mantle melting.pdf

EES 303: IGNEOUS PETROLOGY UNIT-4 GENERATION OF BASALTIC MAGMAS AND THEIR DIVERSIFICATION OUTLINE TOPICS THAT WE WILL COVER IN THIS COURSE: o Mantle Petrology o Mantle melting o Generation of Basalts o Magma differentiation processes o Tectonic influences on magma gene...

EES 303: IGNEOUS PETROLOGY UNIT-4 GENERATION OF BASALTIC MAGMAS AND THEIR DIVERSIFICATION OUTLINE TOPICS THAT WE WILL COVER IN THIS COURSE: o Mantle Petrology o Mantle melting o Generation of Basalts o Magma differentiation processes o Tectonic influences on magma generation Petrology of the Mantle MANTLE PETROLOGY Direct access to rocks from the mantle: 1. Ophiolites o Fragments of oceanic lithosphere thrust onto the edge of continents and/or incorporated into mountain belts, during tectonic collision o Erosion then exposes a characteristic section of sedimentary, mafic, and ultramafic rocks o Show a considerable range in size, thickness, and degree of structural integrity o Lowermost ultramafic part Lithospheric mantle 2. Alpine-type units o Smaller slivers of oceanic lithospheric mantle, dismembered and incorporated into deformed mountain belts o Tend to be highly metamorphosed and tectonically deformed MANTLE PETROLOGY Direct access to rocks from the mantle: 3. Dredge samples o Oceanic lithospheric mantle sections exposed in transform faults o Slow-spreading ridges have extensive detachment zones exposing deeper mantle o Drag bottom-sampling dredges along the scarps 4. Nodules/xenoliths in basalts o Mantle samples entrained and carried to the surface by basalts o Can give access to deeper mantle rocks o Can be metamorphism/deformation free o Alkali basalt hosted shallower mantle Kimberlite hosted deeper mantle MANTLE PETROLOGY Petrology of the upper mantle of the Earth Dominantly peridotitic lithologies- olivine, clinopyroxene and orthopyroxene upper mantle formation of deeper rock types within the earths upper mantle. the mineral assemblages present in a rock depend on the specific conditions of p and t that it has experienced deeper mantle lherzolite as pressure and temperature increase with depth in the earths mantle the rock type transitions from plagioclase peridotite to spinel peridotite and eventually to garnet peridotite Mantle Melting MANTLE MELTING What tells us that melting takes place in the mantle? Partial Melting o Transformation of a certain fraction of the 1. Direct evidence solid (rock) into a liquid phase (melt) Volcanism at oceanic crust o Different minerals have different melting Melting happens temp. lower T minerals start to melt while Mid-Atlantic ridge, Hawaii etc. locally and higher T remain solid distribution of melt 2. Indirect evidence o Melts start forming at grain margins and is heterogeneous Seismic imaging of the Earth’s slowly creates an interconnected network interior of melt in pore spaces 3-D X-Ray Tomography of a partially molten rock MANTLE MELTING What tells us that melting takes place in the mantle? Partial Melting o Transformation of a certain fraction of the 1. Direct evidence solid (rock) into a liquid phase (melt) Volcanism at oceanic crust o Different minerals have different melting Melting happens temp. lower T minerals start to melt while Mid-Atlantic ridge, Hawaii etc. locally and higher T remain solid distribution of melt 2. Indirect evidence o Melts start forming at grain margins and is heterogeneous Seismic imaging of the Earth’s slowly creates an interconnected network interior of melt in pore spaces o When significant vol. is reached, melt is “Extracted”, leaving behind the “Residue” Melt Partially molten Residue MANTLE MELTING Geothermal gradient and P-T phase diagrams under “normal” mantle conditions, partial melting shouldn’t take place There must be changes in some of the intrinsic properties of the mantle which will allow partial melting 1. Raising the temperature 2. Lowering the Pressure 3. Changing the composition MANTLE MELTING 1. Raising the temperature Local perturbations in heat flow Accumulating heat to intersect the geotherm with the i. Labelled “Hotspots” solidus of mantle peridotite ii. Narrow pipe-like conduits of basaltic magma that appear to have a stationary source in the mantle iii. Plate motion produces Volcanic Tracks iv. Heat source attributed to column of hotter material rising up from the lower mantle or core-mantle boundary and causing melting in shallower areas of the mantle How to accumulate heat?? Decay of radioactive elements (U, Th and K) i. Extremely low concentrations in the mantle ii. 107 years to raise the temperature of a peridotite by 1° C iii. Generated heat would dissipate iv. Extraction melt would further deplete residue of U, Th, K no further melting MANTLE MELTING 1. Raising the temperature Accumulating heat to intersect the geotherm with the solidus of mantle peridotite Local perturbations in heat flow i. Labelled “Hotspots” ii. Narrow pipe-like conduits of basaltic magma that appear to have a stationary source in the mantle iii. Plate motion produces Volcanic Tracks iv. Heat source attributed to column of hotter material rising up from the lower mantle