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Ilhas Oceânicas e Platôs (seamounts)Ilhas Oceânicas e Platôs (seamounts)

Comumente associados com Comumente associados com hot spotshot spots

Vulcanismo Oceânico IntraplacaVulcanismo Oceânico Intraplaca

Figure 14-1. After Crough (1983) Ann. Rev. Earth Planet. Sci., 11, 165-193.

Tipos de Magmas OIBTipos de Magmas OIBDuas series de magmas principais Duas series de magmas principais

Série TholeiiticaSérie Tholeiitica (tipo dominante) (tipo dominante) Basalto toleiitico de ilhas oceanicas parental, ou Basalto toleiitico de ilhas oceanicas parental, ou OITOIT Similar ao MORB, mas com algumas Diferenças Similar ao MORB, mas com algumas Diferenças

químicas e mineralógicasquímicas e mineralógicas Série AlkalineSérie Alkaline (subordinada) (subordinada)

Basalto alcalino Parental de ilhas oceânicas, ou Basalto alcalino Parental de ilhas oceânicas, ou OIAOIA Duas principais sub-séries alcalinasDuas principais sub-séries alcalinas

silica undersaturatedsilica undersaturated slightly silica oversaturatedslightly silica oversaturated ( (less common series) less common series)

O HawaiiO HawaiiPadrão cíclico da história eruptivaPadrão cíclico da história eruptiva

1.1. Estágio Pré-construção do edíficio Estágio Pré-construção do edíficio vulcânico vulcânico predomina rochas alcalinas e predomina rochas alcalinas e variaveisvariaveis

2. Estágio construção do edíficio vulcânico 2. Estágio construção do edíficio vulcânico inicia-se com enorme quantidade de inicia-se com enorme quantidade de basalto tholeiiticobasalto tholeiitico

HawaiiHawaii3.3. Diminuição da atividade mais Diminuição da atividade mais alcalinaalcalina, ,

episodica, e violenta (Mauna Kea, Hualalai, episodica, e violenta (Mauna Kea, Hualalai, and Kohala). Lavas são também mais and Kohala). Lavas são também mais diversas, com a maior proporção de liquidos diversas, com a maior proporção de liquidos differentiados.differentiados.

4.4. Um longo periodo de dormência, seguido por Um longo periodo de dormência, seguido por um estágio um estágio post-erosional tardiopost-erosional tardio. . Caracterizado por magmas Caracterizado por magmas fortemente fortemente alcalinoalcalino and silica-undersaturated magmas, and silica-undersaturated magmas, incluindo alcali basaltos, nefelinitos, melilitos incluindo alcali basaltos, nefelinitos, melilitos basaltos, and basanitosbasaltos, and basanitos

Evolução em SeriesEvolução em SeriesToleiitico, alcaline, e fortemente alcalineToleiitico, alcaline, e fortemente alcaline

Figure 14-2. After Wilson (1989) Igneous Petrogenesis. Kluwer.

Table 14-4. Alkali/silica ratios (regression) for selected ocean island lava suites.

Island Alk/Silica Na2O/SiO2 K2O/SiO2

Tahiti 0.86 0.54 0.32Principe 0.86 0.52 0.34Trinidade 0.83 0.47 0.35Fernando de Noronha 0.74 0.42 0.33Gough 0.74 0.30 0.44St. Helena 0.56 0.34 0.22Tristan da Cunha 0.46 0.24 0.22Azores 0.45 0.24 0.21Ascension 0.42 0.18 0.24Canary Is 0.41 0.22 0.19Tenerife 0.41 0.20 0.21Galapagos 0.25 0.12 0.13Iceland 0.20 0.08 0.12

Alcalinidade é altamente variavel Alcalinidade é altamente variavel Alcalis são elementos incompativeis, não afetado por Alcalis são elementos incompativeis, não afetado por

menos do 50% cristalização fracionada rasa, isto menos do 50% cristalização fracionada rasa, isto também reforça a existência de também reforça a existência de distintas fontes distintas fontes mantelicas oumantelicas ou diferentes mecanismos de geração diferentes mecanismos de geração

Elementos TraçosElementos Traços Os elementos traços Os elementos traços LILLIL (K, Rb, Cs, Ba, Pb (K, Rb, Cs, Ba, Pb2+2+ e Sr) e Sr)

são incompativeis e são são incompativeis e são todos enriquecidos em todos enriquecidos em magmas OIB magmas OIB com respeito aos MORBscom respeito aos MORBs

