Dry Mantle Melting and the Origin of Basaltic Magma

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Dry Mantle Melting and the Origin of Basaltic
Magma

• GEOS 508 Lec 14-15-16

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Mantle melting

• Needs special condition to melt, usually solid
• Melt fractions usually low, under 2 at any given
  time
• Regardless of melting conditions, yields one or
  another variety of basaltic (low silica) melts -
  andesites, dacites, do not come out of the mantle

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2 principal types of basalt in the ocean basins
Tholeiitic Basalt and Alkaline Basalt
Common petrographic differences between
tholeiitic and alkaline basalts
Tholeiitic Basalt
Alkaline Basalt
Usually fine-grained, intergranular
Usually fairly coarse, intergranular to ophitic
Groundmass
No olivine
Olivine common
Clinopyroxene augite (plus possibly pigeonite)
Titaniferous augite (reddish)
Orthopyroxene (hypersthene) common, may rim ol.
Orthopyroxene absent
No alkali feldspar
Interstitial alkali feldspar or feldspathoid may
occur
Interstitial glass and/or quartz common
Interstitial glass rare, and quartz absent
Olivine rare, unzoned, and may be partially
resorbed
Olivine common and zoned
Phenocrysts
or show reaction rims of orthopyroxene
Orthopyroxene uncommon
Orthopyroxene absent
Early plagioclase common
Plagioclase less common, and later in sequence
Clinopyroxene is pale brown augite
Clinopyroxene is titaniferous augite, reddish rims
after Hughes (1982) and McBirney (1993).
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Each 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

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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

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Lherzolite is probably fertile unaltered
mantle Dunite and harzburgite are refractory
residuum after basalt has been extracted by
partial melting
Tholeiitic basalt
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Partial Melting
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Wt. Al2O3
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Brown and Mussett, A. E. (1993), The Inaccessible
Earth An Integrated View of Its Structure and
Composition. Chapman Hall/Kluwer.
Lherzolite
Harzburgite
Residuum
Dunite
0
0.8
0.4
0.6
0.2
0.0
Wt. TiO2
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Lherzolite A type of peridotite with Olivine gt
Opx Cpx
Olivine
Dunite
90
Peridotites
Wehrlite
Lherzolite
Harzburgite
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Pyroxenites
Olivine Websterite
Orthopyroxenite
10
Websterite
10
Clinopyroxenite
Orthopyroxene
Clinopyroxene
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Phase diagram for aluminous 4-phase lherzolite
Al-phase

• Plagioclase
• shallow (lt 30 km)
• Spinel
• 30-80 km
• Garnet
• 80-400 km
• Si VI coord.
• gt 400 km

Phase diagram of aluminous lherzolite with
melting interval (gray), sub-solidus reactions,
and geothermal gradient. After Wyllie, P. J.
(1981). Geol. Rundsch. 70, 128-153.
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How does the mantle melt??

• 1) Increase the temperature

Melting by raising the temperature.
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• 2) Lower the pressure
• Adiabatic rise of mantle with no conductive heat
  loss
• Decompression melting could melt at least 30

Figure 10-4. Melting by (adiabatic) pressure
reduction. Melting begins when the adiabat
crosses the solidus and traverses the shaded
melting interval. Dashed lines represent
approximate melting.
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HW 7

• Use pMELTS to determine if the sub SWUS upwelling
  mantle would melt along an adiabat that contains
  the PT point of 14500C and 15 kbar. A typical
  composition of the SWUS mantle is given via a San
  Carlos peridotite calculate the fraction of melt
  and phases in equilibrium with the liquid?
• What is the composition and melt fraction at 10
  kbar (at the Moho), assume 1390C?

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• 3) Add volatiles (especially H2O)

Dry peridotite solidus compared to several
experiments on H2O-saturated peridotites.
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Fraction melted is limited by availability of
water
15 20 50 100
From Burnham and Davis (1974). A J Sci., 274,
902-940.
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• Heating of amphibole-bearing peridotite
• 1) Ocean geotherm
• 2) Shield geotherm

Phase diagram (partly schematic) for a hydrous
mantle system, including the H2O-saturated
lherzolite solidus of Kushiro et al. (1968), the
dehydration breakdown curves for amphibole
(Millhollen et al., 1974) and phlogopite
(Modreski and Boettcher, 1973), plus the ocean
and shield geotherms of Clark and Ringwood (1964)
and Ringwood (1966). After Wyllie (1979). In H.
S. Yoder (ed.), The Evolution of the Igneous
Rocks. Fiftieth Anniversary Perspectives.
Princeton University Press, Princeton, N. J, pp.
483-520.
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Melts can be created under realistic circumstances

