Total alkalis (Na2O+K2O) versus silica (all in wt.%) diagram for Kerguelen Archipelago lavas (inset data for Kerguelen Plateau-Broken Ridge). Lavas from the archipelago are shown relative to the Hawaiian boundary between tholeiitic and alkaline lavas (MacDonald and Katsura, 1964) and range from transitional to alkalic lavas with alkalic lavas becoming dominant with decreasing eruption age. Newly determined whole rock compositions from the Mt. Crozier and Ravin du Charbon sections confirm this trend. The Mt. Bureau and Rabouillère fields are from Yang et al. (1998). Also shown for comparison are the data fields for the Upper Miocene basanite to phonolite series (Weis et al., 1993), the Pleistocene alkalic basalt to trachyte series forming Mt. Ross (Weis et al., 1998) and the Lower Miocene alkalic basalt to trachyte series from the SE Province (Weis et al., 1993). Mt. Ross lavas are more alkalic than the flood basalts from the northwest sections and they are intermediate between the Upper and Lower Miocene groups in the SE Province. Thus, they show a reversal of the trend of increasing alkalinity with decreasing age defined by the Miocene lavas from the SE Province and the 28-29 Ma flood basalts from the northwestern part of the archipelago. Lavas from Kerguelen Plateau ODP sites 747, 749 and 750 are tholeiitic basalts. The above-basement lavas from ODP Site 748 are alkalic basalts. The lavas dredged from the central Kerguelen Plateau (open circles) and ODP Site 738 straddle the boundary line, largely because the alkali contents of these lavas were increased during post-magmatic alteration. As an extreme example, the solid triangle indicates a highly altered sample (750B, 19R-1, 47-50) from ODP Site 750. Lavas from Dredges 9 and 10 on Broken Ridge are also tholeiitic, whereas lavas from Dredge 8 overlap with the field for ODP Site 738. Data from Davies et al., 1989; Storey et al., 1992 and Mahoney et al., 1995.
(87Sr/86Sr)i and MgO (wt.%) histograms for Kerguelen Archipelago lavas. The Sr histogram shows that the large majority of the Kerguelen lavas have (87Sr/86Sr)i = 0.7050-0.7058, i.e. in the range inferred for the Kerguelen Plume (Weis et al., 1993; 1998; Frey and Weis, 1996). Only the group D samples from Mt. Bureau and Mt. Rabouillère have distinctly lower values resulting from interaction with previously formed oceanic crust (Yang et al., 1998). The inset is a histogram with all lavas, including the more evolved lavas from Mt. Ross and the SE Province (Weis et al., 1993; 1998). The MgO histogram clearly reflects the predominance of low MgO basalts (3-5 wt.%) on Kerguelen. This could indicate either that the plume flux was relatively low during formation of the flood basalts, i.e. during formation of the archipelago, or the involvement of a non-peridotite source. One of the goals of this research proposal is to identify the processes responsible for the formation of high 87Sr/86Sr and low MgO lavas.
Weis et al. proposed that the purest representation of the Sr, Nd and Pb isotopic characteristics of this plume are in the Upper Miocene lavas erupted from 10.2 to 6.6 Ma in the Southeast Province of the archipelago. In contrast to the conclusion of Weis et al. (1993), Class et al.(1993) proposed that some recently erupted Heard Island lavas are the purest representation of the Kerguelen plume. The pros and cons of these different interpretations were discussed by Class et al. (1996) and Frey and Weis (1995, 1996). In an effort to further evaluate these hypotheses and to identify the petrogenetic processes that were important in constructing the archipelago, we are determining the spatial and temporal geochemical variations in archipelago lavas.
Initial 87Sr/86Sr, 143Nd/144Nd and measured 206Pb/204Pb
versus age for plutonic and volcanic rocks of the Kerguelen Archipelago.
The oldest, ~40 Ma, rocks are from gabbroic plutons. As observed in the
flood basalts from Mt. Bureau and Mt. Rabouillère, the gabbros range
widely in initial 87Sr/86Sr, from <0.7045 to >0.7055. In contrast,
none of the <28 Ma samples have relatively low initial 87Sr/86Sr; they
range from 0.70483 to 0.70573. The same, but reverse, observation can be
made for 143Nd/144Nd. Pb isotopes appear to show stronger differences and
preliminary data for Mt. Crozier confirm the trend towards higher 206Pb/204Pb,
>18.3, for the flood basalt sections in the eastern part of the archipelago
(Weis et al., 1997). Mt. Bureau and Rabouillère also differ in 207Pb/204Pb.
The 206Pb/204Pb variation is not a simple temporal trend as younger lavas
in the SE Province (<10 Ma) and Ross Volcano (<2 Ma) also have relatively
low 206Pb/204Pb. Either the plume is more heterogeneous in Pb than Sr or
Nd, or Pb is more sensitive to a role for other components. Some of the
older (>40 Ma) submarine basalts forming the Kerguelen Plateau (Weis
et al., 1989a; Mahoney et al., 1995) and Ninetyeast Ridge (Weis and Frey,
1991; Frey and Weis, 1995) have incorporated a lower 206Pb/204Pb component
as a result of interaction of the plume with oceanic lithosphere (Sites
750, 758) or continental lithosphere (738). Presumably these lithospheric
components are present at depth beneath the Kerguelen Archipelago and their
heterogeneous distribution may explain the variations in Pb isotopic ratios.
Our proposed detailed study of the various stratigraphic sections should
define these spatial and temporal variations during the last 40 million
years on the archipelago. Data for plutonic rocks from Weis and Giret (1994);
data for volcanic rocks from Gautier et al. (1990, basalt survey), Yang
et al. (1998), Weis et al., (1993; 1998), and x symbols indicate 4 Mt. Bureau
samples analyzed by White (White and Hofmann, 1982; White, 1985).