Liquidus Projections for haplo-basalts The Basalt Tetrahedron at 1 atm: The olivine - clinopyroxene - plagioclase plane 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.

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:

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. 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.

The Effect of Pressure 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 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. 1 atm

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.

Dry Solidus Simple Peridotite

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

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

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

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.

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

Y 2 TO 4 = Mg 2 SiO 4 - Fe 2 SiO 4 dashed lines: Kd = (Fe/Mg)olivine / (Fe/Mg)liquid = 0.3  0.03 olivine line: Fe + Mg = thin tie-lines: j oin coexisting olivine and liquid curved arrow: l iquid line of descent dotted lines: i sotherms, slope = -Kd

Dry Solidus Simple Peridotite

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

Only two known rock types have the required density to match that inferred for the mantle underlying the MOHO. Eclogite (  = ), 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 (  = ), a rock consisting predominantly of olivine ( %), 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 (Y 2 TO 4 ) predominates over pyroxene (YTO 3 ) as the ferromagnesian mineral, and feldspar is minor or absent. MOHO

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

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

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

Dry Solidus Simple Peridotite

Hyperstene Normative Basalts MgO > 8 wt.%

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