Primitive shoshonitic melts from Fij
9.3 Trace element geochemistry
9.3.3 Chalcophile elements (Cu and Zn)
The Cu concentrations of primitive Tavua inclusions (normal and anomalous hosted by olivine >Fo88) range from 120-218 ppm (average = 1 77 ppm), whereas inclusions hosted by evolved olivine « F084) contain higher eu of up to 474 ppm (average
=
336 ppm Cu). Figure 9.3.3A shows that the Cu contents of inclusions exhibit �2-fold increase with decreasing forsterite over the range F090-8 1 . Primitive inclusions from Tavua (hosts >F088) contain similar to slightly higher concentrations of Cu relative to primitive host lavas (Figure 9.3.3A). With decreasing host forsterite, inclusion Cu content increases to levels significantly higher than shoshonitic host rock concentrations, and appreciably higher than the average values for MORB and arc lavas of (Stanton, 1 �94, 71 and 1 08 ppm Cu respectively) (Figure 9.3.3A).Astrolabe Group inclusions (hosts F086.6-91 .7) with anomalous major element " composition display a wide range of Cu concentrations (40-329 ppm, average = 1 05 ppm), lying below and above the field of host rocks on Figure 9.3 .3A. Unlike Tavua, the. Cu concentrations of Astrolabe Group inclusions do not appear to vary with host forsterite.
Vatu inclusions, like those in Astrolabe Group olivines are predominantly anomalous with respect to their major element composition and display a range of Cu (253-5 19 ppm, average = 355 ppm) over a restricted range of host forsterite (Fo84.2-84.8).
In
general terms, (ie. with respect to average values) the behaviour of Cu in Vatu inclusions conforms to the overall evolutionary trend exhibited by Tavua inclusions (Figure 9.3.3A). However, the wide range of inclusion Cu concentration at essentially constant host forsterite (Figure 9.3.3A) suggests that DRM-type processes involving monzonitic lithologies (Chapter 8) have had an effect on the absolute Cu content of Vatu inclusions. Accordingly, the 2 lowermost Cu values ofVatu inclusions (253 and 278ppm) are likely to represent the least perturbed concentrations, conforming closely to the average Cu content of Tavua' inclusions (262 ppm) in hosts F088-84.
It should be noted that because Zn is an olivine-compatible element, the Zn content of inclusions were recalculated using measured olivine Zn values (,.., 1 50 ppm, Appendix 2.2).
The Zn concentrations of primitive Tavua inclusions (hosts >F088) range from 20- 62 ppm (average = 45 ppm) and are significantly (�2x) lower than the Zn concentrations of shoshonitic whole rocks, average values for MORB and arc rocks of Stanton (1994) (Figure 9.3.3B). Like Cu, inclusion Zn concentrations increase with decreasing host forsterite, with evolved Tavua inclusions (ie. hosts �F081) containing ,.., 2x the Zn concentration of primitive inclusions (average = 82 ppm Zn), overlapping with whole rock Zn 'abundance.
Astrolabe Group anomalous inclusions (hosts F086.6-91 .7) contain similar Zn abundances to primitive Tavua inclusions, with an average Zn concentration of 36 ppm. Anomalous inclusions from Vatu (hosts F084.2-84.8) display a range of Zn from 67-1 04 ppm (average = 78 ppm) and, like Cu (Figure 9.3.3A) have similar Zn abundance as evolved (hosts ::sF084) inclusions from Tavua (Figure 9.3.3B).
The majority of melt inclusions from Tavua and Vatu display a CU/Zn of ,..,4 (Figure 9.3.3C), which is notably higher than CU/Zn values of shoshonitic host lavas from the studied suites (1 .7), and average MORB (0.845) and arc rock values (1 .4) of Stanton (1 994). Astrolabe Group inclusions display a wide range of CU/Zn ( 1 . 1 -7.9, average = 3) at constant host forsterite (Figure 9.3.3C), which may be related to the assimilation of brine-altered lower crustal lithologies containing variable Cu and Zn contents (see Section 9.2. 1).
On the basis of melt inclusion data from Tavua, it appears that both Cu and Zn display an �2-fold increase in absolute concentrations with decreasing host forsterite from �F090-8 1, broadly consistent with the amount of fractionation estimated for the evolution of Tavua melt inclusions in Section 9.1 .4.
