[181] 
C3:  Spinels as Petrogenetic Indicators
Spinels are potentially useful petrogenetic indicators because they show marked compositional variations in terms of both divalent and trivalent cations, and therefore reflect sensitively the physicochemical conditions under which the host rocks have formed (Irvine, 1965). Recent experimental work (Hill and Roeder, 1974) on the crystallization of spinel from basaltic liquids has demonstrated that the composition of spinels (chromite and titaniferous magnetite) crystallizing under near-surface conditions is a function of several variables, the most important being the chromium, oxygen (ferrous/ferric ratio), MgO and TiO2 contents of the liquid, temperature, and the presence or absence of pyroxene as a stable phase during crystallization of the spinel.  The details of Hill and Roeders' findings will be discussed later in the light of the compositional variations displayed by the spinels of the Bay of Islands cumulates and massive gabbro.

C3a:  Chemical variation.
Chemical analyses of 18 chromian spinels and 3 magnetites are given in Table 4, Appendix III,and the variation in chemical composition of chromian spinels in the Bay of Islands cumulates is illustrated in figure [182] 34 in terms of variations in trivalent cations and 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 Cr3+ 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 Fe  /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 which characterizes the dunite-wehrlite 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 Fe2+   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 [183] [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) Al2O3 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 indicate that in order to understand the significance of chemical variations of spinel crystallizing from a   differentiating magma it is necessary to relate their chemical changes to those of the coexisting silicates. Figure 35 A illustrates the relationships between Fe2+/Mg ratios of spinels and Fe2+/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 H 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 pairs (table 11), Fe+2/Mg ratios and mole fractions of  Ce3+ of spinels is shown in figure 35B

Several interesting features emerge from diagrams 35 A and B:
1) The Fe2+/Mg ratios of group 1 and 2 spinels increase with increasing Fe/Mg ratios of the coexisting olivines;
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 [186] [187]  [188] spinel-olivine pairs have higher K values (Table 11).
3) 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.
4) Anomalously low temperatures (table 11, samples 52 B  and 44 B) reflect the coexistence of olivine having nomal 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 Fe /Mg ratios of the disseminated spinels 52 B and 44 B are considered to have been produced by 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 
                           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 early 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 crystallization.
                           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 useful indicator of the degree of fractionation under-gone prior to extrusion. 
              Of the other massive spinels analyzed, two samples (33 and 47) have Cr contents within the range of that of the disseminated spinels, whereas one sample (32) is extremely low in Cr (2.04 wt Cr2O3.  Since the spinel occurs as large grains partially embayed where in contact with olivine or as small subhedral grains enclosed in olivine, there is no reason to question its magmatic origin. At present the only logical genetic interpretation is that the spinel represents a metastable phase that crystallized from an Al-rich and Cr-poor batch of magma, and that it was preserved because it was immediately removed from the magma (see.Osborn and Tait, 1952; Irvine, 1967).
                             Disseminated and massive spinels from the North Arm, Table Mountain, and Blow Me Down masses contain very little Fe (less than 1 cation per 32 oxygens) within the forsterite range 90-87 of coexisting olivines. The Fe3O4 content increases rapidly up to 2.44 cations per 32 oxygens  at forsterite 84 (olivine-spinel 44) and jumps to 12.88 cations per 32 oxygens at forsterite 76 (olivine-spinel 53) . The latter spinel is a magnetite which contains minor chromite and hercynite solid solutions (analysis 53, table 4,  Appendix III). On the basis of the available experimental results relating spinel composition to the oxygen fugacity of the magma (Hill and Roeder, 1974) , the Fe3+ 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 divines (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 cumulate sequence.
              The Fe3+ content of disseminated spinels from the Lewis Hills mass (group 2) coexisting with olivine Fo82 (olivine-spinel pair 38) is distinctly lower than Fe3+  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 and fractionation trends.

D SUMMARY AND CONCLUSIONS
                           Detailed studies of clinopyroxenes indicate that the cumulate sequences of the Bay of Islands ophiolite were derived from an "almost alkalic" transitional basalt whereas the TiO2 variations displayed by ultramafic and mafic cumulates, gabbros, dykes, and pillow lavas, are compatible with the possibility that the magma was generated along a mid-ocean ridge. Fractionation of the parental magma led to the development of more saturated compositions as is indicated by 1) the chemical variations of the clinopyroxenes, 2) the presence of orthopyroxene reaction rims within olivine-clinopyroxene-plagioclase , cumulates and 3) the presence of cumulus orthopyroxene in the more differentiated olivine-free gabbros.
                          The differentiating magma was periodically mixed with new batches of primitive magma as is indicated by frequent fluctuations in the Fe/Mg ratios of the cumulus minerals. Anomalous Ti/Al ratios of some clinopyroxenes (samples 44, 55) also suggest that each new batch of magma might have had a slightly different chemical composition. 
[193]  [194] The presence of "calcic plagioclase phenocrysts" implies that some plagioclase may have been separated by flotation; and that remelting of plagioclase may also have been an operative process during mixing of magmas.  Several lines of evidence such as the Al-rich, Cr-poor nature of the clinopyroxenes and spinels and the moderate iron enrichment of spinels relative to coexisting olivines indicate that the magma which gave rise to the Lewis Hills cumulate sequences was richer in Al and was formed by a lower degree of partial melting of Iherzolite than the magma which gave rise to the Blow Me Down, North, Arm, and Table Mountain cumulates. The absence of a well defined harzburgite-dunite tectonite sequence underneath the Lewis Hills cumulates is in further support of this. The Fe content and overall chemistry of spinels within the dunite-wehrlite sequences of the Bay of Islands ophiolite indicate that the magma from which they crystallized fractionated under conditions of low oxygen fugacity.  However the oxygen fugacity of the magma rapidly increased within the dunite-troctolite sequence as evidenced by the marked increase in the Fe content of chrome spinels and the early appareance of cumulus magnetite. Minor amounts of interstitial amphibole present in otherwise unaltered cumulates suggest that oxidation of the magma was caused by the introduction of water. The early separation of  [195 ] magnetite coupled with the mixing of undifferentiated batches of magma were responsible for the limited iron enrichment which characterizes the cumulus minerals.
                         Although the ultramafic-mafic cumulate sequences of the Bay of Islands are roughly conformable, there is some evidence to suggest that a major compositional break may exist between ultramafic and gabbroic cumulates (units 4 and 5, respectively): 1) the order of crystallization of cumulus minerals within unit 4 is olivine (chromite)-clinopyroxene-orthopyroxene (the latter is rare but does occur in certain layers toward the top of the sequence). On the contrary the order of crystallization of cumulus minerals of unit 5 is olivine-plagioclase-clinopyroxene-orthopyroxene; 2) blocks of ultramafic cumulates are embedded in layered gabbros; 3) the oxygen fugacity of the magma was very low during crystallization of the ultramafics cumulates but increased rapidly during cyrstallization of the gabbroic cumulates.
                         The significance of the "amphibole gabbro" which represents the upper part of the gabbroic section in the Blow Me Down and North Arm masses (unit 6) is at presently not well understood due to lack of analytical data.  As a working hypothesis it is suggested that the upper gabbro unit may have formed from above downward by a process of slow congelation due to loss of heat into the overlying lavas.