Terrestrial analogues are often investigated to get insight into the geologic evolution and magmatic processes of planetary bodies. Theo's flow (Ontario, Canada) is a 120m thick magmatic unit containing (bottom to top) the following layers: peridotite (up to 9m); clinopyroxenite (up to 60m), gabbro (up to 40m) and hyaloclastite (up to 10 m thick). The pyroxenite is regarded as the best terrestrial analogue of nakhlite, including Miller Range (MIL) 03346. Based on a careful petrological and stratigraphic analysis Lentz et al., (2011) concluded that the lithologic diversity observed in Theo's flow (TS) results from a differentiation processes of a single, thick magma pulse, and was not generated by multiple magmatic injections of distinct composition. Their conclusion is mainly based on the absence of sharp boundaries between adjacent units and on the progressive compositional changes observed in mineral and bulk-rock compositions (Lentz et al., 2011). These authors also observed that the average composition of the whole magmatic pile matches the average composition of the top, quenched hyaloclastite. Given the analogies of the clinopyroxenite with Martian lava flows, the mechanism and timing of differentiation of Theos's flow is of great interest and yet, it is still not fully understood thus deserving further investigation. Aiming at better understanding magmatic evolution and the cooling history of whole Theo's flow, we use a geothermometric calibration to calculate the equilibrium closure temperature (Tc) of the Fe2+-Mg intracrystalline exchange reaction between M1 and M2 sites in augites separated from four samples collected at the lower (TSC 3.9; TSC 3.12), middle (TSC 3.22) and upper (TSC 3.31) part of the flow. All selected clinopyroxene have Fs composition within the compositional range reported in Murri et al. (2016) calibration. For each crystal we performed a high-resolution single-crystal X-ray diffraction analysis followed by structural refinement. The resulting Tc, calculated for all investigated clinopyroxenes, are almost identical within error (682°C ± 30) and are also similar to the clinopyroxene Tc previously obtained from TS7 Theo's sample (Domeneghetti et al., (2013) located in lower middle part of the flow. This relatively uniform Tc appears inconsistent with the differential cooling rate expected for samples collected at significantly different height (bottom to top) across a ca. 120 thick lava flow and cannot support a scenario of a single magma unit differentiated in situ.
NEW INSIGHTS ON THEO'S FLOW LAVA USING INTRACRYSTALLINE THERMOMETRY ON AUGITES
Fioretti AM;
2016
Abstract
Terrestrial analogues are often investigated to get insight into the geologic evolution and magmatic processes of planetary bodies. Theo's flow (Ontario, Canada) is a 120m thick magmatic unit containing (bottom to top) the following layers: peridotite (up to 9m); clinopyroxenite (up to 60m), gabbro (up to 40m) and hyaloclastite (up to 10 m thick). The pyroxenite is regarded as the best terrestrial analogue of nakhlite, including Miller Range (MIL) 03346. Based on a careful petrological and stratigraphic analysis Lentz et al., (2011) concluded that the lithologic diversity observed in Theo's flow (TS) results from a differentiation processes of a single, thick magma pulse, and was not generated by multiple magmatic injections of distinct composition. Their conclusion is mainly based on the absence of sharp boundaries between adjacent units and on the progressive compositional changes observed in mineral and bulk-rock compositions (Lentz et al., 2011). These authors also observed that the average composition of the whole magmatic pile matches the average composition of the top, quenched hyaloclastite. Given the analogies of the clinopyroxenite with Martian lava flows, the mechanism and timing of differentiation of Theos's flow is of great interest and yet, it is still not fully understood thus deserving further investigation. Aiming at better understanding magmatic evolution and the cooling history of whole Theo's flow, we use a geothermometric calibration to calculate the equilibrium closure temperature (Tc) of the Fe2+-Mg intracrystalline exchange reaction between M1 and M2 sites in augites separated from four samples collected at the lower (TSC 3.9; TSC 3.12), middle (TSC 3.22) and upper (TSC 3.31) part of the flow. All selected clinopyroxene have Fs composition within the compositional range reported in Murri et al. (2016) calibration. For each crystal we performed a high-resolution single-crystal X-ray diffraction analysis followed by structural refinement. The resulting Tc, calculated for all investigated clinopyroxenes, are almost identical within error (682°C ± 30) and are also similar to the clinopyroxene Tc previously obtained from TS7 Theo's sample (Domeneghetti et al., (2013) located in lower middle part of the flow. This relatively uniform Tc appears inconsistent with the differential cooling rate expected for samples collected at significantly different height (bottom to top) across a ca. 120 thick lava flow and cannot support a scenario of a single magma unit differentiated in situ.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
