1. Generation of Basaltic Magma GLY 4310 - Spring, 2019
Petrology Lecture 6
Generation of Basaltic Magma
GLY Spring, 2019
How has the crust of the earth formed? This is an extremely important question in our quest to understand the earth. Oceanic crust is basalt. Many other igneous rocks can be generated from the basaltic magma by processes such as fractional crystallization and assimilation. Thus, the generation of basaltic magma serves as a good starting point in trying to understand the evolution of the crust.

2. Magma Series Definition
A group of rocks that share some chemical, and sometimes mineralogical, characteristics
They share patterns on chemical variation diagrams which suggests a genetic relationship – beyond the scope of this course

3. Major Magma Series J.P. Iddings, 1892
Alkaline
Sub-alkaline
C.E. Tilley (1950) split sub-alkaline into:
Tholeittic
Calc-alkaline
J.P. Iddings,
(left)
There are three major magma series. In a classic paper called "The Origin of Igneous Rocks", J.P. Iddings, in 1892, first divided magmas into two series, the alkaline and the sub-alkaline.
The sub-alkaline series has been further divided into the tholeiitic and calc-alkaline series by C.E. Tilley in Tholeiitic magmas are found at divergent plate margins, almost to the exclusion of the other types. Alkaline rocks are sometimes found in the initial stages of rifting. Calc-alkaline rocks are restricted to subduction related plate tectonic processes. The relationship of each series to tectonic environments is shown in table 8-7. Because calc-alkaline rocks are limited to convergent plate margins, we won't consider them at this point.
More information about J.P. iddings at:
More information about C.E. Tilley at:
C.E. Tilley,
(right)

4. Relationship of Magma Series to Plate Tectonics
Tholeiites are far more voluminous than alkaline rocks. They form at mid-ocean ridges (MORB), as well as in intra-plate volcanic centers (OIB, or Oceanic Island Basalt), where they created oceanic islands, such as Hawaii. Alkaline basalts are restricted to intra-plate occurrences, or occasionally to eruptions at a convergent boundary which pass through oceanic rock on the way to the surface. The alkaline rocks are sometimes called alkaline-olivine basalts, because the alkaline rocks commonly have olivine in both the phenocrysts and groundmass, whereas tholeiitic rocks never have olivine in the groundmass, and only rarely in the phenocrysts. Thus, any magma generation model proposed must be able to account, at least, for tholeiitic and alkaline basalts separately.
Since shear seismic waves pass through the crust and the mantle, these regions are essentially solid. The generation of magma necessarily involves melting rock. Since the crust is derived from the mantle, we need to examine melting within the mantle. Basaltic magmas are known to erupt in the ̊C range. They have cooled as they rose.

5. Geotherms
Estimated ranges of oceanic (hatched) and continental (solid) steady-state geotherms to a depth of 100 km using upper and lower limits based on heat flows measured near the surface
Examination of the geotherms shows these melts must have come from at least 100 km down. Seismic evidence, which often shows deep seismic activity prior to an eruption, supports this conclusion. This region is in the upper mantle.
We have never penetrated the mantle with a drill, so direct samples of the mantle are unavailable. Nevertheless, there are four sources of rocks believed to have originated in the upper mantle, as well as one extra-terrestrial source that yields information:

