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34 in terms of variations in trivalent cations and 2+ titanium relative to the Fe/Mg ratios of the spinels. The spinels are subdivided into three groups: Group 1, disseminated spinels from the Table Mountain, North Arm, and Blow Me Down masses (samples 27, 34, 35, 36, 43, 44A, 44B, 48); Group 2, disseminated spinels from the Lewis Hills (samples 29, 37A, 37B, 38, 52A, 52B); Group 3, massive spinels (samples 31, 32, 33, 47).
         The compositions of Group 1 spinels contrast markedly with those of the Lewis Hills spinels (figure 34) . They are significantly richer in Cr + and poorer in Al2O3. Furthermore the Cr content of group 1 spinels from the dunite-wehrlite sequence (samples 25, 27, 34, 36) increases with increasing Fe2+/Mg ratio whereas group 2 spinels from the same type of sequence (samples 29, 37A, 37B, 38) become poorer in Cr with increasing Fe/Mg ratios.  In group 1 spinels the chromium-enrichment trend whicli characterizes the dunite-v/ehrlite sequence is reversed within the troctolite-gabbro sequence (samples 48-43-44). Here the chromium content decreases from a maximum value of 6.83 cations per 32 oxygens (sample 48) to 5.51 cations per 32 oxygens (sample 44).
The concentration of Fe   and Ti in all spinels is generally low, less than 1 cation per 32 oxygens and less than 0.1 cation per 32 oxygens respectively.  However Fe and Ti concentrations increase markedly in the more differentiated spinels of group 1 reaching maximum values

184
of 2.44 cations per 32 oxygens (Fe3+) and 0.39 cations per 32 oxygens (Ti) in sample 44 (sample 44B is even richer in Fe3+ but this enrichment reflects late oxidation along the grain boundary). The marked Fe3+-Ti enrichment in spinels 43 and 44 is counterbalanced by a decrease not only in Cr3+ , as previously mentioned, but also in Al3+ . This is a direct consequence of the formation of FeFe2O4 and Fe2TiO4 solid solutions at the expense of Fe2Cr2O4 and MgAl2O4 solid solutions (spinel analyses 43 and 44, Table 4, Appendix III).
                 The four spinels of group 3 (massive spinels) are highly variable with respect to Cr3+, Al3+ concentrations  and Fe2+/Mg ratios. They range in composition from almost pure spinel-hercynite solid solutions containing only 0.32 Cr cations per 32 oxygens and very low (0.22) Fe2+/Mg(ratio) (sample 32) to picrochromite-chromite solid solutions with relatively high FeO/TiO2 ratio (1.5) and low (2.15) cations per 32 oxygens) Al3+ concentrations (sample 31).   

                                            C3b: Mg-Fe distribution between coexisting  spinels and olivines

Irvine (1965) showed that Cr-rich spinels co-existing with olivine of a given composition have high Fe2+/Mg ratios and, viceversa, Al-rich spinels coexisting with the same olivines have lower Fe2+/Mg ratios. During magmatic differentiation, the early removal of Cr-spinel and Mg-rich phases greatly reduces the Cr and Mg concentrations 

185 
of the liquid so that the next spinel phase to crystallize becomes poorer in Cr and both spinels and silicates become enriched in Fe2+ relative to Mg. These relationships   J  indicate that in order to understand the significance   R of chemical variations of spinel crystallizing from a   J differentiating magma it is necessary to relate their   K chemical changes to those of the coexisting silicates. 
        Figure 35 A illustrates the relationships between Fe2+/Mg ratios of spinels and Fe/Mg ratios of  coexisting olivines. Olivine was chosen because both olivines and spinels occur as cumulus phases, because the Fe/Mg ratios of olivines are a function of the Fe/Mg                    content of the magma from which they crystallize (Roeder and Emslie, 1970), and because the Fe2+/Mg partitioning between olivine and spinel can be used to determine formation temperatures (Jackson, 1969). The relation between the temperature of formation of spinel-olivine (table 11) , Fe2+/Mg ratios and mole fractions of Cr3+ of spinels as shown in Figure 35B.
     Several interesting features emerge from diagrams 35A and B: 
1) The Fe2+/Mg ratios of group 1. and 2 spinels increase with increasing Fe/Mg ratios of the coexisting olivines  (figure 35A).
 2) Olivines coexisting with group 1 spinels become enriched in Fe at a slower rate than olivines coexisting with group 2 spinels (figure 35 B) . Consequently group 1                  I

