46. MAGNETIC PROPERTIES OF BASALT SAMPLES FROM HOLES 504B AND 505B ON THE COSTA RICA RIFT, DEEP SEA DRILLING PROJECT LEGS 69 AND 70 1

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1 46. AGNETC ROERTES OF BASALT SALES FRO HOLES 504B AND B ON THE COSTA RCA RFT, DEE SEA DRLLNG ROJECT LEGS 69 AND 70 1 Tshi Furuta, Ocean Research nstitute, University f Tky, Tky, Japan ABSTRACT re than 60 basalt samples frm tw Deep Sea Drilling rject hles n the Csta Rica Rift were studied fr magnetic prperties and were fund t have n prperties significantly different frm ther DSD basalts. Opaque mineralgical and thermmagnetic prperties f these samples, hwever, t sme extent shw differences frm nrmal submarine basalts; a new type f thermmagnetic curve and wide range f chemical cmpsitins were recgnized. Oxidized samples pssibly cntaining incipient ilmenite exslutin lamellae were reduced and re-equilibrated during heating. The Curie temperatures f the re-equilibrated titanmagnetites are interpreted t be thse f the riginal crystallized phase befre xidatin. NTRODUCTON This wrk presents the results f magnetic and paque-mineral studies f basaltic rcks recvered frm Hles 504B and B during DSD Leg 69 and Leg 70 in the Csta Rica Rift area (Fig. 1). Site 504 is lcated in an area f unifrmly high heat flw. Site, n the ther hand, is lcated in an area where heat flw is highly variable, but n the average much less than that predicted by thery (Langseth et al., this vlume). The ages f Sites 504 and are estimated t be 5.9 m.y. and 3.9 m.y., respectively, based n the magneticanmaly chart cnstructed during the pre-drilling survey (Langseth et al., this vlume). The cntrasts between the tw sites raise sme interesting questins: (1) Hw different are the magnetic prperties f basement rcks between the high-heat-flw and lw-heat-flw areas? (2) Hw much are the magnetic prperties f basalts in the hles affected by hydrthermal alteratin? n Hle 504B, the effects f hydrthermal alteratin n the basalts were evident in the frm f secndary minerals bserved during shipbard petrgraphic study. agnetic and paque-mineral studies f Hle 504B basalts indicate that they represent relatively yung ceanic crust in the eastern acific. Detailed paque-mineral examinatin indicates that the Csta Rica Rift basalts have unique prperties in mineralgy and rck magnetism. EXERENTAL ROCEDURES The samples used fr labratry magnetic measurements were the same as thse used fr palemagnetic measurement n bard Glmar Challenger during Leg 69. Hwever, the samples recvered during Leg 70 are nt the same as thse treated by the shipbard party. Saturatin magnetizatin and cercive frce were measured by a vibratin magnetmeter with a strng field f 15 koe, which is sufficient t saturate these ceanic basalts. Thermal changes in saturatin magnetizatin were measured with a hrizntal beam magnetic balance, n which a small chunk f rck (~ mg) was heated at 6 C/min in a magnetic field f 4.5 koe. The samples were cled t rm temperature at the Cann, J. R., Langseth,. G., Hnnrez, J., Vn Herzen, R.., White, S.., et al., nit. Repts. DSD, 69: Washingtn (U.S. Gvt. rinting Office). same rate at which they were heated. The cycle f heating and cling was accmplished in a vacuum f 10~ 5 t 0" 6 trr. Sme samples were measured in air t examine the differences between thermal behavir under different xygen pressures. The paque minerals in the basalts were examined in plished thin-sectins using a reflectedlight micrscpe, and the chemical cmpsitins f the minerals were determined with tw electrn-prbe micranalyzers, JXA-5 (JEOL) and EX-S (Shimadzu C. Ltd.). EXERENTAL RESULTS The results f the magnetic measurements fr individual samples are listed in Tables 1 and 2 in rder f increasing depth in each hle. n this sectin, we present the data in terms f the vertical distributin f the magnetic prperties in the hles. Saturatin agnetizatin, J s The bserved whle-rck saturatin magnetizatin, / s, ranges frm emu/g t 54 emu/g. These values are nt significantly different frm thse f the ther DSD basalts (Ade-Hall et al., 1976; Day et al., 1978; Haman et al., 1979) and