6. Sources of Upper Mantle Rocks
Ophiolites
Dredge Samples from oceanic fracture zones
Nodules in basalts
Autoliths
Restites
Xenoliths in kimberlites
Stony meteorites – analogous to upper mantle of a broken planet
1. Ophiolites - Sheet-like mafic to ultramafic rocks, sometimes thrust onto continental borders or incorporated into continental mountain ranges. They are composed of oceanic crust and upper mantle. Erosion has exposed these rocks in some areas. The ultramafic rocks in the lower portion of the ophiolite sequences are believed to represent the upper mantle. In the mountains, we sometimes find slivers referred to as alpine peridotites. These also contain ultramafic portions. The ultramafic rocks in both regions contain peridotites, usually harzburgite and dunite, and subordinate rocks such as wehrlite, lherozolite, and pyroxenite.
2. Dredge samples from oceanic fracture zones - The fracture zones are transform faults, and significant fault scarps are sometimes found there. At the base of the scarps, broken rocks accumulate. In the 1960's and 70's, much dredging of these samples occurred. These samples, like the ophiolites on the continents, are restricted to the uppermost mantle. The rocks seen are very similar.
3. Nodules in basalts - Nodules are found in some basalts, usually fist-sized or smaller. They are xenoliths, rocks that felt into the magma and survived. They are found only in alkaline basalts, never in tholeiites. Many types of nodules are found. They include gabbro, dunite, and harzburgite, which are common rocks in the crust and upper mantle. There are also lherzolite in three varieties (spinel, plagioclase, and garnet) and eclogite. Nodules may be autoliths, rocks genetically related to the magma, rather than picked up from the wall or roof rocks. Restites, which are refractory residuum left by partial melting, are also found. (Picture next slide) The garnet lherzolites are of the most interest, because they are high-pressure rocks found in the most alkaline, silica-deficient rocks. This suggests there origin in deep in the mantle, and that they rose very quickly to the surface.
4. Xenoliths in kimberlites - Kimberlite pipes are always found in continental areas. They appear to tap upper mantle sources between km in depth, with the magma rising very rapidly to the surface. Along the way, xenoliths of both upper mantle and crustal rocks are incorporated. The least altered deep samples are composed of spinel or garnet lherzolites. (Photo two slides forward)
5. Stony meteorites - Recent meteorite collections in Antarctica have enhanced our knowledge of the stones. Some of these specimens are believed to represent the interior, but not the core, of a broken planet.
The general conclusion from all of these sources is that the mantle is composed of ultramafic rocks. Spinel and garnet lherzolites, if partially melted, would yield a magma of basaltic composition. The density and seismic properties of the lherzolites also corresponds to that observed in the mantle. Lherzolites are often composed of four phases: olivine, opx, cpx, and an aluminous phase such as garnet, spinel, or plagioclase. The dunites and harzburgites of the shallow mantle are probably residuum left by the partial melting of lherzolite.

7. Restite Restite inclusions: Right of the knife,
Two smaller left of the knife,
Midsize to the bottom left of the knife,
All inclusions are sub-parallel, most likely an orientation due to flow of granite
Source:

8. Kimberlite
Premier Kimberlite Pipe, Mesoproterozoic, ~1.2 Ga; Premier Mine, near Cullinan, NE South Africa
The Premier Kimberlite Pipe erupted about 1.2 billion years ago, during the Mesoproterozoic
It is significantly diamondiferous
Diamond are xenolithic inclusions believed to form in the mantle 
Photo:

9. Basaltic Partial Melt TiO2 vs. Al2O3
Extraction of a basaltic partial melt from lherzolite can result in solid refractory harzburgite or dunite
Brown and Mussett, A. E. (1993), The Inaccessible Earth: An Integrated View of Its Structure and Composition. Chapman & Hall/Kluwer.
Figure 10-1 shows the relationship. Application of the lever rule to figure 10-1 allows us to estimate that about 20-25% of the lherzolite melts, leaving 75-80% as Al-free ultramafic residuum.
Caution: Some olivine/opx mixtures have textures which indicate they are the product of fractional crystallization in magma chambers at or near the top of the mantle.

