]) fr grundmass spinels and spinels included in livine (F cntents as indicated) and plagiclase. B. Calculated Fe + /Fe- + vs. Cr# fr all spinels. were als bserved in the least-mdified naturally quenched inclusins. livine Inclusins f < 00 µm diameter were selected fr experiments as it was fund that larger inclusins ften leaked during heating. A slwer heating rate was initially required than fr plagiclase t nucleate a fluid bubble at 750-900 C, and recrystallizatin f the inclusin ccurred frm 000 t 080 C; melting began at ~l 0 C. Unlike inclusins in plagiclase, hmgenizatin culd be achieved after keeping inclusins at a cnstant temperature fr mre than 0 min, indicating slwer melting kinetics in inclusins in livine. Fr example, after hmgenizatin f the smaller 0-40 µm inclusins, up t 0 min was required t hmgenize larger inclusins. After - min at T,,, small (<l µm) black minerals (mst likely magnetite, the result f xidatin; Sblev and Danyushevsky, 994) crystallized in all inclusins, althugh the relative amunt varied frm inclusin t inclusin. n quenching, mst inclusins were glassy, with scattered magnetite(?) dust and vapr bubbles in sme f the larger inclusins. Hmgenizatin was achieved in six experiments, with T h ranging frm 90 t 00 C fr hst livines f F 87y _ X87. As with plagiclase, n crrelatin exists between livine cmpsitin and T h, prbably because f the very narrw range f experimental temperatures and livine cmpsitins. Temperatures f livine saturatin calculated frm inclusin cmpsitins, using the calibratin f Frd et al. (98), agree well with experimental temperatures (Fig. ), indicating that equilibrium was btained during the runs and that n significant quench mdificatin ccurred. Cmpsitins f hmgenized inclusins are shwn n Figure. Cntrary t the mineralgical evidence, which suggested that livines f ~F 88 shuld be in equilibrium with mre magnesian melts than the hst glasses, the cntents f hmgenized inclusins are significantly lwer than thse f the hst glasses. Als Na, Ti, A, and Fe cntents f these inclusins differ significantly frm the field f pillwrim glasses, a feature nt bserved in the least-mdified naturally quenched inclusins. In additin, the Mg#s f these inclusins are far t lw t be in equilibrium with their hsts (Table 9). These cmpsitinal features may be prduced if T h is lwer that the trapping temperature. This wuld be the case fr a fluid-undersaturated melt; as n fluid inclusins were fund in livine phencrysts, we suggest that these inclusins were trapped frm such a melt. T btain the true cmpsitin f the trapped melt, mre livine shuld therefre be added t these inclusin cmpsitins. As magnetite precipitatin during the run culd intrduce additinal cmpsitinal variatins, we estimated the melt cmpsitin in equilibrium with livine phencrysts by simulating the reverse f livine crystallizatin n inclusin walls fr the least-mdified naturally quenched inclusins. Calculatins were perfrmed using the livine-melt mdel f Frd et al. (98) and a melt Fe + /Fe + f 7.9 (Fe^/LFe = 0.). This value was btained frm the average spinel Fe + /Fe + (.; frm Fig. 5B), using the equatin f Maurel and Maurel (98). Results are shwn n Figure. Calculated cmpsitins are cnsistent with their being mre primitive melts f the same suite as the hst glasses, with calculated equilibrium temperatures f 40-60 C.,X