or core-mantle boundary and causing melting in shallower areas of the mantle Local phenomenon and have been usually observed away from plate boundaries MANTLE MELTING Decompression melting is a process where hot mantle rock ascends towards the Earth’s surface, leading to a decrease in 2. Lowering the Pressure pressure o Melting can be achieved in a peridotite system by lowering the pressure at constant temperature o However, localized pressure lowering would cause ductile mantle material to flow from higher-pressure areas and reinstate lithostatic equilibrium o More plausible way moving mantle material upwards, decreasing the pressure but keeping their heat content intact o If the rate of rise is low, heat content dissipates o Rate of rise has to be sufficiently rapid zero heat loss Adiabatic Process o Rising material would follow 10° C/GPa geotherm Mantle Adiabat o Decompression Partial Melting o Divergent plate boundary Mid-Ocean Ridge o Once melting starts, latent heat of fusion will absorb (MOR) o Calculations (Langmuir et al., 1992) show that mantle heat from the rising mass shallowing of the mass has to rise ~ 150 km and suffer 20-30% geotherm limited quantities of melting decompression partial melting to produce amount of basalt in MOR MANTLE MELTING 3. Changing the composition Amphibole o Observations show that mostly elemental composition of mantle is fixed o Volatile composition can vary H2O and CO2 How do we know that mantle contains any volatiles at all? o Mantle rocks sometimes contain hydrous minerals Amphibole and Phlogopite Phlogopite Structurally bounded water (OH- ions) MANTLE MELTING 3. Changing the composition o Observations show that mostly elemental composition of mantle is fixed o Volatile composition can vary H2O and CO2 How do we know that mantle contains any volatiles at all? o Mantle rocks sometimes contain hydrous minerals Amphibole and Phlogopite Structurally bounded water (OH- ions) o Micro-scale volatile inclusions in mantle minerals Fluid inclusions (H2O and CO2) Vapor bubble MANTLE MELTING 3. Changing the composition o Observations show that mostly elemental composition of mantle is fixed o Volatile composition can vary H2O and CO2 How do we know that mantle contains any volatiles at all? o Mantle rocks sometimes contain hydrous minerals Amphibole and Phlogopite Structurally bounded water (OH- ions) o Micro-scale volatile inclusions in mantle minerals Fluid inclusions (H2O and CO2) o Carbonate inclusions in mantle minerals and in the groundmass of kimberlites MANTLE MELTING 3. Changing the composition o Observations show that mostly elemental composition of Ocean geotherm mantle is fixed a b c f o Volatile composition can vary H2O and CO2 Adding H2O drastically lowers the solidus temp. (more at higher P) geotherm intersects the wet-solidus partial melting However, “normal” mantle contains only 0.1-0.2 wt.% H2O That H2O Structurally bound water Hence, two criterions need to be satisfied: i. Free H2O must be present ii. P-T must be sufficient for melting MANTLE MELTING 3. Changing the composition o Observations show that mostly elemental composition of Ocean geotherm mantle is fixed a b c f o Volatile composition can vary H2O and CO2 Adding H2O drastically lowers the solidus temp. (more at higher P) geotherm intersects the wet-solidus partial melting However, “normal” mantle contains only 0.1-0.2 wt.% H2O That H2O Structurally bound water Hence, two criterions need to be satisfied: i. Free H2O must be present ii. P-T must be sufficient for melting MANTLE MELTING 3. Changing the composition o Observations show that mostly elemental composition of Ocean geotherm mantle is fixed a b c f o Volatile composition can vary H2O and CO2 Shield geotherm d e f Adding H2O drastically lowers the solidus temp. (more at higher P) geotherm intersects the wet-solidus partial melting However, “normal” mantle contains only 0.1-0.2 wt.% H2O That H2O Structurally bound water Hence, two criterions need to be satisfied: i. Free H2O must be present ii. P-T must be sufficient for melting MANTLE MELTING 3. Changing the composition o Observations show that mostly elemental composition of Ocean geotherm mantle is fixed a b c f o Volatile composition can vary H2O and CO2 Shield geotherm d e f Adding H2O drastically lowers the solidus temp. (more at higher P) geotherm intersects the wet-solidus partial melting However, “normal” mantle contains only 0.1-0.2 wt.% H2O That H2O Structurally bound water Hence, two criterions need to be satisfied: i. Free H2O must be present ii. P-T must be sufficient for melting MANTLE MELTING 3. Changing the composition o Observations show that mostly elemental composition of mantle is fixed o Volatile composition can vary H2O and CO2 Breakdown of amphibole/phlogopite releases low amounts of H2O f o Partial melting amount will be small~ 1% o Not extractable; creates a interconnected film We need more adsorbing to grain surfaces free H2O to o Low-Velocity Layer (LVL) retards seismic wave generate more velocity amounts of melt! o Depth range 60-200 km matches with breakdown experiments and seismic refraction data