Mantle melting and basalt generation

Melting in the mantle2 principal types of basalt in the ocean basinsEach basalt type is chemically distinct and erupts at 1000-1200 CEvolve via fractional crystallization as separate series along different pathsImage source: Winter, 2001Tholeiites are most commonly generated at mid-ocean ridges, but alsooceanic islands and subduction zonesAlkaline basalts are commonly generated at ocean islands, (some inoceanic islands).

Sources of knowledge of mantleHow do we know what the mantle is made of?(2) Dredge samples from oceanicfracture zones(3) Xenoliths in some basaltsSpinel harzburgite xenolith in alkali basalt. Image: Darrell Henry(4) KimberlitexenolithsDiamond-bearingpipes blasted up fromthe mantle carryingnumerous xenolithsfrom depth

Sources of knowledge of mantleWhat is the mineralogy of the mantle and what happensto it when it melts?Lherzolite:A type of peridotite withOlivine > Opx + Cpx.Other major peridotites ofthe mantle includeharzburgite and dunite.Image: John Winter (2001)

Sources of knowledge of mantleWhat is the mineralogy of the mantle and what happensto it when it melts?Image: John Winter (2001)For peridotites with similarcompositions, the stablealuminous phase changeswith ihdepth h(P) (P). Thealuminous phase is a modifierto peridotite.Al-phase• Plagioclaseshallow (< 30 km)Pl + Ol = Opx + Cpx + Sp• Spinel30-80 kmSp + Opx = Ol + Grt• Garnet80-400 km

Sources of knowledge of mantleWhat is the mineralogy of the mantle and what happensto it when it melts?Lherzolite is probably fertileundepleted mantle i.e. themantle that has little basalticmelt extracted from it.Dunite (20-25% 25% partial melting)and harzburgite (15-20% partialmelting) are refractory residuaafter basalt has been extracted bypartial meltingImage: John Winter (2001)

Sources of knowledge of mantleHow does the mantle melt??(1) Increase T above normalgeotherm.• e.g. Radioactive decay of K, U orTh - but, these are in lowconcentration in mantle• local high heat flux at hot spots -however, these are localphenomenonImage: John Winter (2001)2) Lower the pressure• Adiabatic rise of mantle with noconductive heat loss• Decompression melting could meltup to 30% - particularly at MORs

Sources of knowledge of mantleHow do we generate tholeiitic and alkaline basalts from achemically uniform mantle?Variables (other than X)• Temperature• Pressure• Minimum meltPressure changesIncreasing P moves the ternaryeutectic to more highly silicaundersaturated compositions i.e.will generate the alkali basaltsImage: John Winter (2001)

Sources of knowledge of mantlePrimary magmasMagmas formed at depth and not subsequently modified by fractionalcrystallization or assimilation.• generates derivative magmas.• Also the most primitive of a suite of magmas is termed parental, but itmay not be primary.Criteria to evaluate the possibleprimary nature of the magmaImage: John Winter (2001)• Highest Mg# (100Mg/(Mg+Fe))g or lowest SiO 2 , only definesparental magma• Experimental results of lherzolitemelts• 100*Mg/(Mg+Fe) g rock = 66-75%• Cr > 1000 ppm• Ni > 400-500 ppm

Sources of knowledge of mantleOceanic magmasThe positive slopes of theMORB must represent meltsderived from mantle that waspreviously depleted inincompatible elementsImage: John Winter (2001) increasing incompatibilityThe negative slopes of the OIBis due to partial melting of asource undepleted or enriched inincompatible elements

Sources of knowledge of mantleOceanic magmasThese potential melt sourcerocks indicate that there must beLREE enrichment in some ofthe peridotites rather thannondepletion increasing incompatibilityImage: John Winter (2001)

Sources of knowledge of mantleSummaryImage courtesy of Donald W. Forsyth, Brown University.(1) A chemically homogeneousmantle can yield a variety ofbasalt types(2) () Alkaline basalts are favoredover tholeiites by deepermelting and by low % partialmelt(3) Fractionation at moderate tohigh depths can also createalkaline li basalts from tholeiites(4) REE are very useful fordistinguishing gthe sourcerock/minerals of melting