188  
  spinel-olivine pairs have higher K values (Table 11). 
4) Figure 35 B relating the temperature of formation of spinel-olivine pairs to the spinel composition indicates that temperatures of formation of disseminated spinels (groups 1 and 2) -olivine pairs  decrease with increasing Fe2+/Mg ratios of spinels (and olivines). On the contrary, the temperatures of formation of the massive spinel-olivine pairs (group 3) depends primarily on both the Cr3+ /Cr3+ + Al3+ + Fe3+   and Fe2+/Mg ratios of spinels.  
5) Anomalously low temperatures (table 11, samples 52 B and 44 B) reflect the coexistence of olivine having normal Fe/Mg ratios with spinels which have Fe2+/Mg ratios which do not fall along the main Fe2+/Mg trend  (e.g. plot point 52 B, figure 35 A). The anomalously high Fe2+/Mg ratios of the disseminated spinels 52 B and 44 B are considered to have been produced by its subsolidus reequilibration.  
             In summary, the above observations show that variations in the chemical compositions of spinels in cumulates may reflect variations in the composition of the initial "undifferentiated" magma, as well as changes in magma composition during differentiation and subsequent subsolidus reequilibration.    
                             C3c: Interpretation of relationships between spinel composition and parental magma composition
                     Hill and Roeder (1974) have shown experimentally  
 189
that chrome spinels crystallizing from a near-surface magma of low oxygen fugacity (less than 10~8) contain approximately 1,000 times more Cr than the coexisting liquid.   In the Bay of Islands cumulate section earlyl formed chrome-spinels contain little Fe3+ so that their Cr203 contents can be used to derive the Cr content of the parental magma. If the 1,000 conversion factor is correct this then indicates that early formed group 1  spinels crystallized from a magma containing 238ppm Cr whereas early precipitated disseminated spinels from the  Lewis Hills (sample 29) were derived from a magma with  approximately 131 ppm Cr. Since the solubility of Cr in silicate melts is likely to increase with increasing pressure (Burns et al., 1972), the initial lower Cr content of the Lewis Hills magma can only be explained as resulting from a lower degree of partial melting within the mantle source. This lends support to the prior conclusion that the high-Al content of the Lewis Hills clinopyroxenes reflects the high-Al content of the magma rather than high pressure crystallisation. The calculated Cr content of the magma from which sample 31 crystallized is 402 ppm. This value exceeds the maximum Cr solubility of basaltic magmas (200-300 ppm,Hill and Roeder, 1974) and would indicate that the spinel was precipitated very early during the ascent of the mantle-derived liquid to near surface. In this respect the Cr

190 
content of chemically equivalent basaltic magmas can be a useful indicator of the degree of fractionation under-gone prior to extrusion.  Of the other massive spinels analyzed, two[I  ^Jsamples (33 and 47) have Cr contents within the range(  ^Bof that of the disseminated spinels, whereas one samplej  ^J(32) is extremely low in Cr (2.04 wt Cr.O-J . Since the|  ^Jspinel occurs as large grains partially embayed where in|  ^Hcontact with olivine or as small subhedral grains enclosed|  ^Jin olivine, there is no reason to question its magmatic|i|  ^Borigin. At present the only logical genetic interpretation||  ^His that the spinel represents a metdstable phase that||  ^Hcrystallized from a Al-rich and Cr-poor batch of magma,||  ^Band that it was preserved because it was immediately removed|||  ^Bfrom the magma (see.Osborn and Tait, 1952; Irvine, 1967).||  ^RDisseminated and massive spinels from the North3  ^HArm, Table Mountain, and Blow Me Down masses contain very^  ^Hlittle Fe (less than 1 cation per 32 o::ygens) v/ithin the. ',  ^Hforsterite range 90-87 of coexisting olivines. The Fe34"; ;  ^Hcontent increases rapidly up to 2.44 cations per 32 oxygens  ^Bat forsterite 84 (olivine-spinel 44) and jumps to 12.88(  ^Jcations per 32 oxygens at forsterite 76 (olivine-spinel 53) .J  ^JThe latter spinel is a magnetite which contains minor!•  ^Bchromite and hercynite solid solutions (analysis 53, table 4,'||  ^JAppendix III). On the ^asis of the available experimental|  •Bresults relating spinel composition to the oxygen fugacityj|  ^Jof the magma (Hill and Roeder, 1974) , the Fe34' variations||

191
of the above spinels can be interpreted to indicate that initial crystallization took place under conditions| of very low oxygen fugacity followed by a drastic increase in the oxygen fugacity during the early stages of fractionation. The increase in the oxygen fugacity was rapid enough| to allow primary magnetite to coprecipitate with relatively Mg-rich olivines (Fo76) but not rapid enough to allow the crystallization (within the interval Fo84-Fo76) of spinels with compositions intermediate between Ti-magnetite and chromite. The compositional break between samples 44, (Fe-rich chromite) and 53 (Cr-poor magnetite) may indicate the presence of a liquid-pyroxene-spinel reaction (Irvine,1967; Hill and Roeder, 1974) within the cumulate sequence. The sudden increase in oxygen fugacity of the magma was probably caused by the introduction of water into the magma chamber. This allowed not only early separation of magnetite but also crystallization of some interstitial amphibole (Plate 7-2) in the lower part of the gabbro. The Fe3-1- content of disseminated spinels from the Lewis Hills mass (group 2) coexisting with olivine Fo82 (olivine-spinel pair 38) is distinctly lower than the Fe3O4 content of group 1 spinels coexisting with olivines richer in Mg (olivine-spinel pair 44). This implies either that the Lewis Hills magma was less hydrous than the magma from which group 1 spinels crystallized or, alternatively, that Al-rich spinels (Lewis Hills) are 

192
 less easily oxydized than group 1 spinels. The chemical characteristics of petrogenetically distinct spinel groups studied in this thesis are summarized in figure 36. The diagram clearly reveals the usefulness of spinel studies in discriminating between partial fusion.  

