Ocean Floor Basalts (MORB)
Igneous Petrology 423, Francis 2013
The eruption of MORB basalts is the
dominant form of active volcanism on
the Earth today:
MORB:
OIB:
Arc:
20 km3/yr
2 km3/yr
6 km3/yr
Young Oceanic Crust
Approximately 60% of the surface of the Earth
is composed of basalt or its gabbroic intrusive
equivalent, which are composed largely of
pyroxene and feldspar (< 50%). The oceanic
crust averages about 6 km in thickness, but
ranges from 0 km at mid-ocean ridges to 10
km near the continents. Ophiolites are thought
to be sections of oceanic crust “obducted” onto
land. These sequences suggest that the oceanic
crust consists of a thin layer of sediments
(layer 1) over basaltic pillow lavas (layer 2a)
underlain by basaltic dykes (layer 2b), then
gabbro, and finally layered sequences of mafic
and ultramafic cumulates produced by magma
chambers at the crust - mantle boundary (layer
3). More recent work indicates that the
boundary between 2a and 2b is actually the
transition to zeolite facies conditions in which
primary porosity is filled with secondary
minerals, and that the volcanic – dyke
transition occurs within layer 2b.
Oceanic Crust
glassy
pillow
margin
palagonite
Characteristic features of MORB:
• Dominant phenocrysts are olivine followed by plagioclase. Only the more evolved compositions have
clinopyroxene phenocrysts, commonly after olivine phenocrysts have disappeared, presumably because of
the olivine reaction relationship.
• Hypersthene and olivine normative tholeiitic basalts with a relatively restricted compositional range
corresponding to a density minimum along their liquid line of descent.
• Primitive lava compositions (high MgO) are the most Fe-poor tholeiitic basalts on the Earth.
• Combined high Al (Al2O3 = 15-18 wt.%) and high Ca (CaO = 11-13 wt.%), primitive OIB or Hotspot
tholeiitic basalts commonly have low Al (< 15 wt.% Al2O3) and low Ca ( 10 wt.% CaO).
• The Lowest oxidation states (Fe3+/Fe = 0.05 - 0.10) of all basaltic magmas.
• N-MORB are the most depleted of modern lavas in terms of incompatible trace elements, especially
LIL elements (K, Rb, Ba) and LREE. They are, however, relatively rich in HREE, which are relatively
unfractionated.
•
N-MORB defines an isotopic end-member of the terrestrial volcanic array characterized by low
87Sr/86Sr ratios and high 143Nd/144Nd ratios, indicating that their source has experienced a long term
depletion in Rb and Sm. This is interpreted to be the signature of the convecting asthenosphere.
MORB
Petrographic Characteristics of MORB
Picrites (MgO > 12 wt.%)
Relatively rare,
Highly porphyritic lavas (20+%) with olivine megacrysts to Fo 91.
Olivine basalts (MgO = 10 - 12 wt.%)
Porphyritic basalts with more olivine than plag. phenocrysts.
Predominate along the central ridge axis and valley floor.
Olivine - plagioclase basalts (MgO  10 wt.%).
Porphyritic basalts with plag. more abundant than olivine.
More common on central valley scarps and walls.
Plagioclase basalts (Al2O3  19 wt.%)
Highly porphyritic basalts with plag. megacrysts to An 90.
Probably represent plag. cumulates.
Plagioclase - clinopyroxene basalts (MgO  7 wt.%)
Relatively aphyric basalts with minor cpx and plag.
No olivine microphenocrysts.
Most common as diabase dykes and sills.
Melting Experiments
MORB
MORB
has
an
extremely
restricted
range of major element
compositions.
MORB compositions cluster at
the density minimum for
tholeiitiic fractionation trends,
ie. At the point of apearance of
feldspar.
MORB has an extremely restricted range of major element compositions.
Water Depth and Extent of Partial Melting
There is a positive correlation between water depth and the Na2O content of MORB, and
a negative correlation with CaO / Al2O3. All things being equal, these chemical
parameter are measures of the degree of partial melting, whereas water depth is an
inverse measure of the thickness of the oceanic crust. Thus, the higher the degree of
partial melting, the thicker the oceanic crust, and the lower its Na2O content, but the
higher its CaO / Al2O3 ratio. The degree of partial melting is controlled by the depth that
the mantle adiabat crosses its solidus, which is a function of its potential temperature.
CaO / Al2O3 increases with degree of partial melting
Increasing degree of
melting
Increasing degree
of melting
FeO ?
Trace Element
Characteristics of MORB
Whereas the major element and compatible
trace element compositions of MORB exhibit
a
quite
restricted
range
worldwide,
incompatible trace elements exhibit a
relatively large variations, even within single
sites, ranging between two end-members:
• N-MORB that are strongly depleted
in LIL, LREE, Th, U, and Nb
• E-MORB with relatively enriched in
LIL, LREE, Th/U, Nb/Zr, and have
higher H2O contents. Primitive
examples have slightly higher Al and
lower Fe contents than primitive N
MORB.
E and N-MORB are difficult to tell apart in terms of major elements or compatible trace
elements, although the ratio of K/Ti can be used to distinguish them. N-MORB dominates,
but specimens of E-MORB are found everywhere (typically accounting for 2-5% of
samples), although they are most abundant near hot spots, transform faults, and on small
sea mounts peripheral to spreading axes.