shw very limited variatin. The saturatin magnetizatin represents, t a first apprximatin, the amunt f ferrmagnetic minerals in the samples, s that variatin in / s culd indicate variatin in cntent f ferrmagnetic minerals in each sample. The initial susceptibility, x, als represents the cntent f ferrmagnetic minerals induced by weak field, s the relatin between / s and is theretically a simple prprtin. The relatinship between them fr the Hle 504B basalts is shwn in Figure 2, but there is n gd crrelatin between them. Saturatin Remanent agnetizatin, / sr The bserved whle-rck remanence f saturatin magnetizatin, J ST, ranges frm t emu/g. n general, saturatin remanent magnetizatin depends upn the magnetic-mineral cntent and grain size. The rati J ST /J $ is a simple functin f the grain size, independent f magnetic cncentratin. The J S /J S rati ranges frm t 0.468; such a small variatin is due t the small range f grain sizes in the Hle 504B 711

2 T. FURUTA 86 W 82 W 80 W 78 W - 4"S - 6 S 88 W Figure 1. Lcatin f Sites 504 and, near the Csta Rica Rift. 84 W 82 W 80 W 78 W basalts. J ST and J a are rughly prprtinal; as / sr increases, J n als increases (Fig. 3). Cercive Frce, H c, and Remanent Cercive Frce, H K Cercive frce, H c, and remanent cercive frce, H TC, range frm 34 t 450 Oe and frm 75 t 550 Oe, respectively, shwing a wide range f bulk cercivity. H c and DF shw a gd, nrmal crrelatin fr these samples (Fig. 4). Day et al. (1977) demnstrated that a value f H C less than 4.0 is btained frm samples whse titanmagnetites are pseud-single-dmain r single-dmain crystals. The titanmagnetites in all samples frm Hle 504B shuld be f pseud-single r single dmain, as their H C values are all less than 4.0. Thermmagnetic easurement Thermal changes in saturatin magnetizatin were measured n 79 samples frm Hle 504B, and 8 samples frm Hle 5O5B. The results are listed in Table 2. The 712

3 AGNETC ROERTES OF BASALTS, HOLES 504B AND 5O5B Table 1. agnetic prperties f Hle 504B basalts. Sample (interval in cm) 5O4B-3-1, , , , , , , , , , , , , , , , , SA, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , 4-6 Λ (10-3 G) x (0-4 G) DF (Oe) U (emu/g) (emu/g) (Oe) #rc (Oe) H C Rck Type? Ntes: / n = intensity f natural remanent magnetizatin; = initial susceptibility; DF = median destructive field f NR; Js = saturatin magnetizatin; 7sr = saturatin remanent magnetizatin; Hc = cercive frce; HC - remanent cercive frce; rck type: massive flw, = pillw lava. listed parameters are T cl, the first Curie temperature bserved during heating: T c2, the highest Curie temperature during heating; T ch, the secndary Curie temperature during cling; and J h /J 0, the rati f the saturatin magnetizatin bserved at rm temperature and after and befre heating. There are fur types f thermmagnetic curves; a thermally reversible type (Fig. 5A; type ), and three irreversible types (Fig. 5B-D, types -V). Type shws a reversible curve, and its Curie temperature may represent riginal r slightly xidized titanmagnetite f the samples. Type cmmnly is bserved amng submarine basalts (Fig. 5B). Titanmagnetite f type is xidized at lw temperatures, and its curve has a typical dent which has been explained as the inversin pint f titanmaghemite (Ozima and Ozima, 1971). Type is similar t Type ; it has a similar hysteresis, but cling results in very different behavir. The final J s, dented by / h in Table 2, des nt increase by mre than nehalf f the initial / s, dented by J in Table 2 (Fig. 5C). This hysteresis is nt cmmn in submarine basalts. Type V behaves as a single phase during heating, with the Curie temperature arund 400 C, and withut the dent cmmnly bserved in samples xidized at lw temperature (Fig. 5D). During cling, / s slwly increases with T ch f arund C. Fr reversible type, the Curie temperature is easily determined, as shwn in Figure 5A. The Curie temperature f this type slightly decreases after heating in sme cases. n the upper part f Hle 504B, type behavir was fund in nly tw f 62 samples. Althugh there are few samples frm Hle B, half f all samples were recgnized as type. st f the ther