Blend butter, 8 crushed raspberries, minced arugula, 1/2 teaspoon salt, and 1/8 teaspoon pepper in a small bowl with a fork. Spoon mixture onto a sheet of wax paper, and form into a 4-inch-long log. Twist ends of wax paper to seal. Refrigerate at least 1 hour.
Stir together olive oil, 8 crushed raspberries, raspberry vinegar, 1/2 teaspoon salt, and 1/4 teaspoon pepper in a shallow dish; add salmon. Cover and marinate in refrigerator 1 hour or up to 4 hours.
Remove salmon from marinade, discarding marinade. Grill salmon over medium-high heat 3 to 4 minutes on each side or to desired degree of doneness. Place 2 pats of arugula-raspberry log on each fillet, and allow to melt over fish. Garnish, if desired.