Mauna Loa OIB / Hawaiian Volcanism Francis, 2013.

transcript

Mauna Loa

OIB / Hawaiian VolcanismFrancis, 2013

Hot-Spots producing Ocean Island Basalts (OIB)

• Relative abundance of strongly olivine-phyric, picritic and/or ankaramitic, high-MgO lavas ("oceanites"). Clinopyroxene is the second phase to crystallize after olivine at approximately 8 wt.% MgO, while plagioclase does not appear as a liquidus phase in Hawaiian tholeiites until compositions with less than 7 wt.% MgO.

• Primitive OIB lavas (high-MgO) are among the most Fe-rich picritic basalts on the Earth. • Primitive OIB lavas are also characterized by relatively low Al (Al2O3 15 wt.%) and Ca (CaO = 10 wt.%) contents compared to MORB..• OIB lavas are enriched in all incompatible trace elements, including the high field strength elements (Nb, Ta, Zr, Hf), as well as the LIL (K, Rb, Ba) and LREE, compared to MORB. Typically, however, they exhibit a relative depletion in LIL elements (Ba, Rb, K) with respect to HFS (Nb) and LREE elements.

• Unlike MORB, many OIB suites (although not all) exhibit an anti-correlation between trace element enrichment and isotopic enrichment

Characteristic Features of OIB Lavas

~ 8.5 cm/yr

East MolokaiHonolulu

KoolauKoloa

Mauna KeaMauna Loa

KilaueaMauna Ulu

Stages of Hawaiian Volcanism Post-Erosional: Honolulu Series, Kola Series

Renewed volcanic activity following a hiatus on the order of 1 million years. Small cinder cones, explosive tuff rings or maar, and small valley-filling flows of highly undersaturated, primitive lavas. This stage has not started yet on the big island of Hawaii, but is present on Maui and older islands.

Post Caldera: Mauna Kea, East Molokai

At 12,000', thicker flows begin to accumulate in the caldera, eventually filling it and forming a thin cap on the shield. The beginning of this stage is marked by a transition from earlier tholeiitic series to later alkaline series lavas. Initially, these two types of flows are commonly interbedded, and transitional compositions are also erupted. The earliest alkaline lavas are usually relatively evolved hawaiites, but as alkaline magmas become dominant with time, they also become more primitive (AOB) and explosive, finishing with the development of a cinder cone field capping the shield.

Shield building: Kilauea, Mauna Ulu, Mauna Loa, Lanai, Koolau

The repeated eruption of highly fluid, extensive thin flows of tholeiitic basalt builds the main

shield of the volcano, which usually has a well developed central caldera. There appears to be

a progression from early picritic lavas to later olivine and quartz tholeiites (6-9 wt.% MgO) with

height, as each shield builds to an elevation of 12,000 feet (4000 m) above sea level.

Early Submarine: Loihi

Early pillow lavas range from mildly alkaline basalts (AOB's) to tholeiites. There is a direct

correlation between degree of vesicularity (and thus volatile content) and the degree of silica

undersaturation.

Mauna KeaPost Caldera

Mauna LoaLate Shield

Kilauea

LoihiEarly

Submarine

ActiveVolcanoes

Mauna Ulu

Early Shield

Mauna Loa

KoolauKoloa

Mauna Kea

Kilauea Mauna Ulu

Deep Seated Plume?

Tholeitiic Suite: Kilauea, Mauna Ulu, Mauna Loa, Lanai, Koolau (Oahu)

Oceanite (picrite) Ol-tholeiite Qtz-tholeiite

The tholeiitic suite constitutes 98% of Hawaiian lavas. With the exception of one rhyodacite, all lavas of this magmatic suite are basalts, and there is a marked absence of intermediate and evolved lavas such as: andesite, dacite, and rhyolite.

Olivine is the dominant phenocryt and individual tholeiitic suites commonly define tight olivine control lines, with the dominant rock type being Ql-tholeiite. The most magnesian reported olivine has a composition of Fo 91 (most Fo 88 or less). Olivine, however, is absent in the groundmass, presumably because of the olivine reaction relationship.

