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European Journal of Mineralogy; June 2001; v. 13; no. 3; p. 437-452; DOI: 10.1127/0935-1221/2001/0013-0437
© 2001 E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
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Primary magmas and mantle temperatures

David H. GREEN1,*, Trevor J. FALLOON1,2, Stephen M. Eggins1 and Gregory M. YAXLEY1

1 Research School of Earth Sciences, Australian National University, Canberra ACT, Australia 0200
2 School of Earth Sciences, University of Tasmania, Hobart, Tasmania, Australia 7005


Figure 1
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  		Fig. 1. Comparison of calculated liquidus temperature vs experimental temperature using the Liquidus Temperature vs. Composition relationships of Ford et al. (1983). Data are from experiments for liquids with ≤ 3.0 % Na2O and combine Fig. 7a and 7b of Falloon & Danyushevsky (2000).

 

Figure 2
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  		Fig. 2. Role of water-content in silicate melts in depressing liquidus temperature. Heavy curve is from Falloon & Danyushevsky (2000), compiled from literature data in which water content in glass is determined directly. Lighter curve is for olivine-rich compositions which do not readily quench to glass and in which data are obtained by bracketing liquidus temperatures for known, added water contents. Olivine-rich basanite data from Green (1973) (solid dots, based on liquidi for particular water contents) and unpublished data (open circles, mid-point of temperature interval between liquid and olivine + liquid experiments for 1 %, 4.5%, 7 % H2O). Peridotitic komatiite data from Green et al. (1975). Most of the data plotted in Falloon & Danyushevsky (2000) are derived from more polymerized (i.e. normative olivine + hypersthene, or hypersthene + quartz) liquids than the olivine-rich basanite or peridotitic komatiite and it is possible that water has a greater effect on liquidus depression in olivine-rich liquids.

 

Figure 3
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  		Fig. 3. Comparison of compositions of primitive MOR picrites of Table 2 and primitive ‘Hot Spot’ picrites of Table 1 using projections into the normative ‘basalt tetrahedron’ [Ol+Di+Qz+(Jd+CaTs+Lc)]. Inserts show selected areas of the tetrahedral faces. Projections are from:
  1. Olivine on to the surface Di+(Jd+CaTs+Lc)+Qz; and
  2. from Diopside on to the surface Ol+(Jd+CaTs+Lc)+Qz.

The MOR picrites form a tight cluster in both projections, noting that the olivine-addition calculations from the MORB glasses gives a field elongated along the lherzolite melting trend at 1.5 -> 2 GPa and along a similar trend (1.5–2 GPa) in the projection from olivine (Fig. 4).

The Hawaiian picrites show excellent clustering, particularly in the projection from olivine, and their trend and position clearly define a harzburgitic melting trend (Fig. 3a and Fig. 4).

 

Figure 4
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  		Fig. 4. Picrite compositions (Fig. 3) compared with melting trends for Hawaiian Pyrolite and MORB Pyrolite (Green & Falloon, 1998 and references therein). The MOR Picrites lie between the 1.5 to 2 GPa melting trends with lherzolite to harzburgite residue (cusp in Fig. 4a marking clinopyroxene disappearance from residue). Hawaiian Picrites (Loihi, Hawaii) lie on the harzburgite residue trend (4a) or in the harzburgite residue field (4b) for Hawaiian Pyrolite with Loihi and Hawaii possibly derived from a similar source plotting near the Hawaiian Pyrolite composition but Loihi representing a lower degree of partial melting. Koolau picrites clearly require a different source and residue which, from Fig. 4a, must have a lower Ca/Al and may have lower Ca/Na ratio than MORB Pyrolite or Hawaiian Pyrolite. In Fig. 4a and 4b we have plotted (open star) the composition of extremely refractory harzburgite from Papuan Ultramafic Belt (PUM harzburgite) as illustrative of a potentially buoyant subducted slab/wedge composition (Olivine is Mg#80.90), Spinel is Cr#80–90). We have also plotted the composition of a rhyodacite melt derived from coesite eclogite at 3.5 GPa, 1250°C and 1300°C (Yaxley & Green, 1998). Finally, we have plotted the field of high Mg# basanitic and nephelinitic intraplate basalts which are host to spinel or spinel+ garnet lherzolite xenoliths. These compositions are small melt fractions (< 5 % melt) from peridotite (C+H+O) in the incipient melt regime, in equilibrium with Ol+Opx+Cpx+Ga. Source mantle peridotites for Hawaiian Hot Spot magmas can be derived by refertilizing refractory mantle by addition of rhyodacite (eclogite melt) and basanite/olivine nephelinite (peridotite-(C+H+O) incipient melt), with or without asthenospheric mantle (MORB Pyrolite).

 

Figure 5
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  		Fig. 5. Projection from Clinopyroxene (Jadeite+Diopside) on to plane Olivine + Garnet + Coesite in the high pressure normative tetrahedron (Yaxley & Green, 1998). The figure illustrates extraction of picritic melts lying near the lherzolite minimum melt for the composition marked ‘Mantle’. Residues approach the Olivine + Orthopyroxene join. On the coesite-normative side of the figure the composition of average Oceanic Crust is plotted.

 

Figure 6
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  		Fig. 6. The possible Pressure-Temperature-time history of Horoman Peridotite as inferred by Takazawa et al. (2000). Figure modified from Takazawa et al. (2000) Fig. 17, as an illustration of a peridotite with a long history from primary upwelling, through subduction and refertilization to uplift and reactions in a low pressure and high temperature emplacement.

 




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