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: 20 km3/yrOIB: 2 km3/yrArc: 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

glassypillow

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

MORB has an extremely restricted range of major element compositions.

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 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 MeltingThere 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

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.

Element Decoupling Mixing between liquids representing different melt fractions?

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 E-MORB 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.

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.

MORB Isotopes

The incompatible trace element enrichment of E-MORB is associated with elevated 87Sr/86Sr and decreased 143Nd/144Nd isotopic ratios compared to N-MORB

2 types of

Enrichment

The incompatible trace element enrichment of E-MORB is associated with elevated 87Sr/86Sr and decreased 143Nd/144Nd 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 176Hf + e- + + t1/2 = 3.5 1010 years = 1.94 10-11/ yr

2) Depleted HREE: Many MORB basalts (especially EMORB) 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

3) 230Th/238U excess: 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 extremely small degrees of partial melting. Furthermore E-MORB tends to have higher 230Th excesses and lower Lu/Hf ratios than N-MORB.

4) LREE-depleted Some MORB peridotites are so depleted in LREE compared to HREE, that it is difficult

Peridotites 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.

Solidii for Garnet Pyroxenite and Peridotite

Does the “garnet signature” in MORB reflect the presence of veins or blobs of garnet pyroxenite in its mantle source?

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 has a hard time explaining the mantle isochrons defined by E-MORB