Rare Earth Elements in some representative arc lavas

Transcription

1 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.

2 MORB-normalized spider diagrams for selected island arc basalts Normalization and element ordering 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.

3 Sr and Nd isotopic values in island arc lavas Notes: 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. 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) 3. Antilles arc shows increasing sediment component from N to S (due to proximity to S. American craton and Amazon River sediment source Data sources in Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

4 Variation in 207Pb/204Pb vs. 206Pb/204Pb for oceanic island arc volcanics. Included are some of 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 oceanic 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.

5 The Be-B story 10 Be created by cosmic rays + oxygen and nitrogen in upper atmosphere and is eventually incorporated in clay-rich oceanic sediments. 10 Be has a half-life of only 1.5 Ma so after ~6 half lives (10 Ma), 10 Be is no longer detectable. 10 Be/ 9 Be averages about 5000 x in the uppermost oceanic sediments but in mantle-derived MORB and OIB magmas, & continental crust, 10 Be is below detection limits (<1 x 10 6 atom/g) and 10 Be/ 9 Be is <5 x 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 ( ppm), and B is also high in altered oceanic crust ( ppm). In MORB and OIB, B<2-3 ppm. Figure shows 10 Be/Be(total) vs. B/Be for six arcs. After Morris (1989) Carnegie Inst. of Washington Yearb., 88, Note: Suppose oceanic lithosphere is being subducted at 5 cm/yr. It would take ~3 Ma to get to a depth of 100 km. 1250x Be left. Suppose 2% sediment is included in the mantle melt, i,e, 50 x Be. If the magma is transported immediately to surface, we should see this 10 Be. 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. Figure from: Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

6 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 Some questions: 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? 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?

7 Oceanic crust (Basalt + Greenstone + ~4%H 2 O) Serpentinite Trench Fore Arc Accretionary wedge H 2 O Volcanic/Plutonic Arc Plutons heat Back Arc Eruption of calc-alkalic magmas (flows, pyroclastics) Eruption of K-rich magmas Crustal melting zone Underplated gabbro/amphibolite Stable continental crust (~40 Km) Harzburgite Lherzolite lithosphere Greenschist Minor melting of H 2 O rich harzburgite H 2 O 1000ºC 1200ºC Lithosphere Asthenosphere 100 Amphibolite 50 Scale Eclogite Partial melting of Hydrated asthenosphere a. Dehydration of amphibole b. Dehydration of phlogopite a b Asthenospheric flow lines (induced convection) km Schematic representation of processes and magmatic products at a convergent continental margin

8 Oceanic crust (Basalt + Greenstone + ~4%H 2 O) Scale Serpentinite Harzburgite Lherzolite lithosphere Trench Greenschist km Fore Arc Accretionary wedge H 2 O Minor melting of H 2 O rich harzburgite Amphibolite Eclogite Volcanic/Plutonic Arc H 2 O Plutons heat Partial melting of Hydrated asthenosphere a. Dehydration of amphibole b. Dehydration of phlogopite a Back Arc Eruption of calc-alkalic magmas (flows, pyroclastics) Eruption of K-rich magmas Crustal melting zone Underplated gabbro/amphibolite 1000ºC 1200ºC b Stable continental crust (~40 Km) Lithosphere Asthenosphere Asthenospheric flow lines (induced convection) Schematic representation of processes and magmatic products at a convergent continental margin

9 Conditions required for melting of subducted slab P-T-t paths for subducted crust assuming a subduction rate of 3 cm/year. The length of 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, ). Figure from: Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

10 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 of 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

11 1.Dehydration D releases water in mature arcs (lithosphere > 25 Ma) or in old subducted slabs. No slab melting! Model for subducted slab melting 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. Figure from: Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

12 Wedge melting: Some phase equilibria relevant to wet mantle melting Phase diagram (partly schematic) for a hydrous mantle system, including the H 2 O-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 How appropriate is it to talk about melting along the H 2 O-saturated solidus? Figure from: Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

13 Melting of hydrated peridotite 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 of some amphibole in the peridotite. How much? It is unlikely that there is any free H 2 O in the peridotite so the melting process is driven by the breakdown of amphibole and is known as dehydration melting Melting of phlogopite-bearing assemblages at higher pressure may account for the K-rich volcanics in the subsidiary arc behind the main arc Figure from: Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

14 Summary model of island arc petrogenesis 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, and Tatsumi and Eggins (1995). Subduction Zone Magmatism. Blackwell. Oxford. Figure from: Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice