Appendix A3: Discussion of the thermobarometric studies of Group V grt
websterite xenoliths from Marsabit and thermobarometric calculations
The pressure-temperature (P-T) evolution of the pyroxenite xenolith from Marsabit, was
deciphered by a combination of detailed evaluation of mineral zoning patterns and
thermobarometric calculations using the major element composition of constituent minerals
and published thermobarometers suitable for mantle rocks. Additional mineral zoning patterns
(including those provided in the print version of this article) are illustrated in three additional
figures provided as Electronic Appendices A4 to A6.
Quantitative constraints on the temperature evolution were obtained by enstatite-diopside
solvus thermometry (the two-pyroxene thermometer T2Px-BK90 and the Ca-in-opx thermometer
TCa-opx-BK90 of Brey and Köhler 1990) and by the Mg/Fe partitioning between grt, cpx and opx
(Tgrt-cpx-K88: Krogh 1988; Tgrt-opx-H84: Harley 1984 and Tgrt-opx-BK90: Brey and Köhler 1990).
Pressures were calculated based on the Al partitioning between opx and grt (PAl-opx-BK90; Brey
and Köhler 1990). Maximum pressures after grt breakdown were estimated based on the Cr
content in symplectitic spl-IIb (PCr-WW'86; Carroll Webb and Wood 1986). The jadeite-albitequartz barometer from Holland (1980) can be used to constrain maximum pressures for the
formation of the late-stage plagioclase-bearing reaction zones (Pmax because quartz is missing
in the present assemblages). All thermobarometric results are compiled in a Table in the
Electronic Appendix A7.
In general, the P-T calculations yield results in accordance with the qualitative
interpretations (see above), i.e., cores of porphyroclasts still reflect higher P-T conditions
compared to porphyroclast rims and neoblast cores (see Electronic Appendix A7). In detail,
however, the application of different thermometers to single porphyroclast core compositions
reveals considerable temperature variations (up to 200°C). Independent tests of the here
applied thermobarometers have shown that they yield reliable results in the range of 8001000°C (e.g., Brey et al., 1990; Smith, 1999). Thus, the discrepancy obtained by different
thermometers is most likely due to disequilibrium with respect to some compositional
parameters between cpx, opx and grt and not to the thermometer calibration. Estimates based
on Tgrt-opx-BK90 (and Tgrt-opx-H86) yield the highest temperatures and thus might still reflect the
initial high-T stage. In contrast, calculations involving clinopyroxene (Tgrt-cpx-K88 and T2Px-BK90)
always yield lower temperatures, mostly <900°C. An explanation for this is that pyroxene
cores re-equilibrated faster to the lower temperature stage than garnet cores, which is in
agreement with flat Ca/(1-Na) profiles across cpx-I. Diopside-enstatite solvus thermometry
indicates that Ca contents in opx and cpx do not anymore preserve the high-T stage recorded
by Mg-Fe contents of grt cores.
Compared to grt, pyroxenes are smaller sized and underwent intense dynamic
recrystallisation, as indicated by undulous extinction, subgrain formation and kink bands. As a
consequence, recrystallisation would enhance chemical re-equilibration of pyroxenes during
cooling, which may explain the discrepancy between temperature solely derived from opx and
cpx compositions, compared to those taking grt compositions into account.
The arguments outlined above suggest that temperatures based on grt and opx
compositions probably yield the best estimate for the former high-T stage. This is confirmed
by a simple model illustrating the evolution of Fe/Mg ratios in co-existing grt, opx and cpx as
a function of decreasing temperature in a model bulk rock (Olker 2001; the results of the
model are illustrated in Electronic Appendix A8): While Fe/Mg ratios in opx undergo only
little change during cooling, compared to grt, similar change of Fe/Mg ratios is observed in
the case of cpx and grt. Thus, even though Fe/Mg ratios between pyroxenes and garnet are no
longer in equilibrium, Topx-grt-BK90 for mineral cores will only slightly underestimate the
temperature prior to cooling. On the other hand, estimates based on Tcpx-grt-K88 do no longer
represent former equilibrium temperatures.
Both opx-I and cpx-I exhibit well-preserved low-Al cores (see zoning profiles illustrated
in Electronic Appendices A4 and A6), pointing to former equilibrium with grt. Therefore, the
combination of Tgrt-opx-BK90 with PAl-opx-BK90 probably yields the most appropriate estimates for
the P-T stages before and after cooling and decompression (~970-1100°C at ~2.3-2.6 GPa and
~700-850°C at 0.5-1.0 GPa, respectively; see Electronic Appendix A7).
Above, pressures after cooling and decompression were calculated using PAl-opx-BK90 with
the composition of grt and opx rims. Additional maximum pressure constraints for this stage
can be obtained using the Cr content of spinel in symplectites after garnet (spl-IIb), which
yield P<1.5 GPa (Electronic Appendix A7). This is in agreement with the experimentally
derived grt-spl transition in mafic systems (1.3-1.7 GPa; Irving 1974; Hirschmann and Stolper
1996).
However, some pressures estimates based on mineral rims using PAl-opx-BK90 yield very low
values, implying crustal depths (e.g. sample Ke 1960/4: 0.18±0.12 GPa). These values may
reflect disequilibrium between opx and grt rims. Alternatively, they could reflect partial reequilibration of opx and grt during the formation of kelyphites. Pressures estimates for the
formation of these plagioclase-bearing parageneses can be constrained using a barometer
based on co-existing jadeite, albite and quartz (Holland, 1980). As quartz is missing in the
studied samples the results represent maximum values. The pressure range (0.24-1.10 GPa;
Electronic Appendix A7) broadly overlaps with values based on the grt-opx barometry. The
sometimes very low pressures, together with the fine-grained aspect of the reaction zones
growing at the expense of all mineral generations (porphyroclasts, neoblasts and symplectite
assemblages) indicate a very late-stage formation. The presence of plagioclase in the
kelyphite but not in the spl-opx-cpx symplectites led Henjes-Kunst & Altherr (1992) to the
conclusion that the latter still formed within the mantle, whereas the kelyphite formed
presumably during the ascent of the xenoliths to the surface. A likewise formation can be
suggested for the plagioclase+cpxm (low-Na-Ti-Al secondary cpx) reaction zones replacing all
other cpx generations (cpx-I, -IIa and –IIb). These reaction zones are texturally and
compositionally very similar to 'late-stage' textures formed upon decompression during the
ascent and/or heating of the xenolith in the host magma (e.g., Shaw & Edgar, 1997; Carpenter
et al., 2002; Ulianov & Kalt, 2006).
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