10. Lherzolite: A type of peridotite with Olivine > Opx + Cpx
Dunite 90
Peridotites
Wehrlite
Harzburgite
Lherzolite 40
Lherzolite is a type of ultramafic rock in which ol > (cpx + opx). When the pyroxene is almost exclusively opx, the rock becomes a harzburgite, when cpx, it becomes a wehrlite. Rocks composed almost exclusively of olivine are dunites
Olivine Websterite
Pyroxenites
Orthopyroxenite 10
Websterite 10
Clinopyroxenite
Orthopyroxene
Clinopyroxene
Figure 2.2 C After IUGS

11. Aluminous Lherzolite Phase Diagram
Al – phase
Plagioclase
shallow (< 50 km)
Spinel
50-80 km
Garnet km
Si ® VI coord.
> 400 km
Thus, aluminous lherzolite represents the prime candidate for being undepleted (fertile) mantle. The spinel and garnet lherzolites are nearly identical chemically, but different mineralogically. How is this possible? Figure 10-2 shows a phase diagram that clarifies the problem. As pressure increases, plagioclase is replaced by spinel, and then by garnet. The shallow slope of the phase boundaries suggests that ΔV is more important than ΔS (Clapeyron equation). Thus, the reactions are more pressure sensitive than temperature sensitive. In addition, it should be noted that the oceanic geotherm lies below the solidus for lherzolites, so under normal conditions the mantle does not melt.
Melting of the Mantle
How is it possible to heat the mantle above the normal geotherm? The obvious thought is heat from radioactive decay. However, the long-lived radioactive elements are mostly all large (K, U, and Th) and cannot be accommodated in mafic minerals. Indeed, they produce less than 10-8 J g-1 a-1. This is probably not enough heat to compensate for losses due to thermal conductivity, and certainly not enough to raise the temperature significantly. Even very unlikely concentrations of radioactive elements within the mantle would not be nearly enough to cause melting. So what does produce the heat?
Figure Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70,

12. Mantle Melting Increase in temperature Problem: No realistic mechanism
Perhaps with local hot spots, with very limited area
Hot spots
Hot spot volcanism, which occurs above stationary positions in the mantle, are one possible source. The hot spot is a narrow pipe of basaltic magma, originating in the mantle. The source of heat is not known. Many people attribute the heat to heat exchange at the base of the mantle, possibly with the core. The outer core is liquid. Convection cells within the liquid are possible, and the top of such a cell might transfer enough heat to the base of the mantle to cause localized melting.
Lowering the Pressure
The dry peridotite solidus has a positive P/T slope. Thus, lowering the pressure at constant T would eventually produce melting. But how might the pressure be lowered? The mantle is thought to be ductile, so any pressure imbalances would be quickly corrected by flow from high to low pressure.
Raising a solid mass of rock might work. A decrease in pressure would cause a slight expansion, which would lower the temperature. Experimental results show this temperature lowering to be in the range 10-20̊C/GPa, or about ̊C/km. As the mass rose, it would also be moving up the geotherm, to cooler areas, and heat loss by thermal conduction would occur. Unless the rate of rise were high, the temperature would follow the geotherm, with no possible melting.

13. Decompression Melting
Decompression melting under adiabatic conditions
When adiabat crosses solidus, melting begins
Dashed lines represent approximate % melting
At least 30% melting is realistic
If the rise were done under adiabatic (without loss of heat) conditions, then the rock temperature would follow the adiabat, shown in figure This curve is a loss of about 12̊C/GPa. It is impossible to follow a true adiabat, but rapidly rising masses would come close. The melting curve is decreasing about 130̊C/GPa, so it is possible for the rock to initiate melting. Once melting begins, the heat of fusion will consume energy, and keep the adiabatic path to a shallow slope. Melting will occur, but slowly, and with limited quantities of melt. The process is called decompression melting.
Why would the upwelling occur at all? At a divergent plate boundary, the plates are pulling apart, leaving a potential void. The ductile mantle flows upward, creating a rapid upwelling. It has been calculated that 10-20% melting/GPa, or about 0.3%/km of upwelling, will occur. If the upwelling material began at the solidus temperature, rising km would produce the 20-30% melting thought necessary to produce MORB. But since rock is more likely at the geotherm, more heat would be necessary, and the probable starting point is about 150 km deep.

