PETROLOGY AND GEOCHEMISTRY OF CROSSCUTTING DIABASE DIKES, SITES 920 AND 921

Pamela D. Kempton and John F. Casey

ABSTRACT

New major-element, trace-element, and Sr-, Nd-, Pb-, and O-isotope data are presented for <0.75-m.y.-old diabase dikes recovered during Leg 153. Diabase dikes encountered at Site 920 have MgO contents ranging from 13 to 15 wt%, along with high Ni and Cr and low concentrations of most incompatible trace elements. However, concentrations of most major elements and some mobile trace elements have been strongly modified by hydrothermal alteration, leading to enrichment in MgO and Sr, and to depletions in CaO, FeO, and TiO2. Sr isotopes indicate that the fluids responsible for alteration of the diabase at Site 920 had low 87Sr/86Sr compared with seawater; these low values are thought to be typical of the fluid composition during the earliest stages of alteration, not only of the diabase dikes, but also of the peridotite section they crosscut. Depletion in 18O (+4.9 to +5.2) relative to average mid-ocean ridge basalt (MORB) is consistent with hydrothermal alteration involving seawater at moderate temperatures (250-300C).

Estimates of the original composition of the Site 920 diabase before alteration suggest they contained ~11 wt% MgO and were remarkably similar to the primary melt compositions proposed by Sullivan (1991) for the Mid-Atlantic Ridge south of the Kane Fracture Zone (MARK) area. In contrast, diabase dikes at Site 921 are similar in composition to recent MARK basalts (MgO ~8 wt%), but have slightly higher CaO and Al2O3 and lower incompatible trace-element contents at a given MgO.

Trace-element modeling is consistent with the diabase from Site 920 being generated as a pooled melt by <15% melting of mantle peridotite in the spinel facies (i.e., shallow depths of melting). Recent MARK basalts and diabase from Site 921 were generated by slightly larger degrees of melting. Lower 143Nd/144Nd ratios (0.51313) and flatter REE patterns (Ce/Yb = ~1) are consistent with a less-depleted source for the Site 920 diabase than for that giving rise to the majority of MARK gabbros (0.51318-0.51322) and recent MARK basalts (0.51314-0.51318; Ce/Yb = 0.9). The average isotopic composition of the mantle source supplying the MARK magma system has changed over a period of <1 m.y., becoming more radiogenic in Pb, but less radiogenic in its Nd-isotope composition with time. This indicates that, on average, the present mantle source is less depleted than that ~750,000 yr earlier. However, the contemporaneity of rocks derived from relatively depleted sources (i.e., Sites 921-924 gabbros and Site 921 dikes) and those from significantly less depleted sources (i.e., Site 920 diabase and amphibolitized microgabbro) supports a mixing model in which the proportion of enriched to depleted mantle components changes with time, rather than a gradual compositional shift of a homogeneous source.

Date of initial receipt: 4 August 1995
Date of acceptance: 22 March 1996


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