Garnet geotherms: Pressure‐temperature data from Cr‐pyrope garnet xenocrysts in volcanic rocks

Abstract

The temperatures and pressures of equilibration of single peridotitic garnet xenocrysts are estimated using a combination of major-and trace-element data, determined using electron microprobe (EMP) and proton induced X ray emission (PIXE). This new method enables the use of xenocrysts found in kimberlites and other volcanic rocks to determine the local paleogeotherm at the time of eruption of the magma which sampled and transported the xenocrysts. The "Ni thermometer" of Griffin et al. [ 1989], based on the strong temperature dependence of the partitioning of Ni between garnet and olivine, is refined using an expanded database. Pressure is calculated from garnet composition using an algorithm that combines a modification of the geobarometer of Nickel [1989], based on Cr solubility in coexisting garnet and orthopyroxene, with the composition of a hypothetical coexisting orthopyroxene. The orthopyroxene composition is estimated by inverting the geothermometry equations of Gasparik [1987], Brey and KOhler [1990], and Harley [1984], and combining these with empirical relationships describing Cr in orthopyroxene in Crsaturated peridotitc (chromite present). The derived pressure (PCr) gives the equilibration pressure of peridotic garnets provided they were in equilibrium with chromite; garnets from Cr-undersaturated rocks will produce underestimates of pressure. Therefore, the locus of maximum PCr at a given TNi defines the "garnet geotherm", and provides a method for the determination of paleogeotherms based solely on PIXE and EMP analyses of garnet grains in concentrates. The assumption of coexisting chromite is tested by comparing the temperature distributions of garnets and chromites from the same concentrate. Chromite equilibration temperature is estimated using the "Zn thermometer", based on the strong temperature dependence of the partitioning of Zn between chromite and olivine. This thermometer is calibrated against the new Ni thermometer using a suite of garnet-chromite intergrowths. The garnet geotherm technique provides an estimate of the geotherm with an accuracy comparable to xenolith-derived geotherms and provides a means of mapping the thermal state of the lithosphere where xenoliths are rare or absent. display major element partitioning between their constituent min-The use of trace-element patterns measured using proton in-eraIs that is widely regarded as quenched from mantle conditions duced X ray emission (PIXE) on the proton microprobe has proand is routinely used to locate the source of these rocks in pres-vided a partial solution to this problem. The concentration of sure (P) and temperature (73 coordinates. However, these poly-trace Ni in peridotitic garnets can be used routinely to estimate the phase mantle samples are rare in many provinces, although rem-equilibration temperature TNi for each garnet in its source rock. nants of disaggregated xeno!iths can be found as discrete grains in This empirical "Ni thermometer" [Griffin et al., 1989] is refined heavy-mineral concentrate. Garnets in these ...

Mantle transect of the Karelian Craton from margin to core based on P-T data from garnet and clinopyroxene xenocrysts in kimberlites

“…Although Mg# increases uniformly with temperature, Cr# does not, corroborating a temperature control on pyrope Mg#. Since the Ni thermometry methodology of Ryan et al (1996) employed here is calibrated against the gt/ol Fe/Mg K D mentioned above, the strong correlation of Mg# with T Ni is expected in a well-behaved, closed system. The strong correlation in the craton core data contrast with those at the craton mantle edges where there have been considerably more disturbances due to open system behavior and an influx of higher volumes of metasomatic components.…”

“…In the upper parts of these mantle sections at Lentiira and Kuhmo, TiO 2 contents of pyropes vary con- (Ryan et al, 1996) from Kaavi-Kuopio and Lentiira-Kuhmo. Garnet classification after Grütter et al (2004).…”

“…Mg# of garnet also reflects mantle bulk composition but is not as straightforward to interpret because it is also strongly affected by temperature dependent Mg-Fe exchange with olivine, orthopyroxene, clinopyroxene and Mgchromite (Stachel et al, 2003). In addition, Ca has Grütter et al (2004), T Ni caclulated using the calibration of Ryan et al (1996) and depths by extrapolating the T Ni temperatures of pyropes to the Kaavi-Kuopio geotherm determined by Kukkonen et al (2003). Megacryst garnets (G1) with Cr 2 O 3 >1.5 wt.% and MgO>18 wt.% are shown for illustrative purposes, see section 4.3.1 for discussion.…”

“…The garnets were classified based on their major element composition into harzburgitic (G10), lherzolitic (G9), wehrlitic (G12), high-Ti peridotitic (G11), lowCr megacrystal (G1), eclogitic (G3, G4), pyroxenitic (G4, G5), and crustal varieties according to Grütter et al (2004). For equilibration temperatures the Ni thermometer (Griffin et al, 1989a) was applied using the calibration of Ryan et al (1996). Equilibration pressures and temperatures of peridotitic clinopyroxene were calculated using the single-grain cpx thermobarometer of Nimis & Taylor (2000).…”

Ultrabazické Horniny Těšínitové Asociace v Západní Části Slezské Jednotky

“…3). Poněkud širší rozsah teplot (733-904 °C) poskytl spinelový termometr založený na obsazích Zn (Ryan et al 1996).…”