samples were irreversible, as cmmnly fund fr basalts xidized at lw temperatures. Amng the irreversible samples f type, cling results in a steep increase in J s, as seen in typical ceanic basalts (Fig. 5B) and in sme terrestrial vlcanic rcks (Furuta et al., 1980). Althugh type H samples shw curves similar t thse f type samples during heating, their cling curves shw a very slw increase in / s, with a cnvex curve, cmpared with the heating curve (Fig. 5C). rever, a new type f thermal curve is recgnized: / s gradually decreases as temperature increases up t C, but J s stays nearly the same while temperature cntinuusly increases frm t C. After the temperature exceeds C. / s decreases again (Fig. 6). These samples seem t have variable cmpsitin f 713

4 T. FURUTA Table 2. Thermmagnetic and paque-mineral prperties f basalts frm Hles 504B and B. Sample (interval in cm) 5O4B-3-1, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , B-2-1, , , , , , , ,6-8 Td Tc Teh Jh/J Type V V V V V V V V x Values ± ± ±0.021 /0.641 ±0.022\ ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± / / ± Grain Size Ti/Fe l Rati f lmenite Ntes: TQ\ = first Curie temperatures bserved during heating; TC2 = highest Curie temperatures bserved during heating; rcn = secnd Curie temperatures bserved during cling; Jfr/J - rati f saturatin magnetizatin bserved at rm temperature after and befre heating, temperatures in C; type (thermmagnetic curves explained in text); grain size = averages f largest 10 grains; x values = value in the frmula xfe2tic 4 (l - x)fe3θ4; uncertainty f x value represents ne standard deviatin; Ti/Fe ml rati f ilmenite = atmic rati f discrete ilmenite grains X Type samples Type 1 samples Type samples 0 s O 1 X X t * J. r (emu/g) 8 Figure 2. Relatinship between initial susceptibility,, and saturatin magnetizatin, J s, fr basalts frm Hle 504B. 0.0 Figure 3. Relatinship between intensity f NR, /, and saturatin remanent magnetizatin, 7 sr, fr basalts frm Hle 504B. Symbls as in Figure Figure 4. Crrelatin between median destructive field, DF and cercive frce, H c, fr basalts frm Hle 504B. Symbls as in Figure

5 AGNETC ROERTES OF BASALTS, HOLES 504B AND 5O5B 504B-4-2, cm 504B-8-4, cm rπ< c) r(i c) r i. r 504B-62-1, cm r{i Figure 5. rincipal types f thermmagnetic curves. A. Type, nearly thermally reversible. B. Type, typical irreversible curve bserved fr basalts xidized at lw temperature. C. Type, thermally irreversible, with lw r ch. D. Type V, thermally irreversible with high T ci and lw T ch. titanmagnetites whse Curie temperatures range frm t C. The thermal curve fr an air measurement f this sample is als shwn in Figure 6. The curve has tw bends, at temperatures f and C, and the final / s (J h ) is similar t the initial J s (/ 0 ). T ch in air is higher than in vacuum, and its value is the same as the Curie temperature f magnetite. The ratis f J h /J 0 calculated frm the thermal analysis curves are listed in Table 2. These ratis fr type samples are arund 1, fr type samples the ratis are larger than 1, and fr type and V samples the ratis are near r less than 1. Ratis greater than 2, as fund fr example amng typical samples xidized at lw temperature, were nt bserved in any samples frm Hle 504B. icrscpic Observatin f Opaque inerals Opaque minerals were examined in plished thin-sectins using a reflected-light micrscpe. Titanmagnetite is distinguished frm ilmenite and irn sulfide by its clr r anistrpy. Titanmagnetite xidized at lw temperature (titanmaghemite) can be distinguished 715

