Liquidus Projections for haplo-basalts The Basalt Tetrahedron at 1 atm:

1. Liquidus Projections for haplo-basalts The Basalt Tetrahedron at 1 atm: The olivine - clinopyroxene - plagioclaseplane is a thermal divide in the haplo-basalt system at low pressures and separates natural magmas into two fundamentally different magmatic series. Sub-alkaline basaltic magmas with compositions to the Qtz-rich side of the plane fractionate towards Qtz-saturated residual liquids, such as rhyolite. Alkaline basaltic magmas with compositions to the Qtz-poor side of the plane fractionate towards residual liquids saturated in a feldspathoid, such as nepheline phonolite.

2. Since the dominant mineral in the mantle source of basaltic magmas is olivine, we can achieve a further simplification by projecting the liquidus of basaltic systems from the perspective of olivine:

3. Alkaline basalts fall to the Foid-side of the olivine-clinopyroxene-plagioclase plane (1 atm thermal divide) and fractionate to foid-saturated residual liquids. Sub-alkaline basalts fall to the Quartz-side and fractionate towards quartz-saturated residual liquids. Alkaline basaltic lavas are volumetrically insignificant (~1%), but strongly enriched in highly incompatible trace elements profiles compared to sub-alkaline lavas, and low in HREE, Y, & Sc. These characteristics are generally ascribed to small degrees of partial melting at elevated pressures, leaving garnet as a phase in the refractory residue.

5. The Effect of Pressure 1 atm Increasing pressure shifts the oliv-cpx-opx peritectic point towards less Si-rich compositions. At approximately 10 kbs this invariant point moves into the oliv - cpx- opx compositional volume, and the first melt of the mantle has an olivine basalt composition. The invariant point is still a peritectic point, however, because of the extensive solid solution of cpx towards opx. At pressures exceeding 15-20 kbs, this invariant point moves outside the simple olivine - cpx - qtz system, into the Neph-normative volume of the basalt tetrahedron. The first melt of mantle peridotite is an alkaline olivine basalt at these high pressures.

6. Since the dominant mineral in the mantle source of basaltic magmas is olivine, we can achieve a further simplification by projecting the liquidus of basaltic systems from the perspective of olivine: Movement of the invariant point determining the composition of the first melt with increasing pressure.

9. Dry Solidus Simple Peridotite

10. The eclogite plane becomes a thermal divide at pressures above 20+ Kbs for natural basalt compositions

14. Solidus Temperatures Earth Mg no ~ 0.89 Sensitive to the Mg no (Mg/(Mg+Fe)) of the source: δT / δMg no. ~ 7oC Mars Mg no ~ 0.73

15. Solidus Temperatures Sensitive to the levels of the most incompatible elements, eg.: δT / δalkalis. ~ 100oC

16. Increasing degree of melting Behaviour of an understaturated element during partial melting. Al is dissolved in pyroxenes at 10Kbs, but is not essential for the presence of Cpx or Opx. Note the change in behaviour of Al when garnet becomes a stable phase at 30 Kbs.

17. Behaviour of an essential (saturated in the sense that Ca is essential for the presence of Cpx as a phase) element during partial melting. Increasing degree of melting

19. Y2TO4 = Mg2SiO4 - Fe2SiO4 dashed lines:Kd = (Fe/Mg)olivine / (Fe/Mg)liquid = 0.30.03 olivine line: Fe + Mg = 66.67 thin tie-lines: join coexisting olivine and liquid curved arrow: liquid line of descent dotted lines: isotherms, slope = -Kd

24. Dry Solidus Simple Peridotite

25. eclogite Olivine is not on the liquidus of any picritic basalt at pressures above ~ 25Kbs

26. MOHO Only two known rock types have the required density to match that inferred for the mantle underlying the MOHO. Eclogite ( = 3.4 - 3.6), a rock composed of clinopyroxene and garnet which has the same chemical composition as basalt, but a different mineralogy because it has crystallized at high pressure. Peridotite ( = 3.2 - 3.4), a rock consisting predominantly of olivine (60 - 80%), with lesser amounts of orthopyroxene, clinopyroxene, and spinel. The composition of peridotite is much richer in Mg and poorer in Al and Si than basalt, thus olivine (Y2TO4) predominates over pyroxene (YTO3) as the ferromagnesian mineral, and feldspar is minor or absent.

27. The uppermost mantle is seismically anisotropic This is more consistent with a peridotite versus and eclogite upper mantle

29. Does the “garnet signature” in MORB reflect the presence of veins or blobs of garnet pyroxenite in its mantle source? Solidii for Garnet Pyroxenite and Peridotite

30. Mantle Sources for Magmatic End-Members Hy-Norm Basalt Ol- Neph

31. Dry Solidus Simple Peridotite

32. Hyperstene Normative Basalts MgO > 8 wt.%

33. 8+ wt.% MgO Hy-Norm Basalts normalized to 12 wt.% MgO