The first cpx phenocrysts are augitic in composition (at MgO = 8.0 wt.%), but pigeonite and quartz are commonly found in the groundmass.

Xenoliths: dunite and olivine-gabbro xenoliths are relatively rare.

Magmatic Suites

Kilauea

Kilauea Iki

Mauna Loa

Hawaiian Tholeiites

Alkaline Suite: East-Molakai, Mauna Kea

ankaramite AOB hawaiite mugearite benmoreiite trachyte (labradorite) (andesine) (oligocene)

The alkaline suite comprises 2 % of Hawaiian lavas. Both olivine and clinopyroxene (MgO > 10 wt.%) are early phenocryst phases, and primitive lavas are ankaramitic. Unlike the tholeiitic suite, olivine commonly persists in the groundmass, along with titan-augite and interstitial K-spar instead of quartz.

The alkaline suite exhibits a much broader range of Mg contents, which indicates more extensive crystal fractionation involving olivine, clinopyroxene, and plagioclase. The dominant lava type has a relatively evolved hawaiitic composition.

Xenoliths: dunite, wherlite, gabbro; all relatively common.

Magmatic Suites

Mauna Kea

Top of Mauna Kea

Post-Caldera Alkaline Suite

Kilauea

East Molokai

Stages of Hawaiian Volcanism Post-Erosional: Honolulu Series, Kola Series

Renewed volcanic activity following a hiatus on the order of 1 million years. Small cinder cones, explosive tuff rings or maar, and small valley-filling flows of highly undersaturated, primitive lavas. This stage has not started yet on the big island of Hawaii, but is present on Maui and older islands.

Post Caldera: Mauna Kea, East Molokai

At 12,000', thicker flows begin to accumulate in the caldera, eventually filling it and forming a thin cap on the shield. The beginning of this stage is marked by a transition from earlier tholeiitic series to later alkaline series lavas. Initially, these two types of flows are commonly interbedded, and transitional compositions are also erupted. The earliest alkaline lavas are usually relatively evolved hawaiites, but as alkaline magmas become dominant with time, they also become more primitive (AOB) and explosive, finishing with the development of a cinder cone field capping the shield.

Shield building: Kilauea, Mauna Ulu, Mauna Loa, Lanai, Koolau

The repeated eruption of highly fluid, extensive thin flows of tholeiitic basalt builds the main

shield of the volcano, which usually has a well developed central caldera. There appears to be

a progression from early picritic lavas to later olivine and quartz tholeiites (6-9 wt.% MgO) with

height, as each shield builds to an elevation of 12,000 feet (4000 m) above sea level.

Early Submarine: Loihi

Early pillow lavas range from mildly alkaline basalts (AOB's) to tholeiites. There is a direct

correlation between degree of vesicularity (and thus volatile content) and the degree of silica

undersaturation.

Post-Erosional Suite: Honolulu Series, Oahu, and Koloa Series, Kauai

A.O.B basanite nephelinite mellilitite ( 5% norm Ne) ( 5% norm Ne) ( 15% norm Ne) ( 15% norm

Me) (modal feldspathoid) (no modal plag) (modal mellilite)

The post-erosional series comprises 0.1% of Hawaiian lavas. They are characteristically strongly silica undersaturated, and, although they exhibit a wide range of silica saturations, they are all relatively primitive with high Mg contents. They thus do not appear to have suffered significant low pressure crystal fractionation.

Xenoliths: lherzolite, harzburgite, dunite, garnet pyroxenite, all relatively abundant.