• Plates separate and mantle rises at mid-ocean
  ridges
• Adibatic rise decompression melting
• Hot spots localized plumes of melt
• Fluid fluxing may give LVL
• Also important in subduction zones and other
  settings

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Generation of tholeiitic and alkaline basalts
from a chemically uniform mantle

• Variables (other than X)
• Temperature
• Pressure

Phase diagram of aluminous lherzolite with
melting interval (gray), sub-solidus reactions,
and geothermal gradient. After Wyllie, P. J.
(1981). Geol. Rundsch. 70, 128-153.
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Pressure effects
Figure 10-8 Change in the eutectic (first melt)
composition with increasing pressure from 1 to 3
GPa projected onto the base of the basalt
tetrahedron. After Kushiro (1968), J. Geophys.
Res., 73, 619-634.
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Liquids and residuum of melted pyrolite
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Initial Conclusions

• Tholeiites favored by shallower melting
• 25 melting at lt30 km tholeiite
• 25 melting at 60 km olivine basalt
• Tholeiites favored by greater partial melting
• 20 melting at 60 km alkaline basalt
• incompatibles (alkalis) initial melts
• 30 melting at 60 km tholeiite

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Crystal Fractionation of magmas as they rise

• Tholeiite alkaline
• by FX at med to high P
• Not at low P
• Thermal divide
• Al in pyroxenes at Hi P
• Low-P FX hi-Al
• shallow magmas
• (hi-Al basalt)

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Primary magmas

• Formed at depth and not subsequently modified by
  FX or Assimilation
• Criteria
• Highest Mg (100Mg/(MgFe)) really parental
  magma
• Experimental results of lherzolite melts
• Mg 66-75
• Cr gt 1000 ppm
• Ni gt 400-500 ppm
• Multiply saturated

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Multiple saturation

• Low P
• Ol then Plag then Cpx as cool
• 70oC T range

Figure 10-12 Anhydrous P-T phase relationships
for a mid-ocean ridge basalt suspected of being a
primary magma. After Fujii and Kushiro (1977).
Carnegie Inst. Wash. Yearb., 76, 461-465.
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Multiple saturation

• Low P
• Ol then Plag then Cpx as cool
• 70oC T range
• High P
• Cpx then Plag then Ol

Anhydrous P-T phase relationships for a mid-ocean
ridge basalt suspected of being a primary magma.
After Fujii and Kushiro (1977). Carnegie Inst.
Wash. Yearb., 76, 461-465.
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Multiple saturation

• Low P
• Ol then Plag then Cpx as cool
• 70oC 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 eqm with
  the mantle

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Summary

• A chemically homogeneous mantle can yield a
  variety of basalt types
• Alkaline basalts are favored over tholeiites by
  deeper melting and by low PM
• Fractionation at moderate to high depths can also
  create alkaline basalts from tholeiites
• At low P there is a thermal divide that separates
  the two series

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Review of REE
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Review of REE
Rare Earth concentrations (normalized to
chondrite) for melts produced at various values
of F via melting of a hypothetical garnet
lherzolite using the batch melting model
(equation 9-5). From Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
increasing incompatibility
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REE data for oceanic basalts
increasing incompatibility
REE diagram for a typical alkaline ocean island
basalt (OIB) and tholeiitic mid-ocean ridge
basalt (MORB). From Winter (2001) An Introduction
to Igneous and Metamorphic Petrology. Prentice
Hall. Data from Sun and McDonough (1989).
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Spider diagram for oceanic basalts
increasing incompatibility
Spider diagram for a typical alkaline ocean
island basalt (OIB) and tholeiitic mid-ocean
ridge basalt (MORB). From Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall. Data from Sun and
McDonough (1989).
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REE data for UM xenoliths
Chondrite-normalized REE diagrams for spinel (a)
and garnet (b) lherzolites. After Basaltic
Volcanism Study Project (1981). Lunar and
Planetary Institute.
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Review of Sr isotopes

• 87Rb 87Sr l 1.42 x 10-11 a
• Rb (parent) conc. in enriched reservoir
  (incompatible)
• Enriched reservoir

develops more 87Sr over time
After Wilson (1989). Igneous Petrogenesis. Unwin
Hyman/Kluwer.
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Review of Nd isotopes

• 147Sm 143Nd l 6.54 x 10-13 a
• Nd (daughter) enriched reservoir gt Sm
• Enriched res.

develops less 143Nd over time
After Wilson (1989). Igneous Petrogenesis. Unwin
Hyman/Kluwer.
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mantle model II

