Komposisi magma

Convergent Plate Boundaries

Continental Volcanic Arce.g., WA

Volcanic Island ArcBanda, Jepang

Convergent Plate Boundaries

Pemanasan kerak samudra melepaskan fluida (umumnya air) ke mantel di atasnya.Penamahan fluida menghasilkan magma basalt dari pelelehan mantel

Magma naik mencapai dasar kerak dengan gaya buoyant.

basalt naik melalui kerak benua, magma melelehkan dan mengabsorb kerak, menjadi lebih kaya silika

Gunungapi Continental arc erupsi secara eksplosif mengeluarkan magma kaya silika berupa abu dari gunungapi komposit.

Divergent Plate BoundariesPada umumnya berada di mid-ocean ridgesMantle melelehkan karena penurunan tekanan secara adiabatikLelehan mempunyai komposisi basaltikPendinginan basalt membentuk kerak samudra baru

Unusual circumstances can put a piece of the ocean crust on land, where we can observe pillow basalt.

Mid-ocean ridges

Intraplate Volcanism“Head” of hot mantle collects at core-mantle boundaryWhen it reaches the base of the crust it makes a huge amount of basalt (flood basalts)As plate moves over plume, a trail of volcanism forms

Intraplate Volcanism“Head” of hot mantle collects at core-mantle boundaryWhen it reaches the base of the crust it makes a huge amount of basalt (flood basalts)As plate moves over plume, a trail of volcanism forms

Intraplate Volcanism“Head” of hot mantle collects at core-mantle boundaryWhen it reaches the base of the crust it makes a huge amount of basalt (flood basalts)As plate moves over plume, a trail of volcanism forms

Intraplate Volcanism - Ocean

forms basaltic volcanoes (Hawaii)

Intraplate Volcanism - Continentforms flood basalts, then more silica-rich volcanoes (calderas)Columbia River Basalts & Yellowstone

Komposisi magma

Mid Oceanic Ridge: basaltMantle plum (hot spot): basaltZona penunjaman: basaltik - ryolitik

Batuan beku

Klasifikasi QAPF

Klasifikasi QAPF batuan plutonik

Klasifikasi QAPF batuan vulkanik

Seri Bowen

Spider diagram Trace elements

Geokimia trace elements MORB

Perbandingan trace elements OIB & MORB

Magma di zona penunjaman

Proses diferensiasi di zona penunjaman

Modifikasi komposisi magma zona penunjaman.Penambahan material

sumber magmaProses diferensiasi

Major Elements and Magma Series

Figure 16-3. Data compiled by Terry Plank (Plank and Langmuir, 1988) .

a. Alkali vs. silicab. AFM c. FeO*/MgO vs. silica

diagrams for 1946 analyses from ~ 30 island and continental arcs with emphasis on the more primitive volcanics

Recommended Classification Schemebased on K2O and SiO2 contents identifies shoshonitic, high-K, med-K, and low-K seriesdistinguishes basalt, basaltic andesite, andesite, dacite, rhyolite comp. based on SiO2 contents

does not include high-Mg andesites

Figure 16-4. The three andesite series of Gill (1981) Orogenic Andesites and Plate Tectonics. Springer-Verlag. Contours represent the concentration of 2500 analyses of andesites stored in the large data file RKOC76 (Carnegie Institute of Washington).

53 57 63

6 sub-series if combine tholeiite and C-A (some are rare)

May choose 3 most common:

Figure 16-5. Combined K2O - FeO*/MgO diagram in which the Low-K to High-K series are combined with the tholeiitic vs. calc-alkaline types, resulting in six andesite series, after Gill (1981) Orogenic Andesites and Plate Tectonics. Springer-Verlag. The points represent the analyses in the appendix of Gill (1981).

Low-K tholeiiticMed-K CAHi-K mixed

d-K A

Low-K tholeiiticMed-K CAHi-K mixed

From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Figure 16-6. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Turner & Foden (2001)

Quaternary Sunda Banda volcanic rocksWheller et al., (1987)

Quaternary Sunda-Banda volcanic rocks

Radiogenic Isotope Tracerssensitive tracers:

isotopes of Sr, Nd, Pb, Hfwhy?they have long half-lives compared with the time scales of subduction processeslarge contrast between the isotopic composition & element concentrations in source components requisite with usage:isotopic composition of all possible contributing sources is knowndisplay sufficient compositional contrast

2 types of SZM based on Sr- & Nd- ratio behavior

A-typeoccurs in oceanic SZshows horizontal to positive trendsrestricted to low 87Sr/86Sr & high 143Nd/144Nd valuesB-typefrom continental SZ

shows negative trends

extends from field of oceanic crust to very high 87Sr/86Sr & low 143Nd/144Nd values

Figure 16-13. Variation in 207Pb/204Pb vs. 206Pb/204Pb for oceanic island arc volcanics. Included are the isotopic reservoirs and the Northern Hemisphere Reference Line (NHRL) proposed in Chapter 14. The geochron represents the mutual evolution of 207Pb/204Pb and 206Pb/204Pb in a single-stage homogeneous reservoir. Data sources listed in Wilson (1989).

Unsur unsur ringan (Li, Be, B) & isotopesnya digunakan sebagai tracer kontribusi sedimen dan kerak samudra teralterasi ke magma subduksi dan sebagai indikator keterlibatan fluida dalam genesa maga zona subduksi

10Be created by cosmic rays + oxygen and nitrogen in upper atmosphere and accumulates in the upper layers of oceanic sediments

half-life of only 1.5 Ma (long enough to be subducted, but quickly lost to mantle systems). After about 10 Ma 10Be is no longer detectable

10Be/9Be averages about 5000 x 10-11 in the uppermost oceanic sediments

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

10Be/Betotal vs. B/Betotal diagram (Betotal 9Be since 10Be is so rare)

Figure 16-14. 10Be/Be(total) vs. B/Be for six arcs. After Morris (1989) Carnegie Inst. of Washington Yearb., 88, 111-123.