Magmatic Suites

Hy-norm Basalt

Alkali-Olivine Basalt

Basanite Olivine Nephelinite /

Melilitite

NormativeMineralogy

Opx 0% < Foid < 5% 5% < Foid < 15%

Foid > 15%Melilitite (>10% Larnite)

Matrix ModalMineralogy

Plag Plag & K-Spar K-Spar, Plag & Foid

Foid, no Plag

Crystallization Sequence

Ol Plag Cpx Ox

Ol Cpx Plag Ox

Ol Ox Cpx

Lamprophyre equivalent

Camptonite Camptonite Monchiquite

Sodic Alkaline Mafic Volcanism

Eclogite

divide

Eclogite

divide

Fe / Mn

Hawaii and other OIB suites have been shown to have higher Fe/Mn ratios than MORB

The elevated Fe/Mn ratios

of OIB magmas has also

been claimed to reflect the

incorporation of minor

amounts of outer core

material, which has a much

higher Fe/Mn ratio than the

mantle

Hawaiian tholeiites are enriched in all incompatible trace elements in comparison to MORB, and are characterized by distinctive convex-upwards fractionated REE patterns that peak at Pr.

Regardless of the degree of enrichment in the LREE, Nb, and Ta, however, there typically remains a significant relative depletion in LIL elements such as K, Rb, and Ba. This appears to require the present of a residual hydrous phase, such as amphibole or phlogopite, in the mantle source regions of the some of the alkaline magmas. The foidites develop slight negative anomalies for HFSE elements, eg. Nb and Hf.

There is a systematic anti-correlation between degree of incompatible trace element enrichment and degree of Si saturation, and much of the trace element variation in the Hawaiian lavas can be explained in terms of mixing between two components. Going from tholeiite to AOB to basanite and then olivine nephelinite corresponds to a systematic increase in the degree of enrichment in LREE, Nb, and Ta, with little change or a slight decrease in the levels of HREE.

Recent Alkaline Basalts (8+ wt.% MgO)

Hirschfeld

olivine fra

c.minor

dominant

Zr ppm

Lanai / Koolau End-Member

The lavas within many OIB suites define approximately linear arrays between two chemical and isotopic components, one relatively depleted and the other relatively enriched. Originally these were thought to correlate with the MORB source and primitive mantle respectively. However, it rapidly became apparent that these linear arrays were different in different OIB suites.

There are thus many "flavours" of OIB suites, and at least five different components are required to explain them. Furthermore, there are geographic correlations in the isotopic characteristics of OIB suites. For example, the DUPAL anomaly in the south Pacific is defined by the abundance of EM II OIB suites that appears to correlate with a lower mantle seismic tomography anomaly.

Inter-shield variations in Hawaiian tholeiitic picrites

Kilauea, Mauna Kea, Loihi Lanai, Koolau, Mauna Loa

low Si, high Fe, Ti, Ca high Si, low Fe, Ti, Ca

highest IE, Nb/Zr, Th/U lowest IE, Nb/Zr, Th/U

low 87Sr/86Sr, high 143Nd/144Nd. high 87Sr/86Sr, low 143Nd/144Nd .7036 Nd = + 7 .7042 Nd = +1

low 18O 4.7 high 18O 6.0

high 206Pb/204Pb 18.6 low 206Pb/204Pb 17.9

low 187Os/188Os 0.13 ~ MORB high 187Os/188Os 0.145

Tholeiitic End-Members

The primitive tholeiitic lavas of the shield building stage of Hawaiian Islands range in compositions between two end-members:

Despite appearances, the Koolau source component is not equivalent to primitive mantle (low Rb/Sr, high 187Os/188Os, high, 18O), and the Kilauea source component is not equivalent to depleted mantle (DMM) (high 87Sr/86Sr, low 18O), nor Pacific ocean crust and/or lithosphere (high 206Pb/204Pb, low 187Os/188Os). Both of these source components would seem to be from the lower mantle.

Mauna Loa

KoolauKoloa

Mauna Kea

Kilauea Mauna Ulu

The Hawaiian Paradox

The low Al and Ca of most primitive OIB picritic magmas, including those of Hawaii, are consistent with equilibration with a harzburgitic residue at pressures ranging from 1.5 to more than 3.0 GPa. If these OIB parental magmas were derived from the same Pyrolite mantle source that gives rise to MORB, then they would have to represent a greater degree of partial melting, beyond the point at which clinopyroxene disappears in the solid residue. This interpretation is supported by recent melting experiments on a Kilauean picrite (Eggins, 1992a), which is saturated only in olivine and orthopyroxene in this pressure range.

