New perspectives on the origin of olivine-rich troctolites and associated harrisites from the Ligurian ophiolites (Italy)

Abstract

The mid-ocean ridge (MOR)-type gabbroic sequences from the Internal Ligurian ophiolites include olivine-rich troctolite lenses up to tens of metres thick. For one of these lenses, a portion of metre-scale thickness characterized by skeletal to dendritic olivines (harrisites) was observed near the contact with host gabbros. Spinels from the olivine-rich troctolites locally include Ti-pargasite to kaersutite frequently associated with phlogopite to Na-phlogopite. Spinel-hosted amphibole displays rare earth element (REE) patterns characterized by a negative Eu anomaly, thereby recording a magmatic process associated with plagioclase segregation. The amphiboles show variable depletion of light REE (LREE) relative to middle REE (MREE), and heavy REE (HREE) that are weakly enriched to depleted with respect to MREE. Crystallization of the inclusion-bearing spinels is attributed to cooling of hybrid melts that originated by interaction between primitive melts and gabbro-related melts relatively rich in SiO 2 and incompatible elements. Clinopyroxene and amphibole from the harrisites show extensive variability for trace element compositions, albeit characterized by subparallel incompatible element patterns. This chemical variability was most probably acquired in response to rapid crystal growth related to undercooling of the harrisite parental melt. We propose that the melt undercooling was related to interaction of a primitive melt batch with a gabbroic crystal mush. Supplementary material : Details of the analytical technique are available at http://doi.org/10.6084/m9.figshare.c.3284312

Multi-stage Reactive Formation of Troctolites in Slow-spreading Oceanic Lithosphere (Erro–Tobbio, Italy): a Combined Field and Petrochemical Study

“…Clinopyroxene cores from Troctolite A have high Cr 2 O 3 contents (Fig. 11a and c), similar to those described in oceanic gabbroic rocks analyzed at the Mid-Atlantic Ridge (Lissenberg & Dick, 2008;Lissenberg & MacLeod, 2016;Ferrando et al, 2018), easternmost South-West Indian Ridge (Paquet et al, 2016) and Godzilla Megamullion (Sanfilippo et al, 2016b), and in olivine-rich troctolites from the Internal Liguride ophiolite (Renna & Tribuzio, 2011;Renna et al, 2016). Although pMELTS (Ghiorso et al, 2002) does not allow the Cr 2 O 3 compositional modelling of clinopyroxene, the process of partial dissolution and recrystallization of a dunite (olivine þ spinel) described above could well explain the Cr 2 O 3 enrichments observed in the clinopyroxene cores (Fig.…”

“…Within Troctolite A, interstitial minerals often develop embayments on corroded relicts of spinel grains. This indicates partial dissolution of Cr-rich spinel (Cr# ¼ 55-65 in the dunite; Table 6) together with the olivine during the reactive melt percolation process, as was previously described in oceanic settings and in the Internal Liguride ophiolites during replacive formation of olivine-rich gabbroic rocks (Lissenberg & Dick, 2008;Renna & Tribuzio, 2011;Lissenberg & MacLeod, 2016;Paquet et al, 2016;Renna et al, 2016;Sanfilippo et al, 2016b;Ferrando et al, 2018). The corrosion of spinel leads to Cr 2 O 3 enrichments in the reacting melt, therefore explaining the Crrich compositions of clinopyroxenes crystallized from the percolating modified melt (Fig.…”

“…7c and d) (Rampone et al, 2016). Hopper and dendritic morphologies of olivine have been previously described in the Rum Layered Intrusion (Donaldson, 1974(Donaldson, , 1977(Donaldson, , 1982O'Driscoll et al, 2007), in olivine-rich troctolites from the Ligurian ophiolites (Renna et al, 2016) and in crystallization experiments (Donaldson, 1976(Donaldson, , 1977Faure et al, 2003Faure et al, , 2007 resulting from rapid disequilibrium crystallization of an undercooled melt (driven by a difference between the liquidus temperature of the melt and the melt temperature). Olivine CPO in the granular portion of Troctolite B shows random orientations of the [100] axis, strong concentrations of the [010] axis normal to the foliation, and the [001] axis being the strongest axis concentration within the foliation plane (Fig.…”

“…Recent studies have demonstrated that melt-rock interactions can lead to extensive small-scale structural and geochemical heterogeneities within percolated mantle peridotites at different depths (e.g. Quick, 1981Quick, , 1982Dijkstra et al, 2002Dijkstra et al, , 2003Lissenberg & Dick, 2008;Soustelle et al, 2009Soustelle et al, , 2010Soustelle et al, , 2014Collier & Kelemen, 2010;Higgie & Tommasi, 2012Tursack & Liang, 2012;Saper & Liang, 2014;Dygert et al, 2016;Paquet et al, 2016;Renna et al, 2016;Sanfilippo et al, 2017), and can act as a rock-forming process for replacive lithotypes. In extensional settings worldwide, spinel harzburgites and spinel dunites showing decoupled bulkrock and mineral chemistry have been interpreted as the replacive products of open-system reactive melt percolation at spinel-facies depths, driven by pyroxene dissolution and olivine crystallization (e.g.…”

Revisiting the Australian‐Antarctic Ocean‐Continent Transition Zone Using Petrological and Geophysical Characterization of Exhumed Subcontinental Mantle

“…(a) Mg# Fetot vs. TiO 2 wt%; (b) Mg# Fetot vs. Cr 2 O 3 wt%; (c) Mg# Fetot vs. Al 2 O 3 wt%; and (d) Mg# Fetot vs. Na 2 O wt%. Data for the Western Tethys from Rampone et al (1998), Borghini et al (2007), Borghini and Rampone (2007), Rampone et al (2008), Montanini et al (2008), Renna and Tribuzio (2011), Renna et al (2016), McCarthy and Müntener (2019), and Basch et al (2019). Mid‐ocean ridge data from Sanfilippo et al (2013), Dick et al (2010), Lissenberg and Dick (2008), Suhr et al (2008) and Leuthold et al (2018).…”