6 T. FURUTA "b 0.0 n vacuum (10 & trr) n air 504B-6-2, cm Figure 6. A new type f thermmagnetic curve. The intensity f saturatin magnetizatin apparently stays the same during temperature increase frm t C. frm nn-xidized titanmagnetite by its clr. Hwever, because mst grains f titanmagnetite in these samples are very small, we culd nt find bvius differences between titanmagnetite and titanmaghemite under the micrscpe. There apparently are different grain sizes and crystal shapes in the same sectin f sme samples, which can be classified int grups. One grup cnsists f relatively large, skeletal r herring-bne-shaped crystals (Fig. 7), and the ther cnsists f small, platey crystals. These samples shw a new type f thermmagnetic curve, suggesting a mixture f wide cmpsitinal ranges derived frm different crystallizatin stages f titanmagnetites (e.g., 504B-7-5, 3-5 cm). Data frm micrprbe analysis supprt these implicatins. Visually estimated grain size listed in Table 2 is the average f the largest 10 grains bserved in the sample micrscpically. Accrding t ther magnetic measurements, such as / s, J ST, H c, and H TC, these might nt represent the true magnetic grain size, because titanmagnetites ften are divided int several magnetic dmains by the develpment f cracks resulting frm xidatin (Jhnsn and Hall, 1978) and exslutin f fine ilmenite lamellae. Chemical Analysis f Opaque inerals The Ti/Fe rati in titanmagnetite and ilmenite was determined with tw different electrn-prbe micranalyzers. Chemical cmpsitins f titanmagnetite and ilmenite were measured n 19 samples frm Hle 504B and five samples frm Hle B. The cmpsitins f mre than 20 grains per specimen were determined, and the values were averaged (Table 3). The cntents f ther elements besides Fe and Ti were measured als in the same grains. The atmic rati f Ti/Fe was btained, and the x value f the ulvspinel-magnetite slid slutin [xfe 2 TiO 4 (l -.x)fe3θ4] r its xidized prduct was calculated n the assumptin f a stichimetric ulvspinel-magnetite series. Detailed analysis f a few samples indicated that the cntents f ther elements are less than a few percent. n general, it is well knwn that the x value f submarine basalts has a small variatin and is arund the value f 0.6. Jhnsn and Hall (1978) demnstrated that the x value f nn-xidized r xidized titanmagnetites f submarine basalts frm the acific Ocean and the Atlantic Ocean des nt vary significantly frm an average value f 0.62 ± The x value f titanmagnetites in sme samples frm Hle 504B is t sme extent different frm the nrmal value fr submarine basalts. Fr example, Sample 504B-6-2, cm cntains titanmagnetites with a wide range f x values, frm 0.3 t, and these values vary cntinuusly (Fig. 8). The thermmagnetic curve f this sample als suggests a wide range f cmpsitins f titanmagnetites. Crrelatin between A1 2 cntent (representative f minr cmpnents in titanmagnetite) and the x value f titanmagnetites in this sample is gd. As the x value increases r titanium cntent increases, the A1 2 cntent decreases, whereas as the x value decreases, no and go cntents increase. The titanmagnetites in Sample 504B-7-5, 3-5 cm are divided int fur x value grups; grups and have a lw A: value, arund 0.43, but these tw grups are subdivided clearly n the basis f different cntents f A1 2. Grup has a nrmal value f submarine basalts, arund Grup V resembles pure magnetite. The results f chemical analysis f Sample 504B-7-5, 3-5 cm are listed in Table 3 and shwn in Figure 9. DSCUSSON We discuss here tw main prblems cncerning magnetic, thermmagnetic, and paque-mineral prperties. alemagnetic and Rck agnetic rperties The results given in Table 1 and Figures 2 thrugh 4 indicate that these basalts are nt significantly different frm ther DSD basalts. The averaged magnetic results f bth pillw lavas and massive flws are listed in Table 4. Other magnetic prperties, the intensity f natural remanent magnetizatin (/ ), and the initial susceptibility ( ), based n shipbard data, are discussed in Furuta and Levi (this vlume). f the grain size f magnetic minerals is f a single dmain, the intensity f NR is prprtinal t the saturatin magnetizatin. As shwn in Figure 10, there is n crrelatin between J n and J s. This suggests that the grain size f these samples is larger than that f single dmain. Frm examinatin f J ST /J S versus H C (Fig. 11), hwever, the grain size f these samples may be f single-dmain r pseud-single-dmain structures. Therefre, the intensity f NR might be reduced because f strng alteratin, s that the crrela- 716

7 AGNETC ROERTES OF BASALTS, HOLES 504B AND 5O5B 20 µm 40µm 40 µm 20µm Figure 7. htmicrgraphs f typical paque minerals in reflected light. A. Typical titanmagnetite in pillw basalts. These grains were sufficiently xidized at lw temperature (504B-27-2, cm). B. Typical titanmagnetites in massive flw basalts (504B-8-1, cm). C. An example f different crystallizatin f titanmagnetites in the same sample, large grain (right) and small grains (left) (504B-6-2, cm). D. C-existence f hem-ilmenite (light clr) and apparent magnetite (dark clr) (5O4B-7-5, 3-5 cm). Table 3. Chemical cmpsitin f titanmagnetites (analyses 1-21) and discrete ilmenites (analyses 22-23) in Sample 504B-7-5, 3-5 cm. arameter Analysis ean f 1-6 (Grup ) 7 10 ean f 7-10 (Grup ) A1 2 TiO 2 FeO* Ti/Fe (ml %) x values arameter ean f (Grup ) ean f (Grup V) ean f lmenites A1 2 TiO 2 FeO* Ti/Fe (ml %) x values tin between J n and / s is dispersed by the effects f weathering, mainly hydrthermal alteratin and lwtemperature xidatin. The relatinship between J n and 7 r is rughly prprtinal (Fig. 3). 7 sr depends n grain size and shape, because the grain size decreases (i.e., J T increases) as J n increases. Frm the results f tw relatinsj n versus J ST and J n versus / s the intensity f NR f the present samples appears t depend mre upn the grain size than magnetic cncentratin. The effects f magnetic-grain size n varius magnetic prperties were demnstrated by Day et al. (1978). Accrding t their results, the rati f H TC is ne f 717

8 T. FURUTA 1 - Ti/Fe iθ B-6-2, cm θ'.3 seems t be f pseud-single r single-dmain size. Figure 11 shws the relatinship between J ST /J S and H TC ; the data reprted by Day et al. (1978) are als pltted in this figure. The values seem t shift left parallel t synthetic-titanmagnetite values. The cause f this shift is nt yet clear, but xidatin might be respnsible fr this behavir. N crrelatins between J n and visual grain size determined with the micrscpe culd be fund (Fig. 12). There is nly ne weak crrelatin: J n nrmalized by J s decreases as visual grain size increases. Finally, DF and cercive frce are imprtant t determine, in rder t define the palemagnetic stability. Figure 4 shws the strng crrelatin between DF and H c, indicating that the data frm Hle 504B are sufficiently reliable t use fr discussin f palemagnetic prblems. Figure 8. Variatin f x value (r Ti/Fe rati) with cntent f A1 2 fr titanmagnetites in Sample 504B-6-2, cm Ti/Fe Figure 9. Variatin f x values (r Ti/Fe rati) with cntent f A1 2 fr titanmagnetites in Sample 504B-7-5, 3-5 cm. Data are divided int fur grups n the basis f x value and cntents f A1 2. Grup number crrespnds t that in Table 3. the factrs suitable fr determinatin f magnetic-grain size. When the rati f H TC is less than, the grains are single-dmain. The grains whse value f this rati is mre than 4 are multi-dmain, and their critical size is a value f abut 30 µm t 40 µm fr x = 0.6 f titanmagnetite. The rati f H TC f Hle 504B basalts is less than 4, s that the magnetic minerals in these basalts Thermmagnetic and Opaque-ineral rperties There are fur types f thermmagnetic hysteresis amng the present samples, as discussed earlier. Thermmagnetic curves can be used t estimate the kinds f ferrmagnetic minerals that are cntained in the samples. Frm the thermal hysteresis curves, we can cnveniently btain imprtant infrmatin, such as Curie temperature and xidatin state f individual samples. The prprtin f samples with thermally reversible curves (type ) is different in the tw hles. Only tw f 62 samples frm the upper part f Hle 504B are type samples, whereas fur f 8 samples f Hle B are type. This difference suggests that the basalts f the sites have experienced different cnditins f xidatin. Site 504 and are lcated respectively at high- and lw-heat-flw areas. f the surce f hydrthermal circulatin stays fixed relative t basement rcks, which migrate with plate mtin, basalts in high-heat-flw areas shuld always be subjected t alteratin by hydrthermal circulatin (Karat and Becker, in press). st f the basalts frm the upper part f Hle 504B were xidized at lw temperature; the cause f xidatin may be nt nly greater crustal age than thse f Hle 5O5B, but als a higher activity f hydrthermal circulatin. Basalts