Lecture 16 resources 7 diamond Flashcards
what forms can carbon take
Graphite and diamond
Facts about diamond
- hardest structure
- resists scratching, maintains polish
- global reserves 1.2 billion carats
price for one carat is between $1,300 and $16,500
Where are diamonds found
historically South africa, namibia - more recently russiam brazil, canada and australia.
is value related to weight for diamonds
no also related to colour, quality, quantity.
so some mines may produce more diamond but have less value.
Who sells diamonds
Central selling organisation - de beers - london based
cartel - maintains high price and keeps markets stable
represented more than 80% of global trade, up to half of the 20th centaury.
where are diamond sources found/linked to
core of ancient crust - sit in centre of plates we’ve been looking at.
- core of plates and ancient links.
geology of diamond
carrot shaped craters formed by explosive volcanism
- some is eroded and transported to offshore diamond deposits.
primary source - kimberlite pipe
what is a kimberlite (geochemistry)
Ultramafic - primitive, start deep in earths mantle - early magnesium rich melt
ultrapotassic - lots of potasium
near primitive - start as melt and don’t fracture to granite textures.
REE-enrichment
Moderate LILE enrichment
not uniform in composition - olivine mantle like rocks are preserved.
High H2O and CO2
kimberlite - formation
carrot like intrusion
magma comes up and interacts with basement rocks and they are transported towards the surface.
Where are diamonds found?
Xenocrysts within potassium rich ultramafic rocks called Kimberlite. or diamond placers - can be traced to eroded kimberlites.
- thick continental crusts.
what kind of intrusions are kimberlites?
cylindrical - extend upwards from deep tabular dike-like bodies.
pipes include fragments of many rocks and minerals - including diamond, held together by kimberlite magma - many fragments are pieces of rock from the mantle (mantle nodules).
What is the diamond distribution in the lower mantle?
not uniform
- diamonds are not present in all intrusions.
- more than 5000 bodies only 30% contain diamond and 1% minable concentrations.
how old are diamonds
suggests most diamonds are xenocrysts and older than the magma transporting them.
volatility of kimberlite diamond deposit
Highly volatile
rapid accent to the surface
graphite diamond boundary
Where are they stable?
more than 150km below the earths surface and they go down to greater than 300km
magma above 150km (above line) cannot be brought to the surface.
slow accent = diamonds transform to graphite therefore on a diagram can assume if diamond they must have ascended to the surface quickly.
2) part of mantle withtin the magma
3) diamond rich layers int he mantle - probably discontinuous - requires thick cold crust above otherwise occur at too shallow depth.
