Rare Earth Elements in some representative arc lavas Low-K - - PowerPoint PPT Presentation

Rare Earth Elements in some representative arc lavas

Low-K (tholeiitic), Medium-K (calc-alkaline), and High-K basaltic andesites and andesites. A typical N-MORB pattern is included for reference Notes:

• 1. Within each series, slope of pattern is

relative constant. Changes in REE concentrations within each series is primarily due to fractional crystallization.

• 2. Slight positive slope of low-K series is

strongly suggestive of a depleted mantle source.

• 3. Many arc mafic volcanics are more

depleted than MORB, especially in HREE.

• 4. Med-K and high-K mafic volcanics have

negative LREE slopes indicating heterogeneous mantle sources.

• 5. HREE (Dy-Yb) pattern is flat indicating that

there was no residual garnet in source region.

After Gill (1981) Orogenic Andesites and Plate Tectonics. Springer-Verlag.

MORB-normalized spider diagrams for selected island arc basalts

Normalization and element

• rdering scheme of

Pearce (1983) with LIL on the left and compatibility increasing to right from Ba-Th to Yb. Composite OIB REE pattern is shown in yellow. Notes:

• 1. Many element abundances, particularly HFSE and HREE, are lower than MORB.
• 2. Elevated values of LIL elements: Sr, K, Rb, Ba, Th.
• 3. Strongly depleted HFSE [particularly Nb, (Ta) but also Zr, Hf]
• 4. LIL and HFS appear to be decoupled in arc magmas. Why?

Clue: LIL are hydrophilic.

Figure from: Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Sr and Nd isotopic values in island arc lavas

Data sources in Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

• 1. New Britain,

Marianas, Aleutians, and South Sandwich show restricted range close to MORB indicating that DM is the principal magma source although TE indicate some additional components.

• 3. Antilles arc shows increasing sediment component from N to S (due to

proximity to S. American craton and Amazon River sediment source

• 2. Other arcs show the

effect of addition of different continental components to source, most likely Atlantic sediment (Antilles) or Pacific sediment (Banda, New Zealand)

Notes:

Variation in 207Pb/204Pb vs. 206Pb/204Pb for oceanic island arc volcanics.

Included are some

• f the isotopic

reservoirs (EMI, EMII, DM, PREMA) and the Northern Hemisphere Reference Line (NHRL). Notes: 1. Pb in some arcs overlaps MORB (DM source). 2. Data in many arcs trend towards a

• ceanic marine sedimentary reservoir. 3. Sunda arc trends towards EMII. 4. Pb data clearly

show a sedimentary component in many arc magmas (is this a young or old component?

Data sources in Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

10Be created by cosmic rays + oxygen and nitrogen in upper atmosphere and is eventually

incorporated in clay-rich oceanic sediments. 10Be has a half-life of only 1.5 Ma so after ~6 half lives (10 Ma), 10Be is no longer detectable. 10Be/9Be averages about 5000 x 10-11 in the uppermost oceanic sediments but in mantle-derived MORB and OIB magmas, & continental crust, 10Be is below detection limits (<1 x 106 atom/g) and 10Be/9Be is <5 x 10-14

The Be-B story

Figure shows 10Be/Be(total) vs. B/Be for six arcs. After Morris (1989) Carnegie

• Inst. of Washington Yearb., 88, 111-123.

Note: Suppose oceanic lithosphere is being subducted at 5 cm/yr. It would take ~3 Ma to get to a depth of 100 km. 1250x10-11 10Be

• left. Suppose 2% sediment is included in

the mantle melt, i,e, 50 x 10-11 10Be. If the magma is transported immediately to surface, we should see this 10Be. If there is a contribution from altered oceanic crust as well as sediments, we should also see an elevated B. The figure shows some results from the work of Julie Morris.

B is a stable element with a very brief residence time deep in subduction zones since it tends to escape into shallow crust and hydrosphere. B in recent sediments is high (50-150 ppm), and B is also high in altered oceanic crust (10-300 ppm). In MORB and OIB, B<2-3 ppm.

Figure from: Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Petrogenesis of arc magmas

To have any hope of understanding the origin of arc magmas, one has to know (1) the temperature distribution (thermal regime) in subduction zone environments, (2) the flow regime in the convecting mantle wedge, (3) the degree of alteration of the subducted slab, (4) metamorphic reactions in the subducted slab, (5) the fate of fluids expelled during progressive metamorphism of the subducted slab (6) melting processes in the slab and wedge, (7) extent of interaction between magmas and mantle and crust during ascent, (8) extent of fractional crystallization, (9) the role of volatiles in melting and crystallization…

• 1. What conditions are necessary for slab melting? Are such conditions commonly (or ever)

realized? How would one recognize slab melts, assuming they exist?

• 2. Are volatiles involved in arc magmatism? If so, how do we know this? What is the

source of volatiles?

• 3. What conditions are necessary for mantle wedge melting? How often are such conditions

realized? What is the composition of the wedge, depleted or enriched mantle?