D: SUMMARY AND CONCLUSIONS  
                           Detailed studies of clinopyroxenes indicate that•|  Hthe cumulate sequences of the Bay of Islands ophiolitej||  Bwere derived from an "almost alkalic" transitional basalt_|  Hwhereas the TiO^ variations displayed by ultramafic andi||  Hmafic cumulates, gabbros, dykes, and pillow lavas, arej||  Hcompatible with the possibility that the magma was; generated along a mid-ocean ridge. Fractionation of theill  parental magma led to the development of more saturated||  Hcompositions as is indicated by 1) the chemical variationsJS  Bof the clinopyroxenes, 2) the presence of orthopyroxene11  •^reaction rir^s within olivine-clinopyroxene-plagioclase , U  Jcumulates and 3) the presence of cumulus orthopyroxenefl  Kin the more differentiated olivine-free gabbros.1|  ^JThe differentiating magma was periodically mixedIjH  ^Rwith new batches of primitive magma as is indicated byIf  Jfrequent fluctuations in the Fe/Mg ratios of the cumulus|fl  ^Bminerals. Anomalous Ti/Al ratios of some clinopyroxenesif  B(samples 44, 55) also suggest that each new batch of||  Jmagma might have had a slightly different chemical composition. ' f|

194
 The presence of "calcic plagioclase phenocrysts" implies|[   H|that some plagioclase may have been separated by flotation;[l|   ^and that remelting of plagioclase may also have been an|   Hoperative process during mixing of magmas.|   JSeveral lines of evidence such as the Al-rich,^ll   fCr-poor nature of the clinopyroxenes and spinels and the|   ^Rmoderate iron enrichment of spinels relative to coexisting;||   ^Holivines indicate that the magma which gave rise to the|   HLewis Hills cumulate sequences was richer in Al and wasI |   Hformed by a lower degree of partial melting of Iherzolite' ijj    than the magma which gave rise to the Blow Me Down, North,| |   ^HArm, and Table Mountain cumulates. The absence of a well||   ^Bdefined harzburgite-dunite tectonite sequence underneath:j |   Rthe Lewis Hills cumulates is in further support of thisi|j |   JThe Fe content and overall chemistry of spinels|   Jwithin the dunite-wehrlite sequences of the Bay of Islands;||   ^Rophiolite indicate that the magma from which they crystallized |   ^Hfractionated under conditions of low oxygen fugacity.'^B ^H |However the oxygen fugacity of the magma rapidly increased;i:   ^|within the dunite-troctolite sequence as evidenced by the,|   ^Bmarked increase in the Fe content of chrome spinels andIj   H|the early appareance of cumulus magnetite. Minor amounts:|   ^Hof interstitial amphibole present in otherwise unaltered^l   ^Rcumulates suggest that oxidation of the magma was caused I i||   ^|by the introduction of water. The early separation of|jj

195 
magnetite coupled with the mixing of undifferentiated' B  ^Bbatches of magma w^re responsible for the limited ironJ  ^Benrichment which characterizes the cumulus minerals.|g  ^1Although the ultramafic-mafic cumulate sequences'JIM-:  Rof the Bay of Islands are roughly conformable, there is-|||  ^Ksome evidence to suggest that a major compositional breakf  ^Jmay exist between ultramafic and gabbroic cumulates (units|  ^B4 and 5, respectively): 1) the order of crystallization of1  ^Jcumulus minerals within unit 4 is olivine (chromite)-I  ^Bclinopyroxene-orthopyroxene (the latter is rare but does.11  ^Roccur in certain layers toward the top of the sequence).||B  ^JOn the contrary the order of crystallization of cumulusll  ^Jminerals of unit 5 is olivine-plagioclase-clinopyroxene-'SB I  ^Horthopyroxene; 2) blocks of ultramafic cumulates areIB'  ^Hembedded in layered gabbros; 3) the oxygen fugacity of the!• I  ^Jmagma was very low during crystallization of the ultramaficSBsi  ^Bcumulates but increased rapidly during cyrstallization of||  ^fthe gabbroic cumulates.1111  ^BThe significance of the "amphibole gabbro" which|j| 1  ^Brepresents the upper part of the gabbroic section in thef.  ^JBlow Me Down and North Arm masses (unit 6) is at presentIM |  ^Hnot well understood due to lack of analytical data. Asll| |  ^Ja working hypothesis it is suggested that the upper gabbro|j |  ^Junit may have formed from above downward by a process'jm |   ^of slow congelation due to loss of heat into the overlying