frm the lwer part f Hle 504B have nt undergne lw-temperature xidatin. This difference f xidatin in basalts f Hle 504B may indicate the different nature f alteratin, that is, the alteratin in the upper part f the hle was xidative, whereas that f the lwer part was nn-xidative (CRRUST, in press). Sme samples have curves similar t thse f type up t C, but beynd C / s remains cnstant until C (Fig. 6). This new type f thermal curve previusly has nt been fund in any submarine basalts. t suggests that the titanmagnetites in these samples have Table 4. Averaged magnetic prperties f Hle 504B basalts. /n Rck Type N (10~ 3 G) X (10~ 4 G) Js (emu/g) ^sr (emu/g) ^sr/ ^s He (Oe) "re (Oe) H TC illw lava ± 6.4 assive flw ± ± ± ± ± ± ± ± ± ±90 68 ± ± 128 ± 28 8 ± ±0.25 Ntes: N= number f samples; / n = intensity f NR; x = initial susceptibility; / s = saturatin magnetizatin; J sr = saturatin remanent magnetizatin; H c = cercive frce; H TC = remanent cercive frce; uncertainty represents ne standard deviatin. 718

9 AGNETC ROERTES OF BASALTS, HOLES 504B AND 5O5B J. (emu/g) Figure 10. Relatinship between intensity f NR, J n, and saturatin magnetizatin J s, fr basalts frm Hle 504B. Symbls as in Figure 1.. >Vc Figure 11. Relatinship between J ST /J S and H TC fr basalts frm Hle 504B. Circled crsses represent data frm synthetic titanmagnetites (x = 0.6) after Day et al. Other symbls as in Figure 1. l:4µm, 2: µm, 3: 3.3 µm, 4: 6.4 µm, 5: 12 µm, 6: 25.5 µm. a wide range f cmpsitin, and Curie temperatures ranging frm t C. The x values f titanmagnetites having such high Curie temperatures are less than 0.6. The Λ: values f submarine basalts frm several different areas shws a small variatin, being arund 0.6 (Jhnsn and Hall, 1978). Accrding t Jhnsn and Hall (1978), x values higher than 0.6 can be explained by migratin f irn ins int the surrunding materials, such as ther silicates and glass. Hwever, the diffusin rate f titanium ins is cnsiderably less than that f irn ins, s that there is wide variatin in cmpsitin, with x values ranging frm 0.3 t (Fig. 10 Grain Size (µm) Figure 12. Relatinship between intensity f NR nrmalized by saturatin magnetizatin f (J n /J s ) and visual grain size f titanmagnetites. Symbls are the same in Figure 1. 8). t is clear that lw Ti cntent titanmagnetite shuld be generated at crystallizatin stage in sme cases. n type samples, heating curves are similar t thse f type, but cling curves shw an apparent decrease in J s and the Curie temperature. This type f thermmagnetic curve is abundant in the upper part f Hle 504B, but has been reprted nly rarely elsewhere (arshall, 1978; Haman et al., 1979). Haman et al. (1979) carried ut detailed experiments with type samples and reprted that this type f thermmagnetic behavir depends upn grain size f measured samples, which may reflect the xygen fugacity. The Curie temperatures f type V curves are relatively high, ranging frm t C, and these samples are als fund in the lwer part f Hle 504B. Type V samples are characterized by high Curie temperature and nt having the characteristic dent in the thermmagnetic curve. The cling curves f type V samples shw a decrease in J s, and have a Curie temperature similar t type samples. n general, the initial high Curie temperatures f submarine basalts can be explained by lw-temperature xidatin, but the high Curie temperature f the present samples cannt be explained nly by xidatin. Because the thermal curves f these samples d nt have a dent, which is the inversin pint f xidized titanmagnetite, we infer that the titanmagnetites in type V samples scarcely experienced lw-temperature xidatin. The x values f type V samples are arund 0.6, the nrmal value f ceanic basalts. We nw discuss details f type V samples, and their decrease f Curie temperature, as well as type thermal behavir. The samples shwing type V curves 719