The foregoing conclusion, however, is inconsistent with the fact that all Hawaiian primitive magmas are enriched in incompatible trace elements compared to MORB. To make matters worse, the buffered levels of heavy rare earth elements in magmas ranging from tholeiites through strongly alkaline basalts has convinced many trace element geochemists (Hofmann et al. 1984, Frey and Roden, 1987) that residual garnet must be present in their mantle source. Inversions of Hawaiian rare earth element data (Watson, 1993) also indicate melting in the presence of residual garnet. But olivine and garnet never coexist on the liquidus of primitive Hawaiian tholeiites.

We are thus presented with a paradox. Melting experiments on both mantle lherzolite (Hirose and Kushiro, 1993, Falloon et al. 1988) and Hawaiian picrites themselves (Eggins, 1992a) indicate that garnet is not stable in melts with compositions of the Hawaiian picrites until pressures greater than 3.0 GPa, after olivine has ceased to be a stable phase. Garnet and olivine are not in equilibrium together with a Hawaiian picritic liquid under any conditions. This Hawaiian paradox is aggravated if the picrites are normalised to coexist with the residue of a more Fe-rich mantle, such as HK-66 with an olivine of composition Fo 86. This leads to higher Si contents and unlikely estimates for the pressures of equilibration (0.7 to 2 GPa), well below any pressure estimates for the stability region of garnet in a lherzolitic bulk composition and less than depths indicated by seismic data, and leaves unexplained the presence of Fo 89+ phenocrysts in some primitive Hawaiian lavas. Eggins (1992b) has demonstrated that the paradox can not be resolved by calling upon dynamic melting processes, such as percolation melting (Ribe, 1988) or accumulated continuous melting (Mackenzie and Bickle, 1988), and that the behaviour of the HREE in primitive Hawaii tholeiites requires melting to have occurred largely in the presence of garnet

Mixing between melts derived from a lower-mantle-sourced plume and small degree partial melts of the

upper mantle asthenosphere, as represented by MORB

Hawaiian Olivines are higher in Ni for any given Mg no. compared to MORB

Olivine Compositions

Multi-Stage Melting Model:

As the plume adiabatically rises:

Peridotite melts at lowest pressures

Hi-Mg Pyroxenite zones melt at lower pressures.

Si-rich melts react with peridotite host to form metasomatic high-Mg Pyroxenite.

Eclogite pods melt first at highest pressures to produce Si-rich melts.

Multi-Stage Melting Model:

Hawaii and other OIB suites have higher Fe contents and Fe/Mn ratios than MORB

The elevated Fe contents and Fe/Mn ratios of OIB magmas has been claimed to

reflect the incorporation of minor amounts of outer core material, which has a much

higher Fe/Mn ratio than the mantle

Mantle

Hot Spot

Only a quite small amount of core material would be required to explain the excess 186Os in OIB magmas

Recycled Mn nodules have been proposed for the anomalous 186Os in hot spot magmas, however, such an explanation conflicts with the actual Mn data for OIB magmas.

Involvement of the Core?

186Pt 186Os + e-

187Re 187Os + e-

The presence of a coupled enrichment in 186Os and 187Os in Hawaii and some other OIB suites has been cited as evidence for the incorporation of core material into the source of the plume that produced them.

Excess 186Os in the outer core is caused by the increase in Pt/Os (parent/daughter) in the fluid outer core because of the growth of the inner core - Os is compatible, but Pt is incompatible.

Hawaiian olivines have been argued to be too Ni-rich to have equilibrated with a peridotite mantle source with ~ 1900 ppm Ni, but could be derived from pyroxenite mantle source with ~ 1000 ppm Ni.

Mauna Loa OIB / Hawaiian Volcanism Francis, 2013.

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