Meter-Scale Chemical and Isotopic Heterogeneities in the Oceanic Mantle, Leka Ophiolite Complex, Norway

Abstract

Mantle peridotites from three 3 x 3 metre grids sampled at kilometer distances from one another in the ca. 497 Ma Leka Ophiolite Complex (LOC), Norway, are examined in order to investigate the chemical and isotopic nature of oceanic mantle domains at the cm- to km-scale. The lithology of each grid locality is predominantly harzburgite, but includes layers and lenses of dunite and pyroxenite. Major and lithophile trace element compositions indicate a history of prior melting at pressures at or slightly below the garnet stability field. The common presence of orthopyroxenite veins likely reflects infiltration of silicic melts associated with supra-subduction zone processes. Osmium isotopes and highly siderophile element (HSE) abundance data for cm-scale sampling of traverses from the pyroxenites into the harzburgites reveal that the formation of the veins had little effect on Os isotopic compositions, and Os, Ir, Ru and Re abundances in the harzburgites. Adjacent to one of the orthopyroxenite veins studied, however, Pt and Pd abundances appear to have been strongly modified by interactions with vein-forming melts or fluids at distances of as much as 4-6 cm from the pyroxenite-harzburgite contact. Leka harzburgites have initial γOs values (% deviation from a chondritic reference) that range from -4.7 to + 2.2 (6.9% variation), with individual uncertainties of ± 0.2 units. Averaged initial Os isotopic compositions for harzburgites from the three grid sites separated by as much as 6 km, by contrast, differ by only a maximum of 2.6%. Isotopic heterogeneity on the cm- to m-scale is, therefore, larger than km-scale heterogeneity, indicating that at least some of the Os isotopic heterogeneity commonly observed globally among mantle peridotites is the result of processes that acted on a local scale. The general uniformity of these isotopic compositions among the three grid sites suggests that the portion of the oceanic mantle sampled by the LOC was relatively homogenous at the km-scale with respect to the long-term Re/Os ratio. The long-term projected Re/Os for LOC harzburgites is similar to the average required for modern abyssal peridotites. This observation strengthens prior interpretations, based largely on data for abyssal peridotites, that the Os isotopic evolution of oceanic mantle is consistent with a long-term 187Re/188Os of ∼0.38. The present ∼3 to 4% difference between the Os isotopic composition of the modern oceanic mantle and estimates for primitive mantle suggests that at least ∼6% of the mass of the oceanic mantle has been removed from it in the form of Re-enriched, mafic oceanic crust. Despite the recycling of this crust back into the mantle, most of it has evidently not been mixed back into accessible portions of the upper oceanic peridotite mantle. Compared to composition estimates for the primitive mantle, the median HSE compositions for the three grid sites are moderately- to strongly-depleted in Pd and Re, consistent with the corresponding lithophile element evidence for 20-30% melt depletion. As with initial γOs values, most harzburgites from a given grid are characterized by greater variations in absolute and relative HSE abundances than the differences between the median abundances of the three grid sampling locales. This observation indicates that as with Os isotopes, the HSE abundance heterogeneity among the harzburgites most strongly reflects cm- to m-scale melting and remobilization effects. Except for Ru, median HSE abundances for grid harzburgites are similar to median abundances for abyssal peridotites. The ∼30% lower median Ru/Ir in the LOC compared to the median ratio for abyssal peridotites suggests that the abundance of Ru in the oceanic mantle may be more variable than generally thought.

Highly Siderophile Element and Os Isotope Results From the Structurally Atypical Batin Dunite in the Wadi Tayin Massif of the Oman Ophiolite

“…constraints, harzburgites and dunites from the same pair were not always collected in close proximity (3-20 m separation) (Table 1). As shown by tightly spaced sampling of other ophiolites (Haller et al, 2021;OʼDriscoll et al, 2018;Snortum & Day, 2020) large Os isotope variations can occur between harzburgites separated by a few meters or even centimeters. The relatively large separation between each dunite and its paired harzburgite thus implies that the compositions of the harzburgites do not necessarily represent those of the harzburgites parental to the associated dunites.…”