10 T. FURUTA might be slightly xidized at high temperatures, and we speculate that they have extremely small ilmenite exslutin lamellae, judging frm their thermmagnetic prperties. These lamellae cannt be detected under ptical micrscpe r electrn micrprbe, because they are "immature." We believe that the Curie temperature is relatively high during the heating prcess, because the x value f titanmagnetite may be lw (~0.2) as a result f titanium exslutin frm the riginal titanmagnetite. Abve the Curie temperature, ilmenite lamellae may be resrbed and mixed int the hst titanmagnetite, s that the curve f cling resembles that f riginal titanmagnetite, but with much lwer Curie temperature (~ C). Haman et al. (1979) reprted that the change in Curie temperature during heating and cling f samples f this type can be explained by reductin and mixing f the decmpsed titanmagnetite and hem-ilmenite. mse (pers. cmm.) suggests that the mixing rate f ilmenite lamellae depends n their size, with the result that it wuld prbably be very hard fr samples with well-develped lamellae having appreciable dimensins t revert t riginal phases. The small range f initial cmpsitins f titanmagnetites suggests that the interval f crystallizatin f titanmagnetites in submarine basalts is very limited; they are the latest phase t crystallize in the magmas. Hwever, there is a wide range f cmpsitin f titanmagnetites in mre than ten samples frm Hle 504B, as implied by thermmagnetic curves. The Λ: values f these titanmagnetites range frm 0.2 t. This may imply that titanmagnetites whse x values are much less than 0.6 exist in submarine basalts, and that the crystallizatin stage f titanmagnetite is variable in sme submarine basalts. t may be that the x value f titanmagnetite in all submarine basalts is nt very limited, as attested by Jhnsn and Hall (1978). SUARY The magnetic prperties that cntrl the intensity and stability f palemagnetic measurements f basalts frm the Csta Rica Rift can be summarized as fllws: 1) The intensity f NR, the initial susceptibility, saturatin magnetizatin, and cercive frce f the basalt samples prcessed in this study are nt significantly different frm previus DSD results. 2) The effects f hydrthermal alteratin, such as lw-temperature xidatin, n magnetic prperties are precisely defined. 3) Thermal curves shw several kinds f behavir, sme f which are rarely bserved in submarine basalts. The causes f differences between the nrmal submarine basalts and these samples is nt yet clear, but may include effects f hydrthermal alteratin. Anther factr may be the cmpsitin f titanmagnetite, which is different frm that in nrmal submarine basalts in sme cases. 4) There is a wide range f cmpsitin in titanmagnetites in these samples, with x values ranging frm less than 0.3 t. These wide ranges f chemical cmpsitin, especially the lw titanium cntent, previusly have nt been fund in submarine basalts. This cntradicts the cnclusin f Jhnsn and Hall (1978). Titanmagnetites with x values much larger than 0.6 may be explained by the migratin f irn ins (Ryall and Hall, 1980), but lw x values can be attributed nly t the initial values f the crystallized phase. ACKNOWLEDGENTS wuld like t acknwledge the able assistance f the shipbard scientific and technical staff n Legs 69 and 70.1 am very grateful t O. Oshima, T. Nishitani, K. Kbayashi, and K. mse fr valuable suggestins and critical reading f the manuscript. REFERENCES Ade-Hall, J.., Jhnsn, H.., and Ryall,. J. C, Rck magnetism f basalts, Leg 34. n Yeats, R. S., Hart, S. R., et al., nit. Repts. DSD, 34: Washingtn (U.S. Gvt. rinting Office), CRRUST, in press. Gethermal regimes f the Csta Rica Rift, East acific, investigated by drilling, DSD-OD Legs 68, 69 and 70. Gel. Sc. Am. Bull. Day, R., Fuller,., and Shmidt, V. A., Hysteresis prperties f titanmagnetites: grain-size and cmpsitinal dependence. hys. Earth lanet. 13: Day, R., Halgedahl, S., Steiner,., Kbayashi, K., Furuta, T., shii, T., and Faller, A., agnetic prperties f basalts frm DSD Leg 49. n Luyendyk, B.., Cann, J. R., et al., nit. Repts. DSD, 49: Washingtn (U.S. Gvt. rinting Office), Furuta, T., Kbayashi, K., and mse, K., agnetic prperties f igneus rcks f the hilippine Sea, Deep Sea Drilling rject Leg 58. n Klein, G. dev., Kbayashi, K., et al., nit. Repts. 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