• 4. What type of melting occurs in the mantle wedge? Anhydrous, V-saturated, dehydration?
• 5. What are the compositions of primary arc magmas? Do we ever see these?
• 6. What processes serve to change the compositions of primary magmas? What are these

processes and where do they occur?

Some questions:

• 7. How and why does the calc-alkalic trend (suppressed Fe-enrichment) occur?
• 8. What do we mean by underplating and why might it be an important process?
• 9. Can the composition of the continental crust be reconciled with the production of arc magmas?

Volcanic/Plutonic Arc Fore Arc Back Arc

Oceanic crust (Basalt + Greenstone + ~4%H2O) Trench Accretionary wedge Eruption of calc-alkalic magmas (flows, pyroclastics) Eruption of K-rich magmas Stable continental crust (~40 Km) Lithosphere Asthenosphere Harzburgite Lherzolite lithosphere Serpentinite Greenschist Amphibolite Eclogite

Schematic representation of processes and magmatic products at a convergent continental margin

1 º C 1 2 º C Asthenospheric flow lines (induced convection) Minor melting of H2O rich harzburgite Partial melting of Hydrated asthenosphere

• a. Dehydration of amphibole
• b. Dehydration of phlogopite

a b Underplated gabbro/amphibolite heat Crustal melting zone Plutons 50 100 50 100 km

Scale

H2O H2O

Oceanic crust (Basalt + Greenstone + ~4%H2O) Trench Accretionary wedge

Volcanic/Plutonic Arc Fore Arc Back Arc

Eruption of calc-alkalic magmas (flows, pyroclastics) Eruption of K-rich magmas Stable continental crust (~40 Km) Crustal melting zone Underplated gabbro/amphibolite Lithosphere Asthenosphere Asthenospheric flow lines (induced convection) Serpentinite Harzburgite Lherzolite lithosphere Greenschist Minor melting of H2O rich harzburgite Amphibolite Eclogite Partial melting of Hydrated asthenosphere

• a. Dehydration of amphibole
• b. Dehydration of phlogopite

H2O a b

Schematic representation of processes and magmatic products at a convergent continental margin

50 100 50 100 km

Scale

Plutons heat H2O 1 º C 1 2 º C

P-T-t paths for subducted crust assuming a subduction rate of 3 cm/year. The length

• f each curve corresponds to

~15 Ma. The curves show various situations of arc age (yellow curves) and age of subducted lithosphere (red curves, for a mature ca. 50 Ma old arc). From (Peacock, 1991, Phil. Trans. Roy. Soc. London, 335, 341-353).

Conditions required for melting of subducted slab

Figure from: Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Solidi for dry and water-saturated melting of basalt and dehydration curves of likely hydrous phases

Subducted crust P-T-t paths for various situations of arc

• age. Superimposed are some

pertinent reaction curves, including the wet and dry basalt solidi, the dehydration

• f hornblende (Lambert and

Wyllie, 1968, 1970, 1972), chlorite + quartz (Delaney and Helgeson, 1978).

Figure from: Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice

• 2. Slab melting M in arcs

subducting young lithosphere and/or young arcs.

• a. breakdown of chlorite +

quartz in subducted slab releases water and may induce some melting: probably very minor proces.

• b. Breakdown of amphibole in

relatively young lithospheric slab releases water above the wet solidus and induces dehydration

• melting. Adakites (slab melts)

probably form by such a

• process. Such rock should show

a “garnet REE” signature since garnet is a stable mineral in the slab under these conditions.

Model for subducted slab melting

1.Dehydration D releases water in mature arcs (lithosphere > 25 Ma) or in old subducted slabs. No slab melting!

Figure from: Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Phase diagram (partly schematic) for a hydrous mantle system, including the H2O-saturated lherzolite solidus [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.

Wedge melting: Some phase equilibria relevant to “wet” mantle melting

How appropriate is it to talk about melting along the H2O-saturated solidus?

Figure from: Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

P-T-t paths for peridotite in the mantle wedge as it follows a path similar to the flow lines in shown in an earlier figure. Included are some P-T-t path range for the subducted crust in a mature arc along with the wet and dry solidi for peridotite. Note that none of the P-T-t paths come close to the dry peridotite solidus. The subducted crust dehydrates, and water is transferred to the wedge (arrow) which results in the formation

• f some amphibole in the peridotite.

How much? It is unlikely that there is any free H2O in the peridotite so the melting process is driven by the breakdown of amphibole and is known as “dehydration melting”

Melting of hydrated peridotite

Figure from: Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Melting of phlogopite-bearing assemblages at higher pressure may account for the K-rich volcanics in the subsidiary arc behind the main arc

Dehydration of slab crust causes hydration of the mantle (violet), which undergoes partial melting as amphibole (A) and phlogopite (B) dehydrate. From Tatsumi (1989), J.

• Geophys. Res., 94,

4697-4707 and Tatsumi and Eggins (1995). Subduction Zone

• Magmatism. Blackwell.

Oxford.

Summary model of island arc petrogenesis

Figure from: Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice