The Mantle Lherzolite xenolith

Structure of the Earth - A reminder (I hope)

Structure of the Earth - A reminder (I hope)

Structure of the Earth

The Sources of Magmas Continental Crust (Andesitic) Oceanic crust (basaltic) Upper Mantle

Ultrabasic (and basic) rocks Mantle Rocks Ultrabasic (and basic) rocks Above the 400km discontinuity With an Al-rich phase Clinopyroxenite!

Equigranular textures Lherzolite Equigranular textures Opx is usually enstatite Cpx is usually Cr-diopside

Al-rich Phases Plag lherzolites imply heating at low pressure Garnet suggests depths of > 60-80 km

Mantle wall rocks transported by rapidly Mantle Xenoliths Mantle wall rocks transported by rapidly ascending mantle-derived magmas. Magma Types Alkaline Magmas - Alkali Basalts, Basanites Highly Alkaline Magmas - Nephelinites, phonolites Ultrabasic Magmas - Kimberlites Megacrysts/ Xenocrysts

San Carlos Olivine Mine Arizona, USA Mantle Xenoliths San Carlos Olivine Mine Arizona, USA

Mantle Xenoliths Ascent rates of 5 m/s (basalts) to 50 m/s (kimberlites) are required to prevent mantle xenoliths settling out.

Mantle Xenoliths: Commonest Types (1) Lherzolites - spinel, garnet bearing. (2) Harzburgites - spinel-bearing. (3) Pyroxenites - sometimes as veins within other xenoliths suggesting they are crystallised magmas that have intruded the lithospheric mantle. (4) Eclogites - basaltic inclusions dominated by garnet + sodic pyroxene. (5) Metasomites - Metasomatised mantle.

Eclogites Essential Minerals Jadeiitic (sodic) pyroxene Pyrope Garnet Accessory Minerals Quartz, coesite, kyanite, rutile, dolomite, diamond

Eclogites: Origin Eclogites have the same bulk composition as basaltic rocks. Eclogite phase transition helps to drive plate motions. Densities Basalt = 3.0 g/cc Eclogite = 3.5 g/cc

Metasomites Basalts & Alkaline Magmas kaersutite (amphibole) suite paragasite (mica) suite Kimberlites K-richterite, phlogopite, associated with MARID suite mica-amphibole-rutile-ilmenite-diopside. Glimmerites! IRPS (ilmenite-rutile-phlogopite-sulphide) suite.

Metasomites

Cumulate Xenoliths Not all xenoliths are from the mantle. Cumulate xenoliths are common in basalts and sample accumulated phenocrysts. Magma Chamber Cumulates Layered, aligned phenocrysts, poikolitic textures, intercumulus liquid. Commonly dunites. Peridotites may contain plag.

Distribution of Xenoliths

Ophiolites Slices of the oceanic lithosphere obducted onto the land.

Ophiolite Mantle Sequences Sequence in the Oman Ophiolite Layered Gabbro GM Dunite cumulate RM Gabbro, dunite pxenite Moho Spinel Harzburgites (plag Sometimes formed during obduction) Spinel Lherzolites

Alpine Ophiolites Ophiolites found in orogenic belts which mainly consist of the mantle sequence (they can, however, contain eclogites where the transformation has been caused by tectonic forces). Alpine peridotites sometimes contain plagioclase lherzolite.

Mantle Melting Under “normal” conditions the mantle doesn’t melt.

Mantle Melting: Friction & LVZ

Mantle Melting: Decompression Melting

Mantle Melting: Volatiles

Mantle Melting: Composition Feldspar and quartz crystals Glass (cooled magma) Rhyolite (seen through a microscope!) Rare Earth Elements are incompatible elements are not easily accommodate in crystal structures due to the HFSE nature Incompatible elements are partitioned into a melt phase.

Compatibility/Incompatibility Bulk Mantle Rock Light Rare Earth Most incompatible Heavy Rare Earth Least incompatible

Compatibility/Incompatibility The most incompatible elements are the most enriched. Melt generated from rock Original Rock Light Rare Earth Most incompatible Heavy Rare Earth Least incompatible

Compatibility/Incompatibility Melt generated from rock Original Rock Residual rock The most incompatible elements are the most depleted. Light Rare Earth Most incompatible Heavy Rare Earth Least incompatible

Compatibility/Incompatibility Enrichment in incompatibles decreases with increasing melting! 10% melting 50% melting 100% melting Light Rare Earth Most incompatible Heavy Rare Earth Least incompatible

Mantle Melting: Affect on Composition Incompatible elements - those that partition into magmas (e.g. K, Ti, S). Compatible elements - in crystal phases

Depleted Mantle Most mantle xenoliths from the lithosphere are depleted indicating basalt melt extraction. Metasomites are enriched in incompatibles due to metasomatism.

Fertile Mantle High temperature sheared garnet lherzolites (from kimberlites) have near bulk Earth compositions and are samples of the asthenosphere. Sheared lherzolite xenolith Deformed lherzolite (from ophiolite)

Mantle Melting: Affect on Mineralogy

Mantle Melting: Affect on Mineralogy

Mantle Melting: Affect on Mineralogy

Mantle Melting: Affect on Mineralogy

Mantle Melting: Affect on Mineralogy Lherzolite Harzburgite Dunite with increasing melting

Structure and Composition of the Mantle K-rich, hydrous metasomites Garnet perid, diamond, sheared lhz