1. The Mantle
Lherzolite xenolith

2. Structure of the Earth - A reminder (I hope)

3. Structure of the Earth - A reminder (I hope)

4. Structure of the Earth

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

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

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

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

9. 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

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

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

12. 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.

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

14. 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

15. 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.

16. Metasomites

17. 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.

18. Distribution of Xenoliths

19. Ophiolites
Slices of the oceanic lithosphere obducted onto the land.

20. 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

21. 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.

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

23. Mantle Melting: Friction & LVZ

24. Mantle Melting: Decompression Melting

25. Mantle Melting: Volatiles

26. 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.

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

28. 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

29. 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

30. 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

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

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

33. 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)

34. Mantle Melting: Affect on Mineralogy

35. Mantle Melting: Affect on Mineralogy

36. Mantle Melting: Affect on Mineralogy

37. Mantle Melting: Affect on Mineralogy

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

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