Emplacement of ultramafic rocks into the continental crust monitored by light and other trace elements: An example from the Geisspfad body (Swiss-Italian Alps)

Abstract

In order to evaluate the influence of continental crustal rocks on trace element budgets of serpentinized peridotites incorporated into the continental crust, we have analyzed the chemical composition of whole rock samples and minerals of the Geisspfad ultramafic complex (Swiss-Italian Alps). This complex represents a relict oceanic succession composed of serpentinites, ophicarbonates and metabasic rocks, emplaced into crustal gneisses during Alpine collision. Following peak metamorphic amphibolite facies conditions, fluid flow modified some of the trace element contents of ophicarbonates and deformed serpentinites close to the contact with country rocks. The fluid originated from the surrounding continental crustal rocks as documented by the increase of Pb in the serpentinites, and by the strongly negative ∂D values (−112‰) of some ultramafic rocks close to the contact with surrounding gneisses. Little or no modification of the fluid mobile elements Li, B or U was observed in the serpentinite. In-situ analysis of light elements of serpentinite minerals indicate redistribution of light elements coupled to changes of mineral modes towards the outer 100-150 m of the massif. In the centre of the massif, Li is preferentially concentrated in olivine, while Be and B are hosted by tremolite. In contrast, at the outer rim of the massif, Li and Be are preferentially incorporated into diopside, and B into antigorite. This redistribution of light elements among the different minerals is visible in the serpentinite, at a maximum distance of~100-150 m from the ophicarbonate-metabasite contact. Our results show that interaction of ultramafic rocks and crust-derived fluids can be easily detected by studies of Pb and ∂D in whole rocks. We argue that small ultramafic bodies potentially record an emplacement-related trace element signature, and that crustal light element values in ultramafic rocks are not necessarily derived from a subducting slab.

Subduction zone metamorphic pathway for deep carbon cycling: II. Evidence from HP/UHP metabasaltic rocks and ophicarbonates

“…The dark grey-shaded area represents the global average of modern day sub-arc depths. Table 2 of Pelletier et al, 2008). Although such rocks experience significant dehydration over the 550-800˚C temperature range, relatively little CO 2 is lost and even the warmest of the P-T paths (Cascadia) results in only ~15% loss.…”

Implications of the serpentine phase transition on the behaviour of beryllium and lithium–boron of subducted ultramafic rocks

“…This indicates that chlo- Table 1. Oceanic chlorites from Scambelluri et al (2004) and Vils et al (2008), Geisspfad field after Pelletier et al (2008a). Only few other studies have so far determined Li and B in chlorite in ultramafic rocks.…”

“…5). In-situ measurements of light elements in antigorite from metamorphic environments have been studied previously (Scambelluri et al, 2004;Marschall, 2005;Pelletier et al, 2008a). They showed that antigorite contains up to 39 lg/g B. Additionally, Scambelluri et al (2004) studied the B and Li composition in serpentine along the subduction path of ultramafic rocks, comparing samples from different localities with different P-T conditions.…”

“…Investigations along the contact between felsic and ultramafic rocks have shown, that the light element content in ophiolite bodies might be influenced over about 100-150 m from the contact (Pelletier et al, 2008a). In order to minimize the effects of fluid rock exchange with country rocks all analysed hand specimens were selected at distance to the surrounding units ( Fig.…”

The Lithium, Boron and Beryllium content of serpentinized peridotites from ODP Leg 209 (Sites 1272A and 1274A): Implications for lithium and boron budgets of oceanic lithosphere

“…As B is abundant in seawater (4.6 lg g À1 , Quinby-Hunt and Turekian, 1983; Jean-Baptise et al, 1991) compared to depleted mantle (0.04 lg g À1 , Table 6), seawater-peridotite interaction should enrich the mantle in B. Various studies on dredged abyssal peridotites have confirmed this enrichment in serpentinite (Thompson and Melson, 1970;Bonatti et al, 1984;Spivack and Edmond, 1987;Boschi et al, 2008), and studies on alpine serpentinite have demonstrated a large variability of B contents in serpentine (Scambelluri et al, 2004;Pelletier et al, 2008).…”