• Upper depleted mantle MORB source
• Lower undepleted enriched OIB source

After Basaltic Volcanism Study Project (1981).
Lunar and Planetary Institute.
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Mantle convection model I
After Basaltic Volcanism Study Project (1981).
Lunar and Planetary Institute.
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Nd and Sr isotopes of Ocean Basalts

• Mantle Array

Initial 143Nd/144Nd vs. 87Sr/86Sr for oceanic
basalts. From Wilson (1989). Igneous
Petrogenesis. Unwin Hyman/Kluwer. Data from
Zindler et al. (1982) and Menzies (1983).
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OAHU
Pali
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Significance of the Koolau component
From Lassiter Hauri, 1996
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Koolau component-recycled crust?
Eiler et al., 1996
Lassiter and Hauri, 1998
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2mm (a,b,c), 1mm (d)
Sp lherzolite-d
Plg-lherzolite-a,b,c,
a
b
d
c
Take a look at hand specimen too!
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Sr-Nd isotopes _at_Pali, Salt Lake Crater Koolau
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Nd and Sr isotopes of Kimberlite Xenoliths
Initial 143Nd/144Nd vs. 87Sr/86Sr for mantle
xenoliths. From Wilson (1989). Igneous
Petrogenesis. Unwin Hyman/Kluwer. Data from
Zindler et al. (1982) and Menzies (1983).
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Dm , bse, em1, em2, himu
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Chemical dynamics- a word of caution

• LAB should be defined based on rheology not
  chemistry T1250 C is where olivine starts
  behaving ductily
• Asthenosphere becomes lithosphere and viceversa -
  thus chemistry is not a good indicator
• Small enriched domains in a chemically
  heterogenous mantle can supply most melts if F is
  small.

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Experiments on melting enriched vs. depleted
mantle samples
1. Depleted Mantle

• Tholeiite easily created
• by 10-30 PM
• More silica saturated
• at lower P
• Grades toward alkalic
• at higher P

Results of partial melting experiments on
depleted lherzolites. Dashed lines are contours
representing percent partial melt produced.
Strongly curved lines are contours of the
normative olivine content of the melt. Opx out
and Cpx out represent the degree of melting at
which these phases are completely consumed in the
melt. After Jaques and Green (1980). Contrib.
Mineral. Petrol., 73, 287-310.
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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

Results of partial melting experiments on fertile
lherzolites. Dashed lines are contours
representing percent partial melt produced.
Strongly curved lines are contours of the
normative olivine content of the melt. Opx out
and Cpx out represent the degree of melting at
which these phases are completely consumed in the
melt. The shaded area represents the conditions
required for the generation of alkaline basaltic
magmas. After Jaques and Green (1980). Contrib.
Mineral. Petrol., 73, 287-310.
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Need to parametrize melting

• Will do this for dry melting only
• Aim to explain major elements
• Assume adiabatic melting
• Need a melting function
• Need a start depth and an end depth
• Assume that SiO2 does not change much
• Use fractionall melting in increments of 1 kbar

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Parametrization

• Melting is linear as a function of depth
• Source is only peridotite
• Shape of melting domain is triangular no extra
  wings to scavenge traces
• Based on McKenzie and Bickle (1988) Langmuir et
  al. (1992) and Wang et al. (2002).

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Assumptions

• Ti is used as a perfectly incompatible element
• Fe and Na will constrain the depth where melting
  starts and the length of melting column
  respectively
• Thickness of melt column is also calculated (e.g.
  for MORB it should be 6 km)
• K influence the calculation - I forget why.

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Comparing against data

• Plot the major elements of your set against MgO
  (Harker type diagrams)
• Find the FeO, Na2O, TiO2 and K2O corresponding to
  the most primitive composition
• Those are the values to compare against the
  forward model
• Works for any adiabatic melting assuming that
  only peridotite is the source. You can mess with
  fertility ( cpx source), amount of MgO, Na2O,
  K2O, FeO in source.

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Na2O2.8 FeO9
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Best match

• Start at 23 kbar
• Stop at 15 kbar
• 8 kbar column of melt, stops exactly at crust
  -mantle boundary (about 50 km under the Puna)
• Predicts 2.5 km of basalt accumulated in the
  crust average melting 7
• Is this any good?

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Hits solidus at around 1450 C
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Other constraints

• Use ol-glass thermometer for magma temp
• Get xenoliths to find out how depleted/fertile a
  peridotite from under the Puna is
• Crustal and lithospheric thickness constraints
  from seismo people

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HW8

• Use Blondes et al major element data for the
  Papoose flows only to determine the FeO and Na2O
  corresponding to the most primitive MgO
• Use LPK model to determine the melt starting
  pressure, ending pressure, melt thickness and
  average F