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GeoPeP 2003 Results 2.

Diffusion in sample E-03-01C (Deudesfeld) / Sonja Pabst

Sample E-03-01C (Fig. 1) is a rounded mantle xenolith from Deudesfeld, at the W´ side of the Meerfelder Maar.

The xenolith contains about 90 Vol. % subhedral olivine, up to 10 Vol. % clinopyroxene (chromdiopside) with some orthopyroxene and few spinel (chromite). The olivine is partially deformed (deformation lamellae) and is crossed by several fluid paths. The pyroxene has a good and slightly orientated fissility. The rim of the xenolith is partially defined by a very thin line of small cpx. (Fig. 2)

Fig. 1: Thin section of the xenolith, long side 3.8 cm.

Fig. 2: Microscopic pictures of the thin section.

For the electron-microprobe analysis we put a measuring line across the rim of an olivine to study the suspected element content changes caused by diffusion. (Fig. 3)

Fig. 3: The large picture is a BSE-image showing 2 zones at the rim (different grey scale),
the small picture presents the measuring points.

The element map (Fig. 4) from the studied rim shows a significant change of the element content partitioned in 2 zones. Especially Mg and Fe, but neither Al nor Ca, show a variation in the element content.

Fig. 4: Element map showing a change in the element content of Mg and Fe at the outer rim.

Like also shown in the diagram in Fig. 5 the Mg concentration decreases in the diffusion zone 1 from 50 wt. % to 45,5 wt. % whereby the Fe concentration increases from about 9 wt. % to 14 wt. %. In the diffusion zone 2 the Mg concentration increases by almost 1 wt. % and the Fe concentration decreases by 1 wt. %. Ca and Ti concentrations (not shown) remain constant. Notice that the “diffusion rim” is not everywhere along the olivine: dissolution removed parts of the selvage.

Fig. 5: This diagram shows the variation in element content of Mg and Fe in dependence to the distance from the centre to the rim of the olivine.

The 2 diffusion zones are caused by 2 diffusion events. The first diffusion at time t1 determined for example an element decrease towards the rim (Fig. 6). The second diffusion event at the time t2, caused by a different magma, lead to an increase of e.g. Mg towards the rim. But the second diffusion zone is not complete, because the element concentrations reach no constant value at the rim. This third event at the time t3 is dissolution of the olivine.

Fig. 6: Schematic behaviour during 2 diffusion events.

For the history of the olivine one can record that t1 > t2 > t3. The fact that the Mg concentration first decreases and secondly increases reveals that the t1-diffusion took place by existence of an intermediate melt and the t2-diffusion by a mafic melt. One cannot say how much the dissolution took away from the mineral. So the maximal width of the rim can only be a minimum assumption.
To calculate the time the xenolith remained in the melt I took different diffusion coefficients (see below). The value 8,6E-17 m2s-1 for D is from CHAKRABORTY (1997). It should be the best approximation.

The realistic time, the xenolith remained in the magma before eruption is a minimum value from 246 days for the both of the two diffusion events calculated with the D-value from CHAKRABORTY (1997).

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