Sulphide inclusions trapped in olivine phenocrysts from arc and back-arc settings evolve from Ni-Fe-rich (eg. pyrrhotite) in primitive, high temperature melts, to separate Fe-and Cu-rich chalcopyrite and pyrite compositions in more evolved lower temperature melts (Danyushevsky et aL, 2001). The average S/Cu in such
=
sulphides (ie. pyrrhotite-pyrite-chalcopyrite) is -3.75, with average S/Zn 318 (Danyushevsky et aI., 2001). On the basis of inclusion data from Tavua (Section 9.3.2) the average amount of S available to form sulphide is -900 ppm. If a S/Cu of 3.75:1 is assumed, then -240 ppm of Cu (ie. 900/3.75) would potentially be removed if sulphide were continually crystallising and separating from the primitive shoshonitic melt Since primitive Tavua inclusions (hosts >F088) contain an average Cu content of 177 ppm, continual removal or separation of sulphide would result in constant to decreasing inclusion Cu content with decreasing host forsterite. However, the correlation of increasing inclusion Cu content with decreasing host forsterite (Figure 9.3.3A) indicates that sulphide did not form a separate immiscible phase, and suggests that the shoshonitic melt (at least with respect to Tavua) remained sulphide undersaturated during fractionation over the range F092.55-81 (-16-4wt% MgO). Although sulphide inclusions are commonly observed in olivine phenocrysts from MORB, back-arc and arc settings (eg Danyushevsky et aI., 2001), their conspicuous absence in olivine phenocrysts from the shoshonitic samples in this study supports the view that Fijian shoshonitic magmas likely remained sulphide undersaturated during their early magmatic evolution.
The observation that evolved melt inclusions have significantly higher Cu content than similarly evolved rock compositions (Figure 9.4.3A), suggests that the late magmatic behaviour of Cu and Zn is strongly decoupled, with Cu preferentially partitioning into a fluid/vapour phase relative to Zn (eg. Lowenstem, 1995).
7.97 5.79 64.49 58.67 38.27 5.00 55.40 27.90 30.33 458.74 7.56 7.20 2.45 1.35 1.30 5.37 5.95
Table 9.4A Recalculated trace element and volatile concentrations of Tavua normal inclusions
sample % olivine added No.1 EL1 0/OL·4125 13.20 No. 2 EL9/0L· 7/137ml 13.57 No. 3 EL9/0L· 7/42 14.47 No.4 EL9/0L·5!9-2 15.13 No. 5 EL9IOL·7/117m1 13.85 NO.6 average stdev FW\% SW\% Clw\% 0.07 0.07 0.14 0.08 0.05 0.11 0.0 76 0.061 0.122 0.01 0.01 0.02 9.60 U 6.58 6.45 7.28 1.52 Zn 57.80 67.66 66.67 63. 06 4.56 So 25.72 33.94 27.78 30.40 31.23 Cu 181.70 167.91 176.53 157.88 145.45 165.85 14.60 Rb 40.67 45.98 38.49 49.13 45.93 6.76 Sr 822.31 952.64 688.88 918.61 683.78 849.28 102.71 Y 11.71 12.40 12.03 11.73 12.61 12.10 0.40 Zr 38.00 44.17 34.50 34.98 6.43 Nb 0.61 0.90 0.51 0.65 0.73 0.68 0.14 sa 570.45 423.39 491.25 491.94 487.16 54.44 La 5.31 6.64 4.96 5.61 6.03 5.69 0.66 Ce 11.63 15.51 10.61 13.40 13.22 12.88 1.87 Nd 10.54 9.66 8.48 8.69 1.40 3.23 2.71 2.72 0.50 2.00 3.15 Sm 2.60 Eu 0.86 0.91 0.57 0.89 0.90 0.83 0.14 Gd 2.38 3.06 2.85 3.67 2.94 2.98 0.46 2.80 1.93 2.52 2.62 2.46 0.33 Oy 1.47 Er 1.44 1.15 1.47 1.38 0.14 Yb 0.94 1.08 1.02 1.17 1.10 0.14 Pb 5.61 7.61 6.23 6.15 0.88 Th U 0.77 0.26 0.84 0.41 0.66 0.38 0.71 0.33 0.70 0.34 0.74 0. 34 0.07 0.06
NOlI. 1-1; trace element ana vOlaUle concentrahons or I avua normal inClUSions recalcul81ea to oe lin eqUilionum With OliVine t-092.55; No. 6 average recalculated trace element and volatile concentration and standard deviation