PRIMARY MELT CMPSITIN AND CRYSTALLIZATIN Table 7. Representative analyses f naturally quenched melt inclusins, Hle 896A. Analysis n.: Plagiclase 4 5 6 livine 7 8 Spinel 9 Si Tiò A Fe Mn Ca Na 7 K7 P5 Cr 50.6 0.59 8.7.90 0.4.59.77. 5 5.9 0.6 9.0 9.4 0.0.47.88 0.7 9 50. 0.75.6.04.04.7.44 6 50.48 0.77 5.9 8.69 0.7 9..6.5 0. 50.6 0.99 5.6 9.89 0.7 7.9..87 0 49. 0.67 8.4 8.79 0. 6.6 4.59.78 6 50.5 0.66 7.66 8.87 5.90 4.74.7 0. 0.4 50.6 0.66 8.70 8.0 0.4.90 6.6.65 6 49.77 0.9 6.9 7. 9 9.98.4.88 0.4 99.65 00.7 00.75 99.60 99.85 00.4 00.50 0 00.59 Mg# Ca/ Hst 67..4 94.0 7. 9. 9. 66. 8.8 9.9 65.5 5.5 88.5 58.9 7.0 85.8 57.4 8. 88.9 54.4 8.6 87.8 46.6 9.9 88.6 7.6 7.0 Ntes: Alphanumeric cdes fllwing sample numbers are identifiers f grain munt and grain number. = Sample 48-896A-9R-I (Piece 4), A9-54 (hsted by plagilcase, Table ). = Sample 48-896A-9R- (Piece 4), A9- (hsted by plagiclase, Table ). = Sample 48-896A-7R- (Piece ), A-6 (hsted by plagiclase 4, Table ). 4 = Sample 48-896A-9R- (Piece 4), -5 (hsted by plagiclase 4, Table ). 5 = Sample I48-896A-9R- (Piece 4), -8 (hsted by plagiclase 5, Table ). 6 = Sample 48-896A-R- (Piece4), A4-0 (hsted by livine, Table 4). 7 = Sample 48-896A-7R- (Piece 5), A 4 (hsted by livine, Table 4). 8 = Sample 48-896A-7R-, Piece, A0-40 (hsted by livine 4, Table 4). 9 = Sample 48-896A-5R- (Piece ), A8-57 (hsted by spinel 7, Table 5). All Fe as Fe. Hst mineral cmpsitin (hst): anrthite = Ca/(Ca + Na) and frsterite = Mg/(Mg + Fe). As with inclusins in plagiclase, bth hmgenized and naturally quenched inclusins in livine have a larger range and extend t higher Ca/Na values than the pillw-rim glasses. Hwever, unlike inclusins in plagiclase, unusually lw Ti, Fe, and Na cntents have nt been fund in inclusins in livine. DISCUSSIN Cmpsitins f bth hmgenized and the least-mdified naturally quenched inclusins in plagiclase and livine have a significantly wider range f Ca/Na than the pillw-rim glasses, althugh there are sme crrelatins with the glass grup. All inclusins frm Grup have lwer Ca/Na than inclusins frm Grup ; hwever, thse frm Grup cver the entire range (Figs. 6, 0). This suggests that magma mixing may have played an imprtant rle during the evlutin f this suite. Mixing is als suggested by livine-plagiclase mineralgical relatinships, where plagiclases, interpreted t be in equilibrium with hst glasses (Fig. 4), are included in livines that are t magnesian t be in equilibrium with the same glasses, suggesting mixing f primitive and mre evlved melts. A detailed examinatin f evidence fr mixing will be presented elsewhere. Many inclusins in plagiclase and sme in livine have significantly higher K (up t 0.74%; Figs. 6, 0) than pillw-rim glasses. Higher K in plagiclase inclusins has als been reprted frm the extrusive sequence f Hle 504B (Natland, et al., 98). The reasns fr this are unclear. Hwever, unlike the suggestin f Natland et al. (98) that they may reflect kinetic effects during trapping, we favr a hypthesis linking high K cntent f sme inclusins with lcal cntaminatin by seawater-derived hydrthermal fluids. Investigatins f the H cntent f inclusins with variable K cntent are currently in prgress. Inclusins in plagiclase shw wide variatins in Ti, Si, and Fe cntents nt seen in the pillw-rim glasses and inclusins in livine and spinel. The Si cntents f inclusins in plagiclase are higher, whereas Fe and Ti cntents are lwer, in cmparisn with pillw-rim glasses. The ccurrence f these variatins nly in plagiclase suggest that they may result frm sme reequilibratin with plagiclase after trapping, althugh the mechanism f this prcess and why it des nt ccur in livine and spinel are nt knwn. The lack f crrelatin between Ti, Fe, Si, and Na and the independence between the magnitude f their variatins and cntents indicate n systematic pattern f these variatins and their independence frm degree f fractinatin. In cntrast t sme recent studies f melt inclusins in plagiclase