E and N MORB Aong Ridge Axis
Element Decoupling
Mixing between liquids
representing different melt
fractions?
The apparent decoupling between
highly incompatible trace elements and
compatible and major elements in
MORB could reflect mixing of melts
produced by different degrees of partial
melting.
The
fractionation
of
highly
incompatible elements such as Ba
(KBa ~ 0.001) and Th (KTh ~ 0.002)
with respect to incompatible elements
seen in such La (KLa ~ 0.01) that is
seen in E-MORB would require
extremely small degrees (~0.2%, F =
0.02) of partial melting, which is
inconsistent with the similar major
element compositions of E and N
MORB.
This leaves us with the requirement that
the trace element enrichment in EMORB is a feature of its mantle source,
rather than very small degrees of partial
melting.
Can E-MORB be produced by
a smaller degree of melting of the mantle source
that produces N-MORB?
Silicate melt inclusions trapped in olivine from single specimens of MORB can exhibit a larger range
in terms of degree of enrichment in incompatible trace elements than the whole rock samples of
MORB from all over the world. These results indicate that the enriched and depleted components in
MORB are intermixed on a very fine scale in their mantle sources.
MORB Isotopes
N-MORB defines an
isotopic end-member of
the terrestrial volcanic
array characterized by
low 87Sr/86Sr ratios and
high 143Nd/144Nd ratios,
indicating that their
source has experienced a
long term depletion in
Rb/Sr and Sm/Nd ratios.
The incompatible trace
element enrichment of EMORB is associated with
elevated 87Sr/86Sr and
143Nd/144Nd
decreased
isotopic ratios compared to
N-MORB
2 types
of
Enrichment
The incompatible trace
element enrichment of EMORB is associated with
elevated 87Sr/86Sr and
143Nd/144Nd
decreased
isotopic ratios compared to
N-MORB, opposite to the
correlation observed at
many hot spots, such as
Hawaii.
The
source
enrichment
responsible in E-MORB must
be old with respect to the
source enrichment in Hawaiian
highly understaturated lavas.
Plots of parent/ daughter ratios against the isotopic composition of the daughter isotope
result in apparent mantle isochrons with ages of approximately 300 ma.
Origin of E-MORB
The similarity of the major and compatible element compositions of E and N MORB
suggest that they both reflect similar degrees of partial melting of sources with similar
mineralogy. The trace element and isotopic characteristics of E-MORB, however, appear
to require a two stage process:
1)
Low degree partial melts locally metasomatized mantle to create an
enriched source for E-MORB that is mixed with the non-metasomatized mantle on
a small scale.
2)
After a significant time period, larger extents (10-15%) of melting of this
enriched mantle source produces E-MORB
The ~ 300 ma mantle isochrons defined by E and N MORB need not reflect a single
enrichment event, but could reflect the residence or survival time of the enriched mantle
reservoir. If the mass of the enriched mantle reservoir is a few percent of the N-MORB
source, then the mass of the MORB system approximates that of the upper mantle and
300 million years is the approximate turnover time for convection in the upper mantle.
Signature of Garnet in MORB?
1)
Hafnium Paradox: MORB's have 176Hf/177Hf ratios indicating derivation from a source with a long term
Lu/Hf ratio greater than chondrites, yet they have a measured Lu/Hf ratio lower than
chondrites. One way of explaining this is to have garnet in the mantle residue with a
complementary high Lu/Hf ratio.
176Lu
2)
Depleted HREE:
3)
230Th/238U
4)
LREE-depleted
Peridotites
excess:

176Hf
+
e- +

+ 
t1/2 = 3.5 1010 years
 = 1.94  10-11/ yr
Many MORB basalts have (Sm/Yb)n ratios of 1.3 to 1.5, suggesting slight depletion in
the heavy rare earths (HREE). This has been interpreted as a residual garnet effect.
The effect is small, however, and MORB have relatively high and unfractionated HREE
in comparison to all OIB basalts.
MORB have 230Th/238U activity ratios greater than 1, implying that residual garnet may
be holding back U preferentially to Th. As the garnet D’s for both these elements are
very small, this explanation will only work for very small degrees of partial melting.
Furthermore E-MORB tends to have higher 230Th excesses and lower Lu/Hf ratios than
N-MORB.
Some MORB peridotites are so depleted in LREE compared to HREE, that it is difficult
to model them as partial melt residues, unless melting occurred in the garnet stability
field.
Features #1 through #3 are, in fact, best developed in E-MORB, and thus the "garnet
signature" may in fact come from the enriched component(s).
A garnet signature in E-MORB?
It is unlikely that residual garnet is
stable along a mantle peridotite
solidus at the depths at which the
bulk of MORB appears to be
generated.
Does the “garnet signature” in MORB reflect the presence of veins
or blobs of garnet pyroxenite in its mantle source?
Solidii for Garnet Pyroxenite and Peridotite
The presence of
small amounts of
water and CO2 might
produce
small
degree partial melts
in
adiabatically
rising mantle which
would then migrate
to enrich parts of the
host mantle.
Such a model has a
hard time explaining
the mantle isochrons
defined by E-MORB
The release of water
due to the break down
in
wadsleyite
to
olivine in a rising
mantle plume could
lead to small degrees
of partial melting at
the top of the 410
discontinuity.
Such a model also has
a hard time explaining
the mantle isochrons
defined by E-MORB