How does the mantle melt?
How does the mantle melt?
1) Increase the temperature
Melting by raising the temperature
Melting by raising the temperature
Solidus:
Temperature of
melting,
increases with
depth (P)
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Temperature - Geochemistry - Lecture Slides, Slides of Geochemistry
In these Lecture Slides, the Lecturer has put emphasis on following key points : Temperature, Increase, Mantle Melt, Lower the Pressure, Adiabatic Rise, Conductive Heat Loss, Decompression, Melt At Least, Decompression, Nearly Isothermal
Typology: Slides
2012/2013
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How does the mantle melt?How does the mantle melt?
1) Increase the temperature
Melting by raising the temperatureMelting by raising the temperature
Solidus:Temperature ofmelting,increases withdepth (P)
2) Lower the pressure
– Adiabatic rise of mantle with no conductive heat loss– Decompression melting could melt at least 30%
Melting by (adiabatic) pressure reduction. Melting begins when the adiabatcrosses the solidus and traverses the shaded melting interval. Dashed linesrepresent approximate % melting.
Adiabatic paths:Nearly isothermaldecompression http://www.whitman.edu/geology/winter/John Winter (2001) An Introduction to Igneous and Metamorphic Petrology
How does the mantle melt?^ How does the mantle melt?
Magma Formation
-^
Two major mechanisms:–
Decompression melting
-^
Passive asthenospheric upwelling at mid
-^
ocean ridges
-^
Rising plume head at “hot spots”
Fluid-fluxed (volatile-aided) melting
-^
Hydrous melting in subduction zones
-^
H
O released from amphibole, mica, serpentine minerals 2 in subducting crust
-^
Lowers solidus
of overlying mantle
-^
Magmas generated are different from decompressionmelts
Pressure-temperature phasediagram showing the soliduscurves for H
O-saturated and 2
dry granite.A H
O-saturated granitoid just 2
above the solidus at
A
will
quickly intersect the solidus asit rises and will thereforesolidify.A hotter, H
O-undersaturated 2
granitoid at
B
will rise further
before solidifying.
Magma Formation^ Magma Formation
The MidThe Mid
Ocean Ridge SystemOcean Ridge System
MORBMORB
Minster et al. (1974)
Ocean islands and seamountsOcean islands and seamounts
OIBOIB
Crough (1983)
MORBs, OIBs and LIP’s sample agreat expanse of oceanic mantle inplaces where crustal contaminationis minimal and provide incomparableevidence to the nature of the mantle
Why are^ Why are
MORB’sMORB’s
,^ ,
OIB’sOIB’s
andand
LIP’sLIP’s
so important?so important?
BasalttypenBasalttypen
•^
Tholeiitbasalt
~50% SiO
2
Mg-reicherOlivin (Fe-reich in Qz-Tholeiiten
Ca-reicher Plg
± Augit (Cpx)
± Opx (Hypersthen)
Pigeonit (Ca-reicherHypersthen, bzw. Ca-armer Cpx)
•^
Alkalibasalt
~45% SiO
2
Olivin (in der Grundmasse)Ca-reicher Plg
Augit
± Nephelin
kein Opx, kein Pigeonit
Diopsid
Pigeonit Hypersthen
Orthopyroxen
Augit
Hedenbergit
Ferrosilit
Enstatit MgSiO
FeSiO
CaFeSiO
CaMgSiO