frm ther MRB suites (e.g., Nielsen et al., 994), which suggest that such variatins, particularly in Ti, preserve primary melt increments befre aggregatin, we suggest that the evlved nature f these melts and the evidence fr mixing presented abve, make preservatin f such features in these samples unlikely. ur data suggest that plagiclase An 94 crystallized frm melts with majr element cmpsitins similar t the mst magnesian (-9.5% ) pillw-rim glasses but with higher Ca/Na values. It appears that variatins in Ca/Na values f Grup and melts are independent f ther element cncentratins and mainly reflect variatins in Na cntent. Thus, differences in cmpsitin f high- An plagiclase reflect differences in Ca/Na values f equilibrium melts rather than degree f fractinatin. Such melts were in equilibrium with livine F X6 _ 87, suggesting (especially as livines f up t ~F 89 5 were fund in all glass grups) that these plagiclases crystallized frm relatively evlved melts. This implies that an interval f livine-nly crystallizatin preceded ctectic livine-plagiclase crystallizatin. As ur experimental results with inclusins in livine have been shwn t be unreliable due t underheating and magnetite precipitatin, these inclusins cannt be used t characterize melt cmpsitins in the livine-nly field. Hwever, cmpsitins f primary melts can be estimated by calculatins f the reverse f livine crystallizatin frm the cmpsitin that crrespnds t the start f ctectic crystallizatin. Primary melts fr each grup can be defined as melts in equilibrium with the mst magnesian livine. Althugh F 9 i 6 was fund in Grup samples nly, we think that the similarity f mineralgical features between all grups indicates that this is a sampling effect. Cmpsitins f primary melts fr Grups and were calculated frm the cmpsitins f the mst primitive glasses frm these grups. Using the Petrlg prgram (Danyushevsky et al., 990), livine was added t the glass cmpsitins until it was in equilibrium with livine f F 96 (the mst primitive livine sampled). Melt Fe + /Fe +, required fr this calculatin, was fund t be 7.9 using calculatins as described abve. The resultant parental melts are given in Table 0. The range f Ca/Na values displayed by inclusins frm these tw grups, being independent f variatins in ther elements, shuld reflect variatins in Ca/Na values f primary melts fr
A.W. McNEILL, L.V. DANYUSHEVSKY 0. 0.4.β 0.8.0..4 Ti (wts) 0. 0.4.β 0.8.0..4 Ti (wt*) i. i.4 ΛΛ Na Figure 7. Tii vs. variatins in A. K and B. Na. C. Variatins in K vs. Na fr naturally quenched inclusins in plagiclase and livine, and pillw-rim glasses. Symbls as n Figure 6D. β 6 0 IS 4 lβ (wt*) é ' é ' lb' ill' ú ft ' it (wt*) Figure 6. Cmpsitins f naturally quenched melt inclusins in plagiclase and livine. vs. A. A, B. Na, and C. Ca/Na. Variatins in Na, A and Ca/Na define trends that are interpreted t reflect differing amunts f pst-trapping crystallizatin. D. Ca/Na vs., fr least mdified inclusins (i.e., thse with similar t that f the pillwrim glasses). vs. E. Ti and F. K. Ti in plagiclase, and K in bth plagiclase and livine, have a much wider range in the naturally quenched inclusins than in the pillw-rim glasses. Symbls are defined as fllws: = pillw-rim glasses; = inclusins in livine;, 4, and 5 = inclusins in plagiclase hsted by Grup,, and glasses, respectively. Pillwrim glasses n Figure 6D are subdivided using symbls as n Figure. each grup. Fr Grup, this range is 6.9 t 7.9; fr Grup, the range is frm 8. t 8.4. We have n evidence, hwever, t suggest that ther elements varied in primary melts fr these tw grups. Glasses frm Grup are mre evlved, making calculatins f the