Magma Generation

• Know the mineralogic characteristics of the upper mantle at different depths

Phase diagram for aluminous 4-phase Lherzolite

Aal-phase:

Plagioclase

-shallow (<50km)

Spinel

-50-80 km

Garnet

-80-400km

Si->vi COORD

>400 km

Classification of Volcanic Rocks Geochemical

Tholeiitic Basalt

Groundmass: Usually fine-grained, intergranular

No olivine

CPX= augite (plus possible pigeonite)

OPX (hypersthene) common, may rim OL

No alkali feldspar

Interstitial glass and/ or quartz common

Phenocrysts: Olivine rare, unzoned, and may be partially reorbed or show reaction rims of OPX

OPX uncommon

Early plagioclase common

CPX is pale brown augite

Alkaline Basalt

Groundmass: Usually fairly course, intergranular to ophitic

olivine common

titaniferous augite (reddish)

OPX absent

interstitial alkali feldspar or feldspathoid may occur

interstitial glass rare, and quartz absent

Phenocrysts: Olivine common and zoned

OPX absent

PLag less common, and later in sequence

CPX is titaniferous augite, reddish rims

Each Magma Series is chemically distinct
Evolve via FX as separate series along different paths

Tholeiites are generated at mid-ocean ridges

-Also generated at oceanic islands, subduction zones
Alkaline basalts generated at ocean islands
-Also at subduction zones

Magma generation

-What is melting? – Mantle lithology
-How does melting happen?
Heating, decompression, hydration
-What is produced by melting? – primary
magma at different pressure/ H2O contents
-Mantle heterogeneity and plate tectonics

Sources of mantle material

Ophiolites:

-slabs of oceanic crust and upper mantle

-thrust at subduction zones onto edge of continent

Dredge samples from oceanic fracture zones

Nodules and xenoliths in some basalts

Kimberlite xenoliths

-Diamond-bearing pipes blasted up from the mantle carrying numerous xenoliths from depth

Know the different mantle melting mechanism

Melts can be created under realistic circumstances:

Hot spots -> localized plumes of melts

Plates separate and mantle rises at mid-ocean ridges

-Adiabatic rise -> Decompression melting

Fluid Fluxing may give LVZ

-Also important in subduction zones and other settings

More complicated melting scenarios

Fluid induced melting and metasomatism of mantle wedge - arc magmatism

Crustal melting after basalt ponding at level of neutral buoyancy (LNB)

Rift related too

What happens to the solid when a rock is heated to ~ 1275°?

Partially melts

What will the first comp. of the first melt be?

-At E more Di-rich melt

What happens to the solid comp as melt is continuously formed?

-Solid becomes depleted in Di moves to An

Mantle melting

Any composition in this system starts to melt at the ternary eutectic E

-melt is multiply saturated with PL,OPX and CPX

As eutectic melt E composition is removed- solid residue moves toward OL-OPX boundary

Harzburgite melting at Ol- OPX cotectic after around 30% melt

Accumulated melt moves up to cotectic

Decompression melting:

Can happen at mantle plume, beneath a rift, or beneath a mid-ocean ridge

Fluid induced melting and metasomatism of mantle wedge- arc magmatism

Crustal melting after basalt ponding at level of neutral buoyancy (LNB) Rift related too.

How does the mantle melt?

1.     Increase the temperature- thermal plume

2.     Lower the pressure

-Adiabatic rise of mantle with no conductive heat loss

-Decompression melting could melt at least 30%

3.     Add volatiles  (especially H20) subduction zones

 

Melting at convergent margins:

Flux melting due to slab dehydration and water lowering the solidus of the overlying mantle

The Effect of water on melting

Dry melting: solid -> liquid

Add water, water enters the melt

The more mafic the rock the higher the melting point

All solidi are greatly lowered by water

 

Conclusion:

All though the addition of water can drastically reduce the melting point of rocks, the amount of melt produced at the lower temperature may be quite limited, depending on the amount of water available

We know the behavior of water-free and water-saturated melting by experiments, which are easy to control by performing them in dry and wet sealed vessles. What about real rocks?

Some may be dry, some saturated, but most are likely to be in between these extremes

-a fixed water content < saturation levels

-a fixed water activity (H20 relative to other volatile species, mostly CO2)

The Albite-Water System

Red curves = melting for a fixed mol % water in the melt (X)

Blue curves tell the water content of a water saturated melt

Conclusion:

All though the addition of water can drastically reduce the melting point of rocks, the amount of melt produced at the lower temperature may be quite limited, depending on the amount of water available`

Raise a melt with a ratio of albite:water 1:1

X melt water = 0.5

from a point a at 925C and 1GPa pressure, toward the earth’s surface under isothermal conditions

Conclusions:

-A rising magma with a fixed % water will progressively melt

-At shallower levels it will become saturated, and expel water into its surroudings

Melting of Albite with a fixed activity of Hfixed activity of H22OO

Fluid may be a CO2-H2O mixture with Pf=Ptotal

In many situations CO2 only acts to dilute the effects of water

Heating of amphibole-bearing peridotite

Island Arc Petrogenesis

Effect of Pressure, Water, and CO2 on the position of the eutectic in the basalt system

Increased pressure moves the ternary eutectic (first melt) from silica-saturated to highly undersaturated alkaline basalt

 

Water moves the eutectic towards higher silica, while CO2 moves it more to alkaline types

What’s the difference between a lherzolite and a harzburgite?  A dunite?