14. Addition of Volatiles
Figure Dry peridotite solidus compared to several experiments on H2O-saturated peridotites
Remember solid + water = liq(aq)
LeChatelier principle leads to dramatic lowering of melting point of peridotite
Volatiles in the Mantle
Ultramafic rocks like those in the mantle ordinarily do not contain volatiles. Minerals like olivine, pyroxene, and calcic plagioclase are anhydrous. In mantle xenoliths, there are small amounts of phlogopite, an amphibole, and sometimes serpentine, all of which are hydrous. None are stable above 600̊C, so all are clearly alteration products. There is also the possibility of carbon dioxide. Small fluid filled inclusions are often filled with liquid carbon dioxide. Carbonate minerals are also found. Although H2O saturated melts would be below the geotherm in the mantle, the mantle is very far from being saturated with either water or carbon dioxide. Estimates of water concentration are in the range of up to % maximum. In areas around subduction zones, water content is clearly higher. The amount of melt that can be produced with so little water is very limited, probably no more than 1%. This is not enough to form a separate melt, but may account for at least part of the seismic low velocity zone. The depth of the seismic low velocity zone is 60 to 220 km, which agrees well with where some water might be generated, at depths near 90 km. Similar considerations for carbon dioxide show small levels of melting in the kilometer depth range.
Basalts from a Chemically Uniform Mantle
Assuming the chemical composition of the mantle is constant does not imply that the mineralogy is. Garnet, spinel, and plagioclase lherzolites all have the same composition, but melting will not begin at the same temperature, since garnet, spinel, and plagioclase all have different melting points. The effect of pressure will also differ on different minerals, since they have different compressibilities.

15. Effect of Pressure on Initial Melting
Change in eutectic position with increasing pressure
First melting occurs at the eutectic
After Kushiro, 1968
Increased pressure moves the ternary eutectic minimum from the oversaturated tholeiite field to the under-saturated alkaline basalt field
Alkaline basalts are thus favored by greater depth of melting
Figure 10-8 shows the effects of increasing pressure on the ternary eutectic in the system Neph-Fo-Silica. As pressure increases from 1 atm to 3 GPa, the eutectic shifts from the silica oversaturated region, to a greatly undersaturated area. At one atmosphere, albite is melting, but at 3 GPa, albite is unstable, and is replaced by jadeitic pyroxene. This tells us that saturated, tholeiitic basalts will be produced by shallow melting, whereas alkaline basalts are favored by deep melting, because the latter are silica undersaturated.

16. Pyrolite Melting
A.E. Ringwood,
In 1967, the late A.E. Ringwood of Australian National University performed a series of experiments on a synthetic material called pyrolite, which was supposed to represent the earth’s mantle. Pyrolite is a mixture of olivine, clinopyroxene, orthopyroxene, plagioclase, and small amounts of other phases. In these experiments, they varied the pressure to simulate various depths below the surface, down to about 140 kilometers below the surface. Figure 10-9 summarizes the results.
The most notable aspect of their work was the lack of a separate spinel phase at any depth. The melt at 25 km (0.9GPa) is more silica-saturated than the initial melt at 60 km. As temperatures were increased on the 60 km runs, the melts were initially alkaline, but as more material melted, it diluted the early alkalinity, producing a tholeiite basalt. Alkaline elements are highly incompatible, and tend to fractionate strongly into the early formed melts. As the alkali is exhausted, more silica melts. The residuum beyond 20% melt is harzburgite (ol + opx). A dunite residuum would require a much higher % of melting. Also present is a thermal divide which separates the alkaline magma from the tholeiitic magma at low pressure. This disappears at higher pressures.
Ringwood’s experiments considered only the effects of partial melting.
More information about A.E. Ringwood:
Nature of the liquids and refractory residua associated with partial melting of pyrolite
After Green and Ringwood (1967)