primary melt fr this grup impssible. Hwever, as Mg-Fe cvariatins f these glasses are indistinguishable frm thse f Grup, we think that the primary melt fr this grup had similar t Grup but lwer Ca/Na. By cmparisn with the data f Falln and Green (988), the estimated primary melts can be prduced by melting MRB mantle surces at -5 kbar. SUMMARY Glass samples frm pillw rims shw systematic cmpsitinal variatins with depth, particularly in Ca/Na and Ti ; hwever, they generally are characterized by high Ca and and lw Na and Ti, when cmpared with glasses frm nearby Hle 504B (Nat- land et al., 98). Water cntents (<. %) are cnsistent with ther MRB suites f similar Mg# and K. Seven samples were selected fr detailed mineralgical and melt inclusin studies n the basis f these majr element variatins. The phencryst assemblages f the selected samples are dminated by plagiclase An 78 _ 945, with lesser livine F 80 _ 9, 6, Cr-Al spinel, and clinpyrxene (Mg# = 85-9). A crrelatin between plagiclase cmpsitin and pillw-rim glass Ca/Na is interpreted t indicate their equilibrium. Textural and chemical crrelatins between majr phencryst phases and glasses suggest that the mst cmmn livine cmpsitins (F 87 _ 89 ) crystallized frm mre primitive melts than recrded in the pillw-rim glasses. Heating-stage experiments indicate trapping temperatures f 95-5 C fr melt inclusins in plagiclase phencrysts f An 8 _94. 5. The cmpsitins f these inclusins are cmparable t thse f the hst glasses, with the exceptin f anmalus Ti and K, and inclusin cmpsitins mre primitive than the mst magnesian glass ( = 9.5%) were nt fund. The Ca/Na f melt inclusins hsted by plagiclase f ~An 94 is -9.5, which is cnsiderably lwer than the value required by current experimental data t crystallize plagiclase f >An 9() (i.e., Ca/ Na >0). Crystallizatin f plagiclase An 94 ccurred frm relatively evlved melts, the result f ~ 5% livine-nly fractinatin. Primary melts fr this suite had 5 wt%; these melts are characterized by variable Ca/Na values but relatively cnstant cntents f ther elements. Such melts can be prduced by melting MRB mantle surces at ~ 5 kbar. ACKNWLEDGMENTS We wish t thank Wieslaw Jablnski, Graham Rwbttm, and Keith Harris fr technical assistance. D. Christie and A. Sblev are als thanked fr helpful reviews. The authrs were supprted by an Australian Research Cuncil fellwship t L.V.D., and an Australian 0
PRIMARY MELT CMPSITIN AND CRYSTALLIZATIN _0.β- Δ é i (wtx) (wtx) t<u 8 Qft 8q (wt*) Δ 0 0 Figure 8. Cmpsitinal variatins f all experimentally hmgenized inclusins in plagiclase. Pillwrim glasses are pltted fr cmparisn. Symbls as fr Figure 6A. B» : 80 40-0- 60- i 0 If / ó σ' 60 80 00 0 40 T PL re) Figure 9. A. Calculated plagiclase temperatures (using the mdified plagiclase-melt gethermmeter f Weaver and Langmuir, 990) vs. run temperature fr experimentally hmgenized inclusins in plagiclase. B. Calculated livine temperature (using the gethermmeter f Frd et al., 98) vs. calculated plagiclase temperature fr all hmgenized inclusins in plagiclase. Slid symbls represent inclusins thught t be representative f trapped melt cmpsitins. T CALC = calculated temperature, T RUN = run temperature, L = livine, and PL = plagiclase. See text fr discussin. pstgraduate research award t A.McN. Funding fr the research was thrugh an Australian Research Cuncil Large Grant t Dr. A.J. Crawfrd. REFERENCES Allan, J.F., Sack, R.., and Batiza, R., 988. Cr-rich spinels as petrgenetic indicatrs: MRB-type lavas frm the Lamnt seamunt chain, eastern Pacific. Am. Mineral., 7:74-75. Andersn, A.T., Jr., 974. Evidence fr a picritic, vlatile rich magma beneath