“…This model is unavoidably speculative, as it is based on a small number of samples. As several studies have shown (Haller et al, 2021;OʼDriscoll et al, 2018;Snortum & Day, 2020), large heterogeneities in Os isotopic composition and HSE abundances can exist on length scales of a meter or less in ophiolites. Therefore, drawing conclusions from a few widely spaced samples may be unwise.…”

Modification of Lithospheric Mantle by Melts/Fluids With Different Sulfur Fugacities During the Wilson Cycle: Insights From Lesvos and Global Ophiolitic Peridotites

“…Tick marks indicate 5% melting increments. Also plotted are the ophiolitic peridotites from the settings of ocean‐continent‐transition (OCT; data from Becker et al., 2006; Fischer‐Gödde et al., 2011; Lorand et al., 1999, 2000, 2008, 2010; Luguet et al., 2007; Rampone et al., 1995; Reisberg & Lorand, 1995; Riches & Rogers, 2011; Saal et al., 2001; Snow et al., 2000; van Acken, Becker, Hammerschmidt, et al., 2010; van Acken, Becker, Walker, et al., 2010; van Acken et al., 2008; Wang et al., 2013), mid‐ocean‐ridge (MOR; data from Aldanmaz et al., 2012; Batanova et al., 2008; Dijkstra et al., 2010; Gong et al., 2020; Hanghøj et al., 2010; Hu et al., 2020; Schulte et al., 2009; Sergeev et al., 2014; Uysal et al., 2012) and supra‐subduction‐zone (SSZ; data from Agranier et al., 2007; Aldanmaz et al., 2012, 2020; Büchl et al., 2002, 2004; Chen et al., 2020; Haller et al., 2021; C. Z. Liu et al., 2018; O'Driscoll et al., 2012, 2015, 2018; Prelević et al., 2015; Secchiari, Gleissner, et al., 2020; Snortum & Day, 2020; Uysal et al., 2012; Xu & Liu, 2019). Arrows show the possible additional processes to account for the data.…”

“…HSE systematics and Re‐Os isotopes further support the conclusion that the Lesvos ophiolite was formed in an OCT environment: (a) Most of the Lesvos lherzolites have PUM‐like HSE patterns (Figure 4) and plot in the modeled near‐fractional partial melting trend (Figure 5), both of which can well match their relatively fertile “Alpine” character coupled with low degrees of melt extraction (Koglin et al., 2009); (b) with regards to absolute and relative HSE abundances and Re‐Os isotopic compositions (Figures 5–7), almost all the Lesvos peridotites fall within the field of the OCT and MOR peridotites, but mostly within the OCT field, indicating that the Lesvos peridotites have not experienced subduction‐related processes, in contrast to the SSZ setting of most Mesozoic ophiolites exposed in the Eastern Mediterranean Tethyan region (A. H. F. Robertson, 2002); (c) given that the impact of melt‐rock interaction on 187 Os/ 188 Os ratios is relatively limited (see Section 5.2.1), it is more likely that the near‐horizontal 187 Os/ 188 Os versus Al 2 O 3 variation of the Lesvos peridotites, especially the depleted harzburgites (Figure 7) was due to recent melt depletion during rifting and thinning of the Lesvos lithosphere (Koglin et al., 2009), along with the continuous creation of oceanic lithosphere by an incipient Neo‐Tethyan rift; (d) the ancient T RD age (1.6 Ga) of sample 17AC09 can reflect the mantle Os‐isotope heterogeneity of the type observed in other oceanic lithosphere (e.g., Brandon et al., 2000; Haller et al., 2021; Harvey et al., 2006; C. Z. Liu et al., 2008). Focusing on the Alpine‐Apennine peridotites typical for the OCT setting, McCarthy and Müntener (2015, 2019) have argued that those ancient and variably depleted, refractory domains might be previously recycled into convective mantle and subsequently exhumed during continental breakup and rifting.…”

“…Arrows show the possible additional processes to account for the data here and in the rest figures. reflect the mantle Os-isotope heterogeneity of the type observed in other oceanic lithosphere (e.g., Brandon et al, 2000;Haller et al, 2021;Harvey et al, 2006;C. Z. Liu et al, 2008).…”