17. Fractional Crystallization of Basaltic Magmas
Minerals fractionating are listed near arrows
After Wyllie, 1971
Peter Wyllie took the process a step further, considering both partial melting and the removal of a phase by fractional crystallization. At a given depth of melting, fractional crystallization is simply the reverse of partial melting. By allowing the melt to rise to a shallower depth, the fractional crystallization process removes a different material than was originally melted, producing a wider spectrum of products. For example, using the 30% melt of Ringwood at 60 km, and removing an aluminous enstatite phase results in an alkaline basalt, the same phase produced by 20% partial melting at 60 km. The thermal divide again disappears at higher pressures.
Another notable feature is the solubility of Al in the pyroxene phase at high pressure. As the magma rises, pyroxene containing little Al crystallizes. This enriched the resulting liquid in Al. At higher pressures, where the Al remains in the pyroxene phase, an Al and Si poor (undersaturated) magma results. The product of this liquid is a nephelinite.
Other experiments, although differing in details, have confirmed the general ideas found in the Ringwood and Wyllie papers. Summarizing this work:
More information about Peter Wyllie:
Peter Wyllie,
1930-

18. Partial Melting and Fractional Crystallization

19. Trends in Partial Melts
Spinel lherzolites, from 6 to 35% partial melt
Low melt leads to alkaline basalt, higher % melts to more tholeittic compositions
After Hirose and Kushiro, 1991
Figure shows the work of Hirose and Kushiro on the melting of dry natural spinel lherzolite. High pressure melts are undersaturated (nephelinite) at low degrees of partial melting, and increasing silica saturated at higher degrees of melting. Higher pressures also favor olivine.
Basalt petrogenesis can be summarized in another table.

20. Basalt Petrogenesis

21. Magma Types
Primary Magma - one that forms by melting at depth, without any later modification
Derivative - A primary magma that has been modified by some magma differentiation process on the way to the surface
Parental - Most primitive magma type within a given magma series, it may or may not be primary
A primary magma is one that forms by melting at depth, without any later modification. A derivative magma is a primary magma that has been modified by some magma differentiation process on the way to the surface. Since it is difficult to tell if such processes have occurred, a parental magma is defined as the most primitive magma type within a given magma series.
Solutions may be unsaturated, singly saturated (one phase precipitating), or multiply saturated. In a two-component system, multiply saturated means two phases are simultaneously crystallizing. In a ternary system, two (cotectic line) or three (ternary eutectic) phases can simultaneously crystallize.
Multiply saturated magmas might appear to be primary magmas. When a magma is produced by partial melting, it is necessarily at the eutectic. However, as magmas rise, the pressure decreases, and the eutectic composition changes. Figure 10-8 showed that considerable changes in the eutectic position occur as pressure is changed. Therefore, any magma which is multiply saturated at low pressure is in equilibrium with the solid phases at low pressure, and the corresponding depth is too shallow for magma generation. It has been modified by fractional crystallization and therefore cannot be primary.
Forward vs. Inverse Methods
We can use melting experiments to determine the properties of rocks and magmas, and their generic relationships. If we synthesize a mantle material like pyrolite to determine potential derivatives, as Ringwood did, we are said to be using the forward method. Melting of surficial rocks to try and see what their parent rock was is called the inverse method.

22. Multiple saturation Low P Ol then Plag then Cpx as cool ~70oC T range
Figure shows the result of one set of inverse experiments. The rock in question was a glass from MORB. At one atmosphere, it is saturated with olivine at 1215̊ with plagioclase and then cpx being added as the sample cooled.
Figure 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,

23. Multiple saturation High P Low P Ol then Plag then Cpx as cool
70oC T range
High P
Cpx then Plag then Ol
At high pressure, cpx would be followed by plagioclase, then olivine.
Figure 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,