Mt. Shasta, Califrnia. J. Petrl., 5:4-67. Aument, F., Melsn, W.G., et al., 977. Mt. Repts. DSDP, 7: Washingtn (U.S. Gvt. Printing ffice). Bender, J.F., Hdges, F.N., and Bence, A.E., 978. Petrgenesis f basalts frm the Prject FAMUS area: experimental study frm 0 t 5 kbars. Earth Planet. Sci. Lett, 4:77-0. Byers, CD., Garcia, M.., and Muenw, D.W., 986. Vlatiles in basaltic glasses frm the East Pacific Rise at N: implicatins fr MRB surces and submarine lava flw mrphlgy. Earth Planet. Sci. Lett., 79:9-0. Christie, D.M., Carmichael, I.S.E., and Langmuir, C.H., 986. xidatin states f mid-cean ridge basalt glasses. Earth Planet. Sci. Lett., 79:97-4. Clcchiatti, R., 977. Les liquides silicates pièges dans les cristaux d'livine, de plagiclase et de pyrxene: prises d'essai du magma. Applicatin à un basalt à affinitie thleiitque de la ride ceanique émergée d'asal (T.F.A.I.). C.R. Acad. Sci. Ser., 84:0-06. Danyushevsky, L.V., Chizhv, S., Kuzmin, V., Pugachv, R., and Sblev, A.V., 990. Petrlg v..0: Mscw (SeLeSft C.). Danyushevsky, L.V., Falln, T.J., Sblev, A.V., Crawfrd, A.J., Carrll, M., and Price, R.C., 99. The H cntent f basalt glasses frm Suthwest Pacific back-arc basins. Earth Planet. Sci. Lett., 7:47-6. Danyushevsky, L.V., Sblev, A.V., and Dmitriev L.V., 988. rthpyrxene-bearing lw-ti thleiites as a new type f mid-cean ridge thleiite. Trans. USSR Acad. Sci., Earth Sci. Sect., 9:0-05., in press. Estimatin f the pressure f crystallizatin and H cntent f MRB glasses: calibratin f an empirical technique. Mineral. Petrl. Dnaldsn, C.H., and Brwn, R.W., 977. Refractry megacrysts and magnesium-rich melt inclusins within spinel in ceanic thleiites: indicatins f magma mixing and parental magma cmpsitin. Earth Sci. Planet. Lett., 7:8-89. Duncan, R.A., and Green, D.H., 987. The genesis f refractry melts in the frmatin f ceanic crust. Cntrib. Mineral. Petrl, 96:6-4. Falln, T.J., and Green, D.H., 986. Glass inclusins in magnesian livine phencrysts frm Tnga: evidence fr highly refractry parental magmas in the Tnga arc. Earth Planet. Sci. Lett, 8:95-0., 988. Anhydrus partial melting f peridtite frm 8 t 5 kbar and the petrgenesis f MRB. J. Petrl., 9:79-44. Fisk, M.R., 984. Depths and temperatures f mid-cean ridge magma chambers and the cmpsitin f their surce. In Gass, I.G., Lippard, S.J., and Sheltn, A.W. (Eds.), philites and ceanic Lithsphere. Gel. Sc. Spec. Publ. Lndn, :7-. Frd, C.E., Russell, D.G., Craven, J.A., and Fisk, M.R. 98. livine-liquid equilibria: temperature, pressure and cmpsitinal dependence f the crystal/liquid catin partitin cefficients fr Mg, Fe +, Ca, and Mn. J. Petrl, 4:56-65. Gaetani, G.A., Grve, T.L., and Bryan, W.B., 994. Experimental phase relatins f basaltic andesite frm Hle 89B under hydrus and anhydrus cnditins. In Hawkins, J., Parsn, L., Allan, J., et al., Prc. P, Sci. Results, 5: Cllege Statin, TX (cean Drilling Prgram), 557-56. Grve, T.L., and Bryan, W.B., 98. Fractinatin f pyrxene-phyric MRB at lw pressure: an experimental study. Cntrib. Mineral. Petrl, 84:9-09. Grve, T.L., Gerlach, D.C., and Sand, T.W., 98. rigin f calc-alkaline series lavas at Medicine Lake Vlcan by fractinatin, assimilatin and mixing. Cntrib. Mineral Petrl, 80:60-8. Haskall, K., Frsyth, L., Nielsen, R.L., and Fisk, M.R., 99. Experimental cnstraints n the parental magma fr the high-an feldspar bearing Lamnt seamunt lavas. Es, 74:57. Jarsewich, E.J., Nelen, J.A., and Nrberg, J.A., 980. Reference samples fr electrn micrprbe analysis. Gestand. NewsL, 4:4-47. Jhnsn, K.T.M., Fisk, M.R., and Naslund, H.R., 995. Gechemical characteristics f refractry silicate melt inclusins frm Leg 40 diabases. In