As As razõesrazões de elementos incompativeis tem sido de elementos incompativeis tem sido empregadas para distinguir entre reservatórios fonteempregadas para distinguir entre reservatórios fonte

N-MORB: a razão K/Ba é alta (usualmente > 100)N-MORB: a razão K/Ba é alta (usualmente > 100) E-MORB: a razão K/Ba é em torno de 30’sE-MORB: a razão K/Ba é em torno de 30’s OITs variam de 25-40, e OIAs são maiores do 20’sOITs variam de 25-40, e OIAs são maiores do 20’s

Assim todos parecem ter fontes distintasAssim todos parecem ter fontes distintas

Trace ElementsTrace Elements Elementos HFSElementos HFS (Th, U, Ce, Zr, Hf, Nb, Ta, and Ti) (Th, U, Ce, Zr, Hf, Nb, Ta, and Ti)

também são incompativeis, e são enriquecidos emtambém são incompativeis, e são enriquecidos em OIBs > MORBsOIBs > MORBs As razões destes elementos também são usadas para As razões destes elementos também são usadas para

distinguir fontes mantélicasdistinguir fontes mantélicas A razão Zr/NbA razão Zr/Nb

N-MORBN-MORB geralmente bem alta (>30)geralmente bem alta (>30) OIBs são baixas (<10)OIBs são baixas (<10)

Trace Elements: REEsTrace Elements: REEs


Trace Elements: REEsTrace Elements: REEs

La/Yb (REE slope) correlates with the degree of silica La/Yb (REE slope) correlates with the degree of silica undersaturation in OIBs undersaturation in OIBs

Highly undersaturated magmas: La/Yb > 30Highly undersaturated magmas: La/Yb > 30 OIA: closer to 12OIA: closer to 12 OIT: ~ 4OIT: ~ 4 (+) slopes (+) slopes E-MORB and all OIBs E-MORB and all OIBs N-MORB N-MORB

(-) slope and appear to originate in the lower (-) slope and appear to originate in the lower enriched mantle enriched mantle

MORB-normalized Spider DiagramsMORB-normalized Spider Diagrams

Figure 14-3. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).

Isotope GeochemistryIsotope Geochemistry Isotopes do not fractionate during partial Isotopes do not fractionate during partial

melting of fractional melting processes, so melting of fractional melting processes, so will reflect the characteristics of the sourcewill reflect the characteristics of the source

OIBs, which sample a great expanse of OIBs, which sample a great expanse of oceanic mantle in places where crustal oceanic mantle in places where crustal contamination is minimal, provide contamination is minimal, provide incomparable evidence as to the nature of incomparable evidence as to the nature of the mantlethe mantle

Simple Mixing ModelsSimple Mixing ModelsBinaryBinary

All analyses fall All analyses fall between two reservoirs between two reservoirs

as magmas mixas magmas mix

TernaryTernaryAll analyses fall within All analyses fall within

triangle determined triangle determined by three reservoirsby three reservoirs

Figure 14-5. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Sr - Nd IsotopesSr - Nd Isotopes

Figure 13-12.Figure 13-12. Data from Ito et al. (1987) Data from Ito et al. (1987) Chemical Geology, 62, 157-176; Chemical Geology, 62, 157-176; and LeRoex et al. (1983) and LeRoex et al. (1983) J. Petrol., 24, 267-318.J. Petrol., 24, 267-318.

Figure 14-6. After Zindler and Hart (1986), Staudigel et al. (1984), Hamelin et al. (1986) and Wilson (1989).

Mantle ReservoirsMantle Reservoirs

1.1. DMDM (Depleted Mantle) (Depleted Mantle) = N-MORB source= N-MORB source


2.2. BSEBSE (Bulk Silicate Earth) or the Primary (Bulk Silicate Earth) or the Primary Uniform ReservoirUniform Reservoir


3.3. EMIEMI = enriched mantle type I has lower = enriched mantle type I has lower 8787Sr/Sr/8686Sr (near Sr (near primordial) primordial)

4.4. EMIIEMII = enriched mantle type II has higher = enriched mantle type II has higher 8787Sr/Sr/8686Sr (> Sr (> 0.720, well above any reasonable mantle sources0.720, well above any reasonable mantle sources


5.5. PREMAPREMA ( (PREPREvalent valent MAMAntle)ntle)


Pb IsotopesPb IsotopesPb produced by radioactive decay of U & ThPb produced by radioactive decay of U & Th