Lherzolite: A type of peridotite with Olivine > OPX +CPX

 

Lherzolite is probably fertile unaltered mantle

Dunite and harzburgite are refractory residuum after basalt has been extracted by partial melting

Which is considered “fertile undepleted” mantle.  Which is the most refractory?

Understand the different origins of alkaline vs tholeiitic basalts from melting of either enriched or depleted mantle sources (i.e depth or melting, fraction of melting, etc.), and the petrographic distinctions between them

1.     Depleted Mantle

-Tholeiite easily created by 10-30% PM

-more silica saturated at lower P

-Difficult to generate alkaline basalt

2. Enriched mantle

-Tholeiites extend to higher P than for DM

-Alkaline basalt field at higher P yet

And lower % Partial melt

REE data for oceanic basalts

Spider diagram for oceanic basalts

Review of Sr Isotopes

87Rb->87Sr

-Rb (parent) concentrated in enriched reservoir (incompatible)

-Enriched reservoir develops more 87Sr over time

-Depleted reservoir (less Rb) develops less 87Sr over time

 

Nd and Sr Isotopes of ocean basalts: “Mantle Array”

Sm-Nd: Evolution curve is opposite to Rb - Sr

What conditions (chemical characteristics) are necessary for a magma to be considered a primary mantle melt?

Primary magmas:

-Formed at depth and not subsequently modified by FX or Assimilation

-Criteria

·       Highest Mg# (100Mg/(Mg+Fe)) really -> parental magma

·       Experimental results of Lherzolite melts

-Mg#= 66-75

-Cr> 1000 ppm

-Ni >400-500ppm

-Multiply saturated

-low % SiO2, highest Mg # (100Mg/(Mg+Fe))(parental magma)
-high extrusion temperature

Multiple saturation

-Low P

Ol then Plag then Cox as cool

70C T range

-High P

CPX then plag then Ol

-25 km get all at once

=multiple saturation

suggests that 25 km is the depth of last eq with the mantke

Heterogeneous Mantle Models

Magmatic product erupted at different settings are different in their concentrations of incompatible trace elements and are assumed to be derived from different regions of the mantle

What are the different “reservoirs”?

-Portions of mantle with different compositions

-Defined based on isotopes and trace elements

What is the scale (size) of heterogeneity?

-MORB- depleted mantle

-OIB- mixing of depleted mantle and three other reservoirs (dated 1-2 Ga old, but younger than the Earth)

-Some OIBs picked up a high He3/hE4 reservoir- primitive mantle?

Experiments on melting enriched vs. depleted mantle samples:

1. Depleted mantle

-tholeiite easily created by 10-30% PM

-more silica saturated at lower P

-difficult to generate alkaline basalt

Experiments on melting enriched vs. depleted mantle samples:

2. Enriched Mantle
-tholeiites extend to higher P than for DM

-alkaline basalt field at higher P yet

and lower% PM

Summary

A chemically homogeneous mantle (unlikely) can yield a variety of basalt types
-Alkaline basalts are favored over tholeiites by deeper (hydrous or carbonated) melting and by low % PM
-Mantle is heterogeneous on many scales
-Different types of melting occurring in different tectonic environments result in significantly different rock series

How can mantle geochemical heterogeneity be preserved with mantle convection?

Mantle convection model needs to satisfy:
1) Different geochemical reservoirs
2) Seismic tomographic models
3) Thermal balance
4) Geodynamically feasible

Layered model

Many geochemists have favored
models in which the mantle is
chemically and dynamically
layered, with the upper mantle
being DMM and the lower mantle
being primitive (A).

Geodynamic Model

Seismic tomography showing subducted
down-going oceanic plate in the lower mantle

Recent calculations at an Earth-like
convective vigor find that the lower mantle
becomes well mixed and outgassed over
billions of years

Primitive blob model

Plate tectonics introduces a toroidal
component to mantle flow, which can
cause chaotic mixing even in steady-
state 3D flows, resulting in efficient
mixing, although unmixed “islands” can
occur within efficiently mixed regions.

Blobs with a viscosity 10 to 100 times
that of normal mantle deform slowly
enough to remain unmixed after billions
of years

Complete recycling model

l D’’ layer at core-mantle
boundary
l Mixing of recycling lithosphere
and crust for High He3/4
reservoir

high-3He/4He reservoir (primative piles)

The high-3He/4He reservoir is a primitive,
primordial layer (containing the missing
heat-producing elements) with a deeper
boundary.
l primitive material forms a discontinuous
layer (E) consisting of two piles
corresponding to the two megaplumes
observed under Africa and the Pacific in
seismic tomographic models

undulating layer (deep primiatuve layer)

The second study proposed a global,
undulating layer of average thickness
1300 km (but varying by ;1000 km)
and constituting ;30% of mantle mass
(F), overlain by a recycled crust layer

Various mantle convection and reservoir models