24. Multiple saturation High P 25 km get all at once 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 equilibrium with the mantle
Multiple saturation implies that the liquid corresponding to the melted basalt in the experiment was in equilibrium with Ol + Cpx + Plag at 25 km depth
This is the appropriate mineralogy for a lherzolite at this depth
The phase diagram derived from experiments shows the sample would be saturated with four phases (ol, cpx, opx, plag) at about 1250̊C and 0.8 GPa. This is then a quaternary eutectic. The phases seen are those associated with plagioclase lherzolite, and the pressure is consistent with what we saw previously about plagioclase lherzolite, that it is stable above 30 kilometers depth. Thus this appears to be a magma that was in equilibrium with the mantle at 25 km depth. It probably represents the point where the magma separated from the mantle solids, and began to act as a separate system. Thus, it might actually be a parental magma, rather than a primary magma.
Proving that a magma is primary is thus quite difficult. Showing that it is derivative, and therefore not primary, is considerably easier. Any magma in equilibrium with a low-pressure eutectic is likely derivative.
Other criteria for the determining that a magma is primary have been suggested. Magmas containing dunite or peridotite nodules may be primary. These nodules are extremely dense, and their presence indicates the magma rose rapidly, with little chance to fractionate. The composition of the olivine is another indicator. If the olivine is in the range Fo86-91, the magma may be primary. Extremely magnesium rich olivines are rare, and had to form at high temperatures, deep within the earth. High contents of Cr (> 1000 ppm) and Ni (>400 ppm) are also indicative of high temperature of formation. A basalt in equilibrium with olivines of this composition should have a MgO/(MgO + FeO) ratio of
Basalts from a Chemically Heterogeneous Mantle
By the end of the 1970's it was clear that all common types of basalts could form from a homogeneous mantle. But the samples we believe came from the mantle do show some variety, belonging to either the lherzolite, harzburgite, or dunite types. The major element composition of these types is reasonably constant, but there is enough variation to separate samples into two groups, the fertile and the depleted xenoliths. Fertile xenoliths, sometimes termed “enriched”, are higher in Al, Ca, Ti, Na, and K than depleted xenoliths. In addition, their Mg/(Mg+Fe) and Cr/(Cr + Al) ratios are lower than depleted xenoliths. The fertile xenoliths thus have higher concentrations of incompatible elements, which are likely to separate quickly into an early formed magma. The most fertile samples are garnet and spinel lherzolites, in that order. The most depleted is dunite.
The terms fertile and enriched may not be synonymous. Some xenoliths contain phlogopite and amphiboles, which are hydrous phases. They may be primary, but the textures sometimes indicate addition of volatiles via an external fluid phase. Thus, fertile means undepleted, whereas enriched implies undepleted with added components.

25. OIB and MORB Upper, REE diagram Lower, Spider diagram
Data from Sun and McDonough, 1989
The increasing use of microprobe techniques of analysis has added considerable knowledge of trace element chemistry, complicating the picture further. Figure illustrates one problem. The diagrams are called REE diagrams, for REE, and Spider diagrams, for multi-elements, respectively. If the concentration of each element is plotted versus increasing atomic number, trends, such as a positive or a negative slope, are often noted. So is the increasing-decreasing sawtooth pattern produced by the Oddo-Harkins effect. This can eliminated by normalizing the concentrations relative to a standard. CI chondrites are often used for this purpose. A spider diagram is similar to the REE diagram, except it uses a wide range of trace elements.
The abbreviation CI is derived from the C for carbonaceous and from the I for Ivuna, the type locality in Tanzania. CI chondrites are sometimes called C1, with the 1 in C1 standing for the type 1 meteorites in the classification scheme of Van Schmus-Wood. Type 1 meteorites normally have no recognizable chondrules.
Figure shows a depletion of trace elements as the atomic number increases for the OIB rocks. This corresponds to an enrichment of incompatible elements in the OIB by partial melting of a peridotite, or fractional crystallization of a peridotite melt.
MORB presents a real problem, however. It shows a pattern of Heavy Rare Earth Element (HREE) enrichment, as well as enrichment in the heavier trace elements. Neither partial melting nor fractional crystallization can produce this effect. This suggests the mantle is already Light Rare Earth Element (LREE) depleted! The mantle is thought to have a composition like the CI chondrites. If so, the plots should all have a value of 1 or each element, and no slope. Only if the mantle were previously depleted in LREE could we get a melt like MORB, with a positive slope.
The OIB show no similar pattern, and appear to be differentiated from a fertile mantle source. This strongly suggests that there are at least two distinct reservoirs, one depleted in LREE, and the other undepleted.