9-209-20 238238U U 234234U U 206206PbPb

9-219-21 235235U U 207207PbPb

9-229-22 232232Th Th 208208PbPb

Pb is quite scarce in the mantlePb is quite scarce in the mantle Mantle-derived melts susceptible to contaminationMantle-derived melts susceptible to contamination

U, Pb, and Th are concentrated in continental crust (high radiogenic U, Pb, and Th are concentrated in continental crust (high radiogenic daughter Pb isotopes)daughter Pb isotopes)

204204Pb is non-radiogenic, so Pb is non-radiogenic, so 208208Pb/Pb/204204Pb, Pb, 207207Pb/Pb/204204Pb, and Pb, and 206206Pb/Pb/204204Pb Pb increase as U and Th decayincrease as U and Th decay

Oceanic Oceanic crustcrust has elevated U and Th content (compared to the mantle) has elevated U and Th content (compared to the mantle) as will as will sedimentssediments derived from oceanic and continental crust derived from oceanic and continental crust

Pb is a sensitive measure of Pb is a sensitive measure of crustalcrustal (including sediment) components in (including sediment) components in mantle isotopic systemsmantle isotopic systems

93.7% of natural U is 93.7% of natural U is 238238U, so U, so 206206Pb/Pb/204204PbPb will be most sensitive to a will be most sensitive to a crustal-enriched componentcrustal-enriched component

= = 238238U/U/204204Pb (evaluate uranium enrichment)Pb (evaluate uranium enrichment) HIMUHIMU reservoir has a very high reservoir has a very high 206206Pb/Pb/204204Pb ratio, suggestive of a Pb ratio, suggestive of a

source with high U, yet not enriched in Rb, and old enough source with high U, yet not enriched in Rb, and old enough (> 1 Ga) to develop the observed isotopic ratios (> 1 Ga) to develop the observed isotopic ratios

HIMU models: subducted and recycled oceanic crust (possibly HIMU models: subducted and recycled oceanic crust (possibly contaminated by seawater), localized mantle lead loss to the contaminated by seawater), localized mantle lead loss to the core, and Pb-Rb removal by those dependable (but difficult to core, and Pb-Rb removal by those dependable (but difficult to document) metasomatic fluidsdocument) metasomatic fluids

The high Sr ratios in EMI and EMII also require a The high Sr ratios in EMI and EMII also require a high Rb content and a similarly long time to high Rb content and a similarly long time to produce the excess produce the excess 8787SrSr This signature correlates well with continental This signature correlates well with continental

crust (or sediments derived from it)crust (or sediments derived from it) Oceanic crust and sediment are other likely Oceanic crust and sediment are other likely

candidates for these reservoirscandidates for these reservoirs The The 207207Pb/Pb/204204Pb data, especially from the northern Pb data, especially from the northern

hemisphere ~ a linear mixing line between DM hemisphere ~ a linear mixing line between DM and HIMU, a line called the and HIMU, a line called the Northern Hemisphere Northern Hemisphere Reference Line (NHRL) Reference Line (NHRL)

Figure 14-8. After Wilson (1989) Igneous Petrogenesis. Kluwer. Data from Hamelin and Allègre (1985), Hart (1984), Vidal et al. (1984).

Can map the geographic distribution of the isotopic data Can map the geographic distribution of the isotopic data

Figure 14-9. From Hart (1984) Nature, 309, 753-756.

Isotopically enriched reservoirs (EMI, EMII, and Isotopically enriched reservoirs (EMI, EMII, and HIMU) are too enriched for any known mantle HIMU) are too enriched for any known mantle process, and they correspond to process, and they correspond to crustalcrustal rocks and/or rocks and/or sedimentssediments

EMIEMI (slightly enriched) correlates with (slightly enriched) correlates with lower lower continental crustcontinental crust or or oceanic crustoceanic crust

EMIIEMII is more enriched, especially in radiogenic Sr is more enriched, especially in radiogenic Sr (indicating the Rb parent) and Pb (U/Th parents) (indicating the Rb parent) and Pb (U/Th parents) correlates with the correlates with the upper continental crustupper continental crust or or ocean-ocean-island crustisland crust

A Model for Oceanic MagmatismA Model for Oceanic Magmatism

DMDM

OIBOIB

ContinentalContinental

ReservoirsReservoirs

EM and HIMU from EM and HIMU from crustalcrustal sources (subducted OC + CC seds) sources (subducted OC + CC seds)

Figure 14-10. Nomenclature from Zindler and Hart (1986). After Wilson (1989) and Rollinson (1993).

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