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GeoPep 2003 Results 1.

Diffusion in a CPX-crystal (West-Eifel, Brück) / Michel Bechtold

The analysed 4 x 2,5 x 2,5 cm clinopyroxene (cpx), an isolated euhedral mega-crystal, is part of pyroclastic deposits near Brück (West-Eifel).
The cpx is in transmitted and reflected light brownish grey and riddled with glass-filled micro cracks and fluid inclusion paths. There is an in reflected light white, partly resorpted thin growth-seam around the euhedral sides of the crystal with abundant inclusions of other minerals, especially phlogopite. After this growth-seam follows an accretion of magma at the rim. Inside the rim-covering magma are the same minerals like in the growth-seam. At one side the crystal is broken and the white growth-seam is missing.

In reflected light there is a striking zonation crossing the whole crystal. The abundant micro cracks all intersect this boundary, a proof of their younger genesis.
The origin of this intra-crystal boundary is obscure, but its existence can be used to estimate the time that has passed since its formation until the eruption of the crystal, the so-called residence time. Only when the crystal is surrounded by magma, the temperature is high enough to generate an effective diffusion. After eruption the diffusion processes stop. Thus the diffusion zone at the intra-crystal boundary of the cpx can deliver information about its residence time in the magma.
The diffusion zone was characterized by ICPMS-Laser-Ablation, microprobe and optical grey scale analysis.

Using the Laser-Ablation ICP-MS we measured a profile of some 45 points through the whole CPX perpendicular to the noticeable boundary crossing the crystal.
The systematic variations in major and trace elements indicate a typical differentiation pattern (Fig. 1+2).


Fig. 1: Thin section of the analysed cpx-megacrystal, the black line shows the measured profile.

Fig. 2: Both the Mg/Fe-ratio and the Ni/Zr- and Sc/Ti-ratios clearly show the differentiation trend in the light grey part of the crystal in growth direction. The dark grey part is characterised by a relatively constant composition, similar to that at the quite differentiated outer region of the light grey part.

To get a better resolution of the intra-crystal boundary, this part was also analysed by the electron microprobe (Fig. 3). The spatial resolution of the electron microprobe is insufficient. Thus the diffusion penetrating depth can be limited to 40 µm, but the real value is probably much shorter.



Fig. 3: Mg is chosen here as the best proxy of the intra-crystal boundary. The diffusion penetrating depth is limited to <40 µm.

An alternative to better characterize the diffusion penetrating depth is the gradient of the grey scale at the intra-crystal boundary (Fig. 4). This method yields a maximum value of <10 µm.



Fig. 4: Gray scales as proxy of the intra-crystal boundary.

BRADY and MCCALLISTER (1983) report a temperature-dependent interdiffusion coefficient of Mg in clinopyroxene DMg,cpx, illustrated in Fig. 5.
By defining the temperature range to 1200-1300 °C, the D-value is limited by a maximum and a minimum (Fig. 5). The following calculation, illustrated in Fig. 6, is based on the simplified equation:

d: one-dimensional diffusion penetrating depth [m]

D: diffusion coefficient [m2/s]
t: time [s]

Considering the gray scale estimate for diffusion depth as the more adequate method, the maximum residence time of the crystal in the magma ranges from 4 to 25 years.




Fig. 4: Temperature dependence of the interdiffusion coefficient of Mg in clinopyroxene
D(Mg,cpx) (Brady and McCallister, 1983)



Fig. 5: Calculation of the residence time t using the simplified equation d = (2Dt)^1/2

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