26. LREE enriched
LREE depleted
or unfractionated
REE Variation
Chondrite-normalized REE diagrams for spinel and garnet lherzolites
After Basaltic Vocanism Study Project, 1981
LREE depleted
or unfractionated
LREE enriched
Figure shows REE diagrams for spinel and garnet lherzolites from OIB and MORB rocks. The same patterns emerge. It appears the LREE-enriched rocks must be truly enriched, rather than simply non-depleted. Some process must have added non-compatible LREEs to these rocks.
We can also look at isotopic chemistry. Isotopes are normally compared using isotopic ratios, like 143Nd/144Nd or 87Sr/86Sr. Since the ions are chemically identical for a given element, and are very heavy, they will not mass fractionate during either melting or crystallization processes. The initial isotopic ratio, corrected for age, will be identical to that of the source from which they were originally melted.
The neodymium isotopes are part of the samarium-neodymium system. Both elements are LREE, and are incompatible. Neodymium has a lower atomic number than samarium, and is therefore slightly larger (lanthanide contraction). It is slightly preferentially concentrated into the liquid phase, relative to samarium. Thus the Sm/Nd ratio decreases in partial melts, or in late liquids undergoing fractional crystallization. 147Sm is radioactive, and alpha decays to 143Nd. 144Nd is not a radiogenic product, so its concentration is constant over time. The rate of change of 143Nd/144Nd thus depends on the amount of 147Sm present.
The strontium isotopes are part of the rubidium-strontium system, very commonly used in the radiometric dating of rocks. 87Rb decays to yield 87Sr via beta decay. 86Sr is not affected by any radiogenic decay. Thus the ratio 87Sr/86Sr will change with time, depending on the concentration of 87Rb . 87Rb behaves like K, so it is concentrated in micas, amphiboles, and to some extent in K-spar. Sr is similar to Ca, so it concentrates in plagioclase and apatite, but is too large to fit in cpx. Rubidium will thus concentrate in a magma, and be enriched in crustal rocks derived from mantle melts. The slope of line plotted as 87Sr/86Sr versus time will be higher for magma derived by partial melting, and lower for the depleted residuum.
The initial concentration ratio of 87Sr/86Sr in melt and residuum will be identical, since no mass fractionation occurs. With time, the 87Sr/86Sr will change, as the 87Rb decays. The slopes of the 87Sr/86Sr will change after a melting event. The slope for the rocks formed from the melt will be steeper, while the slope for the residuum will be less.
The neodymium system is just the opposite. Neodymium is a larger ion than samarium, and will concentrate in the liquid. Because the parent isotope (samarium) is concentrated in the residuum, the 143Nd/144Nd ratio will increase more in the residuum than in the melt.

27. 143Nd/144Nd vs. 87Sr/86Sr Upper, oceanic basalts
Lower, ultramafic xenoliths from subcontinental mantle
MAR = Mid-Atlantic Ridge
EPR = East Pacific Rise
IR = Indian Ocean Ridge
Plotting neodymium isotope ratios versus rubidium-strontium ratios gives Figure Depleted mantle will evolve to high values of 143Nd/144Nd and low values of 87Sr/86Sr. If MORB is derived from depleted mantle, it should show similar ratios. Figure 10-16a shows that MORB plots exactly where we would expect it to. The star, which is the chondritic abundance we believe the original earth should have had, plots below and to the right. Hawaii, which is OIB, plots part way down from the MORB area.
Figure 10-16b shows the same sort of plot, using data from mantle xenoliths. These xenoliths are enriched in strontium relative to the chondrite value. A few of the oceanic islands also show the same sort of enrichment (Figure 10-15a). This suggests that there are regions within the mantle, including some
but not all regions beneath the ocean, which are enriched in strontium, and less enriched in neodymium.
The sub-oceanic mantle is thought to be separated into two regions. One is above 660 km in depth, and has Si4+ in IV coordination. The other, below 660 km, has Si4+ in VI coordination (perovskite structure). The upper region is depleted, the lower region less depleted, possibly even enriched. If this idea is correct, what are the consequences?

