Abstract

It is now widely accepted that the long-lived oceanic magmatism is a surface
manifestation of “mantle plume”—narrow column of hot, buoyant, low-density material
that flow upward from the deep region in the Earth’s mantle. The volcanic products ofsuch magmatism could potentially give some important insights into the chemical andphysical structure of the mantle plume, and its origin, evolution and dynamics occurringin the deep mantle through geological time.

In my PhD study, I have been trying tounderstand the evolution of long-lived magmatism in order to give important insights
into the chemical evolution and dynamics of the mantle plume.

My PhD study focusedon the temporal geochemical variations of the Icelandic magmatism.

In order tounderstand the evolution of the Icelandic magmatism, I have been devoted to two main
works: (1) analytical development for measurement of potassium in geologicalmaterials using an inductively coupled plasma mass spectrometry (ICP-MS) for K-Ar
dating, which could be used to calibrate the history of the Icelandic magmatism, and (2)establishment of the detail petrological and geochemical record of the Icelandic
magmatism during last 13 Myr for putting constrains on the nature of source materials
contributing to magmatism and examining the evolution and dynamics of the Iceland
mantle plume.

i(1)I report here a simple and rapid analytical technique to determine potassium
concentrations in geological materials by utilizing an inductively coupled plasma mass
spectrometer (ICP-MS) equipped with the collision reaction cell.

The standard

addition method was applied for the determination of potassium in the mass
spectrometric procedure. The samples were digested with HF-HClO4, followed bydissolution of potassium with 0.5 M HNO3. This solution was further diluted with 0.5
M HNO3 into dilution factor (DF) of ca. 3000. Three aliquots were made from thissolution, and then the standard potassium stock solution (ca. 30 µg/g) was added into
two aliquots in different amounts.

In ICP-MS analysis, I found that the noises of 39

amu (atomic mass unit) derived from the argon-based polyatmic species were reducedby the positioning the torch far from the sampling orifice and introducing H2 gas into
the collision reaction cell.

In the best analytical condition, the noise effect(noise/signal intensity ratio) on 39 amu could be suppressed below 0.01%.

In thiscondition, the standard addition method for determination of potassium concentration
was developed.

First, the analyte signal intensities of 23Na, 24Mg, 25Mg, 26Mg, 27Al, 39K

and 44Ca, were measured for sample solutions, and then the ratios of 39K/23Na, 39K/24Mg,
39

K/25Mg, 39K/26Mg, 39K/27Al and 39K/44Ca are calculated for the net intensities possessedby subtractions of the backgrounds.

It is plotted on coordinates with 39K/nM (e.g. 23Na)

intensity ratio as the ordinate and standard/sample weight ratio as the abscissa.

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In thisdiagram, the three data for the sample should be plotted on the straight regression line.
I usually could obtain well-correlated linear trends (R2 values usually yield
0.999~1.000).

At this time, the concentration of potassium in samples could becalculated from the absolute values of abscissa intercept of this regression line.

To

demonstrate the applicability of this method, some of the reference materials of thegeological survey Japan (GSJ) were analyzed to obtain the potassium contents in thesematerials. The reproducibilities of < 1.5% for volcanic rock samples (basalts: JB-2and JB-3; andesites: JA-2 and JA-3) were obtained and thus this method is considered
to be usefully applicable to measurement of potassium in geological materials.

(2)

The first comprehensive geochemical (major and trace element concentrations)and Sr-Nd-Hf-Pb isotope data of the Tertiary lavas from eastern Iceland, ranging in age
from ca. 13 to < 1 Ma, together with published geological, geochronological andgeophysical data, are used to evaluate the temporal changes in mantle sources
contributing to the Icelandic Tertiary magmatism and their relative roles on magma
productivity.

The trace element and isotopic composition could clearly discriminate

the involvement of three distinct mantle end-components in the Icelandic Tertiary
magmatism. The temporal variation in lava geochemistry is attributed to the changesiii

in relative contribution of these three end-components to the erupted magma.

I foundthat the temporal variation of magma productivity is well correlated with the lava
geochemistry.

The extrusion of lavas with geochemically enriched composition ismarked in the periods of 13-11 and 8-7 Ma, which are coincident in time with higher
magma productivity.

However, the geochemical characteristics of lavas erupted

among these two discrete periods are quite different. The lavas of the former period
have relatively more radiogenic 176Hf/177Hf and less radiogenic 206Pb/204Pb ratios thanthose produced during the latter period. The extrusion of relatively depleted lavas
around 10 Ma and younger than 6.5 Ma are coincident with lower magma productivities.
The coincidence of the productivity and compositions of the Icelandic Tertiary lavas
could be interpreted that the periodic involvement of geochemically enriched
end-component and subsequent participation of depleted end-component in magma
generation with ca. 4 Myr intervals as the progress of magmatism. This periodicity
may be controlled by thermochemical interactions induced by the presence of dense
recycled crustal materials in the plume source region at the thermal boundary layer indeep mantle.

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