28. Mantle Convection Models
After Basaltic Vocanism Study Project, 1981
The traditional model of circulation within the mantle shows convection from the top to the bottom of the mantle. (Figure 10-17a) The convection cells are thought to be the major driving force of plate tectonic movement. Such circulation would homogenize the two proposed reservoirs. We should see roughly equivalent isotopic ratios for rock formed from any mantle source.
A second possibility is a two regions do not mix. There may be separate convection cells within each region, but density differences would be strong enough to prevent mixing. (Figure 10-17b) The density differences between the layers need not be large to prevent mixing. Various estimates run from 3% down to 0.1%. The generation of MORB at the spreading centers is sufficient, over time, to deplete the upper zone. The lower layer remains undepleted, and even be slightly enriched relative to chondritic abundance. The oceanic islands fed by hot spots get their magma from the lower region, and thus show a different isotopic composition than the MOR's. Some, like Tahiti, are actually enriched. Most, like Hawaii, the Azores, and St. Helena are simply less depleted than MORB. As the magma for these latter volcanoes rises, mixing must occur with the depleted upper mantle material. Indeed, it would seem very unlikely that some mixing didn't occur on the way up from the 660 km division. The xenoliths seen in ultramafic rocks are more enriched than even Tahiti and Gough, suggesting that the magma for these volcanoes has had a minor degree of mixing.
It is possible that some mantle plumes stall in the upper region. Subduction will also return both depleted, and some enriched, magmas to the upper region. Thus it is possible this region is heterogeneous, with layers or lenses of enriched material, despite the stirring. The rigid lithospheric mantle may preserve many of the inhomogeneities imposed upon it since its formation.

29. Partial Melting Experiments
“Opx out” and “Cpx out” = degree of melting at which these phases are completely consumed
Since the isotopic evidence strongly supports a two layer mantle, we need to consider the implications for basalt generation. Figure shows partial melting experiments on both depleted (a) and fertile (b) lherzolite. Steep, dashed lines are the percentage of partial melting, whereas the sharply curved lines give the olivine content. Melting of either depleted or enriched sources at depths of km can yield tholeiitic basalts at 10-40% partial melting. Alkaline basalts are harder. They can only be generated by 5-20% partial melting of fertile lherzolite, at depths of between 40 to 90 kms. This generally agrees with earlier results, except that the depleted magmas are unable to generate alkaline basalts under any conditions.
A 1992 paper by Wyllie indicates a further complication exists. His results indicate that dry lherzolites melts at depth are nearly incompressible, so the ΔV approaches zero on going from solid to liquid. Indeed, beyond a pressure of 7 GPa (more than 200 km), ΔV may even be negative, like water. This causes both the solidus and liquidus to change slope (for example in figure 10-2 – next slide).
Left, depleted lherzolites; Right, fertile lherzolites
Dashed lines = % partial melt produced
Shaded area = condition required for the generation of alkaline basaltic magmas

30. Changing ΔV
ΔV approaches zero on going from solid to liquid as pressure increases
Causes both solidus and liquidus to change slope as depth increases
Figure 10-2 As long as they geotherm remains below the solidus, no melting would occur. The buoyancy of any melt produced would be very low, and the magma might even sink it ΔV were negative. Thus, hot spot plumes forming below 660 km face another obstacle. They may be solid diapirs rising until decompressive melting can occur at much shallower depths.